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会理拉拉地区是我国著名的铁铜矿区之一,区内前震旦纪河口群是拉拉式铜矿最主要的赋矿岩石。前人利用多种同位素方法对河口群进行过年龄测定,但获得数据差异大,造成河口群在康滇地轴前震旦基底地层划分中存在争议,因而无法约束河口群成岩及变质过程与成矿的关系。通过对河口群钠长岩中的锆石采用高精度SHRIMP U-Pb年代学研究,获得年龄为1 680Ma±13Ma,因而,将河口群归为早元古代晚期。此外,岩石有一致高的Nb/Y比值(0.7～3.0),属碱性玄武系列,在Nb/Th-Zr/Nb和Zr/Y-Nb/Y图上,HFSE比值成分点落入洋岛玄武岩(OIB)范围,具有较陡的REE分布型式和“驼峰”状微量元素原始地幔标准化分配型式,缺乏Nb,Ta负异常,与大陆玄武岩十分吻合,以及岩石形成于板内裂谷环境,这些地球化学特征表明河口群钠长岩形成于陆间裂谷,来源于地幔深部。
Huili Lala area is one of the famous iron-copper mine in our country. The former Sinian estuarine group is the most important ore-bearing rock in Lala copper mine. The predecessors used many isotopes to measure the age of the estuarine population. However, the differences in the obtained data caused the estuarine population to be controversial in the stratiformization of the anterior Sinian basement in the Kang-Dian region. Therefore, it is impossible to restrain the diagenesis and metamorphism of the estuarine and mineralization relationship. By using high-precision SHRIMP U-Pb dating of zircons in the alluvion of Hekou Group, the age of 1 680 Ma ± 13 Ma was obtained. Therefore, the estuarine group was classified as the Early Early Proterozoic. In addition, the rocks have consistently high Nb / Y ratios (0.7-3.0) and belong to the alkaline basaltic series. On the Nb / Th-Zr / Nb and Zr / Y-Nb / Y maps, Basalt (OIB) range has a steeper REE distribution pattern and a standardized distribution pattern of “hump ” trace elements in the original mantle, which lacks the negative Nb and Ta anomalies and fits well with the continental basalts, and the rock is formed in an intraplate rift environment These geochemical characteristics indicate that the estuarine albite formed in the inter-continental rift and originated from the deep mantle.