Zircon U-Pb ages, geochemical features and constraints on regional tectonic evolution of gabbro-diabase in Xiaoyuanshan area Qimantage of East Kunlun Mountains

This paper discusses magmatism, source area, diagenetic tectonic environment, age and geological significance of Xiaoyuanshan gabbro-diabase (XGD) in Qimantage, East Kunlun, China, through the study of petrography, chronology and rock geochemistry.The test data indicated that the content ranges of SiO₂, Na₂O+K₂O, MgO, TFe₂O₃, and TiO₂ were 48.27%~50.06%, 3.10%~4.04% (Na₂O > K₂O), 7.81%~8.60%, 10.69%~12.00%, and 1.32%~1.76% respectively, supporting that XGD should belong to tholeiitic series rocks.The average value of ∑REE, in addition, was 58.13×10^-6, and the δEu value was 1.02~1.24 with its average of 1.12, showing slightly negative Eu anomaly.The normalized distribution pattern of REE chondrites presented a nearly flat curve with a slight loss of Gd, and had weak enrichment characteristics of LREE with no obvious differentiation between LREE and HREE.The rock was obviously enriched in large ion lithophile elements (Cs, Rb, Ba, K and Sr), active incompatible elements (U and Th), relatively depleted in high field strength elements (Nb, Ta and P), LREE, and Pb.The crystallization age of XGD and the zircon crystallization age formed by the late deep magma rising and emplacement into XGD were respectively obtained to be 415±16 Ma (MSWD=5.0) and 243±11 Ma (MSWD=4.1) by SIMS zircon U-Pb method.And the latter indicated a response to the extension activity at the end of the Middle Triassic.Conclusively, it was considered that XGD was a product from partial melting of the depleted lithospheric mantle balance in the extensional island arc background, and was contaminated by crustal materials to a certain extent.