PRELIMINARY STUDIES IN THE LARAMIDIC CERRO BACHOCO INTRUSIVE AND RELATED DIKES FROM HERMOSILLO, SONORA, MEXICO

Hermosillo city geology in central Sonora, Mexico is characterized by the exposure of different plutonic felsic outcrops and related dikes, as well as at least two different lamprophyre dikes and felsic volcanic rocks. The area, located in the NW urban part of the Sonora capital is part of the El Bachoco Range and its morphology is dominated by the Basin and Range physiographic province. Field relations in Cerro Bachoco show an intrusive granodioritic body with abundant NW-SE structures, cut by leucocratic dikes emplaced during the final phases of the Laramide Orogeny; and by lamprophyre dikes that are believed to be synchronous with the Basin and Range province. In the area we can also find felsic tuffs overlaying the Laramide Intrusive and the dikes mentioned before; these supposedly Cretaceous volcanic specimens are still being studied by comparing them to similar rocks in the area to stablish a more specific age by correlation. In the thin sections the petrography shows five different groups that are divided as: 1) Granitoids with phaneritic texture composed primarily of quartz, feldspar and plagioclase with minor biotite and muscovite. 2) Leucocratic dikes with an aplitic texture with a mineralogy of plagioclase, quartz, feldspar, and micas, as well as a similar paragenesis of the mineral alteration seen in the granitoids; in some cases, carbonate related alteration minerals are found. 3) The mid-Tertiary lamprophyre dikes have a micro-porphyritic texture and are divided into two types: spessartites and kersantites, depending on the content of amphiboles and mica in each sample. 4) The felsic pyroclastic rocks, resemble felsic tuffs, composed mainly of quartz and felspar with vesicles and amigdules filled with quartz. 5) The metamorphic rocks show a variety of alteration minerals such as chlorite, epidote, calcite, and oxides replacing preexisting minerals as seen in a sample described as a meta-lamprophyre. Results from our ongoing petrographic and geochemistry study allow us to classify the lamprophyres and contribute to the description of the Granite-Granodiorite intrusive, leucocratic dikes, felsic tuffs and metamorphic samples that were found in the area. Further PXRF analyses are being conducted to compare the intrusives bodies.