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The Pecos greenstone belt, dated at ca. 1,720 Ma, consists of metamorphosed subaqueous basalts together with locally important felsic volcanic rocks, iron-formation, and metasedimentary rocks, some of volcanic provenance. Volcanic rocks define a compositionally bimodal suite of basalt (80 percent) and dacite-rhyolite (20 percent). Mafic volcanics are mostly fine-grained, massive to well-foliated amphibolite that locally display relict amygdules, pillows, and pillow breccia. Felsic volcanic rocks are mainly porphyritic flows and crystal-rich volcaniclastic units that contain variable amounts of quartz, K-feldspar, and plagioclase phenocrysts. The greenstone belt also includes a compositionally bimodal subvolcanic complex that intrudes, and is locally overlain by,...

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