THERMOCHRONOLOGY AND DEFORMATIONAL HISTORY OF METAPELITES FROM THE SOUTHWESTERN ADIRONDACKS: ELECTRON MICROPROBE ANALYSIS OF MONAZITES

In situ electron microprobe elemental U-Th-Pb analyses of monazite grains from multiply deformed metapelites in the southwestern Adirondack Mountains have revealed a complex thermal history over at least 200 Ma. Cores, collected with a portable diamond core drill, were taken down the axial plunge of folds so that sections cut parallel to the cores would lie in primary (S-1) foliation and sections cut normal to the cores would show later, axial-planar (S-2) foliation. After monazite grains were located by Ce mapping of polished thin sections, selected grains were mapped individually for the distribution of Y, Th, Ca, and U with a Cameca SX50 electron microprobe to identify compositional domains. These domains were then analyzed for U, Th, and Pb following the protocols of Jercinovic and Williams (2005). Two hundred and seventy-four in situ analyses on seventeen individual grains revealed as many as three age domains for single monazite grains and a total of four well-defined age domains throughout the seventeen grains. These age domains include 1190-1170 Ma and 1150-1110 Ma, commonly involving the cores of monazite grains, and 1090-1020 Ma and 1000-970 Ma, commonly derived from rims and embayments. These age domains indicate monazite growth during the Shawinigan orogeny (1190-1170 Ma) followed by further growth during or just post intrusion of the AMCG suite (1150-1110 Ma); new monazite growth during high-grade metamorphism associated with Ottawan orogeny (1090-1020 Ma); and finally a period of monazite formation in the interval 1000-970 Ma, perhaps indicating thermal or hydrothermal activity during the Rigolet orogeny. Because almost all of the monazite grains with age domains greater than 1100 Ma occur in sections cut parallel to the cores, we believe that the primary S-1 foliation was formed during Shawinigan events whereas secondary, axial planar (S-2) foliation was developed during later Ottawan folding.