Integrated provenance analysis of a complex orogenic terrane: Mesozoic uplift of the Bogda Shan and inception of the Turpan-Hami Basin, NW China

摘要

We employ petrographic and advanced geochemical techniques to better document the evolution of the Turpan-Hami basin based on the unique geologic histories of the are terranes that served as potential sources of Turpan-Hami deposits. First, a provenance study of Permian through Cretaceous sandstone of the Turpan-Hami basin reveals temporal and spatial changes in dominant source terranes that provided detritus to the basin. Volcanic-lithic-rich Upper Permian sandstone (mean Qm(19)F(18)Lt(63); Qp(7)Lvm(89)Lsm(4); QM(48)P(39)K(13)) followed by more quartzose compositions in Triassic sandstone (mean Qm(41)F(19)Lt(40;) Qp(20)Lvm(75)Lsm(5); Qm(68)P(21)K(11)) indicate progressive unroofing of the extinct northern and central Tian Shan arc terranes to the south of Turpan-Hami. A sharp change to sedimentary-lithic-rich Lower Jurassic sandstone (mean Qm(47)F(16)Lt(37); Qp(16)Lvm(42)Lsm(42); Qm(75)P(12)K(13)) overlain by a return to volcanic-lithic-rich Middle Jurassic sandstone (mean Qm(39)F(21)Lt(40); Qp(14)Lvm(51)Lsm(35); Qm(65)P(21)K(14)) points to the initial uplift and unroofing of the largely andesitic Bogda Shan to the north, which first shed its sedimentary cover as it emerged to become the partition between the Turpan-Hami and southern Junggar basins.Second, geochronological, trace-element, and Sm-Nd isotopic variations among granitoids in the late Paleozoic Tian Shan orogenic belt provide a further test of Mesozoic uplift of the Bogda Shan. On the basis of previous models of crustal compositions throughout the South, Central, and North Tian Shan, Bogda Shan, and East and West Junggar terranes, we infer that isotopically enriched granitic cobbles (average epsilonNd(i) = -0.50, n = 6) contained in Lower Triassic deposits in the north-central Turpan-Hami basin were derived from the continental crustal Central Tian Shan terrane, south of Turpan-Hami, and not from the more oceanic North Tian Shan, Bogda Shan, and East and West Junggar terranes, north of the Turpan-Hami basin. We therefore infer that the ancestral Bogda Shan had not been uplifted by the Early Triassic, and that prior to this time, a unified Junggar-Turpan-Hami basin existed during Late Permian deposition of extensive lacustrine deposits.