Metamorphic devolatilization of basalts across the greenschist-amphibolite facies transition zone: insights from isograd mapping, petrography and thermodynamic modelling

Abstract

ABSTRACT The greenschist-amphibolite facies transition zone within metabasites is of great importance due to the common occurrence of this transition within greenstone belts, and the proposition that fluids released across the transition may be involved in orogenic gold deposit formation. In this work, the nature of devolatilization reactions occurring across the greenschist-amphibolite facies transition zone is assessed from petrological examination and thermodynamic modelling of an exceptionally exposed part of the Flin Flon Greenstone Belt (Manitoba/Saskatchewan). The sequence at Flin Flon comprises an intact metamorphic sequence of ~10 km length spanning the greenschist and lower amphibolite facies, the latter of which can be subdivided into three zones (from S to N): the hornblende-actinolite zone, the hornblende-actinolite-oligoclase zone, and the hornblende-oligoclase zone, demarcated by the hornblende-in, oligoclase-in and actinolite-out isograds. The crossing of the hornblende-in isograd is associated with small amounts of hornblende growth, relatively little change in the proportions of chlorite, and negligible fluid loss. By contrast, significant changes in the modal mineralogy occur across the 1500 m wide hornblende-actinolite-oligoclase zone, with the breakdown of approximately 75% of the chlorite and the loss of approximately 1-2 wt% H2O. There is no textural or modal evidence for significant loss or gain of carbonate or sulphide minerals going through this interval. Petrological estimates of devolatilization across the greenschist-amphibolite facies transition zone are compared with predictions from thermodynamic modelling. Although there are differences between the two, both indicate that the fluid loss is strongly tied to variation in bulk composition, the latter predominantly the result of pre-metamorphic alteration processes. High-Mg and high-Ca basalts, representative of pillow rim and core material, undergo an average of 1.8 wt% (modelling estimate: 2.2 wt%) and 1.1 wt% H2O loss (modelling estimate: 1.8 wt%), respectively, across the hornblende-actinolite-oligoclase zone. T-XCO2 modelling predicts that the XCO2 content of fluid produced from the hornblende- and oligoclase-producing reactions is low ( 0.2) above the greenschist-amphibolite facies transition zone within samples containing high carbonate contents (>5%). However, in contrast to the modelling, the majority of chlorite (>75%) breaks down at the main greenschist-amphibolite facies transition zone in the Flin Flon sequence, and most samples have low carbonate contents (1.5% average), limiting the volumes of fluids with higher XCO2 compositions that can be generated at higher grades. Thus, whilst the greenschist-amphibolite facies transition zone at Flin Flon is the site of significant devolatilization (1-2 wt%) over a small interval of P-T space, is does not appear to have been accompanied by significant carbonate and sulphide breakdown, and the fluids generated across this interval were CO2-poor. If the Flin Flon sequence is representative of other metamorphosed greenstone belts, it may be that metabasites metamorphosed across the greenschist-amphibolite facies transition zone do not, in general, release the CO2– and gold hydrosulphide-bearing fluids characteristic of orogenic gold deposits.