Volatile elements in alkaline and calc-alkaline rocks from the Colima graben, Mexico: Constrains on their genesis and evolution

Abstract

Based on major element chemical composition, three groups of rocks having different parental magmas can be recognized in the Colima graben region. The most mafic rocks belong to the alkaline group, which display the highest incompatible and compatible element contents among the rocks studied. The second group, the calc-alkaline volcanics have the largest compositional range among the rocks studied due probably to low pressure evolutionary processes. A third group straddles the subalkalinealkaline field boundary, and has KzO abundances intermediate between alkaline and calc-alkaline volcanics. Primitive rocks were neither found in calc-alkaline nor in high potassium calc-alkaline rocks: All the rocks studied have similar hydromagmaphile patterns with high LILE/HFSE values. Halogens (F and Cl), S and C analyses have been performed on the majority of the samples from the three groups of rocks of the Colima region. All the most primitive rocks of the three groups are characterized by comparable low abundances of C and S. Cl is initially relatively low, but tends to increase with the degree of evolution. On the contrary, the most primitive rocks of the three groups have distinctly different F contents. Alkaline rocks have the highest values and calc-alkaline the lowest F contents. Fluorine, as well as chlorine, tends to increase with the degree of evolution in calc-alkaline and high potassium calc-alkaline rocks, although some samples seem to have lost halogens by ex solution in a shallow level magmatic reservoir. The fluorine and potassium abundances seem to indicate phlogopite as one of the mineral phases in the Colima sub-arc mantle. A mica-bearing mantle source affected by different degrees of partial melting can be hypothesized for the genesis of alkaline and calc-alkaline parental magma. Nevertheless, this has to be still considered as a working hypothesis, and further mineralogical, geochemical and petrological studies must be performed to better constrain the role of evolution in the genesis of studied rocks.