Greenschist-Amphibolite Transition Equilibria at Low Pressures

In low P/T progressive metamorphism of basaltic rocks in the Yap Islands, western Pacific, and in Kasuga, Japan, quartz-bearing mineral assemblages change from albite + actinolite + epidote + chlorite (greenschist) to plagioclase + hornblende (amphibolite). Characteristic transitional assemblages (1) albite + oligoclase + actinolite + hornblende + epidote + chlorite, (2) albite + oligoclase + hornblende + epidote + chlorite, and (3) plagioclase + hornblende + epidote + chlorite occur in between those of greenschist and amphibolite. Accompanying this paragenesis, all solid solution minerals vary systematically in composition. With increasing metamorphic grade, plagioclase changes from albite (An 00–07) through two Na-plagioclases to andesine (An 12–50), epidote grades from Ps 35 to Ps 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 the progressive sequence indicates that the so-called immiscibility between actinolite and hornblende may not be a solvus but a transition loop. Polymorphic transformation of actinolite was estimated to be at about 420 °C, 2 kb and at about 450 °C, 3 kb P_fluid. Paragenetic relations of greenschist–amphibolite minerals including plagioclase(s), epidote, chlorite, Ca-amphibole(s) and quartz in the model system Na₂O–Ca0–Mg0–Al₂0₃–SiO₂–H₂O are delineated. The results subdivide the transition zone into a high-pressure albite–epidote amphibolite facies and a lower-pressure actinolite–calcic plagioclase hornfels facies (Miyashiro, 1973). In natural metabasites, Fe³⁺/Al and Fe²⁺/Mg ratios are significant in determining the compositions and parageneses of minerals. The invariant assemblage of the above model system may appear over a relatively wide P–T range for natural parageneses, and compositions of epidote and chlorite of the invariant assemblage can be used to monitor the P–T conditions for metamorphism in the low-pressure facies series. P–T relations of the greenschist–amphibolite transition deduced from natural parageneses are compared with those from experimental studies. They are quite compatible in terms of variations in mineral assemblages and mineral compositions as well as in P–T boundaries for the transition. However, experimental data yield much higher temperatures and a wider temperature range for the transition zone due to the equality of $PH2O$ and P_total and possible lower $fO2$ compared to natural metamorphic conditions.