ISOTOPIC AND GEOCHEMICAL EVIDENCE FOR BIMODAL COMPLEXITY IN THE GOULDSBORO PLUTON, COASTAL MAINE

Isotopic, major and trace element data from the ~419 Ma bimodal Gouldsboro pluton in eastern coastal Maine suggests that both partial melting of peri-Gondwanan crust and extensive interaction with mantle-derived magmas created significant spatial variability of isotopic and geochemical signatures. Whole rock epsilon Nd data from the Gouldsboro show a range of values from +1.18 to -6.03. The geometry of the system, a series of on-lapping, SW-dipping sheets, preserves striking evidence of felsic-mafic interaction near the loci of latest injections (NE). Homogenous granites comprise the center with intermediate to aplitic hybrids, nearing the roof (SW). In areas of clear mechanical and chemical mingling between kfs+plg+qtz+hbl granite and ol+opx+cpx+plg+hbl gabbros, epsilon Nd = -0.93. Less obviously contaminated kfs+qtz+plg+hbl±bt±ep granites from the center show significant localized variation, -6.03 to +1.18. Aplites intruding volcanic breccias near the roof have a value of +0.23. Positive epsilon Nd values and trace element variation in areas without obvious evidence of mingling support a model of long-lived chamber evolution via thermal and chemical inputs from multiple mafic and felsic injections. Spatial Nd variation suggests zones of localized isotopic and chemical disequilibrium, in mesoscopically homogenous granites, can be extensive despite the >kilometer distances from mingling zones. This data set contrasts the Gouldsboro with mafic and felsic rocks from the Cadillac Mountain intrusive complex (+7.5 to +4.5 and +1.6 to +2.8 (Becker et al., 1998; Waight et al., 2001)) and Pleasant Bay (+5.7 and +2.2 (Waight et al., 2001). These systems are contemporaneous with the Gouldsboro; however, younger plutons, in close proximity to the Gouldsboro, such as Tunk Lake (+2.17 to +2.46 (Gylling et al., in review), also show considerably different epsilon Nd signatures. Data from Waight et al., 2007 from the southernmost exposures of Gouldsboro granite (+2.8 to +3.1) are also in marked contrast to data from the center and lower portions of the Gouldsboro pluton. This new data set suggests that multiple, unique magma pulses were injected during system construction, but are only poorly preserved in major and trace element geochemical signatures, microscopic textures, or macroscopic geometry.