STRUCTURAL CONTROL ON INTRUSION OF GRANITE SHEETS IN THE HUDSON HIGHLANDS, NY

The Hudson Highlands basement massif in NY consists of sequences of metavolcanic, metasedimentary and plutonic rocks that experienced polyphase metamorphism and deformation during the Grenville orogenic cycle. A suite of granite sills in the Sterling Forest area (Byram Intrusive Suite), consists of dozens of individual sheets that intruded parallel to the regional foliation. Between the granite sheets, the country rock consists of narrow belts of metavolcanics (interlayered felsic, intermediate and mafic gneisses) and metasediments (pelitic and calcsilicate gneisses) that were metamorphosed to granulite facies. Individual granite sheets are parallel to penetrative foliation (S1) in the country rock, and generally strike 015-030 and dip 15-45 WNW. Thickness of the granite sheets ranges from a 10 to 100’s of meters, and the thickest sheets were mapped up to 8 km long. The contacts with the country rock are typically very sharp, and occasionally show finer grain size suggesting minor chill margins. Most contacts show little textural or mineralogical evidence for contact metamorphism. Texturally the granite varies from medium grained (thinnest sheets) to coarse grained with the thickest sheets containing metacrystic K-feldspar and plagioclase. Most of the granite sheets contain minor amounts of mafic phases such as hornblende or biotite. Regionally the granite sheets occur within a map-scale recumbent fold (F1) with the concentration of granite sheets in the hinge region, and the structural stacking shows a pattern of thin sheets (<10 m) progressively giving way to thicker ones from the limb to the hinge of the fold. The lack of substantial contact metamorphic zones suggests the country rock was thermally comparable to the magma upon intrusion, pointing to probable intrusion during metamorphism. The tie between distribution of granite sheets and the hinge of a map-scale fold suggests the granite was concentrated in the nose of the fold during deformation.