INVESTIGATION OF THE GONIC FORMATION AS A STRUCTURAL AND PETROLOGIC BOUNDARY BETWEEN THE CENTRAL MAINE TERRANE AND MERRIMACK GROUP, SOMERSWORTH QUAD, SOUTHWESTERN MAINE

The boundary between rocks of Laurentian and Avalonian origin has traditionally been interpreted to be the boundary between the Central Maine Terrane and the Merrimack Group, west of the Massabesic Gneiss. Recent studies by Thompson et al. (2005) and Kerwin et al. (2005) have re-interpreted this boundary by re-classifying rocks southeast of the Massabesic Gneiss as continuous with the Central Maine Terrane. The research presented here investigates the Gonic Fm in southwestern Maine as a possible location for the above mentioned boundary. The Gonic Formation crops out as a silvery grey, coarse-grained, garnet, biotite, muscovite, ± chlorite schist. SE of the Gonic Fm, the Berwick Fm, the Merrimack Group, crops out as a purple biotite granofels. The absence of graded beds or a pelitic component distinguishes the Berwick Fm from the Gonic Fm. Rocks to the NW of the Gonic Fm crop out as a rusty weathering, biotite staurolite meta-greywacke with graded beds preserved, characteristics more common of the Central Maine Terrane. Along with a petrologic change, structural characteristics change across the study area. Bedding in the rocks NW of the Gonic Fm are overturned and openly folded. In contrast, bedding in the Berwick Fm dips more steeply, suggesting tighter folding. Both units display an increase in deformation near their contact with the Gonic Fm, where fold limbs become parallel with the foliation in the Gonic Fm. The Gonic Fm records a more complex deformational history. To the SE where the Gonic Fm is in contact with the Berwick Fm, 2 foliations are discernable. S1 is defined by alignment of biotite and muscovite. S2 is a spaced crenulation cleavage. Garnet porphyroblasts grow preferentially along S2. Further to the NW, S1 foliation is completely transposed leaving S2 as the dominant foliation. A younger crenulation cleavage develops in S2 which rotates the porphyroblasts about a horizontal axis. The rapid change in structural and petrologic characteristics on either side of the Gonic Fm, support the interpretation of the Gonic Fm as an important boundary and perhaps a candidate a terrane boundary.