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| Titel |
Lu-Hf and Sm-Nd geochronology of garnet gneisses in the central Appalachians, U.S.: Implications for the timing and duration of Grenville Orogeny |
| VerfasserIn |
Jeff Vervoort, Molly Ramsey, Sean Mulcahy, John Aleinikoff, Scott Southworth |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250100219
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| Publikation (Nr.) |
 EGU/EGU2014-16111.pdf |
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| Zusammenfassung |
| The Grenville orogeny is one of the most significant geological events in Earth’s history with
remnants of this event prominent on virtually every continent. Constraining its timing and
duration is important not only for understanding the tectonics of the Grenville itself, but also
for understanding supercontinent cycles and other questions of Earth’s evolution. In order to
provide better constraints on the timing of Grenvillian metamorphism, we analyzed
garnet-bearing Mesoproterozoic ortho and paragneisses, collected along a 150 km
transect in the northern Blue Ridge Province, using combined Lu-Hf and Sm-Nd
geochronology. The orthogneisses have U-Pb zircon crystallization ages of ~1140 and
1100 Ma. The paragneisses have maximum depositional ages ~1050 to 1020 Ma,
based on the youngest detrital zircon populations. Zircon overgrowths and monazite
ages suggest metamorphic events between ~1050 and 960 Ma. The Lu-Hf and
Sm-Nd data for these samples both yield robust garnet ages with large spread of
parent/daughter ratios, low age uncertainties, and low MSWD values. Lu-Hf ages define a
narrow time span (1043±12 Ma to 1016±4 Ma; wtd. mean, 1024±7 Ma, 2Ïă).
The Sm-Nd ages, determined on the same solutions as Lu-Hf, also define a narrow
time range but are systematically younger (974±11 Ma to 932±5 Ma; wtd. mean,
957±10 Ma). The average difference between Lu-Hf and Sm-Nd ages is 67 Ma; the
oldest Sm-Nd age is 40 Ma younger than the youngest Lu-Hf age. These large
systematic differences in the ages are enigmatic. While Sm-Nd ages younger than
Lu-Hf are not uncommon, these differences are typically small. There are, however,
potential explanations for these differences. (1) Lu partitions strongly into garnet
during growth resulting in high Lu/Hf ratios in the core and yielding ages weighted
toward the beginning of growth (e.g., Skora, 2006); no similar partitioning exists in
Sm/Nd and these ages reflect mean garnet growth. (2) Lu diffuses much faster than
Hf at elevated temperatures resulting in Lu diffusing from high Lu regions after
garnet formation, potentially leading to anomalously old ages (e.g., Ganguly et al.,
2011). (3) The Lu-Hf system has a higher closure temperature than Sm-Nd (e.g.,
Scherer et al., 2001; Smit et al., 2013) and younger Sm-Nd ages could reflect a
later start of their isotopic clocks. Based on our data, the first two explanations
are unlikely to generate large and systematic differences in the ages. None of the
Blue Ridge garnets have significant Lu/Hf or Sm/Nd zoning, which likely indicates
equilibration of the garnets subsequent to their growth; differences in elemental
partitioning during garnet growth cannot explain the age differences. The Lu-Hf ages,
while much older than the Sm-Nd ages, are not anomalously old and overlap with
the timing of zircon growth. Ti-in-quartz thermometry performed on 7 samples
yield a weighted average temperature of 828±54°C which broadly overlaps with
estimates from the Ti-in-zircon thermometer by Tollo et al., (2010) of 740±40°C.
Therefore, we interpret the younger Sm-Nd ages as due to differences in closure
temperatures; the Lu-Hf system closed soon after garnet growth at ~1024 Ma whereas
Sm-Nd closed at ~ 970 to 930 Ma. These data require that the rocks remained
at elevated temperatures and pressures for tens of millions of years, presumably
deep within thickened crust, during the culmination of the Grenvillian orogeny. |
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