Geochronology and geochemistry of the Neo-Proterozoic amphibolites in the Muqitan Formation: Implications for the tectonic evolution of the North Qinling Orogen

The Muqitan ophiolitic melange found within the Muqitan Formation near Muqitan region, the North Qinling Orogen. They mainly consist of amphibolite with minor meta-gabbro and meta-chert. Zircon LA-ICP-MS U-Pb dating yields an age of 762.5±4.6Ma for the amphibolites, interpreted as crystallization time. The amphibolites are characterized by low TiO2 (0.57%~2.16%), relatively low K2O/Na2O ratios (0.06~0.55) and moderate-high Mg# (45.3~68.7). They are geochemically similar to low.Ti tholeiites. The Muqitan amphibolites have low REE contents, with differentiated LREE/HREE patterns and no Eu anomalies on the chondrite-normalized REE diagram, analogies to E-MORB. The amphibolites display an enrich elements Rb, Ba, Th, and La pattern, low (87Sr/86Sr)i ratios of 0.7038 to 0.7040 and high eNd(t) values of +4.1 to +6.9, suggesting a mantle origin, similar in composition to FOZO-like source. Their (Th/Nb)N, Nb/La and Ba/La ratio features indicate that these amphibolite have been experienced varying degrees of crustal contamination. All these data, combined with the regional geological features demonstrate that the Muqitan amphibolites are interpreted as remnants of the Shangdan ancient oceanic crust that would have formed during the inception of the oceanic open at ca. 762.5±4.6Ma.