Retrograde fluids in the Archean Shawmere anorthosite, Kapuskasing Structural Zone, Ontario, Canada
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The Archean Shawmere anorthosite lies within the granulite facies portion of the Kapuskasing Structural Zone (KSZ), Ontario, and is crosscut by numerous linear alteration veins containing calcite + quartz dolomite zoisite clinozoisite margarite paragonite chlorite. These veins roughly parallel the trend of the Ivanhoe Lake Cataclastic Zone. Equilibria involving clinozoisite + margarite + quartz calcite plagioclase show that the vein minerals were stable at T <600C, XCO2 <0.4 at P = 6 kbar. The stabilities of margarite and paragonite in equilibrium with quartz are also consistent with T <600C and XCO2 <0.4 at 6 kbar. Additional assemblages consisting of calcite + clinochlore + quartz + talc + margarite indicate T <500C with XCO2 >0.9. Thus, vein formation, while clearly retrograde, spanned a range of temperatures, and fluid compositions evolved from H2O-rich to CO2-rich. The calcite in the retrograde veins has 18O values that range from 8.4 to 11.2 (average = 9.7 0.9) and 13 values that range from -3.9 to -1.6 (average = -3.1 0.6). These values indicate that the fluids from which calcite precipitated underwent extensive exchange with the anorthosite and other crustal lithologies. The fluids may have been initially derived either from devolatilization of metamorphic rocks or crysallization of igneous rocks in the adjacent Abitibi subprovince. Vein quartz contains CO2-rich fluid inclusions (final melting T = -57.0 to -58.7C) that range in size from 5 to 17 m. Measured homogenization temperatures (Th) range from -44.0 to 14.5C, however for most inclusions (46 of Sl), Th = -44.0 to -21.1C (pCO2 1.13 to 1.05 g/cm3). At 400 to 600C these densities correspond to pressures of 3.5 to 7 kbar, which is the best estimate of pressures of vein formation. It has been argued that some high density CO2-rich fluid inclusions found in the KSZ were formed during peak metamorphism and thus document the presence of a CO2-rich fluid during peak granulite facies metamorphism (Rudnick et al. 1984). The association of high density CO2-rich fluid inclusions with clearly retrograde veins documents the formation of similar composition and density inclusions after the peak of metamorphism. Thus, the coincidence of entrapment pressures calculated from fluid inclusion density measurements with peak metamorphic pressures alone should not be considered strong evidence for peak metamorphic inclusion entrapment. All fluid inclusion results are consistent with an initially semi-isobaric retrograde P-T path.
