滇西点苍山印支期花岗岩的成因及其大地构造意义

Petrogenesis and Tectonic Significance of the Indosinian Granites in the Diancangshan Massif,Western Yunnan,China

滇西三江造山带属于特提斯构造域的重要组成部分,金沙江-哀牢山构造带作为古特提斯的演化产物之一,记录了古特提斯洋演化的重要信息。然而,关于该带古特提斯洋的属性、时空演化方式和最终闭合的精细时间仍没有得到很好的约束。本文选择金沙江与哀牢山构造带衔接处的点苍山中部偏东北和南部地区的花岗质岩石开展详细的LA-ICP-MS锆石U-Pb定年、微量元素和Hf同位素分析以及全岩主-微量元素研究。锆石定年和Hf同位素结果显示,点苍山南部的片麻状花岗岩的形成时代为240.3±1.4Ma,对应的ε_(Hf)(t)值为-16.2~-8.7;中部偏东北的两个淡色花岗岩的形成时代分别为241.2±2.3 Ma和240.1±2.2 Ma,对应的ε_(Hf)(t)值分别为-12.0~-5.5和-15.2~-11.7。元素和锆石Hf同位素特征显示,点苍山南部的片麻状花岗岩属于A型花岗岩,起源于该区新元古代变基性岩的部分熔融;而中部偏东北的淡色花岗岩显示出典型的S型花岗岩属性,为泥质碎屑岩在中-低温条件下脱水部分熔融而成。进一步结合整个金沙江-哀牢山岩浆-变质-构造和沉积作用的研究结果,精细地刻画了金沙江-哀牢山古特提斯洋的属性及其闭合和碰撞后开启的时限,即金沙江-哀牢山洋在晚二叠世之前开始发生俯冲,整个洋盆的闭合在254Ma已经完成,并不是前人认为的由东南向西北剪刀式逐步闭合;洋盆闭合后的陆-陆碰撞发生在254~247Ma,247Ma峰期碰撞后俯冲的特提斯洋壳发生断离,247~220Ma整个造山带进入碰撞后的伸展阶段。我们在点苍山新识别的A型花岗岩为造山带碰撞后伸展作用提供了至关重要的约束。

The Sanjiang orogenic belt in West Yunnan is an important component of the Tethyan tectonic domain.The Jinshajiang-Ailaoshan tectonic belt,as one of the evolution products of the Paleo-Tethys Ocean,recorded important information on the evolution of the Paleo-Tethys Ocean.However,the attribute,spatial-temporal evolution mode,and final closing time of the Paleo-Tethys Ocean in the Jinshajiang-Ailaoshan tectonic belt are still not well-constrained.In this paper,we present zircon U-Pb ages,trace element and Lu-Hf isotopic compositions,and whole-rock major and trace element concentrations of the granitic rocks in the Northeast and South Diancangshan massif.The zircon U-Pb dating and Hf isotope results show that the granitic gneiss in the South Diancangshan was formed at 240.3±1.4 Ma with correspondingε_(Hf)(t)values of-16.2 to-8.7,while the leucogranites in the Northeast crystallized at 241.2±2.3 Ma and 240.1±2.2 Ma with correspondingε_(Hf)(t)values of-12.0 to-5.5 and-15.2 to-11.7,respectively.These geochemical signatures indicate that the gneissic granite belongs to A-type granite and was formed by partial melting of the Neoproterozoic metabasites.In contrast,the leucogranites show an affinity to S-type granite,and were formed by partial melting of the argillaceous clastic rocks under middle-low temperature conditions.Additionally,our results,coupled with previously reported magmatism-metamorphism-tectonism-sedimentation data in the region,illustrate that the Jinshajiang-Ailaoshan Paleo-Tethys Ocean began to subduct before the late Permian,and the closure of the ocean was simultaneously completed at 254 Ma,which is not consist the Southeast to Northwest gradual closure pattern proposed by previous researchers.Subsequently,the continent-continent collision occurred during 254 Ma to 247 Ma,and then the Paleo-Tethys oceanic slab began to break off at 247 Ma.Since then,the whole orogenic belt entered the post-collision extension stage during 247 Ma to 220 Ma.Consequently,the new identification of 240 Ma A-type granite in the Diancangshan massif provides a crucial constraint on the post-collisional extension of the orogenic belt.