MAGMATIC DIFFERENTIATION AND MONAZITE GEOCHRONOLOGY OF CALC-ALKALINE METAVOLCANICS AND METAINTRUSIVES IN THE ADIRONDACK LOWLANDS, NEW YORK

Mesoproterozoic rocks of the Adirondack Lowland portion of the Grenville Province consist of three NE-SW trending lithostratigraphic units, the Lower Marble, Popple Hill Gneiss (PHG), and Upper Marble, that were metamorphosed (peak T = 710-740^oC, P ~ 6-7 kb) by multiple thermotectonic events during a protracted period of collisional and post-orogenesis. They were intruded by a variety of syn-orogenic, chemically heterogeneous, calc-alkaline magmas, including the Hermon Granite (HG). The PHG is primarily a biotite-quartz-two feldspar gneiss that is locally migmatitic. Variation diagrams for ~100 samples collected along three traverses, yield magmatic differentiation trends, with compositions ranging from rhyolite to andesite. This data supports the interpretation that the PHG is a calc-alkaline metavolcanic rather than a metapelite, and that these units represent carbonate and volcanic deposition into an arc-basin. U-Th-Pb dating of 26 polygenetic monazites (957 analyses) indicates that the PHG contains inherited Elzevirian orogeny xenocrysts or detritus (~1270-1240 Ma) and experienced multiple thermotectonic events, ~1195-1150 Ma (Shawinigan-orogenesis and possibly the impact of AMCG magmatism), ~1115-1110 Ma (only found locally when PHG is intimately associated with HG), and ~1085-1080 Ma (early Ottawan-orogenesis). With the exception of the ~1115-1110 Ma ages, these results are consistent with those reported by others. The HG is a chemically and texturally heterogeneous intrusive predominantly within the PHG. It ranges from granite to tonalite and from megacrystic to fine-grained. Despite this textural disparity, variation diagrams from ~90 samples yield differentiation trends compatible with those for the PHG. U-Th-Pb dating of 11 polygenetic monazites (325 analyses) has identified three age domains: ~1150 Ma (Shawinigan/AMCG), ~1120 Ma, and ~1060 Ma (Ottawan). Structurally, the bulk of the HG is the shallowest of the calc-alkaline intrusives within the Lowlands and the lack of Elzevirian ages in the HG suggests that it was derived from melting of a different source, possibly PHG. The ~1120-1110 Ma monazite ages in some HG and PHG may be the result of very late Adirondack AMCG anatectic magmatism and contact metamorphism, mixing, or very early Ottawan regional thermal activity.