Finding of Neoproterozoic (∼775 Ma) magmatism recorded in metamorphic complexes from the North Qilian orogen: Evidence from SHRIMP zircon U-Pb dating

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.

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The Age and Origin of the Jinfosi Biotite Granite, North Qilian, NW China: Evidence from U–Pb Zircon Age Data, Geochemistry, and Nd–Sr–Pb Isotopes

Advanced Materials Research ◽

10.4028/www.scientific.net/amr.616-618.3 ◽

2012 ◽

Vol 616-618 ◽

pp. 3-18 ◽

Cited By ~ 1

Author(s):

Xing Li Sun ◽

Xiao Huang Liu ◽

Jiu Feng Liu ◽

Bai Nian Sun

Keyword(s):

Minor Component ◽

Biotite Granite ◽

Pb Isotopes ◽

Zircon Age ◽

Nw China ◽

The North ◽

Material Values ◽

A Minor ◽

Shrimp Zircon ◽

North Qilian

New geochemistry, Nd–Sr–Pb isotopes and U–Pb zircon data from the Jinfosi Biotite granite provide important constraints on the evolution of the crust in this part of the North Qilian, NW China. The Jinfosi Biotite granite have the following properties: SiO2 > 65%, A/CNK(Molar Al2O3/(CaO + Na2O + K2O) ratios generally > 1.1, Na2O generally < 3.2%, Sm/Nd values between 0.17 and 0.27, and high Rb/Sr values. A chondrite-normalized rare earth element (REE) pattern shows negative Eu anomalies and depletion in heavy REEs. 143Nd/144Nd values are relatively low, and values of εNd(t) and εSr(t) are indicative of continental lithosphere. (87Sr/86Sr)i values are between 0.69952 and 0.70962, corresponding to continental crust mixed with a minor component of mantle material. Values of 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb are 18.9–19.0, 15.59–15.85, and 38.00–and 39.00, respectively, corresponding to S-type collision-related granites. The Jinfosi Biotite granite yield a SHRIMP zircon U–Pb age of 416.7 ± 4.1 Ma. R1–R2 and Rb versus (Yb + Nb) discrimination diagrams indicate that the Jinfosi biotite granite was produced during continental collision following closure of the paleo-North Qilian Ocean.

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