Proto-Tethys Research Articles

Early Cambrian forearc ophiolite-hosted VMS-type Cu deposit in the North Qaidam belt, northern Tibetan Plateau

The North Qaidam belt is an important polymetallic metallogenic belt in the northwestern region of China. However, the tectonic setting and age of related VMS deposits remain debated. Here we performed an integrated analysis of field relationship, geochemistry, and geochronology for hosting rocks of the Lüliangshan VMS-type Cu deposit and surrounding mafic-ultramafic rocks. These rocks, including serpentinite, pyroxenite, chromitite, mafic dykes with associated meta-plagiogranite, lava, chert, and limestone, constitute a relatively complete ophiolite complex, indicating that the Lüliangshan Cu deposit can also be introduced as an ophiolite-hosted VMS deposit. Geochemical data show that the meta-dolerite and ore-hosting lava exhibit geochemical features similar to tholeiitic forearc basalt and are probably generated by partial melting of a depleted mantle source metasomatized by hydrous fluids. Some lavas have boninitic compositions and are formed by partial melting of residual mantle after extraction of forearc basalt. Some ore-hosting lavas also have geochemical affinities to island arc tholeiites as a result of more SSZ components involved in their magma source. The chert samples have remarkably high Fe2O3T contents and are classified as iron-rich one of hydrothermal origin, which is deposited in a ridge-proximal environment. These rocks, together with chromitites with subducted-related geochemical features, collectively indicate that the ophiolite-hosted VMS-type Cu deposit was formed in the forearc setting. Meta-gabbros intruding the ore-hosting lavas yield zircon U-Pb ages mainly ranging from 527 Ma to 518 Ma. The new ages of forearc ophiolite and the oldest age of island-arc rocks (514 Ma) suggest that the Lüliangshan Cu deposit formed in the early Cambrian during early-stage subduction of Proto-Tethys Ocean.

Ediacaran stratigraphy and paleogeography in the north Yangtze Block, South China

The Ediacaran gas exploration of the Sichuan Basin in the northwest Yangtze Block, South China, indicates that the paleogeography and lithofacies are of great significance for understanding sedimentary environments and further exploration. There are several contrarian views, however, regarding the Ediacaran paleogeographic configuration and lithofacies distribution in the north Yangtze Block. In this study, we systematically analyzed the Ediacaran stratigraphic, paleogeographic, and facies distributions by integrating outcrop observation, well correlation and seismic interpretation. The Ediacaran in the Micangshan-Hannan terrane is dominated by progressive onlap successions from west to east. We suggest that during the Ediacaran an overall paleogeographic configuration evolved from a shallow-water platform to deeper slope-basin and finally into an open ocean (i.e., the Proto-Tethys ocean) in the north Yangtze Block. The platform margin is roughly distributed along the north margin of the Micangshan-Hannan terrane and the present-day Chengkou fault. During the Doushantuo time, the paleogeographic configuration was characterized by a distally steepened ramp passing seaward into slope-basin facies. During the Dengying time, a rimmed carbonate platform developed on the Doushantuo Formation ramp. The platform margin facies predominantly contained microbially laminated dolomites and stromatolite reef complex. The slope-basin facies were dominated by thin-bedded and slump dolomites. The Ediacaran of the north Yangtze Block experienced a sedimentary evolution from a mixed carbonate-siliciclastic ramp of the Doushantuo Formation to a rimmed carbonate platform of the Dengying Formation, primarily controlled by tectonic subsidence differences and sea level changes.

Tectonic evolution of the Proto-Qiangtang Ocean and its relationship with the Palaeo-Tethys and Rheic oceans

Abstract An evaluation of the potential geodynamic connections between the evolution of Paleozoic oceans in NW Gondwana and NE Gondwana is challenging. Until recently, most syntheses emphasized only two Paleozoic oceans (the Proto-Tethys and the Palaeo-Tethys) in the east Tethys realm. However, the discovery of early Paleozoic ophiolites along Palaeo-Tethys sutures located south of Proto-Tethys sutures challenges these traditional views. After a comprehensive review of relevant early Paleozoic tectonomagmatic events, we herein recognize and propose a model for the tectonic evolution of a hitherto unrecognized early Paleozoic ocean, which we call the Proto-Qiangtang Ocean. This ocean was short lived; it opened in the late Cambrian, began to subduct in the Middle Ordovician, and closed diachronously westwards between the Late Ordovician and the middle Silurian. Its closure by middle Silurian time indicates that was a spatially and temporally distinct ocean from the Palaeo-Tethys Ocean. The early tectonic evolution of the Proto-Qiangtang Ocean shares many characteristics with that of the Rheic Ocean. Both opened in the late Cambrian in the back-arc region of the Iapetus–Proto-Tethys Ocean, and the Proto-Qiangtang Ocean is considered to represent the eastern extension of the Rheic Ocean. This correlation has important implications for the Paleozoic tectonic evolution and palaeogeography of northern Gondwana.

Geochronology and petrogenesis of the early Silurian Zeluo mafic-ultramafic intrusion, eastern Tibet: implications for the tectonic setting and evolution of the eastern Proto-Tethys Ocean

ABSTRACT The Tibetan Plateau is a key region to understand the evolution of the Tethys Oceans. To better constrain the tectonic evolution of the Proto-Tethys Ocean on the western margin of the Yangtze plate, we present an integrated petrography, geochemistry, and zircon U-Pb-Lu-Hf isotope study on newly recognized early Silurian gabbro and serpentinite rocks from the eastern Yidun terrane of the Tibetan Plateau. Zircon U-Pb dating of the gabbro yields an Early Silurian age of 438.2 ± 2.8 Ma. Zircon εHf(t) values of 5.4 to 8.5 suggest a single-stage model age (TDM1) ranging from 729 to 858 Ma. The gabbros exhibit low total rare earth element abundances but are moderately enriched in the light rare earth elements and the large-ion lithophile elements (e.g. Rb, Ba, and Sr), and display representative negative high-field strength elemental anomalies for Nb, Ta, Zr, and Hf on spidergrams. The gabbro and serpentinites were derived from a depleted mantle-like source made of garnet-spinel lherzolite composition, from a sub-arc mantle wedge that was metasomatized by slab dehydration. Thus, the gabbro and serpentinites record an Early Silurian subduction event of the Proto-Tethys Ocean under the Yangtze plate. Furthermore, this study confirms that the Yidun terrane on the western margin of the Yangtze plate is underlined by a Precambrian crystalline basement.

Growth of early Paleozoic continental crust linked to the Proto-Tethys subduction and continental collision in the East Kunlun Orogen, northern Tibetan Plateau

The East Kunlun Orogen (EKO) in the northern Tibetan Plateau records two continental collisional orogenic events and magmatism in early Paleozoic and early Mesozoic. However, possible magmatic additions to the continental crust growth of the EKO in different tectonic stages of early Paleozoic collisional orogeny have been overlooked. Three phases of early Paleozoic plutons from the Xiangride-Kuhai area in the east of the EKO have been chosen for detailed investigation and the results are reported here. The oldest magmatic suite (Stage 1) includes the ca. 471 Ma Qurelong Monzodiorite and ca. 454 Ma granodiorite in the Zhiyu Intrusive Complex. The monzodiorite has a sanukitoid-like composition with high TiO2 and Y contents and is interpreted as being derived from partial melting of metasomatized mantle wedge lherzolite. The granodiorite is typified by its high SiO2 content, high Sr/Y ratio, and depleted Hf isotope, and is interpreted as an adakite-like melt derived from the melting of a subducted Proto-Tethys oceanic crust. The magmatism can be linked to northward subduction of the Proto-Tethys Ocean between 520 and 450 Ma. Stage 2 magmatism is represented by a plutonic suite emplaced during ca. 450−431 Ma with an I-type granitic composition. Of these, the ca. 447 Ma Kengdenongshe Intrusion composed of peraluminous granite with enriched Nd-Hf isotopes is indicative of a Mesoproterozoic igneous source in the orogen. The ca. 450−434 Ma monzogranite and granodiorite in the Walega and Zhiyu intrusive complexes exhibit variable element and isotope compositions. They would have been generated by magma mixing of felsic melts from the old crust and mafic magmas derived from the metasomatized lithospheric mantle, with a mafic melt proportion of >30%. The ca. 431 Ma quartz diorite in the Walega Intrusive Complex is formed through crustal assimilation and fractional crystallization of mafic magmas derived from the metasomatized lithospheric mantle, with a mafic melt proportion >60%. Stage 2 suite was emplaced during the closure of Proto-Tethys oceanic branches and subsequent continental collision during 450−426 Ma. Magmatism diminished between ca. 426 and 410 Ma during exhumation of the continental lithosphere as indicated by the presence of retrograde eclogites in the EKO. Stage 3 magmatic suite includes the ca. 408 Ma Langmuri Intrusion and ca. 403 Ma Niantang Syenogranite. These plutons are adakite-like or have an A-type granitic composition and are enriched in Nd-Hf isotopes. They might have been derived from the remelting of old and juvenile continental crust in a post-collisional extensional setting during 410−390 Ma. Identification of partial melts, derived from the subducted Proto-Tethys oceanic crust and metasomatized lithospheric mantle in stage 1 and 2 plutons, show that the subcrustal materials have been significantly transferred to the overlying continental crust. Hence the magmatism in oceanic subduction (Stage 1) and continental collision (Stage 2) settings contributes to the early Paleozoic juvenile continental crust growth of the EKO. The post-collisional extensional setting (Stage 3) is dominated by the reworking of a pre-existing continental crust. The early Paleozoic continental crust growth processes in the EKO are different from the previous view in which the continental collision orogens have no crust growth, and inconsistent with the proposal that crust growth is significant only in a continental collision setting.

Detrital zircons from high-pressure trench sediments (Qilian Orogen): Constraints on continental-arc accretion, subduction initiation and polarity of the Proto-Tethys Ocean

The Qilian Orogen in the northern Tibetan plateau records the tectonic history of the Proto-Tethys Ocean from its initial spreading, subduction to final closure. However, the timing of subduction initiation, subduction polarity and arc volcanic processes of the Proto-Tethys Ocean remain controversial. In this contribution, we present geochronological and Hf isotopic studies for 812 detrital zircons from blueschist- to eclogite-facies metamorphic sedimentary rocks from the trench and accretionary wedge in the Proto-Tethys subduction zone in the North Qilian suture zone. Out of all the U-Pb age data, the Neoproterozoic to Early Paleozoic zircons reveal the timing of opening and closure of the Proto-Tethys Ocean. We show that ∼50 % of the original sediments were derived from the arc-related magmas, with ages ranging from 535 Ma to 441 Ma. This range represents the time from subduction initiation to closure of the Proto-Tethys Ocean in the Qilian Orogen. Based on the closer resemblance of statistical ages to the crust to the north (Alxa) than the south (Central Qilian), we believe the initial subduction polarity of the Proto-Tethys Ocean is northward. The corresponding εHf(t) values (−11 ∼ +15) shows characteristics of continental arc formed by subduction process. The massive intrusion of arc magma during subduction led to a significant crustal growth, and the crustal thickness estimates based on Eu anomalies indicate that the thickness of the continental margin varies from ∼60 km at 500–490 Ma to ∼45 km at ca. 450 Ma, suggesting an evolution process from compression to back-arc extension.

Tracing tectonic processes from Proto- to Paleo-Tethys in the East Kunlun Orogen by detrital zircons

The East Kunlun Orogen (EKO), a crucial Tethys composite accretionary belt in the northern Tibetan Plateau, experienced a polyphase tectonic history during an orogenic evolution from the Proto-Tethys to the Paleo-Tethys oceans. It is thus an ideal natural laboratory for tracing the Tethys orogenic cycles. Although numerous studies have been done on the geologic evolution of the East Kunlun Orogen, the timing of initial subduction, final closure, and the transition from the Proto-Tethys to the Paleo-Tethys still remain controversial. This study presents U-Pb-Hf isotopic analyses of 794 detrital zircons from three modern river drainage samples and two sedimentary samples in the EKO. The analysed zircons cluster in five age groups: 2900–2100 Ma, 1900–1300 Ma, 1200–730 Ma, 550–370 Ma, and 290–200 Ma, in which almost 67 % are of Phanerozoic age. The εHf(t) values of the 550–370 Ma detrital zircons have a broad range from −28.8 to + 15.9 and exhibit a trend towards positive values, suggesting that subduction of the Proto-Tethys Ocean initiated at ca. 550 Ma and accretion of the continental arc occurred with increasing addition of mantle material. The εHf(t) values (−8 to + 13) of 290–200 Ma detrital zircons have no obvious evolutionary trend, reflecting the continuous crust-mantle interaction during the evolution of Paleo-Tethys arc. Our results, combined with previous research, reveal two discontinuous and distinct orogenic cycles from the Proto-Tethys to the Paleo-Tethys in the EKO: the Proto-Tethys orogenic cycle from oceanic subduction to continental collision and orogenic collapse at ca. 550–370 Ma, and orogenesis of the Paleo-Tethys Ocean at ca. 290–200 Ma in the EKO, with a gap of ∼ 80 Myr, during which time there was no subduction, and the extensional setting resulted in rare identifiable zircon record.

Tectonic affinity and significance of the Qilian Block: Evidence from river sediments in the Central Qilian Belt

The tectonic affinity of the Qilian Block is crucial to reconstructing the ocean-continent configuration of Rodinia. This study integrates previous studies with new whole-sample Nd and zircon U-Pb-Hf isotopic data from modern river sediments to identify the origin of the Qilian Block and its tectonic role in the Rodinia supercontinent cycle. Zircon U-Pb-Hf data in combination with whole-sample/whole-rock Nd and Pb isotopic data indicate that the Central Qilian Belt (CQB) shows close tectonic affinity with the Hualong and North Wulan terranes and together constituted a coherent Qilian Block crustal domain, whereas the Quanji Block likely underwent an independent evolution prior to the early Paleozoic orogeny. A comprehensive assessment of geochronological and whole-rock geochemical-isotopic data reveal that the Precambrian basement of the Qilian and Alxa blocks experienced similar historical evolutions and possibly formed a coherent Qaidam-Altyn-Qilian-Alxa continent before the breakup of Rodinia. Moreover, the Qaidam-Altyn-Qilian-Alxa continent was probably once part of the Yangtze Block, laying along the periphery of the supercontinent Rodinia. Therefore, the northwestern boundary of the Yangtze-affinity block can extend to the Alxa Block and the Qilian Ocean is a branch of the proto-Tethys Ocean. A micro-block can serve as a window into reconstructing the ocean-continent configurations of supercontinents during certain periods.

Subduction initiation of the Proto-Tethys Ocean that facilitated climate change and biodiversification

The effect of plate interactions on surface processes, environmental changes of the earth and their contributions to biodiversification during Gondwana assembly has been hotly debated in the last decades. The late Neoproterozoic-early Paleozoic was a key period in the tectonic assembly of Gondwana, global climate change and metazoan evolution. During this interval, the amalgamation of Greater Gondwana was succeeded by the subduction initiation of oceanic slabs in the Proto-Tethys Ocean. Here, we use whole-rock geochemical data, detrital zircon U-Pb and Hf isotopic of Cambrian and Ordovician sedimentary rocks from the Baoshan block in the Southeastern Tibetan Plateau combined with existing data in the literature to constrain the timing of subduction initiation of the Proto-Tethys Ocean at 540-500 Ma. The results show that the Baoshan block, adjacent to the Pinjarra Orogen on the northern margin of India, witnessed the transition from passive to active continental margin in East Gondwana. After the subduction initiation, the source of the sedimentary rocks experienced more intense weathering, causing a higher sediment flux. Contemporaneously, the Cambrian Explosion and stepwise increase in atmospheric oxygen accompanied the Greater Gondwana assembly at the late Neoproterozoic-early Paleozoic boundary. Subduction of the Proto-Tethys Ocean and occurrence of peripheral orogens further increased atmospheric O2 content, transported nutrients to the ocean and reduced the greenhouse effect, which provided major propulsion for biological evolution.

The Early Paleozoic Subashi ophiolite in the West Kunlun Orogenic Belt (northwestern Tibetan Plateau): Implication for the tectonic evolution of the Proto-Tethys

The West Kunlun Orogenic Belt (W-KOB) in the northwestern Tibetan Plateau retains critical information about the tectonic evolution of the Proto- and Paleo-Tethys during the Early Paleozoic to Mesozoic. Although a trench-arc-back arc basin model was used to address the tectonic evolution of Proto-Tethys, records about the evolution of this ocean basin remain ambiguous, particularly no studies about ages and tectonic affinities of ophiolites in the W-KOB are available. In this paper, we present new results on whole-rock elemental and Sr-Nd isotopic composition of basalts, and U-Pb zircon ages of gabbro, diabase and basalt from the Subashi ophiolite. The basalts are characterized by low REE concentrations, depleted LREEs in the chondrite normalized REE patterns, and unfractionated HFSEs in the primitive mantle normalized trace element spider diagrams, resembling normal mid-ocean ridge basalts. They have constant 87Sr/86Sr(i) values of 0.70472–0.70590 and positive εNd values of +7.5 to +7.9. Accordingly, it can be concluded that the Subashi ophiolite was derived from a depleted spinel lherzolite mantle source in a mid-ocean ridge setting. Zircon grains separated from basalt, diabase and gabbro have 206Pb/238U ages of 455 ± 4 Ma, 451 ± 6 Ma, and 446 ± 6 Ma, respectively, which are taken to be the crystallization ages of these rocks. Together with previously published data, our new data propose that the Mazha-Kangxiwa-Subashi ophiolitic mélange zone marks the existence of the Proto-Tethys ocean basin during the Early Paleozoic.

Grenville-age basement and early Paleozoic reworking in the Qilian Orogen, NW China: Implications for paleotectonic reconstruction of Rodinia and Gondwana

• Grenville-age terrane is identified in the Qilian orogen. • The Precambrian basement of the Qilian orogen is divided into four distinct mega-sequences. • ∼475 Ma metamorphism was related to the subduction of the Proto-Tethys Ocean. The Qilian orogen at the northeastern margin of the Tibetan Plateau records a complete history from continental breakup to oceanic basin evolution and back to continental collision during the Neoproterozoic to Paleozoic interval. The Qilian Ocean, as a branch of the Proto-Tethys Ocean, was connected to the Iapetus Ocean but is lesser known than its two more famous counterparts. This paper presents zircon U-Pb and biotite-muscovite 40 Ar/ 39 Ar isotope data from the Yemananshan Complex in the western Central Qilian belt to assess the age and provenance of late Mesoproterozoic-early Paleozoic successions in the Qilian orogen. The results indicate the Central Qilian belt is characterized by Late Mesoproterozoic-early Neoproterozoic orogenesis with magmatism from 1200 Ma to 900 Ma and clastic metasedimentary rocks deposited between 1050 and 920 Ma. Statistical analysis of detrital zircon data and auxiliary temporal information from the Qilian orogen affirms the notion that sedimentation occurred in four distinct megasequences: (1) 1950–1850 Ma, the oldest known strata, (2) 1250–920 Ma, during Grenvillian-Sveconorwegian orogenesis, (3) 840–760 Ma, the disintegration of Rodinia, and (4) 650–550 Ma, which is typical of the orogenic belts that sutured the Gondwana continental blocks. Metamorphic zircon and 40 Ar/ 39 Ar isotope data indicate early Paleozoic metamorphism at ca. 475 Ma, which overlaps with magmatic activity associated with arc-back-arc basin development in the Central Qilian belt, which was associated with subduction of the Proto-Tethys in eastern Gondwana.

Geochronology, ore-forming processes and fluid sources of the Qinglonggou gold deposit, North Qaidam (NW China): Constraints from in-situ U-Pb dating of monazite and geochemistry of pyrite

The Tanjianshan goldfield is the most important gold producer in the North Qaidam, NW China. Genesis of gold deposits in Tanjianshan is enigmatic, primarily because of uncertainties as to timing, mineralization processes and fluid sources. We investigate these uncertainties through a comprehensive study of the texture and geochemistry (LA-MC-ICPMS trace elements and sulfur isotopes) of pyrite and in-situ U-Pb dating of monazite from the Qinglonggou gold deposit. Six types of pyrite with distinct textures, trace element contents and sulfur isotopes have been identified, and provide constraints on two overprinting gold-related hydrothermal events. Stage I consists of Py1, occurring as veinlets parallel to S1 deformation in a schist, is characterized by extremely low Co/Ni ratios with high contents of Ni, and high δ34S values (+14.9 ‰ to +16.1 ‰), and were likely derived from metamorphism of the sedimentary strata in the Tanjianshan and Wandonggou Group. In stage II, Py2 occurs in the pyrite-quartz veins that cut Py1 veinlets. They have similar δ34S (+13.1 ‰ to +17.8 ‰) to Py1, indicating that sulfur was likely derived from the dissolution of Py1. The stage I and II veins formed during an early gold pre-concentration event, which is constrained at 429.1 ± 5.9 Ma by in-situ U-Pb dating of monazite that coexists with Py1. Stage III includes Py3 and Py4 which occur in the altered carbonaceous schist and accompanying quartz veins. Trace metals are highest in Py3 (Ag, Au, Sb, and Bi) which represent the most economic ore stage in the deposit, whereas the following Py4 shows low concentrations of most trace elements due to abundant Py3 precipitation. In contrast, trace elements contents increase in Py5 to similar levels as those seen in Py3, suggesting a new pulse of fluid in stage IV. Minor Py6 grains precipitated in the youngest quartz-calcite veins associated with stage V, and contain low concentrations of trace elements. The δ34S values of Py3 (+5.2 ‰ to +6.0 ‰) and Py4 (+4.2 ‰ to +6.1 ‰) are within the range of magmatic sulfur, suggesting a dominantly magmatic source. The δ34S values increase from Py3-4 to Py5 (+6.3 ‰ to +7.4 ‰), and to Py6 (+8.7 ‰ to +9.6 ‰), probably due to increasing contribution from the host rocks, as well as decreasing fO2, through incremental fluid-rock interaction. Stages III and IV represent a late gold-mineralizing event during Late Paleozoic to Early Mesozoic, while stage V is post-mineralization. The overprinting gold pre-concentration and mineralization events in the Tanjianshan goldfield correspond to orogenic evolution of the Proto-Tethys Ocean and Paleo-Tethys Ocean, respectively.

Evolution of the Paleo-Tethys Ocean in Eastern Kunlun, North Tibetan Plateau: From continental rift-drift to final closure

The dynamic evolution of the Paleo-Tethys Ocean from continental rift-drift and seafloor spreading to initial subduction and final closure has been hotly debated. This paper tries to bring new constraints to this longstanding debate based on the compiled the latest data and investigations into Carboniferous-Triassic magmatism in the westernmost segment of the Eastern Kunlun orogenic belt, North Tibetan Plateau. New results indicate that the ca. 335 Ma appinite stocks were derived from the asthenosphere mantle wedge slightly modified by slab-derived materials. The ca. 244 Ma dioritic appinite stocks were sourced from the partial melting of hydrothermally altered upper oceanic crust following the closure of the Paleo-Tethys Ocean. The ca. 231 Ma adakitic granite stock originated from the partial melting of the thickened eclogitic lower crust in the syn -collisional setting, and the ca. 219–215 Ma granite batholith had a mafic source in the post-collisional setting. By integrating records available, new scenarios of the evolution of the Paleo-Tethys Ocean in Eastern Kunlun were established. The ca. 390–380 Ma continental rifting leading to the opening of the Paleo-Tethys Ocean is attributed to the post-collisional gravitational instability subsequent to the closure of the Proto-Tethys Ocean and the far-field extensional stress induced by the retreating subduction of the Paleo-Asian Oceanic lithosphere along the northern margin of Tarim-Qaidam. The Paleo-Tethys Oceanic lithosphere initially northward subducted at ca. 366–335 Ma. The closure of the Paleo-Tethys Ocean leading to the “soft” collision between the Eastern Kunlun and Bayanhar terranes occurred at ca. 235–225 Ma followed by the ca. 225–200 Ma post-collisional extension. • The ca. 390–380 Ma opening of the PTO due to gravitational instability and retreating subduction. • The initial northward subduction of PTO occurred at ca. 366–335 Ma. • The closure occurred at 235–225 Ma followed by 225–200 Ma post-collisional extension.

Petrology and P-T-t Path of Huangyuan Group and Maxianshan Group in the Central Qilian Block, NW China: Implications for Tectonic Evolution of the Proto-Tethys Ocean

AbstractThe Qilian orogen marks the northern boundary of the Proto-Tethys Ocean and consists of the North Qilian belt, the Central Qilian block and the South Qilian belt. The metamorphic P–T–t path of the Central Qilian block and the tectonothermal processes that affected it are crucial to understanding the closure of the ocean. This paper presents a systematic investigation of the metamorphic P–T–t path of amphibolites from the Precambrian Huangyuan Group and Maxianshan Group in the Central Qilian block. An amphibole 40Ar/39Ar age of 456.7 ± 5.2 Ma from a garnet amphibolite and a 427.8 ± 3.8 Ma muscovite 40Ar/39Ar age from an orthogneiss in the Huangyuan Group, and amphibole and biotite 40Ar/39Ar ages of 408.7 ± 2.2 Ma and 349.9 ± 2.0 Ma from an amphibolite and a 361.4 ± 2.0 Ma biotite 40Ar/39Ar age from an orthogneiss in the Maxianshan Group record cooling stages after peak metamorphism. Phase equilibrium modeling of all amphibolite samples indicates clockwise P–T paths. A garnet amphibolite from the sillimanite zone in the Huangyuan Group recorded peak metamorphic conditions of 5.6–6.6 kbar and 730–770°C, reflecting granulite-facies metamorphism. Peak P–T conditions for a garnet amphibolite from the garnet zone are constrained to 580–600°C and 6–7 kbar. The amphibolite sample from the Maxianshan Group indicates peak P–T conditions of ~5.2 kbar and ~630°C. The Central Qilian block was affected by both southward subduction of the North Qilian Ocean and northward subduction of the South Qilian Ocean, which resulted in paired metamorphic belts along the northern margin of the Central Qilian block and accretion of the Hualong block to its southern margin. The subducted North Qilian oceanic slab detached and the remainder was exhumed at 453–445 Ma, accompanied by the extension and retrograde metamorphism of the overlying crust of Huangyuan Group at 457–428 Ma. Subsequent cooling of the Maxianshan Group indicates that the thermal regime relaxed during post-collisional extension at c. 409–350 Ma between the Hualong block and the Central Qilian block.

Petrogenesis and tectonic setting of Early Silurian island-arc-type quartz diorite at the southern margin of the East Kunlun orogenic belt: analysis of the evolution of the Proto-Tethyan Ocean

Petrogenesis and tectonic setting of Early Silurian island-arc-type quartz diorite at the southern margin of the East Kunlun orogenic belt: analysis of the evolution of the Proto-Tethyan Ocean

Geochronology, geochemistry, and Hf isotopic compositions of the Early Devonian Heihaibei granite in the East Kunlun Orogen, Northwest China

The East Kunlun Orogen (EKO) is located in the western part of the Central Asian Orogenic Belt, and it records the Neoproterozoic–Early Devonian tectonic evolution involving the subduction and extinction of the Proto-Tethys Ocean. The Heihaibei granite is located in the South Kunlun belt within EKO. In this study, geochronology, geochemistry, and Hf isotope analysis of Heihaibei monzogranite and granodiorite were carried out. Their LA-ICP-MS zircon U-Pb results yielded ages of 414.1 ± 2.0 and 417.8 ± 2.0 Ma. The Heihaibei granite is peraluminous (A/CNK = 0.99–1.30), with high alkali contents (7.76–9.14 wt.%) and 10 000 Ga/Al (2.60–3.38), FeOT/MgO (27.6–49.5), and Y/Nb ratios (1.93–2.93), suggesting that the Heihaibei granite is an A2-type granite. The Heihaibei granite samples have zircon εHf(t) values ranging from −6.3 to −0.3, which suggests that the Early Devonian granite was derived from partial melting of Mesoproterozoic ( TDM2 = 1797–1415 Ma) crust. When considering data on contemporaneous granites, geochronological and geochemical data for the Heihaibei granite indicate that it was emplaced in a post-collisional extension environment following the closure of the Proto-Tethys Ocean. The Heihaibei granite was formed by extension of the southern subduction accretionary complex as part of the post-collisional setting during the closure of the Proto-Tethys Ocean.

Timing, Provenance, and Tectonic Implications of Ore-Hosting Metasedimentary Rocks in the Giant Liba Gold Deposit, West Qinling Belt, China

The closure time of the Shangdan Ocean is critical for understanding the tectonic evolution of the Proto-Tethys Ocean. However, the proposed closure time was prolonged from Ordovician to Devonian. In the present study, detrital zircon from the metasedimentary rocks of the Liba Group in the West Qinling Belt was analyzed to constrain the closure time of the Shangdan Ocean. The three youngest grains from the Liba Group yield a maximum deposition age of 418 ± 13 Ma, indicating the Middle Devonian deposition. Detrital zircon grains show two main U–Pb age peaks of 810 Ma and 440 Ma with εHf(t) values spanning from −24.3 to +8.8 and −6.3 to +4.1, respectively, suggesting that the sediments of the Liba Group were derived from both the North and South Qinling Belts. The Lower Devonian in the South Qinling Belt shows similar provenance to the Liba Group, whereas sediments from the North Qinling Belt are absent in the Silurian strata of the South Qinling Belt. From Late Silurian to Early Devonian, the tectonic setting changed from subduction to collision. This evidence consistently suggests the disappearance of the Shangdan Ocean. The noticeable decrease in magmatism from 510–420 Ma to 420–390 Ma and the shrinking of εHf(t) values from −15.5–+12.8 to −8.4–+4.2 reveal that the Shangdan Ocean, as the eastmost embranchment of the Proto-Tethys Ocean, was closed at ca. 420 Ma.

Detrital zircon age signatures of the basal Cambrian sandstone unit in North China: implications for drainage divides during global Sauk transgression and separation from Gondwanaland

ABSTRACT The basal Cambrian sandstone unit in the North China craton (NCC) is an example of globally widespread siliciclastic succession that resides on the Great Unconformity and deposited on a hypothesized low-lying peneplain during the Cambrian global eustatic sea-level rise. Detrital zircon age signatures from this distinct sequence enable recognition of the ancient drainage system of the NCC in deep time and track its potential linkage with the Gondwana landmass. LA-ICP-MS U–Pb dating of the fossil-calibrated basal Cambrian (Series 2) detrital zircon samples from seven measured sections reveal marked spatial changes in their age signatures that can be divided into three distinct types. The first is generally characterized by the bimodal age populations with broad peaks at ~1.85 Ga and ~2.5 Ga that correlate with the Archean to Paleoproterozoic basement inboard of the NCC. The second is featured by multimodal distribution with diagnostic Neoproterozoic peaks that correspond to subregional magmatic record. The third also shows multiple-zircon age populations, but yields significant crystallization ages close to the early Cambrian age. Comparing our new data with existing age spectra for the Cambrian strata across the NCC and the northern Gondwana demonstrates that separate drainage systems did exist in the peneplained basement during global Cambrian transgression and the basal Cambrian unit in the NCC was not a part of the far-travelled sand sheet across the northern margin of Gondwana. The most suitable source for Cambrian-aged grains constrained by paleogeographic restoration is the arc terrane developed along the northern margin of the NCC as a result of subduction of the Paleo-Asian oceanic plate. Our new continental-scale detrital zircon provenance signatures in the basal Cambrian unit suggest that the NCC should be considered a discrete continental block separated by the Proto-Tethys Ocean in the Cambrian, rather than an integral part of the northern Gondwana.

Cambrian–Silurian sediments in the southeastern Qilian Orogen, NE Tibetan Plateau: Constraints on crustal and tectonic evolution of microcontinents in the northern Proto-Tethys Ocean

The Qilian Orogen preserves abundant Early Paleozoic records, revealing the tectonic evolution of the northern Proto-Tethys Ocean on the periphery of the northern margin of eastern Gondwana. We present detrital zircon U–Pb and Hf isotopes, and whole-rock element and Nd isotopic geochemical data on the Early Paleozoic sedimentary rocks in the southeastern Qilian Orogen. Detrital zircon grains from these rocks are clustered into seven distinct age groups: 495–426 Ma, 710–501 Ma, 1043–726 Ma, 1347–1054 Ma, 2009–1609 Ma, 2874–2274 Ma, and >3100 Ma. Detrital zircon and geochemical results suggest that their provenances are Early Paleozoic subduction–accretion related rocks, Neoproterozoic basements of the Qilian Block, and recycled sediments from Precambrian units. The whole-rock εNd(t) values of the sedimentary samples range from –13.0 to 2.5, and the εHf(t) values of Neoproterozoic–Early Paleozoic detrital zircon grains range from –28.6 to 15. Hf and Nd isotopes indicate marked growth and reworking of the continental crust in the early Neoproterozoic and Ordovician–Early Silurian. As revealed by age spectra of detrital zircon, we infer that the Qilian Block was tectonically related to blocks on the northern margin of eastern Gondwana during the Early Paleozoic, such as the North Qinling, Indochina and South China blocks. We also suggested that the Qilian Block experienced double-sided subduction of the North and South Qilian oceans, and developed sedimentation in the inter-arc and back-arc basins during the Ordovician to earliest Silurian. Finally, sedimentation occurred in an Early Silurian retro-arc foreland basin and Middle Silurian foreland basin during the syn-collisional stage.

Cambrian intra-oceanic subduction within the southern branch of the Proto-Tethyan Ocean: Constraints from rhyolites in the Lajishan suture, NE Tibetan Plateau

The South Qilian suture in the Qilian Orogen is an early Paleozoic belt that developed as a result of closure of the southern branch of the Proto-Tethyan Ocean. It is characterized by widespread Cambrian intermediate-basic volcanic rocks which record the early-stage evolution of the paleo-ocean basin. However, Cambrian evolution of the paleo-ocean basin remains controversial due to the lack of research on the age and tectonic affinity of these volcanic rocks. In this study, we identified rhyolites from the intermediate-basic volcanic rocks in the South Qilian suture. They occur as pebbles in the breccias, or as veins and interlayers within the intermediate-basic volcanic rocks. Zircon U-Pb datings yield ages of 514.0 ± 2.8 Ma, 521 ± 4 Ma, 523 ± 5 Ma, and 513 ± 9 Ma for four rhyolites, respectively. The Cambrian rhyolites show variable La/Sm ratios and Rb contents and positive zircon εHf(t) values (+6.8 to +14.6) and whole-rock εNd(t) values (+2.1 to +4.6), suggesting that they were derived from partial melting of juvenile mafic crust. They also have high Al2O3 and CaO contents, as well as low K2O and trace element concentrations, which are similar to those of oceanic arc rhyolite. These early Cambrian oceanic arc rhyolites, along with the SSZ-type ophiolite (ca. 530–520 Ma) to the north and late-stage granitoid intrusions (ca. 474–450 Ma), indicate that the southern branch of the Proto-Tethyan Ocean experienced southward intra-oceanic subduction during the early Cambrian-early Ordovician period.