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Geochronological and paleomagnetic constraints on the early Middle Permian Paleolatitude of the Lhasa block and its tectonic implications
HangJun LI, ZhenYu LI, Lin DING, TengGe ZHAO, Jing XIE, YaHui YUE
Prog Geophy ›› 2026, Vol. 41 ›› Issue (1) : 54-82.
PDF(2581 KB)
PDF(2581 KB)
Geochronological and paleomagnetic constraints on the early Middle Permian Paleolatitude of the Lhasa block and its tectonic implications
A cast of east-west trending continental slivers, which were composed of the Cimmerian Supercontinent, including the Lhasa continental block, started to rift apart from the northern margin of Gondwana in the late Paleozoic. According to this comprehensive tectonic scenario, several points of view have been proposed on when and from which continent the Lhasa block rifted away from the northern margin of Gondwana over the past decades, however, the issue still remains controversial. To this end, in this study, we aim to provide quantitative constraints on this topic by employing isotope-based geochronological dating and paleomagnetic techniques. Through carrying out an integrated study including zircon U-Pb geochronology, paleomagnetism, petrography, and rock magnetism on the early Middle Permian Luobadui Formation basalts and andesitic basalts developed in and around the northwest part of the Linzhou County of south Xizang (i.e., Tibet), it reveals that the Luobadui Formation volcanics was formed at ca. 272 Ma (i.e., Guadalupian stage of Middle Permian). Based on rock magnetic results carried out by previous scholars and petrographic investigations performed in this study, it shows that the Luobadui Formation volcanic samples experienced low-grade metamorphism along margins of some rock-forming minerals like plagioclase, quartz. Typical accessory minerals like magnetite particles (is also a kind of iron-bearing oxides) distributed in Luobadui Formation volcanic rock samples was also impacted by this low-grade metamorphism. Rock magnetic data sets also suggest that the dominant remanence carriers in the Luobadui Formation volcanic rocks are pseudo-single domain (PSD) magnetite grains in this study. Statistical analysis on filtered characteristic remanent magnetizations from 21 paleomagnetic specimens of the Luobadui Formation volcanic rocks reveals that the Fisherian mean direction and associated parameters over the sampling unit of the Luobadui Formation volcanic rocks are Ds±ΔD=124.2°±8.8°, Is±ΔI=34.5°±7.2°, ks=20.3, α95 s=7.2° after bedding correction. Thus, based on already obtained statistical mean direction over the selected 21 samples of the Middle Permian Luobadui Formation volcanics, the paleomagnetic pole can be computed as λp=16.8°N, φp=325.5°E, K=20.7, A95=7.2°. The paleosecular variation has been adequately averaged out according to paleosecular variation (PSV) evaluation criteria in paleomagnetism for the sampled Luobadui Formation volcanics. Therefore, in combination of positive fold test results for the paleomagnetic data sets with rock magnetic and petrographic results, we believe that our obtained paleomagnetic results are primary origin. It can be calculated (reference site: 30.01°N, 90.99°E) that the Lhasa block was at 19.7°±7.2°S in southern hemisphere at ca. 272 Ma. In other words, the Lhasa block was still in middle-low latitude of southern hemisphere then. Combined with previously published results from the Lhasa block, we intend to think that the Lhasa continental block along with the Cimmerian supercontinent likely rifted away from northern periphery of Gondwana and started its long-lasting northward travel journey in early Permian or even much earlier period. Combined with previously published paleomagnetic and geological results, it shows that the total northward convergence is estimated as 1776±700 km of the Lhasa block travelling from low-middle latitude band of 19.7°±7.2°S at ca. 272 Ma to the equatorial area of 3.7°±3.4°S at ca. 180 Ma. This further indicates that the average motion rate of the Lhasa block was around 1.9 cm/a during the time period of 272~180 Ma. In other words, the Lhasa block likely rifted away from northern margin of Gondwana in Early Permian and immediately started its northward drift and motion journey.
Qinghai-Xizang Plateau / Lhasa block / Middle Permian / Paleolatitude / Neo-Tethys Ocean
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实验过程得到了黄宝春教授、颜茂都研究员以及刘双迟博士等人的帮助.原北京大学地球与空间科学学院赵千博士在部分样品测试过程中给予了有益帮助.期刊审稿专家提出了很多有益的修改建议,帮助提升了稿件质量.中国科学院青藏高原研究所张利云博士及博士生刘林对锆石年代学结果提出了积极的建议,在此一并表示感谢.
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