Petrology and geochemistry of ultramafic rocks in the Mogok belt, Myanmar: Cumulates from high-pressure crystallization of hydrous arc melts
Petrology and geochemistry of ultramafic rocks in the Mogok belt, Myanmar: Cumulates from high-pressure crystallization of hydrous arc melts
复制标题
缅甸抹谷带超镁铁质岩石的岩石学和地球化学：含水电弧熔体高压结晶的积累
DOI：
10.1002/gj.4238
复制
发表时间：
2022
影响因子：
1.8
通讯作者：
Dong Xiao-Tao
中科院分区：
文献类型：
--
作者：
Liu Tong;Liu Chuan-Zhou;Chen Yi;Guo Shun;Sein Kyaing;Dong Xiao-Tao
缅甸中部的抹谷变质带（MMB）以其复杂的构造、岩浆作用和特提斯俯冲和印度-亚洲碰撞框架内的变质作用而闻名。它还因宝石级红宝石和蓝宝石而闻名于世。因此，识别和区分该地区的岩石单元对于了解宝石矿化非常重要。超镁铁质地块也与 MMB 中含宝石的变质岩一起出露。关于它们的起源（即侏罗纪-白垩纪蛇绿岩或堆积岩的地幔橄榄岩）及其与宝石生成的关系仍然缺乏约束。在这里，我们报告了抹谷超镁铁质岩石的岩石学和地球化学数据。岩相观察表明，它们具有典型的堆积结构，结晶顺序为橄榄石/尖晶石-斜方辉石-单斜辉石。它们的全岩 Al2O3 和 CaO 含量较低，而尖晶石 Cr# 极高（=Cr/[Cr + Al]；高达 0.86）。当归一化为 CI 球粒陨石时，这些岩石的单斜辉石显示出轻稀土元素富集模式。结果表明，抹谷超镁铁质岩石的全岩和矿物成分与缅甸蛇绿岩的地幔橄榄岩（例如 Kalaymyo 和 Myitkyina 蛇绿岩）的成分不同，但与典型的阿拉斯加型超镁铁质堆积体相似。因此，抹谷超镁铁质岩石是含水弧熔体高压结晶产生的堆积物，可能是在特提斯洋板俯冲过程中形成的。我们认为，尽管需要进一步限制这些岩石的年龄，但这些弧熔体的就位可能为早期岩浆岩和区域高温变质作用提供了额外的热量。这与碰撞后延伸过程中的岩浆作用和变质作用可能共同促成了世界级彩色宝石矿化的产生。
The Mogok metamorphic belt (MMB) in central Myanmar is well known for its complex tectonics, magmatism, and metamorphism in the framework of Tethyan subduction and India–Asia collision. It is also world renowned due to the gemstone‐class rubies and sapphires. Identification and discrimination of those petrological units in this region therefore are important for understanding the gemstone mineralization. Ultramafic massifs also crop out together with the gemstone‐bearing metamorphic rocks in the MMB. It is still poorly constrained about their origin (i.e., mantle peridotites of Jurassic–Cretaceous ophiolites or cumulate rocks) and their relationship with the gemstone generation. Here, we report petrological and geochemical data for the Mogok ultramafic rocks. Petrographic observations show that they have typical cumulate textures, with a crystallization order of olivine/spinel‐orthopyroxene‐clinopyroxene. They have low whole‐rock Al2O3and CaO, and extremely high spinel Cr# (=Cr/[Cr + Al]; up to 0.86). Clinopyroxenes of these rocks show light rare earth element enriched patterns when normalized to CI chondrite. The results suggest that the Mogok ultramafic rocks have different compositions, both whole‐rock and mineral, from mantle peridotites of Myanmar ophiolites, such as the Kalaymyo and Myitkyina ophiolites, but resemble typical Alaskan‐type ultramafic cumulates. Therefore, the Mogok ultramafic rocks are cumulates generated by high‐pressure crystallization of hydrous arc melts, probably during the subduction of the Tethyan oceanic slab. We argue that the emplacement of these arc melts may have provided additional heat for the earlier magmatic rocks and regional high‐temperature metamorphism, although there is a need to further constrain the ages of these rocks. This, along with magmatism and metamorphism during post‐collisional extension, probably collectively contributed to the generation of world‐class coloured gemstone mineralization.