Phase relations of pyroxene and amphibole in greenstone,blueschist and eclogite of the Franciscan Complex,California |
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| Authors: | E H Brown J Y Bradshaw |
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| Institution: | (1) Department of Geology, Western Washington University, 98225 Bellingham, Washington, USA;(2) Present address: Department of Geology, University of Otago, Dunedin, New Zealand |
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| Abstract: | The phase relations of pyroxenes, amphiboles and associated minerals in metamorphic rocks of the Franciscan Complex can be graphically depicted on a ternary diagram which has at its apices the metamorphic clinopyroxene end members, viz NaAl-NaFe3+-Ca(Fe2+, Mg). Phases are plotted by projection from a constant subassemblage of minerals. This analysis allows interpretation of the effects of pressure, temperature, bulk rock composition and fluid composition on stability of minerals within the Franciscan.Pyroxenes in meta-igneous rocks and metagraywackes have a limited compositional range and fall into two groups: the omphacites, with 50±5% diopside +hedenbergite component; and the jadeitic pyroxenes with 10±5% diopside+hedenbergite. Pyroxenes intermediate between these two groups are unstable relative to assemblages containing Na-amphibole+other minerals.Coexisting pyroxenes and amphiboles in eclogites and associated coarse blueschists comprise equilibrium assemblages, and the proportion of pyroxene to amphibole is a function of rock composition. Eclogites are stable at higher temperature than regionally developed fine-grained greenstones and blueschists in the Franciscan, and at higher pressure than amphibolites. X
H2O
fluid is not an important factor in the stability of Franciscan eclogite relative to amphibolite. |
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