Abstract

The Benavides-Pozos area of eastern Chihuahua, Mexico, is composed of a field of mid-Tertiary lavas and ash-flow tuffs with a composite thickness of ∼1 km. This volcanic field contains some of the easternmost, arc-related volcanic rocks in northern Mexico. The oldest volcanic rocks in Benavides-Pozos are volumetrically minor, undated, alkalic basalts that are interbedded with Tertiary clastic sedimentary rocks. Calc-alkalic rocks ranging in age from ∼34 to 30 m.y. overlie these basalts. Rocks of the calc-alkalic suite are predominantly silicic in composition, although intermediate and mafic rocks are also present. In areas adjacent to Benavides-Pozos, the calc-alkalic rocks are overlain by a chemically distinct, “ferroaugite-rhyolite,” ash-flow tuff ∼28 m.y. old. This unit is in turn overlain by extension-related alkalic basalts dated between 26 and 17 Ma.

The volumetrically dominant calc-alkalic-suite rocks are high-K (K57.5 = 3.2), and rocks have initial 87Sr/86Sr ratios that range from 0.7036 to 0.7070 and εNd of about +1.5 to −2.2. Petrogenetic processes inferred for generation of the calc-alkalic basalt to rhyolite series include fractionation of mantle-derived basalts coupled with minor to moderate amounts of assimilation of lower crust, magma mixing (especially among intermediate magmas), low-pressure fractional crystallization, and possible upper-crustal interaction. The andesitic and dacitic lavas show considerably more mineralogic and isotopic evidence for mixing and crustal interaction than do the voluminous rhyolitic ash-flow tuffs. Isotopic, trace-element, and mineralogic data demonstrate that mixing of basalt and rhyolite was not the process responsible for genesis of the intermediate calc-alkalic rocks. Assimilation-fractional crystallization modeling of 87Sr/86Sr suggests that the most contaminated andesites and dacites contain ∼25% crustal component.

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