Yu-Xiu Zhang

The blueschist-bearing Qiangtang metamorphic belt (northern Tibet, China) as an in situ suture zone: Evidence from geochemical comparison with the Jinsa suture

Metasiliciclastic rocks and metabasalts from the blueschist-bearing Qiangtang metamorphic belt and the Jinsa suture zone were analyzed for major and trace elements in an attempt to evaluate the affinities of these two tectonic entities. Tholeiitic mid-oceanic-ridge basalts (MORBs) from the Jinsa suture can be distinguished from metabasalts of Qiangtang, which have alkalic compositions and exhibit a range of characteristics typical of many within-plate oceanic islands. The Qiangtang metasiliciclastic rocks were derived from a passive continental margin source, whereas those from the Jinsa suture zone were sourced from a continental island arc or an active continental margin source. The geochemical distinction of metasiliciclastic rocks and metabasalts of the Qiangtang metamorphic belt from their counterparts within the Jinsa suture indicates that there is no affinity between these two tectonic entities and that the Qiangtang metamorphic belt could not have been underthrust from the Jinsa suture. It most likely represents an exhumed accretionary complex composed of sediments derived from a passive continental margin and fragments of seamount sequences, and marks an in situ suture zone that separates northern and southern Qiangtang terranes.

U–Pb and Lu–Hf isotope systematics of detrital zircons from the Songpan–Ganzi Triassic flysch, NE Tibetan Plateau: implications for provenance and crustal growth

We conducted in situ U–Pb and Lu–Hf isotope analyses of 401 detrital zircons collected from the Songpan–Ganzi Triassic turbidite complex in an attempt to understand the provenance variations of the siliciclastic rocks and the crustal growth history of central China. These detrital zircons exhibit a wide age spectrum with three major peaks at 1.7–2.0 Ga, 750–1050 Ma, and 210–500 Ma. They are dominated by negative ϵHf(t) values with a large range. Synthesis of the zircon U–Pb and Lu–Hf isotopic data indicate that the Triassic Songpan–Ganzi turbiditic succession could have been derived dominantly from the Tibetan terrains + the Kunlun and Qinling orogens. Our samples are characterized by a common, prominent group of Hf crust formation model ages at 0.8–4.1 Ga with a peak at 2.7–3.4 Ga. This fact indicates that (1) Phanerozoic magmatism in central China could have been predominantly products of crustal reworking with insignificant formation of juvenile crust and (2) the Neoarchaean was an important period of continental growth in central China. In addition, our data set also reveal that three widespread tectonothermal events could have occurred in the region during the late Mesoproterzoic, Palaeozoic, and early Mesozoic, respectively.

Newly discovered eclogites from the Bangong Meso–Tethyan suture zone (Gaize, central Tibet, western China): mineralogy, geochemistry, geochronology, and tectonic implications

ABSTRACT We report the occurrence, mineralogy, whole-rock geochemistry, and geochronology of two types of eclogites newly discovered in the western Bangong Meso–Tethyan suture zone (Gaize, central Tibet, western China). Type 1 eclogites contain a peak metamorphic mineral assemblage of garnet + clinopyroxene whereas Type 2 ones are characterized by a peak metamorphic mineral assemblage of garnet + clinopyroxene + rutile. Type 2 clinopyroxene and garnet are relatively more enriched in Mg contents than Type 1 ones. Type 2 eclogites are significantly more enriched in TiO2, P2O5, and light rare-earth elements, but more depleted in Al2O3 contents and Mg numbers. P–T estimates for both types of eclogites are consistent with minimum pressures of 23–25 kbar at 830–920°C, with the retrograde amphibolite facies assemblage of 15.2–17.5 kbar and 500–732°C. Zircon U–Pb dating indicates that the protolith of Type 1 eclogites, with normal mid-ocean ridge basalt affinity, emplaced at ~260 Ma, whereas the protolith of Type 2 eclogites, geochemically familiar with oceanic-island basalt (OIB), erupted at ~242 Ma. 40Ar/39Ar analyses of hornblende from the retrograded eclogites indicate that the Gaize eclogite-bearing high-pressure rocks were exhumed at ~194 Ma. The occurrence of these high-pressure rocks implies the subduction of oceanic crust to a depth of ~85 km along the southern margin of the Qiangtang block during the Late Triassic.

Late Permian–Triassic siliciclastic provenance, palaeogeography, and crustal growth of the Songpan terrane, eastern Tibetan Plateau: evidence from U–Pb ages, trace elements, and Hf isotopes of detrital zircons

In order to constrain the detrital provenance of the siliciclastic rocks, palaeogeographic variations, and crustal growth history of central China, we carried out simultaneously in situ U–Pb dating and trace element and Hf isotope analyses on 368 detrital zircons obtained from upper Permian–Triassic sandstones of the Songpan terrane, eastern Tibetan Plateau. Two groups of detrital zircons, i.e. magmatic and metamorphic in origin, have been identified based on cathodoluminescence images, zircon Ti-temperatures, and Th/U ratios. Our data suggest that the derivation of siliciclastic rocks in the Songpan terrane was mainly from the Qinling, Qilian, and Kunlun orogens, whereas the Yangtze and North China Cratons served as minor source areas during late Permian–Triassic times. The detrital zircons from Middle–Late Triassic siliciclastic rocks exhibit wide age spectra with two dominant populations of 230–600 Ma and >1600 Ma, peaking at ~1.8–1.9 Ga and ~2.4–2.5 Ga, suggestive of a derivation from the Qinling, Qilian, and Kunlun orogens and the Yangtze Craton being the minor source area. The proportions of detrital zircon populations from the northern Qinling, Qilian, and Kunlun orogens distinctly decreased during Middle–Late Triassic time, demonstrating that the initial uplift of the western Qinling occurred then and it could have blocked most of the detritus from the Qilian–northern Qinling orogens and North China Cratons into the main Songpan–Ganzi depositional basin. The relatively detrital zircon proportions of the Yangtze Craton source decreased during Early-Middle Late Triassic time, indicating that the Longmenshan orogen was probably being elevated, since the early Late Triassic and gradually formed a barrier between the Yangtze Craton and the Songpan terrane. In addition, our Lu–Hf isotopic results also reveal that the Phanerozoic magmatic rocks in central China had been the primary products of crustal reworking with insignificant formation of a juvenile crust.

Late Palaeozoic and early Mesozoic tectonic and palaeogeographic evolution of central China: evidence from U–Pb and Lu–Hf isotope systematics of detrital zircons from the western Qinling region

The western Qinling region of central China is situated centrally in the Kunlun, Qilian, Qinling, Longmenshan, and Songpan–Ganzi orogens. Late Palaeozoic and Early Mesozoic sediments deposited here may provide keys to understanding the tectonic evolution of the Palaeo-Tethys and collision of the North China and Yangtze Cratons. We conducted in situ U–Pb and Lu–Hf isotope analyses of 568 detrital zircons collected from Upper Palaeozoic to Mesozoic sandstones in the central Qinling block, Taohe depression, and Bailongjiang block in western Qinling to constrain the sources of these sandstones. Our results reveal that the Bailongjiang block has affinities with the Yangtze Craton, from which it may have been rifted. Therefore, the Palaeo-Tethyan Animaqen suture between the two cratons lies north of the Bailongjiang block. We identified the North China Craton as the main source for Triassic flysch in central China. It is possible that the Bailongjiang block could have blocked detritus shed from the North China Craton into the main depositional basins in the Songpan–Ganzi area. The dominance of 300–200 Ma detrital zircons of metamorphic origin in Lower Jurassic sandstones indicates that the Dabie–Qinling orogen was elevated during Early Jurassic time. In addition, our Lu–Hf isotopic results also reveal that Phanerozoic igneous rocks in central China were mostly products of crustal reworking with insignificant formation of juvenile crust.

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.

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.

Early Permian Qiangtang Mantle Plume, Northern Tibet, China: Evidence from Geochemistry, Geochronology and Geological Responses

in Qiangtang terrene, including west Qiangtang (WQT), east Qiangtang (EQT) and the central Qiangtang (CQT) metamorphic belt. The igneous rocks distributed in WQT display the typical characters of rifted igneous rock (e.g. Wang et al., 1987, 2001; Deng et al. 1996; Zhang and Zhang, in press). The blueschistand eclogite-bearing CQT metamorphic belt is characterized by ubiquitous blocks of OIB-type meta-basalts (Zhang et al., 2006a, 2006b, 2007a; Tang and Zhang, 2012a, 2012b, 2014; Zhang and Zhang, in press). However, the distribution, age and geological responses of these igneous rocks remain open to be discussed. We report petrography, geochemistry, geological responses, and correlate the Early Permian basaltic rocks from WQT and CQT, with those in EQT, which totally covered an area of > 3.3 × 10 km, with a maximum thickness of 1.5–2.0 km. Similar Early Permian basalts are also present in the Lhasa terrane (e.g. Jiangrang and Ranwu), the present-day South Tibet–northern India (including Panjal Traps, Selong–Bhote Kosi and Abor), Sanjiang area (e.g. Woniusi) and the Tarim basin (Zhang and Zhang, in press, and references therein). These basalts originally have an extent of > 8.84 × 10 km and a volume of > 6.76 × 10 km. A Sakmarian–Kungurian mantle plume is proposed to be responsible for such largescale eruption of flood basalts, which could have initiated the disintegration of northern Gondwana (Zhang and Zhang, in press). The SiO2 contents of the WQT basaltic samples range from 37.34% to 57.18%, while the CQT basalts samples contain 35.55%~53.76% SiO2. The remarkable high ratios of Ti/Y and Lan/Ybn preclude these Early Permian basalts being derived from a depleted mantle source. And most of the sample have relative high LOI value and variable initial Sr/Sr ratios, suggesting rocks have undergone alteration. The East Qiangtang basalts are characterized by high TiO2, low Al2O3, significant HREE concentrations, apparent LREE enrichment, and lack Eu anomaly. The basalts and mafic dykes also display similar low MgO, Mg and Ni contents, and the strong correlations between Mg and major and trace element indicate they had experienced fractional crystallization process (Zhang and Zhang, 2016). Compared with ocean island basalts (OIB) of primitive mantle-normalized incompatible trace element patterns, most of the Qiangtang basalt samples show a notable resemblance and suggest a mantle plume origin. Moreover, the geochronological and paleontological data suggest most of the Early Permian widespread magmatism occurred in a few million years, with a peak at ~ 287 Ma (e.g. WQT mafic rocks). The Permian magmatism is widespread occurring over 1000 km from the continental margin, and implies that anomalously hot mantle extended over a very wide area and melted extensively. Generally, the basalts in our research coincide with models that suggest an upwelling plume head was trapped beneath the lithosphere and separated from the plume tail (Chung et al., 1998; Leitch et al., 1998), with plume material spread over a very large area by ambient mantle flux (Wilson, 1997). Early Permian OIB-type basalts also distribute in three main Tibetan–Himalayan suture zones (the Bangong– Nujiang, Shuanghu, and Yarlung-Zangpo sutures), or in their nearby continental margins. Therefore, this mantle plume could have not only initiated the separation of Tarim and entire Tibet from Gondwana, but also initiated the corporate separations of the WQT, EQT, Lhasa, and Tarim terranes during the Early Permian, and could be responsible for the formation of main suture zones within the Tibetan plateau (e.g. Zhang et al., 2007, 2012, 2014a, in press). The middle Permian warm-water limestone bear rich warm-water faunas, while glacial-water fauns exist in the ZHOU Xiaoyao, JIN Xin, ZHANG Yuxiu, 2016. Early Permian Qiangtang Mantle Plume, Northern Tibet, China: Evidence from Geochemistry, Geochronology and Geological Responses. Acta Geologica Sinica (English Edition), 90(supp. 1): 138-140.

Newly discovered Late Triassic Baqing eclogite in central Tibet indicates an anticlockwise West–East Qiangtang collision

The Triassic eclogite-bearing central Qiangtang metamorphic belt (CQMB) in the northern Tibetan Plateau has been debated whether it is a metamorphic core complex underthrust from the Jinsha Paleo-Tethys or an in-situ Shuanghu suture. The CQMB is thus a key issue to elucidate the crustal architecture of the northern Tibetan Plateau, the tectonics of the eastern Tethys, and the petrogenesis of Cenozoic high-K magmatism. We here report the newly discovered Baqing eclogite along the eastern extension of the CQMB near the Baqing town, central Tibet. These eclogites are characterized by the garnet + omphacite + rutile + phengite + quartz assemblages. Primary eclogite-facies metamorphic pressure–temperature estimates yield consistent minimum pressure of 25 ± 1 kbar at 730 ± 60 °C. U–Pb dating on zircons that contain inclusions (garnet + omphacite + rutile + phengite) gave eclogite-facies metamorphic ages of 223 Ma. The geochemical continental crustal signature and the presence of Paleozoic cores in the zircons indicate that the Baqing eclogite formed by continental subduction and marks an eastward-younging anticlockwise West–East Qiangtang collision along the Shuanghu suture from the Middle to Late Triassic.

Garnet amphibolites from the Ganzi–Litang fault zone, eastern Tibetan Plateau: mineralogy, geochemistry, and implications for evolution of the eastern Palaeo-Tethys Realm

ABSTRACT The Ganzi–Litang fault zone, an outstanding tectonic element in the eastern Tibetan Plateau has been intensively debated as an in-situ suture zone marking relict of a subducted Palaeo-Tethyan oceanic crust or a failed intracontinental rift. This paper reports the garnet amphibolites discovered along the Ganzi–Litang fault zone, eastern Tibetan Plateau. These garnet amphibolites are characterized by the garnet–hornblende–rutile–sphene–plagioclase–quartz assemblage. Conventional geothermobarometry figures out the metamorphic temperature and pressure conditions at 582–626°C and 1.61–1.82 GPa, respectively. Geochemical analysis (no Nb–Ta deletions and left-inclined to flat patterns of rare-earth elements) indicates that the garnet amphibolites could represent metamorphic product of the mid-ocean-ridge (MORB)-type mafic rocks that were contaminated by a mantle plume. The protolith of the garnet amphibolites was dated at 236 Ma using in-situ U–Pb zircon method, and the retrograde metamorphism was dated at 218 Ma using in-situ U–Pb sphene method. A comprehensive analysis combined with the development of the Palaeo-Tethys Ocean and the Yidun arc through geologic time indicates a Triassic to Early Jurassic age (236–195 Ma) for the metamorphism of the garnet amphibolites. The low geothermal gradient of 9.8ºC/km and the N-MORB nature of the garnet amphibolites suggest a subduction-zone environment for the high-pressure metamorphism. Therefore, the Ganzi–Litang fault zone is a Palaeo-Tethyan suture separating the Yidun arc and the Songpan–Ganzi terrane, representing the relics of a branch of the Palaeo-Tethys Ocean that was contaminated by a mantle plume.