College of Earth Sciences, Jilin University, Changchun 130061, China

In order to confirm the formation age, protolith characters and petrogenesis, the LA-ICP-MS zircon U-Pb dating, petrochemistry and Hf isotope analysis are carried out for the gabbro in the Xitieshan mining area. The results show that the gabbro has the zircons with weighted mean age of (514.7±1.1)Ma, implying that the gabbro emplaced in Middle Cambrian. Geochemical data indicate that the gabbro has such major element contents as SiO₂: 47.88%~49.87%, MgO: 4.66%~5.21%, TiO₂: 0.47%~0.51%, P₂O₅: 0.04%~0.05%, K₂O: 0.34%~0.56%, Na₂O: 2.67%~3.24%, Na₂O>K₂O and Mg^#: 61.67~65.84, which show the characters of high aluminum calc-alkaline rocks. Slightly right-leaning REE distribution patterns and obvious positive Eu anomaly (δEu = 1.74~1.92) are also indicated. The ε_Hf(t) values of the gabbro range from 4.1 to 7.67 and single-stage Hf model ages (T_DM1) range from 950 Ma to 812 Ma, suggesting that the magma originated from depleted mantle. The rock is enriched in LILE (e.g., K, Ba, Sr) and incompatible elements U, but depleted in HFSE(e.g., Nb, Zr, P and Ti), showing the typical island-arc magmatic rock characteristics. The gabbro in Xitieshan area was formed in the island-arc setting where the northern margin of Qaidam oceanic crust subducted beneath the southern Qilian block. It further confirms that oceanic crust subduction occurred in the northern margin of Qaidam Basin during the Middle Cambrian.