摘要:
侏罗纪/白垩纪之交是中国中东部乃至东亚地区构造活动的重要时期,发生了构造变形、岩浆-成矿、气候环境突变等一系列重大的构造事件,被称为“燕山运动”,但对于其启动时限的认识却一直存在着争议。本文以晋东北侏罗纪盆地为研究对象,在早中侏罗世沉积序列划分和重矿物组合特征变化研究的基础上,结合重点层位火山岩夹层锆石U-Pb同位素测年,详细刻画了这期构造事件启动的具体时限。研究认为,中侏罗统云岗组(J2yg)底部砾岩沉积时期,沉积序列由早期的湖进序列向湖退序列转换,碎屑岩的成分由早期的以石英、长石为主过渡为以长石、岩屑为主,重矿物组合由早期的以锆石为主过渡为以石榴子石为主,预示着侏罗纪构造活动进入了启动阶段。通过下侏罗统永定庄组(J1y)火山碎屑岩的年龄(179.2±0.79) Ma和中侏罗统云岗组(J2yg)顶部凝灰质碳酸盐岩年龄(160.6±0.55) Ma的制约,推测中侏罗统云岗组(J2yg)底部砾岩沉积时间大约为168 Ma,这个时间代表了侏罗纪构造活动(燕山运动)的起始时限。该成果在沉积学方面为燕山运动的起始时限提供了新的制约。
Abstract:
A series of significant geological processes termed as the “Yanshanian movement”, comprising such activities as tectonic deformation, magmatic-metallogenic systems, abrupt changes of climate and environment, occurred in the central and eastern parts of China as well as eastern Asia in the transition period of Jurassic and Cretaceous, but the initiation timing as well as the development process of the “Yanshanisn movement” has not been well established, and some controversies still exist. In this study, the timing of Yanshanisn movement initiation was revealed by investigating the Jurassic basin of northeastern Shanxi, and the initiation timing of the Yanshanian movement was analyzed in detail by determining the boundaries of sedimentary cycles, identifying the heavy minerals assemblage type in clastic rocks and conducting zircon U-Pb isotope dating. The results indicate that the Jurassic orogeny was probably initiated during the sedimentation period of basal conglomerate of the middle Jurassic Yungang Formation (J2yg). The switch from transgressive lacustrine deposition to regressive lacustrine deposition in the sedimentary successions of the Middle Jurassic Yungang Formation (J2yg) indicates that the regional stress field might have changed from early expansion to late compression. In addition, the change in primary constituents from early quartz-feldspar to late feldspar-detritus was observed in clastic rocks. Meanwhile, the heavy mineral assemblage was predominated by stable zircon at the early stage and by relatively stable garnet at the late stage. The timing of the conglomerate development at the bottom of the Middle Jurassic Yungang Formation (J2yg) is probably ~168.0 Ma, as inferred from the age of zircons in tuffaceous micrite ((160.6±0.55) Ma) on the top of the Middle Jurassic Yungang Formation (J2yg) and the age of zircons in pyroclastic rock((179.2±0.79) Ma)in the middle of lower Jurassic Yongdingzhuang Formation (J1y). The age of 168.0 Ma is believed to be the timing of Yanshanian movement initiation. The results presented in this study provide a new sedimentary constraint on the timing of Yanshanian movement initiation.
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