U–Pb isotopic age of Longwangdong ophiolitic mélange in the Tianshui Area, West Qinling Orogenic Belt and its time limit of subduction–collision

This paper is the result of geological survey engineering.

Objective The newly discovered Longwangdong ophiolitic mélange as a typical ophiolite suit in the Tianshui area of West Qinling Orogenic Belt witnessed the westward extension of Shangdan suture zone in Qinling Orogenic Belt. Studying its petrology, geochronology, and geochemistry will help improve our understanding of the tectonic evolution of this region.

Methods This study demonstrates through petrological and whole−rock geochemical analyses that this ophiolite suite is mainly composed of amphibolite, metamorphosed basalt, and pyroxenite.

Results LA–ICP–MS zircon U–Pb dating shows the meta−gabbro was formed at (474.6±6.7) Ma, constraining the formation time of the mélange. The whole−rock geochemical analysis reveals that the meta−basalts are subalkalic tholeiite in composition and rich in Ti, Mg and Fe. The total REE of the meta−basalts are low and characterized by flat type distribution pattern without obvious Eu anomalies. Their trace element distribution yields flat patterns with negative anomalies of Cs, Th, Nb, and lowed Zr/Nb ratios, consistent with that of the E−MORB.

Conclusions The geochemical features of the meta−basalts suggest the Longwangdong ophiolitic mélange was potentially generated in the extended volcanic arc environment, which was considered to be the back−arc basin based on the regional geological data. It further implies that Shangdan Ocean experienced a period of subduction process in the Early Ordovician in the West Qinling region.