Growth and demise of chloritoid along a metamorphic P-T path: an example from the South Carpathians 1
1
Elena Negulescu , Gavril Săbău and Hans-Joachim Massonne
2
1
Geological Institute of Romania, Bucharest, Romania,
[email protected],
[email protected] 2 Universität Stuttgart, Azenbergstr. 18, D-70174 Stuttgart, Germany,
[email protected]
California 2014
The Leaota Massif basement (Fig. 1) consists of a flat-lying sequence of five concordant units (Negulescu, 2013) displaying internal lithologic and metamorphic contrasts (Fig. 2). Metapelites with Mg-rich chloritoid occur associated with eclogites and metagabbronorites in the Bughea Complex (BC) which is interpreted to represent a subduction mélange (Săbău, 2000; Negulescu et al., 2007, 2009). The BC matrix comprises micaceous gneisses, amphibole-garnet schists and Fe-Mg-chloritoid - bearing micaschists that experienced up to 8.5-9.5 kbar and 555-585°C (Negulescu, 2013) while the calculated metamorphic conditions of the HP-blocks range between 550-780°C, 15-32 GPa (Fig. 2), illustrating the P,T-incompatibility between these blocks and their hosts.
gbn GN, MS
Chloritoid, ideally (Fe2+,Mg)(Al,Fe3+)2OSiO4(OH)2 is a common index mineral in metapelites. Fe-rich end-members are widespread in low-grade metapelites whereas Mg-rich members are exclusively connected with high-pressure rocks (e.g. Chopin & Schreyer, 1983). Only a few number of natural occurrences of Mg-chloritoid (XMg>0.50) are known, mostly of them located in Alps. Fe-Mg-chloritoid is a common mineral in the metapelites from the Leaota Massif units. It was identified in: - the matrix of mica-gneisses (XMg = 0.16-0.23) and in garnet porphyroblasts (XMg = 0.26-0.34) of the upper Voineşti Formation (Fig. 3); - garnet porphyroblasts of some schists (XMg = 0.24-0.3) belonging the Lerești Formation(Fig. 3). Mg-richer compositions of chloritoid were identified in: - garnet porphyroblasts of the Bughea Complex matrix (XMg = 0.32-0.39) (Fig. 3) - inclusions in garnets of the retrograde kyanite-chloritoid-garnet assemblage (Negulescu et al., 2009) in the marginal zone of the VHPE (the Bughea Complex)(XMg = 0.40-0.47)(Fig. 3). CrE
The Iezer Complex The Voineşti Formation
0.6
The Lereşti Complex
0.55
VHPE
The Bughea Complex inclusions in garnet from VHP-eclogite
kwm
CrE
500 m m
cld
0.5
chl grt grt
inclusions in garnet from micaschists
cld cld
0.45 0.4
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Fig. 1 Simplified geologic map of the Leaota Massif (modified after Gheuca & Dinică 1996) with the high-pressure block occurences
0.35 0.3
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in mica-gneisses matrix
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Căpitanu Complex
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Fig. 2 PT distribution in the Leaota basement units. Input data acquired from this study (MS samples) and from literature: VHP-eclogites (Săbău, 2000), metagabbronorites (Negulescu et al., 2007), common eclogites (Săbău, 2000; Negulescu et al., 2007), Cr-ek (Negulescu et al., 2007), Bughea matrix, Iezer and Călușu (Negulescu, 2013).
E
D chl
C/ Km
MS
Fig. 3 Mg/(Mg+Fe) variation in the chloritoid from pelitic rocks belonging to different units of the Leaota Massif (modified after Negulescu, 2013)
The Mg-richest compositions of chloritoid were identified in the schists of the Bughea Complex (MS - Fig. 1). They are composed of garnet porphyroblasts embedded in a matrix of chlorite, phengite, paragonite, epidote, ilmenite, rutile, and quartz. Garnet contains inclusions of chloritoid, phengite, paragonite, chlorite, quartz, epidote, and rare amphibole and kyanite. The internal structure of the garnet porphyroblasts is outlined by their chemical zoning, the array of mineral inclusions and the chemical variation of included chloritoid. Compositional maps of garnets (Fig. 4) and core-to-rim chemical profiles (Fig. 5) document a prograde zoning characterized by Mn and Ca decrease and Mg increase towards the rims. The array of mineral inclusions portrays three zones which are well correlated with the chemical compositions of the garnet porphyroblasts: (I) high-Mn core rich in chloritoid and small epidote inclusions; (II) inner mantle also rich in chloritoid inclusions; (III) high-Mg rims, free of chloritoid but containing Mg-chlorite along with kyanite appearing for the first time in the assemblage. The chemical composition of chloritoid is characterized by gradual Mg increase from the inclusions located in garnet cores (XMg=0.26-0.3) towards those placed near rims (XMg=0.33-0.38, sample 05Bg2) - fig. 4D, I; fig. 6. The highest-Mg chloritoid compositions (XMg=0.40-0.44) were identified in the II-zone of garnet gt1 (fig. 4N) of sample 12BgC.
100
Approximate depth (km)
0.2
inclusions in garnets from schists
VHP-eclogites metagabbronorites common eclogites Mg-chloritoid schists Cr-ek Bughea matrix Albeşti granites
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1 2 3 4 5 6 7 8 9 10111213141516171819202122232425262728293031
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Fig. 6 XMg=Mg/(Fe+Mg) variation in chloritoid Fig. 5 Chemical variation of the gt2 and gt1 garnet compositions along the core-to- inclusions: A) 05Bg2 sample (fig. 4D, I), and B) rim profiles. 12BgC sample (II-zone, fig. 4N).
III Fig. 4 Compositional maps of garnet porphyroblasts of the Mg-rich chloritoid-bearing schists, the Bughea Complex. The Mg distribution maps scaled for inclusions (D, I, N) show Mg increase from the inclusions located in garnet cores (I) towards those located near
rims (II).
Na2O = 2.440, MgO = 5.467, Al2O3 = 24.327, K2O = 1.516, CaO = 2.070, TiO2 = 1.365, MnO = 0.13, FeO = 12.692
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Chl Phe Carp Amp Gt law ru
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Chl Phe Amp MMP law ru
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Bio Pl Gt MMP ru
1. Phe Ctd Carp Amp Gt law ru 2. Phe Ctd Amp Gt ky law ru 3. Phe Ctd Amp Gt ky ru 5. Phe Ctd Amp Gt MMP law ru 6. Phe Ctd Amp Gt MMP ru 7. Chl Phe Ctd Carp Amp Gt law ru 8. Chl Phe Ctd Amp law ru 9. Chl Phe Ctd Carp Amp law ru 10. Chl Phe Ctd Amp Gt MMP ru 11. Chl Phe Ctd Carp Amp MMP law ru 12. Chl Phe Ctd Amp Gt MMP law ru 13. Phe Gt Cpx ky ru 14. Phe Amp Gt Cpx MMP ky ru 15. Phe Amp Gt Cpx MMP ru 16. Bio Phe Amp Gt MMP ru 17. Chl Phe Amp Pl MMP zo ru 18. Chl Phe Pl MMP zo ru 19. Chl Phe Pl MMP zo sph 20. Chl Phe Amp Pl Gt MMP zo ru 21. Chl Phe Amp Pl Gt MMP ru 22. Chl Phe Pl MMP ru 23. Chl Phe Pl MMP ru ilm 24. Chl Phe Pl MMP ilm 25. Bio Amp Amp Pl Gt MMP ru 26. Bio Chl Amp Amp Pl Gt MMP ru 27. Bio Chl Pl Gt ky ru
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Chl Phe Amp Gt MMP zo ru
stability field of chloritoid stability field of kyanite
Bio Chl Pl Gt MMP ru
Chl Phe Amp MMP zo ru
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The investigated Mg-rich-chloritoid - bearing schists represent pelitic rocks (Negulescu et al., 2014) metamorphosed under high-pressure conditions. The calculated PT-path is consistent with the other PT-conditions calculated for the eclogites and gabbronorites from the Bughea Complex, indicating a geothermal gradient ranging between 7-10°C/km (fig. 2), comparable to geotherms of subduction zones and in concordance with other literature data like the Alpine-type HP to UHP-LT metamorphic rocks from peri-Pacific and peri-Mediterranean fold belts of Paleozoic to Tertiary ages which are characterized by geothermal gradients of 4-10°/km (e.g. Maruyama et al., 1996). The occurrence of chloritoid, the gradual change in the Mg-Fe partition with the surrounding phases in response to changing PT-conditions, and finally its thermal breakdown represent a valuable tool in constraining the prograde path of subducted pelitic sediments, leading to a better understanding of the dynamics of subduction channels.
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.2 06
Solid solutions: Amph(DPW) - abbrev. Amp Mica(M) - abbrev. MMP (muscovitemargarite-paragonite solid solution) Chl(HP) - abbrev. Chl Pheng(HP) - abbrev. Phe Gt(HP) - abbrev. Gt Bio(TCC) - abbrev. Bio Pl(h) - abbrev. Pl Ctd(HP) - abbrev. Ctd Carp St(HP)- abbrev. St hCrd - abbrev. Crd T Omph(HP) - abbrev. Cpx
H2O saturated Y(CO2) = 0.00 Component saturation hierarchy: SiO2
0.38
Sample 05Bg2 was modelled in the systems Na2O-K2O-CaO-MnO-FeO-MgO-Al2O3-SiO2-TiO2-H2O. P2O5 is neglected in the input data for simplification purpose, also considering that this component occurs only in apatite which is not involved in the calculations. A PTpseudosection was computed for the whole-rock compositions using the PERPLE_X computer package (Connolly, 1990), Perplex 6.6.8 version available at http://www.perplex.ethz.ch/perplex/ibm_and_mac_archives/WINDOWS/ using an updated internally consistent thermodynamic data set of Holland & Powell (1998). The solid-solution models used are those identified by the following abbreviations in the 'solution_model.dat' file of the above-mentioned package, modified from Holland & Powell (1998) according to the references therein: Chl(HP) for chlorite, Gt(HP) for garnet, Pheng(HP) for phengite, Mica(M) for paragonite, Bio(TCC) for biotite, Ctd(HP) for chloritoid, Omph(HP) for clinopyroxene, Amph(DPW) for amphiboles, T for talc, Carp for carpholite, St(HP) for staurolite, Pl(h) for plagioclase, hCrd for cordierite, the pure phases lawsonite, rutile, kyanite, and the compensated Redlich-Kwong fluid equation of state from Holland & Powell (1998). Quartz and water were considered as excess phases. The calculated pseudosection for the 400-700°C and 4-24 kbar range (fig. 7) predicts the occurrence of chloritoid at temperatures of 400590°C and P>12 kbar. The XMg isopleths of chloritoid and garnet, and Si isopleths of phengite constrain a prograde path in the range 520550°C and 16-18 kbar. The rare kyanite inclusions in garnet are in equilibrium with garnet rims at about 600°C and P>21 kbar, beyond the predicted P-T field of chloritoid.
05Bg2
P(kbar)
F
Bio Chl St Pl Gt ru Chl Phe Amp MMP zo sph
20 21 17 18
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19
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460
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Bio Crd Pl Gt ilm
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Gt sill ru Bio Crd Pl ll ilm Bio Crd Pl Gt si
T(oC)
580
640
700
Fig. 18 P-T pseudosection calculated for the 05Bg2 sample using the XRF-derived bulk composition in the Na2O-K2O-CaOFeO-MgO-MnO-Al2O3-SiO2-TiO2-H2O system with excess of quartz and water.
References Chopin, C, Schreyer, W., 1983, Magnesiocarpholite and magnesiochloritoid: two index minerals of pelitic blueschists and their preliminary phase relations in the model system MgO-Al2O3-SiO2-H2O: Am. J. Sci., 283-A, 72-96. Connolly, J.A.D., 1990, Multivariable phase diagrams: an algorithm based on generalized thermodynamics, American Journal of Science 290, 666-718. Holland, T. J. B., Powell, R., 1998, An internally consistent thermodynamic data set for phases of petrological interest, Journal of Metamorphic Geology 16, 309-343. Gheuca, I., Dinică, I., 1996, The metamorphic basement of the Getic Nappe in the Eastern margin of the South Carpathians (Leaota and Iezer Mountains): IGR 90 Symposium, Excursion Guide C3, An. Inst. Geol. Rom. 69, Suppl. 5, 15 pp. Maruyama, S., Liou, J.G., Terabayashi, M., 1996, Blueschists and eclogites of the world and their exhumation. Int. Geol. Rev. 38, 485-594. Negulescu, E., Săbău, G., Massonne, H.-J., 2007, The Origin and Significance of Eclogite and Metagabbronorite Knockers from the Bughea Complex, South Carpathians, Eos Trans. AGU, 88(52), Fall Meet. Suppl., Abstract V13A-1141. Negulescu, E., Săbău, G., Massonne H.-J., 2009, Chloritoid-Bearing Mineral Assemblages in High-Pressure Metapelites from the Bughea Complex, Leaota Massif (South Carpathians), Journal of Petrology, 50, 1, 103-125. Negulescu, E., 2013, The significance of minerals and mineral assemblages in deriving the metamorphic history of the Leaota Massif crystalline basement (in Romanian, English Summary), Tehnopress, Iaşi, 100p. Negulescu, E., Săbău, G., Massonne, H.-J., 2014, Protoliths of the high-pressure tectonic blocks from the South Carpathians basement units. Geophysical Research Abstracts Vol. 16, EGU2014-4454, 2014. Săbău, G., 2000, A possible UHP-eclogite in the Leaota Mts. (South Carpathians) and its history from high-pressure melting to retrograde inclusion in a subduction melange, Lithos, 52, 253-276.
Acknowledgments This contribution was financially supported by Deutscher Akademischer Austauschdienst (DAAD) and Romanian Executive Unit for Financing Higher Education, Research, Development and Innovation grant PN-II-ID-PCE-2011-3-0030.
