GEOCHEMICAL RECYCLING AND MAGMA GENERATION IN THE EASTERN TRANS-MEXICAN VOLCANIC BELT: THE CASE STUDY OF THE SERDÁN-ORIENTAL BASIN

We present a preliminary geochemical study of the Quaternary igneous varieties of the Serdán-Oriental Basin (SOB) in the eastern Trans-Mexican Volcanic Belt. Mafic-intermediate monogenetic cones and explosion craters, as well as some silicic domes, are built on >45 km thick crust at ~360-420 km distance from the trench, in a region under which the Cocos plate lays at >120 km depth. For these features, the volcanic sequences of the SOB offer a good opportunity to understand element recycling and magma generation in the Mexican arc, and the relationship between magmatic diversity and geodynamic context.

Our data reveal the existence of different mafic varieties in the study area, and permit to observe a relationship between the geochemical diversity of magmatism and its geographic distribution. Most cones emplaced towards the front vary in composition from calc-alkaline basalt to andesite, and display typical arc-like geochemical features, such as high LILE/HFSE (Ba/Nb=61-133) and moderate REE ratios (La/Yb=6-14; Gd/Yb=2-3). The southern part of the basin also hosts a few high-K mafic rocks with stronger LILE enrichments at similar HFSE (Ba/Nb=152-169) and higher REE ratios (La/Yb=24-43; Gd/Yb=4-6). In contrast, the volcanic products of the northern sector are basalts and basaltic andesites with higher TiO₂ and Na₂O, and higher HFSE at similar LILE (Ba/Nb=25-62), than those of the volcanic front.

The magmatic distribution pattern recognized in the SOB is consistent with different degrees of mantle melting produced by variable contributions from the oceanic plate. In particular, decreasing Ba/Nb and Zr/Nb (from 19-41 to 13-23) ratios in the volcanic products emplaced from the front to the rear-arc reflect a gradual decrease in slab fluxes added to the wedge, and hence lower degrees of mantle melting, as the Cocos plate sinks to higher depths. On the other hand, the coexistence of calc-alkaline and high-K rocks in the arc front suggest the involvement of different subduction components: LILE-enriched aqueous fluids likely contributed to the generation of the more typical calc-alkaline magmas; whereas stronger incompatible elements enrichments and REE fractionations in high-K rocks suggest derivation from a garnet-phlogopite-bearing source, formed by reaction of mantle peridotites with silicic slab melts.