New Mexico Geological Society Annual Spring Meeting — Abstracts

Hembrillo Canyon area of the cen ral San Andres Mountains is comprised chiefly of quartzites and phyllites with minor amounts of felsic volcanics and quartz wackes. The base of the succession is unexposed. The succession is intruded with diabase sills and dikes and post-deformational (1350-1400 Ma) granites. Quartzites are crossbedded and are comprised chiefly of quartz with variable amounts of K-feldspar and small amounts of seririite, zircon and magnetite. Quartz wa ckes and associated mixtures, which occur near the lower exposed portion of the succession, have preserved graded bedding and appear to represent turbidity-current and submarine debris-flow deposits. Provenance studies of clastic sediments indicate a source in which granitic rocks dominate.

Felsic volcanics and diabases comprise a bimodal volcanic suite. The diabases are tholeiites (Mg No. = 37 to 62) which have undergone olivine plus or minus clinopyroxene fractionation. Incompatible element distributions including REE and high field strength elemetits are similar to distributions in young arc-related basalts. Depletion in Ta and Nb relative to REE and in Ta relative to Hf and Th indicate a significant subduction-zone component in the source. Felsic volcanics have relatively high contents of heavy REE and high field-strength elements, a feature characteristic of felsic volcanics from continen tal rifts and back-arc basins in or near continental crust.

Geochemical data are consistent with a continental margin back-arc tectonic setting. The relatively large proportion of quartzites in the Hembrillo succession may reflect cratonic sources on the inland side of a back-arc basin.