喜马拉雅造山带始新世和中新世的地壳深熔条件:来自花岗岩的地球化学证据  

The change of crustal anatectic conditions for granitic magmatisms from Eocene and Miocene in the Himalayan orogen:Geochemical evidences of granites

作　　者：高彭 王艳 尹常青 张健[2] 钱加慧 GAO Peng;WANG Yan;YIN Chang-qing;ZHANG Jian;QIAN Jia-hui(Guangdong Provincial Key Laboratory of Geological Processes and Mineral Resources,School of Earth Sciences and Engineering,Sun Yat-Sen University,Guangzhou510275,China;Department of Earth Sciences,The University of Hong Kong,Pokfulam Road,Pok Fu Lam,Hong Kong SAR,China)

机构地区：[1]中山大学地球科学与工程学院,广东省地质过程与矿产资源探查重点实验室,广州510275 [2]香港大学地球科学系,中国香港

出　　处：《矿物岩石地球化学通报》2023年第5期998-1016,I0004,共20页Bulletin of Mineralogy, Petrology and Geochemistry

基　　金：国家自然科学基金资助项目(41973025,92155306,42025204)。

摘　　要：与花岗岩岩浆作用相关的地壳深熔条件与汇聚板块边缘的构造体制和热结构密切相关。对喜马拉雅新生代花岗岩来说,不同时代的地壳源区深熔条件被两组花岗岩记录下来。第一组形成于43~44 Ma的始新世;第二组形成于17~18 Ma的中新世,具有典型高喜马拉雅淡色花岗岩的地球化学组成。始新世花岗岩经历了分离结晶作用,形成了两个亚组,一组具有高镁铁度、高CaO含量和高Na2O/K2O和Sr/Y值,另一组则恰恰相反。Sr-Nd同位素组成表明,始新世花岗岩的源区物质由角闪岩和变泥质岩组成。相平衡模拟结果显示,始新世花岗岩的原始熔体由角闪岩和变泥质岩构成的混合源区在(850±50)℃和(0.85±0.05)GPa条件下部分熔融产生。而中新世花岗岩的源区熔融条件与典型高喜马拉雅淡色花岗岩的一致,为750~770℃和0.6~0.8 GPa,源岩为变泥质岩。综合包括变质岩石学在内的多学科研究结果,碰撞造山带从同碰撞阶段向碰撞晚期和碰撞后阶段的演化过程中发生了热结构的变化。同碰撞阶段的超高压变质岩记录了低的地温梯度(<10℃/km),碰撞晚期的始新世花岗岩形成于高的地温梯度(约30℃/km),碰撞后阶段的中新世花岗岩也形成于高的地温梯度(约25~35℃/km)。地温梯度的改变可以通过碰撞造山带构造体制的变化来解释,即由同碰撞阶段的挤压体制转变为始新世碰撞晚期和中新世碰撞后的伸展体制。Crustal anatectic conditions associated with granitic magmatisms are closely related to the tectonic regime and thermal structure of the convergent continental margin. For Cenozoic granites in the Himalayan orogen, their different crustal anatectic at different times conditions were recorded by two groups of granites. The first group is the Eocene granite which was formed at 43~44 Ma. The second one is the Miocene granite which was formed at 17~18 Ma, with typical geochemical compositions of the Higher Himalayan leucogranites. The Eocene granite formed through fractional crystallization has two subgroups with high and low maficities, respectively. The Eocene granites with high maficities have high CaO contents and high Sr/Y and Na_(2)O/K_(2)O ratios, while the Eocene granites with low maficities have low CaO contents and low Sr/Y and Na_(2)O/K_(2)O ratios. Sr-Nd isotopic compositions indicate that both amphibolites and pelites were source rocks of the Eocene granite group. Phase equilibrium modellings suggest that the initial melts of the Eocene granites were produced by the partial melting of a mixture composed of amphibolite and pelite at t=(850±50)℃ and p=(0.85±0.05) GPa. However, the Miocene granite magmas were sourced from partial melting of pelite at t=750~770 ℃ and p=0.6~0.8 GPa which were consistent with those of the typical Higher Himalayan leucogranites. Combining results from multidisciplinary studies including metamorphic petrology, it is indicated that the thermal structure of the Himalayan orogen had changed from the syn-collisional to late-collisional and post-collisional stages. The UHP(ultra-high pressure) metamorphic rocks in the syn-collisional stage were formed at low geothermal gradients(<10 ℃/km), whereas the Eocene granites in the late-collisional stagewere formed at high geothermal gradients(ca. 30 ℃/km) and the Miocene granites in the post-collisional stage were formed at high geothermal gradients(ca. 25~35 ℃/km). The change of geothermal gradients can be explained by the chan

关 键 词：花岗岩 喜马拉雅 碰撞造山带 张裂造山 相平衡 构造体制 

分 类 号：P581[天文地球—岩石学] P597[天文地球—地质学]