GEOLOGIC HISTORY OF THE HUDSON HIGHLANDS BASEMENT MASSIF: RESULTS FROM NEW GEOLOGIC MAPPING IN THE MONROE & SLOATSBURG QUADS

Recent bedrock mapping in the Monroe & Sloatburg quads (Hudson Highlands, NY) delineates belts of metasedimentary, metavolcanic and quartzofeldspatic rocks that experienced a common intrusive, metamorphic and deformational history. The metasedimentary belts consist of interlayered pelitic, semipelitic and calcsilicate gneisses. Pelitic and semipelitic rocks contain bio-gt gneiss with K-spar, plag, sil, qtz and local cordierite. The calcsilicate gneiss contains diopside, plag, gt, apatite, sphene, scapolite, and hornblende, and are commonly migmatitic. Metavolcanic rocks consist of interlayered mafic, intermediate and felsic gneiss at the scale of cm to meters. The gneiss contains varying amounts of pyroxene, hornblende, plag & quartz with minor magnetite and bio. The quartzofeldspathic gneiss contains massive and layered quartz-plagioclase gneiss with minor amounts of biotite & hornblende. These three lithofacies occur in belts 0.5-5 km wide with gradational contacts. There are at least two deformational events that impacted this region of the Hudson Highlands. The first event (D1) is characterized by penetrative foliation (S1) that affects all the lithofacies described above. S1 is associated with intrafolial asymmetric isoclinal folds (F1) with consistent vergence in some areas. Meso- & megascopic folds are recumbent to shallowly reclined, commonly asymmetric with sheared lower limbs. The fold asymmetry consistently indicates NW transport. The structures of D1 are crosscut by diorite that contains xenoliths of local gneiss, and a suite of granite sheets intruded parallel to S1. The second event (D2) is a network of anastomosing shear zones that transect the entire area. These shear zones transpose the D1 structures and locally deform the diorite and granite bodies. The shear zones strike NE, are steeply dipping, and range from 0.5-2 km wide. Mineral lineations plunge shallowly NE, and kinematic indicators show dextral shear sense. Late deformation on these shear zones was dilational with mineralized veins form zones 1-5 km long. Upright open folds associated with shear zones have oblique hinge axes consistent with dextral shear and suggest a transpressional strain history.