浙江徐家墩鹅湖岭组含铜多金属矿火山岩的地球化学与年代学

Geochemistry and Geochronology of Copper and Polymetal-Bearing Volcanic Rocks of the Erhuling Formation in Xujiadun,Zhejiang Province

浙江江山徐家墩鹅湖岭组流纹质熔结凝灰岩的断裂构造中含有铜多金属矿物,然而其形成时代及成因目前尚不清楚。该岩石为浅粉红色,具有流纹构造和斑状结构。其化学成分显示出富硅(SiO2=72.99%～77.56%)、富碱(Na2O+K2O=6.57%～8.61%),低钙、镁(CaO=0.03%～0.05%和MgO=0.01%～0.08%)的特征。稀土配分显示强烈负铕异常(δEu介于0.028～0.038之间)的"海鸥型"特征。岩相学和地球化学特征均显示其为具有A型花岗岩特征的流纹质熔结凝灰岩,推测属板内构造环境。其内锆石SHRIMP的U-Th-Pb测年结果显示该火山岩的形成年龄为138.0±2.4Ma,显示其就位时间为中生代白垩纪燕山运动的中晚期。中生代具有A型花岗岩特征的流纹质熔结凝灰岩的发现有助于建立所在盆地岩浆-构造活动序列及理解该区铜多金属成矿作用和选定找矿靶区。

Copper-polymetal minerals occur in the cracks of the Erhuling Formation rhyolitic ignimbrite in Xujiadun, Jiangshan District, Zhejiang Province, but the origin and age of rhyolitic ignimbrite are still unclear. Based on the field observation, it is pink in color and of rhyotaxitic and porphyritic structures. It is characterized by enrichments in Si and alkali （SiO2 =72. 99%77. 56%, Na2O＋K2O=6. 57%-8.61%） and depletion in Ca and Mg （CaO=0.03%-0.05%, MgO=0.01%-0.08 %）. REE geochemical analysis shows high negative Eu abnormalities （δEu=0. 028-0. 038） with a ＂seagull shaped＂ distribution in REE patterns. Petrography and geochemical characteristics indicate that the rhyolitic ignimbrite is of A- type granite, suggesting it formed in an intraplate structural environment. SHRIMP zircon U-Pb dating yields an erupting age of 138.0±2.4 Ma, indicating that the volcanic rocks likely emplaced at the middle late stage of Yanshanian movement during the Mesozoic Era. Discovery of Cretaceous rhyolite with A-type granite geochemical feature will do help to establish a magmatism-tectonic sequence and understand copper and polymetal mineralization in the Xianxialing Basin.