Calculated stabilities of sodic phases in the Sambagawa metapelites and their implications
Calculated stabilities of sodic phases in the Sambagawa metapelites and their implications
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Sambakawa 变泥岩中钠相的计算稳定性及其意义
DOI：
10.1111/j.1525-1314.2010.00919.x
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发表时间：
2011
影响因子：
3.4
通讯作者：
Enami M.
中科院分区：
文献类型：
--
作者：
Kouketsu;Y.;Enami M.
利用Perple_X 07对Sambagawa带的变岩进行了压力（P）-温度（T）假截面分析，以确定高压（0.5-2.5 GPa/400-600 °C）条件下MnO-Na 2 O-K2 O-CaO-FeO-MgO-Al 2 O3-SiO2-H2O-（CO2）模型系统中的矿物平衡和钠相的稳定性。 还计算了500 °C下的压力-XNA [=Na/（Na+Al-2K-1.5Ca）]假截面，以检查块体岩石组成的Na/Al值对含钠矿物稳定性的影响。Sambagawa变岩的块体岩石成分在XNA值上是可变的。蓝片岩相和榴辉岩相条件下变岩稳定组合的计算结果表明：钠长岩和蓝闪石在寄主岩石的整个岩石成分范围内都是稳定的;钠质辉石的稳定P-T条件随XNA值的增大而增大;变质流体中CO2的存在使钠长岩的稳定场增大。变岩中钠云母的广泛稳定性以及钠质辉石的稳定性与大块岩石成分之间的关系解释了以下原因：（i）几个俯冲相关岩石中的变岩中绿辉石的出现是罕见的;以及（ii）Sambagawa带Besshi地区的变岩中的石榴石中通常存在钠云母作为包裹体。钠长岩是高压变质岩的重要钠质相，石榴子石中钠长岩与高余压石英的组合可能是验证高压变质岩中榴辉岩相变质作用的有用标志。
Pressure (P)–temperature (T) pseudosection analyses were carried out on metapelites from Sambagawa belt by using Perple_X 07 so as to determine mineral equilibria and the stability of sodic phases, in the model system MnO–Na2O–K2O–CaO–FeO–MgO–Al2O3–SiO2–H2O–(CO2) under high‐pressure (HP) conditions (0.5–2.5 GPa/400–600 °C). A pressure–XNA[=Na/(Na+Al–2K–1.5Ca)] pseudosection at 500 °C is also calculated to examine the effect of Na/Al value of the bulk‐rock composition on the stabilities of sodic minerals. The bulk‐rock compositions of Sambagawa metapelite are variable inXNAvalues. The calculation results of stable assemblages in metapelites under the blueschist and eclogite facies conditions indicate that: (i) paragonite and glaucophane are stable throughout the wideXNArange of bulk‐rock compositions of host rocks; (ii) stableP–Tconditions of sodic pyroxene enlarge with increasingXNAvalue; and (iii) the stability field of paragonite enlarges with the presence of CO2in the metamorphic fluid. The suggested wide stability of paragonite in metapelites and the relationships between the stability of sodic pyroxene and the bulk‐rock compositions explain the reasons why (i) the occurrence of omphacite in metapelites from several subduction‐related terranes is rare; and (ii) paragonite commonly occurs as inclusions in garnet of metapelites from the Besshi region of the Sambagawa belt. Paragonite is an important sodic phase of HP metapelites, and a combination of paragonite and quartz with high residual pressure included in garnet may be a useful indicator to verify the evidence for the eclogite facies metamorphism recorded in metapelites.