西藏羌塘中部亚丹高压变质岩年代学、地球化学特征及其构造意义

羌南和羌北地块沿龙木错-双湖缝合带碰撞对接,标志着古特提斯洋的最终闭合,然而古特提斯洋的构造演化,尤其它的打开时限一直争议不断。本文对龙木错-双湖缝合带以南荣玛地区的变质杂岩进行了岩相学、年代学和地球化学特征研究,进而约束古特提斯洋的开、合时限及其关闭后的构造演化特征。通过LA-ICP-MS锆石U-Pb定年,本文首次报道了1件新元古代斜长角闪岩,其加权平均年龄为717±7Ma(n=61,MSWD=1. 9),表明原岩形成于新元古代中期,反映羌南地区存在前寒武基底;另获得1件中侏罗世斜长角闪岩的锆石加权平均年龄为163±2Ma(n=26,MSWD=1. 9),表明其原岩形成于中侏罗世;获得2件石榴石多硅白云母石英片岩锆石边部谐和年龄范围分别为267～1349Ma和214～2050Ma;另对片岩中2组多硅白云母样品进行~(40)Ar/~(39)Ar定年,分别获得224. 2±1. 5Ma和223. 9±1. 5Ma的一致坪年龄,暗示区域变质作用可能从～224Ma持续到214Ma之后,且～214Ma的岩浆事件可能形成于碰撞过程中。新元古代和中侏罗世斜长角闪岩的岩石地球化学特征显示,前者原岩具E-MORB特征,后者原岩趋向于OIB特征。基于区域岩石组合与地球化学特征,推断二者均形成于伸展环境,前者可能形成于陆内伸展背景,响应Rodinia超大陆的裂解,后者可能形成于大陆裂谷环境,响应羌南、羌北地块碰撞结束后的裂解。综合前人与本文研究成果,认为新元古代中期(～717Ma),某未知陆块可能就已逐渐开始从羌南-印度大陆裂解,导致"古特提斯洋"在ca. 717～517Ma之间的某个时间点就已打开;古特提斯洋可能在～224Ma才完成闭合,羌南、羌北陆陆碰撞挤压持续到214Ma之后;中侏罗世区域为裂谷伸展环境,并发育OIB特征的碱性玄武岩和双峰式火成岩。

Geochronology, geochemistry and tectonic significance of high-pressure metamorphic rocks from Yadan area in Central Qiangtang, Tibet

The collision of the southern and northern Qiangtang massifs along the Longmucuo-Shuanghu suture zone marks the final closure of the Paleo-Tethyan Ocean in the Central Qiangtang. However, the tectonic evolution of the Paleo-Tethyan Ocean between the two massifs, especially the opening time, has always been ambiguous. In this study, we carried out the study about the petrography, chronology and geochemistry of the metamorphic complexes in Rongma area, southern Longmucuo-Shuanghu suture zone, to restrict the opening and closing time of the Paleo-Tethyan Ocean in northern Tibet and its tectonic evolution after closure. The plagioclase amphibolite yields a weighted average age of 717±7Ma (n=61, MSWD=1.9) by LA-ICP-MS zircon U-Pb dating, which indicates the Precambrian basement exists in the southern Qiangtang massif. In addition, a plagioclase amphibolite also was obtained with yielding a weighted average age of 163±2Ma (n=14, MSWD=1.9), indicating that the protolith was formed in the Middle Jurassic. The LA-ICP-MS zircon U-Pb dating shows that two garnet-polysilicon-muscovite quartz schists yielded concordant ages ranging from 267Ma to 1349Ma and from 214Ma to 2050Ma, respectively. And two groups of polysilicon-muscovite samples show a similar plateau age of 224.2±1.5Ma and 223.9±1.5Ma, through ⁴⁰Ar/³⁹Ar dating, respectively, implying that regional metamorphism may sustained from~224Ma to 214Ma. The whole-rock geochemical characteristics of Neoproterozoic and Middle Jurassic plagioclase amphibolites show that the former has affinity to E-MORB but the latter is characterized by OIB. Based on the regional rock assemblages and geochemical characteristics, it indicates that both of the two plagioclase amphibolites are formed in extensional environments. The former may be formed in an intracontinental extension environment, responding to the breakup of Rodinia supercontinent and the latter may be formed in a continental rift setting. Combined with previous data, we propose that an unknown continental block might have begun to rift from the Qiangnan-Indian unite block in the Meso-Neoproterozoic (~717Ma), and "the Paleo-Tethys" Ocean might have opened at some time from~717Ma to~517Ma. Furthermore, the opening time of the Paleo-Tethyan Ocean may be earlier than previously thought. The Paleo-Tethyan Ocean might be closed at~224Ma, and the collisional orogeny between the northern and southern Qiangtang massifs might last after 214Ma. Then the region occurred in an intraplate extension setting in the Middle Jurassic, with occurring of alkaline basalts characterized by OIB and bimodal igneous rocks in the region. We proposed the evolution of the ancient ocean might be analogous to the Paleo-Asian Ocean.