扬子板块西缘早中生代“绿豆岩”成因及构造启示:锆石U-Pb年龄、微量元素及Hf同位素约束  

Origin and tectonic implication of early Mesozoic“mung bean rock”in the western margin of the Yangtze Platform:Zircon U-Pb age,trace element and Hf isotope constraints

作　　者：钏茂山 胡乐 蔺如喜 毛崇祯 李仕忠 李锁明 袁永盛 CHUAN Maoshan;HU Le;LIN Ruxi;MAO Chongzhen;LI Shizhong;LI Suoming;YUAN Yongsheng(Kunming Natural Resources Comprehensive Investigation Center,China Geological Survey,Kunming 650100,China;Institute of Earthquake Forecasting,China Earthquake Administration,Beijing 100037,China)

机构地区：[1]中国地质调查局昆明自然资源综合调查中心,云南昆明650100 [2]中国地震局地震预测研究所,北京100037

出　　处：《地学前缘》2024年第2期204-223,共20页Earth Science Frontiers

基　　金：中国地质调查项目(ZD20220318)。

摘　　要：为了研究早中生代“绿豆岩”的火山灰来源,及其与古特提斯弧火山活动的时空联系,对滇东北乌蒙山地区两件绿豆岩样品进行了锆石U-Pb年代学及Hf同位素分析。结果显示:嘉陵江组绿豆岩谐和年龄为(250.0±2.8)Ma,εHf(t)值为-18.1~-9.5,二阶模式年龄T_(DM2)为2141~1674 Ma;关岭组底部绿豆岩锆石谐和年龄为(247.0±1.2)Ma,εHf(t)值为-16.4~-7.5,对应的T_(DM2)范围为2050~1559 Ma。两层绿豆岩相似的年代学数据、Hf同位素组成和模式年龄分布,表明两层绿豆岩具有相同的物质来源,均为古—中元古代地壳物质熔融产物。锆石微量元素具有酸性岩浆锆石特征,表明绿豆岩原岩为长英质岩石。通过与区域数据对比,确定“绿豆岩”火山灰来源于三江造山带,且有证据显示在早—中三叠世期间,沿西南三江—宋马—十万大山一带存在一条由古特提斯洋发生俯冲消减形成的岩浆弧,两层绿豆岩都是古特提斯洋俯冲背景下的火山活动记录。结合野外地质特征及项目研究成果,限定古特提斯弧火山作用至少于嘉陵江组绿豆岩形成前(254.9 Ma)启动,至关岭组底部绿豆岩形成时(247.1 Ma)仍在活动。In order to clarify the source of volcanic ash found in early Mesozoic“mung bean rocks”and the spatiotemporal relation between the volcanic ash and Paleotethyan arc volcanism,geochronology,trace elements and Hf isotope of zircons from samples collected from two mung bean rock layers,Wumengshan area,northeastern Yunnan,are analyzed.For rock sample from the Jialingjiang Formation,it yields zircon U-Pb age of(250.0±2.8)Ma,εHf(t)values from-18.1 to-9.5 and T_(DM2)ages of 21411674 Ma,and from the Guanling Formation it yields zircon U-Pb age of(247.0±1.2)Ma,εHf(t)values from-16.4 to-7.5 and T_(DM2)ages of 20501559 Ma.The similarity of results between samples indicates the mung bean rocks are derived from the same source,and are products of Paleo-Mesoproterozoic crustal materials.The trace element characteristics of zircons from mung bean rocks are consistent with data from acidic magmas,indicating protoliths of mung bean rocks are felsic.Combined with previous results,it is determined that volcanic ash found in mung bean rocks comes from the Sanjiang orogenic belt.Evidences have shown that there exists a magmatic arc along the Sanjiang-Songma-Shiwandashan area during the Early-Middle Triassic,arisen from the subduction of the Paleo-Tethys Ocean,and the mung bean rocks are records of volcanic activities caused by the subduction of the ancient Tethys oceans.Combined with geological data,it is considered the ancient Tethys arc volcanism starts before mung bean rock diagenesis in the Jialingjiang Formation(254.9 Ma)and lasts until sedimentation at the bottom of the Guanling Formation(247.1 Ma).

关 键 词：绿豆岩 锆石U-PB年龄 HF同位素 古特提斯 岩浆弧 

分 类 号：P588.14[天文地球—岩石学] P542.4[天文地球—地质学]