Coupling thermodynamic modeling and high-resolution in situ LA-ICP-MS monazite geochronology: evidence for Barrovian metamorphism late in the Grenvillian history of southeastern Ontario

摘要

The Flinton Group is a greenschist to upper amphibolite facies package of metasediments in southeastern Ontario that was metamorphosed during the Ottawan Orogeny. Thermodynamic modeling of metapelitic mineral assemblages suggests an increase in peak conditions of metamorphism across the 40 km wide study area from 3.5 to 7.9 kbar and 540 to 715 degrees C. Garnet isopleth thermobarometry applied to the cores of compositionally zoned porphyroblasts reveals remarkably similar P-T conditions of initial crystallization at approximately 3.7-4.0 kbar and 512-520 degrees C, corresponding to a relatively high geothermal gradient of ca. 34-45 degrees C km(-1). It is inferred from modeling and reaction textures that metamorphism was along Barrovian P-T paths. Major and trace element zoning in garnet from one sample records a complex growth history as evidenced by major and trace element zoning and the distribution of xenotime, allanite and monazite inclusions. High-resolution (6 mu m) LA-ICP-MS U-Pb geochronology performed on monazite in the rock matrix and included in the outer 150 mu m of garnet rim-ward of a Y annulus revealed an age of 976+/-4 Ma. The age is interpreted to reflect monazite growth at the expense of allanite and apatite late in garnet's growth history over the P-T interval 4.5-6.8 kbar and 540-640 degrees C. This new age estimate for near peak metamorphism fits well into the regional framework but is significantly younger than previously reported ages for Ottawan metamorphism. Based on microstructures this new age suggests that compressional tectonics were operating much later in the history of the Grenville of southeastern Ontario than previously thought.