MODELING THE DECOMPRESSION P-T-t PATH IN SAPPHIRINE GRANULITES ASSOCIATED WITH ECLOGITES, EAST ATHABASCA MYLONITE TRIANGLE, NORTHERN SASKATCHEWAN, CANADA

Sapphirine and spinel are common minerals in Mg- and Al-rich granulites. In Qtz-absent rocks, Spr and Sp (± Crn) commonly form as symplectitic reaction products with anorthite at the expense of earlier aluminous phases, such as Ky. Such reaction textures can be used to constrain P-T evolution and tectonic history. However, unlike the petrogenetic grids that are used to constrain the evolution of UHT Spr-Qtz assemblages, grids that allow the interpretation of plagioclase-bearing assemblages are less common. Two varieties of Spr-bearing rocks (Qtz-present and Qtz-absent) occur in the East Athabasca mylonite triangle, northern Saskatchewan, Canada, both of which are associated with Ky-bearing eclogites. The first is a 1-2 m reaction zone structurally above the eclogite unit. The peak assemblage of Grt-Ky-Qtz occurred near 20 kbar, 900°C and developed during eclogite facies metamorphism ca. 1904 Ma. A near-ITD path is inferred from reaction textures that developed around primary porphyroblasts. Opx and Pl rims occur around Grt and Spr-Sp-An reaction rims occur around Ky. A second variety of Spr-bearing assemblages that are Qtz-absent occur in Ky veins within the eclogite. The peak assemblage in the veins contains Grt, Opx, Ky, and Pl, and formed following the eclogite facies metamorphism. Evidence for decompression includes the replacement of Ky by Sil and the development of reaction rims involving Spr, Sp, Crn, and An around Ky. Using Perplex and the Holland & Powell (1998) dataset, we have constructed a new grid for this assemblage in the CFMAS system. The Spr-Sp-Crn-An symplectites in the veins are interpreted to have formed at ~6 kbar and 600-700°C. This suite of rocks preserves a P-T history of initial near-isothermal decompression from ~20 to ~10 kbar followed by cooling from ~900 to ~600°C during decompression to ~6 kbar. Geochronological constraints on the development of these reaction textures bracket the interval between the high-P metamorphism and decompression to ~20 Ma, and suggest that the initial decompression from 20 to 10 kbar occurred at 1890-1895 Ma, followed by more protracted exhumation to mid-crustal depths by 1880 Ma. These results suggest that our approach may be useful in other Si-undersaturated Mg-rich granulites and have implications for quantifying decompression rates in high-P granulites.