The Study Of Late Paleozoic Basaltic Magmatism And Associated Nickel-Copper Metallogeny In The Baoshan Block,Sanjiang Tethys Belt,SW China

The Baoshan block,northern tip of the Sibumasu block,is one of the important components of the Sanjiang Tethys belt in southwest China.The Baoshan block was rifted from the northern margin of the Gondwana supercontinent in the Late Paleozoic and marks the opening of the Meso-Tethys Ocean basin.However,there has been a lack of systematic research on the driving mechanism of the opening of the ocean basin,reconstruction of the northern Gondwana supercontinent and regional metallogenic systems.In this work,I carried out geochronological,petrological,and geochemical studies for the late Paleozoic flood basalts,and associated dolerites and mafic-ultramafic intrusive rocks that are widely developed in the Baoshan block and for Daxueshan nickel-copper sulfide deposit hosted in one mafic-ultramafic intrusive rocks.Based on the results,I discussed the late Paleozoic basaltic rocks spatial-temporal distribution,magma nature,and petrogenesis-mineralization in order to provide important implications.Magmatic zircon crystals from seven selected dolerite/gabbro intrusions in the Baoshan block yield U-Pb ages from?3 10 to?280 Ma.These new ages,together with previous zircon U-Pb ages,define a protracted(-30 myr)episode of basaltic magmatism in a small area of this region,which is inconsistent with the typical temporal-spatial distribution of mantle plume magmatism.The trace element compositions of the Baoshan dolerite and associated lavas show light REE enrichments and negative Nb-Ta anomalies,similar to arc basalts as well as continental flood basalts worldwide but different from the oceanic island basalts.Mixing calculations using the whole-rock Sr-Nd and zircon Hf isotope data of the Baoshan mafic rocks indicate that their intriguing trace element characteristics can be explained by contamination of mantle-derived magmas with crustal materials.Together with the lack of Late Paleozoic arc-related calc-alkaline rocks and granitoids in the Sibumasu block and other contemporaneous Gondwana-derived micro-continental blocks,strongly support the premise that the?310-280 Ma basalts and dolerites in the Baoshan region are the products of continental rift-related magmatism rather than arc magmatism.Based on the temporal correlation of the?310-280 Ma rift-related magmatism in several related Gondwana-derived micro-continents(Sibumasu,South Qiangtang,Lhasa and Himalaya),plus other independent constraints such as paleoclimate biotas and paleolatitudes from the literature,I provide an improved model for the configuration of the Gondwana supercontinent in the Early Permian.It is common for some intrusions associated with continental flood basalts to host large magmatic Ni-Cu-PGE sulfide ore deposits.The Daxueshan deposit hosted in a mafic-ultramafic intrusion is the first case of magmatic Ni-Cu sulfide deposit that has been discovered in the eastern part of the Tethyan orogenic belt.The intrusion is composed of lherzolite,harzburgite,and gabbro.SHRIMP U-Pb dating of zircon crystals from gabbro samples yields a crystallization age of 300.5±1.6 Ma.The mafic-ultramafic intrusion shares the similar light rare earth element(REE)enrichments and pronounced negative Nb-Ta anomalies with the coeval basalts and dolerites in the Baoshan block,together with high fugacity of the parental magma,indicating some extent of modification by previous subduction processes.Such close temporal-spatial relationship and similar geochemical characteristics indicate that the late Paleozoic basaltic rocks,dolerites and the Daxueshan mafic-ultramafic intrusions are the products of the same tectonic-magmatic event.Olivine chemistry,whole-rock trace elements and Nd-Hf-Os isotope data indicate that fractional crystallization and crustal contamination played a role in triggering sulfide saturation in the Daxueshan magma,although their relative significance is unclear based on this study.The platinum group element(PGE)tenors of the Daxueshan deposit are extremely low,indicating a severe PGE depletion of the parental magma due to previous sulfide segregation at depth.Based on the results from this study,I conclude that further investment in the exploration of magmatic Ni-Cu sulfide deposits associated with the?310-280 Ma mafic-ultramafic intrusions in northern Baoshan and the other related Gondwana-derived micro-continental blocks is warranted.