Late neoproterozoic P-T evolution of HP-UHT Granulites from the palni Hills (South India): New constraints from Phase Diagram Modelling, LA-ICP-MS Zircon dating and in-situ EMP monazite dating

Brandt, S. and Schenk, V. and Raith, M. M. and Appel, P. and Gerdes, A. and Srikantappa, C. (2011) Late neoproterozoic P-T evolution of HP-UHT Granulites from the palni Hills (South India): New constraints from Phase Diagram Modelling, LA-ICP-MS Zircon dating and in-situ EMP monazite dating. JOURNAL OF PETROLOGY, 52 (9). pp. 1813-1856. ISSN 0022-3530

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Official URL: doi:10.1093/petrology/egr032


Late Neoproterozoic (c. 555 Ma) high-pressure-ultrahigh-temperature (HP-UHT) metamorphism has been documented for MgAl-rich migmatitic granulites from the Palni Hills in the Southern Granulite Terrane (South India). Conspicuous reaction textures indicate a clockwise P-T evolution, which is constrained through P-T pseudosection modelling and thermobarometry. The transformation of sillimanite to kyanite, which coexisted with orthopyroxene and/or garnet, records an early stage of loading. During subsequent heating to UHT conditions at deep-crustal levels (c. 1000 degrees C, 13 center dot 0 kbar) kyanite was transformed to sillimanite, and distinct peak-temperature assemblages (orthopyroxene + sillimanite + mesoperthite + rutile +/- garnet +/- quartz +/- sapphirine, garnet + biotite + sillimanite + spinel + corundum + rutile + plagioclase and garnet + orthopyroxene + rutile + plagioclase +/- quartz) formed in specific bulk compositions through biotite-dehydration-melting reactions. A sequence of corona and sapphirine-bearing symplectite textures records subsequent isothermal decompression of the order of c. 6 kbar at persistent extreme temperatures (1010-920 degrees C). UHT decompression is consistent with the uniformly high Al contents of porphyroblastic, coronitic and symplectitic orthopyroxene (up to 10 center dot 4 wt % Al2O3). Regrowth of garnet and biotite documents post-decompressional cooling to subsolidus conditions of < 800 degrees C at mid-crustal levels (c. 6 kbar). HP-UHT metamorphism and the clockwise P-T path of the Palni Hills granulites is attributed to a single late Neoproterozoic tectono-metamorphic event, which has been consistently dated at c. 555 Ma through laser ablation inductively coupled plasma mass spectrometry U-Pb analyses of zircon and in situ electron microprobe U-Th-total Pb analyses of monazite. The MgAl-rich granulites occur as enclaves in enderbitic orthogneiss. The intrusion of the orthogneiss in the late Archean (2534 +/- 28 Ma) marks the beginning of voluminous granitoid emplacement in the Southern Granulite Terrane between 2530 and 2440 Ma, which presumably caused a first high-grade metamorphic event in the early Paleoproterozoic (2469 +/- 13 Ma), recorded by zircon cores in the MgAl-rich granulites. The clockwise P-T-t evolution indicates that HP-UHT metamorphism in the central part of the Southern Granulite Terrane is related to collisional tectonics during the final assembly of Gondwana in the late Neoproterozoic. Extreme heating is ascribed to upwelling of the asthenosphere during delamination of the thickened lithospheric mantle. Fast uplift of the rocks followed by mid-crustal isobaric cooling reflects extension of the hot overthickened crust and its subsequent cooling to a normal geotherm.

Item Type: Article
Uncontrolled Keywords: P-T pseudosection; P-T-t path; MgAl-rich granulite; Southern Granulite Terrane; ultrahigh-temperature (UHT) metamorphism
Subjects: F Earth Science > Geology
Divisions: Department of > Geography
Depositing User: Users 23 not found.
Date Deposited: 07 Aug 2019 11:47
Last Modified: 07 Aug 2019 11:47

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