Petrologic and geochemical evolution of the Oligocene bimodal volcanism in the San Luis Potosi volcanic field (Mexico)

This work reports field, petrographic and geochemical data (major and it-ace elements, including the rare-earth elements - REE) of a late Oligocene rhyolitic ignimbrite (Riolita Panalillo, Trp) and lavas of intermediate composition (basalto La Placa, Tbp) that were concurrently emplaced in the Son Litis Potosi volcanic field (SLPVF), and more specfically, within the structure known as Bledos graben. Both volcanic units (Trp and Tbp) belong to a volcanic event of Oligocene age, with a restricted age range between 2 7 and 28 Ma. This volcanism generated sub-alkaline rocks that form a typical bimodal series, composed mainly, by basaltic andesite and rhyolitic ignimbrite. The regional tectonic environment is basically distensive and associated with extension that occurred during the Oligocene. The rhyolitic ignimbrite has a sub-alkaline and hyperaluminous character The chondrite-normalized REE patterns of this rhyolitic unit (Trp) display a negative Eu anomaly, which si qgesisfiraciional crystallization of plagioclase. This is also supported by the behavior of some elements such as Sr, Ba, and Ti. Moreover, the Rb vs. Y+Nb relationship suggests an intraplate tectonic environment. The mineralogy of this ignimbrite is relatively simple and mainly consists of potassic feldspar > quartz > plagioclase > opaque minerals. Regarding the intermediate lavas (Tbp), they. have a slightly sub-alkaline character and differ from the rhyolitic ignimbrite in their metaluminous character their REE paterns have relatively flat slopes, with only a slight enrichment in light-REE, and do not show Eu anomalies. The behavior of trace elements such as Sr Ba, Rb, Nb, and Th, suggest that partial melting of the upper mantle played an important role in the genesis of these magmas. Similarly, the Ti/Y-Zr/Y and Zr-Zr/Y relationships also suggest an intraplate tectonic environment. The petrographic analysis of these lavas shows that the main rock-forming minerals are: plagioclase> olivine> Fe-Ti oxides > quartz. The combined, petrographic, geochemical and field data, cis well as the ages of these units, suggest that the rhyolitic ignimbrite and the intermediate lavas may be considered as a group within the SLPVF, spatially, as well as temporarily associated. Possible magma mixing processes are revealed by the presence of reaction rings in plagioclase of the Tbp unit, as well as by reaction halos in olivine and mafic rock fragments within the Trp unit. Another important point worth mentioning is that these units could be co-magmatic.