Geochemical characteristics and zircon U-Pb ages of porphyroclastic lava in the Bayaerhushuo area, the south-central segment of Great Xing'an Range, and its geological significance

The porphyroclastic lava is located in the Bayaerhushuo area, the south-central segment of Great Xing'an Range, which can be divided into two parts, the central facies of fine-grained quartz monzonites (-quartz monzonitic porphyry) and the margin facies of dacitic rhyolitic porphyroclastic lava. Based on the SHRIMP zircon U-Pb dating, the porphyry monzonitic granite and dacitic ryholititic porphyroclastic lava samples yielded U-Pb ages of 137.4±0.9Ma (MSWD=1.13) and 135.2±0.8Ma (MSWD=1.17) respectively, indicating that the porphyroclastic lava samples in the study area were formed during the Early Cretaceous. The results of geochemical analyses show that the six samples are all high-K calc-alkaline A-typed granites and have the same characteristics in the trace element spidergrams and REE patterns. The features of evident negative Eu anomaly, enrichment in the LILEs Rb and Pb, LREE and Th, depletion in HFSE Nb, Ba, Sr and Eu, reveal that these samples may originate from the partial melting of crust. By tectonic environment discriminations, the porphyroclastic lava are plotted into the A2-typed granite, revealling the post-collisional extension environment. Combined with the Mesozoic tectonic evolution of the Great Xing'an Range, the formation of the Early Cretaceous porphyroclastic lava should be related to the lithospheric extensional envrinment caused by the closure of MongoliaOkhotsk ocean.