| 2. |
- Rao, C.V. Dharma, et al.
(författare)
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New constraints on the metamorphic evolution of the Eastern Ghats Belt, India, based on relict composite inclusions in garnet from ultrahigh-temperature sapphirine granulites
- 2011
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Ingår i: Geological Journal. - : Wiley. - 0072-1050 .- 1099-1034. ; 46:2-3, s. 240-262
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Tidskriftsartikel (refereegranskat)abstract
- The sapphirine granulites from Gangaraju Madugula, Eastern Ghats Belt (EGB), India preserve a rich variety of mineral assemblages and unique isolated and composite mineral inclusions within garnet that provide robust evidence for extreme crustal metamorphism at ultrahigh temperature (UHT) conditions (>900°C). Diagnostic UHT assemblages in these rocks include sapphirine + quartz, spinel + quartz and high alumina orthopyroxene + sillimanite + quartz. The stability of spinel + quartz, sapphirine + quartz and orthopyroxene + sillimanite + quartz assemblages provides evidence for temperatures exceeding 960°C at moderate pressures. The mineral association of garnet–orthopyroxene is indicative of a subsequent high P–UHT metamorphic event as indicated by the high alumina contents of orthopyroxene (>10 wt% Al2O3) coexisting with garnet. Peak P–T conditions of ∼970°C and 9.5 kbars are calculated from conventional garnet–orthopyroxene geothermobarometry. Calculated isochemical sections constructed in the model system Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 (NCKFMASHTO) for the sapphirine granulites and garnet–orthopyroxene granulites adequately predict phase relationships that are consistent with those observed in the rocks. An evaluation of the assemblages and textures and P–T estimates indicate a three-stage evolution of the sapphirine granulites and associated garnet–orthopyroxene granulites: (1) inclusion assemblages with sapphirine, spinel and quartz on the low-pressure prograde path (M1 stage); (2) a peak UHT assemblage of porphyroblastic garnet–orthopyroxene (M2 stage) and (3) a retrogression that resulted in orthopyroxene–cordierite intergrowths and biotite rims on Grt (M3 stage). The sapphirine granulites and associated garnet–orthopyroxene granulites indicate that they grew during the prograde and retrograde stage. The thermo-barometric estimates from mineral compositions and the complimentary isochemical sections approached from bulk rock compositions allow tighter constraints to be placed on the P–T evolution of this sector of the EGB.
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| 3. |
- Rao, C.V. Dharma, et al.
(författare)
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Sapphirine granulites from Panasapattu, Eastern Ghats belt, India : Ultrahigh-temperature metamorphism in a Proterozoic convergent plate margin
- 2011
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Ingår i: Geoscience Frontiers. - : Elsevier BV. - 1674-9871. ; 3:1, s. 9-31
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Tidskriftsartikel (refereegranskat)abstract
- We report equilibrium sapphirine + quartz assemblage in biotite–orthopyroxene–garnet granulites from a new locality in Panasapattu of Paderu region in the Eastern Ghats granulite belt, which provide new evidence for ultrahigh-temperature (UHT) metamorphism at 1030–1050 °C and 10 kbar in this region. The development of migmatitic texture, stabilization of the garnet–orthopyroxene–plagioclase–K-feldspar association, prograde biotite inclusions within garnet and sapphirine as well as sapphirine and cordierite inclusions within garnet in these granulites indicate that the observed peak assemblages probably formed during prograde dehydration melting of a Bt–Sill–Qtz assemblage, and constrain the prograde stage of the P–T path. The core domains of orthopyroxene porphyroblasts have up to wt(Al2O3) 9.6%, which suggest that the temperatures reached up to 1150 °C suggesting extreme crustal metamorphism. These conditions were also confirmed by the garnet–orthopyroxene thermobarometery, which yields a P–T range of 1012–960 °C and 9.4 kbar. The P–T phase topologies computed using isochemical sections calculated in the model system Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O (NCKFMASH) for metapelites, garnet-free sapphirine granulites and garnet-bearing sapphirine granulites match the melt-bearing assemblages observed in these rocks. Isochemical sections constructed in the NCKFMASH system for an average sub-aluminous metapelite bulk composition, and contoured for modal proportions of melt and garnet, as well as for the compositional isopleths of garnet, predict phase and reaction relations that are consistent with those observed in the rocks. Garnet and orthopyroxene contain Ti-rich phlogopite inclusions, suggesting formation by prograde melting reactions at the expense of phlogopite during ultrahigh-temperature conditions. These P–T results underestimate ‘peak’ conditions, in part as a result of the modification of garnet compositions in the domains where some melt was retained. The post-peak evolution is constrained by a succession of melt-present reactions that occur at P < 10 kbar, inferred from micro-structural relations among various minerals. After high-temperature decompression from the metamorphic peak, the P–T path followed a near isobaric cooling stage to T < 900 °C. The UHT rocks investigated in this study occur within a continental collision suture which witnessed prolonged subduction–accretion history prior to the final collision. We correlate the extreme metamorphism and the stabilization of UHT mineral assemblages to heat and volatile input from an upwelled asthenosphere during subduction–collision tectonics in a Proterozoic convergent plate margin.
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