2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 174-7
Presentation Time: 4:30 PM


JOHNSON, Thomas A., School of the Environment, Washington State University, Pullman, WA 99163, RAMSEY, Molly J., Washington State University, PO Box 642812, Pullman, WA 99164, VERVOORT, Jeffrey D., School of the Environment, Washington State University, Pullman, WA 99164, MULCAHY, Sean R., Geology Department, Western Washington University, Bellingham, WA 98225, SOUTHWORTH, Scott, U.S. Geological Survey, MS 926A National Center, Reston, VA 20192-0001 and ALEINIKOFF, John N., US Geological Survey, MS 963, Denver, CO 80225, thomas.johnson@wsu.edu

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 Ma1,2,3. The maximum depositional ages of the paragneisses are between 1100-10204. Zircon overgrowths and monazite ages indicate metamorphic events between ~1050 and 960 Ma1,2.

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 chronometers5,6. Partitioning of Lu in garnet cores can result in Lu-Hf ages reflecting older core nucleation and Sm-Nd ages reflecting later growth5,6. 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.

[1] Aleinikoff et al., Precam. Res. 99 (2000) 113-146. [2] Tollo et al., GSA Mem. 197 (2004) 647-678. [3] Tollo et al., Precam. Res. 151 (2006) 224-264. [4] Southworth et al., GSA Abstr. w. Prog. 40, 4 (2008) 18. [5] Lapen et al., EPSL 215 (2003) 57-72. [6] Skora et al., Con. Min. Petr. 152 (2006) 703-720. [7] Smit et al., EPSL 381 (2013) 222-233.