HIGH TEMPERATURE HYDROTHERMAL ALTERATION AND THE ORIGIN OF QUARTZ-SILLIMANITE NODULAR GRANITE NEAR PORT LEYDEN, ADIRONDACK HIGHLANDS, NEW YORK

OLSON, Christopher, and BRACKETT, Chapin, Dept. of Geology, Colgate University, 13 Oak Drive, Hamilton, NY 13346.
colson@center.colgate.edu
cbrackett@center.colgate.edu

In a continuing investigation of the origin of quartz-sillimanite nodules in the Adirondacks, outcrops of water polished nodular granite were mapped at a scale of 1"=10'. The exposures lie along the bed of the Moose River near Lyonsdale Bridge, N.Y. The country rock is a pink, medium grained leucogranite previously dated at 1032ñ7 Ma (U-Pb zircon). The granite hosts a swarm of quartz- sillimanite veins and nodules which commonly comprise ~50% of the outcrop. Also present are zones rich in rounded calc-silicate xenoliths containing scapolite, diopside, grossular, and in at least one case, wollastonite. Although rare, quartzite xenoliths have also been identified. In numerous cases foliation in the xenoliths is crosscut by the granite establishing its intrusive origin. The xenoliths trend N50E, parallel to ellipsoidal, cm- scale quartz-sillimanite nodules. The intervening granite exhibits no grain shape fabric, and the alignment of the inclusions is interpreted to be the result of magmatic flow (established as N50E by AMS). N50E features are crosscut by N20E quartz-sillimanite veins and nodules which are themselves crosscut and disrupted by magma and a late set of N50E quartz-sillimanite veins. The latest set of quart-sillimanite veins have no regular orientations and exhibit very ductile features. The sequence of veins an nodules establishes their formation within the granitic matrix rather than as incorporated material. They are crosscut by weakly deformed quartz-albite veins and by an undeforemd pegmatite (1031ñ4 Ma) thus fixing their time of origin within the late magmatic history of the granite. The configuration of the veins and nodules, including folds that commonly exhibit disharmonic patterns, reflects a "soft" rheology during their evolution. Absence of grain shape fabric or foliatin in the granite indicates that it contained a significant liquid component during vein formation, disruption, etc.. The veins and nodules are interpreted to be the result of cation leaching by hydrothermal fluids active at pressures and temperatures within the sillimanite stability field. HCL and H2CO3 facilitated the transport of aluminum over relatively short distances whereas Na, K, Ca, Si were transported much farther. Veins of bull quartz (30-40m width) reflect Si transport and the presence of sericite alteration within them is interpreted to be the result of K-alteration.

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