Zircon U-Pb dating, geochemical characteristics and tectonic setting of intermediate-basic volcanic rocks from Middle Cambrian Shengou Formation, eastern Qilian orogen

There are andesites intercalated with metabasalts in the Mogou area, southeastern Lajishan mélange. Their protoliths are an-desite and basalt belonging to the sub-alkaline tholeiite series. LA-ICP-MS zircon U-Pb dating of the andesite yielded an age of 503.1±6.6Ma, suggesting Middle Cambrian. ∑REE values of basalt are 93.40×10^-6~135.39×10^-6, (La/Yb)_N=2.76~3.64, and δEu=0.87~1.00. The spider diagram of trace elements shows similar characteristics to OIB or continental rift volcanic rock, with no obviously anomaly of δEu and enrichment of incompatible elements. The andesites have lower REE content than basalt, and the negative Eu anomalies are not obvious.The spider diagram of trace elements of andesites show the enrichment of LILEs (Cs, Rb, Ba) and depletion of HFSE (Nb, Ta, Ti). The genetic analysis suggests that the basalt resulted from low degree partial melting of garnet+spinel lherzolites without obvious crustal contamination, and andesite was derived from partial melting of the crust. Tectonic environment discrimination shows that Shengou Formation was formed in a continental rift environment. These data further indicate that southeastern termination of Lajishan belt might not have had a mature system of ocean basin in the central part of Central Qilian block. With the South Qilian Ocean subdcution in the Early Paleozoic, the Shengou Formation was emplaced on the southern margin of central Qilian block as fragments of continental rift lithosphere.