GEOLOGICAL AND GEOCHEMICAL CHARACTERIZATION OF THE GRANITIC ROCKS AROUND HERMOSILLO, CENTRAL SONORA, MEXICO

A large fraction of the rocks exposed in northwestern Mexico consist of granitic batholiths of Laramide age. However, no systematic studies attempting to understand the anatomy of these bodies have been done so far. The Hermosillo batholith is being studied as a model to understand the different features observed along the rest of the Laramide belt. This is a distinctive well-exposed NW-SE geomorphologic structure that was emplaced mostly in carbonate rocks of late Paleozoic age. Exposures of these rocks that survived to tectonic uplifting and erosion consist of wollastonite-bearing marble patches, locally displaying tungsten skarn mineralization. The Hermosillo batholith is essentially of granodioritic composition but it also shows internal variations from medium to very coarse-grained granitic facies. Lateral variations from mafic outer envelopes to more felsic cores are frequently observed. The batholith is everywhere crosscut by felsic and mafic dikes, particularly dominated by pegmatite and a variety of lamprophyric rocks. Xenoliths revealing a more mafic source or underlying component are commonly seen throughout most of its exposure. The granitic rocks studied indicate high-K calc-alkaline metaluminous compositions, typical for most North America-Farallon plate subduction-related magmatic products along the western North American Cordillera. Chondrite-normalized patterns of samples from this batholith are characterized by light rare earth element-enriched slopes, which show little or no europium anomalies. Compared to other nearby Laramide plutons displaying pronounced negative europium anomalies, the Hermosillo batholith appears to have undergone a period of europium enrichment relative to the rest of the rare earth elements, possibly in equilibrium with hornblende. K-Ar isotopic geochronology yielded a crystallization age of ~64 Ma. Radiogenic isotopic data show initial strontium ratios and epsilon neodymium values ranging from 0.7070 to 0.7088 and from -4.6 to -5.3, respectively. These relatively evolved isotopic signatures suggest that the source for the Hermosillo batholith may have incorporated an important old crustal component in the melt.