Magmatic-hydrothermal processes of the Laojunshan metamorphic massif in Southeastern Asia: Evidence from chemical and B-isotopic variations of deformed tourmalines
Magmatic-hydrothermal processes of the Laojunshan metamorphic massif in Southeastern Asia: Evidence from chemical and B-isotopic variations of deformed tourmalines
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DOI：
10.1016/j.lithos.2022.106609
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发表时间：
2022-01
期刊：
影响因子：
3.5
通讯作者：
Wei Li;S. Cao;Eizo Nakamura;T. Ota;Zhongyuan Liu;Yanlong Dong;T. Kunihiro
中科院分区：
文献类型：
--
作者：
Wei Li;S. Cao;Eizo Nakamura;T. Ota;Zhongyuan Liu;Yanlong Dong;T. Kunihiro
在东南亚老君山变质岩体中，多期流体流入导致了具有重要体积意义的与花岗岩相关的电气石赋生，该变质岩体表现出多变的变形构造和分解结构。其中包括浸染状碧玺斑岩碎屑（ⅰ型）、横切碧玺-石英脉（ⅱ型）和碧玺细脉（ⅲ型）。三种类型电气石的化学变化反映了从早期富硼熔体到晚期热液流体的来源转变，并伴有不同的流体-岩石相互作用。与岩浆型电气石相比，热液型电气石具有较高的Mg/(Mg + Fe)比、较低的Na/(Na + Ca)比、较明显的Eu正异常、较高的Sr和重稀土元素（HREE）浓度、较低的Li、Nb、Zr、Hf和轻稀土元素（LREE）浓度等特征。型电气石δ11B值为−13 ~−7.9‰，型电气石δ11B值为−15.5 ~−7.5‰，型电气石δ11B值为−18.6 ~−11.6‰。这种变化主要是由于多重流体析出、瑞利分馏作用和两种同位素差异源的混合作用所致。两个含碧玺片麻岩的岩浆锆石单键pb定年结果显示岩浆侵位和结晶年龄在445 ~ 420 Ma之间。志留系过铝质花岗岩释放的岩浆热液是浸染型电气石的主要成因。在挖掘过程中进一步进行水力压裂，形成了电气石-石英脉（ii型）和电气石脉（iii型），可能来自白垩纪花岗岩岩浆流体，与变质岩相互作用。变质岩体在挖掘过程中，由于碎裂作用和超压流体的作用，使寄主岩石沿裂缝逐渐变弱，最终导致变形。电气石的化学和同位素组成表明，可能通过流体沸腾和流体-岩石相互作用产生的氧化条件可能促进了区域的锡成矿作用。
Multi-stage fluid influx has led to the volumetrically important occurrence of granite-related tourmalines at the Laojunshan metamorphic massif in Southeast Asia, which exhibits variable deformed structures and decomposition textures. These include disseminated tourmaline porphyroclasts (type-I), crosscutting tourmaline-quartz veins (type-II) and tourmaline veinlets (type-III). The chemical variations of these three types of tourmalines reveal a source transition from early boron-rich melt to late hydrothermal fluid, accompanying with variable fluid-rock interaction. Hydrothermal tourmalines are characterized by higher Mg/(Mg + Fe) ratios, lower Na/(Na + Ca) ratios, more pronounced positive Eu anomalies, higher Sr and heavy rare earth element (HREE) concentrations, and lower Li, Nb, Zr, Hf, and light rare earth element (LREE) concentrations compared to magmatic tourmalines. The tourmalines exhibit δ11B values ranging from −13 to −7.9‰ for type-I, from −15.5 to −7.5‰ for type-II, and from −18.6 to −11.6‰ for type-III. This variation is mainly due to multiple fluid exsolution, Rayleigh fractionation and the mixing of two isotopically distinct sources. Usingle bondPb dating results of magmatic zircons from two tourmaline-bearing gneisses reveal the ages of magma emplacement and crystallization from 445 to 420 Ma. Magmatic-hydrothermal fluids released from Silurian peraluminous granites are responsible for the formation of the disseminated type-I tourmalines. Further hydraulic fracturing during exhumation led to the formation of the tourmaline-quartz veins (type-II) and the tourmaline veinlets (type-III), possibly from Cretaceous granitic magma-derived fluids that interacted with metamorphic rocks. The decreasing grain size resulted from decomposition and cataclasis coupled with the overpressured fluids weakening the host rocks along fractures and eventually promoting the deformation during exhumation of the metamorphic massif. The chemical and isotopic compositions of tourmaline suggest that the oxidized conditions possibly through fluid boiling and fluid-rock interaction may promote the regional Sn-mineralization.