Geochemical Characteristics and Genesis of Oceanic Plagiogranites Associated with South Andaman Ophiolite Suite, India: A Late Stage Silicate Liquid Immiscible Product

Plagiogranite is a leucocratic low-K member of the Andaman Ophiolite Suite (AOS) that occurs as bands (dykes?) of irregular shapes and sizes, particularly in the cumulate sequence of the suite. It is medium- to coarse-grained and commonly shows hypidiomorphic granular texture. At places granophyric texture (intergrowth between feldspar and quartz) is also seen. Thc main mineral constituents are plagioclasc (mainly albite), quartz and minor mafic minerals. Mineralogically and chemically these rocks are classified as low-K dacite (trondhjemite/tonaIite type). Low Rb concentrations (< 5) and low RblSr ratios (<0.01 5), observed in studied plagiogranites and associated Fe-Ti enriched rock, are characteristic of oceanic derivatives. All elements, plotted on the primordial mantle normalized multi-element spidergrams show enriched patterns. Variations in the concentrations of Nb, Sr, Zr, and Y arc limited but other elements show wide variations. Strong negative anomalies for Nb are also observed. These rocks show characters analogous to those of orogenic granites, emplaced in volcanic arc settings. On the basis of observed petrological and gcochernical characteristics and their spatial association with Fe-Ti enriched mafic rocks, it is concluded that the Andaman plagiogranites are the integral part of AOS having crystallized from late stage immiscible silicate liquid.