SHAWINIGAN SUBDUCTION AND TRANSCURRENT ASSEMBLY OF THE ADIRONDACK MASSIF, NEW YORK

New evidence suggests that northward subduction under the Laurentia margin (ca. 1205-1180) resulted in the emplacement of the Piseco Lake granitoid arc plutonic suite into the Highlands of the Adirondacks. This suite of arc plutonic rocks occurs throughout the Adirondacks, including the Lowlands, however, the most voluminous bodies form a broad belt within the Piseco Lake shear zone (PLsz), spanning the width of the southern Adirondack window. The PLsz is proposed to divide 1350-1300 Ma tonalitic gneisses in the southernmost Adirondacks from the AMCG dominated region of the Highlands. The timing of granitoid intrusion and deformation suggest that intensely developed tectonites in the PLsz developed within a continental arc batholith during the collision with the Southern Adirondack Terrane. This terrane was therefore accreted to Laurentia during the Shawinigan Orogeny, and the PLsz is the cryptic suture between Mesoproterozoic continental blocks. The PLsz includes anastomosing domains of well-developed L-, L>S, and L-S tectonite that form a series of mylonitic domes in the northern portion of the zone. The domes merge with a 10-15 km wide, subvertical zone of steeply dipping mylonite with subhorizontal mineral lineations. Megacrystic granite is the protolith for most of these tectonites, and all of these rocks contain evidence for sinistral shear throughout the PLsz. The intrusion and deformation of the arc granitoids immediately predates anataxis that impacted the entire Adirondack Region from 1180-1160 Ma, and this tectonic sequence occurred just prior to the intrusion of 1165-1145 MA AMCG suite (~65% of the Adirondack Highlands). A model is proposed that involves delamination or slab detachment during subduction that resulted in the ascent of enriched asthenosphere, and ultimately served as the mechanism for the production of AMCG massifs in the Highlands, north of the PLsz. The timing of this model is consistent with other anorthosite massifs in the southern Grenville Province that have been linked to Shawinigan convergence.