HR: 1330h
AN: V32C-1032    [PDF]
TI: Garnet, zircon, and apatite (U-Th)/He thermochronometry of the Ocna de Fier (Romania) skarn ore district
AU: * Nicolescu, S
EM: stefan.nicolescu@yale.edu
AF: Yale University, Geology and Geophysics, PO Box 208109, New Haven, CT 06520-3421 United States
AU: Reiners, P W
EM: peter.reiners@yale.edu
AF: Yale University, Geology and Geophysics, PO Box 208109, New Haven, CT 06520-3421 United States
AB: The Ocna de Fier (ODF)- Dognecea ore district in SW Romania, formed at the contact of Upper Cretaceous granodiorites (75.5 $\pm$ 1.6 Ma SIMS U-Pb zircon age; 76.6 $\pm$ 0.3 Ma Re-Os molybdenite age) with Upper Jurassic - Lower Cretaceous limestones, has been continuously mined for more than 4000 years, and is part of the group of deposits first recognized as being of contact-metasomatic (skarn) origin. Due to its ore reserves, mineral wealth (in excess of 140 valid minerals, including three new mineral species) and genetic type, for the last 250 years the deposits have been the focus of a wide interest in the scientific community, resulting in close to 300 district-related papers published to date. To better constrain the medium- and low-temperature thermal history of the ODF and examine the utility of skarn minerals for (U-Th)/He dating, we have measured He ages of garnet, epidote, zircon, and apatite from this deposit and for zircon and apatite from the adjacent (4 km north) Bocsa intrusive. Replicate zircon U-Th/He (ZHe) ages of 76 $\pm$ 6 Ma for the ODF intrusive and 71 $\pm$ 6 Ma for the related Bocsa intrusive, are indistinguishable from or slightly less than U/Pb, Re/Os, and zircon fission track (ZFT) age of 72 $\pm$ 6 Ma, suggesting very rapid cooling ($\sim$$83\deg$ C/Myr) through $\sim$$200\deg$ C. However, for apatite the difference between fission track (AFT) and U-Th/He (AHe) ages is significant: 68 $\pm$ 5 Ma vs. 36 $\pm$ 2 Ma respectively, suggesting a protracted cooling through low temperatures. These data suggest that following intrustion and skarn formation, the region experienced extremely slow cooling ($<$ $2\deg$ C/Myr) until at least $\sim$30 Ma. We have also developed analytical methods for (U-Th)/He dating of garnet, involving laser He extraction in Nb foil that is subsequently dissolved along with post-degassing garnet residue for U-Th measurement. Multiple replicates of interior cores of large ($\sim$1 cm) grandite garnets using this method yielded strongly contrasting U contents (20 vs. 0.3 ppm), but similar (U-Th)/He ages of 75.2 $\pm$ 2.5 Ma and 77.9 $\pm$ 4.3 Ma, respectively. Due to the apparently rapid cooling of this system, we cannot estimate a He-in-garnet closure temperature from these results, except to say that it is greater than about $200\deg$ C. Preliminary data on hydrothermal epidote from the deposit, using the same procedures, show poor reproducibility. Epidote He ages on six aliquots range from 15-29 Ma. Although we currently have no explanation for the discrepant epidote results, analyses are ongoing, and preliminary indications suggest a relatively low closure temperature for this phase.
DE: 1035 Geochronology
DE: 1040 Isotopic composition/chemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting