CONSTRAINTS ON THE TIMING AND DURATION OF THE GRENVILLE OROGENY
To better constrain the timing and duration of metamorphism–and identify latitudinal variation in the temporal record if it exists–we determined Lu-Hf and Sm-Nd garnet and U-Pb monazite ages of garnet-bearing rocks exposed in the Virginia Blue Ridge. Multiple periods of Mesoproterozoic metamorphism and deformation have been previously recognized with a major fabric-forming event occurring prior to ~1065 Ma and subsequent local episodes occurring until at least 1000 Ma. Zircon overgrowths and monazite ages suggest metamorphic events between ~1050 and 960 Ma.
Our new Lu-Hf garnet dates range from 1088 ± 27 to 1019 ± 3 Ma. Sm-Nd dates, determined on the same garnet dissolutions, define a narrower range of dates from 954 ± 14 Ma to 949 ± 6 Ma. Our monazite dates are consistent with previously measured ages (~1050 Ma) and often overlap the Lu-Hf garnet dates from the same samples. The crystallization ages of the rocks analyzed (U-Pb zircon) are as much as 130 my different. There is a large and systematic difference (80-60 Ma) in the garnet dates from the two systems. Although Sm-Nd ages slightly younger than those recorded by Lu-Hf are not uncommon, the large discrepancies between the two systems that we see here are atypical. While there are multiple explanations for such a difference between the two chronometers, the garnets are typically unzoned and yield Lu-Hf ages that coincide with the zircon overgrowth and monazite ages. Our interpretation is that the discrepancy in garnet ages reflects the difference in closure temperature between the two isotopic systems, with Lu-Hf closing at a higher temperature than Sm-Nd as the rocks slowly cooled from high temperatures following orogenesis. These new dates are consistent with a major tectonothermal event during the Ottawan orogeny and indicate that regional metamorphism in the Blue Ridge occurred later than previously suggested.