摘要
琼嘉岗伟晶岩型锂矿作为喜马拉雅稀有金属成矿带的标志型伟晶岩矿床,其成矿后的剥露保存研究对喜马拉雅带内稀有金属矿床的形成与保存、以及后续矿产勘查方向具有重要指导意义。本文通过采集不同海拔的地表花岗伟晶岩样品,对琼嘉岗矿区开展磷灰石(U-Th)/He(AHe)低温热年代学研究,结合热历史模拟,限定了琼嘉岗矿区抬升剥露历史。结果显示,琼嘉岗AHe年龄呈现出两期年龄数据,位于藏南拆离系内部肉切村群中的样品的AHe年龄集中于约16~13Ma,位于高喜马拉雅结晶序列中的样品年龄集中于~3Ma,揭示出琼嘉岗地区存在三期剥露事件。第一期剥露为25~16Ma,基于前人限定的锂辉石伟晶岩侵位年龄(25Ma)和侵位深度(6.7~8.9km),根据磷灰石(U-Th)/He封闭温度(35~80℃),结合古地温梯度(约30℃/km)和地表温度(约2~4℃),计算得到约16~13Ma时矿床位于距地表~2.7km,由此限定第一期剥露量为约4~6km,剥露速率为400~600m/Myr;第二期剥露集中于16~3Ma,剥露量为2.5 km,剥露速率为180m/Myr;第三期剥露主要发生于高喜马拉雅地段,时代为~3Ma至今,根据封闭温度和古低温梯度,第三期剥露量为2.7km,剥露速率为900m/Myr。由于第一期剥露与藏南拆离系活动时间相符,且样品主要产出藏南拆离系内,指示第一期剥露事件为藏南拆离系所导致。通过古应力分析可知,琼嘉岗矿区的最小主应力(σ1)为东西向(E-W),同时,南北向裂谷的活动峰期(20~5Ma)与琼嘉岗矿区第二期剥露作用基本一致,且琼嘉岗矿区位置靠近大型南北向裂谷岗嘎-当惹雍错裂谷,因此南北向裂谷可能为导致琼嘉岗矿区发生第二期剥露的主要构造因素,对成矿后剥露抬升起主导作用。第三期剥露(~3Ma)的主要体现在高喜马拉雅海拔较低且靠近河谷的样品,反映出第三期剥露主要与河流切割相关,是青藏高原对气候变化(包括降雨量增加以及河流袭夺事件)的响应,不利于含矿伟晶岩的保存。本次研究中获得的琼嘉岗高海拔地区(>5000m)样品剥露量(约6~9km),指示花岗伟晶岩总体剥露量较大,因此高海拔地区(>5000m)更有利于花岗伟晶岩的保存和出露,是未来喜马拉雅琼嘉岗地区稀有金属勘查的有利地段。
The Qiongjiagang granitic pegmatite-type lithium deposit,as a characteristic pegmatite deposit of the Himalayan Metallogenic Belt,provides important guidance for understanding the formation,preservation,and future exploration of rare metal deposits in the Himalayas.This study conducted(U-Th)/He(AHe)low-temperature thermochronology on surface granite pegmatite samples from different altitudes in the Qiongjiagang area.Combined with thermal history modeling,the exhumation history of the Qiongjiagang area was constrained.The results show two distinct age groups in the AHe dating of Qiongjiagang.Samples from the Rouqiecun Group within the South Tibetan Detachment System yield AHe ages concentrated around ca.16~13Ma,while samples from the Greater Himalayan Crystalline Sequence show AHe ages concentrated around~3Ma,indicating three distinct exhumation events in the region.The first exhumation event occurred at 25~16Ma.Based on previous studies on the age(25Ma)and depth(6.7~8.9km)of lithium pegmatite intrusion,and using the AHe closure temperature(35~80℃),combined with the paleo-geothermal gradient(~30℃/km)and surface temperature(ca.2~4℃),it was calculated that the deposit was located~2.7km below the surface at ca.16~13Ma.Therefore,the first exhumation phase is estimated to have a total exhumation amount of ca.4~6km,with an exhumation rate of 400~600m/Myr.The second exhumation phase occurred at 16~3Ma,with an exhumation amount of 2.5km and an exhumation rate of 180m/Myr.The third exhumation phase primarily occurred in the Greater Himalayan region around~3Ma to present day,with an exhumation amount of 2.7km and an exhumation rate of 900m/Myr.The first exhumation phase,which coincides with the activity period of the South Tibetan Detachment System,and the fact that the samples mainly occur within the detachment system,suggests that the first exhumation event was caused by the South Tibetan Detachment.Paleo-stress analysis indicates that the minimum principal stress(σ1)in the Qiongjiagang area is oriented E-W.Furthermore,the peak activity period of the N-S extensional faults(20~5Ma)is roughly consistent with the second exhumation event in the Qiongjiagang area.The area is also located near the large north-south Gangga-Tangra Yumco Rift,suggesting that the N-S rifting likely played a major role in the second exhumation phase.The third exhumation phase(~3Ma)is primarily reflected in the samples at lower elevations in the Greater Himalaya Unit,closer to river valleys,indicating that the third exhumation was mainly related to river incision.This phase reflects the response of the Tibetan Plateau to climate change(including increased rainfall and river capture events),which was detrimental to the preservation of mineralized pegmatites.The study also shows that the exhumation amount of granite pegmatites in the high-altitude regions(>5000m)of Qiongjiagang is ca.6~9km,suggesting that the total exhumation of granite pegmatites is relatively large.Therefore,high-altitude regions(>5000m)are more favorable for the preservation and exposure of granite pegmatites.These areas represent promising regions for future rare metal exploration in the Qiongjiagang area of the Himalayas.
作者
皮静怡
李俊瑜
赵俊兴
秦克章
吴林
何畅通
朱丽群
曹明坚
PI JingYi;LI JunYu;ZHAO JunXing;QIN KeZhang;WU Lin;HE ChangTong;ZHU LiQun;CAO MingJian(State Key Laboratory of Lithospheric and Environmental Coevolution,Institute of Geology and Geophysics,Chinese Academy of Sciences,Beijing 100029,China;College of Earth and Planetary Sciences,University of Chinese Academy of Sciences,Beijing 100049,China)
出处
《岩石学报》
北大核心
2025年第3期771-788,共18页
Acta Petrologica Sinica
基金
国家重点研发计划青年科学家项目(2023YFC2908400)
第二次青藏科考(2019QZKK0806)
国家自然科学基金项目(42372093)联合资助.
关键词
琼嘉岗锂矿
锂辉石伟晶岩
古应力分析
低温热年代学
南北向裂谷
Qiongjiagang lithium deposit
Spodumene pegmatite
Paleo-stress analysis
Low-temperature thermochronology
N-S rift
