Tracking fluid/melt flow in lower crustal mafic gneiss using zircon isotopic zoning, kapuskasing uplift

Coordinated cathodoluminescence (CL) imaging and ion microprobe (SHRIMP and CAMECA 1280) analysis (U-Pb dating, oxygen isotope and trace element) of metamorphic zircon from a granulite-facies mafic orthogneiss in the Kapuskasing Uplift of the Archean Superior Province reveal episodes of fluid-rock interaction. Metamorphic zircon grains were isolated from six sample sites along a 10 m traverse away from contact with a high delta18O paragneiss unit. CL images reveal that most zircons have bright cores surrounded by darker rims. The 207Pb/206Pb ages of metamorphic zircons generally range from 2660 to 2550 Ma. Age differences between cores and rims are not always resolvable within the analytical precision of SHRIMP measurements (+/- 8 to 247 Ma, 2sd) due to very low U concentrations in many zircon cores. Ages do not vary systematically with distance across the traverse. Ion microprobe measurements in most zircons reveal isotopic and trace element zoning that record three major geochemical changes in the mafic gneiss: (1) an 18O/16O enrichment in the protolith prior to 2660 Ma, preceding or during the early stages of regional metamorphism; (2) localized 18O/16O enrichment in the margin of the mafic gneiss adjacent to the paragneiss prior to 2660 Ma, likely during the early stages of metamorphism; (3) trace element enrichment and localized decrease in delta18O values within the traverse. Major depletions in 18O/16O (to delta18O values <8.5‰) occur after 2625+20 Ma. Thus, these 18O/ 16O depletions and trace element enrichments likely occurred during, or persisted into the initial stages of retrograde cooling. These geochemical changes are interpreted to result from three distinct periods of fluid infiltration and fluid-rock interaction during the metamorphic history of the mafic gneiss. Coordinated U-Pb, oxygen isotope and trace element analysis of metamorphic zircon, in addition to oxygen isotope data of garnet and quartz, serve as a powerful tool to constrain the timing of metamorphic events in the lower crust and to reveal episodes of fluid infiltration and fluid-rock interaction that drive the geochemical evolution of the Archean crust in the Superior Province.