Mid-tertiary siliceous igneous activity above cratonic and accreted basement in northern Mexico; comparison of two localities

Abstract

The southern edge of the North American craton is located in northern Mexico, but it cannot be exactly located due to lack of basement outcrops. This papers investigates the chemistry of the 1 000 m thick cover of Tertiary igneous rocks dominating the Mexican Sierra Madre Occidental as a tracer of variations in basement composition. For this purpose two localities have been studied. San Buenaventura, located in northwestern Chihuahua, represents igneous activity above Precambrian cratonic basement. The Copper Canyon section at EI Divisadero in southwestern Chihuahua represents igneous activity above accreted terranes with unknown basement. Both areas are dominated by siliceous lava flows and ignimbrites. 'Fhe 600 m section at Buenaventura is characterized by an 11 km large caldera structure associated with a resurgent granitic intrusion. The area adjacent to the caldera structure is formed by several major ignimbrites and intermediate lava flows of basaltic andesitic to rhyolitic composition. The entire suite plots in the field of high K calc-alkaline rocks. The siliceous rocks are characterized by K-feldspar, plagioclase, biotite, clinopyroxene, amphibole and opaque phases. The RblSr age is 33.2 Ma with an initial 87Sr/86Sr ratio of 0.706577 ± 0.000425. As all siliceous rocks plot on the isochron they are interpreted as being derived from a common magmatic source, probably a large composite pluton. The existence of cumulative rocks within the granite complex suggest that the final stage of differenciation did not occur at greater depth. A linear increase of LlL element abundance with Si02 exists and has to be taken into account when comparing the Buenaventura suite with other suites. The Copper Canyon section at EI Divisadero is characte~ed entirely by siliceous rocks of dacitic to rhyolitic composition. The entire section of 1 400 m consists of horizontal layers of siliceous lava flows and ignimbrites. The section has been subdivided into a lower and upper unit. The upper unit has a RblSr age of 29.0 Ma old with an initial 87Sr/86Sr ratio of 0.705644 ± 0.000335. Mineralogically the siliceous rocks are similar to the Buenaventura rocks, with the exception of a larger amphibole abundance, suggesting similar petrographic evolution. CompariSon of geochemical data from EI Divisadero with the Buenaventura suite shows that besides the difference in Sr-isotopic composition a significant difference exists in the K-, Ti-, and Th-group elements, REE abundances, and interelement ratios. The Tertiary igneous rocks can therefore be used as indicators of basement variations.