The Precambrian Tectonic Affinity Of The Chinese Central Tianshan And Its Paleozoic Tectonic Evolution

The Chinese Central Tianshan terrane (CCTT) constitutes part of the Central Asian Orogenic Belt, locating in the northern margin of the Tarim block. The research on the derivation, formation and evolution of the CCTT will shed light on the formation history of the Chinese Tianshan belt and the Central Asian Orogenic Belt. Lots of achievements have been gained by previous researchers, such as the Precambrian history of the Tarim block, the accretionary feature of the Chinese Tianshan belt and the structural geology of the CCTT. However, many questions remain to be solved, particularly the Precambrian tectonic affinity of the CCTT, the emplacement timing and formation setting of the voluminous granitoids in the CCTT region, the closure timing of the Paleo-Tianshan ocean and the beginning timing of the post-collisional phase in the Chinese Tianshan region.Based on the analysis of the regional geological data and geological background, we chose several typical and important lines for fieldtrip, such as Urumqi-Baluntai-Korla, Baluntai-Houxia-Gangou, Tuoksun-Kumux, Korla-Yuli-Xingdi etc. In the Central-Southern Tianshan region, the lines of kinematic indicators (such as north-vergent folding-thrusting, sigmoidal volcanic clasts and shear bands, asymmetrically sheared layers and asymmetric clasts, etc.) showing top-to-the-north ductile shearing have been reported by previous researchers, and we had an observation on them again. These ductile north-directed deformation suggests a north vergent accretionary complex and a south-directed subduction of the Paleo-Tianshan ocean below the Tarim block and its subsequent rifted-part of the Central Tianshan terrane. Two pre-Permian regional unconformities have been discovered in the Chinese Tianshan belt:one between the lower Carboniferous Maanqiao Group and the schistose Ordovician-Silurian rocks in Central Tianshan; and the other between the lower Permian conglomerates and the pre-Permian sequences in the whole Tianshan belt. The first unconformity indicates that the earlier collisional orogeny in the northern margin of the Central Tianshan terrane was terminated before early Carboniferous. The second unconformity between the lower Permian conglomerates and the pre-Permian rocks probably suggests that the orogeny in the Chinese Tianshan belt ended before Permian, and then the whole Chinese Tianshan entered into the post-collisional regime phase.A suite of Paleozoic granitoids in Central Tianshan was studied for both geochemistry and geochronology in an effort to constrain their origin and tectonic setting. We combined LA-ICPMS dating of zircon, standard geochemical analyses and Hf-isotopic studies of zircon to develop our tectonic model. Based on our analysis, the granitoids formed in three distinctive stages:450-400 Ma,370-350 Ma and 340 Ma. The first stage (450-400 Ma) granitoids exhibit metaluminous, magnesian, high-K to shoshonitic characteristics of I-type granitoids (arc-setting), that are enriched in LREE relative to HREE with high (La/Yb)CN values, show negative Eu anomaly and are depleted in HFSE, such as Nb, Ta and Ti. This phase of granitoid emplacements was most likely related to the southward subduction of the Paleo-Tianshan Ocean beneath the Tarim block and the subsequent Central Tianshan arc. In contrast, the second stage granitoids (370-350 Ma) are distinctly different and are classified as calc-alkaline or shoshonitic plutons with a weak positive Eu anomaly. Within the second stage granitoids, it appears that the earlier (365 Ma) granitoids fit within the A-type field whereas the younger (352 Ma) granitoids plot within the post-collisional potassic field. These granitoids formed during collisions between Central Tianshan and the Northern Tianshan arc that occurred along the northern margin of Central Tianshan. Lastly, the 340 Ma granitoids are typical of volcanic arc granitoids again that probably formed by subsequent southward subduction of the residual Paleo-Tianshan Ocean.We present U-Pb-Hf isotopic and elemental data for granitoid intrusions formed during the early Permian in Eastern Tianshan (eastern part of Chinese Tianshan), with the aim of deciphering their magma sources and geodynamic evolution. These granitoids were formed at ca.300-270 Ma and generally exhibit alkali-calcic to alkali compositions, and metaluminous and high-K calc-alkaline to shoshonitic characteristics. Commonly, steep REE patterns with significant enrichment of LREE relative to HREE showing (La/Yb)cN (4.4-43), distinctive negative Eu anomalies and depletion of Nb, Ta and Ti are noteworthy features of these granitoids, akin to the typical I-type granitoids. The tectonic setting of these granitoids in combination with the regional geology, suggests that the granitoids were generated in a post-collisional setting. During the post-collisional process, the slab breakoff played a key role in the genesis of the granitoids.The Precambrian tectonic affinity of the CCTT has been hotly debated for long time. In order to examine the above puzzle, we performed detrital zircon U-Pb dating of Devonian greywackes and Neoproterozoic metasedimentary rocks in the CCTT. Some detrital zircons of the Devonian sandstones have also been performed for Hf isotopes. Most analyzed zircon grains of Devonian greywackes show oscillatory zoning and have Th/U ratios>0.4, suggesting that they were mainly derived from igneous rocks and sourced from a near-source region. A total of about 400 detrital zircon analyses yielded five Precambrian age populations, namely, early Paleoproterozoic (peak at 2470 Ma), middle Paleoproterozoic (peak at 1858 Ma), early Mesoproterozoic (peak at 1541 Ma), early Neoproterozoic (peak at 952 Ma), and late Neoproterozoic (820-750 Ma). The Neoproterozoic metasediments here are dominated by zircons with major Precambrian age populations of 2544-2394 Ma,1900-1500 Ma and 1070-752 Ma. Similarly, most of these zircons are euhedral to subhedral, featured by typical magmatic oscillatory zoning and high Th/U ratios (>0.4) (acounting for 85% of the total).Based on the above discussion, we infer that these detrital zircons are mainly derived from the CCTT and the prominent age peaks are corresponded to the Precambrian magmatic events in the CCTT. These magmatic events represented by notable age peaks are remarkably consistent with the polyphase tectonothermal events that occurred in Tarim. So, we think that the CCTT was part of Tarim during the Precambrian time and has a Precambrian tectonic affinity with the Precambrian basement of Tarim.The age population of 2470-2450 Ma indicates the presence of late Neoarchean to early Paleoproterozoic magmatism in Tarim. The age populations at 1860-1850 Ma and 1570-1540 Ma probably coincide with the assembly and breakup of the Columbia supercontinent, respectively. The age populations of 950 Ma and 820-750 Ma may represent the time of the assembly and breakup of Rodinia, respectively.We present U-Pb age data on detrital zircons in Paleoproterozoic schists from Central Tianshan and employ the magmatic age spectra to probe the Precambrian history of the region. We use the U-Pb ages of metamorphic zircons to constrain the Paleoproterozoic collisional event of the Tarim block, and to evaluate its link with the Columbia supercontinent. The age populations of the detrital igneous zircons and inherited cores show ages at range of 2544-1882 Ma. Combined with the results from previous studies, we conclude that the northern Tarim was an active continental margin from late Neoarchean to late Paleoproterozoic. The tightly constrained age range of 1830-1788 Ma (weighted mean 1808 Ma) obtained from the metamorphic zircons and overgrowth mantles mark the timing of the thermal event associated with the final collisional orogeny along the northern margin of Tarim, coinciding with the assembly of Columbia.