Textural and Compositional Relationships of Ca-Amphiboles in Metabasites of the Cape Smith Belt, Northern Qubec: Implications for a Miscibility Gap at Medium Pressure     

BGIN  NORMAND J.; CARMICHAEL  DUGALD M. 《Journal of Petrology》1992,33(6):1317-1343

Textural and chemical relationships in the Ca-amphibole serieshave been examined in progressively metamorphosed mafic schistfrom the Povungnituk Group, northern Qubec. In the hornblendezone, coexisting actinolite and hornblende are characterizedby sharp optical and chemical boundaries. They also displayan epitaxial intergrowth texture, consistent with contemporaneouscrystallization and the presence of a miscibility gap (solvus)between actinolite and hornblende. At higher grade, in the oligoclasezone, actinolite is always rimmed by hornblende. Across theoligoclase isograd, the compositional break between actinoliteand hornblende remains relatively constant in width (6–10wt.% Al₂O₃). Accordingly, the concentric texture and the lackof chemical gradients are interpreted as a local equilibriumat the scale of the interfaces between the two amphiboles. Throughoutthe hornblende zone and the lower part of the oligoclase zone,both amphiboles are characterized by a progressive Mg-enrichment,with no consistent change in edenite and Tschermak components.The hornblende zone can be characterized by a continuous reactionwith Mg-enrichment of reactants (biotite and chlorite) and products(actinolite and hornblende). However, this reaction requiresa mixed-volatile phase with a much higher X_CO2 ({small tilde}0.5) than those calculated from equilibrium curves in the NCMASH—CO₂system. The oligoclase isograd can be represented by a discontinuousoligoclase—hornblende-forming reaction which coincideswith the change from epitaxial to concentric texture at theoligoclase isograd. At the upper limit of the oligoclase zone,gradational boundaries and a continuous solid solution betweenhornblende and actinolite document a disequilibrium texture.Although a miscibility gap can be documented in the hornblendezone, its closure at higher grade is not observed, based onequilibrium crystallization of actinolite and hornblende. Theresults of this study suggest that the crest of the solvus mustbe crossed between the oligoclase and actinolite-out isograds,based on zoned profiles between hornblende rims and relic actinolite. "The term exsolution refers to the process whereby an initiallyhomogeneous solid solution separates into two (or possibly more)distinct crystalline minerals without the addition or removalof material to or from the system. Exsolution generally, althoughnot necessarily, occurs on cooling. The miscibility gap representsa temperature—composition field in which solid solutionbetween the end members decreases gradually from higher to lowertemperatures."  相似文献