Lamprophyres, Basanites, and Basalts of the Western Mexican Volcanic Belt: Volatile Contents and a Vein–Wallrock Melting Relationship

Abstract

We present geochemical data for Quaternary basalts, basanites, and lamprophyres within the Colima and Mascota volcanic fields at the western end of the Mexican Volcanic Belt. On the basis of data for 11 whole-rock samples plus 124 glass inclusions and olivine host crystals, we evaluate a vein–wallrock melting relationship between the lamprophyres (vein-dominated melts) and the mafic calc-alkaline rocks (diluted by partial melting of peridotite wall-rock after exhaustion of phlogopite and other vein minerals). Whole-rock Fe3+/Fe2+ and glass-inclusion %S6+ indicate relatively high fO2 in these magmas, up to several log units above the Ni–NiO buffer. The highest concentrations of water and most other volatile elements (7% H2O, 1460 ppm CO2, ∼2% SO3Total, 2400 ppm Cl, and ∼ 1% F) were recorded for a glass inclusion from a Colima minette with 48·2 wt % SiO2, 6·0 wt % K2O, and 1·2 wt % P2O5 (normalized anhydrous). This sample's volatile composition corresponds to a depth of entrapment of 24 km (calculated pressure of 6660 bars). This inclusion (trapped within olivine with Mg-number 91·5) represents the most primitive melt in this study and has a composition that can be attributed to partial melting of phlogopite-pyroxenite veins in the mantle wedge with minor dilution (possibly as little as ∼25%) by partial melts from the surrounding peridotite wall-rock. However, there are indications that even this inclusion has undergone degassing, suggesting that primary vein melts have even higher H2O and CO2 contents. Further dilution of the vein-dominated lamprophyre melts by wallrock melts yields basanites and ultimately calc-alkaline basalts. Mafic calc-alkaline whole-rock and glass-inclusion compositions are consistent with formation through mixing of ∼ 5% vein melts with ∼ 95% peridotite wallrock melt. Among the calc-alkaline glass inclusions, the Mascota basaltic andesite has the highest concentrations of water and most other volatile elements with 49·6 wt % SiO2, 1·0% K2O, 0·3% P2O5 (normalized anhydrous), 2·8% H2O, 296 ppm CO2 (1425 bars pressure and 5·2 km depth of entrapment), 0·8% SO3Total, 870 ppm Cl, and 720 ppm F. Such mafic calc-alkaline melts are envisioned as parental to the volumetrically dominant andesites of western Mexico.