Zircon U-Pb-Hf Isotopes, Biotite ⁴⁰Ar/³⁹Ar Geochronology, and Whole-Rock Geochemistry of the Baogeqi Gabbro in the Northern Alxa, Southernmost Central Asian Orogenic Belt

The final closure time of the Paleo-Asian Ocean and the Permo-Carboniferous tectonic settings in the northern Alxa are very important but controversial tectonic issues. The geochronology and petrogenesis of mafic igneous rocks are superior in clarifying regional tectonic settings. Here, we report on zircon U-Pb-Hf isotopes, biotite 40Ar/39Ar geochronology and whole-rock geochemical data of the hornblende gabbro from the Baogeqi gabbro pluton in the northern Alxa. The LA-ICP MS U-Pb analysis of zircon grains from the hornblende gabbro yield a weighted mean age of 262.7 ± 2.3 Ma (2σ, MSWD = 0.74), manifesting that the Baogeqi gabbro pluton emplacement was during the late Middle Permian (Capitanian). The 40Ar/39Ar dating of biotite grains from the hornblende gabbro yields a plateau age of 231.3 ± 1.6 Ma (2σ, MSWD = 0.55), indicating that the Baogeqi gabbro pluton cooled to below 300 ℃ in the Triassic. The hornblende gabbro samples are calc-alkaline with metaluminous character, and show enrichment in large ion lithophile elements (e.g., Rb, Ba, Sr, and K) but depletion in Nb, Ta, P, Th, and Ti relative to primitive mantle. Combined with the positive zircon εHf(t) values (+4.9–+9.4), we suggest that the magmas formed from the partial melting of depleted mantle were metasomatized by slab-derived fluids. Together with regional geology, these geochemical data suggest that the Baogeqi gabbro pluton was formed in an intracontinental extension setting, further indicating that the Paleo-Asian Ocean in the northern Alxa was closed prior to the late Middle Permian (Capitanian), and this region was in a post-collision extensional setting during the Capitanian-Late Permian. In addition, the Triassic cooling of the gabbro pluton may be a record of the decline of the Capitanian-Late Permian post-collisional extension basin due to the far-field effect of subduction-collision during the closure of the Paleo-Tethys Ocean.