Petrology of Franciscan Metabasites Along the Jadeite-Glaucophane Type Facies Series, Cazadero, California
Petrology of Franciscan Metabasites Along the Jadeite-Glaucophane Type Facies Series, Cazadero, California
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沿翡翠-绿闪石型相系列的方济会变基岩的岩石学，卡萨德罗，加利福尼亚州
DOI：
10.1093/petrology/29.1.1
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发表时间：
1988
影响因子：
3.9
通讯作者：
J. Liou
中科院分区：
文献类型：
--
作者：
S. Maruyama;J. Liou
卡萨德罗蓝片岩异地岩位于加利福尼亚州北部海岸山脉方济各会组合的中央混杂岩带内。对来自 Ward Creek 的 200 多块蓝片岩的矿物成分和组合进行了调查。结果描绘了硬钠石、泵贝利石和绿帘石带。硬钠长岩和泵贝利岩带相当于 Coleman & Lee (1963) 的 II 型变碱岩，其特征是保存完好的火成岩结构、残辉辉石和枕状结构，而绿帘石带岩石相当于 III 型强变形和片状变碱岩。绿泥石、多硅白云母、文石、榍石以及少量石英和钠长石普遍存在。一些岩石具有两种不同的单斜辉石，它们之间由成分间隙隔开。最低品位变碱岩的单斜辉石含有非常低的Xjd。在泵贝氏石带变碱岩中，最常见的组合包含 Pm + Cpx + Gl + Chl，一些具有较高 Al2O3 和/或 Fe2O3 的样品具有 Pm + Lw + Cpx + Chl，阳起石在上泵贝氏石带中加入上述组合，其中阳起石-蓝绿闪石组成间隙明确。绿帘石带变碱岩的特征是组合 Ep + Cpx + 两个角闪石 + Chl、Lw + Pm + Act + Chl 和 Ep + Pm + 两个角闪石 + Chl，具体取决于岩石的 Fe2O3 含量。在上部绿帘石带中，出现了白云石，无铁硬钠石稳定，绿辉石消失，绿辉石含有极低的Ac成分。因此，常见的组合是 Ep + winchite + Lw 和 Lw + Omp + winchite。随着变质程度的进一步增加，绿帘石变得富铝，而绿柱石不再稳定。因此，Ep + winchite + 绿辉石±石榴石是特征。矿物组合和共生序列描绘了三个不连续反应：（1）泵辉石-in； (2)阳起石； (3)绿帘线反应。使用 Taylor & Coleman (1968) 估计的温度和 Ca-Na-辉石的相平衡，可以定位这些反应的 P−T 位置和变质梯度。所有三个变碱石区都出现在霰石稳定场内，并以 Ab = Jd + Qz 的最大压力曲线以及泵辉石和硬钠石的最大稳定性为界。硬钠石区在 200°C 以下、压力范围为 4–6·5 kb 时似乎是稳定的； 200~290°C之间为泵贝石带，290°C以上为绿帘石带，压力变化范围为6·5~9 kb。变质场梯度似乎具有朝向更高压力的凸性质。海底山底侵推测模型被用来解释这一特征。
The Cazadero blueschist allochthon lies within the Central Melange Belt of the Franciscan assemblage in the northern Coast Range of California. Mineral compositions and assemblages of more than 200 blueschists from Ward Creek were investigated. The results delineate lawsonite-, pumpellyite-, and epidote-zones. The lawsonite and pumpellyite zones are equivalent to the Type II metabasites of Coleman & Lee (1963) and are characterized by well-preserved igneous textures, relict augite, and pillow structures, whereas epidote zone rocks are equivalent to the Type III strongly deformed and schistose metabasites. Chlorite, phengite, aragonite, sphene, and minor quartz and albite are ubiquitous.The lawsonite zone metabasites contain lawsonite ( < 3 wt. per cent Fe2O3), riebeckite-crossite, chlorite, and Ca-Na-pyroxene; some rocks have two distinct clinopyroxenes separated by a compositional gap. The clinopyroxene of the lowest grade metabasites contains very lowXjd. In pumpellyite zone metabasites, the most common assemblages contain Pm + Cpx + Gl + Chl and some samples with higher Al2O3and/or Fe2O3have Pm + Lw + Cpx + Chl, Actinolite joins the above assemblage in the upper pumpellyite zone where the actinolite-glaucophane compositional gap is well defined. The epidote zone metabasites are characterized by the assemblages Ep + Cpx + two amphiboles + Chl, Lw + Pm + Act + Chl, and Ep + Pm + two amphiboles + Chl depending on the Fe2O3content of the rock. In the upper epidote zone, winchite appears, Fe-free lawsonite is stable, pumpellyite disappears and omphacite contains very low Ac component. Therefore, the common assemblages are Ep + winchite + Lw, and Lw + Omp + winchite. With further increase in metamorphic grade, epidote becomes Al-rich and lawsonite is no longer stable. Hence Ep + winchite + omphacite ± garnet is characteristic.Mineral assemblages and paragenetic sequences delineate three discontinuous reactions: (1) pumpellyite-in; (2) actinolite-in; and (3) epidote-in reactions. Using the temperatures estimated by Taylor & Coleman (1968) and phase equilibria for Ca-Na-pyroxenes, theP−Tpositions of these reactions and the metamorphic gradient are located. All three metabasite zones occur within the aragonite stability field and are bounded by the maximum pressure curve of Ab = Jd + Qz and the maximum stabilities of pumpellyite and lawsonite. The lawsonite zone appears to be stable atTbelow 200°C with a pressure range of 4–6·5 kb; the pumpellyite zone between 200 and 290°C and the epidote zone above 290°C with pressure variation between 6·5 and 9 kb. The metamorphic field gradient appears to have a convex nature towards higher pressure. A speculative model of underplating seamounts is used to explain such feature.