Middle Cambrian-Early Ordovician ophiolites in the central fault of the East Kunlun Orogen: Implications for an epicontinental setting related to Proto-Tethyan Ocean subduction

Abstract

The Qingshuiquan-Tatuo ophiolites (QTOs), which are located in the central fault of the Kunlun Orogen, can provide critical information regarding the subduction processes of the Proto-Tethys Ocean. This study presents petrological, whole-rock geochemical, SrNd isotopic, and zircon UPb data on these ophiolites. The QTOs consist of mantle peridotites, pyroxenites, and fine-grained low- and high-Ti (alkaline) gabbros (LTGs and HTGs, respectively) and lack mafic lava units. The QTO mafic units can be geochemically subdivided into LTGs and HTGs. The LTGs have high MgO contents (mostly >8.0%), V/Ti ratios (37.41–158.77) and low TiO2 (mostly <0.5%), REE contents and variable ɛNd(t) values (−3.48 to +3.89), and they exhibit geochemical similarities to forearc mafic rocks. Zircon UPb data yield a crystallization age of 516 Ma for the LTGs. In contrast, the HTGs show relatively high total alkali (1.72–5.66%), TiO2 (1.45–3.68%) and REE contents, high (La/Yb)N (3.37–14.06), Ti/Y (290.78–1058.94), and low ɛNd(t) values (−3.02 to −3.55), which suggest a geochemical signature of oceanic island basalt (OIB) and continental rift-like basalt. The zircon UPb data yield crystallization ages of 486–485 Ma for the HTGs. Petrogenetic analyses suggest that the LTGs were derived from remelting of forearc mantle peridotites and that the serpentinites (harzburgites) represent the forearc mantle after the extraction of low-Ti mafic melts. In contrast, the HTGs were derived from a fertile asthenospheric mantle upwelling through a slab window followed by minor subcontinental lithospheric mantle (SCLM)/crust contamination. The findings suggest that the QTOs were generated in an epicontinental environment above the subduction zone, which was geodynamically related to slab rollback at 516 Ma and subsequent oceanic-ridge oblique subduction at 485 Ma.