ZIRCON U-Pb DATING OF THE NANGUSHANZI INTRUSIVE ROCK IN BENXI COUNTY, LIAONING PROVINCE:Tectonic implication

The Nangushanzi intrusive rock, located in the southeastern Benxi County of Liaoning Province, is lithologically medium-fine grained alkali-feldspar granite. The zircon U-Pb isotopic dating result shows that the rock was formed in the Early Cretaceous Epoch of 120-130 Ma, but not the Jurassic Epoch as previously thought. The major element features indicate that the rock belongs to high-K calc-alkaline series. The chondrite-normalized REE patterns show enrichment of LREEs, depletion of HREEs, and strongly negative Eu anomalies. The trace elements are characterized by enriched largeion lithophile elements (LILE, such as Rb, Th and K) and depleted high field strength elements (HFSE, such as Ta, P and Ti). The trace element compositions of alkali-feldspar granite suggest that the magma pressure be less than 0.8-1.0 GPa, derived from the depth of 30-40 km in the crust, belonging to thinning crust, when the Liaodong massif is under intracontinental extension environment. The study fully proves that the Early Cretaceous magmatism in eastern China developed in an extensional tectonic environment which was induced by thinning lithosphere.