The discovery and geological implications of the komatiites in the Luhuo-Daofu rift zone,northwestern Sichuan

The komatiites have recently been found in Qiuluo,Sertar,northwestern Sichuan,and occur as lenticular or lumpy structures and spinifex textures in the basalts from the Triassic Runiange Formation in the Luhuo-Daofu rift zone in the Bayan Har block of the Nujiang-Lancangjiang-Jinshajiang orogenic zone. The komatiitic rock series consist of basaltic komatiite,basalt,gabbro and olivine pyroxenolite,and are characterized by higher MgO contents,lower CaO,Al₂O₃ and Fe₂O₃ contents. SiO₂=39.78%-49.85%,MgO=23.28%-32.65%,TiO₂=0.21%-0.85%,K₂O=0.04%-0.98%,and P₂O₅=0.04%-0.12%. The (FeO + Fe₂O₃)/MgO ratios range from 0.41 to 0.77. The above-mentioned characteristics are consistent with those of the transitional (T-type)mid-oceanic ridge basalts (MORB),and the contents of individual elements agree with those of the typical komatiites. The primitive mantle-normalized transitional element distribution patterns display the asymmetrical "W" type patterns with a distinct peak at the element Th and a remarkable trough at the element Sr. These geochemical signatures show that the komatiites may be derived from the mid-oceanic ridge thermal mantle plume,and formed under the conditions of low pressure,high temperature and partial melting. The spinifex textures in the rocks may result from the high-pressure decomposition of antigorite, indicating that some metamorphic peridotite blocks may once be affected by the eruption of the melted materials. The Qiuluo area is rich in gold resources. The study of the relationship between the komatiitic rocks with the spinifex textures and the mineral resources is of important economical values for the study area.