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岩石学报 2011
Geochronology and geochemistry of the Dabanshan intrusion in Panxi district and its constraints on the metallogenesis of Cu-Ni sulfide deposits
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Abstract:
The Dabanshan mafic-ultramafic intrusion, situated in the northwestern part of the Panxi district, is confined by the Jinhe-Jinghe fault. It consists predominantly of gabbros, noritegabbros and lherzolites. This study focuses on the petrogenesis of noritegabbros and lherzolites, in which the cumulus texture is pronounced and the olivine has maximal forsterite content (Fo) of 81.4%. The SiO2 content of lherzolites ranges from 42.93% to 44.18%, whereas SiO2 of noritegabbros varies between 44.89% and 52.76%. The total REE contents of the two types of rocks are relatively low (23.2×10-6~61.7×10-6), and are enriched in LREE. The similar REE patterns suggest that they are co-magmatic. Most of the samples are slightly enriched in large ion lithophile elements (LILEs, e.g. Rb, Ba and Sr) and moderately depleted in high field strength elements (HFSEs, e.g. Nb, Ta and Ti), suggesting that the magmas had not experienced significant crustal contamination. The petrographic characteristics and the geochemical signatures indicate an intensive fractional crystallization. On (Tb/Yb)PM versus (Yb/Sm)PM diagram, the primitive magma which formed the Dabanshan intrusion was likely derived from the incompatible elements-enriched spinel-facies lherzolite of lithospheric mantle. Most plots of NiO versus Fo contents in olivine show that the sulfides separated from the parental magma by liquid immiscibility, which is also supported by the bulk-rock Cu/Zr ratios in lherzolite (7.04~103) and noritegabbro (0.88~5.56). These suggested that the noritegabbros-lherzolites underwent undersaturation to oversaturation of sulfur, which could be attributed to fractional crystallization in high-level magma chamber, accounting for the sulfide segregation. LA-ICP-MS zircon data yield U-Pb age of 259.69±0.61Ma, being consistent with the ages of other mafic-ultramafic intrusions in Emeishan large igneous province.