从洋壳俯冲到陆壳俯冲和碰撞：来自羌塘中西部地区榴辉岩和蓝片岩地球化学的证据

羌塘中部晚三叠世低温/高压变质带是目前青藏高原内部延伸规模最大的高压变质带,但大量关键高压变质岩石出露地区地球化学资料匮乏,严重制约了对高压变质带原岩建造以及构造演化的全面认识。本文以羌塘中西部地区尚无地球化学资料的果干加年山榴辉岩和红脊山蓝片岩为研究对象,进行了系统的地球化学以及原岩恢复工作。研究表明,果干加年山榴辉岩呈透镜状产于围岩石榴石多硅白云母片岩和少量大理岩中,其原岩为亚碱性玄武岩,具有较低的稀土总量(∑REE=51.19×10-6~59.43×10-6)和轻稀土亏损的特征(La/Yb)N=0.59~0.70],不具有Nb、Ta、Ti的亏损,与典型的N-MORB特征一致,暗示其原岩可能来源于亏损的地幔源区,形成于洋中脊环境。红脊山地区基性蓝片岩的原岩为碱性玄武岩-亚碱性玄武岩,具有高的TiO2(2.97%~4.14%)和P2O5(0.29%~0.48%)含量,富集轻稀土元素(La/Yb)N=6.10~11.6]和高场强元素,地球化学特征类似于OIB。但是这些基性蓝片岩与大量的陆源碎屑岩伴生产出,且具有明显的硅铝质上地壳物质混染的特征,与南羌塘地区二叠纪大陆板内基性岩墙的产出特征以及地壳混染特征一致,可能是其俯冲消减的产物。通过本文研究结果并结合区域内已识别出的E-MORB型洋壳和洋岛/海山物质深俯冲的证据,我们认为羌塘中部晚三叠世高压变质带以洋壳物质深俯冲为主,同时亦保留了部分陆壳物质俯冲的证据,暗示大洋向北俯冲消减结束之后,又牵引至少一部分南羌塘北缘陆壳物质经历了随后的俯冲过程。

A record of complex histories from oceanic lithosphere subduction to continental subduction and collision: Constraints on geochemistry of eclogite and blueschist in Central Qiangtang, Tibetan Plateau

A 500km-long high-pressure metamorphic belt occurs in the Qiangtang Block of northern Tibet. Due to little geochemical investigation, the primary rock formation and original geodynamic setting of the high-pressure metamorphic rocks (eclogite and blueschist) remain unconstrained so far. In this paper we present the results of a geochemical investigation on eclogite and blueschist from this belt, and discuss their geodynamic implications. The eclogites collected from the Guoganjianian localities occurs as blocks and lenses in Grt-Phn schist and marble. Geochemically, the Guoganjianian eclogite is tholeiitic and has low rare earth element abundances (−REE=51.19×10^-6~59.43×10^-6) and display light REE (LREE) depleted patterns with (La/Yb)_N values of 0.59~0.70,without negative Nb, Ta and Ti anomalies. These features are comparable with those of normal mid-ocean ridge (N-MORB). The blueschist collected from the Hongjishan localities occur as blocks and lenses in metasandstone and Phn-Qtz schist. These metamafic rocks are alkaline-tholeiitic and have high TiO₂ (2.97%~4.14%), high P₂O₅ (0.29%~0.48%) contents, and are enriched in light rare earth elements (LREEs) with (La/Yb)_N values of 6.10~11.6 and high field strength elements (HFSEs), similar geochemically to Ocean Island basalt (OIB). Geochemical signatures also indicate that upper continental crust contamination played insignificant role in the generation of protolith of the Hongjishan blueschist. The features of field relations and geochemistry are comparable with those of Permian mafic dyke swarms in south Qiangtang terrane. These rocks, altogether, record a complete history from ocean crust subduction, to continental subduction, and to continental collision.