Neoproterozoic thermal events and tectonic implications in the Beishan orogenic belt: Geochemical and geochronological evidence from two sets of granitic rocks from southern Beishan orogenic belt, Gansu Province

As an important tectonic terrane in the southern margin of Central Asian Orogenic Belt(CAOB), the Beishan orogenic belt has undergone the accretion and stitching process of long-time and multi-stage, where Precambrian basements were universally distributed and Paleozoic crust was remarkably accreted. Therefore, based on the previous works, it is necessary to reconstruct the tectonic evolution of this belt in Neoproterozoic, research the ascription of basements, and discuss its role in the evolution of Paleo-Asian Ocean and even larger-scaled tectonic evolution. Two set of samples were collected from porphyritic biotite monzogranite and rhyolite in Daoban copper-Honggoushan-Dawan iron mine in the southern Beishan belt, Gansu Province, which were used for the study of petrography, geochemistry and zircon U-Pb geochronology. The samples are characterized by high silicon, rich alkali and potassium, peraluminous, low magnesium and calcium. The rare earth elements(REE) are characterized by differentiation of light and heavy rare earth elements and the negative anomaly of Eu. Trace elements manifest depletion of high field strength elements(HFSE) such as Nb and Ta, and enrichment of large ion lithophile elements(LILE) such as Sr and Ba. U-Pb dating of porphyritic biotite monzogranite yields the weighted average age of 892.3±5.1 Ma, and that of rhyolite yields 870.4±4.5 Ma, which shows similar geochronology. Typically featuring S-type granite, the porphyritic biotite monzogranite was formed in collision environment, and its magmatic source was generated from recycling of ancient crustal sediments. Characterized by A₂-type granite, the rhyolite was formed in post-collisional extensional environment, and its magma was originated from partial melting of deep crustal material. The formation of two types of intrusives indicates that Beishan orogenic belt experienced a tectonic transformation from collision to extension. The collision before 890 Ma indicates its response to Rodinia supercontinental converging. The extensional event at 870 Ma reflects the beginning of Paleo-Asian Ocean in Beishan orogenic belt, which may be related to the breakup of the Rodinia supercontinent.