Mesozoic Sandstone Composition in the Qiangtang Block, Tibet: Implications for Tectonic Setting

Key words: 
• sandstone /
• Mesozoic /
• Qiangtang /
• Tibetan plateau /
• Tethys

Abstract: Onehundredandfiftyfour Mesozoic sandstone samples from the Qiangtang block (ca. 3３°~35°N, 85°~93°E) were conducted for analysis of detrital modes, in an attempt to evaluate their provenance and the type of tectonic setting in which they were deposited. The LowerMiddle Triassic sandstones (Q61±8F27±13L12±10, Qm59±8F28±13Lt13±9, Qp28±22Lvm72±22Lsm0±0) in central Qiangtang are characterized by complex derivations from both recycled orogen and continental block provenances. The most of the Upper Triassic (Q78±12F7±7L15±11, Qm77±12F7±7Lt16±11, Qp16±11Lvm30±35Lsm54±33), Lower Jurassic (Q83±20F2±2L15±22, Qm79±23F2±2Lt19±24, Qp35±19Lvm0±0 Lsm65±19), Middle Jurassic (Q69±22F6±6L24±25, Qm67±22F6±6Lt26±24, Qp14±22Lvm32±38Lsm54±38), and Upper Jurassic (Q80±17F6±4L14±17, Qm78±17F6±4Lt16±16, Qp17±23Lvm27±35Lsm56±34) sandstones are dominated by recycled orogen origin. However, the sandstones that were derived from continental block source also exist in the southern margin of southern Qiangtang and in the Lower Jurassic strata in the southern margin of northern Qiangtang. It could have contained abundant ophiolitic fragments and other readilydecomposed ferromagnesian minerals across the entire anticlinorium. These mafic minerals point to the presence of the Shuanghu suture along the entire central Qiangtang anticlinorium and the blueschistbearing metamorphic complexes in central Qiangtang could be accommodated in a largescale enclave in an irregular central Qiangtang continental margin. The distribution of Mesozoic recycled orogenderived sandstones on Qiangtang indicates a contraction background, supporting〖KG*2〗a tensile mechanism for backarc spreading along the BangongNujiang belt. 