Temporal variations of mesozoic sandstone compositions in the Qiangtang block, northern Tibet (China): Implications for provenance and tectonic setting

One hundred and forty-four and fifty-two Mesozoic sandstone samples from the Qiangtang block (ca. 32 degrees 20'-35 degrees N, 85 degrees-93 degrees E; northern Tibet, China) were conducted for analysis of detrital modes and heavy-mineral spectra, respectively, in an attempt to evaluate their provenance and the type of tectonic setting in which they were deposited. The Lower-Middle Triassic sandstones in central Qiangtang are characterized by complex derivations from both recycled orogen and continental block provenances. Most of the Upper Triassic and Jurassic sandstones are dominated by a recycled-orogen origin. However, sandstones derived from a continental-block source also occur in Upper Triassic strata in the southern margin of southern Qiangtang, and in Lower Jurassic strata in the southern margin of northern Qiangtang. Although the heavy-mineral spectra of the Upper Triassic and Jurassic sandstones are occupied mostly by the stable heavy minerals zircon, tourmaline, and rutile (ZTR), most of the samples are generally accompanied by unstable heavy minerals, such as apatite, chlorite, chrome spinel, amphibole, epidote, garnet, and pyroxene, except for those from Upper Triassic strata in the southern margin of southern Qiangtang and from Lower Jurassic strata in the southern margin of northern Qiangtang. The detrital modes and heavy-mineral spectra of the Mesozoic sandstones, along with the lithologic evidence, indicate that the central Qiangtang anticlinorium must have been elevated and eroding due to orogenesis and most likely has contained rolling topographic relief at least since Early Triassic time. It could have contained abundant ophiolitic fragments as evidenced by rich chrome spinel, amphibole, and other readily decomposed ferromagnesian minerals across the entire anticlinorium. These mafic minerals may point to the presence of the Shuanghu suture along the entire central Qiangtang anticlinorium. The blueschist-bearing metamorphic complexes in central Qiangtang could be accommodated in a large-scale enclave in an irregular central Qiangtang continental margin. The Upper Triassic-Jurassic recycled-orogen-derived sandstones in northeastern Qiangtang suggest continued orogenesis due to the suturing between Qiangtang and Asia. Such a contractional background does not support models that relate a tensile mechanism for back-arc spreading along the Bangong-Nujiang belt.