Spectacular exposures of the world's largest megacrystic garnets (to 35 cm diameter) occur in a coarse-grained amphibolite at the Barton Garnet Mine in the Adirondack Highlands (Gore Mountain, New York State, USA). Over the years, numerous geologists have concluded that the large size of the garnets resulted from an influx of fluids during ca. 1050 Ma upper amphibolite facies metamorphism of a ca. 1155 Ma olivine metagabbro. The presence of fluids under such mid-crustal pressure-temperature conditions is anomalous and warrants explanation. Evidence indicates that the fluids were introduced along, and close to, a steep border fault that juxtaposes charnockite against the garnet ore at the southern margin of the mine. Granitic pegmatites and quartz veins are present in the border zone and locally intrude the garnet ore.

Geochronology has played a critical role in resolving the genesis of the Gore Mountain garnets. Over the past 20 yr Sm-Nd and Lu-Hf techniques have been used to date the crystallization of the garnets as 1049 ± 5 Ma, an age that coincides with the termination of the contractional phase of the Ottawan orogeny, the onset of extensional orogen collapse, and the emplacement of the Lyon Mountain Granite. New U-Pb zircon age determinations of 1045 ± 7.5 (Barton Garnet Mine) and 1055 ± 7.4 (New York State Route 3 near Cranberry Lake) for Lyon Mountain granite pegmatites directly associated with megacrystic garnet amphibolites corroborate the synchronicity of emplacement of Lyon Mountain magmas and the growth of the garnet megacrysts.

It is argued that during the ca. 1050 Ma extensional collapse of the Ottawan orogen, fluids gained access to extensional fault networks and interacted with country rocks. We further suggest that increasing temperature due to advected heat from Lyon Mountain Granite carried the Gore Mountain megagarnet amphibolite into granulite facies conditions that resulted in reactions between hornblende and garnet that produced orthopyroxene and calcic plagioclase intergrowths, both as symplectites and coarsely textured pods developed in pressure shadows. Geothermal modeling of garnet zoning in metapelites (Storm and Spear, 2005) and oxygen isotope zoning in titanite (Bonamici et al., 2011) require a short period of rapid cooling ca. 1050 Ma, which we interpret to be related to the extensional collapse of the Ottawan orogen at that time (Rivers, 2008; McLelland et al., 2010a, 2010b). Reconnaissance of the southern and central Adirondacks reveals that a number of megacrystic garnet occurrences similar to those at Gore Mountain are present in areas that contain both metagabbros and megacrystic garnet amphibolites, and we propose that all of these formed during orogen collapse, intrusion of Lyon Mountain Granite, and fluid-related alteration at high temperature.

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