Geochemistry, geochronology and Hf isotopic compositions of metagabbro dykes on the northwestern margin of Oulongbuluke micro-block on the northern margin of Qaidam Basin

Metagabbro dykes were emplaced in the Dakendaban Rock Group on the northwestern margin of the Oulongbuluke Block, northern margin of Qaidam. They are characterized by higher Al₂O₃, CaO but lower P₂O₅, TiO₂, belonging to subalkalic calcalkaline series. Meanwhile, the rocks have lower Σ REE and LREE/HREE ratios between 1.82~2.77, with the chondritenormalized REE showing right-inclined patterns. The rocks are characterized by low Ti and Nb/La(0.44~0.84), but high Th_N/Nb_N (3.75~8.03). The ε_Hf(t)values vary in the range of 9.25~15.11, ¹⁷⁶Hf/¹⁷⁷Hf ratios vary from 0.282813 to 0.282979, and Hf model ages vary from 378Ma to 610Ma. Comprehensive researches show that the magma of metagabbro dykes was derived from asthenosphere mantle, and was probably contaminated by the crust. LA-ICP-MS zircon U-Pb dating of metagabbro dykes in the Dakendaban Rock Group yielded the formation age of 357±4Ma. Combined with regional geological background, the authors hold that the northwestern margin of the Qaidam Basin was still in the extensional tectonic stage in Early Carboniferous, and the rocks were formed under the condition of unrooting of the north of Qaidam orogenic belt and the upwelling of asthenospheric mantle.