陈蔡岩群下河图斜长角闪岩年代学、地球化学特征及其构造意义

Geochronological and geochemical characteristics of the Xiahetu amphibolites from Chencai Group and their tectonic implications

浙江诸暨市下河图村的陈蔡岩群中出露一套斜长角闪岩,空间上与块状大理岩相伴产出。地球化学研究表明,下河图斜长角闪岩SiO_2含量为43.22%~46.56%,MgO为3.23%~7.87%,TiO_2为1.90%~2.98%,与碱性洋岛玄武岩特征类似。稀土元素总量为114.47×10^(-6)~192.39×10^(-6),(La/Yb)N为5.93~12.13,稀土元素球粒陨石标准化配分模式为轻稀土元素富集的右倾型;原始地幔标准化微量元素蛛网图表现出向上隆起的富集形态,微量元素特征表明其可能形成于洋岛构造环境,陆壳物质对其混染的可能性较小,岩石成分主要受熔融源区控制。推测下河图斜长角闪岩原岩很可能形成于靠近消减带的海山环境,来源于洋壳俯冲过程中增生的海山碎片,陈蔡岩群很可能为一套俯冲增生杂岩。LA-ICP-MS锆石U-Pb法获得斜长角闪岩变质年龄为420.6±1.8 Ma,可能代表了扬子和华夏两大地块碰撞拼合的时代。

A suite of amphibolites associated with marble from Chencai Group is exposed at Xiahetu Village in Zhuji County of Zhejiang Province. Detailed geological and geochemical studies were carried out on the Xiahetu amphibolites. The SiO2 content of the amphibolites ranges from 43.22% to 46.56%, MgO content ranges from 3.23% to 7.87% and TiO2 content ranges from 1.90% to 2.98%. All these geochemical features are similar to the average value of oceanic island basalt （OIB）. Moreover, the major elements characteristics of the amphibolite suggest that it belongs to the alkali basalt series. The total REE values of the amphibolite range from 114.47×10-6 to 192.39×10-6, and （La/Yb）N ratios range from 5.93 to 12.13. The chondrite-normalized REE patterns show right-inclined shape, which suggests the enrichment of LREE. The primitive mantle normalized trace element spider diagram shows obvious uplift, which is also similar to the feature of oceanic island basalt. Some tectonic discrimination diagrams suggest that Xiahetu amphibolites were formed in an oceanic island tectonic environment. Petrogenetic research shows that the protolith of the amphibolite had experienced a little crust contamination during the magmatic events. So the geochemical features of the amphibolite can be used to infer its original mantle features. It is held that the protolith of the Xiahetu amphibolits possibly formed in a marine volcanic geological setting near the subduction zone and sourced from the seamount fragments accretion during the subduction of the oceanic crust. Thus rocks in Chencai Group are probably a suite of subduction-accretionary complexes. U-Pb dating results of zircons in the amphibolite yield a mean age of 420.6±1.8 Ma which represents the metamorphic time of its protolith. In conjunction with other field evidence, the authors consider that the amalgamation between the Yangtze and Cathaysia Blocks was completed in Caledonian.