ORIGIN OF BIG GARNET AMPHIBOLITES AT GORE MOUNTAIN AND OTHER LOCALITIES, ADIRONDACK MOUNTAINS, NEW YORK STATE, USA: WHOLE ROCK GEOCHEMICAL CONSTRAINTS

Big garnet amphibolites have been mined in the Adirondacks since 1878. The Barton Mine is well-known for large garnets commonly 15-30 cm across, developed in the southern margin of a metamorphosed post-Shawinigan AMCG suite olivine gabbro (1165-1145 Ma). Previous workers concluded that the ore body and parental gabbro have similar compositions, except for ~2x higher H₂O and Fe³⁺/Fe_total in the ore. Whole rock chemical analyses of gabbro, amphibolite ore, and the transition between show that the ore is highly variable for some trace elements compared to the gabbro. For example, Li is 4x higher in the ore, Cs 0.07x lower, Th 0.15x lower, and concentrations of La, Ce, and Pr are reduced. Whole rock oxygen isotope values from mineral data of other workers indicate a range of δ¹⁸O VSMOW from ~6.8‰ (same as unaltered Adirondack olivine gabbro) to as low as ~4.1‰. Dating by others indicates that ore body garnet amphibolite and gabbro pyroxene granulite assemblages developed simultaneously during Ottawan (~1090-1040 Ma) metamorphism. Thermodynamic calculations show that H₂O-rich metamorphic fluids could not have existed in the ore body during peak Ottawan metamorphism because garnet-plagioclase assemblages would have been unstable and melting would exceed 20%, yielding ore body rock very different from that observed. Instead, H₂O contents at Ottawan high P-T conditions were just sufficient to stabilize the 50-70% hornblende in the ore, plus minor silicate liquid during garnet growth. Variable trace element concentrations suggest channelized fluid flow, and low δ¹⁸O values imply ore protolith hydrothermal alteration by pre-metamorphic meteoric water. High Li concentrations suggest a post-Shawinigan landscape of arid intermontane basins during magma emplacement. The less-studied Warrensburg garnet amphibolite exposure is similar to that at Barton Mine and was probably also derived from a gabbro protolith, but garnets are less abundant and smaller. Trace element differences compared to nearby olivine gabbro include: Li 1.4x higher in the amphibolite, Cs 6x higher, Th 0.6x lower, and reduced La, Ce, and Pr. This differs from Barton Mine but also suggests hydrothermal alteration (but no available oxygen isotopes). Thermodynamic calculations for Warrensburg also indicate no involvement high P-T H₂O-rich fluid.