Qilian Terrane Research Articles

An early Palaeozoic double-subduction model for the North Qilian oceanic plate: evidence from zircon SHRIMP dating of granites

The North Qilian Orogenic Belt (NQOB), which consists of ophiolitic mélange and island-arc assemblages containing many granites, blueschists, and eclogites, lies between the Alax and Qilian terranes in northwestern China. The Minleyaogou and Niuxinshan granitoids occur at the northern and southern margins, respectively, in the middle segment of the NQOB. The Minleyaogou pluton is granodiorite in composition, whereas the Niuxinshan pluton consists mainly of red granite with minor grey quartz diorite. Geochemically, the Minleyaogou granite differs from the Niuxinshan granite in that it contains a smaller range in SiO2, has lower total alkalis, and is more peraluminous. Both granitoids are magnesian but the Niuxinshan granite is alkali-calcic, whereas the Minleyaogou granodiorite is calcic. Both granitoids have similar chondrite-normalized rare earth element patterns with light rare earth element enrichment and negative Eu anomalies. They have pronounced negative Ba, Nb, Sr, P, and Ti anomalies indicating that they have an affinity to island-arc or active continental margin magmatism. SHRIMP U–Pb dating of zircons from the granitoids yields a formation age of 477 Ma for the Niuxinshan granite and 463 Ma for the Minleyaogou granodiorite. These ages, combined with the geochemistry and locations of the plutons, suggest that they formed by the double subduction of the North Qilian oceanic plate during early Palaeozoic time. Formation of the Niuxinshan granite may be related to southward subduction under the Qilian terrane at 477 Ma, whereas the Minleyaogou granodiorite was formed by northward subduction at 463 Ma under the Alax terrane.

Integrated analyses constraining the provenance of sandstones, mudstones, and conglomerates, a case study: the Laojunshan conglomerate, Qilian orogen, northwest China

The Qilian orogenic belt in the northern Tibetan plateau connects the Altaids to the north with the Tethyan orogenic system to the south and occupies a key tectonic position in the evolution and assembly of Asia. The belt contains a wide range of subduction–accretion-related petrotectonic units. The Early–Middle Devonian Laojunshan conglomerate, deposited unconformably upon Cambrian–Silurian strata along the northern margin of the North Qilian terrane, contains a record of the late Paleozoic tectonism of the Qilian orogen. Its provenance and tectonic setting are critical in understanding not only the tectonic evolution of Tibetan plateau, but Paleozoic global reconstructions as well. The composition of clastic conglomerates and heavy mineral assemblages of sandstones suggests that coeval mafic, felsic, metamorphic, and sedimentary rocks were the main sources. The geochemistry of volcanic clasts and paleocurrent and paleogeographic data suggest derivation from subduction–accretion complexes in the North Qilian terrane. The geochemistry of siltstones and mudstones indicates that the Laojunshan conglomerate was derived from an arc and accumulated in an active continental margin. Geochemical data of granitoid clasts suggest that they were derived from Ordovician–Silurian subduction-related magmatic rocks. Mafic and ultramafic clasts, chromite, and magnetite decrease upwards in the stratigraphy whereas metamorphic, sedimentary and granitoid clasts, and garnet increase. These data imply that mafic rocks were the predominant source during initial deposition. Regional studies suggest that the North China plate subducted southwards and produced subduction-related arc magmatism along the southern margin of the North Qilian terrane during the Early–Middle Devonian. Therefore, we interpret the Laojunshan conglomerate as a fore-arc basin fill.

Siberia, the wandering northern terrane, and its changing geography through the Palaeozoic

Siberia, the wandering northern terrane, and its changing geography through the Palaeozoic

The Crustal Poisson Ratio and S‐Wave Velocity Structure Beneath the NE Tibetan Plateau Inferred from Receiver Function and Their Implications

AbstractWe selected broadband teleseismic waveform data recorded by the Gansu Digital Seismic Network to study the crustal and upper mantle structure beneath the Northeastern Tibetan Plateau. The crustal thickness and VP/VS ratio are estimated by the grid‐search method using crustal conversion and multiples, assuming an average P‐wave velocity in the crust. Teleseismic P waveform modeling is carried out to investigate the S‐wave velocity structure, assuming the fixed VP/VS value inferred from the grid‐search method. Our results show that the crust is 50~55km thick beneath Qilian terrane, and about 45 km beneath the Hexi Corridor and Qaidam terrane. The VP/VS ranges from 1.66~1.85, and the Poisson's ratio is about 0.215~0.294. Our results also show that the Moho interface is sharp beneath the Northeastern Tibetan Plateau. Both of the low‐to‐normal Poisson's ratio and the sharp Moho do not support the argument of significant partial melting in the upper‐to‐middle crust and underplating beneath the lower crust of the Northeastern Tibetan Plateau. The existence of anticorrelation between VP/VS and the crustal thickness strongly suggests that the crust is mainly composed of felsic mineral in the northeastern Tibetan crust. The seismic evidence may indicate that the thickened crust, mostly in its upper part, is probably due to the successive intracrustal thrusts.

The neodymium isotopic and geochemical composition of serpentinites from ophiolitic assemblages in the Qilian fold belt, northwest China

Serpentinites are a common component of ophiolitic assemblages in the Qilian fold belt. Samples associated with MORB-like basalts in the northern low-grade blueschist belt of the North Qilian terrane have lherzolitic and pyroxenitic compositions, high ε Nd( T) values of +5.5 to +6.9, and likely represent residues from melting along spreading ridge segments. Despite evidence for incompatible element enrichment by hydrothermal fluids which had interacted with the ocean floor sediment pile, samples at the Ayougou locality retain original isotope Sm–Nd systematics and yield a Silurian age of 422±20 Ma with ε Nd( T)=+6.8. Serpentinites in the southern high-grade blueschist belt of the North Qilian terrane and from the Lha-Jyi terrane of the South/Central Qilian belt are associated with volcanic arc rocks, and include lherzolitic, harzburgitic and dunitic compositions characterised by low ε Nd( T) values of +3.8 to −2.2. Harzburgitic samples from these localities display U-shaped REE profiles, and weakly developed negative Nb-anomalies superimposed on incompatible element enrichments, and are interpreted as remnants of oceanic lithosphere metasomatised by slab-derived fluids in a supra-subduction zone environment. Anomalous enrichments in the mobile elements U, Sr and Ba in serpentinites throughout the Qilian belt may be attributed to post-obduction alteration by groundwater.

Magmatic history of the northeastern Tibetan Plateau

The northeastern margin of the Tibetan Plateau is underlain by the Qaidam and Qilian terranes, which consist primarily of mid‐Proterozoic through lower Paleozoic oceanic and arc‐type assemblages that have been accreted to the southern margin of the Tarim/Sino‐Korean craton. Most previous models suggest that these assemblages formed along a northeast dipping subduction system constructed along the margin of the Tarim/Sino‐Korean craton during early Paleozoic time. The main components are interpreted to have formed either as an archipelago of volcanic arcs and back arc basins, or as a broad expanse of accretionary complexes. Our geochronologic data support a model, suggested by Sobel and Arnaud [1999], in which the Qaidam and Qilian terranes are separated from the Tarim/Sino‐Korean craton by a mid‐Paleozoic suture that closed along a southwest dipping subduction zone. The basement to these terranes consists of oceanic assemblages that were amalgamated into a coherent crustal fragment prior to emplacement of ∼920–930 Ma granitoids. Early Paleozoic arc‐type magmatism occurred between ∼480 and ∼425 Ma, apparently sweeping southwestward across much of the Qilian and Qaidam terranes. Accretion‐related magmatism along the inboard margin of the Qilian terrane occurred between ∼423 Ma and ∼406 Ma. Following Silurian‐Devonian accretion, the region has experienced late Paleozoic and Mesozoic uplift and erosion and has been severely overprinted by Tertiary thrusting, uplift, and strike‐slip motion along the Altyn Tagh fault. Correlation of geologic features and magmatic histories between the Altun Shan and the Nan Shan suggests that the eastern Altyn Tagh fault has a total left‐lateral offset of ∼375 km.

Detrital-zircon geochronology of the northeastern Tibetan plateau

U-Pb geochronologic analyses have been conducted on 413 detrital-zircon grains collected from 16 samples in the Altun Shan, Nan Shan, and Qilian Shan. The samples come primarily from quartz arenites and metaturbidites of Middle to Late Proterozoic age and from feldspathic and volcanic clast-rich sandstones of early Paleozoic age. Zircon grains in Proterozoic strata resting on Tarim basement yielded mainly 2.0–1.9 Ga ages, whereas Proterozoic strata of the Qaidam and Qilian terranes yielded mainly ca. 930–820 Ma and ca. 1.9–1.1 Ga ages. The younger grains were apparently shed from local igneous rocks, whereas the grains older than 1.1 Ga were shed from an undetermined continental source. Grains in the lower Paleozoic strata are mainly ca. 500–430 Ma and were shed from nearby plutonic and possibly volcanic rocks that formed in a magmatic arc setting. Our detrital-zircon ages are consistent with a model (first proposed by E.R. Sobel and N. Arnaud) in which early Paleozoic magmatism occurred within a single northeast-facing magmatic arc that was constructed across an assemblage of Middle to Late Proterozoic accretionary complexes, remnants of magmatic arcs, and shallow-marine strata. This arc system was accreted to the Tarim and Sino-Korean cratons during Silurian–Devonian time. The resulting suture has been reactivated as Tertiary thrust faults that currently define the structural and topographic margin of the Tibetan plateau. Our data also provide two new estimates for the offset along the eastern Altyn Tagh fault. A belt of Middle Proterozoic shallow-marine strata is offset by ∼400 km, whereas a belt of 490–480 Ma magmatic arc rocks is offset by ∼370 km. These values are generally similar to the 350–400 km offset reported in most previous studies.

Ages and Compositions of the Precambrian High‐grade Basement of the Qilian Terrane and Its Adjacent Areas

Abstract Based on geological, chronological, geochemical and Nd isotopic studies of the high‐grade basement of the Qilian terrane, the authors have drawn the following main conclusions: (1) the high‐grade basement of the Qilian terrane consists mainly of meta‐argillo‐arenaceous rocks and granites and its bulk part was formed in the period of 0.8–1.0 Ga (the Jinningian period); (2) most of the meta‐argillo‐arenaceous rocks and granitic rocks have strong negative Eu and Ba anomalies (Eu/Eu*= 0.47–0.71 and Ba/Ba*=0.16–0.64), with fDM and ENd (1.0 Ga) ranging from 1.87 to 2.26 Ga and from −8.54 to −4.06 respectively, showing relatively high maturity; and (3) the Jinningian granitic rocks are a typical product of continent‐continent collision, being probably related to the formation of the supercontinent Rodinia. These studies, combined with the study of high‐grade basement rocks near the Qilian terrane, suggest that before the Jinningian period, the Qilian‐Qaidam northern‐margin terrane and Dunhuang‐Alxa terrane were separated from each other, belonging to different plate systems of the North China craton and Yangtze platform respectively. The Qilian orogenic belt was the same as or similar to the Qiling orogenic belt in terms of the geological evolution history at least before the Jinningian period.

The shoshonitic volcanic rocks at Hongliuxia: Pulses of the Altyn Tagh fault in Cretaceous?

Two periods of magma activities, i.e. 106–112 Ma shoshonitic eruption and 82 Ma trachytic intrusion, are recognized at the intersection of the Altyn Tagh fault and the northern marginal fault of the Qilian terrane, northeast of the Tibetan Plateau, which were previously considered to be Quaternary magma activities. The two rock-types have significant differences in mineral assemblage and chemical composition, and formed during two different pulses of magma activity. The Cretaceous magma activities and deformation in or near the Altyn Tagh fault zone suggest a possibly tectonic reactivation after the intense activity of the fault during Indusinian.

The automatic determination of nitrate in sea water : Brewer P. G. and J. P. Riley, 1965. Deep-Sea Res., 12 (6): 765–772

The automatic determination of nitrate in sea water : Brewer P. G. and J. P. Riley, 1965. Deep-Sea Res., 12 (6): 765–772