IMPACT OF FLUID EVOLUTION ON GARNET ZONING IN PELITIC ROCKS FROM HARPSWELL NECK, MAINE

Complex, time-varying compositional zoning in pelitic garnets from Harpswell Neck, Maine results from growth in the presence of fluids of variable composition. A field study of the relative timing of garnet growth and emplacement of multiple vein generations was combined with microchemical analyses of fluid inclusions (FIs) in vein quartz and garnet; all evidence is consistent with the growth of garnet interiors in the presence of CO₂-rich fluids (producing zoning that is patchy for Mn, Mg, and Fe but concentric for Ca and Y), and with the growth of garnet rims in the presence of H₂O-rich fluids (producing concentric zoning for all elements). The change can be explained by differences in rates of intergranular diffusion, governed by solubility in these two fluids.

Regional structures record shortening in NNE-trending folds and subsequent NNE shearing along km-scale strike-slip faults. Three generations of quartz veins were discriminated in the field on the basis of their relationship to these structural features and their cross-cutting relationships. Initial garnet growth was contemporaneous with the emplacement of the first generation of veins, as garnets are found both within the veins and lining their edges; a correspondence of shear fabrics shows that growth continued during the emplacement of the second generation. FIs in both generations revealed two distinct fluids, one H₂O-rich and one CO₂-rich, with CO₂ dominant in earlier veins and H₂O in later ones. Multiple CO₂-rich FIs were identified in quartz inclusions captured in garnet interiors, but no evidence for H₂O was found there. However, two H₂O-rich FIs were found in a garnet rim, with minor CO₂ suggesting H₂O-CO₂ mixing.

These observations argue that early garnet growth was synchronous with trapping of CO₂-rich fluid in early quartz veins and in quartz inclusions within garnet interiors, whereas later garnet growth was related to an H₂O-rich fluid trapped in later quartz veins and in garnet rims. The change in garnet zoning thus coincides with a distinct shift in fluid composition. High solubility for Ca and Y and low solubility for Mg, Mn, and Fe in early fluids explain the large differences in length scales of equilibration, whereas elevated solubility for all five elements in the late H₂O-rich fluid is responsible for equilibrating all of them in garnet rims.