巴颜喀拉中部三叠纪花岗岩类的岩石成因及其地质意义

Petrogenesis and Geological Significance of Triassic Granites in the Central Bayanhar

巴颜喀拉-松潘甘孜造山带三叠纪花岗岩是研究青藏高原中部古特提斯造山作用过程与地壳生长演化机制的重要探针。本次研究对巴颜喀拉中部的花岗质岩体进行了详细的岩石学、锆石U-Pb年代学和全岩地球化学研究,探讨其岩石成因和动力学背景。锆石U-Pb年代学研究表明,本次研究的花岗质岩体形成于209~217 Ma,是晚三叠世岩浆活动的产物。岩石地球化学分析揭示,岩体由高Sr/Y值和低Sr/Y值花岗岩类组成。其中,高Sr/Y值岩体具有中等K_(2)O(2.13%~3.75%)、较高的Sr(373×10^(–6)~521×10^(–6))含量和Sr/Y值(47.5~69.4),较低的Yb(0.42×10^(–6)~0.62×10^(–6))、Y(6.79×10^(–6)~7.99×10^(–6))、Cr(5.86×10^(–6)~13.0×10^(–6))和Ni(1.91×10^(–6)~6.63×10^(–6))含量,La/Yb值为48.5~64.1,与起源于加厚地壳部分熔融的埃达克质花岗岩类似。低Sr/Y值岩体具有较高的K_(2)O(3.84%~4.27%)和SiO_(2)(63.94%~70.44%)、较低的Na_(2)O/K_(2)O值(0.78~0.95)和Sr/Y值(10.2~14.5),岩石起源于正常厚度地壳的部分熔融。综合分析表明,晚三叠世巴颜喀拉-松潘甘孜造山带并未发生明显地壳减薄,区域三叠纪花岗岩质岩石起源于碰撞环境下不同深度地壳的部分熔融,即加厚地壳熔融形成高Sr/Y值埃达克质岩,而正常厚度的地壳则熔融形成低Sr/Y值普通花岗岩类。不同深度地壳的重熔是巴颜喀拉-松潘甘孜造山带三叠纪时期地壳演化的重要方式。

The Triassic granites in the Bayanhar-Songpanganzi orogen are key probes to understand the Paleo-Tethyan orogeny and mechanisms of crustal growth in the central Qinghai-Tibetan Plateau.To constrain the petrogenesis and geodynamic setting of the granites in the Central Bayanhar area,an integrated petrologic,geochronologic,and whole-rock geochemical study was conducted.Zircon U-Pb dating results show that the granites were formed at 209–217 Ma,belonging to the Late Triassic magmatism.The geochemical study reveals that the plutons can be divided into high Sr/Y granites and low Sr/Y granites.The high Sr/Y granites exhibit intermediate K2O(2.13%–3.75%),high Sr(373×10^(–6)–521×10^(–6))and Sr/Y ratios(47.5^(–6)9.4),low Yb(0.42×10^(–6)–0.62×10^(–6)),Y(6.79×10^(–6)–7.99×10^(–6)),Cr(5.86×10^(–6)–13.0×10^(–6)),and Ni(1.91×10^(–6)^(–6).63×10^(–6)),enriched light rare earth,and depleted heavy rare earth elements,resembling those of adakitic rocks derived from melting of thickened crust.In contrast,the low Sr/Y granites have high contents of K2O(3.84%–4.27%)and SiO2(63.94%–70.44%)with low ratios of Na2O/K2O(0.78–0.95)and Sr/Y(10.2–14.5),which were originated from partial melting of the crust with normal thickness.The comprehensive study shows that there is no obvious crustal thinning in the Bayanhar-Songpanganzi area during the Late Triassic,therefore,the Triassic granites were derived from partial melting of the crust at different depths in the collisional setting,i.e.,the high Sr/Y adakitic rocks derived from melting of the thickened crust,but the low Sr/Y granites were formed by melting of the crust with normal thickness.It can be concluded that crustal remelting at different depths is an important mode of the Triassic crustal evolution in the Bayanhar-Songpanganzi orogen.