The Jinchuan Ultramafic Intrusion: Cumulate of a High-Mg Basaltic Magma |
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| Authors: | CHAI GANG; NALDRETT ANTHONY J |
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| Institution: | Department of Geology, University of Toronto Toronto, Ontario, Canada, M5S 3BI |
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| Abstract: | The Jinchuan intrusion, situated in Gansu province, China, isan ultramafic dyke-like body emplaced in the Longshoushan upliftedterrain on the southwest margin of the Sino-Korea platform.The intrusion is 6 km long, 350 m wide and hosts a major Ni-Cusulfide deposit. It comprises three subchambers: the west, west-central,and east. The two western subchambers are narrow and deep, andboth are laterally zoned from dunite in the core through Iherzoliteto olivine pyroxenite toward the margins. The eastern subchamberis shallow and wide, and it shows vertical stratification gradingfrom dunite at the base upward into Iherzolite and plagioclaseIherzolite, then back to Iherzolite at the top. Sixty rock samples from the Jinchuan intrusion have been analyzedfor major and trace elements, and 54 samples were also analyzedfor the REE. All samples contain > 24 wt.% MgO, with themajority having > 35% when recalculated to 100% anhydrous.Negative linear correlations are observed between MgO and mostother constituents (except for a few such as Na2O, K2O, Sr,and Rb, which may have been affected by alteration), and itappears that the rocks were essentially formed as mixtures ofcumulus olivine and primary magma. Electron microprobe analyses show olivine compositions fromFo79 to Fo86, with most between Fo83 and Fo85. The MgO/(MgO+ FeO) value of the primary magma is calculated to have been 0.64, and its MgO content is estimated to have been 12 wt.%.Thus, the Jinchuan igneous body is probably the ultramafic cumulateportion of an intrusion of a high-magnesium basaltic magma relatedto continental rifting. We suggest that the two western subchambers of the Jinchuanintrusion represent the main conduit to the original magma chamberand that their zoning was formed by flow differentiation. Theeastern subchamber probably represents a higher level of themagma chamber, where crystallization was marked by convectionand periodic replenishment. After consolidation, the Jinchuanintrusion was tilted to the east so the deeper parts of thewestern subchambers are now exposed to the same erosion levelas the shallower part of the eastern subchamber. |
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