OLIGOCENE ASH FLOW VOLCANISM, NORTHERN SIERRA-MADRE OCCIDENTAL - ROLE OF MAFIC AND INTERMEDIATE-COMPOSITION MAGMAS IN RHYOLITE GENESIS

Field, geochemical, and isotopic data from the Tomochic volcanic center in Chihuahua, Mexico, are interpreted to indicate a genetic relationship between large-volume rhyolite ash-flow tuffs and associated more mafic lithologies. These lithologies include (1) porphyritic, two-pyroxene andesite (> 35 Ma) that was extruded mostly before ash-flow volcanism, and (2) crystal-poor basaltic andesite that was erupted mostly after ash-flow activity (approximately 30 Ma) but which was also extruded earlier (approximately 34 Ma) with hybrid intracaldera lavas. Major silicic units at Tomochic include the Vista (approximately 34 Ma) and Rio Verde (approximately 32 Ma) rhyolite ash-flow tuffs; also present are ash-flow tuffs (approximately 38, 36, and 29 Ma) erupted from other sources. A model of rhyolite genesis by closed-system crystal fractionation of andesite is consistent with geochemical and isotopic data. The least evolved Vista rhyolite was formed by fractionation of approximately 65% the original mass of andesite; an additional approximately 55% fractionation of plagioclase, alkali feldspar, quartz, biotite, homblende, FeTi oxides, and sphene generated the most evolved Vista sample. Rio Verde rhyolites were generated from andesite by approximately 50% mass fractionation of an assemblage dominated by plagioclase, pyroxene, and FeTi oxides. Initial Nd and Sr isotope ratios of andesite-dacite lavas (epsilon-Nd = -2.3 to -5.2; Sr-87/Sr-86 = 0.7060 to 0.7089) and of rhyolites (epsilon-Nd = +0.5 to -2.7; Sr-87/Sr-86 = 0.7053 to 0.7066) partly overlap and extend from values near the mantle array toward values typical of old continental crust on an epsilon-Nd-Sr-87/Sr-86 diagram. These isotope ratios, which do not correlate with indices of differentiation, are interpreted to indicate that parental andesite already contained a crustal component (possibly > 20%) before fractionation to rhyolite. The isotopic and geochemical signatures of andesites apparently reflect the incorporation of crust by subduction-related, mafic melts represented by (but more primitive than) exposed basaltic andesites, which have isotope ratios (epsilon-Nd = +1.0 to -0.1; Sr-87/Sr-86 = 0.7044 to 0.7053) near "bulk earth". The pattern of volcanic evolution at the Tomochic center, specifically the transition from andesitic to rhyolite dominated, with late extrusion of basaltic andesite, also occurred in other parts of the volcanic field, and roughly coincided with a sharp decrease in the rate of Farallon plate subduction. This change in subduction rate apparently resulted in a decreased flux of mafic melts into the crust from below, and was associated with the onset of crustal extension and hence, shorter residence times for mafic melts formerly ponded in the deep crust. These, in turn, resulted in (1) the change from andesitic to rhyolite-dominated volcanism as ascending intermediate-composition magmas stalled, coalesced, and differentiated to produce rhyolite, (2) extrusion of basaltic andesite upon brittle failure of the shallow crust, and (3) subsequent termination of calc-alkalic volcanism throughout the Sierra Madre Occidental.