IDENTIFYING THE SEQUENCE OF MULTIPLE FLUID EVENTS IN METAMORPHIC TERRANES: AN EXAMPLE FROM THE LYON MOUNTAIN GNEISS, ADIRONDACK MOUNTAINS, NEW YORK

The combination of whole rock geochemistry, cathodoluminescence (CL) imaging and cross-polarized light petrography, and calculation of the chemical index of alteration (CIA) has proven useful in recognizing the type and sequence of fluid alteration in igneous and metamorphic terranes. The Lyon Mountain Gneiss (LMG) in the Adirondack Mountains of New York State is the host of economic Fe-oxide deposits and was extensively altered by K and Na metasomatism Whole rock geochemistry shows that the LMG may contain up 10 weight percent K or Na but not both in the same sample. This indicates that alteration took place by cation substitution in feldspar. Samples away from alteration zones have average granite alkali ratios, and contain perthitic feldspar consisting of microcline with albite lamellae. Extended periods at elevated temperatures are required for Al-Si ordering in feldspar and the development of microcline. This need for very slow cooling is suggestive of a metamorphic environment. These perthites are texturally consistent with other regional samples that have undergone metamorphism. High K samples plot well outside the field of average granites on a CIA diagram. The presence of microcline instead of adularia as the main K bearing phase indicates alteration was prior to metamorphism. High Na samples are nearly pure albite, posses a red luminescence in CL due the presence of Fe3+, and crosscut both the microcline-rich facies and the perthitic facies. Late hematite alteration on magnetite supports a late oxidizing environment for the Na fluid. These results suggest that the LMG experienced at least two periods of fluid alteration (one or both may be related to the Fe mineralization) and that Ottawan metamorphism in the Adirondacks lasted longer than has been previously reported. The termination of Ottawan metamorphism in the Adirondacks was proposed to have ceased no later than ~1040 Ma and that the rocks in the present study were post-orogenic and undeformed.