POLYMETAMORPHISM RECORDED IN THE PHYLLONITE AT CHURCH RD; BERWICK, MAINE

The Phyllonite at Church Rd (Scr; formally the Gonic Fm) is a NNE-trending meta-pelitic unit that crops out near Berwick Maine between rocks of the Shapleigh Group (SG) and the Berwick Fm (Sb). This unit is truncated in the north by the Permian Lyman Granite, and in the south by the Devonian Barrington Granite. The poly-deformed and vertical Scr seperates shallow-dipping and overturned SG rocks to the west from moderate to steep-dipping Sb rocks to the east. This study investigates polymetamorphism in the Scr to understand better the tectonic events recorded.

Equilibrium mineral assemblages and crystal morphologies support the interpretation for three distinct metamorphic events. The first two metamorphic events are similar throughout the field area; however the third varies along strike. The first event (M1) resulted in euhedral to subhedral staurolite, garnet, and biotite that grew during development of the earliest preserved foliation (S1), but prior to folding of S1, which wraps around M1 porphyroblasts. The second event (M2) post-dates folding of S1 and involves overgrowths on staurolite and garnet, with new biotite and chlorite growth that crosscut the folds in S1 and are aligned with a crenulation cleavage (S2). The third event (M3) varies from north to south. In the north, where the degree of recrystallization is greatest, the mineral assemblage consists of euhedral andalusite, mimetic staurolite, and blocky poikilitic chlorite porphyroblasts overgrowing S2. In the south, the degree of recrystalization is significantly less, and new mineral growth is limited to poikilitic chlorite.

The mineral assemblages and textures for M1 and M2 support the interpretation that both occurred during a regional tectonic event where M1 is late- to syn-S1 development, but pre-dates folding of S1, and M2 occurred after both folding of S1 and development of S2. However, the decrease in both degree of recrystallization and metamorphic grade for M3 with increasing distance from the Lyman Granite supports the interpretation for M3 as contact metamorphism in a static environment associated with emplacement of this pluton, and therefore M3 is Permian.