拉萨地体大型剥蚀面形成时代及过程的低温年代学示踪

The research of timing and evolution of surfaces on the Tibetan Plateau is one of the key questions to understand the uplift history of Tibetan Plateau.However,privous studies are almost qualitative rather than quantitative. We focus on the large-scale denudation surfaces between the Linzizong volcanic rocks and the Takena Formation in the Lhasa terrene, southern Tibet. Surfaces west of Lhasa were buried by the Linzizong volcanic Formation whereas erosion surfaces east of Lhasa were exposed on the granitic bedrocks.Detailded field investigations will be processed in the western of Lhasa to take paleosoils near the surfaces for modeling paleo-elevations during surfaces formation.The low temperature thermochronology such as zircon and aptite dating by both fission-track and (U-Th)/He methods are processed during this study,accomppanied by DEM analysis of exposed erosion surfaces in eastern Lhasa and field investigations in order to select the granitic batholiths cut by this large-scale denudation surfaces.By thermal modeling,we try to depict its T-t thermal history in details and it will shed light on the timing and evolution of this large-scale denudation surfaces.Combined with paleo-elevations suggested by paleosoil oxygen isotopes,uplift history of Lhasa terrane will be constrainted after the India-Asia continental collison.

青藏高原地貌面的形成时代及过程作为青藏高原研究的核心问题之一，对恢复青藏高原的隆升历史具有重要意义。不过前人对青藏高原面的认识大多基于定性研究，而缺乏定量化的约束。本研究拟聚焦在拉萨地体夹持于林子宗火山岩与塔克那组地层之间的大型剥蚀面，该剥蚀面在拉萨以西被林子宗火山岩所埋藏，而在拉萨以东为基岩暴露的剥蚀面。本项目拟对拉萨以西埋藏剥蚀面进行详细的野外调查，通过对剥蚀面中保存的古土壤进行氧同位素研究以重建该剥蚀面形成时的古高度。同时，对拉萨以东暴露基岩剥蚀面进行数字高程地貌分析，甄选沿该剥蚀面暴露于地表的花岗岩体，通过野外系统采样，以低温热年代学实验技术为主要手段（锆石和磷灰石的裂变径迹和（U-Th）/He测年），借助热史模拟方法，定量刻画该剥蚀面T-t演化轨迹，揭示拉萨地体以东大规模剥蚀面的形成时代及演化过程，结合该剥蚀面形成时的古高度，制约碰撞后拉萨地体隆升幅度和机制。

造山带中残留的微起伏地形表面（残留古地面或古夷平面）常常作为大陆构造演化的被动标识，常常被用来推算造山后地表隆升的幅度，对于研究青藏高原隆升过程具有重要指示意义。拉萨地体上林子宗火山岩与塔克那组之间不仅为区域角度不整合面，也是大规模的剥蚀面。林塔大型剥蚀面的形成时代和演化过程，对制约碰撞后拉萨地体隆升幅度和机制具有重要意义。本研究通过对拉萨以东12个岩体的热年代学研究发现，林塔大型剥蚀面在尼洋河以北地区尚有残留，其形成时代不早于97Ma，而在雅鲁藏布江流域，林塔大型剥蚀面已经被始新世早期（56-34Ma）和中新世（22-18Ma）两期快速冷却事件所改造。根据含绿帘石花岗岩的侵位时代与深度，估算出局部地区自33Ma以来剥蚀了近25-30km地壳物质，剥蚀速率可达1mm/yr。同时，拉萨地体以北格拉丹东岩体低温热年代学数据结果显示，自36Ma以来，该岩体经历了极低的冷却速率（4.7-1.9°C/Ma）。遗憾的是，拉萨以西林塔剥蚀面的古土壤，可能由于受到林子宗火山岩的覆盖烘烤或者流体交代等，其氧同位素发生重置，已经不适用于古高度的建立，需要进一步寻找尚未被改造的古土壤。