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Phase equilibria and geochronology of Triassic blueschists in the Bikou terrane and Mesozoic tectonic evolution of the northwestern margin of the Yangtze Block (SW China)
Journal of Asian Earth Sciences ( IF 3 ) Pub Date : 2020-10-01 , DOI: 10.1016/j.jseaes.2020.104513
Chenguang Xu , Michael L. Wells , Dan-Ping Yan , Liang Qiu , Yu Zhou , Yixi Zhang

Abstract The northwestern margin of the Yangtze Block is an important part of the northeastern-most margin of the Tibetan Plateau and is composed of several terranes. However, how and when many terranes or microplates accreted to the Yangtze Block remain to be resolved. Among these, the Bikou terrane consists of Archean to Paleoproterozoic basement and Neoproterozoic volcanic rocks. Blueschists occur as lenses along a ductile shear zone in the southern margin of the Bikou terrane. The age of blueschist facies metamorphism remains an unsolved issue but is important to understand the tectonic evolution. The mineral assemblage of the blueschists is sodic amphibole, epidote, chlorite, albite and quartz. The calculated pseudosection constrains the peak P-T conditions of the blueschist-facies metamorphism to be 5.2–6.5 kbar at 350–385 °C. The blueschists contain zircon grains with an age of 765.2 ± 11 Ma, representing the age of the protolith similar to mafic volcanic rocks in the Bikou terrane. Deformed granitic veins in the blueschists have been subjected to blueschist facies metamorphism and contain phengites with Si contents of 3.5–3.6 per formula unit (pfu). The deformed veins contain zircon grains with ages of 232.3 ± 6.8 Ma and 230.3 ± 4.3 Ma, indicating that blueschist metamorphism is younger than 230 Ma. The new data, combined with previous studies, suggest that the Bikou terrane was accreted to the Yangtze Block in the Late Triassic during the collision between the North China and South China blocks. The blueschist belt, which marks the Late Triassic accretionary boundary between the Bikou terrane and the Yangtze Block, merged with the Qinling-Dabie Orogen to the northeast.

中文翻译:

碧口地体三叠纪蓝片岩相平衡及年代学与扬子地块西北缘中生代构造演化

摘要 扬子地块西北缘是青藏高原最东北缘的重要组成部分,由多个地体组成。然而,许多地体或微板块如何以及何时增生到扬子地块仍有待解决。其中碧口地体由太古代至古元古代基底和新元古代火山岩组成。蓝片岩在碧口地体南缘的延展性剪切带上以透镜体的形式出现。蓝片岩相变质作用时代仍然是一个未解决的问题,但对于理解构造演化很重要。蓝片岩的矿物组合是钠闪石、绿帘石、绿泥石、钠长石和石英。计算的假截面将蓝片岩相变质作用的峰值 PT 条件限制在 350-385 °C 时为 5.2-6.5 kbar。蓝片岩中含有锆石颗粒,年龄为 765.2±11 Ma,代表了与碧口地体中基性火山岩相似的原岩年龄。蓝片岩中变形的花岗质脉经受了蓝片岩相变质作用,并含有每分子式单位(pfu)Si含量为3.5-3.6的phengites。变形脉中含有锆石颗粒,年龄分别为 232.3±6.8 Ma 和 230.3±4.3 Ma,表明蓝片岩变质作用年龄小于 230 Ma。新数据结合以往的研究表明,碧口地体是在晚三叠世华北地块和华南地块碰撞过程中增生到扬子地块的。标志着碧口地体与扬子地块之间晚三叠世增生边界的蓝片岩带,
更新日期:2020-10-01
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