THE JINCHUAN ULTRAMAFIC INTRUSION - CUMULATE OF A HIGH-MG BASALTIC MAGMA

The Jinchuan intrusion, situated in Gansu province, China, is an ultramafic dyke-like body emplaced in the Longshoushan uplifted terrain on the southwest margin of the Sino-Korea platform. The intrusion is 6 km long, 350 m wide and hosts a major Ni-Cu sulfide deposit. It comprises three subchambers: the west, west-central, and east. The two western subchambers are narrow and deep, and both are laterally zoned from dunite in the core through lherzolite to olivine pyroxenite toward the margins. The eastern subchamber is shallow and wide, and it shows vertical stratification grading from dunite at the base upward into lherzolite and plagioclase lherzolite, then back to lherzolite at the top. Sixty rock samples from the Jinchuan intrusion have been analyzed for major and trace elements, and 54 samples were also analyzed for the REE. All samples contain > 24 wt.% MgO, with the majority having > 35% when recalculated to 100% anhydrous. Negative linear correlations are observed between MgO and most other constituents (except for a few such as Na2O, K2O, Sr, and Rb, which may have been affected by alteration), and it appears that the rocks were essentially formed as mixtures of cumulus olivine and primary magma. Electron microprobe analyses show olivine compositions from Fo79 to Fo86, with most between Fo83 and Fo85. The MgO/(MgO + FeO) value of the primary magma is calculated to have been approximately 0.64, and its MgO content is estimated to have been approximately 12 wt.%. Thus, the Jinchuan igneous body is probably the ultramafic cumulate portion of an intrusion of a high-magnesium basaltic magma related to continental rifting. We suggest that the two western subchambers of the Jinchuan intrusion represent the main conduit to the original magma chamber and that their zoning was formed by flow differentiation. The eastern subchamber probably represents a higher level of the magma chamber, where crystallization was marked by convection and periodic replenishment. After consolidation, thc Jinchuan intrusion was tilted to the east so the deeper parts of the western subchambers are now exposed to the same erosion level as the shallower part of the eastern subchamber.