Equilibrium and disequilibrium processes across the greenschist-amphibolite transition zone in metabasites

Documentation of textures, modes and compositions of minerals in metamorphosed basalts across two greenschist-to-lower amphibolite facies sequences (Flin Flon, Manitoba and Rossland, British Columbia) was combined with a compilation of the literature data to assess equilibrium and disequilibrium processes across this important transition zone. At Flin Flon and Rossland, the greenschist-amphibolite facies transition occurs over a narrow spatial interval marked by hornblende-in, oligoclase-in and actinolite-out isograds. The data suggest the existence of stable miscibility gaps separating coexisting actinolite and hornblende, and coexisting albite and oligoclase, in the lowermost amphibolite facies. However, actinolite and hornblende show a divergence in compositions going upgrade across the oligoclase-in isograd, suggestive of disequilibrium between the new, progressively more aluminous hornblende and metastably persisting actinolite. Likewise, coexisting albite and oligoclase compositions show no evidence for converging compositions at higher temperatures, suggesting they do not remain in equilibrium across a miscibility gap. Compositional gaps within epidote phases are attributed to disequilibrium preservation of lower grade epidote compositions to higher grade conditions, rather than the existence of a miscibility gap at greenschist and amphibolite facies conditions. Recognition of equilibrium and disequilibrium relations highlights the difficulty of using natural compositional datasets to extract information on miscibility gaps and more generally to extract a-X relationships. The greenschist-to-amphibolite facies transition is controlled principally by the consumption of chlorite, which primarily occurs upon crossing the oligoclase-in isograd, concomitant with conversion of actinolite to hornblende and albite to oligoclase. The result is a focused pulse of devolatilization over a small spatial and thermal interval.