Constraints on the timing and duration of orogenic events by combined Lu–Hf and Sm–Nd geochronology: An example from the Grenville orogeny

Abstract

Grenville age basement rocks of the Virginia Blue Ridge Province preserve a record of over 150 million years of tectonism and crustal reworking associated with the construction of the supercontinent Rodinia. Although the igneous origins of these rocks have been the subject of much previous work, few studies have examined the complex metamorphic history of these rocks during the Grenville orogeny.To better constrain the timing and duration of metamorphism, we determined Lu–Hf and Sm–Nd garnet and U–Pb zircon rim dates of garnet-bearing rocks exposed in the Virginia Blue Ridge. Multiple periods of Mesoproterozoic metamorphism and deformation have been previously recognized with a major deformational event occurring before ∼1065 Ma and subsequent local episodes occurring at least until 1000 Ma. Zircon overgrowths and monazite dates suggest metamorphic events between ∼1050 and 960 Ma.Our new Lu–Hf garnet dates range from 1032 ± 3 to 1019 ± 3 Ma. New Sm–Nd dates, determined on the same garnet dissolutions, define a younger range, from 965 ± 6 to 949 ± 6 Ma. Our zircon dates are consistent with previously measured ages (∼1090–940 Ma) and often overlap with the Lu–Hf garnet dates from the same samples. There is a large and systematic difference (60–80 m.y.) between the garnet dates from the two isotopic systems. Although Sm–Nd dates slightly younger than those recorded by Lu–Hf are not uncommon, the large discrepancies between the two systems that we see here are unusual. The dated garnets are unzoned and yield Lu–Hf dates that coincide with the zircon overgrowth dates; Sm–Nd ages overlap with the younger monazite dates. We interpret the discrepancy in garnet dates as reflecting the difference in closure temperature between the two isotopic systems. Further, these data require slow cooling following large-scale crustal thickening after the assembly of Rodinia during Grenville orogenesis. These data illustrate that the use of the coupled Lu–Hf and Sm–Nd chronometers in garnet can provide important constraints on the conditions of metamorphism during past orogenic events.