COMPARATIVE ANALYSES OF THE WARRENSBURG AND GORE MOUNTAIN BIG-GARNET AMPHIBOLITES, ADIRONDACK MOUNTAINS, NY

This study presents a detailed comparison of the mineralogy and geochemistry of the Warrensburg and Gore Mountain big-garnet amphibolites as a way to further constrain the formation of mega-garnet deposits in the Adirondacks.  While the large garnets of Gore Mountain are world-famous, big-garnet amphibolites at other localities have been largely overlooked in the Adirondack literature.  Located approximately 30 km southeast of Gore Mountain, the Warrensburg big-garnet amphibolite occurs at the edge of a meta-gabbro body, with excellent 3-dimensional exposures in two roadcuts.  This occurrence is associated with several syenite dikes, and garnetiferous anorthosite thought to be correlative with the Buck Mt. anorthosite.  While hornblende-rimmed porphyroblasts up to 10 cm across are found at this locality, most garnet occurs in pegmatite-like coarse patches of plagioclase and OPX set in a hornblende-plagioclase-OPX-biotite ± CPX matrix.

From SEM-EDS microanalysis, we have determined chemical compositions for the major minerals of the Warrensburg and Gore Mountain occurrences.  Warrensburg garnet has an average composition of Al₆₂Py₂₃Gr₁₃Sp₃.  The matrix hornblende has an average composition of (Na_0.57K_0.11)Ca_1.6(Mg_2.7Fe_2.7Mn_0.04Ti_0.20)(Si_5.8Al_2.4)O₂₂ (OH)₂.  The average composition of matrix plagioclase is An₄₃, nearly identical to plagioclase in the coarse patches with garnet.  The average OPX composition is En₅₂ in the matrix, and En₅₄ in the coarse patches.  By comparison, the average garnet composition at Gore Mountain is Al₄₅Py₃₉Gr₁₃Sp₂.  The Gore Mountain matrix hornblende has an average composition of  (Na_0.70K_0.16)Ca_1.8(Mg_3.1Fe_1.5Mn_0.08Ti_0.16)(Si_6.0Al_2.4)O₂₂ (OH)₂.  The average matrix plagioclase is An₅₉, and OPX is En₆₅.

We conclude the following: 1) mineral compositions do not vary significantly between the Warrensburg matrix and coarse patches, 2) the Warrensburg big-garnet amphibolite is more Fe-rich than that at Gore Mountain, and 3) plagioclase is more Na-rich at Warrensburg than at Gore Mountain.  We will use the mineralogy and bulk chemistry at Warrensburg and Gore Mountain as the basis for thermodynamic modeling, with the goal of testing whether silica-rich partial melt was involved in the formation of the big-garnet porphyroblasts.