南澜沧江带曼兵岛弧花岗岩成因与构造启示:元素地球化学、锆石U-Pb年代学及Hf同位素约束

昌宁-孟连缝合带是中国西南三江特提斯重要的古特提斯洋残余，但其消减过程仍然存在争议。南澜沧江构造带曼兵岛弧花岗岩体为探究古特提斯洋俯冲过程提供了新的线索。本文对曼兵花岗岩体内的片麻状黑云花岗闪长岩和斜长角闪岩进行了锆石U-Pb年代学、Hf同位素和全岩元素地球化学研究。二者的LA-ICP-MS锆石U-Pb年龄分别为251.5±2.7Ma、253.8±1.1Ma和241.8±0.8Ma、259.5±1.9Ma，指示其为晚二叠世-早三叠世的岩浆活动产物。曼兵花岗闪长岩属于钙碱性到高钾钙碱性系列I型花岗岩，富集轻稀土元素和大离子亲石元素，亏损Nb、Ta、Ba、Sr和高场强元素，具有Eu的负异常（δEu=0.45~0.69）。花岗闪长岩和斜长角闪岩¹⁷⁶Hf/¹⁷⁷Hf平均值分别为0.2829035和0.2829762，锆石ε_Hf（t）平均值为11.0和12.5；地壳模式年龄t_DM^C加权平均值分别为578.4Ma和469.3Ma，类似俯冲相关的新生下地壳演化的岛弧花岗岩。曼兵花岗岩具有低的Ce/Pb（3.89~6.57）、Th/La（0.32~0.58）和Sm/La（0.11~0.15）指示了新生下地壳被上部围岩的混染。综合区域古特提斯洋演化历史，认为曼兵岛弧花岗岩体形成于洋陆俯冲向地块增生转换的构造环境，提出三江地区古特提斯主洋盆的闭合延续至早三叠世才完成。

Petrogenesis and tectonic implications of Manbing granitoids, southern Lancangjiang zone: Constraints by geochemistry, zircon U-Pb chronology and Hf isotope

The Changning-Menglian suture is the main remnant of Paleo-Tethys in Sanjiang area, SW China, whereas the subduction process is still a mystery. The Manbing arc-like granitoids in the southern Lancangjiang zone provide a new clue to explore the subduction process. In this paper, zircon U-Pb geochronology, Hf isotopes and whole-rock trace element geochemistry of the gneissic biotite granodiorites and plagioclase amphibolites in Manbing granitic body were studied. The LA-ICP-MS zircon U-Pb ages of those samples are 251.5±2.7Ma, 253.8±1.1Ma and 241.8±0.8Ma, 259.5±1.9Ma, respectively, indicating the related to Late Permian-Early Triassic magma activity. Menbing granodiorites belong to calc-alkaline to high-K calc-alkaline series, which are enriched in light rare earth elements and large ionic lithophile elements, with depleted in Nb, Ta, Ba, Sr and high field strength elements, and have negative Eu anomalies (Eu=0.45~0.69). The mean values of 176Hf/177Hf for granodiorite and anorthosite amphibolite are 0.2829035 and 0.2829762, respectively. The mean values of zircon εHf(t) were 11.0 and 12.5. The weighted mean crustal model ages of tDMC were 578.4Ma and 469.3Ma, which were similar to arc-like granitoids within the juvenile lower crust. The low Ce/Pb (3.89~6.57), Th/La (0.32~0.58) and Sm/La (0.11~0.15) of Menbing granitoids indicate the juvenile lower arc crust was replaced by fluids from pelagic sediments. Combined with the previous research results, it is believed that the Manbing arc-like granitoids were formed in the tectonic environment transition from subduction to continental collision, indicating that the closure of the main oceanic basin in the Sanjiang area may not be completed until the Early Triassic.