那丹哈达地体及周缘中生代变形与增生造山过程

Mesozoic deformation and accretionary orogenic processes around the Nadanhada Terrane

东亚陆缘中生代增生造山过程及变形响应一直以来都是中国区域地质研究的重大课题,也是东亚地质构造演化的一个难点和热点。其中一个最为关键的科学问题就是,古太平洋板块(Izanagi)何时开始启动俯冲?对中生代东亚大陆边缘产生何种影响?那丹哈达地体出露于中国东北,为一套构造混杂岩系,是中国境内由古太平洋板块俯冲-增生形成的唯一证据,为解决这一问题提供了可能。本文通过总结大量前人最新的岩石学、同位素年代学、沉积岩石组合、主干断裂、岩浆活动、古生物及古地磁等资料,试图厘定那丹哈达地体构造属性、增生过程、拼贴时间以及古太平洋板块开始俯冲的时间,并与周缘地体进行对比。结果表明:(1)那丹哈达增生杂岩分为饶河杂岩和跃进山杂岩,饶河杂岩具有洋岛玄武岩(OIB)的特征,不是前人认为的蛇绿岩,可称之为洋岛(海山)杂岩;跃进山杂岩具有洋中脊玄武岩(MORB)的特征,是典型的蛇绿岩,同时暗示古太平洋板块可能于晚三叠世开始启动俯冲,并在136~131 Ma期间就位于现今位置。(2)那丹哈达与日本丹波—美浓—尾足地体都是侏罗纪增生楔,在沉积岩石组合和年龄、放射虫类型及分布、地质构造等特征上都非常相似,在中新世日本海打开之前应是一个统一的超级地体。

The Mesozoic accretionary orogenic processes and deformation responses in the East Asian continental margin have been a major subject of regional geological research in China,and also been a difficult and hot topic on tectonic evolution in East Asia.One of the most crucial scientific problems is the kinematic relationship between the East Asian Margin of the Eurasia Plate and the Paleo-Pacific（Izanagi）Plate in the Mesozoic.The Nadanhada Terrane exposed a set of tectonic mélanges,which is the only evidence of PaleoPacific Plate subduction in China,providing the possibility of solving this problem.Based on a large number of the previous and latest studies of petrology,isotope chronology,sedimentary rock assemblages,main faults,magmatic activity,paleontology and paleomagnetic characteristics,this paper tries to determine its tectonic nature,accretionary orogenic processes,emplacement age of the Nadanhada Terrane and the initial time of the Paleo-Pacific Plate subduction.The result is as follows：（1）the Nadanhada accretionary complex consists of the Raohe Complex and the Yuejinshan Complex.The Raohe Complex has the characteristics of the oceanic island basalt（OIB）,and there is no ophiolite in the complex,which is not as that previously considered.The Yuejinshan Complex as a typical ophiolite is characterized by the mid-oceanic ridge basalt（MORB）,indicating that the Paleo-Pacific subduction was in the latest Triassic and the time of the emplacement of the Nadanhada Terrane in the present location was during the 136-131 Ma.（2）The Nadanhada and Mino-Tamba-Ashio terranes as Jurassic accretionary wedges have similar sedimentary rock assemblages and ages,radiolarian types and distribution,and structural history.There should have been a unified super-terrane before the opening of the Japan Sea in the Miocene.