Discovery of abundant pyroclasts in the Namunagarh Grit, South Andaman: evidence for arc volcanism and active subduction during the Palaeogene in the Andaman area

Abstract

The predominant clastic rocks of the late Eocene Namunagarh Grit, South Andaman Island are coarse to fine-grained and pebbly immature greywacke sandstones. These rocks have long been known and interpreted as epiclastic deposits formed from the weathering and erosion of accreted ophiolite in a trench-slope setting. This study documents abundant juvenile vesiculated fragments, pumice clasts, shards, unbroken vesicles, cognate lithics of andesite, broken and whole euhedral fresh plagioclase, and chlorite-replaced glassy matrix in these sandstones. The sandstones evidently have a pyroclastic origin. Based on structures and textures, a coarse-grained and massive, and a finer-grained, bedded and graded sandstone facies are identified. The coarse-grained facies occurs in the lower part of the section, while the finer-grained one, interstratified with mudstone, occurs in the upper part. Major element chemistry indicates a predominantly andesitic composition for these rocks. The facies characteristics indicate that the coarse-grained facies was emplaced as debris flows while the finer-grained facies was transported by turbidity currents. Preservation of intact glassy constituents and their abundance, imply direct sedimentation, or very rapid re-sedimentation, with limited traction and limited time for alteration during transport. These pyroclast-rich sandstones were laid down in a submarine forearc environment on an accretionary complex. The petrology of the juvenile clasts indicates the subaerial or shallow subaqueous explosive eruption of crystal-rich vesicular magma from andesitic arc volcanoes located on an arc massif, inferred to have existed on the western margin of the Burma–Malaya continent during Eocene–Oligocene times. This evidence of arc volcanism indicates active subduction, accretion and emplacement of ophiolite and the associated sediments from the Andaman-Java trench. The pyroclast-rich sandstone represents a distinct and hitherto unrecorded unit in the Tertiary stratigraphy of the Andaman Islands.