Diplomarbeit zur Erlangung des akademischen ...

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Thanks are due to Drs. Thomas Zack and Matthias Barth for supply .... focus of this thesis centers on a rock group known as the Ancient Gneiss Complex (AGC; ...... particularly for mobile elements such as K, Rb, and Cs. The degree of alkali loss ...... Shang, C.K., Satir, M., Siebel, W., Nsifa, E.N., Taubald, H., Liégeois, J.P., ...


Johannes Gutenberg Universität Mainz Institut für Geowissenschaften Fachrichtungen Metamorphe Geologie/Isotopengeologie

Diplomarbeit zur Erlangung des akademischen Grades Diplom-Geologe    

Petrography, petrology, as well as zircon and garnet geochronology of Archaean granulites from Swaziland, southern Africa    

eingereicht bei:

Prof. Dr. Richard White Prof. Dr. Alfred Kröner

von:

Nils Suhr

wohnhaft:

Auf dem Hewwel 2 55129 Mainz

E-Mail:

[email protected]

Ort/Datum:

Mainz, 02.01.2013

Contents Abbreviations……………………………………………………………...……………..………...1 Acknowledgements………………..…………………………………...…………………………..3 Abstract…………………………………………………………..……………………..………….4 Zusammenfassung………………………………………………...………………………………..5 1. Introduction……………….…………….……………...………..………………………..………….6 2. Geological Overview……………..……………………...……..…..………………………………...9 2.1 Geology of the Kubuta Shiselweni Cattle Ranch……………………..….…….……………..12 3. Petrography……………………………………………………………..…...………………………14 3.1 Hlatikulu granite………………………………………………...……...……………………..14 3.2 Mahamba orthogneiss……………………………………………………..………………….16 3.3 Metasemi-pelite (Kinzigite)………………….………………......………...…………………18 3.4. Gabbro………………………………………………………...……...…...………………….23 3.5 Hornblendite………………………………………………...……………...……..…………..24 3.6 Metavolcanic rocks…………………………………………………...…………….………...25 4. Analytical methods………………………………………...…..……………………………………28 4.1 U-Pb dating on zircons……………………………………………..…...…………………….28 4.2 Whole-rock geochemistry…………………………………………….………………………29 4.3 Lu-Hf/Sm-Nd dating on garnets…………………………………...…..…...……………...….29 4.4 Electron Microprobe analyses………………………………………..….………………...….31 5. Geochemistry……………………………………………………………...……………..………….32 5.1 Geochemical Classification of AGC rocks following Hunter (1978)……………...…………32 5.2 Gabbro…………………..……………………………………...……………..………………35

5.3 Metavolcanic rocks and hornblendite……………..………………………..………………...37 5.4 Granitoid rocks…………………………………………………………..……..…….………........46 5.4.1 Hlatikulu granite………………………………...………..………….………………...48 5.4.2 Mahamba orthogneiss…………………………….…………………..……………......55 5.5 Meta-semipelite……..………………………………...……………...………………...……..60 6. Mineral Chemistry………………………………………………………..…...………….…………67 6.1 Garnet…………………………………………………………...…………..………………...67 6.2 Biotite…………………………………………………………...…………..………………...71 6.3 Feldspar………………………………………………...…………………..…………………72 6.4 Orthopyroxene………………………………………………………...…………...………….74 7. THERMOCALC…………………………….……………………………..………..………..……..76 7.1 Mineral equilibria modelling………………..……………………...………………...……….76 7.2 Interpretation of mineral equilibria modeling………………..…………………...…………..84 8. Geochronology…………………………………………………...……………………...………….90 8.1 U-Pb zircon geochronology…………………..………….…………………..…...…………..90 8.2 Lu-Hf garnet geochronology………………………..……………………………..………...101 8.3 Sm-Nd garnet geochronology………………….………………………………...………….103 8.4 Discussion and interpretation of U-Pb and Lu-Hf ages……………………..……...……….105 9. Summary and conclusions…...…...……………………………………………..…...…………….108 References…………………………...……………………………......…………………………116 Eidesstattliche Erklärung……………………………………………...………...…………...…..129

Abbreviations AGC

Ancient Gneiss Complex

Alm

Almandine

ASI

Aluminium saturation index

BGB

Barberton Greenstone Belt

BSE

Backscattered electron images

CL

Cathodoluminescence

DGB

Dwalile Greenstone Belts

Fig.

Figure

Grs

Grossular

HFSE

High Field Strength Elements

HREE

Heavy Rare Earth Elements

IAT

Island Arc Tholeiite

ICP-MS

Inductively Coupled Plasma Mass Spectrometry

LFC

Large Format Cell

LILE

Large Ion Lithophile Elements

LOI

Loss On Ignition

LP/HT

Low Pressure/High Temperature

LREE

Light Rare Earth Elements

MALI

Modified Alkali-Lime Index

MOR

Mid Ocean Ridge

MORB

Mid Ocean Ridge Basalt

MSWD

Mean square of weighted deviates

MVMS

Mkhondo Valley Metamorphic Suite

NCC

North China Craton

Nd:YAG

Neodymium-doped yttrium aluminum garnet

NMORB

Normal Ocean Ridge Basalt

OIB

Ocean Island Basalt

Suhr, 2013. Petrography, petrology and geochronology of Archean granulites, Swaziland.

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P

Pressure

Prp

Pyrope

REE

Rare Earth Element

syn-COLG

Syn-collision granites

T

Temperature

TIMS

Thermal Ionization Mass Spectrometry

TTG

Tonalite-Trondhjemite-Granodiorite

VAG

Volcanic Arc Granite

WPG

Within Plate Granite

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Acknowledgements First and foremost I thank Profs Alfred Kröner and Richard White for their recommendations and general support while I was working on my thesis. Also, Prof. Kröner for giving me the opportunity to accompany him on one of his journeys to South Africa and Swaziland and for his teaching on Archean geology and Prof. White for his teaching on metamorphic P/T condition estimates, the THERMOCALC software and his patience while reading my thesis and improving the English. I acknowledge the faculty and staff of the Institute of Geoscience at Mainz University, whose assistance was invaluable, and the Geological Survey and Mines Departement of Swaziland for supply and field assistance. Thanks are due to Drs. Thomas Zack and Matthias Barth for supply discussions during U-Pb LA-ICP-MS analyses. Thanks to Dr. Stephan Buhre and Ms. Nora Groschopf for assistance and discussion during EMS and XRF analysis. Special thanks are due to Dr. Elis Hoffmann for assistance during laboratory work at the Steinmann-Institute Bonn and for analysing Lu-Hf garnet separates. Last but not least I want to thank Dr. Yamirka RojasAgramonte for providing me with the software PepiAGE, GCDkit, and CANVAS and for her assistance during heavy mineral separation and the hand picking of zircons and garnets.

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Abstract A petrographic, petrological, geochemical, and geochronological study (U-Pb/Lu-Hf) was carried out on granulitefacies paragneisses and orthogneisses of the Mahamba Gneiss Complex in central Swaziland to provide an insight into the metamorphic conditions and the geodynamic setting from which these rocks were derived. The results of this study show that the metamorphic history of the Mahamba Gneiss cannot be explained by a modern-type subductioncollision model. The data suggest that peak metamorphism occurred at ca. 2.9 Ga. The clockwise P-T path in this study has also no similarity to clockwise P-T paths of modern-subduction-collision tectonic settings. The combination of Lu-Hf dating of garnet, in-situ U-Pb dating of zircon and petrography suggest that Archaean granulites of the Mahamba Gneiss are probably related to a series of short-lived plume events that operated in the Archaean and generated a previously unrecognised Mesoarchaean Large Igneous Province in the southeastern Kaapvaal Craton. The analysis of detrital zircons constrains the time of deposition of the sedimentary protoliths to ca. 3.26 Ga. Furthermore, the rock chemistry suggests that a greenstone terrane was potential sedimentary source, since the Barberton Greenstone Belt consists of mafic and ultramafic lithologies with compositions that may explain the enrichments in Fe, Cr, and Ni in the Mahamba metasediments. A detrital zircon with an age of ca. 3.7 Ga may represent an older crustal source as basement for the greenstones. The Mahamba orthogneiss is a high Al-TTG and the geochemical analysis, as well as ca. 3.5 Ga and 3.2 Ga xenocrystic zircons show that older crustal sources played an important role in the generation of these gneisses. For this reason the arc signature in these rocks is not necessarily subduction-related. The age of crystallization of the Mahamba orthogneiss was dated between ca. 3.3-3.2 Ga, and metamorphic zircons date the same metamorphic event as the paragneiss at ca. 2.9 Ga. Zircons of the Hlatikulu granite show crystallization ages of ca. 2.7 Ga, and the granite shows features common to granites emplaced in stable and cooling regions of thickening lithosphere. Therefore, the zircon ages as well as the generation of the Hlatikulu granite are in agreement with the interpretation that plume processes operated beneath the Kubuta Shiselweni Cattle Ranch area at ca. 2.9 Ga. The geochemistry of a metabasaltic komatiite and a Fe-tholeiite is similar to that of the Dwalile metavolcanics in SW Swaziland and the Onverwacht Group of the Barberton Greenstone Belt. This suggests a proposed genetic link and similar mechanisms of formation between the Barberton Greenstone Belt, the Dwalile Greenstone Belts, and the high-grade mafic-ultramafic lithologies near the Kubuta Shiselweni Cattle Ranch.

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Zusammenfassung In dieser Arbeit wurde eine petrographische, petrologische, geochemische und geochronologische Studie (U-Pb/LuHf) an granulitfaziellen Para- und Orthogneisen des Mahamba Complex in Zentral-Swasiland durchgeführt. Dies ermöglicht es, die metamorphen Bedingungen zu rekonstruieren und somit auch das geodynamische Milieu abzuleiten, in welchem sich diese Gesteine gebildet haben. Die metamorphe Entwicklung des Mahamba Gneises ist nicht mit einem modernen Subduktions-Kollisions Modell erklärbar. Der Höhepunkt der Metamorphose war vor ca. 2.9 Ga und der im Uhrzeigersinn verlaufende P-T Pfad hat keine Ähnlichkeit zu P-T Pfaden moderner Kollisions Orogenene. Die Kombination einer Lu-Hf Datierung an Granat, von in situ U-Pb Datierungen an Zirkon, im Zusammenspiel mit den petrographischen Untersuchungen, lässt darauf schliessen, dass die Entwicklung archaischer Granulite des Mahamba Gneises in Zusammenhang mit einer Serie von kurzlebigen Plumes steht, die während des Archaikums aktiv waren und einer bisher unerkannten Mesoarchaischen Large Igneous Province (LIP) im südöstlichen Kaapvaal Kraton angehörten. Mit Hilfe von detritischen Zirkonen war es möglich, das Ablagerungsalter des sedimentären Protolithen der Mahamba Paragneise auf ca. 3.26 Ga einzugrenzen. Des Weiteren lässt der Gesteinschemismus auf Grünsteingürtel als ein potentielles sedimentäres Liefergebiet schliessen, wobei der Barberton Grünsteingürtel zum Teil aus mafischen und ultramafischen Lithologien besteht, welche die Anreicherungen an Eisen, Chrom und Nickel in den Mahamba Metasedimenten erklären könnten. Ein detritischer Zirkon weist ein Alter von 3.7 Ga auf, was auf eine sehr alte kontinentale Kruste hindeutet, welche die Grünsteingürtel wohl unterlagerte. Der Mahamba Orthogneiss ist ein high-Al TTG. Basierend auf seiner geochemischen Analyse und anhand ca. 3.5 Ga und 3.2 Ga xenocrystischer Zirkone ist es wahrscheinlich, dass ältere krustale Quellen eine entscheidende Rolle in der Genese dieses Gneises spielte. Aus diesem Grund sind InselbogenSignaturen

nicht

zwangsläufig

Indikatoren

für

Subduktionsprozesse.

Zirkonmessungen

ergaben

ein

Kristallisationsalter von ca. 3.3-3.2 Ga für den Mahamba Orthogneiss, jedoch enthält dieser auch metamorphe Zirkone, welche die Metamorphose des Mahamba-Paragneis um ca. 2.9 Ga bestätigen. Zirkone des Hlatikulu Granites haben Kristallisationsalter von ca. 2.7 Ga. Der Hlatikulu Granit zeigt typische Indikatoren eines Granits, welcher in stabile und auskühlende Bereiche verdickter Lithosphäre intrudierte. Die anfängliche Kristallistion von Zirkonen des Hlatikulu Granit und seine abgeleitete Genese passen daher gut in das Schema der Druck-Temperatur Diagramme, die Plume–Prozesse um ca. 2.9 Ga auf der Kubuta Shiselweni Cattle Ranch andeuteten. Die Geochemie eines metabasaltischen Komatiites und eines eisenreichen Basaltes ähnelt denen der Dwalile Metavulkanite in SW Swasiland und der Onverwacht Gruppe des Barberton Grünsteingürtels. Daraus lässt sich ableiten, dass eine genetische Verknüpfung zwischen dem Barberton Grünsteingürtel, den Dwalile Grünsteingürteln und den hochmetamorphen mafischen-ultramafischen Lithologien auf der Kubuta Shiselweni Cattle Ranch bestand und die Bildungsmechanismen ähnlich waren.

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1. Introduction Several rock samples were collected along the Matsanjeni River bed on the Kubuta Shiselweni Cattle Ranch in central Swaziland during field work in May 2010. Table 1 shows the sample localities and main lithologies of the studied rocks. In the next chapter the rocks will be described petrologically. Table 1. Samples of this study with GPS coordinates and lithology.

Sample Number

Coordinates

Lithology

AGC 401

S26°49.244 E31°25.123

Basaltic Komatiite

AGC 402

S26°49.233 E31°25.453

Hlatikulu granite

AGC 405

S26°49.148 E31°25.453

Hlatikulu granite

AGC 408

S26°49.148 E31°25.842

Gabbro

AGC 413

S26°49.148 E31°25.843

Gabbro

AGC 414

S26°49.148 E31°25.843

Metabasalt

AGC 415

S26°49.236 E31°26.034

Kinzigite

AGC 417

S26°49.429 E31°23.618

Kinzigite with leucosome

AGC 419

S26°49.441 E31°23.681

Kinzigite with leucosome

AGC 420

S26°49.440 E31°23.469

Hornblendite

AGC 421

S26°49.520 E31°23.512

Mahamba orthogneiss

AGC 423

S26°49.171 E31°25.803

Mahamba orthogneiss

Geographically this area is located in the Kingdom of Swaziland, a small country of ca. 17,346 km2, located between South-Africa and Mozambique, with a population of some 1.37 million people (estimate of July 2011) (Schlüter, 2006; Internet reference I). The landscape is characterized by hills up to several hundred metres high and intersected by small river valleys. The hills are mostly covered by bushy grassland, whereas alongside the rivers small forests are common. Ca. 2.7 Ga granitic rocks and younger diabase dykes form a high relief and are more resistant to erosion than rocks of the Ancient Gneiss Complex (AGC) and the Usutu Suite. For this reason the rocks of the AGC mostly fill the low-lying valleys (Schoene and Bowring, 2010). The vegetational and morphological features are typical for the central part of Swaziland, known as the Middleveld. Rivers and areas of high relief play an important role in finding pertinent

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outcrops in this region. The best time for fieldwork is during winter (dry season) between May and August, in which rivers have their lowest water level or are dry (Fig.1).

Fig. 1. Strongly deformed tonalitic gneiss with granitic veins in seasonal dessicated Matsanjeni River. Photo taken at location S26°49.196' E31°25.537' at 5/13/10.

Fig. 2 shows the sample localities on a Google Map. The road MR 25 in this map is also shown in Fig. 5 of the geological map of Müller (1989), which may help finding outcrops for future studies. Fieldwork was supervised by Prof. Alfred Kröner, who gave an introduction on the different lithologies in the working area as well as the requirements for collecting fresh samples for geochemical and isotopic analysis. The daily starting point for fieldwork was the Kubuta Shiselweni Cattle Ranch, which is located on a hill near the valley of the Matsanjeni River.

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Fig. 2. Google Earth map of the Matsanjeni River area. Coordinates of sample localities are shown in Table 1. Inset map showing the location of Swaziland in Sout Africa (after www.1000fights.com)

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Fig. 3. View to the South from the Kubuta Sisa Cattle Ranch into the valley of the Matsanjeni River.

2. Geological Overview The study area is part of the eastern Kaapvaal Craton of southern Africa and is well known to contain some of the oldest crustal relicts on Earth (Poujol, 2007). The Kaapvaal Craton records some of the earliest crustal evolution, not only in southern Africa but also globally. The main focus of this thesis centers on a rock group known as the Ancient Gneiss Complex (AGC; Anhaeusser, 1981) and first described by Hunter (1970, 1973, 1974) that has become a prominent name for geologists working in the Archaean of southern Africa. The AGC comprises a diversity of rock types with main occurrences in central Swaziland. It is composed of granitoids and polydeformed granitoid gneisses of the tonalite-trondhjemite-granodiorite (TTG) association ranging in age between Palaeo and Mesoarchaean (~3640 to ~3200 Ma), as well as amphibolites, which are believed to represent metamorphosed gabbroic dykes (e.g. Hunter et al., 1984; Kröner 2007). The four main lithological units of the AGC are the Ngwane Gneiss, the Dwalile Suite, the Tsawela Tonalite and the Usutu Suite (Taylor et al., 2012).

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^

Fig. 4. Modified geological map of Swaziland (Wilson, 1980, 1982; Kröner et al., 1989; Kröner, 2007). The red frame locates the study area.

The oldest rock unit is the Ngwane Gneiss with an age of ca 3.66 Ga (Compston and Kröner; 1988; Schoene and Bowring, 2007) and is one of the two Eoarchaean intrusions that have been found in the Kaapvaal craton (Poujol, 2007). The layered structure is dominated by grey tonalitic gneisses with minor alternations of thin amphibolite layers, which are debated to represent fragments of a proto-continental sequence (Schlüter, 2006). Its metamorphic grade is in the upper amphibolite-facies. However, some metasedimentary intercalations older than 3.3 Ga reach granulite-facies conditions with temperatures of 700-900 °C and pressures of 6.5-7.5 kbar (Milisenda, 1986; Kröner et al. 1993; Condie et al., 1996). Such a metasedimentary unit will be described in more detail in chapter 2.2. The Ngwane Gneiss also contains xenoliths and inliers of variable size from centimetres to kilometres. These inliers, which Wilson (1982) and Jackson (1984) termed Dwalile Supracrustal Suite, are supposed to represent relics of greenstone belt accumulations, composed of mafic-ultramafic metavolcanic rocks, Banded Iron Formations and Suhr, 2013. Petrography, petrology and geochronology of Archean granulites, Swaziland.

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clastic metasedimentary rocks (Kröner, 2007) with an age of ca. 3.45 Ga (see Fig. 2a). In southwest Swaziland, rocks of the Dwalile Metamorphic Suite are correlated with those of the Onverwacht Group, which is part of the Barberton Greenstone Belt (BGB) located in northwest Swaziland and South Africa. However, it is mentioned in Kröner (2007) that the Dwalile rocks have a higher metamorphic grade. Further evidence for direct association between the AGC and greenstone belts can be found in northwest Swaziland, where small intercalations of AGC gneisses crop out in faulted and sheared contact with BGB rocks. This fact, combined with an observation by De Wit et al. (1987), which proves the occurrence of tectonically incorporated tonalitic gneisses in the lower Onverwacht Group of the BGB, allows reconstructing both rock units in coherence. Therefore, the AGC Terrane and the Barberton Mountain Land document the early development of crust in this region. Both are also referred to as Kaapvaal Shield (de Wit et al., 1992), estimated to be the initial development of what later became the Kaapvaal Craton (Poujol, 2007). After emplacement of the lower units of the BGB, the Ngwane Gneiss was intruded by hornblende tonalites (the Tsawela Gneiss, tonalitic to trondhjemitic in composition, and the Mhlatuzane Gneiss), and by the Mponono Anorthosite Suite, which forms a sheet-like structure (Schlüter, 2006). The age of the Tsawela tonalites is ca. 3.45-3.43 Ga (Kröner et al., 1989; Taylor et al., 2012). After this another magmatic phase followed with the emplacement of the ca. 3.23-3.22 Ga tonalities and granodiorites of the Usutu Suite (Schlüter, 2006) with ca. 3.23-3.22 Ga tonalities and granodiorites. This suite is widely believed to represent the beginning of a period of crustal differentiation in the form of episodic calc-alkaline granitoid plutons as a result of a NE-SW dipping subduction zone. Furthermore, Schoene and Bowring (2010) and Taylor et al. (2012) emphasize for the consistency, in time and space, of terrane accretion in an arc-trench setting with a NW-dipping subduction zone in Barberton (e.g. Moyen et al., 2006). Their models suggest that a double-vergent subduction system accounts for the comparable magmatic and deformational history between the AGC and Barberton. However, investigations by Van Kranendonk et al. (2009) and Van Kranendonk (2011) argue for partial convective overturn at ca. 3.26-3.22 Ga in the Barberton Greenstone Belt evolution without the involvment of subduction-accretion processes. The large and sheet-like Mpuluzi Batholith was emplaced at ca. 3.1 Ga (Hunter, 1974; Barton et al., 1983), between the AGC and BGB (Kröner, 2007). This resulted in cratonization of the previously mobile eastern part of the Kaapvaal craton. In the midArchaean the eroded upper part of the Mpuluzi Batholith formed a cratonic basin into which lavas and sediments of the Ponglola Supergroup were deposited (Wilson, 1980; 1982). Suhr, 2013. Petrography, petrology and geochronology of Archean granulites, Swaziland.

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2.1 Geology of the Kubuta Shiselweni Cattle Ranch On the geological map of Swaziland (Wilson, 1982), the study area is mainly part of the Mahamba Gneiss, which is a layered mafic-felsic banded orthogneiss with subordinate paragneiss. The paragneiss consists of the Mkhondo Valley Metamorphic Suite that is located more than 50 km to the west of the Kubuta Shiselweni Cattle Ranch, and of high-grade paragneisses within the Mahamba Gneiss on the Ranch. Both metasedimentary units have an unkown association (Taylor et al., 2012), although there is a suggestion that rocks of the Mkhondo Valley suite were part of the ca. 3.0 Ga Pongola Supergroup (Wilson and Jackson, 1988; Kröner, 2007). In recent studies parts of the orthogneisses in the Kubuta region yielded UPb zircon ages of ca. 3.28-3.24 Ga (Schoene and Bowring, 2010; Taylor et al., 2012), suggesting that these rocks represent a basement for the Pongola Supergroup (Hegner et al., 1994; Taylor et al., 2012). In the western part of the Matsanjeni River the Mahamba Gneiss is in contact with the Nhalangano Gneiss (Schoene and Bowring, 2010). However, the classification of this lithology in this area is erroneous and should be referred to as Mahamba Gneiss, because these gneisses are not related to the gneisses near the town of Nhlangano (pers. comm., A. Kröner, 2012). Other studies named the metasedimentary units of the study area as Shiselweni gneisses (Kröner et al., 1993) or Kubuta gneisses (Taylor et al., 2012). In the eastern part, ca. 3 km away from the Kubuta Shiselweni Cattle Ranch, the Hlatikulu granite is in contact with the Usutu Suite and the Mhlatuzane Gneiss. In addition, gabbros are located ca. 10 km southwest of the Cattle Ranch. In a geological map by Müller (1989; Fig. 5) the Hlatikulu granite is exposed north of the Mahamba gneiss but reaches farther to the south compared to the map of Wilson (1982). In addition, Fig. 5 of Müller (1989) shows mafic to komatiitic dykes with a NE striking direction. The northnortheasterly striking direction is typical for the Granodiorite Suite in central Swaziland that is composed of hornblendites to granodiorites and intrude the AGC in three discrete intrusions (Hunter, 1978). As a consequence of an increase in the geothermal gradient, rocks of the basement were reactivated to build up a series of mantled gneiss domes where the “Nhlangano Gneiss” (Mahamba Gneiss) forms the central core. The Mkhondo Valley Metamorphic Suite together with adjacent outcrops is folded into dome-and-basin-structures because of the diapiric rise of the gneisses (Schlüter, 2006).

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unfoliated Granite, cross-cutting Gneiss, gray

Gneiss, pelitic Gneiss, granodioritic

Nhlangano Gneiss mafic Dykes and Komatiites

pink Orthogneiss, coarse-grained Gneiss, dioritic Gneiss, psammitic

Road

Granulite, dense

AGC415

AGC415

Kinzigite

AGC413+414+408 AGC413+414

Gneiss, granitic + migmatitic

AGC423 AGC423 AGC405 AGC405

AGC402 AGC402 AGC401

AGC401

AGC417 AGC417

AGC419 AGC419

AGC421 AGC421 AGC420 AGC420

Scale 1:20 000

Fig. 5. Geological map of the Kubuta Shiselweni Cattle Ranch area from a mapping of Müller (1989).

In general, the source of TTGs in the oldest shield areas of the world is still controversially debated and remains a significant topic for discussions on geodynamic models and gneissgranite-greenstone belt relationships, respectively. Advocates of a genetic relationship between greenstones and TTG gneisses assume an anatectic melting of mafic-ultramafic greenstone lithologies, which led to diapiric rise of the melting products to form of large plutons (Anhaeusser, 1973; Glikson, 1979; Kröner, 2007). Other scientists suggested that melting of quartz eclogite or garnet amphibolite in the lower crust may generate TTGs-, with no relationship to greenstones (Barker, 1979; Hunter, 1979; Kröner, 2007). Such models have difficulties in explaining the complex relationships found in Swaziland and therefore cannot satisfactorily explain the formation of the present AGC (Kröner, 2007).

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3. Petrography 3.1 Hlatikulu granite Sample AGC 402 The rock sample has a heterogranular texture. K-feldspar (ca. 50%) dominates the quartzofeldspathic minerals, which also consist of plagioclases (ca. 5%) and quartz (15 %). Mafic minerals (ca. 30%) mainly consist of simple twinned clinopyroxene (augite), which may be replaced by amphibole. Minor proportions of biotite can be identified by bird’s eye structures. A pronounced foliation is not evident but quartz subgrains give textural evidence of deformation. Plagioclase and K-feldspar may be sericitized. Accessory minerals are apatite, zircon, monazite, rutile and opaque minerals. In general, sample AGC 402 seems to be similar to sample AGC 405. The lack of a a distinct foliation is probably a reflection of the different mineral assemblages.

1

2

3

4

5

6

Fig. 6. Photomicrographs of sample AGC 402. 1) Perthitc K-feldspar next to microcline. 2) Quartz bird’s eye in biotite. 3) Augite replaced by an amphibole. 4) Monazite with pleochroic halo in biotite. 5) Zircon in sericitized plagioclase. 6) Rutile.

Biotites already shows ductile behaviour during low-T deformation (ca. 200 °C), whereas amphiboles and clinopyroxenes require higher temperatures for ductile behaviour (ca. 700 °C) (Passchier and Trouw, 2005).

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Sample AGC 405 The rock consists of 40 % K-feldspar, 20% of quartz and ca. 10 % of plagioclase. The mafic minerals mainly consist of ca 20% hornblende and biotite with 20% defining the foliation. Accessory minerals are titanite, rutile, apatite, zircon and monazite. Opaque minerals were difficlult to identify, because all thin sections are covered. Most likely opaque minerals in larger sizes represent magnetite and/or ilmenite. The texture is a combination of granoblastic hetereogranular minerals with lepidoblastic biotites.

1

2

3

4

5

6

7

8

9

Fig. 7. Photomicrographs of sample AGC 405. 1,2) Biotite replaces simple twinned hornblende. 3) Green hornblende. 4) Sericitized plagioclase. 5) Metablastic microcline. 6) Simple twinned sericitized K-feldspar next to perthite. 7) Rutile (red) and monazite with pleochroic halo in biotite. 8.) Titanite with ilmenite. 9.) Zircon with pleochroic halo in biotite.

The replacement of amphibole by biotite indicates a retrograde reaction. Some K-feldspars show exsolution and may occur as perthite or microcline. K-feldspar shows breakdown to sericite. Quartz + Orthoklase/Microcline + Plagioclase + Biotite + Hornblende are typical minerals for orthogneisses in medium-grade metamorphism. In addition, the textural occurences of

Suhr, 2013. Petrography, petrology and geochronology of Archean granulites, Swaziland.

15

discontinious biotites at sizes of mm – cm and lenticular shapes of K-feldspars and quartz support this conclusion. No granulite-facies minerals occur in this rock. Sericitization of K-feldspar, together with accessory minerals such as titanite, rutile, zircon, apatite and magnetite may indicate partial overprinting at lower metamorphic grade. Furthermore the replacement of hornblende by biotite, green (richer in magnesium and iron) and brown in colour, together with metablastic microcline may indicate recrystallisation and metasomatism (Wimmenauer, 1985).

3.2 Mahamba orthogneiss Sample AGC 421 K-feldspar, quartz and plagioclase dominate this rock with quartz and biotite defining a foliation. Quartz shows an alternation of coarse to medium sized grains. Coarse quartz grains define a foliation along with biotites. Sericitation of feldspar and exsolution are also common features in this rock. Accessory minerals include apatite, zircon, and opaque minerals. Clusters of stilpnomelane indicate a retrograde reaction. Biotite/ chlorite or hornblende may be replaced by stilpnomelane. The absence of hornblende suggests that the stilpnomelane largely replaced biotite and is consistent with the fact that biotite occurs very close to stilpnomelane. Although no granulite-facies mineral assemblage was found, the textures indicate that this rock experienced high-T metamorphic conditions. Grain boundary migration recrystallisation (GBM-r.) in quartz suggests a high temperature regime. In addition, some plagioclases build an angle of 120° and accordingly a polygonal granoblastic texture.

Suhr, 2013. Petrography, petrology and geochronology of Archean granulites, Swaziland.

16

1

2

3

4

  Fig. 8. Photomicrographs of sample AGC 421. 1) Representative foliation defined by biotite and coarse grained quartz in a finer-grained matrix. 2) Zircon grain. 3,4) Stilpnomelane and biotite. Biotite can be distinguished from stilpnomelane by the bird’s eye structure. Note also the 120° angle of plagioclases with polygonal granoblastic texture in the upper part of photo 4.

Sample AGC 423 The rock has medium- to coarse-grained K-feldspar, quartz and plagioclase with more biotite defining the foliation in comparison to sample AGC 421. Weak sericitization of feldspar is also observed. Accessory minerals are zircon, apatite and opaque minerals. Quartz shows a polygonal texture in places. Biotites may show greenish colours in plane polarised light, which is believed to indicate chloritizitation. As in the other samples exsolution of feldspar occurs. With the exception of some coarser minerals, the texture is more isogranular than in sample AGC 421, and quartz does not show the same elongation. Two microtectonic structures were observed in quartz.

Suhr, 2013. Petrography, petrology and geochronology of Archean granulites, Swaziland.

17

1

2

Fig. 9. Textures of quartz in sample AGC 423. 1) GBM-r. in the centre of the picture. Grain boundaries became highly mobile during high-T and the material was sweept in all directions to remove dislocations. 1,2) Bulging recrystallisation as indicated by small independent grains with round to oval shapes. This process is known as low-T grain boundary migration.

One is showing bulge-recrystallisation structures, the other GBM-recrystallisation structures, indicating two different temperature regimes for deformation (Passchier and Trouw, 2005). Myrmectitic structures of plagioclase, which replace K-feldspars are a rare feature of this rock and characteristic of high-grade facies rocks.

3.3 Meta-semipelite (Kinzigite) Sample AGC 415 This crystalloblastic rock is mainly composed of 1-3 mm diameter garnets (ca. 35%), biotite (ca. 10%) and felsic minerals (ca. 45%) (quartz, K-feldspar, plagioclase) and is also known as kinzigite. Minor occurrences of orthopyroxene were observed, which form the largest grains in thin section. Also, smaller grains of orthopyroxene exist between garnet. Both minerals are poikiloblastic. Inclusions in garnet consist of biotite, apatite, zircon, monazite, quartz and leucosome patches. Inclusions in orthopyroxenes consist of biotite, quartz and plagioclase. Magnetite and ilmenite are common around orthopyroxene grains. Accessory minerals are apatite, zircon, monazite, magnetite and ilmenite.

Suhr, 2013. Petrography, petrology and geochronology of Archean granulites, Swaziland.

18

2

1

Bt breakdown

3

4 Clusters of garnet Bt breakdown

5

6

Opx Fig. 10. Photomicrographs of sample AGC 415. 1) Red arrow indicates alignment of quartz. Note also that garnet, orthopyroxene and perthitic K-feldspar follow the same alignment trend. 2) Elongated biotite with lobate leucosome indicating partial melting. 3) Same as in picture 2, but in addition an orthopyroxene next to quartz, magnetite and ilmenite can be seen in the upper part of the photo. 4) Coalescence of garnets, only slightly separated by a biotite rim. 5) Inclusions of flecky biotite and rounded quartz and melt in garnets next to a large orthopyroxene. 6) Orthopyroxene with plenty of orthopyroxene subgrains, indicating coalescence to form orthopyroxene poikiloblasts.

Garnet shows rims of biotites and in some cases of K-feldspar and quartz. Biotite may occur in lepidoblastic shapes. Exsolution of K-feldspar is rare. Yellowish colours of quartz indicate that the thin section may be too thick. Biotite has reddish colours, which may reflect a high Ti content. Green colours indicate that biotite was weathered in parts and minor amounts of sericite

Suhr, 2013. Petrography, petrology and geochronology of Archean granulites, Swaziland.

19

support this conclusion. However, some biotite rimming garnet has green absorption colours and is not altered. In handspecimen small leucosome patches are visible. The rock fabric element can be subdivided into a mafic (melanosome) and a felsic section (leucosome) defining the rock as migmatite (e.g. Mehnert, 1968; Wimmenauer, 1985). The leucosome consists of quartz, Kfeldspar and plagioclase, but also mafic components of the melanosome can be found in leucosomes, as well as zircon. Garnet and orthopyroxene may both occur as clusters of grains. Retrograde biotite is replacing garnet and orthopyroxene in places. Leucosome, garnet and orthopyroxene occassionaly show the same alignment. Therefore a transport of garnet and orthopyroxene and/or growing in the melt are two options to explain the rock texture. In places orthopyroxene may be altered to anthophyllite. Elongate, cuspate pockets of former melt occur along some of the grain boundaries of biotite and indicate partial melting. No cordierite was observed. According to Milisenda (1986) the absence of cordierite indicates a semipelitic character. This feature is typical of paragneisses at the Mahamba Gneiss locality, whereas the pelitic gneisses of the Mkhondo Valley Metamoprhic Suite include cordierite. Furthermore, Milisenda (1986) described accessory mineral phases such as chalcopyrite, pyrite and xenotime. Although none of these were observed in thin section, their occurrence should not be excluded.

Samples AGC 417 and AGC 419 Samples AGC 417 and AGC 419 have a higher proportion of leucosome compared to sample AGC 415. In addition to leucosome (consisting of plagiclase, quartz and K-feldspar), only apatite, zircon, titanite, magnetite and garnet were identified. Garnet is not poikiloblastic and may show embayements of leucosome. Euhedral shapes at sizes of 4-5 mm are common, whereas garnet in AGC 415 has a more rounded shape and shows a maximum size of 3 mm. Biotite and magnetite were identified in the leucosome. Biotite selvedges of biotite formed from diffusion between the leucosome and the host rock and differ from melanosome in that they are unlikely to represent the residue of partial melting (White and Powell, 2010). Chlorite occasionally replaces biotite and garnet. AGC 419 contains slightly less leoucosome and is similar to AGC 417.

Suhr, 2013. Petrography, petrology and geochronology of Archean granulites, Swaziland.

20

1

2

3

  Fig. 11. Photomicrographs of AGC 417. 1) Zircon in centre of altered plagioclase with chlorite replacing biotite; 2) garnet aligned in the foliation definded by bands of leucosme that alternate with biotite. 3) Cluster of lepidoblastic biotites.

The nature of mineral reactions and textures The metamorphic history of the kinzigite seems to involve complex mineral reactions. Melting experiments on biotite-bearing metapelites and metagreywackes have shown that the occurrence of rounded quartz inclusions in garnets form during fluid-absent melting (Vielzeuf and Holloway, 1988; Vielzeuf and Montel, 1994; Barbey, 2007). Furthermore, the reaction is a response to prograde biotite dehydration melting reactions, occuring at the granulite-facies transition (White et al., 2004; Waters, 2001; Barbey, 2007). In addition, garnet quartz intergrowths are believed to form in the presence of a melt phase. Such an interpretation is supported by field evidence from granulite-facies migmatites from Namaqualand, South Africa, where P-T conditions of such an intergrowth were estimated at 800 °C and 5 kbar (Waters, 1997; Barney, 2007). The absence of cordierite may not only be explained by the whole-rock chemistry. For example, cordieritebearing migmatites in the Velay Massif, France, are interpreted to have formed at a temperature range of 760-850 °C and pressures around 4 kbar (Montel et al., 1992; Barbey, 2007). Higher pressure in the kinzigite may therefore be another explanation for the absence of cordierite. Photomicrographs 1 and 6 in Fig. 10 indicate that growth of garnet and orthopyroxene was restricted to the melt interface, because the diffussity of ionic species in solids is inhibited, whereas faster diffusity can occur in a melt. An interconnected melt network is necessary to facilitate the interdiffussion of Fe-Mg and K-Na-Ca in a crystalline mush (Liang et al., 1996; Barbey, 2007). In the above kinzigite garnet is not only rimmed by biotite but also by both quartz and K-feldspar. The occurrence of biotite and K-feldspar on garnet is a retrograde feature during high temperature. On cooling, the melt reacts with garnet to make biotite (pers. comm., R.W. White, 2012). Excess silicia begins to crystallise to quartz due to melt silicia saturation in concert Suhr, 2013. Petrography, petrology and geochronology of Archean granulites, Swaziland.

21

with garnet growth. Barbey (2007) suggested the combination of two reactions for this case as follows: 1.

K(Fe, Mg)3AlSi3O10(OH)2 + melt = [3(Fe,Mg)O + H2O] + [KAlSi3O8] Fe-Biotite

2.

melt

K-feldspar

[3(Fe, Mg)O] + 0.5 [KAlSi3O8 + NaAlSi3O8 + CaAl2Si2O8] = (Fe, Mg)3Al2Si3O12 melt

feldspar

garnet

+ SiO2 + [0.25K2O + 0.25 Na2O + 0.5CaO] quartz

melt

Both reactions explain why garnet in AGC 415 is intergrown with leucosome patches and quartz. However, the retrograde nature of K-feldspar makes it unlikely that it was a reactand to produce garnet, as suggested by Barbey (2007). Quartz inclusions in garnet can also represent a “restite” of the melting reaction Bt + Qtz + Pl = Grt + Ksp + liq, which I interpret as the reaction that formed garnet. Initial segregation of melt may occur during the the reaction: Bt + Qtz + Pl = Opx + oxides + melt (see also photomicrograph 3 of Fig. 10) and occurs at the opening along grain boundaries, which can result from externally imposed stress or local magma pressure. If no planar fractures exsist, opening of microfractures can form pathways for the melt that can seggregate to leucosome stromata through vein networks. Such a process can also lead to effective drainage of the mesosome, which can be estimated by the amount of minerals such as biotite or orthopyroxene in the leucosome (Sawyer, 2001; Vernon and Clarke, 2008). Indeed, biotite and opaque minerals were observed in the leucosome and garnet is often surrounded by leucosome halos, indicating that both the perithectic minerals garnet and orthopyroxene formed in the presence of a melt. Considering the orthopyroxene reaction above, half of thin-section AGC 415 b seems to represent a melanosome, whereas the other half represents the initial seggregation of melt with a large orthopyroxene poikiloblast. In samples AGC 417 and AGC 419 garnet has a larger crystal size in the leucosome. Powell and Downes (1990) and White et al. (2004) showed that these features (photomicrograph 2 in Fig. 11) likely represent focussing of the melt around

Suhr, 2013. Petrography, petrology and geochronology of Archean granulites, Swaziland.

22

the garnet as it grew (in situ melting). Therefore, AGC 417 and AGC 419 are believed to contain well preserved features of the newly-formed neosome of the kinzigite. After peak metamorphism it is likely that during crystallisation of the melt, forming anhydrous quartz and feldspar, free water was released by these minerals as part of hydroxyl in the melt, which originated from the breakdown of biotite during prograde metamorphism. As a consequence of this, the reaction of anhydrous minerals in the mesosome (AGC 415) with water led to the formation of biotite selvedges and this texture is also evident in the more leucosome parts (Ashworth and McLellan, 1985; Stevens and Clemens, 1993; Stevens, 1997) . In samples AGC 417 and AGC 419, peritectic minerals in the leucosome exhibit this texture only rarely and incompletely. The occurrence of peritectic garnet with biotite rims separating it from the leucosome is a clear microstructural evidence of a retrograde reaction (Kriegsmann, 2001) and is often referred to as ‘back-reaction’ (Kriegsmann and Hensen,1998; Kriegsmann, 2001; Waters, 2001; Vernon and Clarke, 2008). However, the prograde reaction is unlikely to be reversed and the preferable term should therefore be ‘retrograde reaction’ (R.W. White, pers. comm., Vernon and Clarke, 2008). Another way to reveal whether biotite was part of granulite-facies methamorphism, or formed during later retrograde metamorphism is indicated by the absorption colours. Biotite with red-brown absorption colour formed during granulite-facies metamorphism whereas biotite that rims garnet has green absorption colours and formed during later amphibolite-facies retrograde metamorphism (Condie et al., 1996).

3.4 Gabbro Sample AGC 408 This sample is composed of fine-grained magmatic minerals and can be referred to as microgabbro. Ca. 50% of the rock-forming minerals is plagioclase. Clinopyroxene end members are represented by augite and pigeonite. Olivine is also a major mineral and shows some alteration as shown by the presence of serpentine in fractures. In some olivines a transformation to clinopyroxene is observed, and clinopyroxene may be replaced by hornblende or biotite. Accessory minerals are quartz, titanite and opaque minerals.

Suhr, 2013. Petrography, petrology and geochronology of Archean granulites, Swaziland.

23

1

2

3

  Fig. 12. Photomicrographs of microgabbro sample AGC 408 . 1) sub-ophitic strucutre. 2) Transformation of olivine to clinopyroxene. 3) Pigeonite next to augite in thin section.

The rock fabric is hypidiomorphic and contains cumulate olivine and plagioclase. Plagioclase is often large and encloses ferromagnesian minerals. Some plagioclase laths impinge on one another to form sharp angular textures referred to as subophitic-intergranular. Plagioclase varies in mineral size and may show magmatic zoning. Fractures occur in some plagioclase grains and together with previous observations, suggest that the rock is weakly metamorphosed and deformed. Sample AGC 413 This sample is composed of ca. 65% plagioclase and ca. 15% clinopyroxene (pigeonite, augite). Some clinopyroxene is replaced by hornblende, and plagioclase is often sericitized. Opaque minerals and quartz make up accessory minerals. Locally the texture of clinopyroxene resembles spinifex, although the mineral composition does not support such an interpretation. Except for olivine, the mineral composition is similar to AGC 408. However, most minerals are medium- to coarse-grained, and the rock shows a much stronger deformation, whereby smaller grains of plagioclase were deformed almost beyond recognition.

3.5 Hornblendite Sample AGC 420 This sample was taken from a dyke and is composed of 90% amphibole, 5% plagioclase and 5% clinopyroxene, apatite and opaque minerals. In some fractures of both amphibole and

Suhr, 2013. Petrography, petrology and geochronology of Archean granulites, Swaziland.

24

clinopyroxene a rusty alteration is observed. Very dominant for amphibole is an olive-green hornblende.

1

2

Fig. 13. Photomicrographs of sample AGC 420. 1) Simple twinned hornblende; 2) Clinopyroxene (augite).

3.6 Metavolcanic rocks Sample AGC 401 This rock preserves relict igneous textures with some incomplete metamorphic overprint (at most upper amphibolite-facies). Most of the grains are igneous, and the rock is mainly composed of clinopyroxene that commonly has plagioclase rims. In places plagioclase occurs in the core of clinopyroxene grains. Orthopyroxene, amphibole and  minor occurences of small grains of quartz are common. However, amphibole with rims of orthopyroxene occurs in fractures. Together with zonation in some clinopyroxene grains both metamorphic features indicate retrograde metamorphism. In places the rock consists of recrystallised olivine. Kink bands have been identified in plagioclase. Pleochroism in orthopyroxene occurs in white to light greenish colours. Apatite was identified, and some opaque minerals in cubic shape also occur and may be titanomagnetite. Other opaque minerals do not show cubic shapes. A special feature of clinopyroxene in this sample is the development of a spinifex texture that occurs as zoned hollow needles with dark grey cores (pigeonite) and pale grey margins (augite). The intergrowth of irregular elongate plagioclase with clinopyroxene is only possible in slowly cooled parts from deep within thick volcanic flows (Arndt et al., 2008). Clinopyroxene may also form a granoblastic texture of very small grains.

Suhr, 2013. Petrography, petrology and geochronology of Archean granulites, Swaziland.

25

1

2

0.05 mm

 

Fig. 14. Photomicrograph of sample AGC 401. 1) Spinifex texture of clinopyroxene rimmed by plagiocase and sourrounded by a fine matrix of subgrains. 2) Granoblastic texture of polygonal clinopyroxene subgrains.

Sample AGC 414 This sample contains ca. 50% plagioclase that forms a granoblastic texture. A second plagioclase popultation shows an ophitic texture with coarser grain sizes. Twinning may be related to growing pressure in plagioclase. Clinopyroxene and orthopyroxene make up 30 % - 40 % in this rock and have sharp polygonal grain boundaries. In comparison to AGC 401 this sample is more strongly metamorphosed but at most in the upper amphibolite-facies. A common feature of this rock is the replacement of pyroxene by amphibole. In general, pyroxene is completely replaced or the reaction occurred around the grains or along grain boundaries. Therefore, this reaction is believed to be retrograde.

Suhr, 2013. Petrography, petrology and geochronology of Archean granulites, Swaziland.

26

Fig.15. Photomicrograph of sample AGC 414. Orthopyroxene is replaced by dark hornblende. Clinopyroxene is not replaced. Hornblende can easily be recognized by its characteristic angles.

Suhr, 2013. Petrography, petrology and geochronology of Archean granulites, Swaziland.

27

4. Analytical methods 4.1. U-Pb zircon dating on zircons Fresh whole-rock samples were crushed using a splitter, disc mill, jaw crusher and steel roller mill and then sieved to separate the >250µm fraction at the Max-Planck-Institute of Geochemistry, Mainz. Some material from each sample was then pulverized for whole-rock geochemical analysis using an agate vibratory disk mill. The remainder was run over a Wilfley table for heavy mineral separation, followed by concentration of the non-magnetic fraction using a Frantz magnetic separator and panning for zircons. Final handpicking and microscopic inspection of the separates resulted in pure zircon concentrates that were prepared as grain mounts (embedded in epoxy) for isotopic analysis. Zircons were analyzed in the Department of Geosciences, University of Mainz, using an Argilent 7500ce, coupled with a 213 nm New Wave UP213 Nd:YAG laser ablation system. The laser ablation system features a Large Format Cell (LFC), allowing investigation of several samples together with a range of different standard mounts under near-identical conditions at very fast wash-out (one order of magnitude drop in signal in 0.3 sec). Analyses were carried out with a beam diameter of 30 μm and a 10 Hz repetition rate. During each analysis the background was measured for 25 seconds, followed by an ablation time of 30 s, resulting in pits ca. 30 μm deep. In situ zircon trace element measurements were also performed with a beam diameter of ca. 40 µm. The carrier gas of the ablated aerosol consisted of helium, which was mixed with argon before the torch is reached. The flow rates were ca. 0.75 l/min helium and 0.65 l/min argon. The standard zircon GJ-1 (Jackson et al., 2004) was used as a primary standard for correction of 207Pb/235U, 206Pb/238U and 208Pb/232Th ratios as well as for calculation of U and Th concentrations. Off-line data reduction was performed by PepiAGE, version C9 (Internet Reference II). Pooled ages were plotted and calculated using ISOPLOT/Excel “Add-In” version 4.11 (Ludwig, 2012). The accuracy of 207

Pb/235U,

206

Pb/

238

U and

208

Pb/232Th ages is currently given as 1.5 % (2σ), based on long-term

monitoring of several zircon standards (PL, 91500, Mud Tank; see Topuz et al., 2010). The results are presented in Tables 13-17 where the isotopic ratios and ages are shown with 2-σ errors. During data collection, discordant ages for the following zircon standards giving upper intercepts are as follows: North Plug 2702 ±16 Ma (n=9, MSWD=1.00, probability of fit=0.43), Kaap Valley Tonalite 3257±50 Ma (n=6, MSWD=1.8, probability of fit 0.12). TIMS ages for both standards are generally within error: North Plug 2719±1 Ma (Davis, unpublished data), Suhr, 2013. Petrography, petrology and geochronology of Archean granulites, Swaziland.

28

Kaap Valley Tonalite 3227 ±1 Ma (Kamo and Davis,, 1994). The measurement of the High Hill Falls standard with an upper intercept age of 2814±100 Ma (n=5, MSWD=2.5, probability of fit=0.061) is a poor result, because the TIMS age of this standard is 2748±1 Ma (Davis, unpublished data). Instrumental parameters and further analytical details on U/Pb zircon dating in Mainz are described in Zack et al. (2011).

4.2 Whole-rock geochemistry A total of 10 samples was analyzed for both major and trace elements, including some rare earth element (REE) concentrations by wavelength-dispersive X-ray florescence spectrometry, using a Philips MagiX Pro X-ray spectrometer equipped with an Rh-anode tube at the University of Mainz. The rock powders were ignited for ≥ 3 h at 1000 °C to turn all FeO into Fe2O3 and to expel water and CO2, with the aim to gravimetrically determine the loss on ignition (LOI). Glass beads for major element determination were produced by mixing 4.8 g of LiB4O7 with 0.8 g of ignited powder (7:1 dilution) and fused in platinum crucibles at ca. 1200 °C. Trace elements were determined on two powder pellets for each rock sample with calibration based on standard BCR1; Tables 2, 3, 4 and 6 contain the arithmetic average for samples of this study.

4.3 Lu-Hf/Sm-Nd dating on garnets The same sample preparation procedure as for zircons was applied for the garnets up to the magnetic separation stage. To know the necessary amount of garnet for adequate Lu and Hf concentrations, in-situ garnet trace element measurements were performed with an Agilent 7500ce quadrupole ICP-MS, coupled with a NewWave Research UP-213 laser ablation unit in the Department of Geosciences, University of Mainz. Data acquisition was accomplished in a rapid, peak-jumping mode with one point per peak at 10 ms dwell time. Internal standards were 43

Ca and

44

Ca for garnet and standard glass reference material NIST SRM 612 was used as an

internal standard. Data reduction was performed by using the software “Glitter”. During the analytical session, measurement of USGS reference glass BCR-2G was used to monitor instrument performance and stability (Nehring et al., 2010). Garnet monofractions of ca. 100 mg were manually separated under a binocular microscope from a 120-180 µm sieve fractions. Five fractions were prepared with different garnet clarity grades. Three clean garnet fractions with no inclusions and one medium clean fraction with some inclusions and one “dirty” fraction (biotite Suhr, 2013. Petrography, petrology and geochronology of Archean granulites, Swaziland.

29

inclusions and residua of biotite selvedges on garnets) were dissolved using a selective digestion procedure (tabletop digestion). The aim of this procedure is to resolve the major silicate phases of the separates without refractory phases as zircon or rutile (Lagos et al., 2007). This is important because the dissolution of zircon may affect the precision of the garnet Lu-Hf ages and rutile of the garnet Sm-Nd ages (Scherer et al., 2000). The tabletop digestion procedure often results in high precision Lu-Hf ages (e.g., Lagos et al., 2007; Herwartz et al., 2008; 2012; Schmidt et al., 2008; 2011; Smit et al., 2010; Kirchenbaur et al., 2012). A way to control as to whether zircon may alter the Lu-Hf isochron age of garnets is the use of a sub-sample. The sub-sample was digested for three days in steel jacketed high-pressure PARR® bombs (bomb digestion) in a 1:1 mixture of HNO3-HF to dissolve the entire sample with all refractory phases (e.g., Lagos et al., 2007; Herwartz et al., 2008; Kirchenbaur et al., 2012). Subsequently, the powder was dried down with one ml of perchloric acid. Prior to digestion, mineral separates and the whole-rock powder were spiked with mixed

176

Lu-180Lu and

149

Sm-150Nd tracers. Using Ln-Spec resin column

chemistry (Münker et al., 2001) Lu and Hf were then separated from the rock matrix. The Lu-Hf separation and the clean-up for the Hf fraction involves seven different steps that are described in detail in Münker et al. (2001) and Weyer et al. (2002). Sm-Nd separation was carried out using the REE-rich matrix cut left over from the Hf separation, using BioRad® AG50W-X8 cation resin (200-400 mesh) and Eichrom Ln-spec resin (Pin and Zalduegui, 1997; Kirchenbaur et al., 2012). Measurements were carried out on a Finnigan Neptune MC-ICP-MS at the Steinmann Institute, Bonn University. Isobaric interferences of 176Yb and 176Hf on 176Lu as well as 180Ta and 180W on 180

Hf were corrected using interference-free isotopes and the natural abundances. Because the

isotopic composition of Lu in the Hf cuts is essentially unknown (Amelin et al. 2011), both natural and spiked isotopic compositions were used for interference correction of discrepancy was added to the uncertainty in relative to

176

176

Hf/177Hf. The

176

Lu, and the

176

Hf/177Hf values are reported

Hf/177Hf = 0.282160 for the Münster Ames Hf standard (isotopically identical to

JMC-475). For the purpose of calculating Lu-Hf isochron regressions, the 2-σ external uncertainties on 176Hf/177Hf were estimated using the method of Bizzarro et al. (2003). However, in all cases a minimum error of 1.1ε was applied, because this is the long-term reproducibility for the JMC475 standard (Schmidt et al., 2011). The given 2σ external reproducibility for 176Lu/177Hf includes error propagation for over- or under-spiked samples. Ages were calculated using ISOPLOT version 2.49 (Ludwig 2001) and λ176Lu = 1.867 × 10-11 a-1 (Scherer et al., 2001; Suhr, 2013. Petrography, petrology and geochronology of Archean granulites, Swaziland.

30

Söderlund et al., 2004). External error on

147

Sm/144Nd was 0.2% and on

units (0.000020). Daily value for the LaJolla standard (20 ppb) for

143

Nd/144Nd was 0.4 e

143

Nd/144Nd was 0.511835.

Blanks for Lu and Hf were alk= 100+ 4 alk; al < alk = 100 + 3al + alk). Sample

AGC 402

AGC 405

AGC 415

AGC 421

AGC 423

si

210,459

247,005

139,649

344,499

344,389

al

28,421

31,591

14,034

42,127

41,042

fm

41,748

36,501

80,812

17,952

19,984

c

10,855

9,421

2,3

11,359

12,31

alk

18,976

22,487

2,853

28,561

26,664

k

0,464

0,495

0,985

0,293

0,261

mg

0,131

0,127

0,172

0,147

0,17

c_fm

2,338

1,929

1,088

1,114

0,947

ti

0,444

0,345

0,095

0,141

0,121

p

0,26

0,258

0,028

0,633

0,616

qz

34,555

57,056

28,236

130,254

137,733

In the c – (al – alk) – 100 mg diagram (Fig. 23a), the Hlatikulu granite follows a greywacke trend in comparison to the qtz-fsp gneisses of Milisenda (1986). No clear sedimentary source characteristics are shown in the mg vs. c diagram (Fig. 23b, c), and the samples follow a magmatic trend. In the si vs. (al + fm) – (c +alk) diagram (Fig. 23d) both samples plot in the greywacke field together with two quartz-feldspar gneisses and one semi-pelitic gneiss. In the Frost diagram (Frost et al., 2001) of Fig. 24a, the dashed lines show fields of genetically different granitic rocks from the Lachlan Fold Belt, eastern Australia, and a shaded field, which shows the compositional range for Archaean tonalitic gneisses. Sample AGC 405 plots in the Atype field and AGC 402 outside this field because of a lower content in SiO2. Both granitoid rocks have ferroan compositions. Both samples were also plotted in twelve different diagrams (after Whalen, 1987) using the elements Fe, Mg, Na, K, Al, Zr, Nb, Ce, Y, Al, Zn and an Agpaitic index. In all of these plots (not shown here) the samples showed an A-type character, in agreement with the Frost diagram. In the SiO2 vs. MALI (modified alkali-lime index) diagram (Fig. 24b), both samples plot in the alkali-calcic field. In the classification scheme of Frost et al. (2001), the combination of the MALI with the ferroan and the ASI (aluminium saturation index) characteristics show that both samples are derived from an A-type source.

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a)

Quartz - Feldspar Gneisses Semipelitic Gneisses Pelitic Gneisses

100 mg

c

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40

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Quartz - Feldspar Gneisses Semipelitic Gneisses Pelitic Gneisses

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greywacke trend

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acid to intermediate volcanics 423

APPROX LIMIT OF IGNEOUS ROCKS

200

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10 100

100 150 200

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si

0 0

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0.3 0.4

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mg Fig. 23. Niggli diagrams with different Niggli-values to reveal a sedimentary or magmatic origin of the granitoid rocks. Data for quartz-feldspar gneisses, semipelitic gneisses and pelitic gneisses in Fig. a, b, d of Milisenda (1986). Data in c (van de Kamp and Beakhouse, 1979) are from sample numbers AGC 112, -114,- 115, -156, -170, -171, 172 (pelitic gneisses), -131, -132, -133, -154 (semipelitic gneisses), -153, -155, -160 (Qz-Fs gneisses) (Milisenda, 1986).

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1.0

a)

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A-type 405

402

S-type

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421

ferroan

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423

magnesian

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50

70

60

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SiO2 (%)

b)

8

402

4

405

421 423

a 0

Archaean tonalitic gneisses

-4 -8

50

60

SiO2 (%)

70

80

  Fig. 24. Frost et al. (2001) developed a geochemical classification for granitic rocks based upon three variables. These are FeO/(FeO + MgO) = Fe-number, the modified alkali-lime index (MALI) (Na2O + K2O – CaO), and the aluminium saturation index (ASI) [Al/(Ca – 1.67P + Na + K)]. The classification scheme will be used for the granitoid rocks of this study. a) The FeO/ (FeO + MgO) vs. wt % SiO2 - diagram shows the compositional range for granitoid rocks from the Lachlan Fold Belt including the samples of AGC 402, AGC 405, AGC 421 and AGC 423. b) The Na2O + K2O – CaO vs. wt % SiO2 diagram the compositional range for Archaean tonalitic gneisses including the samples of AGC 402, AGC 405, AGC 421, AGC 423, a = alkalic, a-c = alkali-calcic, c-a = calc-alkalic, c = calcic (Frost et al., 2001).

In the A/NK vs. ASI diagram (Schand, 1943; Frost et al., 2001), sample AGC 402 has a metaluminous composition. The ASI 650°C (Ducea et al., 2003) and for the Lu-Hf system consensus is building for a closure temperature in the range of 750–900°C (Scherer et al., 2000; Anczkiewicz et al., 2007; Kylander-Clark et al., 2007; Lagos et al., 2007; Kelly et al. 2011). This could explain why the Lu-Hf age is older than the Sm-Nd age in garnet. The higher error may result from mineral inclusions in some garnet grains (e.g. monazite and apatite; see chapter 3 petrography and Fig. 31), which have a significant effect on the Sm-Nd systematics (Scherer et al., 2000).

a)

± ±

b)

± ±

Fig. 44a) Sm-Nd isochron for AGC 415, medium clean garnets as well as dirty garnets were considered in calculating the best-fit line. b) Dirty garnets are not considered in calculating the best-fit line.

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8.4 Discussion and interpretation of U-Pb and Lu-Hf ages The zircon ages of samples AGC 402 and 405 indicate that both samples are derived from the ca. 2.73 Ga granites in south-central Swaziland, which crosscut the high-grade para- and orthogneisses of the “Nhlangano Gneiss” and Mkhondo metamorphic suite (Wilson, 1982; Schoene and Bowring, 2010). The upper concordia intercept of zircon analyses for orthogneiss AGC 421 yield inferred magmatic crystallization age of 3275±33Ma respectively similiar to the ages reported for the Mahamba gneiss (Schoene and Bowring, 2010). The upper concordia intercept of zircon analyses for orthogneiss AGC 423 yield inferred magmatic crystallization age of 3137±170 Ma.The reason why data for zircons from sample AGC 423 have a larger error in age compared to AGC 421 could be the proximity of the ca. 2.73 Ga granite (see Fig. 5). Furthermore, Schoene and Bowring (2010) advocated partial resetting of zircons in these granites and/or metamorphism. A more complex history is indicated by the zircons of AGC 415. The ages of 2951±71 Ma (spot 228) and 2976±45 Ma (spot 229) are similar to the Lu-Hf metamorphic age (2921±31 Ma) of garnets. In addition, depletion in HREE in both zircons suggests equilibrium with garnet during crystallization. Another metamorphic age of 3098±48 Ma may be indicated by zircon spot 231 due to a low Th/U ratio ≤0.1. This may indicate that AGC 415 underwent two metamorphic events at ca. 3.1 Ga and 2.9 Ga. The correlation of low Th/U ratios and ages at ca. 2.9 Ga is also evident in zircons of the Mahamba orthogneisses. Spot 105 (AGC 421) has a Th/U ratio of 0.03 and an age of 2870±52 Ma and spot 86 (AGC 423) has a Th/U ratio of 0.03 and an age of 2861±48 Ma, suggesting that the orthogneisses were affected by the same metamorphic event as AGC 415. A Th/U ratio of 0.07 and an age of 3295±41 Ma (spot 181) are consistent with a high-grade metamorphic event at ca. 3.3-3.2 Ga as reported by Schoene and Bowring (2010). In addition, a Th/U ratio of 0.04 and an age of 3056±74 Ma (spot 182) may indicate a third metamorphic event that the orthogneisses of the Mahamba gneiss underwent, and this age is similar to the metamorphic age of spot 231 in AGC 415. Rocks with the same age span as above in the Kaapvaal Craton (e.g. Hunter and Reid, 1987; Hegner et al., 1994; Nhelko, 2003; Olson et al., 2010; Gumsley et al., 2012; Taylor et al., 2010) were related to underplating and plume events. A more recent study investigated the Hlagothi Suhr, 2013. Petrography, petrology and geochronology of Archean granulites, Swaziland.

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Complex, which consists of mafic to ultramafic layered sills that are part of the Kaapvaal Craton in northern KwaZulu-Natal (Gumsley et al., 2012). These sills intrude the Archaean crust, and the Mesoarchaean Nsuze Group of the Pongola Supergroup (du Toit, 1931; Groenwald, 1984, 1988, 2006). With a badelleyite age of ca. 2.87 Ga the sills are related, in time and composition, to the Thole Complex (Groenwald, 2006; Gumsley et al., 2012) and to gabbroic portions of the Usushwana Complex. Both complexes are interpreted to represent feeders to volcanic units within the upper Witwatersrand and Pongola supergroups that provide evidence for pulses of plume-related magmatism in the Kaapvaal Craton during the Mesoarchaean (Gumsley et al., 2012). The Usushwana Complex is not situated far from the study area, and both metamorphic zircon ages from samples AGC 421 and 423 correlate with the baddeleyite ages of Gumsley et al. (2012). In addition to these units, the SE trending Badplaas dyke swarm was dated at between 2966±1 Ma and 2967±1 Ma (Olson et al., 2010; Gumsley et al., 2012) and a NW-trending dolerite dyke was dated at ca. 2.95 Ga (Lubnina et al., 2010; Klausen et al., 2010; Gumsley et al., 2012) in the southeastern region of the craton. This suggests that the Lu-Hf ages and both zircon spots 228 and 229 on zircons from AGC 415 are related, in time, to these events in the southeastern Kaapvaal Craton. After this period flood basalt volcanism followed with the deposition of the Ventersdorp Soupergroup, in age between 2714±16 Ma and 2709±8 Ma (Armstrong et al., 1991; Gumsley et al., 2012), but was interpreted to consist of an older basement with 2782±5 Ma by some authors (e.g. Wingate, 1998; de Kock, 2007; Gumsley et al., 2012). As already mentioned earlier, the Ventersdorp magmatism was interpreted by Taylor et al. (2010) to be linked to high-grade metamorphism in the Mkhondo Valley Metamorphic Suite. The remaining question concerns the significance of the older zircon ages of ca. 3.7 Ga in sample AGC 415 (Table 16) that may indicate that older continental crust was already present prior to deposition of the Barberton greenstone succession. Kröner et al. (1989) advocated a continental terrains of at least 100 km in size. Therefore, an older granitoid basement may have been one potential source for the old zircons in AGC 415. The age of zircon spot 111 (3502±44 Ma) is similiar to ages reported from a tonalitic gneiss wedge in the Threespruit Formation (3538+4/-2 Ma and 3538±6 Ma: Armstrong et al., 1990; Kamo and Davis, 1994; Poujol, 2007), the Ngwane tonalitic gneiss (3521±23 Ma and 3490±3Ma;

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Kröner and Tegtmeyer, 1994; Poujol, 2007) and the Steynsdorp Pluton (3510±4 Ma and 3505±5 Ma; Kamo and Davis, 1994; Poujol, 2007). On the other hand, the zircon age of 3308±41 Ma (spot 243) is similar to the age of an Usutu suite granodiorite, in central Swaziland (3306±4 Ma; Maphalala and Kröner, 1993; Pojoul, 2007), to the gabbros and metagabbros of the Komati Formation (3350 Ma; Armstrong et al., 1990; Kamo and Davis, 1994; Pojoul, 2007) and to a volcanic unit within the Mendon Formation of the Barberton Greenstone Belt (3298±3 Ma; Byerly et al., 1996; Pojoul et al., 2007). The age of 3098±48 Ma (spot 231) is similar to ages reported for the granite-monzonite-syenite intrusive group, located southeast of the Barberton Greenstone Belt (e.g., Lowe and Byerly, 2007) and to the Mpuluzi (Lochiel) Granite Batholith (see also Fig. 4). The emplacement of the batholith separated the Barberton Greenstone Belt and the Ancient Gneiss Complex (Kröner, 2007). Since the zircon age of spot 231 may reflect a metamorphic event, the first metamorphic overprint of sample AGC 415 could have been the result of the emplacement of the granitemonzonite-syenite or the Mpuluzi batholith. Therefore, the depositional age of the protolith of sample AGC 415 is older than 3.1 Ga with potential sedimentary sources of ca. 3.7–3.2 Ga. However, a future study needs to clear the role of the 3449±2 Ma dated tonalitic gneiss (Kröner et al., 1989; AGC 261) for a better understanding of the sedimentary history in the Kubuta Shiselweni Cattle Ranch area.

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9. Summary and conclusions Hlatikulu granite (AGC 402 and AGC 405) Both samples AGC 402 and 405 are late Archaean younger A-type/within plate granites (WPG) that intruded the Ancient Gneiss Complex at ca. 2.73 Ga as part of the Hlatikulu granite magmatic event. The chemical similarity with A2-type granites suggests the melting of a depleted crustal source (Eby, 1992; Voordouw and Rajesh, 2012) or that both samples are derived from differentiation of continental tholeiite. However, the high-K characteristics of the samples suggests melting of hornblende- and biotite-bearing tonalite and granodiorite as potential sources (Anderson, 1983; Sylvester, 1989; Creaser et al., 1991, Yuan et al., 2010). Because both samples plot in the WPG and A2-type fields (Fig. 2a, b) they are interpreted as post-orogenic (Eby, 1990; 1992; Bonin, 2007). The absence of xenocrystic zircons in AGC 402 and 405 makes it impossible to identify the source characteristics, via age constraints. One zircon grain (spot 208) of sample AGC 402 gives an age of 2934±73 Ma that may suggest an earlier crystallization event. A-type granites are wellknown for their emplacement in stable and cooling regions of thickened lithosphere and therefore, they require sufficient heat supply, resulting in thermal erosion and/or mechanical delamination of lithospheric roots (Black and Liégeois, 1993; Bonin, 2007). The heat source for generation of the Hlatikulu granite is probably related to a mantle plume, because the crystallization ages of 2728±25 Ma (AGC 402) and 2719±33 Ma (AGC 405) broadly correlate with the age of Ventersdrop magmatism dated between 2714±16 Ma and 2709±8 Ma (Armstrong et a., 1991,; Gumsley et al., 2012; Taylor et al., 2010). Furthermore, the older zircon age (spot 208) correlates with the ages for the Usushwana Complex and the Badplaas dyke swarm (Olsen et al., 2010; Gumsley et al., 2012).

Gabbro (AGC 408 and 413) Samples AGC 408 and 413 are gabbroic dykes. On the geological map of Wilson (1982), two small gabbroic bodies are shown as intrusive into the Mahamba gneiss. One of these lenses is located in the study area, and the GPS coordinates of AGC 408 and 413 coincide with this location. Because of the intrusive contact, the gabbroic dykes are younger than ca. 3.3 Ga (see UPb ages of AGC 421 and 423).

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Metavolcanic rocks (AGC 401 and 414) AGC 401 is a basaltic metakomatiite that has no correlation with the Bimodal Suite of Hunter (1978). However, the sample correlates with a basaltic metakomatiite of the Dwalile greenstone belt and with a 3.45 Ga sample from the Barberton Greenstone Belt (Condie 2005, see Fig. 21). AGC 414 is an iron-enriched tholeiite with a major element distribution pattern similiar to tholeiitic basalts from the Barberton- and the Dwalile greenstone belts, for which Kröner and Tegtmeyer (1994) demonstrated a common origin. AGC 414 compares well with the Bimodal Suite (Hunter, 1978) and therefore it is likely that this sample represents part of the minor mafic intercalations in the Ngwane Gneiss. This is also in agreement with the microscopic observation, showing that the sample was metamorphosed in the upper amphibolite-facies, similar to the Ngwane Gneiss (Kröner, 2007). Both samples AGC 414 and 401, show that the amphibolites exposed in the Matsanjeni River bed may be related to the Barberton and/or the Dwalile greenstone belt. However, the relationship of sample AGC 414 to the Ngwane Gneiss (Figs. 16a, b) indicates that the geochemical interpretation of metabasalts on the Kubuta Shiselweni Cattle Ranch should be taken with caution when interpreting the geodynamic setting. Subduction-related basalts or plume-derived basalts are not distinguishable in cases where the rock contaminated with continental crust or subcontinental lithosphere (Condie, 2005) and Van Kranendonk et al. (2010) investigated a pillowed metakomatiite sample from the Barberton Greenstone Belt with a negative Nd(t) value, either having experienced alteration processes or formed after contamination of the parent magma with older crustal material. This means that the arc or arc-like trace element signatures in trace element discrimination diagrams may be misleading (Fig. 22), because contamination of mantle-derived lavas with continental crust may lead to the arc signature (Arndt and Jenner, 1986, Barley, 1986; Green et al. 2000, Van Hunen and Moyen, 2012).

Hornblendite (AGC 420) Sample AGC 420 is a hornblendite with a basaltic komatiitie composition and was taken from a NNE striking dyke that crosscuts the ca. 3.3 Ga Mahamba Gneiss. Hunter (1978) classified the Granodiorite Suite to consist of hornblendite and granodiorite that intruded the AGC suite.

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The Granodiorite Suite (Hunter, 1978) was renamed as Usutu Suite by Schoene and Bowring (2010), consisting of ca. 3.23 Ga old rocks. The intrusion of the hornblendite dyke into the ca. 3.3 Ga Mahamba Gneiss and the strike direction of the rock on the geological map of Müller (1989) compare well with the description of Hunter (1978) and Schoene and Bowring (2010). Therefore, sample AGC 420 may be interpreted to be part of the Usutu Suite. However, a reliable geochemical classification/identification of AGC 420 is not possible. The plot in the continental tholeiite field (Floyd and Winchester, 1975) may indicate the same problems as in AGC 401 and 414 due to crustal contamination. In addition, the low Sr content of AGC 420 differs from the Granodiorite Suite classification of Hunter (1978), and the sample also compares with a Barberton basaltic komatiites (Fig. 20), the Dwalile greenstone belt serpentinites (Kröner and Tegtmeyer, 1994), and Barberton komatiites with a near-chondritic Ti/Zr ratio (Jahn et al., 1992; Kröner and Tegtmeyer, 1994). Based on the structural observations of the dyke, the sample cannot be related to the Barberton or Dwalile greenstone belt serpentinites, because the youngest komatiites in the Barberton Supergroup are part of the Mendon Formation dated at > 3298 Ma (Byerly et al., 1996; Lowe and Byerly, 2007; Van Kranendonk et al., 2010).

Mahamba orthogneiss (AGC 421 and AGC 423) In the classification diagram of Frost et al. (2001) AGC 421 has a peraluminous, calc-alkalic, ferroan composition and the high silicia content (> 70% SiO2) can be related to partial melting of felsic granulite (Collins et al., 1982, King et al., 2001; Frost and Frost, 2011) or tonalitic to granodioritic crust (Bogaerts et al., 2006; Frost and Frost, 2011). Sample AGC 423 has a peraluminous, calcic, magnesian composition. Experimental studies of Patiño Douce (1997) showed that dehydration melting of granodioritic or tonalitic compositions change the composition from ferroan to magnesian compositions with increasing temperature. In addition, the granodioritc composition would also change and plot closer to the boundary between the calc-alkalic and calcic field in the MALI vs. SiO2 diagram (Frost and Frost, 2011). A difference in the temperature regime of the Mahamba orthogneiss may therefore explain the differences of the samples in the classification diagrams of Frost et al. (2001). In the tectonic discrimination diagrams, AGC 421 and 423 correlate to volcanic arc granites (Rb vs. Y+Nb) and to the volcanic arc granites + syn collision granites (Nb vs. Y; Pearce et al., 1984;

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Figs. 25a, b). Both samples showed correlations to the “Nhalangano Gneiss”, for which Schoene and Bowring (2010) proposed a derivation through mixing of older crustal rocks with mantle derived material. In the TTG source identification diagrams (Moyen, 2011), AGC 421 and 423 compare well with melts formed at low pressure that derived from a mafic source such as Archaean basalts. Both compare also well to melts formed by melting of existing crustal lithologies. For this reason both AGC 421 and 423 are most likely derived through the mixing of two source components (Moyen, 2011). The melting of existing crustal lithologies is also indicated by the presence of a xenocrystic grain (3564±49 Ma), which is older than the xenocrystic ages of the “Nhalangano” orthogneisses (3329 Ma and 3425 Ma; Schoene and Bowring, 2010; ca 3.47 Ga xenocrysts, Condie et al., 1996). This means that the rock was “contaminated” with an older crustal source like the e.g. Ngwane Gneiss, which was dated at 3521±23 Ma and 3490±3 Ma (Kröner and Tegtmeyer, 1994). Several xenocrystic zircons in sample AGC 423 have a younger age of ca. 3.2 Ga. Even though the subduction model of Laurie et al. (2012) show how the Mahamba Gneiss may have formed only with a Cr enrichment, the assimilation with continental crust make it not possible to infer a subduction environment. Therfore, the arc-signature in the Mahamba Gneiss could be originated by (a) continental matter buried in the mantle after subduction or delamination; (b) contamination of a non-enriched basalt by the continental crust through which it ascends (Arndt and Jenner, 1986; Barley, 1986; Green et al., 2000); or (c) the mantle itself being a nondepleted peridotite. The upper concordia of zircons from AGC 421 intercept at 3275±33 Ma and correlates to the crystallization ages between ca. 3240 and 3280 Ma of the “Nhalangano Gneiss” (Schoene and Bowring, 2010), while the upper intercept Concordia in sample AGC 423 intercepts at 3137±170 Ma. In the geochronology chapter the stronger recrystallization processes were already mentioned, which are either the result of stronger high-grade metamorphism or in association with the emplacement of the ca. 2.73 Ga Hlatikulu granite.

Meta-semipelite (AGC 415, 417, and 419) Samples AGC 415, 417 and 419 represent high-grade semi-pelitic garnetiferous gneisses. Detrital zircons in AGC 415 are probably derived from the Ngwane Gneiss, the Usutu Suite and probably

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from the adjacent Barberton Greenstone Belt (see chapter 8.3). For a better understanding of the sedimentary history, it is necessary to clear the role of the 3449±2 Ma dated tonalitic gneiss (Kröner et al., 1989; AGC 261). The tonalite intruded the metapelites and both rocks were later folded. AGC 415 plots between komatiites and basalts (Fig. 30b) and contains a high amount of iron. The Kromberg formation from the BGB consists of komatiites and Fe-tholeiites (Vennemann and Smith, 1999) and could have been a potential source area for AGC 415 based on the whole-rock geochemistry of this sample. The Kromberg Formation is dated at ca. 3416–3334 Ma (Byerly et al., 1996; Lowe and Byerly, 2007; Van Kranendonk et al., 2009) and overlies the 3470 Ma Hooggenoeg Formation, which consists of tholeiitic basalt, komatiitic basalt, komatiite, and felsic volcanic flows and sills (Byerly et al., 2002; Van Kranendonk et al., 2009). On top of the Kromberg formation are komatiites of the Mendon Formation with an age of 3298 Ma (Byerly et al., 1996; Lowe and Byerly, 2007; Van Kranendonk et al., 2009). Because the first metamorphic overprint is indicated by one grain (zircon spot 231) at 3098±48 Ma, zircon spots 109 and 227 best represent the depositional age at ca. 3281±52 Ma and 3267±42 Ma. These ages constrain the deposition to ca. 3.26 Ga. Van Kranendonk et al. (2009) and Van Kranendonk (2011) advocate for a greenstone cover successions in the upper crust that deformed between 3.26-3.22 Ga in association with partly remobilized hot granitic middle crust in response to a partial convective overturn. The deformation resulted without the influence of subduction-accretion tectonics in the BGB and a volcanic plateau setting was inferred for the evolution of the eastern Kaapvaal Craton prior to 3.2 Ga. Taylor et al. (2010) interpreted the metapelitic granulites from the Mkhondo Valley Metamorphic Suite to have a metamorphic origin in relation to the Venterstorp Supergroup flood basalts. They used pseudosections to infer the peak metamorphic conditions. In the geological maps (e.g. Wilson, 1982; Schoene and Bowring, 2010; Taylor et al., 2012) the Mkhondo Valley Metamorphic Suite is close to the Mahamba Gneiss. Despite to the proximity of the two metamorphic suites, Taylor et al. (2012) interpreted the crustal evolution of the metasedimentary granulites in the study area to be related to subduction-collison tectonics with a NW-dipping subduction zone at ca. 3.30-3.23 Ga, in which the sedimentary precursors were deposited in a

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forearc environment south of the “Nhalangano” (Mahamba) gneiss and the Ancient Gneiss Complex. High temperature metamorphism followed at ca. 3.23-3.21 Ga and subduction resumed at ca. 3.18-3.16 Ga, whereby the metasedimentary granulites evolved along a clockwise P-T path at ca. 3.11-3.07 Ga. In the geodynamic model for the Mesoarchaean crustal evolution, the generation of the Barberton Greenstone Belt resulted via a second NW-dipping subduction zone with the closure of an ocean basin at ca. 3.30-3.23 Ga north of the Usutu-Nhlangano (Mahamba) gneiss terrane. During the period of ocean basin closure the Fig Tree group was deposited coevally to the Lubuta-Kubuta sediments in a forearc environment. Final convergence led to collision of the BGB with the AGC.

Discussion Several features from the geological interpretations and the geochronology of this study are not compatible with the interpretation by Taylor et al. (2012): 1) AGC 401 is a metabasaltic komatiite, which correlates to the Barberton Greenstone Belt and Dwalile Greenstone Belts. 2) AGC 414 represents part of the amphibolitic alternations in the Ngwane gneiss (e.g., Schlüter, 2006) and correlates to Fe-tholeiites from the Barberton Greenstone Belt and the Dwalile Greenstone Belts. Because a contamination with a continental crust is not excluded (see above), a subduction or a plume related origin is not clear. In addition, the enrichement in FeO can form at relatively deep and active mantle upwelling regions in oceanic rift zones (Kimura et al., 1993). 3) The orthogneiss samples AGC 421 and 423 of the Mahamba Gneiss give different results. AGC 421 and 423 give evidence that an older crustal source plays an important role in the generation of the gneisses (ca. 3.5 Ga xenocrystic zircon in AGC 421; ca. 3.2 Ga xenocrystic zircons in AGC 423). This is also confirmed by the “enriched” TTG character of the samples (Moyen, 2011). For this reason the arc signature in the Mahamba orthogneiss is not necessarily subduction related and the mixing of “nonarc” material and continental crust yield an apparent arc signature irrespective of the tectonic setting where it takes place (van Hunen and Moyen, 2012). Although in east Pilbara, a slab melting origin for similar high-Al TTGs was also not unequivocally ruled out, Champion and Suhr, 2013. Petrography, petrology and geochronology of Archean granulites, Swaziland.

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Smithies (2007) suggested that their geochemical data represents the derviation from a thickened mafic crust, which is consistent with a plume-like geochemistry (Van Kranendonk and Pirajno, 2004; Smithies et al., 2005; Van Kranendonk et al., 2007b) and series of mantle plume events (Hickman and Van Kranendonk, 2004; Van Kranendonk and Pirajno, 2004; Van Kranendonk et al., 2007a; Van Kranendonk et al., 2007). In the proposed setting, mafic magma may undergo fractional crystallization (± assimilation) (Smithies et al., 2007; Van Kranendonk et al., 2007). In addition, partial melting can follow within the range of crustal levels, resulting in the generation of high-Al and low-Al granites. In such a scenario, the TTG generation occurs on spots, where mantle-derived magmas are ponding and underplating at the base of the crust (e.g. Gromet and Silver, 1987; Atherton and Petford, 1993) or maybe also in hot zones within the crust (Annen et al., 2006; Champion and Smithies, 2007; Van Kranendonk et al., 2007). 4) The whole-rock chemistry of AGC 415 suggest that greenstone belts were potential sedimentary sources, while the Barberton Greenstone Belt consist of mafic and ultramafic lithologies with compositions that may explain the enrichments in Fe, Cr, and Ni in the Mahama metasediments. Moreover, the depositional ages at ca. 3281±52 Ma and 3267±42 Ma correlate to the ca. 3.26-3.22 Ga partial convective overturn of the BGB and the partly remobilized hot granitic middle crust (Van Kranendonk, 2011). One detrital zircon (spot 225) has an age of 3730±45 Ma and may represent an older crustal source, probably the basement for greenstones, as has been suggested by Kröner and Tegtmeyer (1994) and by Van Kranendonk et al. (2009). 5) With the help of mineral equilibria modelling (Figs. 33-36) and mineral textural identifications (Fig. 10), it was possible to constrain a realistic clockwise P-T path, which has no similarity to modern subduction-accretion zones (Fransiscan complex) or collisional orogens (Western Alps) (Van Kranendonk, 2011). The mineral equilibria models of metapelitic granulites from the Mkhondo Valley Metamorphic Suite , which were interpreted to be related to a ca. 2.7 Ga mantle plume marginal to the Kaapvaal craton (Taylor et al., 2010), have strong similarities to the pseudosections and metamorphic gradients of this study. 6) The Lu-Hf age of the garnets, which formed during peak metamorphism have an age of 2921±31 Ma and also two zircons (spot 228, 229) have similar ages at 2951±71 Ma and at 2976±45 Ma. Moreover, each orthogneiss sample has one zircon that gives evidence for a Suhr, 2013. Petrography, petrology and geochronology of Archean granulites, Swaziland.

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metamorphic event at 2870±52 Ma (spot 105; Th/U ratio of 0.03) and at 2861±48 Ma (spot 86; Th/U ratio of 0.03). Therefore, the geochronological data differ with the interpretation of Taylor et al. (2012) that the Luboya and Kubuta granulites evolved along a clockwise P-T path at ca. 3.11-3.07 Ga, which represents modern subduction-accretion tectonics. All listed points show that the proposed geodynamic setting for the Mesoarchaean crustal evolution near the Kubuta Shiselweni Cattle Ranch cannot be explain with a protracted accretionary orogenic event. Moreover, the time of peak metamorphism is in agreement with Gumsley et al. (2012), who argues for an association between the Hlagothi Complex, the Thole Complex, gabbroic phases of the Usushwana Complex and flood basalts within the Mozaan Group and Central Rand Group, forming a previously unrecognized Large Igneous Province in the southeastern Kaapvaal Craton. The Usushwana complex formed by at least two magmatic pulses between 2990 Ma and 2860 Ma (Hunter and Reid; 1987; pers. comm., J.R. Olsson, 2011; Gumsley et al., 2012), may have provided the required mantle heat for metamorphism near the Kubuta Shiselweni Cattle Ranch at ca. 2.9 Ga. After the intrusion of the Hlaghoti Complex, potassium-rich post-Pongola granites derived from partial melting of continental lithosphere and intruded the region (Gumsley et al., 2012). Based on the summarized data in this chapter, the Hlatikulu granites may represent the counterpart in south central Swaziland. Palaeomagnetic data show that the Kaapvaal Craton underwent latitudinal drift during the Mesoarchaean and Neoarchaean with a significant change in the positions of the palaeopoles between 2.95, 2.87 and 2.75 Ga from low latitudes to high latitudes. These ages point to short lived plumes in the southeastern Kaapvaal Craton (Gumsely et al., 2012) rather than to a stationary mantle plume. Based on the data of this study and of Taylor et al. (2010), metasediments near the Kubuta Shiselweni Cattle Ranch and of the Mkhondo Valley Metamorphic Suite give evidence that their high grade metamorphic history is related to these mantle plume events. In addition, this summary shows that a model with two separated subduction zones cannot explain the correlations between the samples of this study and the Barberton greenstone belt.

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Eidesstattliche Erklärung

Hiermit versichere ich, dass ich die Diplomarbeit selbstständig verfasst und keine anderen als die angegebenen Quellen und Hilfsmittel benutzt habe, alle Ausführungen, die anderen Schriften wörtlich oder sinngemäß entnommen wurden, kenntlich gemacht sind und die Arbeit in gleicher oder ähnlicher Fassung noch nicht Bestandteil einer Studien- oder Prüfungsleistung war.

Nils Suhr

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