Implication of geochronological data for the para-metamorphic rocks from Bashekurgan Group of Changcheng System in Munabulake area, South Altyn Tagh

LA-ICP-MS zircon U-Pb dating yielded the protolith ages of 498~840Ma and 500~647Ma respectively for the two-mica monzonitic gneiss in Hongliuquan Formation and the mica quartz schist in Zhasikansayi Formation from Bashikourgan rock group in Munabulake area, southern Altyn Mountains, indicating that the formation ages of the two rocks are Neoproterozoic rather than Mesoproterozoic. The zircon rims of the two-mica monzonitic gneiss exhibit HREE enrichment pattern, with Th/U ratios less than0.1, showing the characteristics of metamorphic zircon. Zircon U-Pb dating yielded the metamorphic age of 498±4Ma, which is identical to the peak metamorphic age of the HP granulite in this area and that of the HP-UHP metamorphic rocks in southern Altyn Mountains, suggesting that the rock should be the product of the deep subduction of southern Altyn Mountains. The two-mica monzonitic gneiss and two-mica quartz schist in this study and the HP granulite in Munabulake area recorded similar age information to the Altyn complex, implying that the three rocks belong to the Altyn complex rather than to the Bashikourgan rock group. Therefore, the Bashikourgan rock group should be disintegrated, and some of this rock group should be assigned to the southwest part of the Altyn complex. The reason for the disintegration of the Altyn complex was that the south Altyn subducted slab was exhumed and broken up at ca.450 Ma, which resulted in the mantle magma upwelling. The upwelling mafic magma formed the present Munabulake mafic-ultramafic rock and separated the Altyn complex into two parts.