北京西山——一个早中生代拗拉谷的一部分

华北地台北部的燕辽带在晚元古代时为一拗陷带,蓟县地区的整个层系厚度在10,000m以上。中生代时发生强烈的岩浆活动和形变。其构造性质与地台的含义并不相符,前人对该区有沉降带、台褶带等提法。

THE XI SHAN OF BEIJING——A PART OF AN ANAULACOGENIENT IN EARLY MESOZOIC

The Mesozoic in the Xi shan of Beijing is composed in ascending order of basalt, coal measures, andesite, coarse clastic rocks and rhyolitic ignimbrite formations. The total thickness of the sequence is more than 8000 m, extending as a trough along NEE direction between the flat-lying Sinian siliceous dolomites on both sides. Essan-tially, it is a graben-horst structure (fig. 1). Following facts that the sequence from bottom to top becomes more and more coarse, from coal deposit changes into red mo-lasse, volcanic materials exist thoughoutly in strata, the volcanic rocks belong to alkaline series petrochemically, and the characteristic of their distribution in space, all indicates that the nature of Mesozoic in Xi shan is similar to those of younger continental rift such as Baikal. Based on the analysis of regional structure the rift probably started from Indonesian Movement near the end of Triassic and was failed during Late Cretaceous to Early Eogene. It developed from east to west (fig. 2).Similar deposits can be traced from northern Shanxi passing through Beijing, northern Hebei, western Liaoning and eastern Jilin towards Asian Continental Margin of that time. The trend of the zone exhibits principlely NEE direction, whole length is over 1,500 km and becomes widen from west to east. In eastern Liaoning the rift zone was cut by Tancheng-Lujiang Fault zone and Songliao Basin. All of the Upper Triassic and Lower to Middle Jurassic in North China and south of Northeast China was seated within this zone. As moving away from the axis of rift to both of north or south side, the strata become more and more incomplete and Middle to Upper Jurassic can directly lie on pre-Mesozoic basement. All identified Mesozoic alkaline plutons in same area are located within this zone too. The initial ⁸⁷Sr/⁸⁶Sr ratio of volcanic rocks taking from middle and east section of the rift zone is 0.7050 and 0.7071 respectively, thus the magmas were generated from upper mantle in general but were contaminated by the continental crust.The growth of Mesozoic in the rift zone presented getting thicker, older and more complicated from west to east. In the most western part of the rift zone only a Middle Jurassic bimodal volcanic series 155 m thick rests on Carboniferous-Permian system. Eastwards to Hebei, a suit of pyroclastic rocks, coal deposit and basalt series whose thickness reaches 4,000 m appears beneath the volcanic formation. Still eastwards to western Liaoning, Late Triassic terrestrial clastic deposit emerged from the bottom of Jurassic. It changs into volcanic series in eastern Jilin and its thickness reached is more than 5,000m in Suifenhe area near the border of Soviet Union (fig. 3). According to the occurrence of T₆-J₁ radiolarian silicolites and submarine basic extrusive rocks in the north of Hulin region, this Yanliao Rift perhaps represented an Early Mesozoic aulacogen which as transverse trough located at reentrant on Eastern Asian continental margin and inserted into cratonized continental interior. The Okhotsk-Mongolia zone and Middle-Lower Yangtze valley zone which are parallel with the Yanliao rift zone possible have a same mechanism.