Greenschist-amphibolite transition equilibria at low pressures.

S. Maruyama, K. Suzuki, J. G. Liou

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Abstract

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 420oC, 2 kbar and 450oC, 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 (M.A. 15-483). 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.

Original languageEnglish
Pages (from-to)583-604
Number of pages22
JournalJournal of Petrology
Volume24
Issue number4
Publication statusPublished - 1983
Externally publishedYes

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ASJC Scopus subject areas

  • Geochemistry and Petrology
  • Geophysics

Cite this

Maruyama, S., Suzuki, K., & Liou, J. G. (1983). Greenschist-amphibolite transition equilibria at low pressures. Journal of Petrology, 24(4), 583-604.