Syn-metamorphic B-bearing fluid infiltrations deduced from tourmaline in the Main Central Thrust zone, Eastern Nepal Himalayas
Syn-metamorphic B-bearing fluid infiltrations deduced from tourmaline in the Main Central Thrust zone, Eastern Nepal Himalayas
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DOI：
10.1016/j.lithos.2019.105175
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发表时间：
2019-12
期刊：
影响因子：
3.5
通讯作者：
Tetsuo Kawakami;H. Sakai;Katsushi Sato
中科院分区：
文献类型：
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作者：
Tetsuo Kawakami;H. Sakai;Katsushi Sato
详细描述了低喜马拉雅层序(LHS)主中央冲断带泥质片岩中电气石的赋存方式和化学成分，旨在推断含硼流体渗入形成的电气石的化学特征，并估算同变质流体的成分。电气岩围岩交代电气石的xCa[=Ca/(Ca + Na)]在xMg[=Mg/(Mg + Fe~(2+))]几乎不变的情况下，从岩芯或地幔到边缘有显著的增加。从黑云母带到蓝晶石带富含电气石(>；1.0 Vol%)泥质片岩中的电气石也表现出XCaat几乎恒定的XMG显著增加，并被解释为含B流体渗透的产物。与变云岩夹层的泥质片岩中形成了丰富的珍珠岩和斜长石，表明富B流体在与变云母层相互作用时变富钙，并交代将钙引入泥质片岩中。这种富含B和Ca的流体渗入泥质片岩可能导致了丰富的电气石的产生，其成分趋势是增加XCaat几乎不变的XMG。富含电气石的泥质片岩中的大部分电气石与斜长石处于平衡状态，表明流体成分受到泥质片岩的缓冲。含-lt；1 体积%电气石的泥质片岩中的电气石成分主要表现为Xa增加，Xmg值范围较大，反映了其在进退变质过程中的生长。通过应用实验确定的流体/电气石化学关系来评估与电气石共存的流体的组成，方法是将实验确定的流体/电气石化学关系应用于与电气石有关的矿脉、含1 体积%电气石的泥质片岩和富含电气石的泥质片岩的组成。假定共存阴离子为氯，估算出这些岩石类型的盐度为∼0.44-0.59molCl2/L +CaCl2，与现今海水相似或略低。与电气石岩化和富电气石泥质片岩有关的矿脉在LHS的MCT带中零星分布，表明MCT带广泛存在同变质、含B的咸水流体渗入，流体通道被局部化和导流。我们的观察支持这样一种假设，即流体渗入高喜马拉雅结晶(HHC)导致HHC的蒸汽饱和部分熔融，从而使电气石淡色花岗岩熔体与MCT带的反转变质作用同时发生。
Mode of occurrence and chemical composition of tourmaline in pelitic schists from the Main Central Thrust (MCT) zone of the Lesser Himalayan Sequences (LHS) are described in detail with the aim of deducing the chemical characteristics of tourmaline formed through B-bearing fluid infiltration and of estimating the composition of thesyn-metamorphic fluids. Metasomatic tourmalines from the tourmalinized wall rocks show significant increases in XCa[=Ca/(Ca + Na)] at almost constant XMg[=Mg/(Mg + Fe2+)] from the cores or mantles to the rims. Tourmaline in tourmaline-rich (> 1.0 vol%) pelitic schists from the biotite zone to the kyanite zone also show marked increase in XCaat almost constant XMg,and are interpreted as a product of B-bearing fluid infiltration. Abundant margarite and anorthite formed in the pelitic schists intercalated with the metadolostone layers suggesting that the B-rich fluid became Ca enriched as it interacted with metadolostone layers, and metasomatically introduced Ca into the pelitic schists. Infiltration of such B- and Ca-rich fluids into pelitic schists likely resulted in production of abundant tourmaline with the compositional trend of increasing XCaat almost constant XMg. Most of the tourmaline in tourmaline-rich pelitic schists are in equilibrium with plagioclase, suggesting that the fluid composition was buffered by the pelitic schists. Composition of tourmaline in the pelitic schists with <1 vol% tourmaline mostly show increase in XCaand wider range of XMgvalues, reflecting its growth during prograde metamorphism. The composition of fluids that coexisted with tourmaline is estimated by applying experimentally-determined fluid/tourmaline chemical relationships to the composition of tourmaline from veins associated with tourmalinization, pelitic schists with <1 vol% tourmaline and tourmaline-rich pelitic schists. Assuming coexisting anion to be Cl, the salinity estimated for these rock types was ∼0.44–0.59 mol/l NaCl + CaCl2that is similar to or slightly lower than the present-day seawater. Veins associated with tourmalinization and tourmaline-rich pelitic schists are sporadically distributed in the MCT zone of the LHS, suggesting that thesyn-metamorphic, B-bearing saline fluid infiltrations took place widely in the MCT zone, and the fluid pathways were localized and channeled. Our observation supports the scenario whereby infiltration of fluid into the High Himalayan Crystallines (HHC) caused vapor-saturated partial melting of the HHC to give tourmaline leucogranite melts contemporaneous with inverted metamorphism in the MCT zone.