A recent petrological study on the lesser-known Sholayar alkaline syenite complex (10°17 0 59.9 0... more A recent petrological study on the lesser-known Sholayar alkaline syenite complex (10°17 0 59.9 00 N: 076°47 0 26.3 00 E) accommodated in the high-grade rocks of the Southern Granulite Terrane reveals the presence of three distinct petrographic types namely leuco syenite, melanofelsic syenite and mela syenite. The syenites, in general, have the following constituent minerals, namely alkali feldspar (Or 76.8 Ab 23.2-Or 93.4 Ab 6.6), plagioclase (An 4.1-An 17.3), calcic amphibole (edenitic hornblende/ferropargasitic hornblende), quad pyroxene (diopside-hedenbergite), biotite and opaque minerals (magnetite and ilmenite). Mode-based statistical studies indicate that a highly significant correlation exists among certain mineralogical parameters including quartz%, total feldspar%, colour index, alteration index and hydration index. Isopleth plots of these parameters strongly suggest maintenance of a magmatic regime throughout, with increasing water content towards the end stage of crystallization. Several geothermobarometric methods point to shallow to moderate level emplacement (*9-23 km) of the alkaline/subalkaline syenite magma which was facilitated by a 'hydration event'. The steep change in water gradients (in localized pockets) of the syenite intrusives has been corroborated by textural evidences.
A recent petrological study on the lesser-known Sholayar alkaline syenite complex (10°17 0 59.9 0... more A recent petrological study on the lesser-known Sholayar alkaline syenite complex (10°17 0 59.9 00 N: 076°47 0 26.3 00 E) accommodated in the high-grade rocks of the Southern Granulite Terrane reveals the presence of three distinct petrographic types namely leuco syenite, melanofelsic syenite and mela syenite. The syenites, in general, have the following constituent minerals, namely alkali feldspar (Or 76.8 Ab 23.2-Or 93.4 Ab 6.6), plagioclase (An 4.1-An 17.3), calcic amphibole (edenitic hornblende/ferropargasitic hornblende), quad pyroxene (diopside-hedenbergite), biotite and opaque minerals (magnetite and ilmenite). Mode-based statistical studies indicate that a highly significant correlation exists among certain mineralogical parameters including quartz%, total feldspar%, colour index, alteration index and hydration index. Isopleth plots of these parameters strongly suggest maintenance of a magmatic regime throughout, with increasing water content towards the end stage of crystallization. Several geothermobarometric methods point to shallow to moderate level emplacement (*9-23 km) of the alkaline/subalkaline syenite magma which was facilitated by a 'hydration event'. The steep change in water gradients (in localized pockets) of the syenite intrusives has been corroborated by textural evidences.
Experimentally determined Ca-Mg-Fe pyroxene phase relations at E00-1200'C and from less than one ... more Experimentally determined Ca-Mg-Fe pyroxene phase relations at E00-1200'C and from less than one atmosphere up to 15 kbar are combined with calculated phase equilibria for the diopside-enstatite and hedenbergite-ferrosilite joins to produce a graphical two-pyroxene thermometer applicable to a wide variety of rocks from the earth, moon, and meteorites. Samples with appreciable contents of "others" components require special projection onto the Di-En-Hd-Fs pyroxene quadrilateral; Wo, En, and Fs components as normally calculated will not yield correct temperatures. The projection approximates the activities of those components in natural pyroxenes but is largely empirical, and may not be appropriate for large contents ofnonquadrilateral components. Therefore, the thermometer should be used only for pyroxenes having Wo + En + Fs > N%.The effects of pressure are <E'C/kbar; graphs are presented for relations at one atmosphere and at 5, 10, and 15 kbar, along with approximate formulas for interpolating between these pressures. The occunence of granule exsolution-the coalescence of exsolved pyroxene, with possible migration to grain boundaries-can complicate the use of the thermometer for slowly cooled rocks. The primary pyroxene compositions must be reconstructed from textural evidence before correct igneous or peak-metamorphic temperatures can be inferred. The experimental data and inferred temperatures for three-pyroxene assemblages permit calibration of an improved pigeonite thermometer.
African orogeny events. Whereas clinopyroxene, amphibole, titanite and apatite fractionation seem... more African orogeny events. Whereas clinopyroxene, amphibole, titanite and apatite fractionation seems to have affected the nephelinite, nepheline syenite and syenite, carbonatite is affected by fractionation of calcite, dolomite, ankerite, pyroxene, apatite, magnetite, mica, and pyrochlore. Trace elements and Sr-Nd-Pb-CO isotopic compositions of these ARCs strongly suggest a subcontinental lithospheric mantle source, that is enriched either by distribution of subducted crustal material or by metasomatism of mantle-derived fluids, for the generation of ARCs. Despite some isotopic variability that can result from crustal contamination, a trend showing enrichment in 87 Sr/ 86 Sr i (0.702 to 0.708) and depletion in ε Nd(i) (-1.3 to-14.1) over a 2 Gyr duration indicates temporal changes in the lithospheric/ asthenospheric source of ARCs, due to periodic enrichment of the source by mantle-derived fluids. ARC generation starts in an intracontinental rift setting (beginning of Wilson cycle). These early-formed ARCs are carriedto 100 km depths during continental collision (termination stage of Wilson cycle) and undergo extensive
The Southern Granulite Terrane (SGT), the wedge-shaped southern termination of Peninsular India, ... more The Southern Granulite Terrane (SGT), the wedge-shaped southern termination of Peninsular India, is a mosaic of several crustal blocks and intervening collisional sutures/shears which developed through multiple orogenic cycles during Mesoarchean to late Neoproterozoic-Cambrian. The SGT has been the focus of global geoscience world for more than four decades mainly with regard to deep crustal processes, crust-mantle architecture, polyphase structural evolution, extreme crustal metamorphism, growth and recycling of continental crust, and the assembly-evolution-disruption of supercontinents through time, among other aspects. The earliest felsic crust in the SGT includes the 3.5-Ga granitoids in the Coorg Block, followed by successive maturation stages marked by younger magmatic phases in the various blocks. The zircon δ 18 O values from the Coorg Block also fingerprint the emergence of continental crust above sea level in the early Earth. Multiple arc-magmatic pulses along the margins of the different crustal blocks of the SGT during Neoarchean and Neoproterozoic, together with accreted remnants of ophiolites and other oceanic components, suprasubduction zone complexes, and exhumed high-to ultrahigh-temperature metamorphic orogens suggest subduction-accretion-collision tectonics that constructed the various crustal blocks through time, and incorporated them within Precambrian supercontinent assemblies.
A recent petrological study on the lesser-known Sholayar alkaline syenite complex (10°17 0 59.9 0... more A recent petrological study on the lesser-known Sholayar alkaline syenite complex (10°17 0 59.9 00 N: 076°47 0 26.3 00 E) accommodated in the high-grade rocks of the Southern Granulite Terrane reveals the presence of three distinct petrographic types namely leuco syenite, melanofelsic syenite and mela syenite. The syenites, in general, have the following constituent minerals, namely alkali feldspar (Or 76.8 Ab 23.2-Or 93.4 Ab 6.6), plagioclase (An 4.1-An 17.3), calcic amphibole (edenitic hornblende/ferropargasitic hornblende), quad pyroxene (diopside-hedenbergite), biotite and opaque minerals (magnetite and ilmenite). Mode-based statistical studies indicate that a highly significant correlation exists among certain mineralogical parameters including quartz%, total feldspar%, colour index, alteration index and hydration index. Isopleth plots of these parameters strongly suggest maintenance of a magmatic regime throughout, with increasing water content towards the end stage of crystallization. Several geothermobarometric methods point to shallow to moderate level emplacement (*9-23 km) of the alkaline/subalkaline syenite magma which was facilitated by a 'hydration event'. The steep change in water gradients (in localized pockets) of the syenite intrusives has been corroborated by textural evidences.
A recent petrological study on the lesser-known Sholayar alkaline syenite complex (10°17 0 59.9 0... more A recent petrological study on the lesser-known Sholayar alkaline syenite complex (10°17 0 59.9 00 N: 076°47 0 26.3 00 E) accommodated in the high-grade rocks of the Southern Granulite Terrane reveals the presence of three distinct petrographic types namely leuco syenite, melanofelsic syenite and mela syenite. The syenites, in general, have the following constituent minerals, namely alkali feldspar (Or 76.8 Ab 23.2-Or 93.4 Ab 6.6), plagioclase (An 4.1-An 17.3), calcic amphibole (edenitic hornblende/ferropargasitic hornblende), quad pyroxene (diopside-hedenbergite), biotite and opaque minerals (magnetite and ilmenite). Mode-based statistical studies indicate that a highly significant correlation exists among certain mineralogical parameters including quartz%, total feldspar%, colour index, alteration index and hydration index. Isopleth plots of these parameters strongly suggest maintenance of a magmatic regime throughout, with increasing water content towards the end stage of crystallization. Several geothermobarometric methods point to shallow to moderate level emplacement (*9-23 km) of the alkaline/subalkaline syenite magma which was facilitated by a 'hydration event'. The steep change in water gradients (in localized pockets) of the syenite intrusives has been corroborated by textural evidences.
Experimentally determined Ca-Mg-Fe pyroxene phase relations at E00-1200'C and from less than one ... more Experimentally determined Ca-Mg-Fe pyroxene phase relations at E00-1200'C and from less than one atmosphere up to 15 kbar are combined with calculated phase equilibria for the diopside-enstatite and hedenbergite-ferrosilite joins to produce a graphical two-pyroxene thermometer applicable to a wide variety of rocks from the earth, moon, and meteorites. Samples with appreciable contents of "others" components require special projection onto the Di-En-Hd-Fs pyroxene quadrilateral; Wo, En, and Fs components as normally calculated will not yield correct temperatures. The projection approximates the activities of those components in natural pyroxenes but is largely empirical, and may not be appropriate for large contents ofnonquadrilateral components. Therefore, the thermometer should be used only for pyroxenes having Wo + En + Fs > N%.The effects of pressure are <E'C/kbar; graphs are presented for relations at one atmosphere and at 5, 10, and 15 kbar, along with approximate formulas for interpolating between these pressures. The occunence of granule exsolution-the coalescence of exsolved pyroxene, with possible migration to grain boundaries-can complicate the use of the thermometer for slowly cooled rocks. The primary pyroxene compositions must be reconstructed from textural evidence before correct igneous or peak-metamorphic temperatures can be inferred. The experimental data and inferred temperatures for three-pyroxene assemblages permit calibration of an improved pigeonite thermometer.
African orogeny events. Whereas clinopyroxene, amphibole, titanite and apatite fractionation seem... more African orogeny events. Whereas clinopyroxene, amphibole, titanite and apatite fractionation seems to have affected the nephelinite, nepheline syenite and syenite, carbonatite is affected by fractionation of calcite, dolomite, ankerite, pyroxene, apatite, magnetite, mica, and pyrochlore. Trace elements and Sr-Nd-Pb-CO isotopic compositions of these ARCs strongly suggest a subcontinental lithospheric mantle source, that is enriched either by distribution of subducted crustal material or by metasomatism of mantle-derived fluids, for the generation of ARCs. Despite some isotopic variability that can result from crustal contamination, a trend showing enrichment in 87 Sr/ 86 Sr i (0.702 to 0.708) and depletion in ε Nd(i) (-1.3 to-14.1) over a 2 Gyr duration indicates temporal changes in the lithospheric/ asthenospheric source of ARCs, due to periodic enrichment of the source by mantle-derived fluids. ARC generation starts in an intracontinental rift setting (beginning of Wilson cycle). These early-formed ARCs are carriedto 100 km depths during continental collision (termination stage of Wilson cycle) and undergo extensive
The Southern Granulite Terrane (SGT), the wedge-shaped southern termination of Peninsular India, ... more The Southern Granulite Terrane (SGT), the wedge-shaped southern termination of Peninsular India, is a mosaic of several crustal blocks and intervening collisional sutures/shears which developed through multiple orogenic cycles during Mesoarchean to late Neoproterozoic-Cambrian. The SGT has been the focus of global geoscience world for more than four decades mainly with regard to deep crustal processes, crust-mantle architecture, polyphase structural evolution, extreme crustal metamorphism, growth and recycling of continental crust, and the assembly-evolution-disruption of supercontinents through time, among other aspects. The earliest felsic crust in the SGT includes the 3.5-Ga granitoids in the Coorg Block, followed by successive maturation stages marked by younger magmatic phases in the various blocks. The zircon δ 18 O values from the Coorg Block also fingerprint the emergence of continental crust above sea level in the early Earth. Multiple arc-magmatic pulses along the margins of the different crustal blocks of the SGT during Neoarchean and Neoproterozoic, together with accreted remnants of ophiolites and other oceanic components, suprasubduction zone complexes, and exhumed high-to ultrahigh-temperature metamorphic orogens suggest subduction-accretion-collision tectonics that constructed the various crustal blocks through time, and incorporated them within Precambrian supercontinent assemblies.
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