Paper No. 40-7
Presentation Time: 4:30 PM-4:50 PM
ESPINOZA, Inocente1, IRIONDO, Alex2, PREMO, Wayne2, PAZ, Francisco1, and VALENCIA, Martin3, (1) Departamento de Geologia, Universidad de Sonora, Hermosillo, Sonora, 83000, Mexico,, (2) USGS, Denver, CO 80225, (3) Instituto de Geologia, UNAM-ERNO, Hermosillo, Sonora, 83000, Mexico



We report new major- and trace-element geochemistry and SHRIMP U-Pb zircon ages from one anorthosite intrusive body, which outcrop at Sierra El Tecolote, 12 km east of Rancho Bamori, NW Sonora, Mexico. These rocks intruded a sequence of strongly deformed Paleoproterozoic gneissic granitoids and amphibolites of the Bámori Metamorphic Complex, in the Caborca Block. The anorthosite bodies display dominant anorthositic to gabbro-anorthosite phases, with typical textural and mineralogical fabrics, consisting of phaneritic cumulate subhedral to anhedral plagioclase grains (An50-70), irregular to prismatic blue-green pyroxenes, and scarce quartz. The mineral assemblage suggests a near-surface, high-temperature, emplacement context. These intrusions are characterized by high Al2O3 and CaO contents, and relatively low Na2O and K2O. They present low abundances of LIL (Sr, Ba, and Rb), and HFS elements (Zr and Y). They show chondrite-normalized REE patterns with overall depletion on LREE and a pronounced positive Eu anomaly due to substitution of Ca++ by Eu++ in plagioclase. Chromium, on the other hand, is relatively high for anothosites (200 and 400 ppm).

SHRIMP U-Pb zircon analyses for centers of primary, magmatic zircons from a sample of Sierra El Tecolote anorthosite yielded an emplacement age of 1095±28 Ma. This age correlates with the Aibo Granite located 17 km NW of Rancho Bámori, which has been dated with the same technique at 1079±18 Ma.

Other anorthositic and gabbroic intrusions reported in Sonora are located near Puerto Peñasco, at Benjamín Hill, and close to Cucurpe. All of these rocks could have been formed from a singular magmatic pulse but unfortunately none of them have been dated. Other similar rocks near the US-Mexico border, yielded Grenvillian ages, such as the anorthosite-syenite complex of the San Gabriel Mountains (1120 Ma) and the diabase sill in the Pahrump Group (1087±3 and 1069±3 Ma), both located in the Proterozoic Mojave province.


Cordilleran Section - 99th Annual (April 1–3, 2003)
General Information for this Meeting
Session No. 40
Proterozoic Basement in Sonora and Adjacent Areas: A Piece of SW Laurentia II
Hotel NH Krystal: Mismaloya
2:15 PM-5:30 PM, Thursday, April 3, 2003

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