Collision and reactivation along the South Tianshan Orogen (NW China) through late Paleozoic to Mesozoic time

<p>The South Tianshan Orogenic Belt in NW China marks the suturing site between the Tarim Craton and the Central Asian Orogenic Belt (CAOB) during late Paleozoic-Mesozoic time. Despite numerous investigations, the amalgamation history along the South Tianshan Orogen remains controversial, especially on the timing and process of the final continental collision between the Tarim Craton and the Central Tianshan (CTS)-Yili Block. We inquire into this issue on the basis of a compiled dataset across the Tarim, South Tianshan and CTS-Yili regions, comprising elemental and isotopic data of magmatic rocks and radiometric ages of regional magmatism, detrital zircons, (ultra-)high pressure metamorphism and tectonothermal events. The data support a continental collision along the South Tianshan belt in 310-300 Ma, in accord with a contemporaneous magmatic quiescence and a prominent decrease of εNd(t) and εHf(t) values of magmatic rocks in the CTS region, and a main exhumation stage of (U)HP rocks in the South Tianshan region. The collisional orogeny along the South Tianshan have most likely been influenced by a mantle plume initiated at ca. 300 Ma underneath the northern Tarim Craton, as evidenced by temporal and spatial variations of geochemical proxies tracing magma source characteristics. The new model of plume-modified collision orogeny reconciles the absence of continental-type (U)HP rocks in the orogen and the insignificant upper-plate uplift during continental collision. In the mid-Triassic (ca. 240 Ma), the Chinese western Tianshan underwent intense surface uplift and denudation, as indicated by sedimentary provenance analysis and tectonothermal events. Paleocurrent and detrital zircon age data from Triassic strata in northern Tarim suggest a provenance change from a single source of the Tarim Craton to multiple sources including the CTS-Yili Block to the north and the Western Kunlun Orogen to the south. We suggest that the mid-Triassic uplifting in Chinese western Tianshan was an intracontinental orogeny caused by far-field effects of the collision between the Tarim Craton and the Qiangtang Block. This research was financially supported by NSFC Projects (41730213, 42072264, 41902229, 41972237) and Hong Kong RGC GRF (17307918).</p>

Neoproterozoic I-type granites geochronology and geochemistry of the Chinese Central Tianshan Block

<p>The Central Asian Orogenic Belt (CAOB), also known as the Altay orogenic belt, is the largest accretionary orogenic belt in the world. It is situated between the Eastern European, Siberian, Tarim, and North China cratons. The CAOB is a large and complex suture zone formed by amalgamation of diverse geologic units including several microcontinents, ophiolites, island arcs, seamounts and accretionary wedges. The evolution of the Precambrian basement in these microcontinents is central to understanding the accretionary and collisional tectonics of the CAOB as well as the evolution of Rodinia supercontinent. The Tianshan block, an important part of the CAOB, is located in the southwestern CAOB, and subdivided from north to south into North Tianshan, Central Tianshan-Yili blocks, and South Tianshan. The Central Tianshan block, located between the Tarim block, the Junggar block and the Kazakhstan block, is one of numerous microcontinental block within the CAOB that overlie Precambrian basement rocks. Constraining the evolution of these ancient basement rocks is central to understanding the accretionary and collisional tectonics of the CAOB, and its place within the Rodinia supercontinent. However, to date, the timing and tectonic settings in which the basement rocks in the Central Tianshan formed are poorly constrained, with only sparse geochemical and geochronological data from granitic rocks within the central segment of the belt. Here, we present a systematic study combining U-Pb geochronology, whole-rock geochemistry, and the Sr-Nd isotopic compositions of newly-identified granites from the Bingdaban area of Central Tianshan. The analyzed samples yield a weighted mean Neoproterozoic <sup>206</sup>Pb/<sup>238</sup>U ages of 975-911 Ma. All have affinities with calc-alkaline, weakly-peraluminous, magnesian I-type granites. The samples are enriched in LREE, display relatively flat HREE patterns with negative Eu anomalies, and show a depletion in the high field strength elements (HFSEs) Nb, Ta, and Ti and enrichment in large ion lithophile elements (LILEs) Rb, U, Th and Nd geochemical characteristics indicative of subduction-related magmatism. All samples show initial (<sup>87</sup>Sr/<sup>86</sup>Sr)<sub>(t)</sub> ratios between 0.705136 and 0.706745. Values for Ɛ<sub>Nd(t)</sub> in the granites are in the range -1.2 to -5.7, corresponding to Nd model ages of 1.6-2.1 Ga, indicating a role for Mesoproterozoic to Paleoproterozoic rocks in the generation of the granitic protoliths. The documented geochemical features indicate the protoliths for the granites had a similar petrogenesis and magmatic source, which may reflect partial melting of thickened crust with the addition of small amounts of mantle-derived material. The Tianshan Block probably constituted part of an exterior orogen that developed along the margin of the Rodinian supercontinent during the early Neoproterozoic, and which underwent a transition from subduction to syn-collision compression at 975-911 Ma. This study reveals that crustal reworking may played a key role in Neoproterozoic crustal evolution in the Central Tianshan block and this block has a tectonic affinity to the Yili block.</p><p>This study was financially supported by the National Natural Science Foundation of China (41772059) and the CAS “Light of West China” Program (2018-XBYJRC-003).</p>