New Mexico Geological Society Annual Spring Meeting — Abstracts

Proterozoic supracrustal rocks in the central San Andres Mountains, New Mexico are dominated by a large volume ofI metasediments, with lesser amounts of mafic metaigneous rocks and minor amounts of felsic volcanic rocks. These rocks are collectively referred to as the Hembrillo Canyon succession. Metasediments are an important part of the succession. Felsic metavolcanic rocks are of local importance in the Hembrillo Canyon area only. The mafic igneous rocks of the area are common throughout the succession and have incompatible element distributions characteristic of basalts erupted in modern volcanic arcs. Incompatible element distributions in felsic metavolcanic rocks are similar to rhyolites from continental margin arcs and associated back-arc basins. Mafic rocks exhibit tholeiitic trends and have undergone olivine plus or minus clinopyroxene fractionation. Depletion of Ta and Nb relative to REE and Th indiate 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 continental rifts and back-arc basins in or near continental crust. The metasedimentary rocks of the area are comprised of quartzites, feldspathic quartzites, arkosites, schists and phyllites with minor conglomerates. Metamorphism has obscured some of the original textures of these rocks, but it appears that most metasediments are quartzites, quartz-rich arkoses or quartz-
intermediate subgraywackes (Moore and Dennen, 1970). The medasediments contain low concentrations of Fe, Mg, Ti and Co, a feature characteristic of clastic sediments deposited on or near continental crust. Chemical and petrographic studies of the sediments indicate a dominantly felsic plutonic-volcanic provenance with minor mafic and andesitic input. Sedimentary structures (e.g. cross bedding and imbricated pellets) indicate two source directions: one dominanat source to the north contributing major amounts of felsic igneous detritus, a second, less important source contributing much finer grained andesitic material from a southerly direction. These same sedimentary structures suggest that the rocks were deposited in a series of submarine fans by turbidity currents in a proximal environment. Coarse grained arkoses, conglomerates and quartzites may represent channel fill deposits associated with the fans.

Overall, data suggest that the Hembrillo Canyon succession was deposited in a continental margin back-arc basin. Continental and arc detritus was rapidly fed into a tectonically active basin. Interbedded with the sediments are intrusive volcanic rocks and felsic tuffs. The mafic rocks in the area are derived from a mantle source with a significant subduction zone component.