ORIGIN OF GRANITIC PROTOLITHS OF MYLONITES OF THE PISECO LAKE SHEAR ZONE

The Piseco Lake Shear Zone (PLSZ) is a ~20-30 km wide, E-W trending zone of intensely deformed granitic rocks which is the boundary between the Central Adirondacks underlain primarily by anorthosite and related rocks and 1300-1400 Ma calc-alkaline tonalitic rocks of the Southern Adirondacks. The age and geochemical characteristics of the granitic rocks within the PLSZ closely resemble those of the 1200-1180 Ma Antwerp-Rossie (AR) and Hermon Granitic Gneiss (HG) suites of the Adirondack Lowlands. The PLSZ rocks have lineated quartz and feldspar megacrysts, and transposed aplite and pegmatite. They 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. They have high Rb, Pb, and K but low Nb, Ta, P, and Ti concentrations, characteristic of arc rocks; chemical traits very similar to those of the AR suite. Their LREE are enriched (La_n/Sm_n = 3.24±1.19; HG = 3.9; AR_felsic = 4.8) and HREE are moderately depleted (Sm_n/Yb_n = 4.86±2.64; HG = 4.9; AR_felsic = 7.0). In addition, they intrude and likely provide the heat source for extensive, Shawinigan (1180-1160 Ma) partial melting of an assemblage of pelitic and psammatic gneisses, compositional similar and with the same provenance, as the Popple Hill Gneiss of the Lowlands. The PLSZ rocks differ from those of the HG and AR suites by displaying more intense fabric development (including L-tectonites) and transpositioning, sporadic opx development, a restricted range of silica percentages, more complex zircons, and late chlorite growth. This suggests the intrusion of vast volumes of granitic melt along a deep crustal suture during active Shawinigan deformation. As recently proposed by Peck et al. (2013) for the AR and HG suites of the Lowlands, these rocks likely represent the transition from terminal arc magmatism to voluminous granitoids associated with the massif anorthosite. Epsilon Nd values at 1200-1180 Ma range from 1.97-3.66 (HGG = 3.31; ARS = 1.5-5.4) and are negatively correlated to T_DM ages of 1366-1544 Ma (HGG = 1424 Ma; ARS = 1288-1634 Ma), compatible with melting of a juvenile arc-related terrane (1300-1400 Ma) and associated metasedimentary rocks and/or a mantle wedge metasomatized by prior north-directed subduction beneath the Adirondack Region.