Wencan Liu

Coupled U-Pb dating and Hf isotopic analysis of detrital zircon of modern river sand from the Yalu River (Yarlung Tsangpo) drainage system in southern Tibet: Constraints on the transport processes and evolution of Himalayan rivers

We conducted coupled U-Pb dating and Hf isotope analysis of detrital zircon in modern sand of the Yalu River in southern Tibet. Our work indicates that the presence or absence of distinctive zircon populations in the Yalu main stream depends critically on the geometric configuration of the tributary rivers. The proportion of upper-stream zircon populations in the Yalu River sand decreases systematically in the downstream direction, which is caused mainly by zircon addition from new source areas in the downstream region. In some extreme cases, the upstream zircon signals can completely be lost in the downstream region due to this dilution effect. Analysis of sand modal composition reveals a downstream increase in the proportion of lithic fragments along the Yalu River, from ∼40% to ∼60% over a distance of ∼600 km. This may be attributed to the combined effect of an eastward increase in the topographic relief and an eastward increase in annual precipitation across the Yalu River drainage basin. Quantitative comparison of detrital-zircon ages between the Yalu River sand and Neogene sediments of the eastern Himalayan foreland supports a previous proposal that the Yalu River once flowed directly over the eastern Himalaya, without going around the Himalaya through its eastern syntaxis. The shortcut appears to have been transient, as it is only recorded in specific stratigraphic horizons of foreland sediments. The inferred Yalu River diversion may have been caused by past advances of glaciers or emplacements of giant landslides that temporarily dammed the Yalu River.

Early Cretaceous A-type granites and Mo mineralization, Aershan area, eastern Inner Mongolia, Northeast China: geochemical and isotopic constraints

The Jiazishan porphyry-type molybdenum deposit is located in the eastern Inner Mongolia Autonomous Region in China. Mineralization occurs mainly as veins, lenses, and layers within the host porphyry. To better understand the link between mineralization and host igneous rocks, we studied samples from underground workings and report new SHRIMP II zircon U–Pb and Re–Os molybdenite ages, and geochemical data from both the molybdenites and the porphyry granites. Seven molybdenite samples yield a Re–Os isochron weighted mean age of 135.4 ± 2.1 Ma, whereas the porphyry granite samples yield crystallization ages of 139 ± 1.5 Ma (Jiazishan deposit) and 133 ± 1 Ma (Taolaituo deposit). The U–Pb and Re–Os ages are similar, suggesting that the mineralization is genetically related to Early Cretaceous porphyry emplacement. Re contents of the molybdenite range from 21.74 ppm to 52.08 ppm, with an average of 35.92 ppm, whereas δ34 S values of the sulphide vary from 1.3‰ to 4.2‰. The ores have 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios of 18.178–18.385, 15.503–15.613, and 37.979–38.382, respectively. We also obtained a weighted mean U–Pb zircon age of 294.2 ± 2.1 Ma for the oldest granite in Jiazishan area. All granites are A-type granites. These observations indicate that the molybdenites and the porphyry granites were derived from a mixed source involving young accretionary materials and enriched subcontinental lithospheric mantle. A synthesis of geochronological and geological data reveals that porphyry emplacement and Mo mineralization in the Jiazishan deposit occurred contemporaneously with Early Cretaceous tectonothermal events associated with lithospheric thinning, which was caused by delamination and subsequent upwelling of the asthenosphere associated with intra-continental extension in Northeast China.

Pre-Cenozoic geologic history of the central and northern Tibetan Plateau and the role of Wilson cycles in constructing the Tethyan orogenic system

In order to better constrain the evolution of the Tethyan orogenic system, we conducted an integrated investigation involving U-Pb dating of igneous and detrital zircon, geochemical analysis of igneous rocks, compositional analysis of sedimentary strata, and a synthesis of existing work across the Qilian Shan, Qaidam Basin, and the Eastern Kunlun Range of central and northern Tibet. This effort reveals five stages of arc magmatism at 1005–910 Ma, 790–720 Ma, 580–500 Ma, 490–375 Ma, and 290–195 Ma, respectively. Arc activities were interrupted by repeated continent-continent collision followed by ocean opening along the older suture zones first created in the Neoproterozoic. This suggests that Wilson cycles have played a controlling role in constructing the southern Asian continent. The magmatic history and regional geologic constraints allow us to construct a coherent tectonic model that has the following key features. (1) The linked South Qilian suture in the west and North Qinling suture in the east formed the northern boundary of the coherent Kunlun–Qaidam–North Qinling Terrane in the early Paleozoic. (2) The Songpan-Ganzi Terrane has been the western part of the Yangtze craton since the Neoproterozoic. (3) Development of the wide (>700 km) Permian–Triassic arc across the Kunlun-Qaidam Terrane was induced by flat subduction and rapid slab rollback, which also caused extreme extension of the Songpan-Ganzi Terrane. (4) The formation of the Anymaqen-Kunlun-Muztagh Ocean (= the Neo–Kunlun Ocean in this study) was created within Laurasia rather than being a preexisting ocean between Gondwana and Laurasia as postulated by most early studies.

Early Cretaceous adakitic granites and mineralization of the Yili porphyry Mo deposit in the Great Xing’an Range: implications for the geodynamic evolution of northeastern China

The Yili porphyry-type molybdenum deposit is located in the Northeastern Inner Mongolia Autonomous Region in China. Mineralization occurs mainly as veins, lenses, and layers within the host porphyry. In order to better understand the link between mineralization and host igneous rocks, we studied samples from underground workings. We report new Sensitive High Resolution Ion Microprobe II (SHRIMP II) zircon U–Pb and Re–Os molybdenite ages and geochemical data from the Yili granitoids. Five molybdenite samples yield a Re–Os isochron weighted mean age of 131.1 ± 3.4 Ma, while two Early Cretaceous adakitic porphyry granite samples yielded crystallization ages of 128.1 ± 1.6 Ma and 129.0 ± 3.5 Ma. The U–Pb and Re–Os ages are analytically indistinguishable, suggesting that mineralization was genetically related to Early Cretaceous magmatism in northeastern China. δ34SV-CDT values of the sulphide vary from 0.3‰ to 3.8‰. We obtained two weighted mean U–Pb zircon ages of 287.7 ± 1.8 Ma for early Permian fine-grained granite and 349.8 ± 2.3 Ma for Early Carboniferous monzogranite in Yili area, respectively. A synthesis of geochronological and geological data reveals that porphyry emplacement and Mo mineralization in the Yili deposit occurred at the same time as Early Cretaceous lithospheric thinning, which was caused by the delamination and subsequent upwelling of the asthenosphere under the intra-continental extension in northeastern China.

Geochronology and geochemistry of Neoproterozoic granitoids in the central Qilian Shan of northern Tibet: Reconstructing the amalgamation processes and tectonic history of Asia

Our understanding of the assembly history of Asia depends critically on the tectonic relationships between its major cratons, including Siberia, North China, South China, and Tarim. The intervening microcontinents between these cratons can provide insight into the paleogeographic and paleotectonic relationships of the cratons, but there is currently a general lack of knowledge regarding the basement geology of these microcontinents. Here we present results from systematic geologic mapping, U-Pb zircon dating, whole-rock geochemical analysis, and synthesis of existing data to establish the Proterozoic to early Paleozoic evolution of the central Qilian basement to the south of the North China craton in northwest China. Our results indicate that the region underwent three major periods of magmatic activity at 960–880, 877–710, and 550–375 Ma. Our geochemical analysis suggests that the ca. 900 Ma plutons were generated during arc magmatism and/or syncollisional crustal melting, whereas the ca. 820 Ma plutons are A-type granitoids, which are typically associated with extensional tectonism. Igneous zircons from a high- and ultrahigh-pressure eclogite in the north-central Qilian Shan have a U-Pb age of ca. 916 Ma, whereas dating of the recrystallized rims suggests that eclogite facies metamorphism occurred at ca. 485 Ma. Our detrital zircon geochronology also indicates that a widespread metasedimentary unit in the region was deposited between ca. 1200 and ca. 960 Ma, prior to the onset of a rift-drift event at ca. 750 Ma. Based on regional geologic constraints and the magmatic history, we propose the following tectonic history: (1) the paleo–Qilian Ocean bound the combined North Tarim–North China craton to the south (present-day coordinates) in the Mesoproterozoic; (2) the paleo–Qilian Ocean closed between 900 and 820 Ma following the collision of North Tarim–North China craton and the South Tarim–Qaidam–Kunlun continent; (3) the younger Qilian Ocean opened at ca. 775 Ma along the previous suture trace of the paleo–Qilian Ocean as a marginal sea within southern Laurasia; and (4) this ocean closed by ca. 445–440 Ma as a result of collision between the Tarim–North China cratons and the Qaidam-Kunlun continent along a south-dipping subduction system.

Tectonic evolution of the Qilian Shan: An early Paleozoic orogen reactivated in the Cenozoic

The Qilian Shan, located along the northeastern margin of the Tibetan Plateau, has experienced multiple episodes of tectonic deformation, including Neoproterozoic continental breakup, early Paleozoic subduction and continental collision, Mesozoic extension, and Cenozoic intracontinental orogenesis resulting from the India-Asia collision. In the central Qilian Shan, pre-Mesozoic ophiolite complexes, passive-continental margin sequences, and strongly deformed forearc strata were juxtaposed against arc plutonic/ volcanic rocks and ductilely deformed crystalline rocks during the early Paleozoic Qilian orogen. To better constrain this orogen and the resulting closure of the Neoproterozoic–Ordovician Qilian Ocean, we conducted an integrated investigation involving geologic mapping, U-Th-Pb zircon and monazite geochronology, whole-rock geochemistry, thermo barometry, and synthesis of existing data sets across northern Tibet. The central Qilian Shan experienced two phases of arc magmatism at 960–870 Ma and 475–445 Ma that were each followed by periods of protracted continental collision. Integrating our new data with previously published results, we propose the following tectonic model for the Proterozoic–Paleozoic history of northern Tibet. (1) Early Neoproterozoic subduction accommodated the convergence and collision between the South Tarim–Qaidam and North Tarim–North China continents. (2) Late Neoproterozoic rifting partially separated a peninsular Kunlun-Qaidam continent from the southern margin of the linked Tarim–North China craton and opened the Qilian Ocean as an embayed marginal sea; this separation broadly followed the trace of the earlier Neoproterozoic suture zone. (3) South-dipping subduction along the northern margin of the Kunlun-Qaidam continent initiated in the Cambrian, first developing as the Yushigou supra-subduction zone ophiolite and then transitioning into the continental Qilian arc. (4) South-dipping subduction, arc magmatism, and the convergence between Kunlun-Qaidam and North China continued throughout the Ordovician, with a trenchparallel intra-arc strike-slip fault system that is presently represented by high-grade metamorphic rocks that display a pervasive right-lateral shear sense. (5) Counterclockwise rotation of the peninsular KunlunQaidam continent toward North China led to the closure of the Qilian Ocean, which is consistent with the right-lateral kinematics of intra-arc strike-slip faulting observed in the Qilian Shan and the westward tapering mapview geometry of Silurian flysch-basin strata. Continental collision at ca. 445–440 Ma led to widespread plutonism across the Qilian Shan and is recorded by recrystallized monazite (ca. 450–420 Ma) observed in this study. Our tectonic model implies the parallel closure of two oceans of different ages along the trace of the Qilian suture zone since ca. 1.0 Ga. In addition, the Qilian Ocean was neither the Protonor Paleo-Tethys (i.e., the earliest ocean separating Gondwana from Laurasia), as previously suggested, but was rather a relatively small embayed sea along the southern margin of the Laurasian continent. We also document >200 km of Cenozoic north-south shortening across the study area. The observed shortening distribution supports models of Tibetan Plateau development that involve distributed crustal shortening and southward underthrusting of Eurasia beneath the plateau. This India-Asia convergence-related deformation is focused along the sites of repeated ocean closure. Major Cenozoic left-slip faults parallel these sutures, and preexisting subduction-mélange channels may have facilitated Cenozoic shortening and continental underthrusting.

First geomorphological and sedimentological evidence for the combined tectonic and climate control on Quaternary Yarlung river diversion in the eastern Himalaya

This study investigates the combined effects of Quaternary climate change and tectonically induced topography on the Yarlung River drainage-system evolution in the eastern Himalaya. Our work integrates field mapping, geomorphological analysis, stratigraphy, sedimentology, optically stimulated luminescence dating, radiocarbon dating, and detrital-zircon dating of a Holocene valley-fill sequence in a Yarlung River tributary. This holistic approach reveals an aggradational event, which started at or soon after 24–20 k.y. B.P. and continued to or after 9.2–8.0 k.y. B.P. across the Himalayan drainage divide between the east-flowing Yarlung River in the north and the south-flowing Subansiri River in the south. The aggradational event was associated with a major phase of glacier advance during a period of warm and wet climate conditions in the eastern Himalaya; it was expressed by the deposition of a valley-fill sequence across the modern Yarlung-Subansiri drainage divide. South-flowing fluvial sediments across the divide and the elevation distribution of the fluvial terraces require the existence of a major glacier dam that either blocked a tributary or the main trunk of the Yarlung River. Although we are unable to differentiate the two competing scenarios, our work reveals that combined Holocene climate change and tectonically induced topography have played a major role in controlling rapid shifts in drainage geometry at a time scale of <10 k.y. across the Himalaya.

Structural and Tectonic Framework of the Qilian Shan‐Nan Shan Thrust belt, Northeastern Tibetan Plateau

Shan-Nan Shan thrust belt (QNS) is the widest thrust belt on the Tibetan Plateau (Fig. 1). Located along the northeastern margin of the plateau, the style and magnitude of deformation in the QNS have important implications for how Cenozoic shortening induced by the Andrew ZUZA , Robin REITH , An YIN , DONG Shuwen , LIU Wencan ,ZHANG Yuxiu and WU Chen, 2013.Structural and Tectonic Framework of the Qilian Shan-Nan Shan Thrust belt, Northeastern Tibetan Plateau. Acta Geologica Sinica (English Edition), 87(supp.): 1-3.

Geochronology and tectonic settings of Late Jurassic – Early Cretaceous intrusive rocks in the Ulanhot region, central and southern Da Xingan Range

Zircon U–Pb dating and whole-rock geochemical analysis have been performed on Late Jurassic – Early Cretaceous intrusive rocks of the Ulanhot area, NE China, with the aim of constraining the tectonic evolution of the central and southern Da Xingan Range. Zircon U–Pb dating indicates that Late Jurassic – Early Cretaceous magmatic events experienced four stages at: c. 155 Ma; c. 144 Ma; 135–130 Ma; and c. 126 Ma. The c. 155 Ma magmatic event consists of quartz diorite and granite-porphyryp with the geochemical characteristic of high Sr and Sr/Y or high A/CNK (1.38), implying the primary magma was derived from partial melting of a thickened lower crust which induced the closure of the Mongol–Okhotsk Ocean. The c. 144 Ma magmatic event consists of quartz monzodiorite with the geochemical characteristics of alkaline series, and indicates the delamination of a thickened crust. The 135–130 Ma magmatic event consists of syenogranite and granite-porphyry with characteristics of both I-type and A-type granites, which induced both the subduction of the Palaeo-Pacific oceanic plate and the post-orogenic extension of the Mongol–Okhotsk Orogenic Belt. The c. 126 Ma magmatic event consisted of highly fractionated I-type biotite granite and alkaline series gabbro, marking the end of the Mongol–Okhotsk Orogen, and implying that the study area was controlled by the circum-Pacific tectonic system during this stage.