天山及邻区燕山期构造岩浆事件

天山及邻区中生代以来发育多个不整合面,其中下白垩统底部的不整合面规模最大,是一个贯穿全区的区域性不整合面。对中新生代沉积地层的碎屑锆石年代学研究和源区分析发现,碎屑物中含有大量晚中生代岩浆岩锆石,说明该地区存在一定规模的同时期岩浆活动。本研究认为,上述晚中生代的构造岩浆事件是燕山运动在天山及邻区的具体表现。早白垩世地层超覆不整合在不同时代的前白垩纪地层之上,不整合面以下的地层发育程度、剥蚀程度和构造变形强度在不同地区表现不同。在现今的天山山脉内部,下白垩统直接覆盖在中侏罗统头屯河组之上,上侏罗统齐古组和喀拉扎组整体缺失。在天山南北两侧的塔里木盆地、准噶尔盆地和吐哈盆地,上侏罗统局部发育,范围远小于中下侏罗统。进一步研究发现,盆地中的上侏罗统通常作为生长地层发育在晚中生代形成的褶皱古隆起翼部,依据上侏罗统齐古组中、下部凝灰岩层锆石U-Pb年代学数据和生物地层学资料,将燕山运动在天山及邻区的启动时间限定在160Ma左右。天山及邻区燕山期的构造变形具有多个方向,推测与中侏罗世晚期以来拉萨地块与亚洲南部边缘的碰撞、蒙古-鄂霍茨克洋的碰撞闭合以及俄罗斯东北部的Kolyma-Omolon地块与西伯利亚克拉通之间陆陆碰撞等构造事件的叠加影响有关,这也是晚中生代东亚多板块汇聚在天山及邻区的具体体现。

Tectono- magmatic events in Tianshan Mountains and adjacent areas during Yanshanian Movement period

There are many unconformities in the Tianshan Mountains and adjacent areas since the Mesozoic, among which the unconformity at the base of the Lower Cretaceous is the largest regional unconformity throughout the whole area. The studies on the chronology of detrital zircons from Mesozoic- Cenozoic sedimentary strata and their source areas have shown that there are a large number of late Mesozoic magmatic zircons in the detrital materials, indicating a certain amount of magmatic activity in the area during this period. This study suggests that the late Mesozoic tectono- magmatic events mentioned above represent the Yanshanian Movement in the Tianshan Mountains and adjacent areas. Unconformities at the base of the Lower Cretaceous overlap the underlying pre- Cretaceous strata. The stratum development, denudation degree and tectonic deformation intensity under the unconformity vary in different areas. In the present Tianshan Mountains, the Lower Cretaceous directly overlies the Middle Jurassic Toutunhe Formation, while the Upper Jurassic Qigu and Kalazha Formations are totally missing. In the Tarim, Junggar and Tuha basins on both sides of the Tianshan Mountains, the Upper Jurassic are locally developed and their ranges are much smaller than that of the Middle and Lower Jurassic. Further study found that the Upper Jurassic strata in the basin are usually developed as growth strata at the limbs of the fold paleouplift formed in the late Mesozoic. Based on previous zircon U- Pb ages of tuff beds in the middle- lower part of the Upper Jurassic Qigu Formation and biostratigraphic data, the initiation time of the Yanshanian Movement in Tianshan and adjacent areas has been limited to approximately 160 Ma. The Yanshanian tectonic deformation in the Tianshan Mountains and adjacent areas has multiple directions. It is possibly related to the superimposed effects of tectonic events such as the collision between the Lhasa block and the southern margin of Asia, the closure of the Mongolia- Okhosk Ocean, and the continent- continent collision between the Kolyma- Omolon block in northeastern Russia and the Siberian craton since the late Middle Jurassic. This is also a concrete manifestation of East Asia multi- plate convergence during the late Mesozoic in the Tianshan Mountains and adjacent areas.