2004 Denver Annual Meeting (November 7–10, 2004)

Paper No. 8
Presentation Time: 9:45 AM


DAHL, P.S.1, HAMILTON, M.A.2, WOODEN, J.L.3, TRACY, R.J.4, LOEHN III, C.W.1, JONES, C.L.1 and FOLAND, K.A.5, (1)Dept. of Geology, Kent State Univ, Kent, OH 44242, (2)Dept. of Geology, Univ of Toronto, Toronto, Canada, (3)USGS-SUMAC, Stanford Univ, Ion Probe Lab, 367 Panama Mall, Stanford, CA 94305, (4)Dept. of Geological Sciences, Virginia Tech, Blacksburg, VA 24061, (5)Geological Sciences, Ohio State Univ, Columbus, OH 43210, pdahl@kent.edu

Monazite (Mnz), titanite (Ttn), and zircon (Zrn) in crystalline rocks from opposite margins of the Archean Wyoming craton have been analyzed by electron and ion microprobe (EMP, IMP) techniques in order to decipher their Early Proterozoic thermotectonic histories. Along the NW margin (Tobacco Root Mtns., SW MT), a quartzofeldspathic gneiss sample contains metamorphic Mnz and Zrn occurring as elliptical matrix grains sub-aligned within a NE-SW-trending deformational fabric. IMP Mnz analyses are concordant, yielding a mean 207Pb/206Pb age of 2454 ± 8 Ma (all ages ±2σ), whereas Zrn (5-17% discordant) yields a less precise intercept age of 2465 ± 18 Ma. These ages predominate in this rock and document a major period of high-grade thermotectonism. Additionally, rare but texturally-distinct Mnz domains yield isotopic age sub-populations of 2785 ± 18 Ma (single truncated core) and ~1710-1735 Ma (overgrowths extending into the main fabric), both of which correspond to known orogenic events. Isotopic Mnz and Zrn ages of ~2780 and ~2460 Ma are also widespread in similar gneisses from the adjacent Ruby Range, whereas U-Th-Pb chemical ages of ~2670, ~2550, and ~2460 Ma characterize metapelitic schists ~80 km to the south, in the Madison mylonite zone and environs. In the eastern cratonic margin (NE Black Hills, SD) a 1-km-thick, layered sill (Blue Draw metagabbro, BDM) intrudes Paleoproterozoic rift succession sandstones (Boxelder Creek quartzite). IMP analysis of magmatic Ttn megacrysts from an upper pegmatitic phase of the BDM yields a 207Pb/206Pb age of 2480 ± 6 Ma, which pinpoints an episode of mafic magmatism and associated rifting in the Black Hills. Coexisting BDM hornblende was disturbed by Tertiary reheating, such that its 40Ar/39Ar spectrum records only a ≥2075 Ma age for this event. The new ~2450-2480 Ma mineral ages, obtained from opposite margins of the Wyoming craton, mirror published ages of mafic dike swarms and layered intrusives in Ontario and Fennoscandia that have been attributed to incipient breakup of supercontinent Kenorland (e.g. Heaman, 1997). This event appears to have affected the E and W margins of the Wyoming craton in the sense that the trailing plate edge reflects rift-dominated sequences and mafic magmatism related to break-up, whereas the leading edge reflects early Paleoproterozoic convergence manifested as collision?-related metamorphism.