GEOCHEMICAL COMPARISON BETWEEN SILICIC VOLCANIC AND PLUTONIC ROCKS IN CHIHUAHUA CITY, MEXICO

Beneath Chihuahua City’s urban growth, in northern Mexico, lies a suite of silicic plutonic, volcanic and pyroclastic rocks. It is difficult to find any field relation or contacts between the different types of rocks, due to the housing developments that already exist over the area. Few outcrops remain, and have been analyzed. The age reported by the Mexican Geological Survey is Oligocene for all the igneous rocks in the surrounding areas of Chihuahua City. Based on whole rock analyses, the silicic rocks are classified as syenite, rhyolite, and rhyolitic and trachytic tuffs. Petrographically, all contain quartz, alkali feldspar, plagioclase, biotite and Fe-Ti oxides, with dendritic pyrolusite in fracture planes. Tuffs contain andesite, rhyolite and basalt lithics and occasionally pumice fragments. Geochemically, the suite of silicic rocks is classified as peraluminous quartz-bearing rocks, ferroan and alkaline, except the rhyolites that are alkali-calcic. There are differences in chemical compositions; the rhyolites are different than the intrusive and pyroclastic rocks. There is a composition variance between intrusive-tuff and rhyolite: SiO₂ 65 and 73 wt.%, CaO 2-3 and 0.4 wt.%, Ba >1000 and 100 ppm, Sr >200 and 40 ppm, Eu 2 and 0.2 ppm respectively. Summarizing, the rhyolite does not relate geochemically to the intrusive or pyroclastic rocks, it is difficult to explain why, due to the lack of field contacts and precise dating for each rock type.