Greenschist-amphibolite transition equilibria at low pressures

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Authors:Maruyama, Shigenori; Suzuki, Kazuhiro; Liou, J. G.
Author Affiliations:Primary:
Stanford Univ., Dep. Geol., Stanford, CA 94305, United States
Nagoya Univ., Dep. Earth Sci., Nagoya, Japan
Volume Title:Journal of Petrology
Source:Journal of Petrology, 24(4), p.583-604. Publisher: Clarendon Press, Oxford, United Kingdom. ISSN: 0022-3530
Publication Date:1983
Note:In English. 2 p. refs.; illus. incl. 18 anal.
Summary:In low P-T progressive metamorphism of basaltic rocks in the Yap Islands, W Pacific, and in Kasuga, Japan, quartz-bearing assemblages change from albite + actinolite + epidote + chlorite (greenschist) to plagioclase + hornblende (amphibolite). This transition can be divided into three subzones characterized, respectively, by two Ca-amphiboles + two sodic plagioclases (B1), one Ca-amphibole + two sodic plagioclases (B2), and hornblende + plagioclase An12-50 (B3) together with epidote, chlorite and quartz. The B1-B2 boundary is sensitive to host-rock Fe/Mg ratio; hornblende and oligoclase appear simultaneously at the onset of zone B1 corresponding with the greenschist-epidote amphibolite facies boundary. With increasing metamorphic grade, plagioclase changes from albite (An0-7) through two Na-plagioclases to andesine (An12-50), epidote grades Ps35-17 and Ca-amphibole passes from actinolite through actinolite-hornblende pairs to hornblende; concurrently, the Al content of chlorite increases. The compositional range shown by amphiboles through this progressive sequence indicates that the so-called immiscibility between actinolite and hornblende may be a transition loop rather than a solvus; polymorphic transformation of actinolite is estimated to be at 420°C, 2 kbar and 450°C, 3 kbar Pfluid. The results subdivide the transition zone into a high-P albite-epidote amphibolite facies and a lower-P actinolite-calcic plagioclase hornfels facies. Experimental data yield much higher T and a wider T range for the transition zone due to the equality of PH2O and Ptotal and possible lower fO2 compared to natural metamorphic conditions. [R.A.H.]
Subjects:Amphibolite facies; Basaltic composition; Facies; Greenschist facies; Low pressure; Low temperature; Metamorphic rocks; Metamorphism; Metavolcanic rocks; Mineral assemblages; Petrology; Phase equilibria; Pressure; Prograde metamorphism; Temperature; Asia; Caroline Islands; Far East; Honshu; Japan; Micronesia; Oceania; Kasuga; Yap
Abstract Numbers:84M/2271
Record ID:1986006246
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom, Reference includes data from PASCAL, Institute de l'Information Scientifique et Technique
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