GRANITIC GNEISSES OF THE PISECO LAKE SHEAR ZONE, SHAWINIGAN OROGEN-PARALLEL DEFORMATION, AND SPATIAL AND TEMPORAL LINKS TO THE AMCG INTRUSIVE EVENT IN THE ADIRONDACK HIGHLANDS, NEW YORK

Mesoproterozoic granitic to charnockitic gneisses form an arching E-W zone transecting the central Adirondacks. The Piseco Lake shear zone (PLsz) is a structural and lithologic discontinuity along the trace of a cryptic suture extending for at least 150 km. It is 20-30 km wide and forms the boundary between the central and southern Adirondacks, underlain respectively by AMCG suite (ca. 1155-1165 Ma) and calc-alkaline tonalitic arc rocks (ca. 1300-1350 Ma). Within the PLsz the Piseco Lake gneisses (PLG) have lineated quartz and feldspar megacrysts attesting to an original coarse-grain size. Along the axis of the zone there are thick (1-2 km) domains of subhorizontal L-tectonite. These rocks have a restricted silica content (66.6-76.6 wt. %), are metaluminous, range from ferroan and magnesian, and range from alkali-calcic to calc-alkalic with increasing silica. Trace elements suggests continental arc magmatism. Epsilon Nd values of the PLG at 1200 Ma range from 2.18-3.95 and are negatively correlated to T_DM ages of 1366-1544 Ma, compatible with melting of a juvenile arc-related terrane (1300-1400 Ma), associated metasedimentary rocks, and/or a mantle wedge metasomatized by prior subduction beneath the Adirondacks. They intrude, and likely provide, the heat source for extensive Shawinigan (ca. 1160-1180 Ma) partial melting of an assemblage of supracrustal gneisses exposed in the shear zone wall rocks. The PLsz forms the core of a region of intense ductile deformation with E-W mineral lineations and sinistral kinematic indicators. The PLG is similar to the 1180-1200 Ma suites in the Adirondack Lowlands and collectively they represent arc magmatism that preceded the emplacement of voluminous granitoids temporally and spatially associated with massif anorthosite. Considered together, the evidence suggests the that PLG developed as suture-stitching arc plutons intruded along a sinistral-convergent, shear system in the deep crust during the Shawinigan orogeny. This orogen-parallel deformation established the crustal architecture for the central and southern Adirondack region and facilitated the disruption, dismemberment, and detachment of a northward subducting slab, allowing asthenospheric rise and decompressional melting along its length to trigger the AMCG intrusive event within the overlying plate.