MESO- TO NEOPROTEROZOIC TRANSPRESSION TO TRANSTENSION IN THE WESTERN HUDSON HIGHLANDS, NEW YORK

Meso- to Neoproterozoic igneous bodies intruded metavolcanics and metasedimentary gneisses in the western Hudson Highlands, NY during a period of major transpression, that transitioned into transtension during Ottawn to Rigolet orogenic phases. The basement rocks of the western Hudson Highlands experienced at least two tectonic events including the intrusion of small igneous bodies and granulite facies metamorphism. All rock bodies contain high-grade gneissocity and intrafolial isoclinal folds. This is D1 for the region, and is cross cut by a network of anastomosing steeply dipping ductile shear zones that range from m to km in width (D2). Metamorphism associated with the shear zones was also high-grade, and in many places it is indistinguishable from the earlier phase. However, detailed zircon geochronology in conjunction with structural and igneous cross cutting relations constrains the transcurrent shear zones being active between ~1026 Ma and 1000 Ma, and in places as late as ~965 Ma. Small mafic and granitic bodies intruded into the shear system during early transpression. A small swam of subvertical granite sheets occur almost exclusively within the cores of antiforms. The antiforms are several kilometers wide, shallowly plunging axes, subvertical axial planes, and developed between shear zone splays. Antiform axes are oblique to shear zone bondaries in places, consistent with overall dextral shear, and in places they merge with the margins of the shear zones. These structures represent early dextral transpression that was accompanied by intrusion of granite sheets and small mafic dikes. Textural evidence is consistent with syn-deformational emplacement, and supported by igneous crystallization ages that overlap metamorphic ages from the shear zones. Several of the shear zones experienced dilation accompanied by extensive mineralization (Rigolet phase). This transition is also marked by intrusion of subhorizontal granitic pegmatite sheets (~10 meters thick) that cross cut the shear zone foliation. Although these pegmatites are not as common as early foliation-parallel pegmatites, they are volumetrically more significant. One example from the central part of the western highlands contains large growth lineated subhorizontal hornblende crystals (trend ~250), and yielded a zircon age of ~980 Ma.