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M. GUEYE, S. SIEGESMUND, K. WEMMER, S. PAWLIG, M. DROBE, N. NOLTE AND P. LAYER

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New evidences for an early Birimian evolution in the West African Craton: An example from the Kédougou-Kénieba inlier, southeast Senegal M. Gueye Institut des Sciences de la Terre, University Cheikh Anta Diop Dakar, Senegal e-mail: [email protected]

S. Siegesmund, K. Wemmer, S. Pawlig, M. Drobe and N. Nolte Geoscience Center, Georg-August University, Goldschmidtstr. 3, 37077 Göttingen, Germany e-mail: [email protected]; [email protected]; [email protected]; [email protected]; [email protected]

P. Layer Geophysical Institute, University of Alaska, Fairbanks, Alaska 99775, United States of America e-mail: [email protected] © 2007 December Geological Society of South Africa

ABSTRACT The Paleoproterozoic rocks in the Kédougou Kénieba Inlier (KKI) consist of volcanic and volcanosedimentary rocks intruded by granitoids with a wide compositional spectrum. Geochronological data based on single-grain zircon Pb-Pb evaporation, U-Pb single grain, Ar-Ar and K-Ar dating of several plutons point to a long lasting emplacement history in the KKI. The zircon data presented here yielded Paleoproterozoic ages for the emplacement/crystallization of the Badon granodiorite (2213 ± 3 to 2198 ± 2 Ma), tonalitic gneiss from Sandikounda (2194 ± 4 Ma), the Tinkoto pluton (2074 ± 9 Ma) as well as for the felsic host rocks of the Mamakono pluton (2067 ± 12Ma). The Badon granodiorite, formerly regarded to be relatively young and post-tectonic, is substantially older than all other known KKI intrusions. As a consequence, this new age may constrain the ongoing discussion on the existence of an Early Birimian cycle in the West African Craton (WAC). Ar-Ar and K-Ar ages on hornblende (2112 ± 12 and 2118 ± 31 Ma, respectively, in the tonalitic gneiss from Sandikounda) and biotite (2098 ± 26 and 2090 ± 21 Ma, respectively, in the Badon granodiorite) define an age of ca 2090 Ma, which is interpreted to mark the major tectonomagmatic episode in this part of the WAC. This is confirmed by the Paleoproterozoic ages of the Saraya, Tinkoto and Mamakono granite. A hydrothermal event at 2020 Ma followed the emplacement of the granitoids and caused the alteration of the Saraya granite. The new zircon ages, reflecting the magmatic and tectonothermal events in the study area correlate well with various magmatic and metamorphic events elsewhere in West Africa.

Introduction Crustal growth processes in ancient Paleoproterozoic terranes can be constrained from the lithology, chemistry and geochronology of plutonic rocks Many studies on Paleoproterozoic granite-greenstone terrains have been carried out in the last decade. However, geochronological and isotopic data are still inadequate. Over the last 20 years numerous attempts have been undertaken to determine the emplacement ages and to refine the intrusion sequence of Birimian granitoids (e.g. Bassot and Caen-Vachette 1984; Dia et al. 1997; Ndiaye et al. 1997) in the West African Craton (WAC, Figure 1). Granitoids constitute a significant proportion of the Paleoproterozoic rocks exposed in the KédougouKénieba Inlier in southeastern Senegal (KKI; Figure 2). Field relationships, petrography, geochemistry, and petrogenesis of granitoids intruded into the high-grade gneisses and low-grade metasedimentary and metavolcanic rocks have previously been studied by Witschard (1965), Bassot (1966), Dia (1988), Dioh (1995), Dia et al. (1997), Gueye (200l), Hirdes and Davis (2002), Pawlig et al. (2006). However, age relationships among the widespread occurrence of granitoids in the

KKI are still not well understood. In particular, systematic geochronological studies are lacking for the southeastern Senegalian Lower Proterozoic greenstone belt, except the recent studies by Dia et al. (1997) as well as Hirdes and Davis (2002), who reported U-Pb, PbPb and Rb-Sr data. The present study is focussed on the magmatic history including intrusion ages and subsequent cooling of calc-alkaline granitoids in the KKI using Pb-Pb and U-Pb single-grain techniques on zircons and Ar-Ar and K-Ar dating on hornblende, whole rock and micas. The results of the study, combined with other regional data will help to clarify the geological evolution of the southern Senegalian Lower Proterozoic rocks in the West African Craton. Geological setting The Kédougou-Kénieba Inlier (KKI) in southeastern Senegal represents the westernmost part of the West African Craton (WAC), between the Reguibat Rise in the north and the Leo Rise in the south (Figure 1). The WAC is a metamorphosed and strongly deformed crustal section which became tectonically stable at ~1.9 Ga

SOUTH AFRICAN JOURNAL OF GEOLOGY, 2007, VOLUME 110 PAGE 511-534 doi:10.2113/gssajg.110.4.511

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NEW EVIDENCES FOR AN EARLY BIRIMIAN EVOLUTION IN THE WEST AFRICAN CRATON

Figure 1. The Birimian terrane of the West African Craton (WAC) with the position of the Kédougou-Kénieba Inlier (KKI). Modified after Gasquet et al. (2003). (1) Limit of the WAC; (2) Post-Paleozoic cover; (3) Late Proterozoic and Paleozoic; (4) Pan-African and Hercynian belts; (5) Archean and Paleoproterozoic basement.

(Bessoles 1977). The KKI at the border of Senegal, Mali and Guinea is a triangular-shaped area bounded by the Mauritanide Hercynian Belt on its western side. It is covered to the north, east and south by Neoproterozoic and Paleozoic formations of the Taoudeni intracratonic basin. The KKI consists of Early Paleoproterozoic formations formed during the Eburnean orogeny at ~2.2 to ~2.0 Ga (Abouchami et al. 1990; Liegeois et al. 1991). These formations are called “Birimian” in West Africa. The Birimian terrains in the WAC represent juvenile crust without any influence of surrounding Archean continents (Abouchami et al. 1990; Boher et al. 1992; Pawlig et al. 2006). The KKI crops out over 16.000 km2 and consists of volcanosedimentary greenstone belts intruded by calcalkaline granites. The volcanosedimentary units are separated into two lithostratigraphic supergroups, the Mako Supergroup (Bassot and Caen-Vachette 1984; Bassot 1987) in the west and the Dialé-Daléma Supergroup in the east (Bassot and Caen-Vachette 1984, Bassot 1987, Table 1 and Figure 2). The Mako Supergroup comprises metamorphosed and deformed volcanic and volcanosedimentary sequences intruded by granitoids of the Kakadian Batholith and smaller massifs as defined by Witschard (1965) and Bassot (1966). From bottom to top, this supergroup consists of dominantly basaltic (mainly pillow basalts), volcano-sedimentary and sedimentary complexes (Diallo 1994). Various gabbroic and

ultrabasic rocks are interlayered. Andesites, rhyodacites and scarce rhyolites are associated with the volcanosedimentary and sedimentary formations (Figure 2). The metabasalts in southern outcrops are either massive or occur as pillow lavas. Geochemical studies on volcanic rocks from the Mako Supergroup indicated that they originated from a mid-ocean ridge (Bassot 1966; Ngom 1985; Dioh 1986; Fabre 1987; Zonou 1987), island-arcs (Dia 1988; Diallo 1994) or oceanic plateaus (Abouchami et al. 1990). More recently Pawlig et al. (2006) confirmed that an oceanic island arc setting is most probable. Dia (1988), Bertrand et al. (1989) and Dia et al. (1997) assumed that an amphibolite-gneiss complex (SAG) represents the base of the Mako Supergroup and is interpreted to be the initial stage of the Paleoproterozoic crustal growth. In the LaminiaSandikounda area, this complex is dated at 2202±6 Ma (Pb-Pb on zircon, Dia et al. 1997). Platform-type sediments characterize the DialéDaléma Supergroup, notably carbonates, which were intruded by intermediate to felsic calc-alkaline rocks. The Dialé formation in the west is separated from the Daléma formation in the east by the Saraya Batholith, which shows arc affinities and was dated at 2079 ± 2 Ma (Hirdes and Davis 2002, U-Pb on zircon). The Dialé formation comprises basal limestones and dolomitic marbles followed by greywackes, sandstones and pelites. The Daléma formation consists of interlayered quartzites, greywackes, schists and marbles that contain rare slump breccias. The boundary between the Mako and the DialéDaléma Supergroups is tectonic and marked by a regional crustal scale shear zone, the Main Transcurrent Shear Zone (MTZ). This shearzone is striking northeast to southwest and rotates to a northwesterly direction as it crosses the Falémé River into Mali (Milési et al. 1989). During the late Eburnean the KKI underwent transcurrent deformation along a network of ductile shear zones (Ledru et al. 1991; Milési et al. 1992). Almost all lithostratigraphic units of the KKI were metamorphosed under greenschist-facies conditions. Amphibolite-facies metamorphism is only observed in the contact aureoles around granitic intrusions. Recently, Kleinschrot et al., (1994), John et al. (1999), Klemd et al. (2002) and Galipp et al. (2003) described regional amphibolite-facies metamorphism in southeastern Ghana, based on mineral-chemical data and geothermobarometry. They suggest that the entire Ashanti Belt, the Kibi-Winnega Belt and the Sefwi Belt in Ghana underwent epidote-amphibolite-facies metamorphism (T = 500 to 650°C and P = 5 to 6 kbar) before experiencing retrograde greenschist-facies. Thus, John et al. (1999) and Klemd et al. (2002) conclude that large parts of the Birimian Supergroup in Ghana and probably West Africa were metamorphosed under peakamphibolite-facies conditions. Furthermore, Debat et al. (2003) show that amphibolite-facies assemblages are locally overprinted by regional greenschist-facies metamorphism.

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Figure 2. Simplified geological map of the Kédougou-Kéniba Inlier (KKI) with sample localities (modified after Ledru et al. 1991) and previous geochronological data. (1) and (2) granitoids, (3) basalts, (4) andesites, (5) volcanosediments and sediments, (6) clastic metasediments, (7) volcanoclastics, (8) intermediate volcanics, (9) andesites, (10) metasediments, (11) limestones, (12) sandstone, (13) undifferentiated Paleozoic and Neoproterozoic. MTZ : main Transcurrent Zone ; SMF : Senegalomalian Fault; dashed line: Senegal – Mali border.

Greenstones of the KKI are intruded by two major batholiths: the Kakadian Batholith in the west and the Saraya Batholith in the east (Witschard 1965; Bassot 1966). The Kakadian Batholith is composed of various granitic rocks dated at 2199 ± 68 Ma (Rb-Sr WR, Bassot and Caen-Vachette 1984). This batholith is a layered plutonic complex mainly composed of

wehrlites, pyroxenites, gabbros and diorites (Dia 1988), and small individual plutons. Some of these intrusions have been dated. The ages cluster in the ~2.2 to ~2.0 Ga interval (Bassot and Caen-Vachette 1984; Dia et al. 1997). Several geochronological studies have been carried out in the KKI including Sm-Nd whole rock, Rb-Sr, U-Pb



Intrusions in Dialé-Daléma

Saraya batholith Intrusion in Dialé-Daléma

Kakadian batholith Intrusion in Mako

Dialé-Daléma supergroup

Mako supergroup

Andesites Quarz rich wacke sediments Andesitic dykes Felsic flow Granite Granodiorite (Laminia) Quarz diorite (Laminia) Orthogneiss (Kaourou) Granodiorite (Kaourou) Layered gabbro Tonalitic gneiss Dioritic gneiss amphibolite Granodiorite Granite Granite Granite Granite Granite Granodiorite Granite Tonalite

Mako volcanosedimentary complex Dialé complex Daléma complex

Daléma complex

Falémé volcanic belt Multiple granitic bodies Laminia-Kaourou Plutonic Complex

Gamaye granite

Boboti granite

Mamakono (pluton)

Sandikounda layered plutonic complex Sandikounda amphibolite-gneiss complex

Rock type Basalt

Complex/Belt Mako volcanic-plutonic complex

Table 1. Compilation of previous geochronological data

Pb/Pb Rb/Sr Pb/Pb U/Pb Pb/Pb Pb/Pb Pb/Pb Pb/Pb Pb/Pb Sm/Nd Pb/Pb Rb/Sr Rb/Sr Pb/Pb Pb/Pb Rb/Sr Pb/Pb Rb/Sr Pb/Pb

Pb/Pb (Zr) detr. (WR) (Zr) (Zr) (Zr) (Zr) (Zr) (Zr) (Zr) (WR) (Zr) (WR) (WR) (Zr) (monazite) (WR) (Zr) (WR) (Zr)

(Zr) detr.

Method Sm/Nd (WR) Sm/Nd (WR) Pb/Pb (WR) Sm/Nd (WR) Pb/Pb (Zr) detr. Pb/Pb (Zr) detr.

Age 2063 2197 2195 2160 2165 2096 2156 2070 2072 2099 2199 2105 2127 2079 2138 2158 2194 2202 2157 2076 1973 2008 2079 2064 1989 2080 2045 2081 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

41 13 11 16 0,9 8 to 10 10 9 4 68 8 6 6 6 8 4 6 67 3 33 35 2 (igneous) 4 (metamorph) 28 0,9 27 1

References Abouchami et. al. 1990 Dia 1988 Dia 1988 Boher 1991 Hirdes and Davies 2002 Milési et. al. 1989 Calvez et. al. 1990 Milési et. al. 1989 Calvez et. al. 1990 Hirdes and Davies 2002 Bassot and Cean-Vachette Dia et. al. 1997 Dia et. al. 1997 Dia et. al. 1997 Dia et. al. 1997 Dia et. al. 1997 Dia et. al. 1997 Dia et. al. 1997 Dia 1988 Hirdes and Davies 2002 Bassot and Cean-Vachette Ndiaye et. al. 1997 Hirdes and Davies 2002 Hirdes and Davies 2002 Bassot and Cean-Vachette Hirdes and Davies 2002 Bassot and Cean-Vachette Hirdes and Davie 1984

1984

1984

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Figure 3. Microfabrics of the Badon Granodiorite, crossed polarizers, plagioclase (Pl), quartz (Qz), feldspar (Kf), biotite (Bi). (A) Rounded quartz inclusion in plagioclase (Pl). Arrow head points to melt a pocket at a plagioclase-plagioclase-quartz grain boundaries. (B) Melt film at grain boundaries: an intergranular film of K-feldspar is developed along quartz-quartz and quartz-plagioclase boundaries (arrows). (C) Cuspate extension of K-feldspar inclusion in two perpendicular directions. (D) Intragranular fine grained clusters of quartz+plagioclase+biotite formed by crystallization of melt. Extension of plagioclase (arrow). (E) Cuspate triangular section through plagioclase grains at the quartz-quartz boundaries (arrow). (F) Small grain of plagioclase occurred at the three grain junction in quartz. Prismatic plagioclase grain at quartz boundaries (arrow).


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Figure 4. Cathodoluminescence (CL) and backscattered-electron (BSE) images of selected zircons from Badon (A, B), Sandikounda (C, D) Tinkoto (E) and Mamakono (F). Prominent cracks are due to sample preparation. (c: core, m: magmatic zoning, r: resorption; rz: reaction zone, z: zoning. (A and B) backscattered-electron (BSE) images of prismatic zircons with an inherited core (c), and variably resorped wide rims. B: Zircon is truncated by secondary overgrowth in the rim. (C and D) Cathodoluminescence (right) and backscattered-electron (left) images of selected zircons from Sandikounda. Prismatic zircons are with an inherited core (c), magmatic zoning (m) and later overgrowth (reaction zone (rz)). (E and F) backscattered-electron images of selected zircons from Tinkoto (E) Mamakono (F).



and Pb-Pb dating on tholeiitic basalt and a variety of granites as well as volcaniclastic sediments. The geochronological data available for Birimian rocks of the KKI are summarized in Table 1 and vary between ~2.2 and ~2.0 Ga (Bassot and Caen-Vachette 1984; Dia 1988; Milési et al. 1989; Abouchami et al. 1990; Calvez et al. 1990; Boher 1991; Diallo et al. 1993; Ndiaye et al. 1997). Sample description and preparation Samples selected for geochronological analyses include granitoids from the southern part of the Kakadian Batholith (M1-02 Badon and M2-02 Tinkoto) and from the Saraya Batholith (M3-02), which intrudes the DialéDaléma Supergroup. The sample from Mamakono (M9-02) belongs to a felsic flow of the Mako Supergroup, while the tonalitic gneiss from Sandikounda (M12-02) represents the assumed base of the Mako Supergroup. Sample M1 was collected from the southern part of the Kakadian Batholith near the Badon village. In the geological maps of Senegal the Badon Granodiorite is mapped as a roundshaped isolated granodiorite within the Kakadian Batholith. This gives the impression of being younger and postkinematic, which is not the case as we will show in the following. The sample localities and additional ones for K-Ar dating are labelled in Figure 2. Zircon morphologies, based on detailed back-scattered electron (BSE) and cathodoluminescence (CL) imaging of representative grains, and petrographic characteristics of the samples are described in Table 3. Textural features and representative zircon crystals are shown in Figures 3 and 4. Analytical procedures We used four different methods in order to determine the age of the rocks from the KKI, namely single zircon analyses using the Pb-Pb evaporation and U-Pb techniques, Ar-Ar and K-Ar analyses on biotite, hornblende and muscovite. Zircon, biotite, hornblende and muscovite were separated from the crushed rocks according to density and magnetic susceptibility. All mineral separates were purified by hand-picking under a binocular microscope. Zircon grains studied by cathodoluminescence (CL) and backscattered electron (BSE) imaging were mounted in epoxy resin and polished. The CL and BSE images were obtained using

a JEOL JXA-8900RL electron microprobe at the University of Göttingen. The Pb evaporation technique for single zircon dating (Kober 1986; 1987) is based on the radiogenic 207 Pb/206Pb* ratios. However, the strength of the evaporation technique is that the zircon may be analyzed in a series of discrete “heating steps”. If the same idealised zircon was analyzed in a large number of discrete heating steps, the initial steps would give a lower plateau age, representing the age of the rim, and the final steps would give an upper plateau age, representing the age of the core. Between these two plateaus, mixing ages can occur. Revealed age frequency spectra of the evaporation technique are consistent with results obtained by U-Pb SHRIMP dating (Foden et al. 1999), implying that the older age increments reflect the age memory of inherited zircon cores. Samples were analyzed at the Department of Geology and Geophysics (University of Adelaide, Australia). Analyses were carried out on both Finnigan MAT 261 and 262 mass spectrometers, each of which is equipped with Faraday cups and a secondary electron multiplier (SEM). Clear and long-prismatic zircons from sample M12-02 were selected from the 100-125 µm fraction for isotope dilution single grain U-Pb dating by thermal ionization mass spectrometry (TIMS) as described by Poller et al. (1997). From this sample, 6 single zircons were transferred into a special Teflon bomb with small holes for each individual zircon (Wendt and Todt 1991; Poller et al. 1997). Prior to dissolution, zircons were cleaned with 7N HNO3 and after quantitative removal of the 7N HNO3, each zircon grain was spiked with 2µl of a mixed 205Pb/233U spike solution. The zircons were then dissolved with concentrated HF by placing the bombs into an oven at 200°C for ~5 days. The samples were dried, placed into the oven again with 6N HCl for one day and dried again. After this procedure, the zircons were ready for measurements without further chemical separation of U and Pb. Pb and U were loaded with Sigel on single Re-filaments and measured on a ThermoFinnigan Triton mass spectrometer at the Department of Isotope Geology (GZG, Göttingen) operating in multiplier mode at temperatures of 1250°C and 1350°C, respectively. The total Pb blank was 10 pg. The following ratios were used for blank corrections: 206 Pb/204Pb = 17.72, 207Pb/204Pb = 15.52 and 208Pb/204Pb =

Table 2. Rock samples selected for age dating Rock Unit

Saraya batholith Dialé-Daléma Dialé-Daléma Mako Belt Sandikounda amphibolite-gneiss complex Mako Belt Dialé-Daléma

Sample No M1-02 M2-02 M3-02 M5-02 M6-02 M9-02 M12-02 MP2-03 KO75-03

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Locality Badon Tinkoto Saraya Wassangara Wassangara Mamakono Sandikounda Diombalou Kakadian


Rock type Granite Granodiorite Episyenite Micaschist Micaschist Rhyolite Tonalitic gneiss Micaschist Granodiorite

M3-02

M9-02

M12-02

Mamakono rhyolite

Tonalitic gneiss

M2-02

Tinkoto granodiorite

Saraya episyenite

Sample number M1-02

Rock name Badon granite


– –

–

– –

–

–

General petrographic features and textural characteristics – medium- to coarse grained granite. Biotite flakes and biotite-hornblende-sphene aggregates define a weak foliation; – melt films along grain boundaries and melt pockets at grain corners (Figure 3B) – small cuspate extensions of plagioclase developed along quartz-quartz graiin boundaries (Figures 3C, 3D and 3E); – small grains of plagioclase commonly occur in triple junctions of quartz (Figure 3F); – corroded grain boundaries, lattice bending and interpenetrating grain boundaries (Figures 3B, 3D).

– equigranular principally composed of K-feldspar, quartz, plagioclase and biotite, with minor amphibole; – well-developed chessboard pattern of quartz grains is present. Quartz grains are flattened to produce quartz ribbons with irregular, serrated edges (migration recrystallization) (Figure 5A); – weak shape preferred orientation (SPO) and non-coaxial magmatic deformation is attested by imbrications of biotite and plagioclase (Paterson et al. 1989; Figure 5B). – the shape of plagioclase was roughly preserved but quartz was entirely dissolved and replaced by secondary albite and carbonate. New albite occurs as an euhedral overgrowth on plagioclase (Figure 6); – the granitic texture is partially obliterated and a wide spread of secondary minerals appears. Petrographic studies of the sample allowed the identification of an intense hydrothermal alteration as already described by Mouthier (1988)(episyenitization). colourless zircon grains (50 µm to 350 µm), with a distinct core-rim – generally porphyritic, composed of phenocrysts of quartz, plagioclase, structure. Zircon cores often contain inclusions of apatite; biotite and amphibole (brown and green hornblende). The groundmass presence of zircons with fine concentric zonation that may is characterized by fine-grained quartz and albite-rich plagioclase; enclose a core without resorbsion (Figure 4F); – the presence of biotite and green hornblende possibly indicates that the upper two growth stages observed in CL image. greenschist-facies conditions were at least locally attained. zircons are slightly elongated and prismatic with concentric, oscillatory – the sample is mainly composed of plagioclase, quartz, diopside (partially zoning in CL images (Figure 4D) and distinct core–rim structure; replaced by hornblende), and green hornblende. Accessory components are magmatic zoning is observed in the rim of some zircons. Zircon cores apatite, sphene, opaque minerals (iron oxides and ilmenite) and zircon; appear as high luminescent but unzoned areas and often contain – plagioclase show deformation lamellae and an undulatory extinction; inclusions of apatite. Some are euhedral and preserve a fine concentric – quartz grains are generally rounded in shape due to low quartz feldspar dihedral zonation that may enclose a discordant core (Figure 4C); angles of the grain boundaries (FigureS 5C, 5D) or sometimes appears as inclusion most grains have at least two to three growth stages (CL photographs); in plagioclase (Figure 5E); many zircons have a thin, highly luminescent rim probably – microcline exsolutions in plagioclase attest to relatively high proportion of melt representing a reaction zone resulting from a common and indicate ternary composition during crystallization (Figure 5C) migmatization (Figure 4D). – intergranular film of quartz developed along quartz-plagioclase-plagioclase boundaries (Figure 5F).

Descriptions (zircons) – short prismatic, displaying complex internal structures with core and rim domains, generally oscillatory-zoned (Figures 4A, 4B); – well-preserved magmatic growth zoning in some cores; others are homogeneous and dominate the major part of the crystal; a third type of core shows corrosion features (Figure 4B); – rim overgrowths associated with sector zonation on oscillatory-zoned cores and evidence for resorption around the cores (in CL images); – presence of perfectly zoned zircons; – simultaneous presence of all three structural types (magmatic zoning resorbed areas, inherited component) in some zircons. – presence of zircons with a distinct morphology; – homogeneous to magmatically zoned cores, and variably thin rims of secondary magmatic overgrowth in the long- to short prismatic euhedra zircons (from 100 to 150µm) show; – needle-like shapes with very high aspect ratios in some grains (Figure 4E).

Table 3. Description of zircons and summary of petrographic and textural characteristics of selected granitoid rocks from the KKI



37.7. The following Pb ratios at 2.1 Ga (Stacey and Kramers 1975) were used for common Pb corrections: 206 Pb/204Pb = 14.742, 207Pb/204Pb = 15.083 and 208Pb/204Pb = 34.394. All ratios were corrected for isotopic fractionation using the NBS981 and U100 standards as reference. Errors for isotopic ratios and calculated ages are at the 2-level. 207Pb/206Pb ages were calculated using the Isoplot/Ex program (Ludwig 2003). All results are presented in Appendix A 40 Ar/39Ar studies were performed at the Geophysical Institute, University of Alaska, Fairbanks, United States. The samples were wrapped in aluminium foil, labelled top and bottom, and arranged in one of two levels within aluminium cans of 2.5 cm diameter and 4.5 cm height. The irradiated samples (one to three crystals) and monitors were loaded into 2-mm-diameter holes in a copper tray, which was then placed into an ultra-high vacuum extraction line. They were fused and stepheated by a 6-W argon-ion laser using the technique described by York et al. (1981) and Layer et al. (1987). Argon purification was achieved using a liquid nitrogen cold trap and a SAES Zr-Al getter at 400°C. The gas was then analyzed in a VG-3600 mass spectrometer. For each sample, a plateau age was determined from three or more consecutive fractions whose ages are within 2 error of each other and total more than 50% of gas release. Full analytical data is given in Appendix C. K-Ar dating was carried out at the GZG, University of Göttingen. The argon isotopic composition was measured in a pyrex glass extraction and purification line coupled to a VG 1200 C noble gas mass spectrometer operating in static mode. The amount of 40 radiogenic Ar was determined by isotope dilution 38 method using a highly enriched Ar spike from Schumacher, Bern (Schumacher 1975). The spike is calibrated against the biotite standard HD-B1 (Fuhrmann et al. 1987). Potassium was determined in duplicate by flame photometry using an Eppendorf Elex 63/61. The samples were dissolved in a mixture of HF and HNO3 according to the technique of Heinrichs and Herrmann (1990). CsCl and LiCl were added as ionisation buffer and internal standard respectivly. Details of sample preparation, argon and potassium analyses are given in Wemmer (1991). Full analytical data of the K-Ar analyses is listed in Appendix D. All Ar-Ar and K-Ar ages are quoted at 95% confidence level (2) and calculated using the constants recommended by the IUGS (Steiger and Jäger 1977). Results Zircon morphologies, based on detailed back-scattered electron (BSE) and cathodoluminescence (CL) imaging of representative grains are described in Table 3. Zircon Zircons from the Badon granodiorite (M1-02) show a bimodal 207Pb/206Pb evaporation age distribution (Figure 7), which suggests an inherited core. The

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inherited components (two grains) reveal a mean 207 Pb/206Pb age of 2213 ± 3 Ma, whereas the rims (four grains) gave a slightly younger age of 2198 ± 2 Ma. 207 Pb/206Pb ages of the Tinkoto pluton (M2-02) vary between 2075 and 2022 Ma with a mean age of 2074 ± 5 Ma. This mean age is dominated by two very precise analyses, while grains three and four exhibit younger ages associated with high analytical errors. 207 Pb/206Pb ages from zircons of the Mamakono rhyolite (M9-02) show scattering with a calculated mean of 2067 ± 12 Ma. A subdivision in inherited cores and magmatic rims is unlikely due to the volcanic origin of the rock. However, the geological significance of the observed spread between c. 2090 and c. 2050 Ma is not easy to explain. Zircons from the Tonalitic gneiss from Sandikounda (M12-02) give ages ranging from 2203 Ma to 2171 Ma. The mean age is 2194 ± 4 Ma (Figure 7). Some grains (data points 1, 6 and 7) yield discordant data that form a non linear array, resulting from the combined effects of inherited components and/or Pb-loss. Data points 2 to 5 are high precision spots. They are more robust analytically and define the weighted mean. The U-Pb single zircon dating gives an age of 2205 ± 15Ma (Figure 8). Even so only two highly discordant grains were analyzed the resulting age is in good agreement with the Pb-Pb evaporation data. Full data for zircon dating is given in Appendix A and B. 40

Ar/39Ar and K-Ar ages for hornblende, biotite and muscovite The 40Ar /39Ar analytical results are presented as age spectra in Figure 9. Detailed data are given in Appendix C and D. Hornblende Two of three runs of hornblende from the Tinkoto pluton (M2-02) have quite flat age spectra with Ca/K ratios greater than 10 (Figure 9). The mean age of the two runs for this sample is 2051 ± 16 Ma and the individual ages are 2047 ± 22 and 2055 ± 20 Ma, respectively. The third single grain shows no plateau and is characterized by a lower Ca/K ratio of 7.6 and a younger integrated age of 1952 ± 18 Ma, which is interpreted as being geologically not significant, evenso a comparable U-Pb age of 1987 ± 20 Ma was obtained by Aït Malek et al. 1998 for the Sidi Said granite in the Moroccan Anti Atlas. The disturbance of the isotope system is indicative for the alteration of this crystal which caused complex argon loss and created alteration products with a lower Ca/K ratio. For the tonalitic gneiss from Sandikounda (M12-02) a mean age of 2112 ± 12 Ma was calculated from three single grain analyses (Figure 9). All three runs have flat age spectra with Ca/K ratios lower than 10. The individual plateau ages are 2104 ± 18, 2109 ± 20 and 2123 ± 20 Ma, respectively. The calculated mean age of 2112 ± 12 Ma is interpreted to date the subsequent


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Figure 5. Microphotographs of the Tinkoto Granite (A and B) and the tonalitic gneiss from Sandikounda (C, D, E and F), crossed polarizers, plagioclase (Pl), quartz (Qz), k-feldspar (Kf), diopside (Di), biotite (Bi). (A) elongate quartz grains indicating a solid state overprint (B) tiling of biotite crystals (C) Cuspate extension of plagioclase in quartz. Note rounded shapes indicating high dihedral angle. (D) K-feldspar melt film along grain boundaries and melt pools at grain junctions. Note the rounded (corroded) outline of quartz and plagioclase against former melt. (E) Diopside (Di) porphyroblast showing inclusion of quartz (F) Melt film at grain boundaries: intergranular film of quartz developed along quartz-plagioclase-plagioclase boundaries. Plagioclase grain is partly replacd by quartz and the lobate quartz and plagioclase boundaries are common for deformation at high temperature conditions.



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Figure 6. Microphotographs of the episyenite from Saraya, plagioclase (Pl), carbonate (Ca), muscovite (Mu). Carbonate is progressively replaced by muscovite and plagioclase. New albite (Pl2) occurs as an euhedral overgrowth on plagioclase (Pl1).

cooling after amphibolite-facies metamorphism. K-Ar dating yield a comparable age of 2118 ± 31 Ma. Biotite Biotite from the M1-02 sample yields a plateau age of 2098 ± 20 Ma (Figure 7), which comprises five steps and 86% of the 39Ar released. K-Ar dating let to an age of 2090 ± 9 Ma. The oldest 40Ar/39Ar biotite plateau age of about 2100 Ma is regarded as a minimum age for cooling of the Badon granodiorite below 300°C (McDougall and Harrison 1999). K-Ar dating on biotite from the Tinkoto pluton (M2-02) yielded an age of 2064 ± 20 Ma. This age being slightly older than the Ar-Ar hornblende age accounts for rapid cooling. Retrograde overprint and disturbance of the K-Ar isotope system is documented in the data of several samples from the Mako and the Dialé-Daléma Supergroup (MP2-03, KO75-03, M5-02 and M6-02, Figure 2). Muscovite Muscovites from M3-02 (Saraya Batholith) give plateaus with release patterns indicating minor disturbance. They yield a mean age of 2022 ± 12 Ma (Figure 9), calculated from three single grain analyses of 2018 ± 20 Ma, 2019 ± 24 Ma and 2027 ± 22 Ma, respectively. Only the last pattern can be regarded as a real plateau age (McDougald and Harrison 1999). The mean muscovite ages of 2022 ± 12 Ma (Ar-Ar) and the K-Ar analyses of 2021 ± 11 Ma reflect cooling through the blocking temperature. Discussion The Eburnean orogeny corresponds to a major crust forming event defined by the formation of Paleoproterozoic supracrustal rocks and associated synvolcanic and syn- to late kinematic intrusive rocks. The Birimian domain is divided into the Birimian sensu stricto at ~2.19 to 2.15 Ga and the Bandamian at ~2.1 Ga (Boher et al. 1992; Hirdes et al. 1996; Lüdtke et al. 1998; 1999; Hirdes and Davis 2002). Slightly younger ages (~2.09 to ~2.05 Ga) were reported for granitoids from the Moroccan Anti-Atlas (Gasquet et al. 2004; 2005),

Guinea (Egal et al. 2002), and Senegal (Hirdes and Davis 2002) respectively, which were related to the late Eburnean event. The term Eburnean refers to all tectonic, metamorphic and plutonic events affecting the Birimian rocks during the Paleoproterozoic at ~2.2 to ~2.0 Ga (Bonhomme 1962; Abouchami et al. 1990; Liegeois et al. 1991). However, Gasquet et al. (2003) report in the Leo Rise an early Paleoproterozoic crustal growth episode at ~2.3 Ga preceding the Birimian s.s and the Bandamian. Peucat et al. (2005) also described an early phase of orogenic activity in the Reguibat Rise (~2.22 to ~2.18 Ga). Age constraints in the KKI Paleoproterozoic or Birimian domains in Senegal are in part represented by the Mako volcanic belt (Figure 2) that consists largely of basaltic rocks with minor intercalations of volcanoclastics, ultramafic (pyroxenitic) subvolcanic intrusions and numerous relatively small biotite- and amphibole-bearing granitoids. The latter plutons were formerly described by Bassot (1966) as the Kakadian Batholith. This batholith is composed of the trondhjemitic calc-alkaline plutons of the Sandikounda Layered Plutonic Complex (SLPC, Dia 1988; Dia et al. 1997), the Laminia Kaourou Plutonic Complex (LKPC, Dia 1988; Dia et al. 1997), the Badon granodiorite and small circular shape plutons (Tinkoto, Mamakono, Diombalou). This classical lithology containing volcanics (Mako Supergroup) at the bottom and volcanosedimentary to sedimentary rocks (DialéDaléma Supergroup) at the top is similar to typical Archean greenstone belt assemblages. Granitoid intrusions in West Africa are often divided into ‘belt-type’ and ‘basin-type’, depending on whether they intrude greenstone belts or metasedimentary basins (Hirdes et al. 1992; 1996; Lüdtke et al. 1998; 1999).Most belt-type plutons from the Leo Rise in Ghana for example, yield U/Pb single zircon ages between 2190 and 2150 Ma, whereas the basin-type plutons are younger, in the range of ~2.11 to ~2.09 Ga (Hirdes et al. 1992; 1996; Davis et al. 1994, Hirdes and Davis 2002).


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Figure 7. Pb-Pb ages for the zircons of samples M1-02, M2-02, M9-02 and M12-02.

Igneous rocks from this study are dated from about 2213 ± 3 Ma to about 2074 ± 5 Ma (Appendix A). This time interval is typical for the Eburnean tectonothermal episode (~2.2 to ~2.1 Ga). However, volcanism in the Mako belt must be older than 2198 ± 2 Ma, which is the age of emplacement of the Badon granodiorite (M1-02, magmatic rims). The older age of 2213 ± 3 Ma is assumed to be the minimum age for zircon cores in the Badon granodiorite (M1-02, inherited cores), which was most probably partially reset by recrystallization processes. These recrystallization features are shown in BSE images as bright reaction fronts truncating the older zonations (Figure 4). Zircons from the Badon granodiorite are xenocrystic and show complex internal structures (Figure 4). The cores and the rims are generally oscillatory-zoned. This preserved oscillatory zoning, is typical for magmatic crystallization environments (Vavra et al. 1996; 1999; Schaltegger et al. 1999). Unzoned or weakly zoned rims of medium brightness are present and are here interpreted as metamorphic overgrowths (Schaltegger et al. 1999). This feature suggests that earlier zircon of igneous origin was resorbed prior to crystallization of the outer growth zones. Some CL images show rim overgrowths associated with sector zonation which is interpreted as solid-state in origin.

Microstructural evidence like intergranular films of K-feldspar or plagioclase developed along quartz-quartz and quartz-plagioclase boundaries, rounded quartz inclusions in plagioclase, cuspate extensions of K-feldspar inclusions in two perpendicular directions, the occurrence of small grains of plagioclase at the three grain junction in quartz, the partly replacement of plagioclase grains by quartz, and the lobate quartz, and

Figure 8. U-Pb concordia diagram of zircon analyses from the tonalitic gneiss from Sandikounda (M12-02).



Figure 9. 40Ar/39Ar age spectra of the samples M1-02, M2-02, M3-02 and M12-02.


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plagioclase boundaries are common for deformation at high temperature conditions (Figures 3 and 5). Corroded grain boundaries, lattice bending and interpenetrating grain boundaries (Figures 3B, 3D and 3E) record high temperature-low strain deformation of plagioclase. Quartz shows chessboard-textured subgrain boundaries, which points to quartz prism--slip as the prominent intracrystalline glide system. Such quartz fabrics are indicative for deformation temperatures exceeding 650 °C (Kruhl 1996). We interpret these textures to be a consequence of partial melting. The foliation in the Badon granodiorite was previously interpreted to be magmatic (Bassot 1966). Our microstructural observations point to intracrystalline plasticity as a minor accommodating mechanism and that the deformation in the Badon granodiorite mainly occurred by granular flow (e.g. Paterson 1995; Rosenberg and Berger 2001). These textures have previously been described in migmatites (Pattison and Harte 1988; Holness and Clemens 1999; Rosenberg and Riller 2000; Gernina and Sawyer 2003). In addition also the zircons from the granodiorite of Badon and the tonalitic gneiss from Sandikounda show sign of metamorphic resorption and most likely represent a reaction zone (Figure 4). In summary, this age is slightly older than the major magmatic event of the Birimian sensu stricto (~2.19 to ~2.15 Ga). A similar age was obtained from clear long prismatic zircons in the SAG (Sandikounda amphibologneiss) which was interpreted as the magmatic crystallization age. The 207Pb/206Pb zircon age of M12-02 (2194 ± 4 Ma) is also supported by the U-Pb single grain zircon age of 2205 ± 15 Ma (Figure 8). Ar-Ar and K-Ar ages from biotite, muscovite and hornblende grains from selected granitoids range from 2112 to 2020 Ma. Ar-Ar and K/Ar hornblende ages for the tonalitic gneiss from Sandikounda indicate that cooling below ca 550° C occurred ~2.1 Ga. Ar-Ar and K-Ar biotite ages from the Badon granodiorite indicate cooling to approximately 300°C, the temperature which is widely accepted for biotite closure to argon diffusion (McDougall and Harrison 1999) around ~2.1 Ga. In sum, the cooling of the oldest intrusions (tonalitic gneiss and Badon granodiorite) lasted over a time span of around 90 Ma, while cooling from 550 to 300°C occurred within 20 Ma. This period (~2.11 to ~2.10 Ga) is coeval with the major Eburnean tectonometamorphic event (~2.1 Ga, Eisenlohr 1989; Eisenlohr and Hirdes 1992; Blenkinsop et al. 1994). By this data it is clearly documented that the KKI stayed hot (T>550°C) until about 2115 Ma. In consequence, the SLPC and LKPC intruded hot crust and led to the migmatization of the rocks, as seen in the Sandikounda tonalitic gneiss and Badon granodiorite. The deformational evolution characterized by the transcurrent tectonics is accompanied by the Late Eburnean magmatism around 2080 to 2070 Ma, for example represented by the plutons from Tinkoto, Mamakono, Saraya etc. New findings on the structural evolution of the KKI will be presented by Gueye et al. (in preparation).

The late kinematic Tinkoto pluton is an excellent example for rapid cooling. Sample M2-02 yieldes an PbPb zircon crystallization age of 2074 ± 5 Ma. Rapid cooling is indicated by Ar-Ar and K/Ar ages on hornblende and biotite, all within error around ~2.06 and ~2.05 Ga. In the Saraya Batholith, Hirdes and Davies (2002) record regional metamorphism expressed by the growth of metamorphic monazite at 2064 ± 4 Ma. Muscovite ages from the Saraya granite do not represent the regional cooling history. Indeed, they reflect a later thermal or hydrothermal event unrelated to, but following, the deposition of uranium (e.g. Mouthier 1988). The K/Ar age of 2021 ± 11 Ma agrees well to an Ar-Ar plateau of 2022 ± 12 Ma. In summary, we can separate the Ar-Ar and K-Ar ages obtained into four groups: (a) an older age group (~2.11 to ~2.10 Ga), cooling after the major magmatic event, (b) a younger age group (~2.05 to ~2.03 Ga), cooling after the late Eburnean magmatism, (c) a group of cooling ages after the hydrothermal overprint in the Saraya Batholith (~2.02 to ~2.01 Ga), (d) retrograde disturbance of the isotopic system. The last group of retrograde disturbed ages is most evident on K-Ar biotite ages, especially biotites from the Mako (MP2-03: ~2.01 Ga) and Dialé-Daléma Supergroup (M5-02: ~1.36 Ga, M6-02: ~0.75 Ga) are partly reset to geologically meaningless ages. Also K-Ar ages on hornblende can be disturbed (KO75-03: ~2.03 Ga). The disturbance in both supergroups is obvious by the simple assumption that in both cases the cooling of the hosts has to be older or equal to the cooling of the oldest intrusive rocks. Age constraints and its regional significance Doumbia et al. (1998) reported ages ranging between 2220 ± 6 Ma to 2133 ± 3 Ma from granitoids of the Central Ivory Coast with a strong crustal signature (numerous inclusions, dark cores and angular or blunt overgrowths). The older ages were discussed as a further evidence for Early Birimian crust. Moreover, a Pb–Pb zircon evaporation age of 2212 ± 6 Ma obtained from a sample of rhyodacite supported the first evidence of Early Birimian volcanic activity in Guinea (Lahondère et al. 2002). In Mauritania the Early Birimian volcanic rocks of the Reguibat Rise correspond to tholeiitic metabasalts dated by Sm–Nd at 2229 ± 42 Ma (Abouchami et al. 1990), whereas Peucat et al. (2005) recorded a metamorphic basement formed during an early stage at ~2.2 Ga (U-Pb). In Ivory Coast, a felsic volcanic rock of the Tehini–Boutourou Belt has been dated at 2195 ± 10 Ma (Siméon et al. 1992), whereas the genesis of the Dabakala tonalite involved recycled components of ~2.3 Ga pre-Birimian crust (Gasquet et al. 2003). It appears likely that the Birimian sensu stricto (~2.17 Ga) and Bandamian (~2.1 Ga) events in Ivory Coast were preceded by an early Paleoproterozoic crustal growth episode at ~2.3 Ga (Gasquet et al. 2003). In Ghana, felsic flow rocks intercalated with tholeiitic



basalts give a U–Pb zircon age of 2189±1 Ma (Hirdes and Davis 1998). Early Birimian volcanic activity is attested in Ghana by Pb-Pb zircon ages from 2266 ± 1 to 2257 ± 1 (Loh and Hirdes 1996). The ages from the Badon granodiorite and the tonalitic gneiss of Sandikounda and those reported above prove the existence of an Early Birimian event. In contrast, Dia (1988) and Dia et al. (1997) considered the rocks of the Sandikounda amphibolite-gneiss complex (SAG) as remnants of a lower level of a Paleoproterozoic juvenile arc-related crust emplaced from ~2.3 to ~2.2 Ga. They interpreted the Mako Supergroup to be younger than this time span. A significant episode of younger magmatic activities represented by the Sandikounda-Layered-PlutonicComplex (SLPC) and the Laminia-Kaourou-PlutonicComplex (LKPC) is evident by ages reported by Dia et al. (1997). For the Sandikounda-Layered-PlutonicComplex (SLPC), ranging in composition from gabbros to diorites, a Pb-Pb single grain zircon age of 2158 ± 8 Ma was reported. The Laminia-Kaourou-PlutonicComplex (LKPC; granodiorites, monzodiorites, tonalites, granites etc.) was emplaced between 2138 and 2105 Ma. In comparison, granitoids east of the Baoulé-Mossi domain (Ghana and Ivory Coast) were dated by Doumbia et al. (1998) and Hirdes and Davis (2002) and yielded Pb-Pb and U-Pb ages between ~2.19 and ~2.11 Ga. Our youngest granite age in the Mako belt is 2074 ± 5 Ma from the Tinkoto pluton and agrees with ages reported for late kinematic plutons in the WAC. This reveals a good estimate of the younger age limit of regional deformation in this area. However, a comparable emplacement age for the Saraya Batholith (2079 ± 2 Ma, Pb-Pb on zircon) is given by Hirdes and Davies (2002). This batholith intrudes the Dialé-Daléma Supergroup and belongs to so-called basin-type granitoids. Plutonism within this time span is also known in Ghana (Eisenlohr and Hirdes 1992; Hirdes et al. 1992). Hirdes and Davis (2002) dated volcanic flows in the Dialé-Daléma Supergroup with a Pb-Pb age of 2099 ± 3 Ma. Our measured 2099 ± 22 Ma Pb-Pb zircon age of the rhyolitic flow, host rock of the Mamakono pluton is also of interest. This is comparable to the emplacement age of the Mamakono pluton of 2076 ± 3 Ma (Hirdes and Davis 2002) in the same belt, thus suggesting the synvolcanic nature of the Birimian belt plutonism. The 2074 ± 5 Ma Pb-Pb single grain zircon age of Tinkoto granodiorite agrees well with ages of basin plutonism elsewhere in Ghana, Mali, Guinea and the Guyana Shield (Liégeois et al. 1991; Vanderhaeghe et al. 1998; Oberthür et al. 1998; Egal et al. 2002). John et al. (1999) described a metamorphic overprint in the Birimian sequence of southern Ghana. The minimum peak conditions of this epidoteamphibolite to amphibolite- facies is given with 500 to 650°C and 5 to 6 kbar. The authors proposed similar conditions for metamorphism in Birimian rocks to be likely for large parts of the WAC. If this holds true for the

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KKI, all cooling data has to be related to postmetamorphic Eburnian cooling. The 2098 Ma age (Ar-Ar and K-Ar biotite ages from the Badon granodiorite) for metamorphism in the KKI is in good agreement with the age of regionally important basin- type plutonism, which occurred in eastern Ivory Coast and Ghana during a late stage of regional deformation and associated greenschist-facies metamorphic overprint. In Ghana, belt-type plutons were affected by metamorphism at 2092 ± 3 Ma, coeval with emplacement of the basin-type pluton at Cap Coast (2090 ± 1 Ma). Monazite and sphene from west Ivory Coast record metamorphism at 2080 ± 10 Ma (Kouamelan 1996), while metamorphic sphene in gneisses from the east Ivory Coast gave 2100 ± 3 Ma (Hirdes et al. 1996). Oberthür et al. (1998) consider the period frp, ~2.12 to ~2.08 Ga in Ghana as a major crust formation event, while Davis (reported in Loh and Hirdes 1996) dated regional metamorphically grown monazite at 2102 ± 1 Ma in the east. Cheillez et al. (1994) described in the southwestern Niger young granitic plutons that were emplaced at 2115 ± 5 Ma. Both the pluton and the surrounding greenstones yield cooling K-Ar ages ~2.12 Ga. Conclusion The aim of this study was to constrain the crystallization ages of different granitoid phases within the KKI. The emplacement of granitoid bodies in the KKI (Senegalian part of the Birimian part of the WAC) signified an important igneous event during the Eburnean orogeny causing deformation and metamorphism of greenstone succession and found cooling and stability of the basement in Senegal. The results of this study constraints the Early Birimian crust forming processes with a minimum age of ~2.2 Ga. From these findings it is obvious that the greenstone association of the Mako Supergroup has to be older. It is worth discussing whether late Archean sequences are involved. Results from Pawlig et al. (2006) support the hypothesis that the new formed crust is juvenile in origin. The Biriminan rocks show uniform juvenile features such as low initial Sr isotopic composition (~0.700 to ~0.704) positive εNd(t) values (2.2 to 4.3) and restricted Nd model ages (~2.0 to ~2.3 Ga). Due to the finings by Pawlig et al. (2006) the Birimian rocks are the product of a large-scale crustal growth process with large amounts of juvenile crust formed from the depleted mantle in a geodynamic environment compared to modern volcanic arcs. These bimodal volcanic arcs were later accreted to the WAC. Similar processes are also reported from other part in the WAC (e.g. Ivory Coast, Guinea, Ghana etc.). We assume that this crust stayed hot over a considerable time span caused by intrusions of several complex composed plutonic sequences (SLPC and LKPK). This heat input originated mainly from ultrabasic and basic intrusions while the melts caused migmatisation clearly documented by microstructural findings and by


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the very complex internal structures in zircons (e.g. Badon granite and tonalitic gneiss). The time interval from ~2.16 to ~2.11 Ga corresponds to the major magmatic activity of the Eburnean Orogeny (Dia et al., (1997). Subsequent cooling is documented by our oldest hornblende and biotite ages (Ar-Ar and K/Ar) from ~2.11 to ~2.10 Ga. Several late-kinematic granitoids were emplaced in the KKI at ~2.08 to ~2.07 Ga (e.g. Mamakono, Tinkoto Saraya). The occurrence of late kinematic granitoids is typical for the Birimian in the whole WAC and was often reported. During this time plutonism and volcanism occur simultaneouly. Cooling history below 300°C is documented by Ar-Ar and K/Ar data on hornblende and biotite. The K/Ar system in biotite is often disturbed and partly resetted by younger processes. A hydrothermal overprint being activ at around 2020 Ma is restricted to the Saraya Batholith leading to the dissolution of quartz and the formation of secondary minerals. The use of the term Kakdian batholith is very confusing because it refers to a widespread heterogenety in composition, structure and timing. The use of this term should be avoided as long as no exact locality and rock description are given. Our findings are in contrast to: • the proposed younging tendency from east to west with respect to Paleoproterozoic belt/plutonism/crust formation in the WAC as postulated by Hirdes et al. (1992); Davis et al. (1994); Hirdes and Nunoo (1984) and Hirdes and Davis (2002). • the separation of the West African region into an eastern and western sub-provinces as proposed by Hirdes et al. (1996) Acknowledgements We are very greatful to the Volkswagen-Stiftung (Az: I/77 340) for the financial support. J. Foden (Adelaide University, Australia) is thanked for providing Pb-Pb zircon ages. We are thankful for the constructive reviews by W.R. Van Schmus and D. Gasquet which significantly improve the manuscript. Editorial handling by R. Klemd is gratefully acknowledged. We very much appreciate the help Claudia Braunschweig. References Abouchami, W., Boher, M., Michard, A. and Albarède, F. (1990). A major 2.1 Ga old event of mafic magmatism in West Africa: an early stage of crustal accretion. Journal of Geophysical Research, 95, 17605-17629. Aït Malek, H., Gasquet, D., Bertrand, J.M. and Leterrier, J. (1998). Géochronologie U-Pb sur zircon de granitoïdes éburnéens et panafricains dans les boutonnières protérozoïques d’Igherm, du Kerdous et du Bas Drâa (Anti-Atlas occidental, Maroc). Comptes Rendus Académie Sciences Paris, 327, 819-826. Bassot, J.P. (1966). Etudes géologique du Sénégal Oriental et de ses confins guinéo-maliens. Mémoires BRGM, 40, 322pp. Bassot, J.P. (1987). Le complexe volcano-plutonique calco-alcalin de la rivère Daléma (Est Sénégal): discussion de sa signification géodynamique dans le cadre de l’orogénie eburnéenne (Protérozoic inférieur). Journal of African Earth Science, 64, 505-519. Bassot, J.P. and Caen-Vachette, M. (1984). Données géochronologiques et géochimiques nouvelles sur les granitoïdes de lÉst du Sénégal: implications sur l´histoire géologique du Birrimien de cette région. In: J.

Klerkx and J. Michot (Editors), African Geology. Tervuren, Belgium, 196-209. Bertrand, J.M., Dia, A., Dioh, E. and Bassot, J.P. (1989). Réflexion sur la structure interne du craton ouest africain au Sénégal Oriental et confins guinéo-maliens. Comptes Rendus Académie Sciences Paris, 309, series II, 751-756. Bessoles, B. (1977). Géologie de l’Afrique: le craton ouest-africain. Mémoires BRGM, 88, 403pp. Blenkinsop, T.G., Schmidt Mumm, A., Kumi, R. and Sangmor, S. (1994). Structural geology of Ashanti Gold Mine. Geologisches Jahrbuch D, 100, 131-153. Boher, M. (1991). Croissance crustale en Afrique de I’Ouest a 2,l Ga. Apport de la geochimie isotopique. Unpuplished PhD thesis University of Nancy I, France, 180pp. Boher, M., Abouchami, W., Michard, A.F. and Arndt N.T. (1992). Crustal growth in West Africa at 2.1 Ga, Journal of Geophysical Research, 97 B1, 345–369. Bohomme, M. (1962). Contribution á l´étude géochronologique de la plateforme de lÓuest africain. Unpuplished PhD thesis University of Nancy I, France 62pp. Calvez, J.Y., Feybesse, J.L., Ledru, P. and Milési, J.P. (1990). Géochronologie du Protérozoïque inférieur du craton ouest africain (méthode d’évaporation directe de zircons isolés). Abstract of the 13e Réunion des Sciences de la Terre, Grenoble, France, 26pp Cheilletz, A., Barbey, P., Lama, C., Pons, J., Zimmerman, J.L. and Dautel, D. (1994). Age de refroidissement de la croute juvénile birimienne dÁfrique de lÓuest. Données U-Pb, Rb-Sr et K-Ar sur les formations à 2.1 Ga du SW Niger. Comptes Rendus Académie Sciences Paris, 319, 435-442. Davis, D.W., Hirdes, W., Schaltegger, U. and Nunoo, E.A. (1994) U–Pb constraints on deposition and provenance of Birimian and gold-bearing Tarkwaian sediments in Ghana, West Africa. Precambrian Research, 67, 89-107. Debat, P., Nikiéma, S., Mercier, A., Lompo, M., Béziat, D., Bourges, F., Roddaz, M., Salvi, S., Tollon, F. and Wemmenga, U. (2003). A new metamorphic constraint for the Eburnean orogeny from Paleoproterozoic formations of the Man Shield (Aribinda and Tampelga countries, Burkina Faso). Precambrian Research, 123, 47-65. Dia, A. (1988) Caractère et signification des complexes magmatiques et métamorphiques du secteur de Sandikounda-Laminia (Nord de la boutonnière de Kédougou, Est du Sénégal): un modéle géodynamique du Birimien de l’Afrique de l’Ouest. Unpublished PhD thesis, University of Dakar, Sénégal 350pp. Dia, A., Van Schmus, W.R. and Kröner, A. (1997) Isotopic constraints on the age and formation of a Palaeoproterozoic volcanic arc complex in the Kédougou Inlier, eastern Senegal, West Africa. Journal of African Earth Science, 24, 197-213. Diallo, D.P. (1994). Caractérisation, dúne portion de croute dáge protérozoïque inferieur du craton Ouest africain : cas de léncaissant des granitoides dans le Supergroupe de Mako (boutonnière de Kédougou)Implications geodynamiques. Unpublished PhD thesis, University of Cheikh Anta Diop, Dakar, Sénégal, 466pp. Diallo, D.P., Debat, P., Rocci, G., Dia, A., Ngom, P.M. and Sylla, M. (1993). Pétrographie et géochimie des roches méta-volcano-detritiques et métasedimentaires du Protérozoïque inférieur du Sénégal oriental dans le Supergroupe de Mako (Sénégal, Afrique de I’Ouest): lncidences géotectoniques. Early Proterozoic symposium, Dakar, Senegal, occasional Publications CIFEG, 23, 1-15. Dioh, E. (1986). Etude des roches magmatiques birimiennes de la région de Sonfara, Laminia, Madina Foulbé (Sénégal Oriental). Unpuplished PhD thesis University of Nancy I, France, 144pp. Dioh, E. (1995). Caractérisation, signification et origine des formations birimiennes encaissantes du granite de Diombalou (partie septentrionale de la boutonnière de Kédougou-Sénégal Oriental). Unpublished PhD thesis, University of Cheikh Anta Diop, Dakar, Sénégal, 445pp. Doumbia, S., Pouclet, A., Kouamelan, A., Peucat, J.J., Vidal, M. and Delor, C. (1998). Petrogenesis of juvenile-type Birimian (Paleoproterozoic) granitoids in Central Côte d’Ivoire, West Africa: geochemistry and geochronology. Precambrian Research, 87, 33-63. Egal, E., Thieblemont, D., Lahondère, D., Guerrot, C., Costea, C.A., Iliescu, D., Delor, C., Goujou, J.C., Lafon, J.M., Tegyey, M., Diaby, S. and Kolié, P. (2002). Late Eburnean granitization and tectonics along the western and



northwestern margin of the Archean Kenema-Man domain (Guinea, West African Craton). Precambrian Research, 117, 57-84. Eisenlohr, B.N. (1989). The structural geology of Birimian and Tarkwaian rocks in Ghana. Report of the Bundesanstalt für Geowissenschaft und Rohstoffe. Hannover, Germany, 106448, 59pp. Eisenlohr, B.N. and Hirdes, W. (1992). The structural development of the early proterozoic Birimian and Tarkwaien rokcs of southwest Ghana, West Africa. Journal of African Earth Science, 14, 313-325. Fabre, R. (1987). Lithostratigraphie du Birimien (Precambrien moyen) dans le centre de la Côte dÍvoire (Afrique de lÓuest), région du Yaouré. Bulletin Société Géologique France, 8, 657-663. Fuhrmann, U., Lippolt, H.J. and Hess, J.C. (1987). Examination of some proposed K-Ar standards: 40Ar/39Ar analyses and conventional K-Ar-Data, Chemical Geology, Isotope Geoscience Section, 66, 41-51. Foden, J., Sandiford, M., Dougherty-Page, J. and Williams, J. (1999). Geochemistry and geochronology of the Rathjien Gneiss, implications for the early tectonic evolution of the Delamerian Orogen. Australian Journal of Earth Science, 46, 377-389. Galipp, K., Klemd, R. and Hirdes, W. (2003) Metamorphism and geochemistry of the Proterozoic Birimian Sefwi Volcanic belt (Ghana, West Africa). Geologisches Jahrbuch, 111, 33-63. Gasquet, D., Barbey, P., Adou, M. and Paquette, J.L. (2003). Structure, Sr-Nd isotope geochemistry and zircon U-Pb geochronology of the granitoids of the Dabakala area (Côte d’Ivoire): evidence for a 2.3 Ga crustal growth event in the Palaeoproterozoic of West Africa? Precambrian Research, 127, 329-354. Gasquet, D., Levresse, G., Cheilletz, A., Azizi-Samir, M.R. and Mouttaqi, A. (2005). Contribution to a geodynamic reconstruction of the Anti-Atlas (Morocco) during Pan-African times with the emphasis on inversion tectonics and metallogenic activity at the Precambrian-Cambrian transition. Precambrian Research, 140, 157-182. Gasquet, D., Chevremont, P., Baudin, T., Chalot-Prat, F., Guerrot, C., Cocherie, A., Roger, J., Hassenforder, B. and Cheilletz, A. (2004). Polycyclic magmatism in the Tagragra d’Akka and Kerdous-Tafeltast inliers (Western Anti-Atlas, Morocco). Journal of African Earth Science, 39, 267-275. Guernina, S. and Sawyer, E.W. (2003). Large-scale melt-depletion in granulite terranes: an example from the Archean Ashuanipi Subprovince of Quebec. Journal of Metamorphic Geology, 21, 181-201. Gueye, M. (2001). Transcurrent fault propogation and granitoid plutonism during Lower Proterozoic transpression (southeast Senegal). Zeitschrift der Deutschen Gesellschaft für Geowissenschaften, 152, 175-198. Heinrichs, H. and Herrmann, A.G. (1990). Praktikum der Analytischen Geochemie, Springer, Berlin, Germany, 669pp. Hirdes, W. and Davis, D.W. (1998). First U-Pb zircon age of extrusive volcanism in the Birimian Supergroup of Ghana/West Africa. Journal of African Earth Science, 27, 291-294 Hirdes, W. and Davis, D.W. (2002). U-Pb Geochronology of Paleoproterozoic rocks in the southern part of the Kédougou-Kénieba Inlier, Senegal, West Africa: evidence for diachronous accretionary development of the Eburnean province. Precambrian Research, 118, 83-99 Hirdes, W. and Nunoo, E.A. (1994). The Proterozoic paleoplacers of Tarkwa gold mine/southwest Ghana: sedimentology, mineralogy, and precise age dating of the Main Reef and West Reef, and bearing of the investigations on source area aspects. Geologisches Jahrbuch D, 100, 247-311. Hirdes, W., Davis, D.W. and Eisenlohr, B.N. (1992) Reassessment of Proterozoic granitoid ages in Ghana on the basis of U/Pb zircon and monazite dating. Precambrian Research, 56, 89-96. Hirdes, W., Davis, D.W., Lüdtke, G. and Konan, G. (1996). Two generations of Birimian (Paleoproterozoic) volcanic belts in northeastern Côte d’Ivoire (West Africa): consequences for the “Birimian controversy”. Precambrian Research, 80, 173-191. Holness, M.B. and Clemens, J.D. (1999). Partial melting of the Appin Quartzite driven by fracture-controlled H2O infiltration in the aureole of the Ballachulish Igneous Complex, Scottish Highlands. Contributions to Mineralogy and Petrology, 136, 154-168. John, T., Klemd, R., Hirdes, W. and Loh, G. (1999). The metamorphic evolution of the Paleoproterozoic (Birimian) volcanic Ashanti belt (Ghana, West Africa). Precambrian Research, 98, 11-30. Kleinschrot, D., Klemd, R., Bröcker, M., Okrusch, M., Franz, L. and Schmidt, K. (1994). Protores and country rocks of the Nsuta manganese deposit (Ghana). Neues Jahrbuch für Mineralogie, Abhandlungen, 168, 67-108.

527

Klemd, R/, Hünken, U/ and Olesch, M/ (2002). Metamorphism of the country rocks hosting gold-sulfide-bearing quartz veins in the Paleoproterozoic southern Kibi-Winneba belt (southeast-Ghana). Journal of African Earth Science, 35, 199-211. Kober, B. (1986). Whole grain evaporation for 207Pb-206Pb-age investigation on single zircons using a double filament thermal ion source. Contributions to Mineralogy and Petrology, 93, 482-490. Kober, B. (1987). Single-zircon evaporation combined with Pb+ emitter bedding for 207Pb-206Pb-age investigations using thermal ion mass spectrometry and implications for zirconology. Contributions to Mineralogy and Petrology, 96, 63-7. Kouamélan, A.N. (1996). Géochronologie et géochimie des formations archéennes et protérozoïques de la dorsale de Man en Côte d’Ivoire. Implications pour la transition Archéen-Protérozoïque. Mémoir Géosciences Rennes, France, 73, 289pp. Kruhl, J.H. (1996). Prism- and basal-plane parallel subgrain boundaries in quartz: a microstructural geothermobarometer. Journal of Metamorphic Geology, 14, 581-589. Lahondère, D., Thieblemont, D., Tegyey, M., Guerrot, C. and Diabaté, B. (2002). First evidence of early Birimian (2.21 Ga) volcanic activity in Upper Guinea: the volcanics and associated rocks of the Niani suite. Journal of African Earth Science, 35, 417-431. Layer, P.W., Hall, C.M. and York, D. (1987). The derivation of 40Ar/39Ar age spectra of single grains of hornblende and biotite by laser step heating. Geophysical Research Letters, 14, 757-760. Ledru, P., Pons, J., Milési, J.P., Feybesse, J.L. and Johan, V. (1991). Transcurrent tectonics and polycyclic evolution in the Lower Proterozoic of Senegal-Mali. Precambrian Research, 50, 337-354. Liégeois, J.P., Claessens, W., Camara, D. and Klerx, J. (1991). Short-lived Eburnian orogeny in southern Mali. Geology, tectonics, U-Pb and Rb-Sr geochronology. Precambrian Research, 50, 111-36. Loh, G. and Hirdes, W. (1996). Geological map of southwest Ghana, 1:100000, sheets Axim and Sekondi. Ghana Geological Survey Bulletin, 49, 63pp. Lüdtke, G., Hirdes, W., Konan, G., Koné, Y., Yao, C., Diarra, S. and Zamblé Z. (1998). Géologie de la région Haute Comoé Nord-feuilles Kong (4b et 4d) et Téhini-Bouna (3a à 3d). Direction de la Géologie, Abidjan. Bulletin, 1, 178-229. Lüdtke, G., Hirdes, W., Konan, G., Koné, Y., Nda, D., Traoré, Y. and Zamblé, Z. (1999). Géologie de la région Haute Comoé Sud-feuilles Dabakala (2b,d et 4b,d). Direction de la Géologie, Abidjan. Bulletin, 2, 167pp. Ludwig, K.R. (2003). Users Manual for Isoplot/Ex Version 3.00: a geochronological toolkit for Microsoft Excel (Special Publication), Geochronology Center, Berkeley, California, United States of America, 41-71. McDougall, J. and Harrison, T.M. (1999). Geochronology and thermochronology by the 40Ar/39Ar method. 2nd Edition, Oxford University Press, United Kingdom, 269pp. Milési, J.P., Feybesse, J.L., Ledru, P., Dommanget, A., Quedraogo, M.F., Marcoux, E., Prost, A., Vinchon., C., Sylvain, J.P., Johan, V., Tegyey, M., Calvez J.Y. and Lagny, P (1989). Les minéralisations aurifères de l’Afrique de l’Ouest. Leurs relations avec l’évolution lithostructurale au Protérozoïque inférieur. Chronique de la Recherche Minière, 497, 3-98. Milési, J.P., Ledru, P., Feybesse, J.L., Dommanget, A. and Marcoux, E. (1992). Early proterozoic ores deposits and tectonics of the birimian orogenic belt. West Africa. Precambrian Research, 58, 305-344. Mouthier, B., (1988). L´épisyénite albitisée uranifère à matière carbonée de Saraya (Sénégal): un exemple de superposition de deux altérations hydrothermales. Comptes Rendus Académie Sciences Paris série II, 307, 761-764. Ndiaye, PM., Dia, A., Vialette, Y., Diallo, D.P., Ngom, P.M., Sylla, M., Wade, S and Dioh, E. (1997). Données pétrographiques, géochimiques et géochronologiques nouvelles sur les granitoïdes du Paléoprotérozoïque du Supergroupe de Dialé-Daléma (Sénégal Oriental): implications pétrogénetiques et géodynamiques. Journal of African Earth Science, 25, 193-208. Ngom, P.M. (1985). Contribution à l´étude de la série birrimienne de Mako dans le secteur aurifère de Sabodala (Sénégal Oriental). Thèse 3e cycle, University of Nancy I, France, 134pp. Oberthür, T., Vetter, U., Davies, W.D. and Amanor, J.A. (1998). Age constraints on gold mineralization and Paleoproterozoic crustal evolution


528


in the Ashanti belt of southern Ghana. Precambrian Research, 89, 129-143. Paterson, M.S. (1995). A theory of granular flow accommodated by material transfer via an intergranular fluid. Tectonophysics, 245, 135-151. Pattison, D.R.M. and Harte, B. (1988). Evolution of structurally contrasting anatectic migmatite in the 3-kbar Ballachulish aureole, Scotland. Journal of Metamorphic Geology, 12, 387-410. Pawlig, S., Gueye, M., Klischies, R., Schwarz, S., Wemmer, K. and Siegesmund, S. (2006). Geochemical and Sr-Nd isotopic data on Birimian Formations of the Kédougou-Kénieba Inlier (Eastern Senegal): implications on the Paleoproterozoic evolution of the West African Craton. South African Journal of Geology, 109, 411-427. Peucat, J.J., Capdevila, R., Drareni, A., Mahdjoub, Y. and Kahoui, M. (2005). The Eglab massif in the West African Craton (Algerie), an original segment of the Eburnean orogeny belt: petrology, geochemistry and geochronology. Precambrian Research, 136, 309-352. Poller, U., Liebetrau, V. and Todt, W. (1997). U-Pb single-zircon dating under cathodoluminescence control (CLC-method): application to polymetamorphic orthogneisses. Chemical Geology, 139, 287-297. Rosenberg, C.L. and Riller, U. (2000). Partial melt topology in statically and dynamically recrystallized granite. Geology, 28, 7-10. Rosenberg, C.L. and Berger, A. (2001). Syntectonic melt pathways in granitic gneisses, and melt-induced transitions in deformation mechanisms. Physics and Chemistry of the Earth (A), 26, 287-293. Schaltegger, U., Fanning, C,M., Gunther, D., Maurin, J.C., Schulmann, K. and Gebauer, D. (1999). Growth, annealing and re of zircon and preservation of monazite in high-grade metamorphism: conventional and in-situ U-Pb isotope, cathodoluminescence and microchemical evidence. Contributions to Mineralogy and Petrology, 134, 186-201. 38 40 40 Schumacher, E. (1975). Herstellung von 99,9997% Ar für die K/ Ar Geochronologie. Geochronologia Chimia, 24, 441-442. Siméon, Y., Delor, C., Vidal, M., Chiron, J.C., Zeade, J. (1992) Mise en évidence d’un épisode tectonique tardi-éburnéen en Côte d’Ivoire. éme RST, Toulouse p 142. In: Comptes rendus 13 Stacey, V.S. and Kramers, J.D. (1975). Approximation of terrestrial lead isotope evolution by a two-stage model. Earth and Planetary Science Letters, 26, 207-221.

Steiger, R.H. and Jäger, E. (1977). Subcommision on geochronology: convention on the use of decay constants in geo- and cosmochronology. Earth and Planetary Science Letters, 36, 359-362. Vanderhaerghe, O., Ledru, P., Thieblemont, D., Egal, E., Cocherie, A., Tegyey, M. and Milesi, J.P. (1998). Contrasting mechanism of crustal growth. Geodynamic evolution of the Paleoproterozoic granite-greenstone belts of French Guiana. Precambrian Research, 92, 165-193. Vavra, G., Gebauer, D., Schmid, R. and Compston, W. (1996): Multiple zircon growth and re during polyphase Late Carboniferous to Triassic metamorphism in granulites of the Ivrea Zone (Southern Alps): an ion microprobe (SHRIMP) study. Contributions to Mineralogy and Petrology, 122, 337-358. Vavra, G., Schmid, R. and Gebauer, D. (1999). Internal morphology, habit and U-Th-Pb microanalysis of -to-granulite facies zircon: geochronology of the Ivrea Zone (Southern Alps). Contributions to Mineralogy and Petrology, 134, 380-404. Wemmer, K. (1991). K/Ar-Altersdatierungsmöglichkeiten für retrograde Deformations -prozesse im spröden und duktilen Bereich - Beispiele aus der KTB-Vorbohrung (Oberpfalz) und dem Bereich der Insubrischen Linie (N-Italien). Göttinger Arbeiten für Geologie and Paläontologie, 51, 1-61. Wendt, I. and Todt, W. (1991). A vapour digestion method for dating single zircons by direct measurements of U and Pb without chemical separation. Terra Abstracts, 3, 507-508. Witschard, F. (1965). Contribution à l´étude géologique pétrologique et métallogénique des massifs granitiques du Sénegal Oriental. Mémoires BRGM, 44, 171pp. York, D., Hall, C.M., Yanase, Y., Hanes. J.A. and Kenyon, W.J. (1981). 40 Ar/39Ar dating of terrestrial minerals with a continuous laser. Geophysical Research Letters, 8,1136-1138. Zonou, S. (1987). Les formations leptino-amphibolitiques et le complexe volcanique et volcanosédimentaire du protérozoique inférieur de Bouroum Nord (Burkina Faso, Afrique de lÓuest). Etude pétrographique, géochimique, approche pétrogénetique et évolution géodynamique. Unpublished PhD thesis, University of Nancy I, France, 299pp. Editorial handling: Reiner Klemd


Sample/grain of scans Badon granodiorite M 1-02 #1 99 M 1-02 #2 96 M 1-02 #3 45 M 1-02 #4 56 M 1-02 #5 51 M 1-02 #6 88 Tinkoto granodiorite M 2-02 #1 58 M 2-02 #2 43 M 2-02 #3 73 M 2-02 #4 84 Mako volcanite M 9-02 #1 28 M 9-02 #2 16 M 9-02 #3 8 M 9-02 #4 17 M 9-02 #5 11 SAG tonalitic gneiss M 12-02 #1 75 M 12-02 #2 87 M 12-02 #3 99 M 12-02 #4 103 M 12-02 #5 117 M 12-02 #6 132 M 12-02 #7 172

Number

0.0017 0.0077 0.0057 0.0011 0.0041 0.0013 0.0034 0.0012 0.0040 0.0014 0.0035 0.0018

0.1163 0.1560 0.1639 0.1035 0.1011

0.1317 0.1081 0.1023 0.1145 0.1191 0.1289 0.0840

2 error

0.0003 0.0125 0.0095 0.0016

Pb

0.1188 0.2836 0.1185 0.1223

206

0.0010 0.0003 0.0001 0.0016 0.0024 0.0049

Pb/

0.1523 0.1479 0.1251 0.1532 0.1395 0.1490

208

Appendix A. Pb/Pb data for single zircon analyses

Pb/

206

Pb

0.000038 0.000067 0.000079 0.000087 0.000098 0.000148 0.000219

0.000257 0.000369 0.000450 0.000132 0.000116

0.000118 0.003461 0.000320 0.000121

0.000052 0.000034 0.000028 0.000026 0.000027 0.000017

204

0.000003 0.000006 0.000005 0.000010 0.000007 0.000010 0.000037

0.000010 0.000040 0.000031 0.000006 0.000009

0.000005 0.000131 0.000190 0.000007

0.000002 0.000001 0.000002 0.000002 0.000001 0.000004

2 error

Measured Istotopic Ratios

207

206

Pb

0.1385 0.1384 0.1386 0.1366 0.1382 0.1385 0.1399

0.1312 0.1342 0.1328 0.1284 0.1310

0.1298 0.1705 0.1306 0.1297

0.1395 0.1392 0.1379 0.1381 0.1380 0.1377

Pb/

0.0017 0.0004 0.0010 0.0014 0.0010 0.0020 0.0006

0.0002 0.0008 0.0003 0.0014 0.0020

0.0003 0.0102 0.0013 0.0013

0.0011 0.0002 0.0002 0.0005 0.0003 0.0031

2 error

2209 2208 2210 2186 2205 2209 2226

2114 2154 2136 2076 2112

2096 2563 2107 2093

2222 2218 2201 2204 2203 2199

Age

21 5 12 18 12 25 7

3 10 4 19 27

4 96 17 17

13 3 2 6 4 39

2 error

Uncorrected

207

206

Pb*

0.1380 0.1375 0.1376 0.1355 0.1369 0.1366 0.1370

0.1278 0.1293 0.1268 0.1266 0.1295

0.1283 0.1245 0.1264 0.1280

0.1389 0.1388 0.1375 0.1378 0.1377 0.1375

Pb/

2203 2197 2197 2171 2189 2185 2190

2068 2089 2055 2052 2091

2075 2022 2049 2072

2213 2213 2197 2200 2199 2196

Age

21 6 13 20 13 27 13

5 17 10 20 28

5 123 53 19

13 3 2 6 4 39

2 error

Common Pb corrected



529

206

Pb/204Pb

±

206

207

Pb/

206

Pb

±

207

Pb/

235

206

U±

Pb/

235

U

206

Pb/

238

U

±

Pb/

238

U

208

rho

7.149 4.154

Pb/

Pb

0.8890 0.7467

206

±

208

U

Pb

235

1966 1552

Pb/

0.032 0.063

Pb/

207

206

±

207

300 600 900 1200 1500 1800 2100 2500 3500 8700 Integrated

(mW)

Laser Power

Ar

0.052 0.364 0.593 0.802 0.867 0.912 0.936 0.949 0.971 1.000

39

Cumulative

39

Ar/ Ar

747.0 935.4 955.8 945.1 928.1 935.7 908.7 907.1 893.7 906.1 929.0

measured

40

5.2 5.6 5.9 5.1 8.4 8.3 12.4 29.9 13.9 14.6 2.6

+/-

-0.005 0.006 0.019 0.020 0.040 -0.015 0.053 0.177 0.048 0.005 0.017

measured

39

Ar/ Ar

37

0.016 0.002 0.004 0.003 0.012 0.017 0.033 0.067 0.041 0.029 0.003

+/-

39

Ar/ Ar

0.021 0.006 0.008 0.008 0.017 0.007 0.034 0.076 0.025 0.025 0.011

measured

36

0.004 0.001 0.001 0.001 0.003 0.008 0.015 0.029 0.016 0.012 0.001

+/-

0.8 0.2 0.2 0.2 0.5 0.2 1.1 2.5 0.8 0.8 0.4

Ar

40

% Atm.

-0.010 0.011 0.034 0.037 0.074 -0.027 0.097 0.325 0.087 0.009 0.031

Ca/K

0.030 0.004 0.007 0.005 0.021 0.031 0.061 0.122 0.075 0.053 0.005

+/-

Cl/K

U

0.0010 0.0020 0.0022 0.0023 0.0031 0.0029 0.0074 0.0080 0.0044 0.0052 0.0025

235

15 40

Pb/

Appendix C-1. Results of Ar/Ar analyses. Ages reported at ± 1 M1-02 Biotite #1 Weighted average of J from standards = 0.002305 +/- 0.000016

0.002 0.004

206

Corrected isotopic ratios *

M12-02 (Tonalite from SAG) A06-4 5.93 0.05 0.314 A06-6 3.61 0.09 0.195 # Initial Pb isotopic composition for 2.2 Ga after Stacey and Kramers (1975)

Sample # and Grain

Pb/206Pb

0.0004 0.0017

207

Measured isotopic ratios

Pb/204Pb

M12-02 (Tonalite from SAG) A06-4 1242.46 39.1 0.1474 A06-6 232.87 5.9 0.1904 * Isotopic ratios were corrected for common lead and isotopic fractionation

Sample # and Grain

Appendix B. U-Pb single zircon data

238

U

0.0011 0.0002 0.0003 0.0003 0.0011 0.0011 0.0028 0.0040 0.0027 0.0021 0.0002

+/-

1759 1147

Pb/

40

±

238

740.7 933.4 953.5 942.7 923.0 933.6 898.8 884.8 886.3 898.6 925.7

39

12 21

Pb/

U

Ar*/ ArK

206

Apparent ages (Ma) # 206

5.3 5.6 5.9 5.1 8.4 8.6 13.1 30.5 14.6 14.9 2.7

+/-

207

206

(Ma)

Age

Pb

1813.0 2088.6 2114.9 2100.9 2074.8 2088.8 2042.0 2022.9 2024.9 2041.8 2078.3

2192 2158

Pb/

±

207

206

Pb

8.2 7.4 7.7 6.7 11.2 11.4 17.8 41.9 20.1 20.3 9.3

(Ma)

+/-

6 30

Pb/



0.002 0.005 0.011 0.017 0.025 0.032 0.041 0.051 0.061 0.081 0.428 0.620 0.684 0.841 1.000

200 400 600 800 1000 1200 1400 1600 1800 2000 2500 3000 3500 4000 8700 Integrated

Ar

39

Cumulative


0.024 0.094 0.505 0.756 0.939 1.000

500 1000 1800 2400 3000 8700

Integrated

Ar

39

Cumulative

(mW)

Laser Power

500 0.052 1000 0.206 1800 0.487 2400 0.726 3000 0.840 8700 1.000 Integrated M2-02 Hornblende #3

(mW)

Laser Power

M2-02 Hornblende #2

Ar

39

Cumulative

(mW)

Laser Power

M2-02 Hornblende #1

39

898.4

943.8 729.5 926.5 887.7 902.3 916.8

measured

Ar/ Ar

40

644.7 787.4 849.2 901.9 856.7 817.7 837.5

measured

39

Ar/ Ar

40

1291.3 1110.9 777.0 722.1 534.2 726.5 649.9 685.1 926.6 950.8 932.3 900.3 912.2 910.4 849.2 898.0

measured

39

Ar/ Ar

40

4.0

39.2 14.9 6.4 7.7 6.3 21.7

+/-

6.7 5.1 4.3 5.4 6.1 4.9 2.2

+/-

942.6 424.0 148.5 152.7 87.8 165.7 112.5 89.7 104.3 60.4 6.9 7.1 18.2 8.2 7.5 5.1

+/-

39

Ar/ Ar

Ar/ Ar

39

39

Ar/ Ar

8.67

16.56 2.78 8.73 8.00 10.19 10.07

measured

37

0.93 0.52 4.23 5.63 6.60 4.64 4.16

measured

37

2.50 6.49 4.77 4.35 1.09 1.32 2.33 3.82 6.50 8.24 7.81 7.70 8.83 8.22 7.01 7.54

measured

37

1.619 1.105 0.560 0.505 0.367 -0.014 0.170 0.094 0.011 0.063 0.031 0.023 0.080 0.022 0.019 0.046

measured

39

Ar/ Ar

36

1.271 0.513 0.156 0.152 0.148 0.169 0.093 0.096 0.105 0.038 0.003 0.007 0.018 0.007 0.006 0.004

+/-

37.0 29.3 21.2 20.6 20.3 -0.6 7.7 4.0 0.3 1.9 0.9 0.7 2.5 0.7 0.6 1.4

Ar

40

% Atm.

4.595 11.960 8.778 8.001 2.000 2.417 4.282 7.029 11.976 15.205 14.395 14.195 16.288 15.155 12.912 13.905

Ca/K

4.597 4.862 1.964 1.993 0.931 1.148 1.023 1.108 1.453 0.995 0.089 0.118 0.335 0.135 0.133 0.080

+/-

0.04

0.70 0.06 0.05 0.06 0.08 0.23

+/-

0.02 0.01 0.03 0.04 0.05 0.03 0.01 Weighted

+/-

0.037

0.405 0.056 0.034 0.027 0.005 0.026

measured

39

Ar/ Ar

36

0.063 0.018 0.013 0.013 0.024 0.024 0.020 average of

measured

39

Ar/ Ar

36

Ar

40

% Atm.

0.003

0.057 0.015 0.002 0.005 0.006 0.019

+/-

1.1

12.5 2.2 1.0 0.8 0.1 0.8

Ar

40

% Atm.

0.006 2.9 0.001 0.7 0.001 0.4 0.001 0.4 0.002 0.8 0.002 0.8 0.001 0.7 J from standards =

+/-

+/-

15.994

30.725 5.101 16.106 14.765 18.819 18.603

Ca/K

0.065

1.307 0.118 0.096 0.105 0.146 0.432

+/-

1.709 0.035 0.956 0.014 7.781 0.046 10.377 0.065 12.157 0.092 8.532 0.055 7.654 0.024 0.002305 +/- 0.000016

Ca/K

Weighted average of J from standards = 0.002305 +/- 0.000016

2.50 2.63 1.06 1.08 0.51 0.62 0.56 0.60 0.79 0.54 0.05 0.06 0.18 0.07 0.07 0.04

+/-


0.0471

0.0327 0.0076 0.0497 0.0467 0.0557 0.0563

Cl/K

0.0097 0.0059 0.0260 0.0380 0.0442 0.0271 0.0272

Cl/K

0.1937 0.0087 0.0805 0.0114 0.0168 0.0033 0.0521 0.0325 0.0297 0.0395 0.0504 0.0494 0.0443 0.0437 0.0380 0.0457

Cl/K

0.0005

0.0071 0.0026 0.0005 0.0008 0.0014 0.0033

+/-

0.0013 0.0005 0.0003 0.0005 0.0007 0.0005 0.0002

+/-

0.1761 0.0725 0.0385 0.0314 0.0286 0.0397 0.0309 0.0244 0.0258 0.0122 0.0008 0.0016 0.0032 0.0015 0.0012 0.0009

+/-

893.1

834.5 714.5 922.4 884.8 907.5 915.9

39

Ar*/ ArK

40

626.5 782.5 847.9 901.7 853.6 813.3 834.3

39

Ar*/ ArK

40

814.4 788.3 613.8 575.0 426.2 731.3 600.7 659.3 927.6 937.8 928.6 898.7 894.4 909.3 848.1 889.3

39

Ar*/ ArK

40

4.1

38.8 15.3 6.4 7.8 6.7 22.5

+/-

6.8 5.1 4.4 5.4 6.1 5.0 2.2

+/-

611.4 314.4 122.5 126.1 80.7 174.3 107.4 91.0 109.4 60.9 7.0 7.4 18.8 8.4 7.7 5.2

+/-

Age

2034.4

1952.4 1772.2 2073.9 2023.0 2053.8 2065.1

(Ma)

Age

1627.5 1876.5 1971.4 2046.0 1979.5 1921.9 1952.1

(Ma)

Age

1923.4 1885.1 1605.7 1537.1 1247.4 1798.5 1582.8 1682.9 2080.9 2094.3 2082.2 2042.0 2036.1 2056.2 1971.8 2029.1

(Ma)

+/-

10.2

55.4 24.2 8.6 10.8 9.0 30.2

(Ma)

+/-

11.7 7.6 6.2 7.4 8.6 7.2 8.9

(Ma)

+/-

888.1 466.4 212.2 227.0 170.6 271.3 188.4 151.0 145.7 80.5 9.4 10.1 25.6 11.4 10.9 11.1

(Ma)

M. GUEYE, S. SIEGESMUND, K. WEMMER, S. PAWLIG, M. DROBE, N. NOLTE AND P. LAYER 531

300 600 900 1200 1500 1800 2100 2500 3500 8700 Integrated

(mW)

Laser Power

Ar

0.000 0.002 0.005 0.008 0.011 0.015 0.074 0.710 0.804 0.818 0.907 0.917 0.919 0.923 1.000

39

Cumulative

Ar

0.000 0.001 0.003 0.007 0.016 0.111 0.865 0.892 0.970 1.000

39

Cumulative

Integrated M3-02 Muscovite #2

200 400 600 800 1000 1200 1400 1600 1800 2000 2500 3000 3500 4000 8700

(mW)

Laser Power

39

39

684.4 581.6 629.9 888.7 894.3 882.8 883.4 853.0 885.2 876.7 881.7

measured

Ar/ Ar

40

878.6

335.1 244.4 331.8 509.8 685.4 816.8 880.3 885.6 867.4 866.8 890.2 889.7 836.7 841.7 886.6

measured

Ar/ Ar

40

235.5 32.4 12.8 17.6 7.6 5.2 6.9 7.7 4.6 6.0 5.2

+/-

3.3

101.9 16.7 20.2 25.7 38.5 27.0 6.2 5.0 5.2 10.9 4.7 26.2 72.7 34.7 5.7

+/-

-0.652 0.116 0.028 0.048 -0.032 0.001 -0.001 0.006 0.000 0.010 0.000

measured

39

Ar/ Ar

37

0.009

0.562 0.310 0.163 0.547 0.160 0.196 0.014 0.001 0.012 0.051 0.006 0.042 -0.031 -0.011 0.013

measured

39

Ar/ Ar

37

-0.446 0.108 0.020 0.039 0.132 0.063 0.020 0.002 0.011 0.022 0.011 0.048 0.148 0.036 0.013

measured

39

Ar/ Ar

36

0.406 0.078 0.056 0.044 0.049 0.046 0.003 0.000 0.002 0.010 0.002 0.011 0.076 0.034 0.002

+/Ar

-39.3 13.1 1.8 2.3 5.7 2.3 0.7 0.1 0.4 0.7 0.4 1.6 5.2 1.3 0.4

40

% Atm.

1.032 0.569 0.299 1.003 0.294 0.360 0.026 0.002 0.021 0.093 0.012 0.077 -0.058 -0.020 0.024

Ca/K

1.975 0.435 0.285 0.235 0.242 0.173 0.011 0.001 0.006 0.042 0.008 0.056 0.296 0.128 0.009

+/-

0.0604 0.0250 0.0079 0.0208 0.0094 -0.0028 0.0016 0.0004 0.0008 0.0015 0.0012 -0.0010 0.0006 -0.0007 0.0013

Cl/K

0.617 0.096 0.079 0.022 0.014 0.001 0.000 0.005 0.002 0.004 0.000

+/-

0.651 0.233 0.092 0.285 0.106 0.016 0.001 0.013 0.005 0.012 0.006

measured

39

Ar/ Ar

36

0.349 0.040 0.020 0.011 0.006 0.000 0.000 0.001 0.001 0.001 0.000

+/Ar

28.1 11.8 4.3 9.5 3.5 0.5 0.0 0.4 0.2 0.4 0.2

40

% Atm.

-1.196 0.214 0.052 0.087 -0.059 0.002 -0.002 0.011 -0.001 0.018 -0.001

Ca/K

1.131 0.175 0.145 0.041 0.026 0.003 0.000 0.009 0.003 0.007 0.001

+/-

-0.0910 0.0068 0.0014 0.0013 -0.0005 0.0008 0.0003 0.0022 0.0006 0.0016 0.0005

Cl/K

0.001 0.008 0.001 0.3 0.017 0.003 0.0008 Weighted average of J from standards = 0.002305 +/- 0.000016

1.075 0.237 0.155 0.128 0.132 0.094 0.006 0.001 0.003 0.023 0.004 0.031 0.161 0.070 0.005

+/-

Appendix C-2. Results of Ar/Ar analyses. Ages reported at ± 1 M3-02 Muscovite #1 Weighted average of J from standards = 0.002305 +/- 0.000016

0.0603 0.0071 0.0043 0.0018 0.0012 0.0001 0.0001 0.0005 0.0002 0.0004 0.0001

+/-

0.0001

0.0611 0.0112 0.0067 0.0068 0.0081 0.0081 0.0006 0.0001 0.0003 0.0013 0.0004 0.0027 0.0168 0.0068 0.0004

+/-

40

40

491.7 512.7 602.7 804.5 862.8 878.1 883.0 849.2 883.8 873.2 879.9

39

Ar*/ ArK

876.3

467.0 212.4 325.8 498.5 646.3 798.4 874.2 885.0 864.0 860.4 887.0 875.6 792.9 830.9 882.8

39

Ar*/ ArK

186.7 30.7 13.6 16.7 7.6 5.2 6.9 7.6 4.6 6.0 5.2

+/-

3.3

181.7 27.2 25.8 28.3 39.1 29.7 6.2 5.0 5.3 11.2 4.7 26.0 72.3 35.7 5.7

+/-

Age

1380.7 1421.4 1586.3 1909.0 1992.4 2013.6 2020.4 1973.3 2021.6 2006.9 2016.1

(Ma)

Age

2011.3

1331.5 727.4 1021.7 1394.0 1661.1 1900.1 2008.3 2023.2 1994.0 1989.1 2026.0 2010.2 1891.9 1947.3 2020.2

(Ma)

+/-

366.4 58.9 23.9 24.4 10.6 7.2 9.5 10.8 6.3 8.3 11.1

(Ma)

+/-

9.6

366.4 76.8 61.9 55.2 65.6 43.7 8.7 6.9 7.3 15.8 6.5 36.0 106.9 51.1 7.8

(Ma)



Ar

39

Cumulative

Ar/ Ar

39

measured

40

+/measured

39

Ar/ Ar

37

+/-

Ar

0.001 0.002 0.003 0.006 0.015 0.080 0.115 0.218 0.363 0.470 0.591 0.649 0.843 0.939 1.000

200 400 600 800 1000 1200 1400 1600 1800 2000 2500 3000 3500 4000 8700 Integrated

39

Cumulative

(mW)

Laser Power

Ar/ Ar

39

779.3 1078.8 798.9 825.8 884.8 940.6 958.2 941.7 948.4 949.6 950.0 935.8 950.1 943.7 950.1 946.1

measured

40

227.2 245.1 163.6 83.5 26.8 8.1 8.2 7.7 6.9 8.4 6.0 9.5 4.9 6.6 7.2 2.3

+/-

3.180 2.989 4.361 3.858 3.987 4.116 3.823 3.648 3.646 3.851 3.835 4.053 4.248 3.844 5.059 3.977

measured

39

Ar/ Ar

37

1.257 1.012 1.130 0.449 0.144 0.034 0.040 0.028 0.025 0.030 0.032 0.042 0.021 0.030 0.038 0.010

+/-

0.141 0.014 0.014 0.000 0.001 0.008 0.010 0.027 0.007 0.009 0.001

+/-

17.8 3.4 2.2 0.3 0.3 -0.2 0.2 1.7 0.5 0.2 0.5

Ar

40

% Atm.

-0.869 0.005 -0.002 0.004 0.007 -0.065 -0.007 0.166 -0.047 -0.051 -0.002

Ca/K

0.555 0.059 0.049 0.002 0.005 0.023 0.036 0.113 0.030 0.034 0.004

+/-

1.747 2.732 1.679 0.336 0.088 0.027 0.016 0.019 0.014 0.013 0.013 0.020 0.011 0.016 0.024 0.023

measured

39

Ar/ Ar

36

0.545 0.659 0.387 0.066 0.020 0.005 0.009 0.003 0.002 0.003 0.002 0.005 0.001 0.003 0.004 0.001

+/-

66.2 74.8 62.1 12.0 2.9 0.8 0.5 0.6 0.4 0.4 0.4 0.6 0.3 0.5 0.7 0.7

Ar

40

% Atm.

5.846 5.495 8.024 7.097 7.336 7.573 7.032 6.710 6.706 7.084 7.054 7.457 7.816 7.072 9.313 7.316

Ca/K

2.317 1.864 2.086 0.828 0.265 0.063 0.074 0.052 0.047 0.055 0.060 0.078 0.039 0.055 0.071 0.018

+/-

average of J from standards = 0.002305 +/- 0.000016

0.639 0.102 0.069 0.010 0.009 -0.007 0.006 0.045 0.016 0.007 0.015

measured

39

Ar/ Ar

36


300 0.002 1061.3 118.5 -0.474 0.303 600 0.021 893.8 17.7 0.003 0.032 900 0.068 917.0 9.9 -0.001 0.027 1200 0.616 892.9 7.3 0.002 0.001 1500 0.866 885.7 4.2 0.004 0.003 1800 0.904 884.4 11.4 -0.035 0.013 2100 0.928 876.0 12.6 -0.004 0.019 2500 0.938 807.8 21.7 0.090 0.061 3500 0.971 887.2 8.9 -0.026 0.016 8700 1.000 900.8 12.8 -0.028 0.018 Integrated 891.1 4.2 -0.001 0.002 Appendix C-3. Results of Ar/Ar analyses. Ages reported at ± 1 M12-02 Hornblende #1 Weighted

(mW)

Laser Power

M3-02 Muscovite #3

0.1207 0.0165 0.1087 0.0394 0.0259 0.0199 0.0175 0.0217 0.0213 0.0231 0.0224 0.0233 0.0203 0.0207 0.0237 0.0217

Cl/K

-0.0046 -0.0018 -0.0010 0.0006 0.0007 0.0006 0.0023 0.0095 -0.0015 0.0017 0.0006

Cl/K

0.0803 0.0481 0.0506 0.0184 0.0063 0.0010 0.0017 0.0007 0.0004 0.0008 0.0008 0.0014 0.0005 0.0009 0.0013 0.0003

+/-

0.0286 0.0021 0.0015 0.0001 0.0002 0.0019 0.0021 0.0056 0.0019 0.0023 0.0002

+/-

40

40

263.9 272.2 304.0 728.5 861.3 935.4 956.1 938.7 946.9 948.5 948.9 932.6 949.7 941.7 946.3 941.9

39

Ar*/ ArK

872.2 863.8 896.5 890.0 882.9 886.5 874.3 794.4 882.5 898.6 886.6

39

Ar*/ ArK

96.4 90.2 81.9 75.7 26.8 8.2 8.6 7.7 6.9 8.5 6.1 9.6 4.9 6.7 7.3 2.3

+/-

103.8 17.6 10.6 7.3 4.2 11.6 12.9 22.8 9.1 13.0 4.2

+/-

Age

867.0 888.6 968.7 1794.2 1990.3 2091.2 2118.4 2095.5 2106.3 2108.5 2108.9 2087.4 2110.1 2099.5 2105.6 2099.7

(Ma)

Age

2005.5 1993.8 2039.0 2030.1 2020.3 2025.2 2008.5 1894.2 2019.7 2041.9 2025.3

(Ma)

+/-

251.4 232.4 202.0 118.1 37.5 10.8 11.2 10.2 9.1 11.2 8.0 12.8 6.4 8.8 9.6 9.2

(Ma)

+/-

144.1 24.6 14.5 9.9 5.8 15.9 17.9 33.6 12.5 17.7 10.3

(Ma)



533

Ar

39

Cumulative

0.002 0.019 0.402 0.740 1.000

500 1000 2000 3000 8700 Integrated

39

Ar/ Ar

39

Ar/ Ar

1273.0 948.5 961.6 956.6 957.8 959.2

measured

40

1195.6 915.5 945.7 952.9 948.1 949.8

measured

40

159.5 14.0 5.2 5.2 5.1 3.0

+/-

489.1 82.3 8.1 5.6 5.0 3.7

+/-

39

Ar/ Ar

39

Ar/ Ar

2.613 5.097 4.230 3.849 3.861 4.017

measured

37

1.563 4.580 4.211 4.276 4.245 4.249

measured

37

0.389 0.080 0.022 0.018 0.020 0.012

+/-

1.172 0.446 0.038 0.025 0.021 0.017 Weighted

+/-

39

Ar/ Ar

39

Ar/ Ar

2.541 0.082 0.008 0.006 0.007 0.012

measured

36

1.765 0.284 0.017 0.011 0.012 0.018 average of

measured

36

Ar

40

% Atm.

0.333 0.009 0.000 0.001 0.001 0.000

+/-

59.0 2.5 0.2 0.2 0.2 0.3

Ar

40

% Atm.

0.786 43.6 0.058 9.1 0.002 0.5 0.001 0.3 0.002 0.3 0.001 0.5 J from standards =

+/-

+/-

4.802 9.383 7.783 7.081 7.102 7.391

Ca/K

0.717 0.147 0.041 0.034 0.037 0.022

+/-

2.871 2.154 8.429 0.823 7.748 0.070 7.867 0.046 7.810 0.039 7.818 0.031 0.002305 +/- 0.000016

Ca/K



M 1-02 Bio M 2-02 Bio M 3-02 Mus M 5-02 Bio M 6-02 Bio M 12-02 Hbl MP 2-03 Bio KO 75-03 Hbl KO 75-03 Bio

Sample

Spike [ No. ] 3028 3042 3018 3024 3020 3022 3213 3223 3230

K2O [ Wt. % ] 8.83 8.61 10.52 6.91 7.36 0.91 8.96 1.57 9.45

Ar * [ nl/g ] STP 1122.34 1071.39 1264.69 453.34 221.96 118.37 1068.74 166.18 1124.52

40

Ar * [%] 99.40 96.02 99.75 98.61 96.05 99.81 99.47 99.58 99.71

40

Appendix D: Results of K-Ar dating Geowissenschaftliches Zentrum der Georg-August-Universität, Goldschmidtstr. 3, 37077 GÖTTINGEN K/Ar - Age Determinations Senegal / Gueye et al Ar - Isotopic Abundance Spike-Isotopic Composition Decay Constants [1/a] 40 Ar : 99.6000% 40 Ar : 0.0099980% le : 5.810E-11 38 Ar : 0.0630% 38 Ar : 99.9890000% lb : 4.962E-10 36 Ar : 0.3370% 36 Ar : 0.0009998% Standard Temperature Pressure (STP) ltot: 5.543E-10 0° C; 760 mm Hg Molar Volume Normal Atmosphere (DIN 1343) [ml] : 22413.8 273,15K; 1013,25 mbar

Ar

39

Cumulative

(mW)

Laser Power

500 0.002 1000 0.011 2000 0.253 3000 0.707 8700 1.000 Integrated M12-02 Hornblende #3

(mW)

Laser Power

M12-02 Hornblende #2

0.0371 0.0151 0.0189 0.0198 0.0201 0.0195

Cl/K

0.1284 0.0156 0.0204 0.0204 0.0207 0.0206

Cl/K

40

40

523.2 927.9 962.2 957.5 958.4 958.4

39

Ar*/ ArK

674.7 834.3 943.6 952.6 947.5 947.4

39

Ar*/ ArK

74.9 14.0 5.2 5.2 5.1 3.0

+/-

291.2 76.8 8.1 5.6 5.0 3.7

+/-

Age [ Ma ] 2089.7 2063.6 2021.4 1362.1 753.4 2118.4 2011.9 1869.9 2009.0

2-Error [ Ma ] 8.7 20.3 10.6 19.0 5.2 30.7 26.7 13.5 12.1

Potassium 40K : 0.011670% K2O/K : 0.8302 Atomic Weight [g/mol] tot Ar : 39.9477 40 Ar : 39.9624 tot K : 39.1027

0.0265 0.0016 0.0001 0.0001 0.0002 0.0001

+/-

0.0970 0.0141 0.0006 0.0004 0.0005 0.0003

+/-

Age

1441.5 2081.2 2126.3 2120.2 2121.3 2121.3

(Ma)

Age

1708.2 1952.2 2102.0 2113.7 2107.1 2107.0

(Ma)

+/-

2-Error [%] 0.4 1.0 0.5 1.4 0.7 1.4 1.3 0.7 0.6

142.1 18.7 6.7 6.8 6.7 9.5

(Ma)

+/-

476.5 109.8 10.7 7.4 6.6 9.9

(Ma)
