2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 46-4
Presentation Time: 9:00 AM-5:30 PM


MITCHEL, Kelford C. and SULLIVAN, Walter A., Department of Geology, Colby College, 5800 Mayflower Hill, Waterville, ME 04901, kmitchel@colby.edu

We focus on deformation mechanisms in an outcrop of the Dyer Long Pond Complex in Jefferson, Maine. The Dyer Long Pond Complex is a high-temperature shear zone that accommodated dextral motion as part of the Norumbega Fault System. Four distinct rock types are present in this outcrop. In order of decreasing abundance, these are: (1) fine- to medium-grained biotite + silliminate gneiss with leucosomes of coarse-grained K-feldspar + quartz + plagioclase, (2) 2-50-cm-wide, coarse-grained leucogranite bodies with both white, and less commonly, pink K-feldspar, (3) 1-15-cm-wide, equigranular, fine-grained biotite + quartz + plagioclase layers with no leucosome material, and (4) 0.1-1.5-cm-wide, fine- to medium-grained biotite + sillimanite selvedges that typically border leucogranite bodies. Foliation strikes NE-SW and is steeply dipping, to vertical throughout the outcrop. Shear-band boudins of leucogranite bodies and asymmetric quartz or K-feldspar clasts that are dismembered and extended within the gneiss record dextral motion in this exposure. The selvedge and equigranular layers are not boudinaged and wrap around the leucogranite bodies. Selvedge layers tend to die out along strike and equigranular layers extend ~8m through the entire exposure. We interpret these rocks as being migmatitic for two reasons: (1) the biotite + sillimanite selvedges indicate residuum from partial melting and (2) the absence of leucosome within the equigranular layers suggests these were refractory bodies during partial melting. The solid-state deformation of leucosome and leucogranite indicates dextral shearing postdates melt crystallization. We sampled each of the four rock types across the exposure including: 10 samples of gneiss, 3 samples of equigranular material, and 1 sample of leucogranite. 4 samples contain pieces of leucogranite bodies and 11 samples contain biotite selvages. We will examine deformation textures and metamorphic paragenesis to determine what deformation mechanisms operated in these rocks and how this deformation is partitioned among different rock types. Interpreting the origin of the poorly understood protolith of the Dyer Long Pond Complex is a second goal of this project. These results will elucidate the Dyer Long Pond Complex’s significance within the Norumbega Fault system.