INTERPRETING REACTION HISTORIES AND P-T-T PATHS FROM IN-SITU MONAZITE AND XENOTIME PETROCHRONOLOGY: IMPLICATIONS FOR THE TECTONIC HISTORY OF NEW ENGLAND

Geochronology and geochemistry of accessory phases are commonly used to constrain the timing of metamorphic reactions, construct tectonic models, and understand orogenic processes. In particular, the Y content of monazite (Mz) has been widely interpreted to be related to reactions involving the growth and breakdown of garnet. However, this interpretation is dependent on the assumption that Mz and garnet are the only Y-bearing phases. To better understand reactions involving garnet and the significance of Y zoning in Mz, we have examined a suite of Mz and xenotime (Xt) bearing metapelites from the New England Appalachians. Multi-scale compositional mapping was employed to identify individual Mz and Xt and to characterize their internal zoning. Compositions and dates from individual domains were obtained using the Cameca Ultrachron. Mz and Xt are found in the matrix and as inclusions within garnet while Xt is also found in haloes around garnet. Mz shows systematic zoning with cores and rims enriched in Y and Y-depleted mantles. While these cores and mantles would typically be interpreted to represent pre- and syn-garnet Mz growth, and rims to represent garnet breakdown, temporal, textural and compositional evidence suggest that cores and rims are in equilibrium with Xt. This interpretation is supported Mz-Xt thermochronology, which is consistent with ⁴⁰Ar/³⁹Ar dates and conventional thermobarometry. Low Y Mz mantles likely grew when Xt was unstable near peak metamorphic conditions. Xt also shows compositional zoning and preserves several dates that are not recorded by Mz which place important constraints on garnet stability. Early garnet growth is constrained by Xt inclusions in garnet cores while breakdown is recorded by Xt haloes on garnet. In the absence of Mz, Xt has higher Th/U while Yb and HREE contents vary with the stability of garnet, behaving in a similar manner to Y in Mz from Xt-absent rocks. In addition to placing new constraints on the polymetamorphic history of the Appalachians, these results emphasize the importance of multi-scale compositional mapping and in-situ geochronology. Further, they suggest that Xt geochronology and geochemistry can provide additional constraints for reaction dating and is critical to interpreting the significance of Mz dates in rocks where both Mz and Xt are present.