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

Feng Qianqian; Qiu Nansheng; Chang Jian; Liu Nian

Abstract：The Fangshan pluton is located on the northern margin of the North China craton,and its uplift-exhumation and tec-tonothermal evolution in Mesozoic and Cenozoic can provide evidence for tectonic evolution of the North China craton.The tec-tonothermal history of the Fangshan pluton is reconstructed by the zircon fission track,apatite (U-Th)/He and zircon (U-Th)/He structural thermochronology based on the high-,middle-and low-temperature geochronological data in this paper.Ac-cording 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 re-veal 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 of 276.0 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 proces-ses in the North China craton.