Talc‐phengite‐albite assemblage in piemontite‐quartz schist of the Sanbagawa metamorphic belt, central Shikoku, Japan
Talc‐phengite‐albite assemblage in piemontite‐quartz schist of the Sanbagawa metamorphic belt, central Shikoku, Japan
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日本四国中部三波川变质带皮蒙特岩-石英片岩中的滑石-多硅白云母-钠长石组合
DOI：
10.1046/j.1440-1738.2000.00268.x
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发表时间：
2000
期刊：
影响因子：
1.5
通讯作者：
D. Nakamura
中科院分区：
文献类型：
--
作者：
J. Izadyar;T. Hirajima;D. Nakamura
摘要在日本四国中部三百川带中高品位部分富MnO总(平均1.65wt%)的片岩中，新发现滑石(TLC)+多硅白云母(PHN)+钠长石(AB)组合。滑石与PHN、AB和绿泥石(Chl)直接接触，边界尖锐，表明这四种相共存。含薄层层析的黑云母-石英片岩的其他主要成分为辉铜矿、闪锌矿、赤铁矿(Ht)、斜长石/重晶石和白云石。金云母(Phl)是一种沿PHN边缘发育的晚期矿物，很少出现。所研究的黑云母-石英片岩具有较高的氧化态，Mg#(=mg/(Mg+Fe2+))~1.0。KNMASH系统中的Schreemaker分析和Sanbagawa带中的矿物组合提出了一个可能的成岩网格，其中TLC-PHN组合在P-T场中是稳定的，周围有以下反应：前人提出的较低压力限制为Chl+Phl+石英(Qtz)=PHn+TLC+H2O；较高压力限制为蓝闪石+Qtz=Tlc+Ab+H2O；较高温度限制为Tlc+Pphn+Ab=Phl+磷灰石+Qtz+H2O。然而，基于Holland&Powell(1998)数据库的热力学计算表明，这些反应定义的TLC-PHN稳定场在11.6-12.0(±0.7)kbar时被不切实际地限制在580-600°C左右。KNMA-Fe3+-SH体系的Schreemaker分析和观察到的矿物组合预测，Chl+Crosite=TLC+AB+Ht+H2O是Sanbagawa带中段较好的TLC形成反应，过量的AB+赤铁矿缩小了TLC-PHN组合的稳定范围。
Abstract The talc (Tlc) + phengite (Phn) + albite (Ab) assemblage is newly confirmed in MnOtotal‐rich (1.65 wt% in average) piemontite‐quartz schists from the intermediate‐ and high‐grade part of the Sanbagawa belt, central Shikoku, Japan. Talc is in direct contact with Phn, Ab and chlorite (Chl) with sharp boundaries, suggesting that these four phases mutually coexist. Other primary constituents of the Tlc‐bearing piemontite‐quartz schist are spessartine, braunite, hematite (Ht), crossite/barroisite and dolomite. Phlogopite (Phl) rarely occurs as a later stage mineral developing along the rim of Phn. The studied piemontite‐quartz schist has mg# (= Mg/(Mg + Fe2+)) ~ 1.0, because of its high oxidation state. Schreinemakers’ analysis in the KNMASH system and the mineral assemblage in the Sanbagawa belt propose a possible petrogenetic grid, in which the Tlc–Phn assemblage is stable in a P‐T field surrounded by the following reactions: lower‐pressure limit by Chl + Phl + quartz (Qtz) = Phn + Tlc + H2O as proposed by previous workers; higher‐pressure limit by glaucophane + Qtz = Tlc + Ab + H2O; and higher‐temperature limit by Tlc + Phn + Ab = Phl + paragonite + Qtz + H2O. Thermodynamic calculation based on the database of Holland & Powell (1998) , however, suggests that the Tlc–Phn stability field defined by these reactions is unrealistically limited around 580–600 °C at 11.6–12.0 (± 0.7) kbar. Schreinemakers’ analysis in the KNMA‐Fe3+‐SH system and the observed mineral assemblage predict that Chl + crossite = Tlc + Ab + Ht + H2O is a preferable Tlc‐forming reaction in the intermediate‐grade part of the Sanbagawa belt and that excess Ab + hematite narrows the stability field of the Tlc–Phn assemblage.