Comparisons Between Ore-bearing And Barren Mafic-ultramafic Intrusions In The Emeishan Large Igneous Province, Southwest China

Mafic-ultramafic layered intrusions constitute important components in the westernpart of the Emeishan large igneous province (ELIP) in SW China. Here we presentzircon U-Pb geochronology, geochemistry and Sr-Nd isotopic data from fiverepresentative barren mafic-ultramafic intrusions in Wupodan, Shangyi, Louyi，Xiayiand Cida. The LA-ICP-MS U-Pb zircon dating shows that Cida mafic-ultramaficrocks and the syentitic rocks were almost coevally formed (243.8±0.8Ma and240.2±0.8Ma). These age data possibly represent the end phase of the Emeishan LIPmagmatism. The other four intrusions were emplaced during the Late Permian,coeval with the formation of Emeishan flood basalts and associated mineralizedintrusions. All intrusions exhibit mineralogical homogeneity and small volume. Withthe exception of Cida mafic-ultramafic intrusion, the other four intrusions are notassociated with intermediate-felsic plutons and were emplaced into the Middle-latePaleozoic strata. Chondrite-normalized REE patterns of Louyi, Shangyi, Wupodanand Cida intrusions exhibit moderately fractionated patterns with (La/Yb)Nof3.40–7.68, whereas the Xiayi intrusion shows pronounced LREE enrichment with(La/Yb)Nof11.61–12.94. Samples from the five intrusions lack any significant Euanomalies. Louyi, Shangyi, Wupodan and Xiayi intrusions have low εNd(t) values of3.0to1.9and variable initial87Sr/86Sr ratios of0.7057–0.7074. Cida complexdisplays variable εNd(t) and (87Sr/86Sr)i, e.g., the mafic-ultramafic unit has low initial87Sr/86Sr ratios (0.7048to0.7064), positive εNd(t)(0.32to2.23, with one exception of0.22), the felsic unit shows lower initial87Sr/86Sr ratios (0.7043) and positive εNd(t)(0.18), and the intermediate unit possess εNd(t) of0.05–0.17and (87Sr/86Sr)iof0.704580–0.705436. All of these Sr-Nd isotopic compositions overlap with the fielddefined by Permian Emeishan basalts. Clinopyroxenes from the SY, WPD and LYintrusions show relative high equilibrium temperatures ranging from~1300℃to~1400℃, which can be regarded as a signature of plume-related origin. Although theXY clinopyroxenes exhibit a relatively low equilibrium temperature (~1200℃), theclose affinity with OIB suggest a plume-related origin. Thus, these intrusions appear to be part of the ELIP. These features, combined with moderate to pronounceddepletion of HFSE (e.g. Nb, Ta and Ti) and enrichment of LILE (e.g. Rb, Ba, K),correlate the Louyi, Shangyi and Wupodan intrusions to the interaction betweenmantle plume and subcontinental lithospheric mantle that had been previouslymodified by subducted-related materials. In contrast, the Xiayi intrusion was derivedfrom the plume source contaminated by subcontinental lithospheric mantle. The Cidamafic-ultramafic unit was derived from plume-head, with the involvement ofconsiderable amounts of enriched subcontinental lithospheric mantle (SCLM); thefelsic unit orignated from the partial melting of juvenile basic materials beneath theYangtze Block; whereas the intermediate unit was resulted from magma mixingbetween the felsic magma and basaltic magma evolved from the parentalmafic-ultramafic magma. We also attempt a comparison between the unmineralizedand mineralized intrusions in terms of their geologic features, parental magma, thedegree of fractionation crystallization, degrees of partial melting of source rocks andthe extent of crustal contamination. Some geochemical criteria that may be useful indiscriminating between mineralized and unmineralized mafic-ultramafic intrusions aresummarized, and insights into the ore potential of the five barren intrusions are given.