Zheng-Xiang Li

Formation of the 1300-km-wide intracontinental orogen and postorogenic magmatic province in Mesozoic South China: A flat-slab subduction model

We propose a flat-slab subduction model for Mesozoic South China based on both new sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon data and a synthesis of existing structural, geochronological, and sedimentary facies results. This model not only explains the development of a broad (∼1300-km-wide) intracontinental orogen that migrated from the coastal region into the continental interior between ca. 250 Ma and 190 Ma, but can also account for the puzzling chain of events that followed: the formation of a shallow-marine basin in the wake of the migrating foreland fold-and-thrust belt, and the development of one of the worlds largest Basin and Range–style magmatic provinces after the orogeny. The South China record may serve as an example of the multiple effects of flat-slab subduction, including migrating orogenesis and foreland flexure, synorogenic sagging behind the active orogen, postdelamination lithospheric rebound, and the development of a Basin and Range–style broad magmatic province.

The breakup of Rodinia : Did it start with a mantle plume beneath South China?

Abstract Mafic to ultramafic dykes and sills in South China, dated as 828±7 Ma old, are identical in age to the 827±6 Ma Gairdner Dyke Swarm in Australia, thought to be of mantle plume origin. These intrusive rocks, accompanied by widespread granite intrusions and rapid unroofing at a lateral extent of ca. 1000 km, and followed by continental rifting, are interpreted to indicate the arrival of a plume head centred beneath South China. This interpretation supports the idea that South China lay between Australia and Laurentia in the Rodinia supercontinent, and suggests that Rodinia breakup may have started with a mantle plume which initiated continental rifting at about 820 Ma ago.

South China in Rodinia: Part of the missing link between Australia–East Antarctica and Laurentia?

Stratigraphic correlations and tectonic analysis suggest that the Yangtze block of South China could have been a continental fragment caught between the Australian craton and Laurentia during the late mesoproterozoic assembly of the supercontinent Rodinia. The Cathaysia block of southeast China may have been part of a 1.9–1.4 Ga continental strip adjoining western Laurentia before it became attached to the Yangtze block around 1 Ga. This configuration provides a western source region for the clastic wedges in the Belt Supergroup of western North America which contain detrital grains of 1.8–1.6 Ga and 1.22–1.07 Ga. The breakup of Rodinia around 0.7 Ga separated South China (Yangtze plus Cathaysia blocks) from the other continents.

Grenvillian continental collision in south China : New SHRIMP U-Pb zircon results and implications for the configuration of Rodinia

The timing of continental collision between the Yangtze and Cathaysia blocks of south China is an issue that bears on the accretion of Asia, as well as on the assembly and configuration of the Neoproterozoic supercontinent Rodinia. We report in this paper SHRIMP (sensitive high-resolution ion microprobe) evidence that suggests a Grenvillian continental collision in south China, including (1) evidence for 1.3–1.0 Ga metamorphism on both sides of the Sibao orogen between the Yangtze and Cathaysia blocks and (2) sedimentary provenance of possible foreland-basin deposits on the Yangtze side of the orogen that were derived from the Cathaysia block and the Sibao orogen during the continental collision. The occurrence of ca. 1430 Ma granodiorites in southern Cathaysia, along with ca. 1800 Ma basement and Archean protoliths in northern Cathaysia, makes Cathaysia a possible western extension of the Mojave province in southwestern Laurentia. Together with regional data, we suggest that the Sibao orogen could be one of the Grenvillian sutures at the center of Rodinia assembly that brought Australia, Yangtze, and Cathaysia-Laurentia together by ca. 1000 Ma.

An outline of the palaeogeographic evolution of the Australasian region since the beginning of the Neoproterozoic

Abstract In the last 1000 million years, Australia has been part of two supercontinents: Palaeozoic Gondwanaland and Neoproterozoic Rodinia. Neoproterozoic Australia was covered by shallow epicontinental seas, and, in the late Neoproterozoic, by low-latitude glaciers. The breakup of Rodinia along the Tasman Line occurred at the end of the Sturtian glaciation (760 Ma) giving rise to the Palaeo-Pacific Ocean. Gondwanaland formed in the Early Cambrian, at the same time as the Tarim block broke away from northwestern Australia. Westward subduction of the Palaeo-Pacific Ocean along the eastern margin of Australia–Antarctica commenced during the Early Cambrian in northern Victoria Land and in the Middle Cambrian in South Australia, and culminated to the Late Cambrian–Early Ordovician Ross–Delamerian Mountains. In the Ordovician, the magmatic arc retreated from Australias then-eastern continental margin, forming a marginal sea and offshore island arc. A shallow seaway across Australia in the Late Cambrian and Ordovician gradually gave way to desert-like conditions in Central Australia and the adjacent Canning Basin by Silurian time. The Silurian to mid-Devonian was an interval of rapidly changing palaeogeography in eastern Australia with deep volcanogenic troughs formed in a dextral transtensional tectonic setting. Widespread deformation in the Tasman orogenic zone in the Middle Devonian to Early Carboniferous, was accompanied by the development of an Andean-style magmatic arc along the Pacific continental margin of Australia. The most widespread Phanerozoic mountain-building stage in Central Australia occurred in the Late Devonian to mid-Carboniferous, as part of a world-wide Variscan orogenic episode associated with the collision of Gondwanaland with Laurussia to form Pangea. In the late Visean, Australia drifted rapidly southward from previous low latitudes to a near-polar position. Glacial conditions dominated the Late Carboniferous and earliest Permian. Transtensional basins associated with dextral oroclinal shear along the Panthalassan eastern margin of Australia developed in the Late Carboniferous and persisted until the Late Permian, when an Andean-style magmatic arc was re-established. Large foreland basins inboard of the Late Permian to Early Triassic magmatic arc accumulated major coal deposits during Late Permian volcanic phases, but drastic climatic changes at the end of the Permian, possibly caused by global greenhouse conditions, led to red-bed deposition in the Early Triassic. Pangea began to rift in the mid-Triassic, and by the Late Triassic, the Cimmerian blocks, which lay off northwestern Australia throughout the Palaeozoic, had departed the northern margin of Gondwanaland. A new Andean-style continental magmatic arc became established along the Pacific Ocean margin of Australia. Breakup between Australia–Antarctica and the northern part of Greater India commenced ca. 130 Ma, and between Australia and Antarctica around 96 Ma. At the beginning of the Palaeogene, Australia commenced its northward drift towards its present position. Seafloor spreading between Australia and Antarctica was at first slow, but increased to ca. 5 cm per year around 45 Ma. By 35 Ma, the circum-Antarctic current became established, thereby triggering glaciation in Antarctica. Northern Australia reached the tropics by the beginning of the Miocene, and Australia has progressively moved northwards at 7 to 8 cm per year since.

U-Pb zircon geochronology, geochemistry and Nd isotopic study of Neoproterozoic bimodal volcanic rocks in the Kangdian Rift of South China: implications for the initial rifting of Rodinia

Abstract SHRIMP U–Pb zircon age, geochemical and Nd isotopic data are reported for the Neoproterozoic Suxiong volcanic rocks in the Kangdian Rift, western South China. These volcanic rocks are bimodal, consisting mainly of mildly alkaline basalts and trachydacites to rhyolites. SHRIMP U–Pb zircon age determination indicates that they were erupted at 803±12 Ma. Most basaltic rocks are characterized by high positive eNd(T) values (+5 to +6), pronounced enrichment in Th, Ta, Nb, LREEs, Sr, P, Zr, Hf, Ti, smooth LREE-enriched patterns and generally ‘humped’ trace element spidergrams. They resemble the alkali basalts of the Hawaiian oceanic island basalts (OIB) and the Ethiopian continental flood basalts (CFB). These features suggest that the basaltic rocks were most probably derived from an OIB-like mantle source without appreciable crustal/lithospheric contamination. Differentiated basalt and trachyandesite samples show relatively low eNd(T) values (+1.7 to +2.4) and Nb–Ta depletion due to contamination by the mafic lithosphere and/or crustal materials. The rhyolite and dacite samples have small positive eNd(T) values (+1.1 to +2.6), general enrichment in most incompatible trace elements (K, Rb, Th, Zr, Hf and REEs) but significant depletion in Nb, Ta, Sr, P, Eu and Ti. They share geochemical characters of A2-type granites, and are likely generated by shallow (P≤4 kbar) dehydration melting of hornblende-bearing granitoids. Geochemical and Nd isotopic characters and high-volcanicity of the Suxiong bimodal volcanic successions are consistent with their formation in a continental rift environment, such as the Ethiopian rift. The Kangdian Rift is considered as part of a wider continental rift system produced by a starting mantle plume beneath South China during the Neoproterozoic breakup of Rodinia.

Paleomagnetic constraints on timing of the Neoproterozoic breakup of Rodinia and the Cambrian formation of Gondwana

Paleomagnetic data from East Gondwana (Australia, Antarctica, and India) and Laurentia are interpreted to demonstrate that the two continents were juxtaposed in the Rodinia supercontinent by 1050 Ma. They began to separate after 725 Ma, allowing the formation of the Pacific Ocean. The low-latitude Rapitan and Sturtian glaciations occurred during the rifting that led to continental breakup. East Gondwana remained in low latitudes for the rest of the Neoproterozoic, while Laurentia moved to polar latitudes by 580 Ma. During the Vendian, a wide Pacific Ocean separated the two continental land masses. The younger Marinoan, Ice Brook, and Varangian glaciations in the early Vendian preceded a second continental breakup in the late Vendian, causing formation of the eastern margin of Laurentia and rejuvenation of its western margin. Paleomagnetic data indicate that Gondwana was not fully assembled until the end of the Neoproterozoic, possibly as late as Middle Cambrian.

Magmatic and metamorphic events during the early Paleozoic Wuyi-Yunkai orogeny, southeastern South China: New age constraints and pressure-temperature conditions

The early Paleozoic Wuyi-Yunkai orogen in South China is a major orogenic belt in East Asia that formed at a similar time as the classic Caledonian orogeny in Europe. Despite the possibility of its being one of the few examples of intraplate orogenesis in the world, details about the orogen remain poorly defined. In this study, we provide age constraints on metamorphic and magmatic events in the eastern segment of the orogen, and the protoliths of the amphibolite-facies metamorphic rocks found there. By combining previous work with our new metamorphic and petrogenetic analyses, we present the following findings: (1) the Wuyi-Yunkai orogeny occurred between mid-Ordovician (>460 Ma) and earliest Devonian (ca. 415 Ma) time; (2) amphibolite-facies metamorphism in the eastern Wuyi-Yunkai orogen occurred between ca. 460 and 445 Ma, whereas cooling below 500–300 °C occurred by ca. 420 Ma; (3) the orogen exhibits a clockwise pressure-temperature ( P - T ) path and a maximum pressure of >8 kbar, indicating crustal thickening during the orogeny; (4) protoliths of the high-grade metamorphic rocks in the eastern segment of the orogen were dominantly Neoproterozoic (840–720 Ma) volcanic and volcaniclastic rift successions and younger deposits formed in a failed rift, and Paleoproterozoic rocks account for only a small proportion of the outcrops; and (5) the analyzed granites indicate a mixed source of Paleoproterozoic basement and Neoproterozoic continental rift rocks, with elevated melt temperatures of >800 °C, which are interpreted as reflecting dehydration melting of basin sediments taken to below midcrustal levels.

Collision between the North and South China blocks: A crustal-detachment model for suturing in the region east of the Tanlu fault

A crustal-detachment model, based on the interpretation of linear aeromagnetic anomalies, surface geological observations, and deep seismic profiles, is proposed for the continent-continent collision between the North and South China blocks east of the Tanlu fault. The model suggests that during the mid-Mesozoic collision between the two continental blocks, the upper crust of the South China block in the Subei-Yellow Sea region was detached from the lower crust and thrust over the North China block for >400 km, whereas the lower part of the lithosphere was subducted under the North China block along a subsurface suture running east from Nanjing. The sinistral offset of the Qinling suture by the Tanlu fault is only 110-120 km in the deep crust—much less than previously suggested.

Positions of the East Asian cratons in the Neoproterozoic supercontinent Rodinia

Three major East Asian crustal blocks, the Tarim, North China and South China Blocks, have records of the Neoproterozoic rifting events that broke up the supercontinent Rodinia. A preliminary tectonostratigraphic analysis suggests that the Tarim Block may have been adjacent to the Kimberley region, the South China Block between eastern Australia and Laurentia, and the North China Block adjacent to the northwestern corner of Laurentia and Siberia during the early Neoproterozoic. All three blocks were probably separated from the larger cratons towards the end of the Neoproterozoic but stayed close to the Australian margins of Gondwanaland from Cambrian until Devonian.