NEW EVIDENCE FOR THE ORIGIN OF THE HERMON GRANITE SUITE BY EXTENSIVE PARTIAL MELTING OF THE POPPLE HILL GNEISS IN THE ADIRONDACK LOWLANDS, NEW YORK

New partial melt modeling provides evidence that the Hermon Granite (HG) suite in the Adirondack Lowlands could have been derived by extensive partial melting of its primary host, the locally migmatitic, biotite-quartz-two feldspar Popple Hill Gneiss (PHG). HG primarily occurs as continuous layers, intimately interwoven and regionally concordant with the NE-SW fabric of the PHG. The HG granitic gneiss texturally varies from megacrystic, porphyritic to fine-grained, equigranular, well foliated varieties with silica content ranging from 53-76 wt. %. Silica content for the majority of the PHG is more restricted, ranging from 65-75 wt. %, but these lithologies are almost indistinguishable on most chemical discrimination diagrams and both produce well delineated calc-alkaline trends. This suggests a petrogenetic relationship, with the HG derived by anatexis of PHG and that melt fractionation and/or assimilation of other lithologies modified the composition of the resulting HG. To test this, partial melt modeling (Winther, 1995) was used to estimate the composition of a partial melt (HG) that could be produced from the PHG. Remarkably compatible modeling results were achieved at 1000-1050^oC, 0.5-1.5 GPa, and <1.5 wt. % water for most of the HG, but the full range of silica was not achieved, again suggesting that subsequent fractionation, assimilation, and/or magma mixing may have occurred. These temperatures indicate that well over 50% of the PHG was melted, which is consistent with field observations that the volume of HG, based on surface area, may occupy 35-50% of the PHG lithostratigraphic unit. This further leads to the proposal that the HG could represent thick sills (possibly 100’s of meters) derived by anatexis in the folded roots of the PHG, with underplating of mantle melts providing some of the heat as well as some melt fraction could have mixed with the PHG partial melt. U-Th-Pb monazite geochronology further supports this model because the HG does not yield Elzevirian (ca. >1250 Ma) and early Shawinigan (ca. 1190 Ma) ages found in the PHG, but does yield middle and late Shawinigan (ca. 1150 Ma and 1115 Ma) and Ottawan (ca. 1080-1065 Ma) ages found in both lithologies, indicating that partial melt temperatures capable of producing large volumes of HG melt were not achieved until the climax of the Shawinigan orogeny.