Geochemistry Of Lamprophyres In The Baimazhai Nickel Deposit, Yunnan Province, China: Implications For Their Origin

Lamprophyres in the Baimazhai nickel deposit are located on the southern segment of Ailaoshan fault belt. These lamprophyres are important parts of Cenozoic potassic rocks on this belt. On the basis of detailed study on their geology, petrology, mineralogy, chronology, elemental and isotopic geochemistry, as well as comparison with other similar rocks on this belt, genetic information of the lamprophyres in Baimazhai nickel deposit has been summed up, genetic relation with regional potassic volcanic rocks has been built, character of the metasomatic fluids and time of the metasomatic enrichment have been preliminarily investigated, partial melting degree, residual mineral and trace elemental composition in the mantle source, fractional crystallization processes have been modeled, and the geodynamical model of the potassic igneous rocks on the whole belt have been preliminarily proposed. Several conclusions have been drawn from this study as followings:1 The age of lamprophyres in the Baimazhai nickel deposit is between 32.01 ±0.60Ma and 32.46 ±0.62Ma, so they are products of early pulse of Cenozoic potassic magmatism in Ailaoshan fault belt.2 All samples are alkaline series, potassic and ultrapotassic calc-alkaline lamprophyres. The fluids derived from dehydration of subducted slab, both continental crust and oceanic crust contributed to enrichment of metasomatic mantle. The lamprophyres was subtly contaminated by crustal substances. Both partial melting and crystallization fractionation play an important role in rock-forming processes. Elemental geochemistry show that low REE group lamprophyres experienced crystallization fractionation of a mineral assemblage of clinopyroxene+olivine+plagioclase±Fe-Ti oxide±apatite, high REE group lamprophyres experienced crystallization fractionation of a mineral assemblage of olivine+clinopyroxene+plagioclase. Low REE group and high REE group lamprophyres are 10% and 4% partial melting of the metasomatic enriched mantle. Petrological mixing calculation show that the ratios of meltingresidue of the primary magma of the low REE group lamprophyres is Ole7.21OpXi6.99Cpxn.s2Gar4.00. Modelling calculation of the REE of the mantle source show that the lamprophyres in Baimazhai nickel deposit are derived from LREE-enriched metasomatic enriched mantle. Modelling calculation of crystallization fractionation of the low REE group lamprophyres show that the low REE group lamprophyres in Baimazhai nickel deposit are products crystallized from primary magma, relatively low crystallization degree (23.74%) and relatively high crystallization degree (44.15%) magma. Geochemica! characteristics of the rock-forming minerals and whole rock show that there are both similarity and difference among lamprophyres in the Baimazhai nickel deposit and those in Machangqing gold deposit, Baiya gold deposit, Yaoan gold deposit and Laowangzhai gold doposit on Ailaoshan fault belt, indicating the regional mantle source being heterogeneous.3> According to regional geology, petrology, mineralogy and geochemistry, genetic model of the lamprophyres has been preliminaryly proposed: about 70~50Ma collision and subduction between India plate and Asia plate initiated, and the fluid (including small-scale melt) metasomatized the Yantze sub-continental lithospheric mantle, forming enriched mantle source of the lamprophyres in Baimazhai nickel deposit. With subduction continued, about 40Ma (the timing of initiation of Cenozoic potassic magmatism on the Ailaoshan fault belt), slab break-off took place between subducting ancient Tethyan slab and India slab, and cause asenospheric mantle upwelling. Under transtensive background, hot asenospheric mantle initiated partial melting of former enriched lithospheric mantle, which formed the lamprophyres in Baimazhai nickel deposit. Regional large-scale potassic magmatism initiated the large-scale strike-slip shear of the Ailaoshan fault belt (about 27-22Ma).