Miocene to Holocene Magmatism in the Eastern Trans-Mexican Volcanic Belt (Palma Sola, Mexico): Mantle Sources, Slab Contributions and Crustal Contamination

The Miocene to Holocene stratigraphic record of the Easternmost part of the Transmexican Volcanic Belt can be divided into 3 main groups in terms of its stratigraphic, geologic and geochemical characteristics: Middle-Late Miocene plutons, Late Miocene-Pliocene plateau basalts and Pleistocene-Holocene cinder cones. The plutonic rocks and cinder cones are calc-alkaline and have trace element patterns compatible with a subduction environment (Ba/Nb= 16-101 and Pb/Ce= 0.07-0.25). In contrast, the plateau basalts are alkaline and have either moderate or negligible subduction contributions (Ba/Nb=6-31 and Pb/Ce=0.04-0.1). Sr, Nd and Pb isotopic compositions of the plutons and plateau basalts do not correlate with indices of differentiation indicating a negligible role of crustal contamination in their petrogenesis. In contrast, isotopic enrichment in the cinder cones correlates with SiO₂ and MgO contents, implying contamination with the local continental crust. The alkaline rocks have higher Nb concentrations (22-58 ppm) and lower Zr/Nb ratios (9.7-5.5) than pacific MORB, suggesting that the background mantle wedge is significantly enriched. The plateau basalts have very radiogenic ²⁰⁶Pb/²⁰⁴Pb ratios (up to 18.963), and form an array in ²⁰⁶Pb/²⁰⁴Pb -²⁰⁷Pb/²⁰⁴Pb space that trends towards the composition of the bulk subducted sediment. These radiogenic Pb isotopes are inversely correlated with Ba/Nb, Th/Nb and Pb/Nb ratios, reflective of the subduction component. Isotopic and trace element-ratio systematics in the plateau basalts require the addition of a subduction component with high Th/Nd and low ¹⁴³Nd/¹⁴⁴Nd, a component consistent with a sediment melt. This component also has lower Pb/Nd and Sr/Nd ratios than the bulk subducted sediment, suggesting that a significant proportion of the fluid mobile elements may have been lost to a fluid phase prior to melting. The geochemical data of the plateau basalts can be modeled by adding less than 4% of a dehydrated sediment melt into the enriched mantle wedge. The Pb isotopes of the plutonic rocks follow a different trend than the plateau basalts, they are bracketed by the subducted sediments and the Pacific MORB. The distinct Pb isotopic composition of the plutonic rocks thus require the participation of an unradiogenic component in addition to the sediments. High La/Yb (46) and Sr/Y (53) ratios in the least radiogenic and evolved plutonic sample (SiO₂=56.8%) might indicate the participation of melts coming from the subducted MORB.