RELATIONSHIPS BETWEEN MIGMATITES OF THE SILURIAN RANGELEY FORMATION AND THE WILDCAT GRANITE: NEW HAMPSHIRE, USA
A gradational contact between stromatic migmatites of the Silurian Rangeley Formation and the 401 Ma Wildcat Granite in the White Mountains, New Hampshire, provides an opportunity to examine a classic problem: is localization of deformation the cause or the result of melt accumulation? The transition from migmatite to diatexite, to granite is accompanied by a gradient in deformation style. The nature of this transition is of interest due to an apparent increase in leucosome fraction in the migmatites, and corresponding increase in disaggregation of compositional layering. If a genetic relationship between the migmatites and the granite can be proven, this area would be ideal to study the rheological effects of variably distributed partial melts, the effects of heterogeneity on melt migration mechanisms, and the processes of granite genesis.
A Sm-Nd isotope geochemistry study was undertaken in an attempt to correlate the Rangeley Formation with the Wildcat Granite. Samples of unmigmatized schists of the Rangeley Formation, Rangeley migmatites, diatexites, and samples of the Wildcat Granite were taken from a variety of locations within approximately 25 km2. All of the samples have eNd (at 401 Ma) ranging from Ð7.7 to Ð9.2 with TDM model ages ranging from 1.62 to 1.96 Ga. The limited range in initial Nd isotopic compositions provides permissive evidence that the Rangeley schist may have been the dominant source material for the Wildcat Granite. The model ages suggest that the crustal precursors of all these rocks were generated during the Early Proterozoic.
3D mechanical models show that melt migration in a rheologically heterogeneous medium is driven by pore-pressure gradients which arise from strain rate localization, resulting in feedback between melt localization and strain rate localization at a variety of scales. Together with the results of 1D thermal modeling, the mechanical models support the idea that melt was mobile within the Rangeley Formation, and that its distribution at the outcrop-scale was likely influenced by deformation. Small-scale processes that lead to melt accumulation may feed into larger-scale melt redistribution and localization, which in turn influence the rheological evolution of an orogen.