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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, portions of the volcano-sedimentary pile. The subvolcanic complex comprises concordant to discordant, hypabyssal intrusions of tonalite-trondhjemite (65 percent) and diabase-gabbro (30 percent). Ultramafic and mafic rocks with possible ophiolitic affinities constitute the remaining 5 percent of the subvolcanic complex.

Greenstone-belt rocks have undergone regional metamorphism of upper greenschist to lower amphibolite grade and show the effects of at least three periods of deformation. The metamorphic sequence is cut by pre- to syn-orogenic granites and quartz porphyries dated at ca. 1,650 Ma and by syn- to post-orogenic granitic rocks dated at ca. 1,500 to 1,450 Ma.

Mafic volcanic and subvolcanic rocks define four distinct chemical populations: (1) ultramafic, (2) high-Mg tholeiite, (3) tholeiite, and (4) calc-alkaline basalt (CAB). Geochemically, the high-Mg tholeiites are similar to Archean high-Mg tholeiites, whereas the tholeiites and calc-alkaline basalts are similar to their counterparts in modern arc systems. The ultramafic rocks and various basalt groups cannot be related to each other by closed-system fractional crystallization or batch melting of a single mantle source. Their incompatible-element ratios seem to demand at least three different mantle sources (one depleted). Felsic volcanics can be divided into three groups (one rhyolitic and two dacitic) on the basis of immobile-element abundances. The rhyolites and one dacite group can be produced by fractional crystallization of CAB. The other dacite group (Doctor Creek dacite) is not obviously or easily related, chemically or genetically, to the rest of the Pecos volcanic and subvolcanic rocks. It instead may belong to a younger (ca. 1,700 Ma) felsic-dominated volcanic succession that is widespread in northern New Mexico.

Rocks of the Pecos greenstone belt may represent a remnant of a back-arc basin that opened far enough to form oceanic crust and tap a depleted mantle source.

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