MONAZITE PETROCHRONOLOGY OF ADIRONDACK LOWLANDS MIGMATITES: IMPLICATIONS FOR THE SIGNIFICANCE OF THE CARTHAGE-COLTON MYLONITE ZONE

The Adirondack Mountains in New York preserve a classic exposure of the hinterland of the Grenville orogenic belt. The region has been broken into two major terranes, termed the Highlands and the Lowlands, separated by the Carthage-Colton Mylonite Zone (CCMZ). The CCMZ is interpreted to have accommodated early thrusting and later extensional collapse that ultimately led to the juxtaposition of the two terranes. Previous zircon geochronology and recent monazite petrochronology of migmatites from the Highlands indicate that melting occurred twice, first at ca. 1150 Ma and second during the 1050 Ma Ottawan phase of the Grenville Orogenic Cycle. The 1150 Ma event may correlate with the period of AMCG magmatism that marks the end of the (1190-1140 Ma) Shawinigan orogeny. In order to compare the tectonic history on both sides of the CCMZ, migmatitic samples from the Popple Hill Gneiss were collected in several locations in the Lowlands for petrochronologic study. Full-section compositional maps were used to locate all monazite grains; high-resolution maps of each grain were used to define monazite domains/generations for dating on the Ultrachron electron microprobe. Core domains are characterized by decreasing Y, interpreted to reflect garnet growth during biotite dehydration melting. These domains yield dates of 1180-1170 Ma, seemingly older than the age of major melting in the Highlands. These data suggest that Shawinigan melting in the Lowlands may not be synchronous with melting in the Highlands, and thus, the CCMZ may juxtapose terranes that have distinct tectonic histories extending as far back as the Shawinigan Orogeny.