Zongqi Wang

Neoproterozoic Subduction and Rifting on the Northern Margin of the yangtze Plate, China: Implications for Rodinia Reconstruction

Located in the southwestern Qinling Mountains of central China near the northern margin of the yangtze plate, the Bikou terrane has long been controversial regarding how it relates to the Late Precambrian tectonic history of the yangtze plate. The igneous and sedimentary succession of the Bikou terrane has been variously interpreted to represent either a Meso-Neoproterozoic rift assemblage, or a subduction arc assemblage. Our recent work involving igneous petrologygeochemistrysedimentary basin analysis strongly supports the subduction arc interpretation. In addition, SHRIMP U-Pb zircon dating of volcanic rocks from the Bikou arc reveal that subduction occurred along the northern margin of the yangtze plate in the Neoproterozoic, ~846-776 Ma. Additionallynew 40Ar/39Ar and SHRIMP U-Pb ages of mafic dike swarms in the Bikou arc indicate emplacement occurred in the Late Neoproterozoic, ~689-660 Ma. Therefore the age of volcanism is coincident with, and thus incompatible with, previously proposed extension-related mantle upwelling and break-up of Rodinia involving the South China Block (SCB). We propose that the yangtze plate collided with the Cathaysian Block at the end of the Grenville orogeny to form the SCB, and subsequently developed a long-lived subduction-related arc along its northern margin in the Neoproterozoic, ~846-776 Ma. Subsequent rifting that formed the passive margin of the SCB may have been initiated ~689-660 Ma.

Opening of the Tethys in southwest China and its significance to the breakup of East Gondwanaland in late Paleozoic: Evidence from SHRIMP U-Pb zircon analyses for the Garzê ophiolite block

SHRIMP U-Pb zircon analyses for a gabbro sample from the Garzê ophiolite block yielded a mean206Pb/238U age of 292 ±4 Ma, which indicated that the spreading time of the Garzê-Litang Tethys was most likely at the earliest Permian. Combined with previous studies, we suggest that the opening of the Tethys in southwest China was derived from breakup of the East Gondwanaland in the late Paleozoic.

Geochemical constraints on the petrogenesis of the Proterozoic granitoid gneisses from the eastern segment of the Central Tianshan Tectonic Zone, northwestern China

The Tianshan orogen is divided into the Northern, Central and Southern Tianshan tectonic zones by the northern and southern sutures on both sides of the Central Tianshan Tectonic Zone. The eastern segment of the Central Tianshan Tectonic Zone is characterized by the presence of numerous Precambrian metamorphic blocks and is unconformably overlain by Ordovician–Silurian and late Palaeozoic strata. The Precambrian Kumishi and Pargantag metamorphic blocks are the largest older blocks in the eastern segment of the Central Tianshan Tectonic Zone, consisting mainly of metamorphic granitoids and sedimentary rocks in greenschist to amphibolite facies. There are two major lithological assemblages of the metamorphic granitoids: (1) quartz dioritic gneisses, and (2) granodioritic–monzogranitic gneisses with a minor amount of tonalitic and syenogranitic gneisses in both the Kumishi and Pargantag blocks. The quartz dioritic gneisses are characterized by low Sr/Ce ( 2 O (2.65–4.04 wt %) contents and e Nd (t) values (−2.37–5.84), and negative Nb and Zr–Hf anomalies, as well as relatively flat chondrite-normalized REE patterns with slightly negative Eu anomalies, suggesting that the quartz dioritic gneisses were derived from partial melting of a depleted mantle source enriched by fluids and sedimentary melts from the subducted slab. However, most of granitic gneiss samples display high K 2 O contents, low Al 2 O 3 /(FeO * + MgO + TiO 2 ) values, and relatively flat chondrite-normalized REE patterns with intensively negative Eu anomalies. Integrated low e Nd (t) values and older T DM model ages suggest that crustal materials played a significant role in the petrogenesis of these granitoid gneisses and that they were mainly derived from the partial melting of calc-alkaline mafic to intermediate rocks in the crust. Also, variations in geochemical features between the Kumishi–Gangou and Pargantag regions, such as Zr and Hf, may reflect geographic variability in the development of coeval granitic magmas. Tectonic discrimination for granitoid, using trace elements, together with Nd isotopic data, demonstrates that these granitoid gneisses in the eastern segment of the Central Tianshan Tectonic Zone formed in a continental margin arc during late Mesoproterozoic times.

Middle Triassic arc magmatism along the northeastern margin of the Tibet: U–Pb and Lu–Hf zircon characterization of the Gangcha complex in the West Qinling terrane, central China

The tectonic setting of Mesozoic magmatic complexes in the northeastern margin of the Tibet plateau is disputed, and hence gives rise to uncertainty concerning the tectonic evolution of the northeastern Tibet Plateau and the timing of the closure of the Palaeo-Tethys ocean. The Gangcha complex is typical of these complexes, consisting of andesite, dacite, gabbro, gabbroic diorite, granodiorite, quartz diorite, and diorite with typical chemical traits of continental margin arc rocks. Andesite, gabbroic diorite, and mineralization-associated potassic-altered diorite yield weighted mean 206Pb/238U ages of 242.1 ± 1.2 Ma, 243.8 ± 1.0 Ma and 234.0 ± 0.6 Ma respectively. Zircon ϵHf(t) for magmatic grains ranges from −3.5 to +5.7, interpreted to demonstrate that the Gangcha complex contains crustally contaminated mantle magmas. Inherited zircons in the complex yield similar U–Pb ages (777–310 Ma) to the A’nyemaqen composite ophiolite assemblage with ϵHf(t) of −17.4 to +11.6. This suggests that components of this older ophiolite melted and contributed to the Gangcha complex magmas. Hence the Gangcha complex is considered to have formed as a continental margin arc in northeastern Tibet by northward subduction during consumption of the Palaeo-Tethys ocean. Regionally, it corresponds to the arc magmatism along the eastern and western Kunlun sutures to the west and the Mianlue suture to the east. Supplementary material: Geochemical data, and zircon U–Pb and Lu–Hf data for the Gangcha complex are available at www.geolsoc.org.uk/SUP18521.

Electron microprobe monazite geochronological constraints on the Late Palaeozoic tectonothermal evolution in the Chinese Tianshan

Electron microprobe monazite data from the early Palaeozoic granitoid gneisses in the Central Tianshan Tectonic Zone provide two main ages: (1) an age of 355 ± 12 Ma obtained on the chemically complex grains, which is good agreement with the high-pressure metamorphic age obtained on eclogite and blueschist rocks from the south Tianshan Tectonic Zone; (2) an age of 323 ± 7 Ma obtained on the complex and the monogenetic grains, which is also closely consistent with the age for a major episode of cooling–recrystallization of these high-pressure rocks. Monazite from the schist of the Xingxingxia complex yields a younger Th/Pb–U/Pb centroid age of 282 ± 9 Ma. This younger age is interpreted as being linked to a magmatic episode, possibly related to underplating–extension, and/or shear activities. These data provide timing constraints on the episodes of metamorphism and high-pressure metamorphism and evidence for transition from compression to extension during the Carboniferous to early Permian, for the Chinese Tianshan region. Moreover, this study shows that electron microprobe dating of monazite, together with recent statistical treatment, can provide a reliable method to reveal a sequence of events within an orogenic cycle.

Devonian Sedimentary Environments and Provenance of the Qinling Orogen: Constraints on Late Paleozoic Southward Accretionary Tectonics of the North China Craton

The Qinling orogen of central China occupies a key position in East Asia, and is of fundamental importance in unraveling its tectonic evolution. Devonian sedimentary basins are located between the North Qinling arc and the Baishuijiang Devonian-Permian accretionary wedge. Paleocurrent indictors and petrological and geochemical analyses show that turbiditic and coastal sandstones and pyroclasts developed in paleo-forearcs. Sedimentation of conglomerates and associated turbiditic and pyroclastic rocks evidently was related to the development of the North Qinling orogen rather than the South China craton. Gravels in the conglomerates were derived predominantly from the North Qinling and partly from its basement. Northward Devonian subduction and subsequent uplift caused successive forearc depocenters and associated facies belts to migrate southwards synchronously with sedimentation. Transpressive and transtensional faults segmented the basins into discrete half-graben sub-basins. Paleocurrent analysis adjacent to thrust faults demonstrates that thrust sheets crests were truncated by erosion and provided detritus for sub-basins. Shallow-marine and turbiditic depositional systems evolved in complex patterns to produce varied facies frameworks associated with complex subduction accretion. These facts, together with other geological and geochemical data, demonstrate that the North China craton grew southwards by arc-accretion processes from the end of the early Paleozoic to the beginning of the late Paleozoic, long before the integration of the North China and Yangtze cratons.

Chronology and petrogenesis of the Hejiazhuang granitoid pluton and its constraints on the Early Triassic tectonic evolution of the South Qinling Belt

The Hejiazhuang pluton is located in the South Qinling Tectonic Belt (SQTB) in the north side of the Mianxian-Lueyang Suture Zone, and consists dominantly of granodiorites. LA-ICP-MS zircon U-Pb dating and Lu-Hf isotopic analyses reveal that these granodiorites of the Hejiazhaung pluton emplaced at ∼248 Ma, and show a large variation in zircon ɛHf(t) values from −4.8 to 8.8. These granodiorite samples are attributed to high-K to mid-K calc-alkaline series, and characterized by high SiO2 (66.6%–70.0%), Al2O3 (15.04%–16.10%) and Na2O (3.74%–4.33%) concentrations, with high Mg# (54.2–61.7). All samples have high Sr (627–751 ppm), Cr (55–373 ppm) and Ni (17.2–182 ppm), but low Y (5.42–8.41 ppm) and Yb (0.59–0.74 ppm) concentrations with high Sr/Y ratios (84.90–120.66). They also display highly fractionated REE patterns with (La/Yb)N ratios of 18.9–34.0 and positive Eu anomalies (δEu=1.10–2.22) in the chondrite-normalized REE patterns. In the primitive mantle normalized spidergrams, these samples exhibit enrichment in LILEs but depletion in Nb, Ta, P and Ti. These geochemical features indicate that the granodioritic magma of the Hejiazhuang pluton was derived from the partial melting of hybrid sources comprising the subducted oceanic slab and sediments, and the melts were polluted by the mantle wedge materials during their ascent. The emplacement ages and petrogenesis of the Hejiazhuang pluton prove that the initial subduction of the Mianlue oceanic crust occurred at ∼248 Ma ago, and the SQTB was still under subduction tectonic setting in the Early Triassic.

Provenance and geotectonic setting of the Palaeoproterozoic Zhongtiao Group and implications for assembly of the North China Craton: whole-rock geochemistry and detrital zircon data

Abstract: Geochemical data reveal the contrasting geochemical characteristics of the metasediments in the Jiepailiang and Bizigou formations of the Palaeoproterozoic Zhongtiao Group, North China Craton. High Zr/Sc and SiO2/Al2O3 values and especially the evolved and uniform nature of εNd(t) values plus detrital zircon U–Pb ages and Hf isotopic signatures for the former are consistent with derivation from an evolved felsic source with sedimentary recycling signs. However, lower values of SiO2/Al2O3 and Zr/Sc, and higher contents of Fe2O3* and MgO for the latter indicate input of more mafic components. These geochemical characteristics, combined with the wide range of εNd(t) values, suggest that these rocks were derived in part from old continental material, dominated by felsic rocks of magmatic origin, and in part from a continental island arc, which accounts for the variable mixture of mafic components. These results suggest that the Zhongtiao Group was deposited after c. 2110 Ma, in a back-arc basin, behind an eastward-directed subduction system. Subsequent deformation and metamorphism reflect closure of the oceanic basin between the Eastern and Western blocks of the North China Craton, along the Trans-North China Orogen, contributing to amalgamation of the supercontinental Columbia. Supplementary material: Data are available at http://www.geolsoc.org.uk/SUP18478.

Timing and setting of Guanjiagou conglomerate in South Qinling and their tectonic implications

Located in the southern Qinling Mountains of central China, the Guanjiagou Formation has been a controversial issue with regard to its depositional age and tectonic implications. Being comprised of an approximately 2050 m thick succession of texturally and compositionally immature, presumed marine turbiditic sandstones and conglomerate, the Guanjiagou Formation consists of an overall prograding- and coarsening-upward megasequence. Although bounded by faults on both its northern and southern margins, it is weakly metamorphosed and deformed. To the north is the Devonian Sanhekou Group and to the south is the Neoproterozoic Hengdan Group. The lower portion of the sequence contains granitic and volcanic clasts (Guanjiagou conglomerate). The feldspars from these clasts were dated using the40Ar/39Ar method. Two cooling ages of 219.69±0.49 and 216.46±0.59 Ma, for K-feldspar from a granitic clast and plagioclase from a volcanic clast, respectively, were obtained. These ages are identical to the time of regional igneous activities (ca. 240–220 Ma) and are interpreted as the products of magmatism associated with collision in the Qinling orogenic belt in the Early Mesozoic, suggesting that the Guanjiagou Formation was deposited in the Norian of the Late Triassic, ca. 220 Ma. Therefore,40Ar/39Ar and sedimentary analyses suggest that the Guanjiagou Formation contains sediments that may have filled in a remnant ocean basin, which might be part of the Anyemaqen-Mianlüe ocean, or Tethys on the southern side of Central Orogenic Belt in China during the Late Paleozoic to Early Mesozoic.

Deformation Structures of the Madao Gneiss in South Qinling： Structural Analysis, Geochronological Constraints, and Tectonic Implications

The tectonic evolution of South Qinling, which is a main part of the Qinling orogenic belt, is still in dispute and deformation history of South Qinling is poorly studied. In this paper, detailed structural, microstructural, quartz c-axis fabric analysis, and geochronology results for the Madao gneiss in South Qinling are presented to characterize the deformation history. Results show that rocks in the northern part (Tiefodian-Laozhanggou) experience general shearing and deform at relative low temperature. The shear sense generally is south to north. In contrast, rocks in the southern part (Laozhanggou-Panjiahe) are weakly sheared with pure shear features and evidence of high-temperature deformation. Based on the analyses, we conclude that there exist two distinct deformation geometries in the Madao gneiss and accordingly we can divide the deformation into two stages. The early stage is represented by regional shortening, while the late stage features northward thrust shearing and evidence shows that it was a progressive process between them. LA-ICP MS U-Pb dating of zircons from pre-deformational migmatite veins yields age of 198.5 ± 2.0 Ma. This result, in combination with the age of post-deformational granite, indicates that the northward thrust shearing of the Madao gneiss occurred in the Late Triassic. In view of these results and other reported data in South Qinling, we propose that deformation in Madao gneiss may result from the initial collision and subsequent northward accretion in Late Triassic.