PETROLOGY AND MONAZITE GEOCHRONOLOGY OF THE SCARBORO FORMATION, JEFFERSON, MAINE

Thermodynamic modeling and monazite geochronology has been carried out on samples from the Ordovician age Scarboro Formation of the Casco Bay Group in Central-Midcoast Maine. This area is located in what is known as the eastern metamorphic lobe of Maine, which experienced deformation and metamorphism during Devonian (Acadian) times. We present constraints on the timing and P-T conditions of metamorphism and suggest that a higher pressure event occurred prior to the widely documented andalusite grade metamorphism that is ubiquitously preserved in these rocks. Preserved metamorphic assemblages include garnet, staurolite, andalusite, biotite, muscovite, plagioclase and ilmenite. Garnet crystals exhibit complex compositional zoning, with clear core and rim domains. Garnet cores show a steadily decreasing Ca content and rims are distinguished by a progressive increase and spike in Ca content. Preliminary thermodynamic phase equilibria modeling suggests garnet cores began to grow at 5-7 kbar and 520-550˚C. Staurolite compositions delineate a slightly decompressive heating path to 4.5-6 bar and 570-600˚C, though sillimanite growth at higher temperatures is known to have occurred in this area. We speculate that andalusite and garnet rim growth occurred during a subsequent phase of metamorphism at <4kbar.

Monazite and xenotime were dated by Laser Ablation ICP-MS, with lower intercept ages and weighted averages of concordant analyses at 375-370 Ma. In some cases, monazite in these rocks overgrows aligned biotite and muscovite that is rotated relative to the enveloping fabric. The calibrated monazite-xenotime thermometer of Heinrich et al. (1997) yields monazite growth temperatures of 575-610˚C. This suggests that ongoing deformation at 375-370 Ma occurred at ~600˚C, in good agreement with existing constraints on the cooling history of the Casco Bay Group. It is currently unclear whether this Acadian monazite growth occurred during staurolite growth or subsequent, lower pressure andalusite-grade metamorphism, though the latter would be consistent with previous suggestions of higher pressure pre-Acadian metamorphism (see Guidotti, 1989; West et al., 2003).