Role of fractional crystallization and partial melting in the generation of intermediate and acidic magmas of the Andaman Island Ophiolite suite of India. Issue 2 (24th November 2021) - Saikia, Ashima
Abstract : 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 Ab98‐99. The acidic rocks show compositional variation from albite to oligoclase (Ab83–99 ). Analysed pyroxenes of intermediate and acidic rocks are augite (Wo46–49 En34–41 Fs13–17 ). The studied calc‐alkaline 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 H2 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. Abstract : The intermediate and acidic volcanic rocks of south Andaman Island Ophiolite suite, SE India, show contrasting generation processes in a supra‐subduction zone. Petrological modelling indicates that the intermediate volcanic rocks were likely derived by fractional crystallization of mafic parental magma and partial melting of hydrothermally altered oceanic crust resulted in the acidic rocks.