GEOCHEMISTRY OF UNDEFORMED GRANITES OF THE SEBAGO MIGMATITE DOMAIN, SOUTHERN MAINE

Previous field and laboratory research has constructed a geologically viable model of Permo-Carboniferous granite magmatism in southwestern Maine. The 293±2 Ma Sebago pluton (c. 400 km²) is fringed by a broad region of largely migmatitic rocks (the Sebago Migmatite Domain; SMD) that extend into the crustal scale Norumbega shear zone system. Felsic igneous rocks within the SMD include texturally homogeneous biotite- and two-mica granites, heterogeneous granites with magmatic foliations and ductilely deformed granites. The latter group grades into schlieric granites and migmatites. The texturally homogeneous granites are analogous to the Group 1 Sebago pluton rocks defined by Tomascak et al. (1996). In this study we are examining the elemental and Nd isotope geochemistry of a suite of undeformed granites from the SMD from outcrop locations dispersed on all sides of the presently-identified Sebago pluton. Goals of the study include definition in detail of the areal extent of the Sebago pluton and its relation to granite magmatism in the SMD as a whole.

The initial major element data show moderate variability but broadly overlap the Group 1 granites. No geographical pattern is apparent from the elemental data. A more significant degree of variability is shown by the trace alkalis and alkali earths, with several samples far exceeding the range in Sr and Ba of the Group 1 granites. A group of samples from outcrops approximately 20 km northeast of the pluton define an initial Nd isotope signature (ε_Nd(293 Ma) = -5.7 to -4.1) that is distinct from that of the Group 1 granites (-3.1 to -1.6). Initial data from samples to both the southwest and southeast of the pluton yield values more similar to the Group 1 granites.

Ultimately the data from this study will be used in tandem with precise U/Pb crystallization ages to map out the spatial-temporal distribution of melt in the crust with exceptional detail. These findings will be linked with further ongoing studies of the petrogenesis, geochronology and structural evolution of migmatites in order to answer first order questions about melt transit and the role of transpression, given the pervasive nature of shear deformation linked to the Norumbega shear zone system.