西藏多龙矿集区热构造演化历史——来自磷灰石(U-Th)/He的证据

The thermal-tectonic history of the Duolong ore district: Evidence from apatite( U-Th )

西藏多龙矿集区是西藏最重要的斑岩-浅成低温热液型矿集区。多龙矿集区经历了多期次的构造抬升-剥蚀事件,成矿后的埋藏-剥蚀历史对矿体的保存至关重要。本文运用磷灰石(U-Th)/He数据对多龙矿集区的低温热年代学进行研究。磷灰石(U-Th)/He年龄平均值分布在85. 1±4. 0Ma到37. 9±2. 5Ma,记录了晚白垩世到古新世、始新世、渐新世的热-构造事件。热历史模拟显示,多龙矿集区主要经历4次冷却事件:Ⅰ) 100～75Ma,冷却速率约为4℃/Myr,剥速率约为0. 16km/Myr,与班公湖-怒江洋的闭合以及拉萨-羌塘地块的碰撞事件有关;Ⅱ) 75～45Ma,冷却速率约为0. 3℃/Myr,剥蚀速率约为0. 01km/Myr,与拉萨-羌塘地块的继续碰撞事件以及由碰撞作用引起的逆冲推覆构造事件有关;Ⅲ) 45～30Ma,冷却速率约为2℃/Myr,剥蚀速率约为0. 08km/Myr,与印度-欧亚大陆的碰撞抬升事件有关;Ⅳ) 30Ma至今,冷却速率约为1℃/Myr,剥蚀速率约为0. 04km/Myr,与渐新世以来印度-欧亚大陆的持续碰撞作用以及渐新世以来青藏高原发育的频繁构造事件有关。多龙矿集区斑岩-浅成低温热液型矿床形成后在强烈的隆升-剥蚀的环境下保存下来,得益于早白垩世美日切错组火山岩的覆盖,由拉萨-羌塘地块碰撞作用引起的逆冲推覆构造引起的上部地层加厚,以及印度-欧亚大陆碰撞事件在多龙矿集区产生的相对较弱的破坏效应。拉萨-羌塘地块碰撞作用引起的地层加厚对多龙矿集区矿床起主要的保护作用。

The Duolong ore district is the most important ore district in the Qiangtang terrane of western Tibet. The Duolong ore district experienced multi-stages of uplift-erosion events, the burial-erosion history after the ore district formation is crucial to the protection of the ore bodies. The apatite (U-Th)/He was studied in this paper to investigate the low temperature history of the Duolong ore district. The average (U-Th)/He ages range from 37.9±2.5Ma to 85.1±4.0Ma, which indicate the thermal-tectonic events from Late Cretaceous to Paleocene and from Eocene to Oligocene. The thermal history shows the Duolong ore district experienced four cooling events:Ⅰ) 100~75Ma with cooling rate of 4℃/Myr and erosion rate of 0.16km/Myr which is related to the closure of the Bangongco-Nujiang ocean and the collision of the Lhasa-Qiangtang terranes;Ⅱ) 75~45Ma with cooling rate of 0.3℃/Myr and erosion rate of 0.01km/Myr which is related to the continued collision of the Lhasa-Qiangtang terranes and the consequent thrust nappe structure caused by the collision;Ⅲ) 45~30Ma with cooling grate of 2℃/Myr and erosion rate of 0.08km/Myr which is related to India-Asia collision;Ⅳ) since 30Ma with cooling rate of 1℃/Myr and erosion rate of 0.04km/Myr, this stage is related to continued collision India-Asia continents and extensive tectonic events since Oligocene. Survival of the Duolong ore district under intensive uplift-erosion environment is attributed to the overlap of the Meiriqiecuo volcanic rocks, thickened overlying stratum caused by thrust nappe structure and the weak destruction imposed by the India-Asia collision. The thrust nappe structure was thought to play a major role in burying and protecting the orebody in this study.