PETROGENESIS OF CONTEMPORANEOUS SODIC AND POTASSIC ALKALINE MAGMAS AT SPANISH PEAKS, COLORADO

Lamprophyric dikes intruded continental crust of the Spanish Peaks region, CO, at c. 22 Ma range from relatively sodic varieties (“camptonites”) to relatively potassic varieties (“minettes”). Typical phenocryst assemblages are dominated by mica and/or amphibole, and megacrysts of Ti- and Al-rich amphibole are widely distributed. These dikes are characterized by relatively high Mg# (typically > 55) and MgO contents (typically 5-9 wt.%) reflecting an origin by melting of mantle lithosphere although many are fractionated and maybe contaminated. Limited isotopic data indicate that the potassic and sodic varieties possibly formed by melting of the same source rock posing a problem in explaining the variable bulk compositions, especially the variation in K₂O/Na₂O which varies from less than 0.5 to ~3 as well as distinct parental magma compositions as shown by distinct alumina contents at high MgO. Apatite and mica halogen contents are distinct in the two dike types: apatites and micas from the potassic dikes are relatively rich in F and could reflect F-rich predecessors in the source region. One possible explanation for the close association in space and time of the two dike types is sequential melting of an amphibole-phlogopite-apatite-bearing source. In this model the parent magmas of the potassic dikes formed late in the melting process after amphibole was exhausted. Phlogopite and apatite were stabilized longer in the melting process by their relatively high F contents and melting was possibly triggered by mantle convection below the eastern margin of the Rio Grande Rift.