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| Titel |
Pan-African granulite facies reworking along Moyar shear zone, south India: Implications for Gondwanaland assembly |
| VerfasserIn |
Subhadip Bhadra, Pritam Nasipuri |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250094195
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| Publikation (Nr.) |
 EGU/EGU2014-9593.pdf |
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| Zusammenfassung |
| The present study documents metamorphic evolution of garnetiferrous quartzo-feldspathic
gneiss from the Moyer shear zone (MSZ), southern granulite terrain (SGT). Quartz (Qtz),
plagioclase feldspar (Pl) and biotite (Bt1) constitute the pre-metamorphic mineral assemblage
in the rock, where as porphyroblastic garnet (Grt) and second generation biotite (Bt2)
characterize the metamorphic mineral paragenesis. Mylonitic fabric in the rock is defined by
biotite (Bt1) and poly-crystalline quartz ribbons that wraps garnet porphyroblast. Core
compositions of the porphyroblastic garnets lie in almandine-pyrope-grossular ternary
(Alm62Prp23Grs14Spss01). In the core to rim traverses within the garnet display variation in
major element zoning patterns that depend on the neighboring mineral phase/phases. Along
traverses where garnet rim shares contact with quartz, a flat Fe, rimwardly decreasing Mg, flat
Mn and rimwardly increasing Ca (referred as Fe0Mg-Ca+Mn0) profile was observed.
Embayed garnet sharing boundary with randomly oriented Bt2, displays rimwardly
increasing Fe, rimwardly-decreasing Mg, rimwardly increasing Ca and flat Mn
(referred as Fe+Mg-Ca+Mn0) profile. Bt2 shows complementary decrease of Fe
and increase of Mg towards the interface with garnet. Garnet sharing contact with
both Bt1 and plagioclase-feldspar displays rimwardly increasing Fe, rimwardly
decreasing Mg, rimwardly increasing Ca and rimwardly increasing Mn (referred as
Fe+Mg-Ca+Mn+) profile. Adjacent biotites show an increase of Fe and Mg towards the
interface with garnet. Anorthite content of plagioclase decreases towards the interface.
While Fe0Mg-Ca+Mn0profile can be interpreted with garnet growth (Bt1 + Pl -
Grt) and compositional homogenization (flat Fe, Mn) during peak metamorphism,
Fe+Mg-Ca+Mn+and Fe+Mg-Ca+Mn0 profiles can be linked with post-peak
compositional modifications respectively via retrograde net-transfer (ReNTR: Grt + Ca-rich
± Qtz - BtII + Ca-poor Plag) and retrograde exchange (ReER: Fe-Bt + Mg-Grt - Mg-Bt +
Fe-Grt) equilibria.
Mineral and isopleth thermobarometry constrain the peak metamorphic condition at 900
oC, 9.5 kbar implying high pressure granulite facies metamorphism. ReNTR and ReER
equilibria were constrained to be closed respectively at 740 oC, 8 kbar and 650 oC, 7
kbar. Geo-speedometric analyses reveal very rapid cooling in the order of >150
oC/Ma, at least during the initial stages of retrograde metamorphism. This estimate is
commensurate with strong Mn kick up near the garnet rim along Fe+Mg-Ca+Mn+profiles
and absence of any symplectectic phases through break down of porphyroblastic
garnet.
High-P granulite facies metamorphism and subsequent cooling decompression path,
documented in this study, in conjugation with reported Neoproterozoic Sm-Nd
garnet ages (624-591 Ma) from the quartzo-feldspathic gneiss bear semblance with
Pan-African HT-UHT reworking along Palghat-Cauvery Shear Zone (521 Ma) that
was culminated with the closure of Paleo-Mozambique ocean during east and west
Gondwanaland assembly. We envisaged that peak granulite facies metamorphism in
MSZ marks the onset of paleo-Mozambique ocean closure in the northern part of
SGT. Rapid exhumation along MSZ was possibly engineered by early Paleozoic
tectonic extrusion related processes during final assembly of the Gondwanaland. |
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