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The lower Datil Group (40–37 Ma) records the early phase of intermediate-composition volcanism in the northern Mogollon-Datil field of New Mexico. Eruptive products consist almost entirely of high-K andesite and dacite that range in SiO2 content from about 58 to 67 weight percent. Lower Datil conglomerate clasts and correlative(?) lavas are characterized by phenocrystic plagioclase (An20–60), amphibole, and titanomagnetite (±biotite, pyroxene) within a groundmass containing alkali feldspar, silica minerals, and plagioclase. Volcaniclastic rocks of the lower Datil Group were deposited by a spectrum of fluvial, debris-flow and lacustrine processes in two semi-arid intermontane basins and differ mineralogically from their volcanic progenitors in the following ways: 1) nonwelded ash is not represented in conglomerate; 2) phenocryst phases are preferentially concentrated in sandstone; 3) groundmass constituents and devitrification products are over-represented in mudstone and detrital matrix in sandstone; 4) opaque reaction rims on free amphibole and biotite grains are poorly developed or not present; 5) amphibole abundance is slightly diminished relative to plagioclase and biotite in sandstone; and 6) potassic riras on plagioclase grains may have been removed by abrasion. Mineralogic fractionation of lower Datil Group volcaniclastic sediments occurred primarily by impact shattering and abrasion during transportation. As a result, provenance determinations for volcaniclastic rocks may be influenced by grain-size effects.

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