UNRAVELING ALKALIC MAGMATISM OF THE JEMEZ LINEAMENT: INTERPRETATIONS FROM HYDROUS MINERAL GEOCHEMISTRY AT TWO BUTTES, COLORADO

Alkalic mantle-derived magmatism at Two Buttes, Colorado, located at the northeastern end of the Jemez Lineament, occurred approximately 37 Ma. The Two Buttes lavas were erupted through thick continental crust. Primitive lavas with hydrous mineral phases (phlogopite and amphibole) are characteristic of this area, which may suggest a metasomatized mantle source.

A suite of hydrous-mineral bearing samples from Two Buttes has been analyzed for whole rock bulk chemical compositions, and separated materials (phlogopites, amphiboles, and groundmass) have been analyzed for hydrogen isotopic compositions. Major and trace elements were also determined for the phlogopites and amphiboles via EMPA and LA-ICP-MS. Whole rock compositions vary greatly, with Mg#s (Mg/Mg+Fe) ranging from 45 to 81. Phlogopites also show a wide compositional variation with Mg#s ranging from 50 to 88. Some phlogopites are zoned, typically with a large core of high Mg#, thinly rimmed with a lower Mg# phlogopite. Hydrogen isotopic compositions for phlogopites show much less variation relative to Mg#s, with a range of about -80 per mil to -75 per mil. A positive correlation exists between the Mg#s and the D/H ratios of the phlogopites. Amphiboles and groundmass are deuterium poor compared to the phlogopites, which suggests that this small range in D/H ratios for phlogopites is a magmatic signature rather than a consequence of post-eruptive exchange. It is expected that post-eruptive exchange would cause the phlogopites to have D/H ratios similar to the groundmass in which they are found. Although prior studies are conflicting, we hypothesize that the groundmass should have initially been deuterium rich, suggesting the low observed values are the result of post-eruptive exchange. In conjunction with hydrous mineral and whole rock compositions, we are using hydrogen isotope compositions to develop a model for interpreting crustal contamination and the evolution of magmatic systems, as well as the potential metasomatic history of the mantle source for alkalic volcanism associated with the Jemez Lineament.