SLOW COOLING: THE TIMING AND DURATION OF METAMORPHISM IN THE BLUE RIDGE PROVINCE, VIRGINIA AND NORTH CAROLINA

Remnants of the Grenville Orogeny, the pervasive tectonothermal event resulting from the assembly of Rodinia, can be found on almost every continent on Earth, including North America. Among the Grenvillian rocks exposed in eastern North America are the granulites of the central Appalachian Blue Ridge. The origins of these rocks have been the subject of much previous work, but few studies have examined their complex metamorphic history during the Grenville.

As a means to better constrain the timing and duration of metamorphism during the Grenville Orogeny, we determined Lu-Hf and Sm-Nd garnet ages of Mesoproterozoic rocks exposed in Virginia and North Carolina. The granitoids and orthogneisses crystallized between ~1180-1140 Ma_1,2,3. The maximum depositional ages of the paragneisses are between 1100-1020₄. Zircon overgrowths and monazite ages indicate metamorphic events between ~1050 and 960 Ma_1,2.

Our Lu-Hf dates range from 1043 ± 12 Ma to 1016 ± 4 Ma. Sm-Nd dates, determined on the same garnet dissolutions, range from 974 ± 11 Ma to 932 ± 5 M— a large (~40-60 Ma) and systematic difference. One possible explanation for this difference is partitioning of Lu and Sm into different parts of the garnet during prolonged growth resulting in different ages for the two chronometers_5,6. Partitioning of Lu in garnet cores can result in Lu-Hf ages reflecting older core nucleation and Sm-Nd ages reflecting later growth_5,6. However, there is little evidence of chemical zonation in the garnets. Our preferred explanation for the discrepancy in garnet ages is the difference in closure temperature between the two systems with the Lu-Hf system effectively closing at a higher temperature than Sm-Nd. Quartz trace element thermometry performed on these rocks yielded a weighted average temperature of 828±54 ˚C, plausibly above the Sm-Nd closure temperature₇. The age discrepancy can thus be explained if the rocks remained at this high temperature and slowly cooled over millions of years.

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