TIMING OF GARNET GROWTH IN THE EASTERN ADIRONDACK HIGHLANDS: IMPLICATIONS FOR THE TIMING AND INTENSITY OF OROGENIC PULSES

Garnet-rich gneisses with abundant monazite are common in the eastern Adirondack Highlands. Many of these rocks contain evidence for multiple periods of monazite and garnet growth, but the timing and significance of garnet growth events are not well constrained. Thermo-tectonic events in the eastern Adirondack Highlands include the ca. 1170 Ma Shawinigan orogeny, emplacement of the ca. 1150 Ma AMCG igneous suite, the ca. 1070 Ma Ottawan orogeny, and subsequent (<1050 Ma) deformation and plutonism, possibly related to extensional collapse. Determining the age of garnet growth and tectonism is critical to understanding the significance, metamorphic grade, and relative intensity of deformation during these tectonic events. We use yttrium concentration in monazite subdomains and monazite inclusions relationships in garnet to constrain the timing of garnet growth events. Because monazite and garnet are the main yttrium-bearing phases in these rocks, the yttrium concentration in monazite decreases during garnet growth and increases during garnet breakdown. Different compositional layers can have dramatically different bulk yttrium concentrations; therefore care must be taken to compare monazite grains only within compositional layers. Our study area is separated into two domains (northern and southern) based on the timing of garnet growth. Samples from the northern area show a significant decrease in yttrium between 1170 and 1150 Ma, which indicates garnet growth occurred during the late Shawinigan or during the emplacement of AMCG magma. There is little evidence for Ottawan garnet growth in these northern samples. Samples from the southern area show a significant decrease in yttrium at approximately 1070 Ma, and a significant increase at approximately 1050-1030 Ma. These ages suggest significant garnet growth during the Ottawan and some garnet breakdown during post-Ottawan deformation. Garnet breakdown is interpreted to signal the beginning stage of unloading or collapse soon after the Ottawan orogeny. If garnet growth is taken to indicate the timing of significant loading and heating, then our results suggest that the two spatial domains may have distinct tectonic histories, and may have been juxtaposed relatively late in the Grenville cycle.