房山岩体构造-热演化:来自(U-Th)/He年龄的约束

Tectonothermal Evolution of Fangshan Pluton:Constraints from(U-Th)/He Ages

房山岩体位于华北克拉通北缘,明确其中-新生代的隆升剥露过程及构造演化史可以为华北克拉通的构造演化提供有力证据.运用锆石裂变径迹、磷灰石(U-Th)/He及锆石(U-Th)/He等构造热年代学研究方法,综合房山岩体高、中、低温热年代学资料,重建了房山岩体的构造-热演化历史,并根据不同矿物的封闭温度差(ΔT)和与之对应冷却年龄差(Δt)的关系,计算侵入岩体在不同构造热演化阶段的抬升冷却速率,分析了岩体隆升速率的变化特征,结合前人研究成果进一步探讨了房山岩体隆升过程的基本特点.研究表明,房山侵入岩体构造热演化分为4个阶段:(1)130.0~123.5Ma,侵位岩浆结晶-固结阶段,岩体平均冷却速率高达88.46℃/Ma;(2)123.5~56.0Ma,岩体相对缓慢冷却阶段,平均冷却速率为0.74℃/Ma,平均隆升速率为29.6m/Ma;(3)56~35Ma,岩体相对快速冷却阶段,平均冷却速率为6.90℃/Ma,隆升速率为276.0m/Ma;(4)35Ma以来,岩体相对缓慢冷却阶段,平均冷却速率为1.0℃/Ma,隆升速率为40.0m/Ma,构造趋于稳定.结合区域构造动力学环境的研究,分析了房山岩体构造热演化可能的动力学成因,认为房山岩体阶段性抬升冷却可能与华北克拉通东部太平洋板块的俯冲作用、南北两侧陆内俯冲造山作用和西南部印度-欧亚大陆碰撞、青藏高原隆升等远程构造挤压有关.房山岩体的形成及相对快速抬升冷却阶段分别对应于华北克拉通两期重要的破坏高峰.

The Fangshan pluton is located on the northern margin of the North China craton,and its uplift-exhumation and tectonothermal evolution in Mesozoic and Cenozoic can provide evidence for tectonic evolution of the North China craton.The tectonothermal history of the Fangshan pluton is reconstructed by the zircon fission track,apatite（U-Th）/He and zircon（UTh）/He structural thermochronology based on the high-,middle-and low-temperature geochronological data in this paper.According to the relationship between the temperature difference（ΔT）and the cooling age difference（Δt）of different minerals,in this paper it calculates the cooling rate at different stages of thermal history and analyzes the characteristics of the cooling rate changes and the characteristics of uplift process of the Fangshan pluton based on previous research results.The results reveal four phases of tectonothermal history of the Fangshan pluton：（1）the crystallization and consolidation stage of emplaced magma during 130.0-123.5 Ma at the average cooling rate of 88.46℃/Ma;（2）the first slow cooling stage of the Fangshan pluton during 123.5-56.0 Ma at the average cooling rate of 0.74℃/Ma and the average uplift rate of 29.6 m/Ma;（3）the fast cooling stage of the Fangshan pluton during 56-35 Ma at the average cooling rate of 6.90℃/Ma and the average uplift rate of276.0 m/Ma;（4）the second slow cooling stage of the Fangshan pluton since 35 Ma at the average cooling rate of 1.0℃/Ma and the average uplift rate of 40.0 m/Ma.The tectonic activity tends to be stable.The possible dynamic causes of thermal history of the Fangshan pluton were analyzed considering regional tectonic dynamics.The differential uplift-cooling may be associated with the subduction of the eastern Pacific plate of the North China craton,the intracontinental subduction orogeny on both sides of the south and the north,India-Eurasia collision in the southwestern China and the tectonic uplift of the Qinghai-Tibet plateau.The formation and the fast cooling stage of the Fangshan pluton respectively correspond to two important uplift erosion processes in the North China craton.