FLUID INCLUSION MICROTHERMOMETRY OF QUARTZ VEINS FROM THE WEPAWAUG SCHIST, SOUTHERN CONNECTICUT

Fluid inclusion microthermometry of quartz in syn- to post-metamorphic veins is used to infer the post-peak metamorphic fluid history and deformation conditions of the Wepawaug Schist in southern Connecticut. Veins were sampled across the steep Barrovian metamorphic gradient, from the chlorite zone, biotite-garnet zone, and staurolite-kyanite zone.

Most fluid inclusions are two-phase, typically less than 10 microns in size, have a vapor bubble occupying <10- 20% volume % of the inclusion, and contain NaCl-rich fluids with less than 6.0 wt. % NaCl equivalent salinity. They occur as primary and pseudosecondary inclusions in chlorite zone veins; and as pseudosecondary and secondary inclusion in the biotite-garnet and staurolite-kyanite zones. In the higher grade zones, most primary inclusions exhibit re-equilibration textures. Primary inclusions in a NE-striking vein in the chlorite-zone have homogenization (T_h) values of 144° - 177°C and 2.9 to 3.5 % salinity, while inclusions in a NW-striking vein have Th values of 220° - 228°C and 5.7 to 6.0 % salinity. Both veins post-date foliation. The NE-striking vein gives trapping conditions (based on lithostatic [25°C km-1 and 26 MPa km^-1]) of 320° - 460°C and 315 – 450 MPa (12.1 – 17.3 km) and the NW-striking vein gives conditions of 415° - 425°C and 405 – 420 MPa (15.6 – 16.2 km).

A folded quartz vein in the biotite-garnet zone has pseudosecondary inclusions that have T_h) values of 201° - 236°C and 5.0 % salinity, corresponding to trapping conditions of 390° - 450°C and 380 – 445 MPa (14.6 – 17.1 km). A foliation-parallel vein in the biotite-garnet zone has coeval pseudosecondary CH₄-CO₂) and two-phase brine inclusions (T_h) of 217° - 249°C and 4.0 to 4.7 % salinity) that were trapped at 505°C and 340 MPa (13.0 km). Secondary inclusions in a recrystallized foliation-parallel vein from the staurolite-kyanite zone has T_h) values of 132° - 145°C and 1.2 % salinity, giving trapping conditions of 295° - 315°C and 290 – 310 MPa (11.2 – 11.9 km).

In general, fluid inclusions were trapped at 300° - 500°C and <450 MPa, significantly less than 400° - 600°C and 600 – 900 MPa at peak metamorphism as determined by Ague (1994). Therefore, fluid inclusions in veins reflect post-peak metamorphic conditions in the Wepawaug Schist, and were probably trapped as the system opened during unroofing and was at a depth of 12-17 km.