Geochemistry, zircon chronology, Sr-Nd-Hf isotopic characteristics and subduction magmatism of Heishidun basic rocks in the East Tianshan

A new set of basic rocks was discovered in the Heishidun in South Kangurtag, Eastern Tianshan. Its lithology mainly consists of gabbro and olive gabbro. The rocks are characterized by relatively low contents of SiO₂ (47.59%-50.79%), K₂O (0.26%-1.15%) and MgO (3.94%-12.55%), high contents of Na₂O (2.72% -4.42%) and Al₂O₃ (14.75% -19.65%), and moderate content of Mg^# (47.6-66.64). Light rare earth is enriched ((La/Yb)_N of 1.83-2.12) with a bit of Eu positive anomaly (1.00-1.25). LILE (Ba, U, Sr) is enriched, while HFS (Ta, Nb, Th, and Ti) is depleted, and Pb is strongly enriched. LA-ICP-MS zircon U-Pb dating yields 342.6±3.2 Ma, indicating that this basic rock is a product of Early Carboniferous magmatic activity. Rocks have lower Sr ratio (⁸⁷Sr/⁸⁶Sr)_i (0.703421-0.704551), positive ε_Nd (t) (7.6-8.1) and ε_Hf (t) (9.82-13.74). Geochemical characteristics and petrography show that it is originated from the lithospheric mantle source of the loss, and the source area was affected by the metamorphism of the dehydration and melting fluids resulted from the subduction of marine sediments rich in large ionic lithophile and light rare earth elements. The separation and crystallization of olivine, pyroxene and plagioclase took place during the emplacement of magma, the degree of contamination by the crustal material during the emplacement was very low. The structural and dynamic background shows that the Heishidun basic rock is the product of the Northern Tianshan Ocean's northward subduction along the Kangugtag-Huangshan fault in the Early Carboniferous.