PETROLOGY OF BIG GARNET AMPHIBOLITES, NORTH CREEK –WARRENSBURG AREA, ADIRONDACKS, NY

Big garnet amphibolites are well-known in the Adirondacks, most prominently the garnet mine at Gore Mtn. (inactive). Despite a century of work, these rocks is still poorly understood in some respects. We compare four big garnet sites: Gore Mountain, the currently active Barton Mines pit on Ruby Mountain (a garnet-rich gabbroic anorthosite), garnet amphibolites at the Hooper Mine (abandoned), and a big garnet amphibolite in Warrensburg. Despite lithologic differences, all have similar assemblages: garnet – pargasite – OPX – plagioclase ± biotite ± CPX). All minerals generally have little zoning and compositions vary little in single thin sections, or even between deposits (but Warrensburg rocks are more Fe- and Na-rich). At all sites some or most garnets are associated with ‘pegmatitic' patches suggestive of partial melts, but these patches are generally richer in OPX than the matrix and are quite mafic overall, contrary to preliminary Perple_X modeling and experimental work by others that predict more felsic liquids. In all four deposits garnets are typically surrounded by 0.5-3 mm thick symplectite rims of plagioclase – pargasite ± OPX ± biotite, with mineral worms typically ~100 µm across. Symplectite minerals mostly have compositions like those in the matrix, but in some cases symplectite plagioclase is more Ca-rich, indicative of a garnet Ca source without diffusive equilibration with a large rock volume (no fluid phase at that time?). At Hooper mine the symplectites on garnet have smaller, second order plagioclase – OPX symplectites only ~15 µm thick on pargasite. Both probably represent decompression breakdown of garnet and pargasite for the first and second order symplectites, respectively, but high variance of assemblages makes calculation of metamorphic conditions problematic. The Gore Mtn. and Warrensburg garnet amphibolites are hydration zones on the margins of corona gabbros. Whole rock chemistry (Gore Mtn.) shows strong Li enrichment in the garnet amphibolites compared to parent gabbro, and depletion in Cu, Cs, Th, and U. In the corona gabbro-garnet amphibolite transition zone, rocks transitional in position, texture, and mineralogy do not have these enrichments and depletions, indicating higher aH₂O and fluid-enhanced diffusion during amphibolitization, but not substantial fluid flux.