The Phillips pluton, Maine, USA: evidence of heterogeneous crustal sourcesand implications for granite ascent and emplacement mechanisms in convergent orogens

The Phillips pluton (age of 403.8 +/- 1.3Ma) was assembled at a crustal lev el below the contemporary brittle-plastic transition during regional dextra l-reverse transpressive deformation. The pluton is composed dominantly of m edium- to coarse-grained leucogranite sensu late (s.l.), but within its bou nds includes decametric massive outcrop of fine- to medium-grained granodio rite (s.l.). In places, the leucogranite contains centimetric enclaves appa rently of the,granodiorite. Granodiorite is host to more biotite than musco vite, and more calcic, oscillatory-zoned plagioclase, compared to the leuco granite. Pegmatitic granite and composite pegmatite-aplite occur as metric sheets within the pluton and as larger bodies outside the pluton to the SW. Magmatic fabrics, defined by biotite schlieren, occur locally in the leuco granite; the attitude of these fabrics and layering within the leucogranite are concordant with the NE-striking, steeply-dipping country rock foliatio n. K2O contents, Rb/Sr ratios, Rb, Sr and Ba covariations, and chondrite-no rmalized rare earth element (REE) patterns of leucogranite are consistent w ith high-to-moderate alpha(H2O) muscovite dehydration equilibrium eutectic melting of a predominantly pelite source similar to metasedimentary rocks o f the surrounding central Maine belt (CMB). The REE patterns and Rb/Sr rati os of granodiorite also suggest derivation From a metasedimentary source, b ut more likely by moderate-to-low a(H2O) (muscovite-) biotite dehydration e quilibrium eutectic to non-eutectic (minimum) melting of a protolith domina ted by greywacke in which garnet and plagioclase were residual phases. Both granite (s.l.) types have heterogeneous initial Nd isotope compositions. S amples of granodiorite define a range in epsilon(Nd) (404 Ma) of -1.8 to +0 .1 (+/-0.3 2 sigma uncertainty), and samples of leucogranite define a range in epsilon(Nd) (404 Ma) of -8.0 to -5.3 (+/-0.3 2 sigma uncertainty). This bimodal distribution suggests that melts were derived from a minimum of tw o sources. The data are consistent with these sources being CMB metasedimen tary rocks (epsilon(Nd) (404,Ma)< -4) for the leucogranite, and Avalon-like (peri-Gondwanan) metasedimentary crust (E-Nd (404 Ma)>- 4) for the granodi orite. The range of Nd isotope compositions within each granite type most l ikely reflects isotopic heterogeneity inherited from the source. These data imply that the integrity of individual melt batches was maintained during ascent, and that extensive mixing of melt batches during emplacement at thi s level in the pluton did not occur, although centimetric enclaves have int ermediate Nd isotope compositions consistent with small-scale interactions between magmas. We infer that the Phillips pluton represents the root of a larger pluton, and that what remains of this larger pluton is the feeder co nstructed from multiple melt batches arrested during waning flow of granite magma through a crustal-scale shear zone system. (C) 1999 Elsevier Science B.V. All rights reserved.