The stability of Ca-Na pyroxene in low-grade metabasites of high-pressure intermediate facies series
The stability of Ca-Na pyroxene in low-grade metabasites of high-pressure intermediate facies series
复制标题
高压中间相系列低品位变碱岩中钙钠辉石的稳定性
作者：
S. Maruyama;J. Liou
次生变质钙钠辉石以微晶形式产出于日本四国三百川带变基性岩中，夹在原生辉石周围的细小晶体和变基性岩石中，介于前驱石-镁铝榴石(PP)和镁铝榴石-阳起石(PA)之间。它们的组成随矿物共生而系统地变化，从辉石到埃米特，翡翠成分恒定，约为20molVo。含钙钠辉石组合中的钙钠辉石成分(AcaAugsJd2s)很低，在含碱性角闪石中经Ac25Aug55Jd2s增加，在含赤铁矿组合中经Ac66AugzsJdrs增加，最终在含磁铁矿组合中获得霓石成分(Ac65Aug1sJd25)。它们与残留的火成辉石在化学上有很低的OPX和二氧化钛含量，在形态上有纤维状结晶习性。含辉石变质基性岩含有绿泥石+辉石+绿泥石(+绿帘石、阳起石、碱性角闪石、辉石、赤铁矿、磁铁矿)组合，与品位较高、发育广泛的绿帘石+绿泥石(XR“：0.50)+阳起石+磁铁矿不相容。提出了这种转变的反应方式为：辉石+绿泥石：镁铝榴石*阳起石+水。从新喀里多尼亚、新西兰和阿巴拉契亚低品位变质基性岩中的共生矿物对比，分析了辉石+绿泥石+纤锌矿的稳定性关系。该组合可能具有一个楔形的P-T空间，通过上述反应与高钙镁铝榴石-阳起石相结合，并通过前驱*阳起石=普通辉石+绿泥石的反应与低T前驱阳起石相结合，可能具有负斜率。根据桑巴加瓦带等地变质基性岩中的矿物共生矿物，认为钙钠辉石赋存于前驱体-钠镁闪石-阳起岩相之间的有限的P-T空间内。
Minor metamorphic Ca-Na pyroxene occurs as minute crystals and as thin rims around primary augites in metabasites intermediate between the prehnite-pumpellyite (PP) and pumpellyite-actinolite (PA) facies in the Sanbagawa belt, Shikoku, Japan. Their compositions vary systematically with mineral paragenesis, from augite to acmite, with a constant jadeite component of about 20 molVo. The Ca-Na pyroxenes in the pumpellyite-bearing assemblages contain a very low acmite component (AcaAugssJd2s), which increases through Ac25Aug55Jd2s in the alkali amphibole-bearing and Ac66AugzsJdrs in the hematite' bearing assemblages, finally attaining an aegirine composition (Ac65Aug1sJd25) in the magnetite-bearing assemblages. They are distinguished from relict igneous augites chemically by very low OPX and TiO2 contents, and morphologically by fibrous crystal habit. The pyroxene-bearing metabasites contain assemblages of chlorite + pyroxene + pumpellyite (+ epidote, actinolite, alkali amphibole, stilpnomelane, hematite and magnetite), which are incompatible with the higher grade and widely developed pumpellyiteactinolite facies assemblage, epidote + chlorite (Xr" : 0.50) + actinolite + pumpellyite. The reaction: augite + chlorite : pumpellyite * actinolite + H2O has been suggested for such a transition. The stability relations of augite + chlorite + pumpellyite were analyzed from comparative mineral parageneses in low-grade metabasites from New Caledonia, New Zealand, and the Appalachians. The assemblage may have a wedge-shaped P-T space bounded against the high-Tpumpellyite-actinolite facies by the above reaction and against the low-T prehnite-pumpellyite facies by the reaction prehnite * actinolite = augite + chlorite with probable negative slope. From the mineral parageneses in the metabasites of the Sanbagawa belt and elsewhere, it is concluded that the Ca-Na pyroxenes occur within a restricted P-T space between the prehnite-pumpellyite and pumpellyite-actinolite facies.