GEOCHEMISTRY AND SHRIMP U-PB ZIRCON GEOCHRONOLOGY OF ANORTHOSITIC ROCKS AT SIERRA EL TECOLOTE IN THE CABORCA BLOCK, NORTHWESTERN SONORA, 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 (An_50-70), 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.