Pertrogenesis and tectonic implications of the late Jurassic basic rocks from the northern Shi-Hang zone, Southeast China

摘要

The tectonic setting of the late mesozoic of South China is in a debate between two schools of thought: an intra-continental rift zone along a passive continental margin or active rifting associated with subduction of the paleo-Pacific Plate. In this study, we present new sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon ages, along with geochemical data of three basic dikes that cross-cut the Dexing porphyry copper deposit. The deposit is the largest of its kind in eastern China and part of large scale mineralization associated with Mesozoic magmatic activity in the area. Our results indicate that the dikes were emplaced in the Late Jurassic with an average U-Pb age of 153.52.4Ma. The intrusions have bulk epsilon(Nd)(t) of ca +0.7 and zircon epsilon(Hf)(t) value of +1.54 to +6.92. Based on relatively enriched light rare earth elements (LREE) and depleted high-field-strength elements (HFSE) abundances with pronounced negative Ta-Nb, Hf-Zr and Ti anomalies in multi-element diagrams, we propose that these dikes were derived from a subduction-modified lithospheric mantle source. The variability in Hf isotopes identifies some degree of crustal contaminations. Our data support a scenario with a back-arc extensional setting or an intra-arc rift environment associated with the westward subduction of the paleo-Pacific Plate at or prior to the late Jurassic as the most likely cause for these subduction signatures.