Abstract:

In this paper, we present petrographical, zircon U-Pb dating and geochemical data for the maficultramafic intrusion in Xiarihamu CuNi deposit to constrain its formation time, petrogenesis and the tectonic setting. Xiarihamu No.1 maficultramafic intrusion locates in the middle of the Eastern Kunlun middle uplifted basement and granitic belt, south of the North Kunlun Fault. The intrusion covers an area of 0.7 km2, in shape of a flat lopolith, with the strikes of NEE. The intrusion is mainly composed of gabbronorite, orthopyroxenite, olivine pyroxenites, plagiolherzolite and orthopyroxene peridotite. Pyroxenite and peridotite are the main host rocks for the Cu-Ni ores. Major element contents of Xiarihamu complex show subalkalic series and are characterized by low SiO2 contents (SiO₂ 36.68%52.58%), low TiO₂ contents (TiO₂ 0.13%0.47%) and alkali contents (Na₂O+K₂O 0.26%1.95%), but high MgO contents (MgO 10.91%35.81%). The m/f values of the rocks range from 3.88 to 6.29, falling into the range of ferrousultramafic rocks. Similar LREErich pattern ((La/Yb)N=1.442.98) and no Eu anomaly indicate that Xiarihamu complex have the same magma source. Trace elements are enriched in LILE (Rb, Th, U, K) and relatively depleted in HFSE (Nb and Ta). The presence of Neoproterozoic granite relicts and the La/Yb, Ce/Yb, Th/Yb, Nb/La, and La/Sm values suggest the limited crustal contamination during the rise of the magma. Zircon LA-MC-ICPMS、U-Pb dating of gabbronorite yielded 423±1 Ma, MSWD=0.14, i.e., the Late Silurian. Zircon 176Hf/177Hf values are in the range of 0.2826280.282833 with corresponding εHf(t) values of 4.010.9. Lu-Hf singlestage modal ages (tDM1) vary from 610 to 875 Ma and mean age (788 Ma) , which is older than U-Pb age. We suggest that the primary magma derived from a depleted mantle, and may have been added metasomatic enriched lithospheric mantle components, and contaminated by crustal material. Combined with regional tectonic evolution, we believe that the intrusion formed in the post-collisional extension setting, perhaps triggered by slab break-off. The fractional crystallization of olivine and orthopyroxene and addition of crustal S appear to be directly related to trigger S saturation.

Key words:  Xiarihamu Cu-Ni deposit, maficultramafic intrusion, zircon U-Pb age, geochemistry, tectonics setting, East Kunlun