Quanren Yan

Paleozoic multiple subduction-accretion processes of the southern Altaids

The formation and development of the southern Altaids is controversial with regard to its accretionary orogenesis and continental growth. The Altay-East Junggar orogenic collage of North Xinjiang, China, offers a special natural laboratory to resolve this puzzle. Three tectonic units were juxtaposed, roughly from North to South, in the study area. The northern part (Chinese Altay), composed of variably deformed and metamorphosed Paleozoic sedimentary, volcanic, and granitic rocks, is interpreted as a Japan-type island arc of Paleozoic to Carboniferous-Permian age. The central part (Erqis), which consists of ophiolitic mélanges and coherent assemblages, is a Paleozoic accretionary complex. The southern part (East Junggar), characterized by imbricated ophiolitic mélanges, Nb-enriched basalts, adakitic rocks and volcanic rocks, is regarded as a Devonian-Carboniferous intra-oceanic island arc with some Paleozoic ophiolites, superimposed by Permian arc volcanism. A plagiogranite from an imbricated ophiolitic mélange (Armantai) in the East Junggar yields a new SHRIMP zircon age of 503 ± 7 Ma. Using published age constraints, we propose the presence of multiple subduction systems in this part of the Paloasian Ocean in the Paleozoic. The intraoceanic arcs became accreted to the southern active margin of the Siberian craton in the middle Carboniferous-Permian. During the long accretionary processes, in addition to large-scale southward-directed thrusting, large-scale, orogen-parallel, strike-slip movements (for example, Erqis fault) in the Permian translated fragments of these intraoceanic arcs and associated accretionary wedges. This new tectonic model has broad implications for the architecture and crustal growth of Central Asia and for other ancient orogens.

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.

Stratigraphic and U‐Pb SHRIMP Detrital Zircon Evidence for a Neoproterozoic Continental Arc, Central China: Rodinia Implications

The Bikou terrane of central China is located within the southwest Qinling Mountains along the northwest margin of the Yangtze Plate. The terrane consists of a volcanic assemblage, the Bikou Group, and a correlative volcaniclastic turbidite sequence, the Hengdan Group. U‐Pb SHRIMP detrital zircon dating of 181 grains collected from the Hengdan Group indicates ages from ca. 850 to 700 Ma, with a peak near 770 Ma. Given the predominance of volcanic, single‐cycle zircons within the Hengdan Group and the thick succession of coarsening‐upward volcaniclastic marine sediments, the sequence is best interpreted as a fore‐arc basin. This interpretation is supported by previous U‐Pb SHRIMP and conventional U‐Pb zircon dating performed within the Bikou Group that indicates subduction‐related magmatism persisted from at least ca. 850 to as recently as 700 Ma. Together, these data indicate that the Bikou terrane is a fragment of a Neoproterozoic continental arc system and that the Yangtze Plate was bordered by a convergent oceanic margin. The South China Block must therefore occupy an outboard position with respect to Rodinia reconstruction models.

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.

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.

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.