Greenschist-amphibolite transition equilibria at low pressures.

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 An_12-50 (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 (An_0-7) through two Na-plagioclases to andesine (An_12-50), epidote grades Ps_35-17 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 P_fluid. 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 P_total and possible lower fO₂ compared to natural metamorphic conditions. -R.A.H.