Geochemical characteristics and zircon U-Pb ages of island-arc basic igneous complexes in the Tianshui area,West Qinling

The Liushuigou intermediate-basic meta-igneous complex at Guanzizhen,Tianshui,is composed mainly of metagabbro-metagabbro diorite-metadiorite,and the Baihua basic meta-igneous complex consists mainly of pyroxenite-gabbro (-diorite)-diorite-quartz diorite.They form a relatively complete comagmatic evolutionary series.The geochemical characteristics of basic to intermediate-basic igneous rocks indicate that they belong to a tholeiite suite.Their chondrite-normalized REE patterns are of nearly flat and slightly LREE-enriched type,and their primitive mantle-normalized and MORB-normalized trace element spidergrams are generally similar;the LILEs Cs,Ba,Sr,Th and U are enriched,while Rb and K and the HFSEs Nb,P,Zr,Sm,Ti and Y are depleted.All these show comagmatic evolutionary and genetic characteristics.The tectonic environment discrimination by trace elements reveals that these igneous complexes formed in an island-arc setting.The TIMS single-zircon U-Pb age for the Liushuigou intermediate-basic meta-igneous rocks in the Guanzizhen area is 507.5±3.0 Ma,representing the age of these igneous complexes,which indicates that island-arc-type magmatic rocks in the north zone of the West Qinling are Late Cambrian in age and also reveals that the timing of subduction of the paleo-ocean basin represented by the Guanzizhen ophiolite and resulting island-arc-type magmatic activity is probably Late Cambrian to Early Ordovician.