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

Paper No. 2
Presentation Time: 1:50 PM


DUMOND, Gregory, Department of Geosciences, Univ of Massachusetts, 611 North Pleasant Street, Amherst, MA 01003-9297, WILLIAMS, Michael, Department of Geosciences, Univ of Massachusetts, 611 North Pleasant Street, Amherst, MA 01003, MAHAN, Kevin, Department of Geosciences, Univ of Massachusetts, Amherst, MA 01003-9297, KARLSTROM, Karl E., Dept. of Earth & Planetary Sciences, Univ of New Mexico, Northrop Hall, Albuquerque, NM 87131 and HEIZLER, Matthew T., New Mexico Bureau of Mines and Mineral Resources, New Mexico Tech, 801 Leroy Place, Socorro, NM 87801, gdumond@geo.umass.edu

Detailed structural analysis and in situ monazite geochronology demonstrate significant Mesoproterozoic reactivation in Paleoproterozoic basement of the Grand Canyon. Ar40/Ar39 data reveal a major thermochronologic break near the 96-Mile shear zone, separating ~1.48-1.4 Ga muscovite apparent ages on the east from ~1.64-1.5 Ga ages on the west. A change in Pb isotopes near the break marks a major terrane boundary between the Mojave and Yavapai provinces. The break coincides with a ~500 m-wide ultramylonite that separates upper amphibolite-grade schists, amphibolites, pegmatite pods, and granitic dikes on the east (Trinity Creek Block) from upper greenschist-grade schists and meta-turbidite sequences on the west (Topaz Canyon Block). Early dip-slip movement occurred on steeply NW-dipping foliation planes with steeply plunging stretching lineations, consistent with sub-vertical extension during Paleoproterozoic NW-SE contraction. Later W-side down, sinistral, oblique- to strike-slip movement occurred on the same foliation along variable shallowly to moderately SSW-plunging lineations.

High-resolution BSE-imaging, X-ray mapping, and EMP monazite geochronology on schists in and near the 96-Mile shear zone reveal several populations of kinematically significant 10-150 µm monazite grains: 1) anhedral sulfur-rich monazite that fills or overgrows fractures in association with apatite, 2) subhedral monazite aligned with shear bands and foliation, and 3) poikiloblastic monazite cut by synthetic and antithetic micro-faults and fractures. Preliminary microprobe dates on low Y-low Th rim domains, interpreted to have precipitated syn- to post-kinematically, are 1505 +/- 24 Ma.

The 96-Mile shear zone records two distinct tectonic episodes: 1) post-peak metamorphic juxtaposition during the Yavapai-Mazatzal Orogeny, and 2) Mesoproterozoic reactivation in the vicinity of a crustal-scale suture (~3.5 km SE of the Crystal shear zone). We suggest the second episode documents an early, intra-continental (100s of km from the active margin, i.e. “Tibetan-scale”) response to the onset of ~1.5-1.35 Ga tectonism. The reactivation may have been localized at deeper crustal levels along an early accretionary boundary.