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Cordilleran Section - 101st Annual Meeting (April 29–May 1, 2005)

Paper No. 10
Presentation Time: 4:40 PM

STRATIGRAPHY, STRUCTURE, AND U-PB GEOCHRONOLOGY OF PALEOPROTEROZOIC GNEISSES IN SIERRA HORNADAY AND CERRO EL AGUILA, NW SONORA, MEXICO: FINGERPRINTS AT THE TRUNCATED MARGIN OF LAURENTIA


ESPINOZA, Miguelangel1, NOURSE, J.A.1, PREMO, W.R.2 and IRIONDO, A.3, (1)Department of Geological Sciences, California State Polytechnic Univ, Pomona, CA 91768, (2)USGS, DFS, Denver, CO 80225, (3)Centro de Geosciences, UNAM, Queretaro, 76230, Mexico, mespinoza@csupomona.edu

New geological mapping and mean 207Pb/206Pb SHRIMP crystallization ages (all MSWDs <2) of zircon from the Pinacate region of NW Sonora reveal two fragments of Paleoproterozoic gneiss with contrasting structural geometries near the southwestern truncated margin of Laurentia.

Situated 125 km southeast of Yuma, Arizona, the ~5 km long, WNW trending Sierra Hornaday is comprised of coarsely porphyritic alkali granite augen gneiss (1649±6 and 1643±4 Ma) and syenogranite gneiss (1651±6 Ma) intruding thin felsic screens of meta-sandstone or meta-rhyolite. Several diabase dikes (1.1 Ga ?) are also present. The gneisses typically display strong S-tectonite fabric and occasional SE trending L-tectonites. Both gneisses and country rocks are tightly folded at mesoscopic and map scale, with the structure dominated by a N75W trending synform. Probable correlative metasediments and granitic gneisses (1644±8 Ma; mean of three samples) located 10-20 km north display northerly fold hinges, discordant to those in Sierra Hornaday, that predate emplacement of 1.43 Ga porphyryitic granite. A separate body of syenogranite gneiss (1645±10 Ma) occurs 20 km northeast along Highway 2.

Four km south of Sierra Hornaday, the ~2 km long Cerro El Aguila also trends WNW. This range consists of unfoliated leucocratic biotite monzogranite sharply intruding a thin layer of coarsely porphyritic granitic augen gneiss (1699±11 Ma) with SW-vergent S-C fabric along its northwestern termination. The southern part of the monzogranite contains an isolated xenolith of L-S tectonized hornblende andesite. We have mapped augen gneisses as old as 1725 Ma, affected by similar noncoaxial strain, at Sierra Los Alacranes ~30 km to the northwest.

Ongoing studies within both ranges are aimed at constraining the age and significance of metamorphism. Potential pre-750 Ma Rodinia ties of the southwestern truncated margin of Laurentia to other continents such as Australia, Antarctica, China, or Siberia are intriguing.

Session No. 16

S2. Tectonics of the U.S. Cordillera, SWEAT Connection and Beyond I: A Session in Honor of Eldridge Moores
Friday, 29 April 2005: 1:15 PM-5:20 PM
Geological Society of America Abstracts with Programs. Vol. 37, No. 4, p.57
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