Sodic amphibole zoning as a petrologic indicator of fenitisation in trachyandesites of the Lichi volcanics, Eastern Himalaya

Abstract

The vestiges of the Kerguelen plume-related magmatism that affected a large part of Eastern India during the Gondwana breakup are also preserved in the Eastern Himalaya. The Lichi volcanics, which are a consequence of the Kerguelen mantle plume activity, are exemplified by basalts, trachyandesites, and trachytes occurring in association with the Gondwana sediments of the Eastern Himalaya. Amphiboles are present in both the studied trachyandesites and trachytes; however, in the trachyandesites, the amphibole compositions range from sodic-calcic (richterite and katophorite) to sodic varieties (riebeckite and arfvedsonite). Sodic amphibole occurs as reaction rims that envelop the pre-existing sodic-calcic amphibole grains. The trachytes are sanidine-bearing, comprising numerous sanidine phenocrysts and are vesicular in nature. Geochemical characteristics of the studied rocks point towards the role of fractional crystallisation for the generation of trachyandesites and trachytes. The presence of calcite and zoned amphiboles along with high alkali contents (Na2O+K2O) and high Ba, Sr values indicate the post-magmatic alteration of trachyandesites by fenitising fluids. Hydrothermal alteration in alkaline complexes is driven by the process of fenitisation. Fenitising solutions enriched in Na, Ca, and volatiles (mainly CO2) have led to the replacement of sodic-calcic amphibole by sodic amphibole along with the crystallisation of calcite in the trachyandesites. The study attempts to understand the magma chamber processes responsible for the development of zoning characteristics in amphibole, the role of fenitisation, and the possible source of these fenitising solutions.