滇东南老君山地区变质岩锆石U-Pb年代学及其构造意义

Zircon U-Pb Geochronology of Metamorphic Complex in the Laojunshan District,Southeastern Yunnan Province and Its Tectonic Implication

滇东南老君山地区出露一套穹窿状变形-变质岩系和花岗岩体,岩体内部因产出丰富的锡钨多金属矿床和特殊的大地构造位置,而备受关注。该套变质岩系亦是研究的热点,但由于缺乏系统详细的年代学限定,其构造演化史和构造指示意义还不清楚。本文选择老君山地区的花岗质片麻岩进行LA-ICP-MS锆石U-Pb定年,获得402.4±1.9Ma、407.7±1.5Ma和416.8~410.6Ma的3组岩浆锆石的结晶年龄,表明老君山地区变质岩系经历了早泥盆世的岩浆事件,很可能是该区陆陆同碰撞造山运动的记录,即广西运动的反映;锆石核部483.5Ma年龄值,可能为区域上加里东运动开始的时间。结合研究区变质岩系已有年代学数据,综合研究认为锆石颗粒古老核部的626.6Ma、715.9Ma和792.4Ma年龄值可以进一步揭示滇东南老君山地区存在新元古代的岩浆活动事件和古老结晶基底,揭示新元古代岩浆活动事件发育范围更广,从桂西-滇东南老君山地区到瑶山-哀牢山地区均有发育,进一步表明扬子地块周缘的华南新元古代裂谷-岩浆岩带极有可能是连成一体的。而锆石颗粒边部年轻的219.4Ma年龄值则记录华南地区广泛的晚三叠世的岩浆和成矿地质事件。

A deformed-metamorphic dome and a great granite body outcrops in the Laojunshan area,Southeastern Yunnan.The study area attracts great attention of many geologists because of its special tectonic position and many Tin-Tungsten polymetallic deposits.The set of metamorphic rocks is also a research focus,and its tectonic evolution history and structural significance are still unclear up to the present.LA-ICP-MS zircon U-Pb dating was conducted on the granitic gneisses from the Paleoproterozoic Mengdong Group,Late Silurian Nanwenhe gneissic granite,and Neoproterozoic Xinzhai Formation.Six samples from granitic gneisses of the Laojunshan metamorphic complex yielded three magmatic zircon groups,and the 206 U/238 Pb concordant age ranges are 402.4±1.9Ma,407.7±1.5Ma,and 416.8±3.5~410.6±0.96 Ma,respectively.Combined with the crystal features,CL images,concordant of U-Th-Pb ages and the corraltion index,the dating ages range from 416.8Ma to 402.4Ma of metamorphic rocks in Laojunshan area,indicating that these rocks experienced an extensively Early Devonian magmatic event,and probably recording the continental syn-collision orogenesis in the study area,i.e.the Guangxi movement;The age of 483.5±7.4Ma from the zircon core probably reveals the beginning time of the regional Caledonian movement.Based on the previous geochronological data of metamorphic rocks in this area,the inherited zircon cores with age group of 626.6Ma,715.9Ma and 792.4Ma further demonstrate that there should have Neoproterozoic igneous rocks and Paleo-proterozoic metamorphic crystal basement in the Laojunshan area,southeastern Yunnan Province.The Neoproterozoic magmatic event,developed extensively from western Guangxi-southeastern Yunnan to Dayaoshan-Ailaoshan region,further suggests that the Neoproterozoic rift and magmatic belt of South China were likely to be integral with each other in the southwestern margin of the Yangtz block.And the 219.4Ma of dating zircon rim records a large-scale extensive magmatism and mineralization event during the Late Triassic in South China block.