Early Paleozoic intermediate-acid magmatic activity in Bairiqiete area along the Buqingshan tectonic melange belt on the southern margin of East Kunlun： Constraints from zircon U-Pb dating and geochemistry

Abstract：Based on 1∶50 000 regional geological mapping on the southern margin of the East Kunlun Mountains and using methods of detailed field section measurement and petrologic analysis as well as LA-ICP-MS zircon U-Pb dating and geochemical investigation, the authors studied the intermediate-acid magmatic rocks in Bairiqiete area of the Buqingshan tectonic melange belt. The results show that the rocks are mainly granodiorite with 206Pb/238U average weighted age of 441±6 Ma（MSWD=0.75）and intermediate-acid volcanic rock such as rhyolite-porphyry with 206Pb/238U average weighted age of 438±3 Ma（MSWD=2.0）. Geochemically, the former is characterized by high content of Si, Al and Na and should belong to the Na-enriched calc-alkali series, whereas the latter is of Si-saturated, aluminous and calc-alkali series. These two kinds of rocks are characterized by low ∑REE content and relatively enriched LREE and depleted HREE, with LREE differentiated more evidently than HREE, and show right-inclination pattern in REE patterns, without or with weak Eu anomalies. All of these features are similar to features of the island-arc calc-alkali series in relation to subduction, representing the volcanic-arc products of the subduction of east proto-Tethys crust in Early Silurian. The results obtained by the authors provide new evidence for the study of the evolution of east proto-Tethys Ocean.