Provenance of Xigaze fore-arc basin clastic rocks (Cretaceous, south Tibet)

Abstract

The Xigaze Group of south Tibet was deposited in a fore-arc basin produced by northward subduction of the Neotethys under Eurasia during middle Cretaceous time. Clasts of andesite from the Gangdese magmatic belt to the north and limestone clasts from a shelf rimming the basin to the north are the near exclusive detrital components in the Xigaze Group in the eastern part of the studied region and in older (Albian) deposits in the west. Younger (Cenomanian–Coniacian) deposits from western and central parts of the Xigaze Group show increasing amounts of tonalite pebbles in conglomerates, and quartz sand. These trends reflect differential arc uplift and erosional dissection, with higher rates in the west. Some arenites show evidence for sediment contribution from an additional source, probably from an orogen to the north of the Lhasa Block. The presence of this orogen indicates that crustal thickening of Tibet is not restricted to Cenozoic times. The middle Cretaceous Gangdese Belt apparently lacked acid volcanic rocks. This suggests a low degree of crustal contamination and/or differentiation of primitive magma, probably because the crust beneath the arc area was thin. A thin crust is in accordance with a low elevation of the arc, deduced from continuous carbonate production on and marine excursions beyond the arc. Magma eruption rates of the arc, estimated from the Xigaze basin sediment volume, were probably low (≈7.5 km 3 /km arc segment per million years), but within the range of modern arcs. The varying sediment compositions of the Xigaze Group may be typical of fore-arc basin deposits in front of continental magmatic arcs. The lack of a correlation of trends in sediment composition with fining-upward sedimentary supercycles in the Xigaze Group is taken as evidence in favor of a eustatic cycle control.