【摘要】 白垩纪期间特提斯喜马拉雅地块与印度克拉通之间的构造关系,是正确认识印度与欧亚大陆碰撞(简称印欧碰撞)模式的关键.本研究通过对特提斯喜马拉雅东部浪卡子县工布学乡下白垩统桑秀组岩石的系统古地磁学和年代学研究,结合早白垩世冈瓦纳大陆的重建和印度板块白垩纪的运动轨迹,在对比分析特提斯喜马拉雅地块早白垩世以来古地磁数据的基础上,提出印欧初始碰撞前存在一个刚性/准刚性的大印度板块,印欧碰撞仍为经典的陆陆碰撞.首先,对采样剖面顶部砂岩和底部安山岩的U-Pb锆石年代学研究限定了桑秀组火山岩的喷发年龄为早白垩世的～134 Ma.其次,岩石磁学研究表明携带高温(高场)特征剩磁分量的主要磁性载体为磁铁矿和钛磁铁矿;进一步古地磁测试分析在采样剖面褶皱两翼共23个采点中获得了可靠的特征剩磁组分,经相邻熔岩流古地磁方向去重检验后的特征剩磁平均方向和对应古地磁极位置分别为D*/I*=308.7°/-57.6°, k=29.5,α95=8.5°(N=11)和6.3°N,308.6°E(A95=9.1°).特征剩磁分量在99%置信水平上通过了褶皱检验和地球磁场长期变模型的检验,指示其为早白垩世末期区域褶皱形成之前获得的剩磁组分,并有效地平均掉了古地球磁场的长期变化.因此,该特征剩磁组分很可能代表了桑秀组火山岩形成时获得的原生剩磁.结果表明,早白垩世特提斯喜马拉雅地块的观测古纬度和古磁偏角与由同时期印度板块古地磁参考极计算获得的期望古纬度和古磁偏角完全一致,指示藏南桑秀组火山岩形成时期特提斯喜马拉雅与印度克拉通之间具有刚性/准刚性板块的特征.更多还原

【Abstract】 Identifying when, where, and how the Indian Plate or the Greater Indian Plate and the Asia continent collided is the most important three key issues for constructing a collision model, and it is also the starting basis for the further understanding of continental dynamics in the Tibetan Plateau. In the past decades, with the deepening of paleomagnetic studies on the terranes or blocks on both sides of the Indus-Yarlung Zangbo suture zone, the blank or deficiency of available paleomagnetic data from the Lhasa and Tethyan Himalaya blocks around the India-Asia collision has been largely filled.However, the timing and paleo-position of the collision between India and Asia still remains controversial on account of the discrepancy of the paleolatitudes of the Lhasa block during the Cretaceous to Eocene times. Meanwhile, tectonic relationship between the Tethyan Himalaya Terrane and the Indian Craton during the Cretaceous period is a key issue to better understanding the collision pattern between India and Asia. Whether or not the Tethyan Himalaya Terrane or the Himalayas kept a rigid or quasi-rigid connection with the Indian Craton, i.e. the Tethyan Himalaya Terrane or the Himalayas and the Indian Craton behaved as a rigid or quasi-rigid plate before the India-Asia collision, plays a key issue for the construction of the India-Asia collision model. In this study we report a combined paleomagnetic and geochronological investigation on the Lower Cretaceous Sangxiu Formation basalts and andesites from the Tethyan Himalaya located in the south of the Gongbuxue Village, Langkazi Country. In the light of the paleogeographic reconstruction of major plates and/or blocks of Gondwana at ～130 Ma, northwards-motion pattern of the Indian Plate during the Cretaceous, as well as published available post-Early Cretaceous paleomagnetic data from Tethyan Himalaya, we confidently argue that the Indian Craton and its northern Himalayas should behave as a rigid or quasi-rigid Greater Indian Plate prior to the India and Asia collision, and the archetypical India-Asia continental collision model is still a convincing scenario. Firstly, the U-Pb zircon chronological dating of the sandstone and andesite samples at the top and bottom of the sampling section yields weighted mean²⁰⁶ Pb/²³⁸ U ages of（136±2） and（133.7±1.6） Ma, respectively, suggesting an eruption age of ～134 Ma of the Early Cretaceous for the sampled Sangxiu Formation volcanic rocks. Secondly, rock magnetic results indicate that the hightemperature or high-field characteristic remanent magnetization（ChRM） components are carried predominately by magnetite and titanomagnetite. As a result, the ChRM directions, successively isolated from a total of 23 sampling sites in both limbs of an anticline, yield a mean direction of D/I=303°/-60.5°（k=48.5, α95=4.4°） after the tilt adjustment,corresponding to a paleomagnetic pole at 1.6°N, 309.5°E（A95=5.4°）. Further site-by-site assessing of the serial correlation between adjacent lava flows indicates that the available site-mean observations could be reorganized into 5 directional groups（DGs） and 6 independent site-mean directions, which yield a new formation-mean ChRM direction and a corresponding paleomagnetic pole of D*/I*=308.7°/-57.6°（k=29.5, α95=8.5°） and of 6.3°N, 308.6°E(A₉₅=9.1°),respectively. Positive fold test and paleosecular variation test results show a pre-folding origin of the ChRM and effectively averaging out the paleosecular variation. A comparison between the observed Early Cretaceous paleomagnetic results and those expected from coeval apparent polar wander path for the Indian Plate shows that the Tethyan Himalaya Terrane has compatible paleolatitude and paleo-declination with the Indian Plate during the Early Cretaceous times, indicating that the Indian Plate and its northern Himalayas should behave as a rigid or quasi-rigid Greater Indian Plate during the formation of the Sangxiu Formation volcanic rocks.更多还原