Origin of breccia in mud volcanoes of the Andaman accretionary prism: Implications for forearc processes

Abstract

Mud volcanoes at convergent margins are important pathways through which clay minerals and fluids, collectively known as mud breccia, originating from deep within forearcs, are ejected at the surface, opening an important window to shallow level processes in subduction zones. Although the mud breccia is the only detachable part of a subducting slab at shallow depths, its origin and implications for the chemistry of crustal material getting recycled into the mantle remain largely unknown. To understand the chemical transformation of slabs within forearcs and the chemistry of recycled components in accretionary subduction zones, we carried out a detailed mineralogical, geochemical and Sr-Nd-Pb isotopic study of matrix of the mud breccia ejected at the mud volcanoes of the Andaman accretionary prism, located at the Indian Plate-Burma Plate convergent margin. Our current and an earlier study on these mud volcanoes reveal that the clay-quartz rich mud matrix, and accompanying water from mineral dehydration and thermogenic hydrocarbons are extracted from depths more than 6 km in the forearc, from tectonic mélanges located at the base of the accretionary prism, in the décollement zone. The mud breccia ascends rapidly through the heavily deformed accretionary wedge and gets emplaced into the overlying soil horizon, incorporating rock clasts from the geological formations of the wedge and minor organic matter from the soil. Major-Trace element contents and isotopic ratios of mud matrix indicate that they contain materials derived from both the altered oceanic crust and terrigenous sediment of the slab, with the former contributing the most (> 80%). All data point to the scenario that only a small fraction of the terrigenous sediment of the slab gets recycled into the mantle at the Andaman subduction zone, which in turn gets reflected in the relatively pristine isotopic compositions of the arc lavas erupted at the Barren Island volcano.