SYNOROGENIC EMPLACEMENT OF GRANITE SHEETS, HUDSON HIGHLANDS, NEW YORK

Grenvillian massifs of the western Hudson Highlands, NY contain meta-igneous and metasedimentary lithotectonic units that were host to complex deformation and metamorphism associated with the assembly of Rodinia. The early deformation (D1), accompanied by granulite facies metamorphism (1.05 Ga), produced west-directed fold nappes and regional gently dipping gneissocity. The second deformation (D2) produced an anastomosing network (>30 km wide) of km-scale steeply dipping shear zones with dextral transpressive displacement. In the western block of the Hudson Highlands, a suite of megacrystic (K-feldspar and plagioclase) granite sheets and small plutons occur in the hinges of D1 nappes defined by folded metavolcanic and metasedimentary rocks. Individual granite sheets range from 10’s to 100’s of meters thick and they extend several kilometers parallel to the hinge of the folds, suggesting a genetic relationship between granite emplacement and fold development. The granite sheets commonly contain xenoliths of the local country rock and they have distinct contacts. Internally, the granite sheets are not foliated, however, the margins of some sheets have foliation that is parallel to the adjacent gneissocity. Typical high-grade ductile deformation textures define the foliation in the granite, such as planar aggregates of recrystallized feldspar and quartz, in addition to alignment of minor hornblende and biotite. The bulk AFM composition of the granite sheets spans the transition between tholeiitic and calc-alkaline. The Aluminum saturation index is peraluminous (1.27-1.40) and the K2O/Na2O and FeOt/(FeOt+MgO) ranges are 0.98-2.25 and 0.87-0.94. The relationship between the granite sheets and D1 fold nappes, in addition to the ductile deformation textures in some granite margins that merge with S1 foliation suggest that the granite sheets were emplaced during deformation. A tectonic surge model is employed to explain the field relations, with granite magma formed by partial melting of regional rock units such pelitic, psammitic or calc-silicate gneisses, all of which are extensively migmatitic. The bulk of the granite occurs as small plutons that precisely overlap the fold hinge regions, and the granite sheets are most likely remnant conduits for the migration of magma during this process.