A 30 Myr record of retrograde metamorphism and multiple generations of monazite and garnet in western MA revealed by coordinated LASS and EPMA

Abstract:

Garnet-kyanite-cordierite pelitic schists with relict phosphatic garnets thought to record ultrahigh-pressure metamorphism (e.g., Snoeyenbos and Koziol, 2008) crop out along the western margin of the Goshen Dome in western Massachusetts. These schists contain monazite inclusions within Grt, Ky, Crd, Bt, Pl, Qz, and Chl. Compositional maps via EPMA-WDS show overgrowths on each monazite grain, indicating that each grain records multiple metamorphic events. We analyzed monazite in situ using laser-ablation split-stream (LASS) ICPMS to correlate the geochemical and age domains and to reconstruct the polyphase metamorphic history. We used WDS compositional maps of Y and Th, and ICPMS Gd/Yb ratios and U/Pb ages to define the timing of major garnet growth and breakdown processes, and melting and/or fluid-rich metamorphism. For example, decreasing Gd/Yb and high Y concentrations are consistent with garnet breakdown; highly variable Gd/Yb indicate melting and/or fluid-rich metamorphism (e.g., Stearns et al., 2013).

The following cumulative histories are interpreted from two samples (B and G). In both samples, decreasing Gd/Yb in monazite suggest garnet breakdown from c. 398 to 387 Ma. Data from sample B (n = 644 analyses) indicate two additional phases of garnet growth: 382 to 377 Ma and 375 to 369 Ma (see figure). Data from sample G (n = 231 analyses) also indicate two additional phases of garnet growth: 387 to 381 Ma and 379 to 369 Ma. The final garnet growth event affecting both samples is marked by the most variable Gd/Yb ratios in monazite from all petrographic contexts and is consistent with melting or a fluid-rich metamorphic event. These data are most consistent with metamorphism during emplacement to crustal levels in the early Acadian, and a substantial overprint during the regional thermal maximum. This complex record can only be unraveled by collecting geochronologic and geochemical data in situ to preserve the petrographic context of each grain, thus underscoring the importance of LASS analysis.