ABSTRACT

A thick sequence of bedded tuff alternated with non-volcanogenic turbidites is present in the Archipelago Group of rocks (Mio-Pliocene age) of the Andaman Islands. The tuff occurs in three different facies: a) Facies A, white massive tuff with ill defined bedding contacts; b) Facies B, dominantly green and white tuff with ill defined turbidite T_abc, T_ab Bouma sequence; and c) Facies C, well defined ash turbidites of T_abc, T_ab, T_abc, T_acde sequence. Dominance of cuspate shards with no pervasive alteration and absence of blocky shards are features of subaerial eruption for the pyroclastic rocks. Absence of glass welding and plastic deformation together with the presence of good sorting and recurrence of ash turbidites indicate that subaerially-erupted ash landed in water and behaved as a cold subaqueous flow. Field features show change of flow character from subaqueous debris flow (for Facies A) to recurrent high to low concentration turbidity current (for Facies B), and ultimately to low particle concentration turbidity current (for Facies C). All the tuff varieties are vitric to crystovitric in character, and contain broken crystals of quartz, plagioclase, mica, and glass shards of delicate shape (crescent, cuspate, curved) without any welding features. The tuffs do not contain any lithic fragments of volcanic rocks. Petrography as well as XRD studies show that glass alteration is common in Facies A and is least in green tuff of Facies B. Alteration of glass to clinoptilolite and analcime has been linked to burial diagenesis. The similarity of chemical composition of glass shards (mainly dacite) and bulk rocks, however, indicates negligible chemical change during diagenesis. The origin of Andaman tuff in a convergent margin tectonic setting is assumed as the basis of high Zr/Nb and Zr/Y.