扬子地台西缘河口群钠长岩锆石SHRIMP年龄及岩石地球化学特征

SHRIMP U-PB ZIRCON CHRONOLOGY AND GEOCHEMISTRY OF ALBITITE FROM THE HEKOU GROUP IN THE WESTERN YANGTZE BLOCK

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。

Hekou Group of Precambrian occurred in the Lala area is the major ore-bearing stratum. Isotopic age dating by previous worker revealed different results and lead to different viewpoints on the age of Hekou Group and the relationship among the diagenesis,metamorphism and mineralization. In this paper,SHRIMP U-Pb method is employed by using zircon from albitite of Hekou Group to obtain its age. The result revealed that the age of volcanic rocks of the Hekou Group is 1 680 Ma± 13 Ma, an age of late Proterozoic. Meanwhile, the testing samples present high Nb/Y ratio （0.7-3.0）, implying that the volcanic rocks of Hekou Group belong to alkali complex. Diagram of Nb/Th-Zr/Nb and Zr/Y-Nb/Y shows that the volcanic rock samples belong to oceanic island basalt （OIB） originated from a mantle plume source. Steep REE distribution pattern inclining slightly toward the right,＂hump＂ shape of primitive mantle-normalized patterns of trace elements,and lacking Nb, Ta negative anomaly, is characteristic of continental rift basalt. Meanwhile,tectonic setting discrimination shows that the volcanic rocks formed in intra--continent pull-apart environment. These geochemical characteristics show that the Hekou group volcanic rocks was formed in the continent rift valley and derived from the deep mantle.