PETROLOGIC AND GEOCHRONOLOGIC EVIDENCE FOR THE SHAWINIGAN OROGENY, INTRUSION OF AMCG-RELATED GABBRO, THE OTTAWAN OROGENY, AND POST-OTTAWAN ACTIVITY IN ROCKS FROM A SINGLE OUTCROP, EASTERN ADIRONDACKS, NEW YORK

High grade garnet-sillimanite gneisses, coronitic olivine metagabbro, and metacarbonates are well exposed in a single outcrop along Hwy 22 in the Putnam quadrangle north of Whitehall, NY. The metagabbro has a clear chilled margin where it is in contact with the garnet-sillimanite gneiss demonstrating that at this locality the gabbro intruded the gneiss. The gneiss-gabbro contact is also discordant to the foliation in the gneiss demonstrating that the foliation in the gneiss formed prior to the intrusion of the gabbro. McLelland et al. (1988) noted the above and reported a U-Pb zircon age from the metagabbro of 1144±7 Ma.

Histograms of microprobe monazite geochronologic data from the garnet sillimanite gneisses in contact with the metagabbro reveal several distinct periods of monazite growth. The oldest period of monazite growth is 1179±9 Ma. We suggest this time is correlative with the Shawinigan Orogeny and corresponds to period of granulite facies metamorphism and fabric development observed in the garnet sillimanite gneisses. Monazites from these gneisses also show evidence for period of growth at 1151±9 Ma. This suggests a period of recrystallization coincident with the intrusion of the AMCG-related gabbro. Interestingly there is virtually no Ottawan (~1050 Ma) signature recorded by monazite growth in the garnet-sillimanite gneiss. There is however, a 1030-1000 Ma phase of monazite growth suggesting some late- to post-Ottawan activity.

The metagrabbroic rocks show a limited amount of recrystallization thought to be related to the Ottawan orogeny. The original igneous mineral assemblage of Pl+Cpx+Ol is recrystallized to the granulite facies mineral assemblage of Grt+Opx+Cpx. Olivine in these rocks typically has a corona of finely crystalline orthopyroxene. Plagioclase crystals with recrystallized margins are typically surrounded by fine crystals of garnet. Cpx is partially replaced by late amphibole and this is followed by later biotite growth. These textural features suggest an influx of H20-rich fluid, possibly during late- to post-Ottawan extension and uplift.