吉林延边开山屯地区地层时代的新证据

吉林延边开山屯地区古生代地层,在构造地层学研究的基础上,取得一些地层时代的新资料在混杂岩中新发现中二叠世北方型动物群化石;对混杂岩中的花岗岩砾石,测得SHRIMP锆石U-Pb年龄(286.8±5.6Ma),代表岛弧岩浆岩结晶时间;在变蛇绿岩和糜棱岩中采取白云母样品,测得40Ar-39Ar年龄,分别为408Ma和205.7Ma。由此,可以得出开山屯地区地层时代和混杂岩形成与演化的时间表泥盆纪洋壳形成;在中石炭世—中二叠世由南向北的运移过程中,洋壳之上发生硅质和碳酸盐沉积,形成海山;洋壳与由北向南移动的兴凯地块相对运动,二叠纪时向大陆俯冲,在兴凯地块西缘(现代方位)发育岛弧活动,并在中、晚二叠世形成滑塌堆积。可能与晚三叠世的洋壳俯冲作用有关,在兴凯地块前陆边缘发生逆冲作用,形成构造岩片。

NEW EVIDENCE OF PALAEOZOIC STRATIGRAPHY IN THE KAISHANTUN AREA, YANBIAN, JILIN

This paper discusses the Palaeozoic strata in the Kaishantun area of Yanbian District, Jilin Province. New stratigraphic data include: 1) a new fossil assemblage characteristic of the Boreal fauna in Middle Permian age has been found in the melange; 2) granite pebbles within the melange give a SHRIMP zircon U-Pb age of 286±5.6Ma, representing the crystallization age of island arc-magmas; 3) muscovites within ophiolite and mylonite were dated by 40 Ar- 39 Ar technique at 351.55Ma and 209.17Ma, respectively. The new data suggest that oceanic crust was formed in the Devonian. Siliceous and carbonate rocks were deposited atop the oceanic crust during the northward movement of the plate in the Middle Carboniferous—Middle Permian. The oceanic crust eventually subducted beneath the southward moving Khanka block, causing the development of Permian island arcs in western margin of the Khanka block and Middle-Upper Permian slump deposits. The oceanic crust kept subducting in the Upper Triassic, leading to eventual obduction at foreland edge, and the formation of the melange zone described in this paper.