Paleomagnetism and Geochronology of the Early Cretaceous Volcanic Rocks in the Eastern Lhasa Terrane, Tibetan Plateau and its Tectonics Implications
Paleomagnetism and Geochronology of the Early Cretaceous Volcanic Rocks in the Eastern Lhasa Terrane, Tibetan Plateau and its Tectonics Implications
复制标题
青藏高原拉萨地体东段早白垩世火山岩古地磁、年代学及其构造意义
DOI：
10.1130/b36648.1
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发表时间：
2023
期刊：
影响因子：
--
通讯作者：
Lei Zhang
中科院分区：
文献类型：
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作者：
Yong Cao;Zhiming Sun;Haibing Li;Zhenyu Yang;Junling Pei;Xiaozhou Ye;Xinwen Cao;Bailing Wu;Lei Zhang
拉萨白垩纪古地理重建对新特提斯洋的演化和亚洲地壳缩短提供了重要的制约。然而，拉萨白垩纪的古地理仍然存在争议。研究这一问题的一个直接途径是对拉萨白垩纪岩石进行古地磁研究，但以往对拉萨白垩纪岩石的古地磁研究大多集中在拉萨白垩纪的中、西段。在印度-亚洲碰撞之后，不同的垂直轴旋转影响了拉萨古地磁的不同部分，因此，拉萨古地磁的西、中段数据不一定能直接约束其东段的古纬度。本文介绍了拉萨盆地东段洛龙地区早白垩世火山岩的古地磁资料。同位素地质年代学显示，这些岩石形成于127 - 124 Ma。17个古地磁测点的平均高温方向为：磁倾角/磁倾角（Ds/Is）= 21.8°/19.0°，其中κs = 65.8，α95 = 4.4°（κ为精度参数的最佳估计，α95为围绕平均方向的95%概率椭圆半径，s为地层坐标）。岩相学调查，一个积极的褶皱测试，一个反转测试，和一个paleosecular变化测试表明这种特征剩磁的主要来源。古地磁极为60.9°N，227.2°E，dp/dm = 2.4°/4.6°（dp/dm为绕平均极的95%概率椭圆的半轴），拉萨高原东段的古纬度为9.2 ± 2.4°N。结合前人古地磁研究的可靠结果，我们得出以下结论。(1)早白垩世，拉萨河整体走向为WNW-ESE，东中段可能为近EW向。(2)早白垩世以来，亚洲沿沿着96°的南北向地壳缩短量为2050 ± 230 km。(3)新特提斯洋南北向宽度的最小值约为1000 m。125 Ma为4185 ± 300 km。
Paleogeographic reconstructions of the Lhasa terrane for the Cretaceous provide important constraints on the evolution of the Neo-Tethys Ocean and crustal shortening within Asia. However, the Cretaceous paleogeography of the Lhasa terrane remains contentious. A direct way to study this issue is to conduct paleomagnetic investigations of the Cretaceous rocks of the Lhasa terrane; however, most previous Cretaceous paleomagnetic investigations of the Lhasa terrane were conducted in the middle and western segments of this terrane. Different vertical-axis rotations affected different parts of the Lhasa terrane following the India-Asia collision, and therefore paleomagnetic data from the western and middle segments of the Lhasa terrane cannot necessarily be used to directly constrain the paleolatitudes of its eastern segment. This study presents paleomagnetic data from the Early Cretaceous volcanic rocks with well-constrained ages from the Luolong area in the eastern segment of the Lhasa terrane. Isotope geochronology reveals that these rocks formed at 127−124 Ma. The mean high-temperature direction obtained from 17 paleomagnetic sites is declination/inclination (Ds/Is) = 21.8°/19.0° with κs = 65.8 and α95 = 4.4° (κ—best estimate of the precision parameter; α95—radius of the 95% probability ellipse around the mean direction; s—stratigraphic coordinates). Petrographic investigations, a positive fold test, a reversal test, and a paleosecular variation test indicate the primary origin of this characteristic remanence. A paleomagnetic pole of 60.9°N, 227.2°E with dp/dm = 2.4°/4.6° (dp/dm—semi-axes of the 95% probability ellipse around the mean pole) yields a paleolatitude of 9.2 ± 2.4°N for the eastern segment of the Lhasa terrane. Combined with reliable results from previous paleomagnetic studies, we draw the following conclusions. (1) During the Early Cretaceous, the Lhasa terrane was oriented WNW-ESE as a whole, and the eastern-middle segments may have been oriented nearly E-W. (2) Asia has accommodated 2050 ± 230 km of N-S crustal shortening along 96°E longitude since the Early Cretaceous. (3) The minimum N-S width of the Neo-Tethys Ocean at ca. 125 Ma was 4185 ± 300 km.