MAGMATIC EVOLUTION OF AN ADAKITIC DOME FROM THE TRANSMEXICAN VOLCANIC BELT

An andesitic dome from the eastern sector of the Transmexican Volcanic Belt, in central Mexico, allocates plutonic xenoliths (biotite-quartz-diorite and phlogopite-clinopyroxenite), and andesitic cogenetic dikes, constituting a metaluminous, medium-K, magmatic suite.

The dome and dikes are high-silica adakites (60-61 wt%), with high values of Na₂O (~4 wt%), Sr (>1000 ppm), and Sr/Y (>60) ratios. They are formed by plagioclase (labradorite-andesine), and zoned Ca-amphibole phenocrysts ranging from edenite to tschermakite. Oxydized amphibole rims are common.

Xenoliths are high silica (70 wt%), high-alumina (Al₂O₃>15 wt%), Ba (599 ppm) and Sr (564 ppm), trondhjemitic (K₂0/Na₂O<1) melts with plagioclase (labradorite-oligoclase), and mica (magnesian siderophyllite and ferroan phlogopite). On the other hand, clinopyroxenites show cumulate texture with high-Ca, green clinopyroxene (ferrian-diopside), phlogopite, and magnesio-hastingsite. The xenoliths are geochemically characterized by high values of MgO (12.55 wt%), Cr (950 ppm), and Ni (190 ppm).

Likely, the dome emplacement is diapiric, formed from a hot, water rich, oxidized magma, with predominant amphibole fractional crystallization, with delayed plagioclase crystallization, at pressures between 7-6 kbar. Quartz-diorite xenoliths crystallized from a water-rich primitive felsic magma. Clinopyroxenites are an ultramafic, primitive cumulate rock, dominated by the fractional crystallization of ferromagnesian minerals.