FILLING IN THERMAL AND STRUCTURAL DETAILS OF THE OTTAWAN PHASE OF THE GRENVILLE OROGENY

Titanite from the Adirondack Lowlands-Highlands boundary preserves oxygen-isotope and U-Pb date zoning that can be linked to the thermal and structural regime of the Ottawan event. Titanites occur in the Diana metasyenite, an AMCG-age pluton that also hosts structures and fabrics of the terrane-bounding Carthage-Colton Mylonite Zone (CCMZ). The AMCG intrusive age of the metasyenite can be recovered from the cores of some titanite grains, suggesting that peak Ottawan metamorphic conditions (~700°C near Harrisville, NY) did not persist long enough for complete diffusive Pb loss and age resetting. Diffusional oxygen-isotope zoning in titanite is consistent with rapid cooling from peak Ottawan temperatures to middle amphibolite-facies temperatures (~500 ºC) in 2-5 my. We propose that rapid cooling occurred in response to exhumation accommodated by extension across the CCMZ. This implies episodic, variable-rate collapse of the Ottawan orogen rather than slow, steady exhumation (Bonamici et al. 2015 Chem Geol).

The Harrisville segment of the CCMZ preserves three deformational fabrics, two of which are tied via titanite microstructures and oxygen diffusion histories to Ottawan collapse. The D2 conjugate shear zone system was active at peak Ottawan temperatures and during the subsequent period of rapid cooling. Emplacement of coeval augite-rich veins was structurally controlled by dilation at the intersections of D2 shear zones. D2 shear zones were transposed by the more penetrative D3 foliation, but U-Pb titanite dates on these fabrics are indistinguishable within error. D2 and D3 are thus inferred to have formed closely in time. The difference in structural style between D2 and D3 may indicate a shift from margin-oblique toward margin-orthogonal collapse. Deformational fabrics along the NE-SW-striking Harrisville CCMZ segment can be related to strike-slip structures along N-S-striking Dana Hill CCMZ segment if Ottawan convergence and extension were NW-SE directed relative to a bent Lowlands-Highlands boundary.