Thermal Modeling as a Test for Internal Consistency Between Thermo- and Geochronology and Thermobarometry: Application to the Acadian Orogeny in SE Vermont.

HR: 1340h
AN: V23C-1550 [Abstracts]
TI: Thermal Modeling as a Test for Internal Consistency Between Thermo- and Geochronology and Thermobarometry: Application to the Acadian Orogeny in SE Vermont.
AU: * McWilliams, C K
EM: ckmcwill@indiana.edu
AF: Indianan University, Dept. of Geological Sciences 1001 E. 10th St., Bloomington, IN 47405, United States
AU: Kunk, M J
AF: U.S. Geological Survey, MS 926A, National Center, Reston, VA 20192, United States
AU: Person, M A
AF: Indianan University, Dept. of Geological Sciences 1001 E. 10th St., Bloomington, IN 47405, United States
AU: Walsh, G J
AF: U.S. Geological Survey, P.O. Box 628, Montpelier, VT 05602, United States
AU: Wintsch, R P
AF: Indianan University, Dept. of Geological Sciences 1001 E. 10th St., Bloomington, IN 47405, United States
AB: We develop a one-dimensional thermal model integrating U-Pb geochronology, ⁴⁰Ar/³⁹Ar thermochronology, and thermobarometry to test tectonic models for internal consistency in a classical region of the northern Appalachians. This model is based on the finite-difference method utilizing an implicit formulation with steady state topography (i.e. sedimentation keeps pace with subsidence and erosion keeps pace with uplift). The Chester and Athens domes of Vermont are Acadian structures cored by Proterozoic gneisses, flanked by Cambrian and Ordovician metamorphic rocks, and unconformably overlain by Silurian-Devonian metasedimentary rocks. The polymetamorphic history of these rocks includes the Grenville (>1 Ga), the Taconic (470-440 Ma), and the Acadian (410-370 Ma) orogenies. ⁴⁰Ar/³⁹Ar cooling ages from core rocks define continuous late Devonian through Carboniferous cooling at ~3.5 ^oC per m.y. Multiple U-Pb SHRIMP analyses of detrital zircons from Devonian Gile Mtn. fm. quartzites yield an age of 409 ± 5 Ma, interpreted as a maximum age of deposition, thus fixing the earliest time at which tectonic loading rates (>2 mm/yr) can commence. Grt-Ms-Bt-Pl and Amp-Pl thermobarometry from core rocks yields temperatures of 650- 700 ^oC and pressures of 13-16 kbars defining a geothermal gradient of 12-13 ^oC per km. In the first model, loading-unloading schedules consistent with detrital zircon ages and ⁴⁰Ar/³⁹Ar data input for a one-dimensional model simulate the Acadian orogeny. Results of this model fail to reach temperature estimates by ~20% and pressure estimates by ~50%. A second model accommodating the Taconic and Acadian orogenies does reach maximum temperatures of ~650 ^oC but still fails to reach pressure estimates by ~50%. Multiple iterations of the second model using a wide variation in geophysical parameters suggest minimum pressure estimates can not be reached given the limited time between deposition of cover rocks (~410 Ma) and the end of metamorphism (⁴⁰Ar/³⁹Ar cooling age of amphibole ~374 Ma). Due to similar time constraints imposed on the Taconic orogeny (e.g. Karabinos et al., 1998, Laird et al., 1984), we conclude that P-T estimates obtained in core rocks of the Chester dome are likely inherited from the Grenville orogeny.
DE: 1140 Thermochronology
DE: 3651 Thermobarometry
DE: 3652 Pressure-temperature-time paths
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting