Ruby Xenocrystals in Dacite from Central Mexico

Abstract

The occurrence of 1-2 cm in diameter, anhedral xenocrystals of red corundum (ruby) in a dacitic lava- flow from central Mexico -in the state of Hidalgo- has been recorded. Geochemically, this dacite nearly resembles adakitic rock (SiO2=63.73 66.64 wt %), with low alumina contents (Al2O3=14.38 14.93 wt %), sodium (Na2O=3.29 3.57wt %) and titanium (TiO2=0.55 0.60 wt %), slightly peraluminous (A/ CNK=1.04 1.17) and moderately potassic (K2O=1.90-2.04 wt %; K2O/Na2O=0.53-0.62), with rare earth spectra enriched in light-rare earth elements (La/Yb)N=9.71 10.98. The origin of the dacite is linked to the early geological evolution of the eastern sector of the Trans-Mexican Volcanic Belt (TMVB) during the Miocene, and its provenance could be a melt of a basaltic and pelitic-sediments slab and magmatic differentiation that generated adakitic magma. The gem-type corundum could have resulted after the addition of refractory products disaggregated from the Precambrian basement, carried, and transported to the surface by ascending magma.