川西高原炉霍-道孚裂谷带丘洛地区科马提质岩的发现及其地质意义

川西高原炉霍-道孚裂谷带丘洛地区首次发现科马提岩,呈透镜状或团块状,具鬣刺结构,产于三江造山系巴颜喀拉地块之炉霍-道孚裂谷的三叠系如年各组玄武岩中。科马提岩由玄武质科马提岩、玄武岩、辉长岩、橄榄辉石岩等组成。岩石具富Mg O,贫Ca O、Al2O3和Fe2O3的特征,其Si O2含量为39.78%~49.85%,Mg O含量为23.28%~32.65%,Ti O2含量为0.21%~0.85%,K2O含量为0.04%~0.98%,P2O5含量为0.04%~0.12%,(Fe O+Fe2O3)/Mg O值为0.41~0.77,符合过渡型(T型)洋脊玄武岩特征,亦符合科马提岩各类元素的地球化学特征。原始地幔标准化的过渡元素配分型式表现为不对称的"W"型,Th元素形成一个明显的正异常"峰",Sr元素形成一个明显的负异常"谷",表明科马提岩系列可能来自于洋脊中心热幔柱,形成于低压高温条件下的部分熔融。岩石中鬣刺结构可能是叶蛇纹石高压分解的结果,表明研究区某些变质橄榄岩岩块曾经受过熔融物质喷发的影响。丘洛地区黄金资源丰富,研究具鬣刺结构的科马提岩岩石系列及其与矿产的关系,具有重要的经济价值。

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.