Petrogenesis of Late Triassic Trachyte and Basic Dike Swarms in Northern Dabashan and Its Geological Significance

An amount of basic dikes and alkaline igneous rock occurred in Ziyang−Pingli−Zhuxi regions of the North Daba Mountain, most of these rock series are believed to have been formed in Early Paleozoic magmatic activity, while the zircon U−Pb isotope dating of the basic dike swarms and trachyte in this study show them formed at 219.5 ± 2.2 Ma and 223.9 ± 2.8 Ma, respectively. The rock series can be regarded as a product of Late Triassic magmatism. The diabase from the basic dike swarms shows a characteristic of high Ti and low Si, trachyte shows a feature of high Ti and is rich in alkali, both of which have some differences for the fractionation of REE. The Pb−Sr−Nd isotopic composition of the diabase shows a source region of OIB, with some additions of enrichment material of EmⅡ and upper crust, the trachyte shows an obvious rise inthe Primitive Mantle normalized diagram. Thus, both of them are formed by the intra-plate magmatism. The Late Triassic trachyte−diabase assemblage in North Beidaba Mountain is considered to have been formed by the continuous intraplate extension after the closure of the Mian−Lue Ocean, also is a geological record of mantle derived magma in South Qinling.