THE ORIGIN OF ACTINOLITE-HORNBLENDE INTERGROWTHS IN THE GREENSCHIST-AMPHIBOLITE TRANSITION ZONE: AN EXAMPLE FROM THE FLIN FLON – GLENNIE COMPLEX, MANITOBA/SASKATCHEWAN

Greenschist-amphibolite facies transition zone mineral assemblages are developed within metamorphosed basalts and basaltic andesites of the Flin Flon-Glennie complex in Manitoba and Saskatchewan, and comprise one of the best exposed sequences of its kind in the world. The area around Flin Flon contains several mineral isograds, the most important of which are the hornblende-in, oligoclase-in and actinolite-out isograds.

Of particular interest are the compositions and textures of the coexisting calcic amphiboles, actinolite and hornblende, due to uncertainty over the postulated existence of a miscibility gap within the actinolite-hornblende series. Detailed compositional analysis of coexisting amphiboles indicates a divergence in the compositions of actinolite and hornblende/ferro-tschermakite with increasing grade, characterised by hornblende/ferro-tschermakite increasing in tschermakite and edenite contents, whilst actinolite remains approximately fixed in composition. The progression in textures and compositions of actinolite-hornblende pairs is best explained by disequilibrium during progressive metamorphism, involving the metastable persistence of actinolite, associated with hornblende that evolves continuously towards higher temperature compositions. This interpretation is also supported by a complex array of textures between the coexisting amphiboles, including ubiquitous patchy intergrowths and core-rim structures. We suggest that: (1) previous studies using textural interpretations of coexisting amphiboles as evidence for equilibrium conditions may require re-evaluation and; (2) if the disequilibrium relationships observed in Flin Flon are typical rather than exceptional, the use of natural amphibole data to define an actinolite-hornblende solvus may be erroneous.