A TRANSECT ACROSS THE NORTHERN MARGIN OF THE MARCY MASSIF, COPPER KILN LANDSLIDE, ADIRONDACK MOUNTAINS, NEW YORK, USA

The Marcy Massif of the Adirondack Mts., New York is a Proterozoic anorthosite batholith of the Grenville Province. The geologic history involves convergent deformation and magmatism (Shawinigan orogeny), emplacement of the AMCG plutonic suite (e.g., massif anorthosites), and continued orogenesis (Ottawan phase) with late syn-to-post kinematic emplacement of the Lyon Mountain Granite. The Marcy Massif behaved as a rigid body during the Ottawan phase localizing deformation at its margins, and study of these areas is key for understanding orogenesis.

Work on southeastern exposures of the massif boundary (Regan et al., 2019) is consistent with an extensional detachment and structurally-controlled emplacement of fluids and melt. This study builds on the work of Regan et al. (2018, 2019) of the northern margin of the Marcy Massif at the Copper Kiln Landslide and surrounding areas where there is less known. From west to east across the margin, the lithologic units vary, starting with the host rock gneiss and a skarn that transitions to a meta-anorthosite with leucogranite layers and gabbroic xenolithic lenses, followed by a more uniform and less deformed meta-anorthosite to the east. Xenolithic bodies variably display features consistent with boudinage, attenuation, and partial melting. Local compositional bands in the host rock gneiss (Catamount Mt.) are folded in outcrop, and strike and dip of gneissosity are consistent with upright folds with the best-fit fold axis of 10->028. The skarn and meta-anorthosite (Copper Kiln Landslide) have north-striking gneissosity, with north-trending, shallowly plunging lineations. There is a subtle but distinct clockwise rotation of strike and a shallowing of dip from the skarn to the metanorthosite as well as weakly defined folding at the hand-sample scale in the skarn. Conjugate fractures, commonly melt-filled, are present throughout the transect and may have played a role in the disaggregation and attenuation of layers and xenoliths in the meta-anorthosite. This study indicates that certain aspects of deformation are shared across the transect that likely preserve part of the Ottawan phase of deformation. However, differences in structures between the meta-anorthosite and skarn/host rock gneiss may reflect the preservation of earlier fabrics.