The Proterozoic Chotanagpur Granite Gneiss Complex (CGGC) at the northern boundary of the Central Indian Tectonic Zone (CITZ) of the eastern Indian shield preserves relics of fossilized oceanic back-arc crust. We describe the field, petrographical and geochemical characteristics of the mafic rocks comprising pillow basalts and dolerites from the Bathani area of the northern fringe of the CGGC, eastern India. The basalts consist of plagioclase feldspar, hornblende, opaque minerals (Fe–Ti oxide) and chlorite, and the dolerite consists of plagioclase, hornblende and opaque minerals. Our data indicate that the Bathani mafic rocks have tholeiitic to transitional composition and are overprinted by greenschist facies metamorphic conditions; however, REE and fluid immobile elements preserve their primary geochemical signatures. The (La/Sm)N ratios (1.38–2.15) and chondrite-normalized REE patterns point to an enriched mid-ocean ridge basalt (E-MORB) mantle source. Geochemical characteristics indicate a mixed signature of MORB and arc tholeiite with enrichment of Ba, Th, Eu and Sr, similar to that of back-arc supra-subduction zone ophiolites. These mafic rocks are the product of MORB-like magma derived from a depleted mantle corresponding to < 2% partial melting of spinel lherzolite, enriched by subduction-induced slab metasomatism and melting. The Bathani mafic rocks are representative of the upper part of a supra-subduction zone columnar ophiolite section, which was emplaced onto the present-day northern margin of the CGGC during suturing of the northern and southern Indian block at c. 1.9 Ga during the Nuna amalgamation.
The Mesozoic-age Andaman Island Ophiolite located in the southeastern fringe of the Indian plate margin is one of the well-preserved supra-subduction-zone oceanic lithospheric slices. It is tectonically associated with the Indonesian arc system and is an extension of the Phanerozoic Indonesian-Myanmar orogenic belt. Thus, it offers a unique opportunity to study oceanic crust formation and specify magma evolution processes in the Tethyan subduction system. The volcanics of the Andaman Islands ophiolite display a wide compositional variation. Rock types vary from basic (basalts) to intermediate (basaltic-trachyandesites, basaltic andesites) to acidic differentiates like dacites and rhyolites. This contribution presents the petrochemistry of the intermediate rock types-basaltic-trachyandesites, trachyandesites and acidic-dacites and rhyodacites of Andaman Island Ophiolite (India). This study examines the role of fractional crystallization and partial melting in the generation of intermediate and acidic magmas of the Andaman Ophiolite suite. Plagioclases and pyroxenes are the major mineral constituents of these rocks on petrographic observation. Plagioclases of the intermediate rocks are of albite composition Ab 98-99. The acidic rocks show compositional variation from albite to oligoclase (Ab 83-99 ). Analysed pyroxenes of intermediate and acidic rocks are augite (Wo 46-49 En 34-41 Fs 13-17 ) . The studied calcalkaline rocks show variable abundance of Large-Ion Lithophile Elements and display strong positive Pb and negative Ti anomalies. The chondrite-normalized Rare Earth Elements display an enriched pattern with varying degrees of enrichment. Clinopyroxene thermobarometry reveals that the intermediate magma and acidic magma becomes saturated with clinopyroxene at 1,062-1,133 C and 1,016-1,079 C, respectively, at 2 kbar and H 2 O = 2 wt%. Petrochemical findings and model calculations using PELE are consistent with a derivation of intermediate rocks from the basaltic melt by fractional crystallization. In contrast, acidic rocks are the product of partial melting of hydrothermally altered oceanic crust. The association of basaltic and intermediate rocks suggests a relatively stable magma chamber to generate differentiated intermediate magmas by fractional crystallization.
Petrological, mineral chemical and geochemical characteristics are reported for the basalts of South Andaman ophiolite suite (India) of Cretaceous age to infer its petrogenesis, magma source characteristics and tectonic setting. In the field, both massive basalts and basalts with wellpreserved pillow morphology are present. The basalts consist of plagioclase, clinopyroxene, Fe-Ti oxide and Cr-spinel. They are tholeiitic in composition with low concentrations of Na 2 O + K 2 O (3 -5 wt.%). The representative samples of these basalts have a nearly flat chondrite-normalized REE pattern [(La/Yb) N = 0.29 -2.27)] with minor Eu anomaly (Eu/Eu* = 0.90 -1). Our findings suggest that the arc-like massive basalts were generated before the back-arc rifting initiated in an earlier stage of back-arc extension as a result of eastward subduction of the Neotethyan oceanic slab. The MORB-like pillow basalts were erupted due to further back-arc spreading in response to the roll-back of the Neotethyan oceanic lithospheric slab. The pillow and massive basalts assemblage in the South Andaman ophiolite records the evolution of a back-arc basin from initial rifting to subsequent spreading, documenting how basaltic magmatism evolves during the formation of the back-arc basin in a suprasubduction zone setting.
The Central Indian Tectonic Zone (CITZ) is a Proterozoic suture along which the Northern and Southern Indian Blocks are inferred to have amalgamated, forming the Greater Indian Landmass. The Chotanagpur Granite Gneiss Complex (CGGC) represents the eastern extension of the CITZ and exposes several granite plutons. Enclaves of diverse origin are present as minor constituents within these granitic bodies. This study reports new major and trace element data for anorthosite enclaves hosted within granites in the Pattharkatti and Rajgir area from the northern margin of CGGC to get modern insights into the petrogenesis of anorthosites. Anorthosite enclaves show sharp contact with the host granite. They contain cumulus plagioclase (An87–94), intercumulus amphibole (magnesiohornblende and ferrotschermakite), and biotite (Mg-biotite and phlogopite) along with minor iron oxides. Amphibole crystallization pressure and temperature are constrained between 0.5 and 6.4 kbar and 653–780°C for the anorthosites. The studied anorthosites display a very gradual and steady increase in whole-rock rare earth element (REE) contents from Lu to La [(La/Yb)N = 1.22–13.08]. They also show a sharp decline in Fe2O3(t) and MgO, whereas Al2O3 increases with increasing silica contents from 45.69 to 51.16 wt%. In the chondrite normalized REE diagram, plagioclase exhibits LREE enriched patterns with strong positive Eu anomaly. The composition of parental liquid for anorthosite from the study area was estimated by adopting the equilibrium distribution method. Parental melt curves from Sm to La are near parallel and constrained broadly between trapped melt fractions (TMF) = ∼5%–15%. Anorthosites of the study area may have formed from the plagioclase-saturated basaltic melt.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.