川西新元古代玄武质岩浆岩的锆石U-Pb年代学、元素和Nd同位素研究:岩石成因与地球动力学意义

U-Pb ZIRCON GEOCHRONOLOGICAL, GEOCHEMICAL AND Nd ISOTOPIC STUDY OF NEOPROTEROZOIC BASALTIC MAGMATISM IN WESTERN SICHUAN: PETROGENESIS AND GEODYNAMIC IMPLICATIONS

扬子块体西缘新元古代岩浆活动非常强烈 ,其成因对研究Rodinia超级大陆的演化有重要意义。目前对这些岩浆岩的成因和形成的构造背景存在地幔柱和岛弧两种截然不同的观点。文中对康定地区的冷碛辉长岩进行了SHRIMP锆石UPb、元素和Nd同位素研究 ,结果表明辉长岩结晶年龄为 (80 8± 12 )Ma ,与康定花岗质杂岩在时空上密切共生。虽然辉长岩浆在上升过程中受到富集岩石圈地幔和 /或基性下地壳物质的混染 ,但其元素和Nd同位素特征总体上与苏雄碱性玄武岩 (典型的板内型玄武岩 )相似 ,形成于板内裂谷环境。与玄武质岩石相反 ,扬子西缘新元古代花岗质岩石地球化学特征没有明确的构造岩石组合关系。目前的研究资料表明扬子块体西北缘在约 95 0～ 90 0Ma期间可能存在一个近东西向的俯冲带和火山弧 ,但在 86 0～ 75 0Ma期间不存在火山弧 ,这个时期的大规模岩浆活动很可能与Rodinia超级大陆下的一个超级地幔柱活动有关。

Neoproterozoic magmatic rocks are widespread in western margin of the Yangtze block. The study of their origin is potentially significant to the study of the evolution of the supercontinent Rodinia. However, there are currently two opposing interpretations, i.e., mantle plume-related and island arc origin, regarding the tectonic environment for these Neoproterozoic magmatism younger than ca. 850 Ma. To further verify these two competing models, SHRIMP U-Pb zircon age determination, geochemical and Nd isotopic analyses are conducted for the Lengqi Gabbro, which is spatially and temporally coeval with the Kangding granitoid complex. U-Pb dating suggests an emplacement age of (808±12)Ma for the Lengqi Gabbro. Despite some arc-geochemical features caused by contamination of an enriched lithosphere mantle and/or mafic lower crust during ascending, it shows general geochemical and isotopic features similar to that of the intra-plate basalts. Therefore, the Lengqi gabbro is considered to have been derived from an OIB-like mantle source in a continental rift environment, similar to the Suxiong alkaline basalts. In contrast to the basaltic rocks that form relatively clear petrotectonic associations with specific tectonic environments, tectonic discrimination using geochemistry of granitoid rocks is ambiguous. Integrated geochemical and isotopic data for Neoproterozoic basaltic and granitoid rocks in South China support an active arc along the northern and northwestern margin of the Yangtze block at ca. 950～900 Ma. The multiple magmatic pulses of 860～750 Ma are likely related to a mantle superplume beneath Rodinia, but not to an active arc.