Plagiogranites derived from high-Mg Andesitic magmas: An example from the Andaman Ophiolite

Plagiogranites are the felsic plutonic rocks occurring amidst a suite of predominantly mafic and ultramafic rocks. Their occurrence ranges from newly formed oceanic crust to Archean ophiolites, and they are usually associated with the crustal section, i.e., gabbros and sheeted dykes. Sometimes, they have been observed in the mantle sections as well. The Andaman ophiolite (AO) is a dismembered ophiolite suite located on the forearc of the Andaman subduction zone, where the Indian plate obliquely subducts beneath the Burma microplate. Plagiogranites of the AO are found to be intruding into gabbros and serpentinized mantle peridotites. They have been dated to 98-93Ma, and are contemporaneous with the other rocks of the ophiolite. Earlier studies propose that these have been generated by crystal fractionation or an immiscible separation from a parental basaltic magma. In this study, we utilize new whole-rock geochemical data and Sr-Nd isotopic ratios of these rocks to constrain their petrogenesis. Geochemically, these rocks are classified as diorites to tonalites-trondhjemites, characterized by plagioclase+amphibole+quartz assemblage. Petrographic observations reveal that euhedral plagioclase and amphiboles were the early crystallizing phases, while anhedral quartz crystallized later in the sequence. The plagiogranites exhibit LREE-enriched patterns on chondrite-normalized plots and negative Nb-Ta and Zr-Hf anomalies on primitive mantle-normalized plots, suggesting derivation from a metasomatized source. Sr-Nd isotopic compositions strongly overlap with other rocks of the ophiolite suite, pointing to a common mantle parentage. Low TiO₂ contents, overlapping trace element patterns with the mafic rocks of the AO, and REE-SiO₂ systematics negate the possibility of plagiogranite formation by fractional crystallization from a basaltic magma. The occurrence of amphiboles in the plagiogranites suggests that the parent magma was hydrous, implying that liquid immiscibility was not the genetic mechanism. Therefore, we explore the possibility that they are crystallized products of a high-magnesian andesitic magma (HMA) derived by the partial melting of a metasomatized mantle source at low pressure, followed by fractional crystallization of plagioclase±amphibole, to explain their genesis and the observed compositional variation. We demonstrate, using the results of alphaMELTS simulations, that compositional variation and the mineral assemblages observed in the plagiogranites of the AO can be explained by this model and suggest that derivation from HMAs is a viable mechanism for the genesis of plagiogranites in similar settings. We propose that the plagiogranites of AO have formed during the initiation of an intra-oceanic subduction, which can explain their geochemical features and geochronological results.