Geochemical Characteristics of An'nage Hornblende Gabbro from East Kunlun Orogenic Belt and Its Constraints on Evolution of Paleo-Tethys Ocean

Zhao Xu; Fu Lebing; Wei Junhao; Zhao Yujing; Tang Yang; Yang Baorong; Guan Bo; Wang Xiaoyun

Abstract：At present,the research on Permian-Triassic basic magmatic rocks in East Kunltm orogenic belt is extremely weak,which not only limits the understanding of the nature of the mantle source in the study area during this period but also restricts the accurate understanding of the evolution processes of Paleo-Tethys Ocean.The zircon LA-ICP-MS U-Pb geochronology and major and trace elements,as well as Sr-Nd-Hf isotope geochemistry of the An'nage hornblende gabbro in the East Kunlun orogenic belt are applied in this paper.LA-ICP-MS zircon U-Pb dating yields a weighted mean age of 242±2 Ma for the intrusion.It is characterized by low SiO2 (46.00％-52.40％),low (Na2O+K2O) (3.07％-3.79％) contents.In addition,it is enriched in Al2O3 (17.35％-20.10％),MgO (4.65％-6.53％) and FeOT(8.77％-11.07％) with high Mg# (68-75),indicating that it belongs to the calc-alkaline series.This intrusion has low (La/Yb)N and Nb/Ta ratios,negative Eu anomalies,with enriched large-ion lithophile elements and depleted high field strength elements.All geochemical characteristics show that this intrusion is the result of typical island arc magmatic activity.Additionally,the samples have relatively uniform (87 Sr/86 Sr)i ratios (0.708 80-0.710 36) and εNd(t) values (-4.8 to-3.4),and relatively high εHf(t) values (-4.9 to-0.4).According to the lithology,geochemical characteristics and isotopic compositions of the intrusion,we propose that the magma has not been significantly contaminated by the crust and it was derived from the low-degree partial melting (2％-10％) of the mantle wedge,which had undergone metasomatism induced by the fluid from the subducted slab.The mantle source was characterized by the spinel phase-phlogopite lherzolite.Slight fractional crystallization of pyroxene and plagioclase might also occur during the evolution of magma.In combination with the tectonic evolution of the East Kunlun orogenic belt and the geochronological and geochemical characteristics of contemporary intrusive rocks,it is concluded that the An'nage hornblende gabbro was produced at the late stage of Paleo-Tethys Ocean subduction,and the final closure timing of Paleo-Tethys Ocean should be the Middle Triassic to Late Triassic.