A detrital heavy mineral viewpoint on sediment provenance and tropical weathering in SE Asia

Abstract

Understanding heavy mineral preservation is important for interpreting generation, pathways, provenance and geochemistry of sediments. Despite this, many assumptions about heavy mineral stability are based on ancient strata and few studies consider modern sediments, particularly those in tectonically active tropical areas such as SE Asia. We report new heavy mineral data on 69 river sand samples from the Malay Peninsula and Sumatra, in which one aim was to find provenance indicators specific to these areas. Identifications were performed using optical microscopy and confirmed with SEM-EDS. In the Malay Peninsula heavy minerals record granitic and contact metamorphic provenance. Variable amounts of zircon, tourmaline, hornblende, andalusite, epidote, monazite, rutile and titanite, and minor amounts of pyroxene, apatite, anatase, garnet, diaspore, colourless spinel, cassiterite and allanite are typical of this source area. The composition of assemblages from Sumatra indicates contributions from two major sources: the modern volcanic arc (I) and the basement (II). Abundant pyroxene, particularly hypersthene (up to 70%), is diagnostic of the volcanic arc source. Vesuvianite, garnet, andalusite, tourmaline, chrome spinel, rutile, anatase and corundum, are present only in small amounts (<3%), and are interpreted as recycled from the basement. Zircon, apatite, hornblende, epidote, and olivine are also common in Sumatra and are likely to have a mixed provenance. Abundance of ferromagnesian silicate minerals suggests mild weathering, possibly reflecting several processes: dilution of natural etching fluids by heavy rainfall, high erosion rates, rapid transport and short grain residence time in the river. The heavy mineral assemblages of modern rivers are very different from those recorded by the few previous studies of Cenozoic sediments of the Malay Peninsula and Sumatra. Assemblages in the Cenozoic basins are significantly more mature than those of modern rivers. The differences cannot be explained simply by dissolution of susceptible minerals during one sedimentary cycle and instead imply rapid source area unroofing.