Petrogenesis and tectonic implications from the Ayishan Group in the South Qilian Belt, NW China

Abstract

The Ayishan Group in the Lajishan area is sandwiched between the South Qilian Belt and the Qilian Block, which is transitionally interpreted to represent the products of the post-collision setting or the continental marginal arc during the Early Ordovician or Silurian and is associated with the evolution of the Proto-Tethyan Ocean. While the detailed information about the age, geochemistry, and tectonic setting of the Ayishan Group is lacking, our geological mapping and analyses demonstrates that the Ayishan Group consists of andesites, rhyolites, ignimbrites, sandstones, and conglomerates, which unconformably overlies the Cambrian-Ordovician arc-ophiolite complex. Zircon U-Pb dating of the rhyolites demonstrated that the Ayishan Group formed between 447 and 442 Ma and is Late Ordovician. Geochemical analyses demonstrated that rhyolites have high and variable SiO 2 contents of 69.6-76.5 wt % and an aluminium saturation index (A/CNK) of 0.85-1.45, belonging to calcalkaline to high K calc-alkaline series. They are subdivided into two subtypes of high-Sr/Y rhyolites (HSRs) and normal rhyolites (NRs) based on the geochemical characteristics. The HSRs show lower total rare earth elements (34.77-62.78 ppm), lower Yb (0.19-0.21 ppm) and Y (1.77-1.99 ppm) contents, and relatively higher Sr contents (159-193 ppm), compared with NRs, and with positive Eu anomalies. Both subtypes exhibit extremely high whole-rock δ 18 O values of 12.63-13.21‰, but various initial 87 Sr/ 86 Sr ratios and ε Nd (t) values, indicating a dominant origin of terrigenous sediments and the HSRs exhibit a more terrestrial composition than the NRs. Various magma sources further caused the assorted residual phase leading to the different geochemistry between the HSRs and the NRs. Combined with the previous data, these results imply that the rhyolites of the Ayishan Group were produced in the process of the amalgamation of the Qilian Block with the Qaidam Block during the Late Ordovician, which further indicated that the Proto-Tethyan Ocean closed 450 Ma ago.

Petrogenesis and tectonic implication of the lower Silurian high-Sr/Y subvolcanic rocks from the South Qilian suture zone in the Qilian Orogen, NW China

“…2019 b ; Sun et al . 2020). Numerous diorites and granodiorites with ages of 470–450 Ma intruded into the Cambrian arc-accretionary complex (Fig.…”

Late early Paleozoic continental collision on the northern margin of the Central Qilian Block, NE Tibetan Plateau: Evidence from a two-stage tectono–metamorphic event

“…For trace elements, NIST610 was used as the external calibration standard and 29 Si as the internal standard. Details of the analytical methods used are given by Sun et al (2020). For the U-Pb isotope analyses, ICPMSDataCal 7.0 (Liu et al, 2010) was used to calculate the zircon isotope ratios and trace elements.…”

“…1b), and it consists mainly of the Neoproterozoic Hualong Complex, volcano-sedimentary rocks of Cambrian-Lower Ordovician age, sedimentary rocks of late Upper Ordovician-Silurian age and sedimentary rocks of Carboniferous-Triassic age (Xiao et al, 2009;Yan et al, 2015Yan et al, , 2019Yan et al, , 2021Fu et al, 2018Fu et al, , 2020. The Cambrian volcano-sedimentary rocks are represented by a dismembered intra-oceanic subduction-accretionary system and a suite of Ordovician volcano-sedimentary rocks that formed in a subduction-collision related system (Song et al, , 2017Fu et al, 2018;Yan et al, 2019;Sun et al, 2020).…”

HT–MP metamorphism in Central Qilian Block, NE Tibet Plateau: Implications on the tectonic evolution of the Qilian Orogen

“…The LA‐ICP‐MS system comprises an Agilent 7500a ICP‐MS and laser ablation system with a COMPex PRO 102 ArF‐Excimer laser source ( λ = 193 nm). The analytical procedures used are described by Sun, Niu, et al (2020). The weighted mean ages and concordia plots of the zircon were calculated using Isoplot v. 3.23 (Ludwig, 2003).…”