The Role of Fluids In the Genesis of Megacrystic Gore Mountain Type Garnet Amphibolite, Adirondack Mountains, New York

Gore Mt mine is the type locality for megacrystic garnet crystals (average diameter ~20cm, largest ~1m). Commonly garnets are rimmed by hornblende and distributed in plagioclase-hornblende-garnet host rocks. Sm-Nd isochron ages are 1059 &plusmn 19 Ma and 1051 &plusmn 4 Ma (garnet core), Lu-Hf 1047 &plusmn 6 Ma. The protolith was 1155 Ma olivine metagabbro, but why the extreme grain size? Clues reside in a common feature of Adirondack megagarnet amphibolites:proximity to late pegmatite-filled faults. The best-exposed example is at Gore Mt Mine where the vertical charnockitic south wall is juxtaposed against the ore deposit along a fault that contains late pegmatite and quartz veins cutting the ca 1050 Ma ore. Away from the fault megagarnet amphibolite grades into coronitic metagabbro with mm-scale garnets around mafic cores. At Speculator and Cranberry Lake minimally deformed pegmatites occur in faults cutting the gabbroic host and are similar to pegmatites dated at 1038 &plusmn 8 Ma at Lyons Falls. At Warrensburg the garnetiferous amphibolite is cut by a coarse quartz-oligoclase pegmatite. It is proposed that at upper amphibolite facies dry, fine-grained garnet coronites were ruptured by steep faults along which ca 1050 Ma Lyon Mt Granite ascended releasing deuteric fluids that crystallized into pegmatites and quartz veins releasing H2O into the hot (~ 600-700 C) country rocks. Original coronites recrystallized into garnet-hornblende-plagioclase assemblages, but because of the highly unusual combination of fluids and high temperature the resulting grain size was extraordinarily large. Veins and layers of megagarnet represent fluid pathways along fractures. Due to an excess of hornblende-forming reactants, black coarse hornblende lag deposits enveloped the megagarnets. As temperatures rose into granulite grade, garnet + hornblende reacted to calcic plagioclase + orthopyroxene and locally plagioclase-orthopyroxene melts may have been produced. These events accompanied the late-tectonic collapse of the Ottawan Adirondacks.