New Mexico Geological Society Annual Spring Meeting — Abstracts

Metabasalts (now amphibolites) with well-preserved pillow structures occur within an extensive early Proterozoic (1.68-1.72 b.y.) compositionally bimodal volcanic terrane in the southern Sangre de Cristo Mountains of north-central New Mexico, and represent the first well-documented occurrence of such structures in the Precambrian of New Mexico.

Narrow, arcuate selvages define individual pillows that are up to 1.5 m in diameter, elliptical to circular in cross-section, and poorly exposed in the third dimension. Horizons of pillow breccia are also present, and consist of smaller (typically < 30 cm), irregularly-shaped fragments and pods, with or without well-developed selvages, set in a mafic matrix. Pillows contain variable amounts of amygdules, and range from massive, non-amygdular varieties to those displaying abundant amygdules arranged in well-defined zones roughly concentric about the core of the pillow. Junctions among pillows may be voids, but commonly are filled with inter-pillow breccia consisting of small (1-3 cm) subangular to subrounded selvage and pillow fragments in a fine-grained mafic matrix.

Chemical analyses of aphanitic to fine-grained amphibolites that are intermixed with and may locally cross-cut pillow-bearing horizons indicate those mafic volcanic rocks are subalkaline, mainly low-K, tholeiites, with an average TiO₂ content of 1.1% and flat REE to slightly light-REE-depleted patterns. The submarine environment, bimodal volcanic suite, chemistry, and nature of the associated felsic volcanic, volcaniclastic, and sedimentary rocks are all compatible with deposition in an immature back-arc basin.