GEOCHEMISTRY OF SILICA-SATURATED INTRUSIVE AND VENT FACIES ROCKS OF THE CHICO SILL COMPLEX, NORTHEAST NEW MEXICO

Building on previous work, this report concentrates on the chemistry of a subgroup of the Chico Sill complex, a diverse suite of Neogene intrusive rocks (sills and dikes) and scant vent facies rocks in northeast New Mexico. Other workers have identified silica-saturated rocks that were emplaced during approximately 7 million years beginning at 37 Ma. Lithologies considered here are: Trachyte (including the Slagle Trachyte and younger trachyte south of Laughlin Peak), spatially-associated vent facies rocks from Turkey Mountain, (quartz monzonite and syenogabbro), biotite-trachyte of the Three Sisters, a “mela-syenite” dike at the eastern edge of the complex and silica-saturated sills of the Philmont Scout Ranch to the west.

The mela-syenite dike is silica-undersaturated and is more chemically and mineralogically similar to phonotephrite found elsewhere in the complex. The syenite/trachyte of Philmont Scout Ranch is chemically and mineralogically distinct from other trachytes and is not considered further.

All other silica-saturated rocks can be related by crystal fractionation, although scatter within clusters suggests separate batches of melt that may have evolved discretely. Turkey Mountain vent rocks (trachyte, syenogabbro, and quartz monzonite) are likely coeval and differ primarily in concentrations of clinopyroxene, and to a lesser extent, amphibole. Mineral composition data provided by scant electron probe analyses support clinopyroxene as a primary control on major-element chemistry, followed by amphibole and biotite. All the silica-saturated rocks show depletion of P and Ti by removal of apatite and a Ti-bearing phase (not titanite), and lack significant Eu or Sr depletions. Zr/Nb is typically in the range of 6 – 8 and decreases with increasing SiO₂ content.