In this study we present field relations, petrology, whole-rock major, trace and rare earth element geochemistry, zircon U-Pb ages, whole-rock Sr and Nd isotopes, and in situ zircon Hf and O isotopes from the Karwar block, western peninsular India. The rocks consist predominantly of tonalite-trondhjemite-granodiorite (TTG), granite and amphibolite. The felsic rocks are grouped into three: 1. TTG-I characterised by low K2O, high Na2O and Al2O3, low Sr/Y and La/Yb ratios, slightly enriched HREEs, negative Sr, Eu and Ti anomalies, a 3.2 Ga crystallisation age, and 3.60 Ga and 3.47 Ga inherited zircons; 2. TTG-II with lower SiO2, higher Sr/Y and La/Yb ratios, stronger REE fractionation with no HREE enrichment, negative Nb and Ta anomalies, a 3.2 Ga crystallisation age, but no inheritance; 3. Granites with high SiO2 and K2O, low Na2O and Al2O3, very low Sr/Y and La/Yb ratios, weak REE fractionation with enriched REEs, negative Sr, Eu and Ti anomalies and a 2.94 Ga crystallisation age. The TTG-I formed from a mantle source, but with a significant component of older crustal material, whereas the TTG-II originated mostly from a mantle-derived juvenile magma. The granite evolved from an enriched source containing a relatively large amount of older crustal material. The precursors of TTG-I and -II are similar to mid-ocean ridge basalts (MORB), whereas the granites are similar to volcanic arc/within-plate sources and the amphibolites are remnants of gabbros/basalts. An initial 3.6 Ga crust likely formed by the underplating of an accreted oceanic plateau-like or island arc-like crust. TTG-I was produced by subduction and slab melting at a moderate depth, induced melting of mafic lower crust and older upper crust at 3.2 Ga. TTG-II formed at 3.2 Ga by subduction and with a higher degree of slab melting at a greater depth than TTG-1, together with more effective mixing with mantle peridotite, followed by intrusion and induced melting of mafic lower crust. Basaltic magmatism at 3.0 Ga and subsequent metamorphism to amphibolite resulted in extensive and thicker crust. Assimilation and melting of TTG crust at a shallow depth during the emplacement of a mantle-derived magma produced the 2.94 Ga granites. The presence of inherited zircons, combined with whole-rock major and trace elements, Nd isotopes and in situ zircon Hf and O isotopes, indicates that older crustal material was incorporated into the source magma of TTG-I and that the Karwar block originally contained 3.60 to 3.47 Ga crust that was subsequently reworked during the Paleo- and Mesoarchean.
Abstract The Ranomena ultramafic complex in NE Madagascar consists of layered gabbro, harzburgite, orthopyroxenite, clinopyroxenite, garnet websterite and chromitite-layered peridotite. This study of the Ranomena chromite chemistry aims to better understand the petrogenesis and palaeotectonic environment of the complex. The chromite from the Ranomena chromitite is unzoned/weakly zoned and has a Cr# (Cr/(Cr + Al)) of 0.59–0.69, a Mg# (Mg/(Fe + Mg)) of 0.37–0.44, and low Al 2 O 3 (15–23 wt %) suggesting derivation from a supra-subduction zone arc setting. Calculation of parental melt composition suggests that the parental magma composition of the Ranomena chromitite was similar to that of a primitive tholeiitic basalt formed at a high degree of mantle melting, suggesting the parental melt composition was equivalent to that of an island-arc tholeiite (IAT). The parental magma of the Ranomena chromite had a FeO/MgO ratio of 0.9 to 1.8, suggesting arc derivation. The parental magma was Al- and Fe-rich, similar to a tholeiitic basaltic magma. The composition of orthopyroxene from the chromitite indicates a crystallization temperature range of 1250–1300°C at 1.0 GPa. The chemistry of the chromite in the Ranomena chromitite further suggests that the complex formed in a supra-subduction zone arc tectonic setting.
The Southern Granulite Terrane (SGT) in India is composed of Archaean to Proterozoic crustal blocks and intervening shear/suture zones. The Nallamalai shear zone lies along the eastern flank of the Shevaroy Block. This study is focused on the eastern part of this shear zone where the basement rocks are dominantly composed of charnockite, quartzo-feldspathic gneiss and metagabbro, all of which preserve original magmatic texture. Zircon U-Pb LA-ICPMS analysis on the charnockite samples yielded an age of 2680 to 2500 Ma for the protolith emplacement followed by metamorphism at 2520 to 2450 Ma. Magmatic zircon grains in the quartzo-feldspathic gneiss and metagabbro yielded weighted mean ages of ca. 2557 Ma and ca. 2583 Ma with metamorphism at ca. 2518 Ma. Zircon Lu-Hf data suggest that the protolith of metagabbro was sourced mainly from the recycled Mesoarchaean crustal component. The ƐHf(t) of charnockite (−0.4 to 4.1) and quartzo-feldspathic gneiss (0.5 to 6.0) represent a combined source of the mantle as well as recycling crust. All the rocks underwent amphibolite to granulite facies metamorphism and petrological studies and pseudosection modelling shows a similar P-T range of 7–8 kbar and 650–830°C. Results from the present study suggest that crustal evolution along the eastern flank of the Nallamalai Shear Zone involved melting of older crustal components (Mesoarchaean age) with significant juvenile input (ƐHf(t) = ~+6.0) within a continental arc setting.
Earth and Space Science Open Archive PosterOpen AccessYou are viewing the latest version by default [v1]Negative Ce Anomaly in the Banded Iron Formation and Associated Clastic Rocks of the Sirsi Shelf Region, Southern India: Inferences on the Fluid-Rock Alteration Event during the Pan-African OrogenyAuthorsPallabiBasuiDIshwar-KumarCSajeevKrishnanRamanandaChakrabartiSee all authors Pallabi BasuiDCorresponding Author• Submitting AuthorIndian Institute of Science BangaloreiDhttps://orcid.org/0000-0002-5094-5470view email addressThe email was not providedcopy email addressIshwar-Kumar CIndian Institute of Technology Kanpurview email addressThe email was not providedcopy email addressSajeev KrishnanIndian Institute of Science Bangaloreview email addressThe email was not providedcopy email addressRamananda ChakrabartiIndian Institute of Science Bangaloreview email addressThe email was not providedcopy email address
Abstract Systematic changes in whole-rock chemistry, mineralogy, mineral textures, and mineral chemistry are seen along a ca. 95-km traverse of late Archean granitoid orthogneisses in the Shevaroy Block, Eastern Dharwar Craton, southern India. The traverse passes from amphibolite-grade gneisses in the north to granulite-grade rocks (charnockite) in the south. Changes include whole-rock depletion of Rb, Cs, Th, and U in the granulite grade rocks as relative to the amphibolite grade gneisses, and oxidation trends regionally from highly oxidised granulite-facies rocks near the magnetite–haematite buffer to relatively reduced amphibolite-facies rocks below the fayalite-magnetite-quartz. Rare earth elements show limited mobility and are hosted a variety of minerals whose presence is dependent on the metamorphic grade ranging from titanite and allanite in the amphibolite-facies rocks to monazite in the vicinity of the orthopyroxene-in isograd to apatite in the granulite-grade charnockite. Cathodoluminescence and back-scattered electron sub-grain imaging and sensitive high-resolution ion microprobe analysis of zircon from 29 samples of dioritic, tonalitic, and granitic orthogneiss from the traverse reveals magmatic zircon cores that record the emplacement of the granitoid protoliths mostly about 2580 to 2550 Ma, along with a few older mid to late Archean tonalites. Protolith zircon was modified during metamorphism by overgrowth and/or replacement. Relative to igneous cores, U-enriched metamorphic zircon, dominant in the amphibolite-grade gneisses, formed at ca. 2530 Ma, predating retrograde titanite growth at ca. 2500 Ma. Uranium-depleted mantles grew on zircon between 2530 and 2500 Ma in granulite-grade samples south of the orthopyroxene-in isograd. In some of these samples, the U-depleted metamorphic zircon is preceded by mantles of U-undepleted zircon, indicating a progression of metamorphic zircon growth with increasingly depleted compositions between 2530 and 2500 Ma. With increasing metamorphic grade (from amphibolite to granulite) and oxidation state, allanite and monazite disappear from the assemblage and zircon became depleted in U and Th. Whole-rock U-Th compositions became decoupled from relict magmatic zircon compositions, reflecting the development of U-depleted metamorphic zircon and indicating that whole-rock chemical differences along the traverse were produced during metamorphism, rather than just reflecting differences in dioritic vs granitic protoliths. Although in situ anatexis and melt extraction may have played a role, whole-rock and zircon depletion of trace elements can be explained by the action of externally derived, oxidising, low-H2O activity hypersaline fluids migrating up through the mid to lower crust. Fluids and element migration during metamorphism may be the end result of subduction related processes that cumulated in the collision and concatenation of island arcs and continental blocks. These tectonic processes assembled the Dharwar Craton at the end of the Archean.
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