川西里伍岩群变质-热事件：年代学证据及构造意义

Songpan-Garze orogenic belt, bounded by east of Tethys-Himalayan tectonic belt and circum-Pacific orogenic belt, experienced Paleo-Tethys and Neo-Tethys atypical collisional orogeny and developed a series of dome structure, whose metamorphic basement has the characteristics of the Yangtze block. The Liwu Group-Complex, a set of volcanic-sedimentary rock，exposed the metamorphic dome core of Jianglang，Taka and Changqiang areas in the Southern of Songpan-Garze orogenic belt. It provides some new information for basement formation in the western margin of the Yangtze block and tectono-thermal evolution in the eastern margin of the Tibetan plateau, for its complex magmatism (sedimentation)-metamorphism-deformation geological record. In this study，we will conduct volcanic tuff zircons U-Pb dating and Hf-O isotopic analyses to constrain the formation time of the Liwu Group-Complex. We will also use mica or amphibole Ar-Ar dating on UV-Laser，and apatite fission track to establish its metamorphic evolution in time sequence and inverse the P-T-t-D path of deformation-metamorphism and its exhumation history based on characteristics of the deformation-metamorphic and the metamorphic protoliths. Finally, compared with adjacent Danba dome, we will discuss the crust-mantle interaction, continental growth and evolution of Yangtze block. In conclusion, the results of this project will provided information for basement evolution and spatial-temporal configuration in the western margin of the Yangtze block, and tectono-thermal evolution in the eastern margin of the Tibetan plateau.

松潘-甘孜造山带位居特提斯-喜马拉雅构造带东缘与环太平洋造山带的交汇部位，其基底具有扬子陆块的特点，经历古特提斯与新特提斯非典型陆陆碰撞造山，发育一系列穹窿构造；其南缘江浪、踏卡和长枪等穹窿核部出露一套火山-沉积杂岩体，命名为里伍岩群，以复杂多样的岩浆（沉积）-变质-变形记录为特征，为研究扬子地块西缘基底组成和青藏高原东缘构造-热演化提供新思路。本项目在查明里伍岩群变质岩原岩性质及变质-变形期次的基础上，采用凝灰岩锆石U-Pb年代学和Hf-O同位素、角闪石和云母等含钾矿物UV-Laser法Ar-Ar定年，辅以磷灰石裂变径迹，建立里伍岩群变质演化时间序列，反演其变形-变质作用的P-T-t-D路径和剥露历史。在上述研究基础上，与邻区丹巴穹窿进行对比，探讨扬子地块西缘的构造属性、地壳生长和演化等动力学信息，为扬子地块西缘基底演化、时空配置及青藏高原东缘构造-热演化等地球动力学过程提供依据。

里伍岩群位于松潘-甘孜地块、扬子地块西缘、义敦岛弧所夹持的江浪、踏卡和长枪穹隆核部，为研究扬子地块西缘和青藏高原东缘构造-热演化提供了一个极佳的窗口。本项目开展了里伍岩群变质岩原岩性质及变形-变质研究，建立了该地区的变质演化时间序列，反演其变形-变质作用的P-T-t-D路径和剥露历史，获得一批重要的结果和数据。里伍岩群变质碎屑岩主要以石英片岩、石英岩夹云母片岩为主，原岩主要为砂岩、杂砂岩或硅质含量较高的砂岩；变质碎屑岩锆石U-Pb同位素年龄存在460Ma、500Ma、540Ma、600Ma、800Ma、900Ma、1000Ma、2500Ma八个峰值，与扬子西缘、康滇古陆的年龄谱系基本一致，暗示碎屑物质来源具近源搬运的特征，为被动大陆边缘沉积的产物。里伍岩群中正变质岩为阳起片岩和阳起石岩，其原岩为变基性岩（玄武岩类），获得锆石U-Pb同位素年龄为254.9Ma、191.5~223.7Ma，暗示基性岩浆活动形成于254.9Ma，并在191.5~223.7Ma遭受区域变质事件。因此，里伍岩群的时代归属限定在254~466Ma，碎屑锆石U-Pb年龄谱系显示原岩形成与华夏和扬子陆块有较强的亲缘关系，与特提斯-喜马拉雅奥陶系地层沉积环境和物源具有相似的特点，证实了里伍岩群变质碎屑岩为古生代的产物。黑云母及角闪石Ar-Ar年龄表明里伍岩群记录了四期变质事件：197Ma~191Ma、~171 Ma、147Ma、123Ma~130Ma，分别对应于低绿片岩相、高绿片岩相-低角闪岩相、低绿片岩相、低绿片岩相变质事件。锆石和磷灰石裂变径迹结果可划分为八个阶段，即4Ma、6.3~9.7Ma、11.1~19.8Ma、27.2Ma、37.7~44.7Ma、49.2~52.9Ma、55~60.8Ma、64~70Ma，暗示自10Ma以来松潘—甘孜地块南缘经历了统一抬升剥露作用。P-T-t-D轨迹显示为近于等温降压的顺时针型，揭示了研究区的变质与甘孜—理塘结合带向西俯冲及雅江盆地的拉张、闭合体制有关。上述结果完整刻画了三叠系以来从碰撞造山、热隆伸展、前陆挤出、陆内造山四大构造体制的转变过程。这一构造—岩浆—变质过程为探讨松潘—甘孜地块的演化、时空配置及青藏高原东缘构造-热演化等地球动力学过程提供大量的数据支撑，并对新生代高原扩展模式及动力学机制等科学问题提供了约束。