Origin of the magmatic varieties of the Serdán-Oriental Basin, eastern Trans-Mexican Volcanic Belt

Quaternary magmatic activity in the Serdán-Oriental Basin (SOB) of the eastern Trans-Mexican Volcanic Belt produced mafic-intermediate monogenetic cones of variable geochemical affinities, that 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 the opportunity to understand the mechanisms of element recycling and the origin of magmatic diversity in the Mexican arc. Our data permit to observe a relationship between the geochemical diversity of magmatism and its geographic distribution. Most cones emplaced at the volcanic front, south of Malinche and Pico de Orizaba stratovolcanoes, vary in composition from calc-alkaline basalt to andesite, and display typical arc-like geochemical features such as high LILE-LREE/HFSE and moderate REE ratios. The southern part of the basin also hosts a few high-K mafic cones with stronger LILE-LREE enrichments at similar HFSE contents, and more fractionated REE patterns; interestingly, high Gd/Yb ratios in these rocks are coupled with high Nb/Ta and Sm/Zr. The basalts and basaltic andesites emplaced at larger distance from the trench display progressively higher Ti and HFSE contents than those of the volcanic front at similar LILE. On the other hand, the mafic cones emplaced north of Malinche display the lowest LILE-LREE/HFSE ratios, with high-Nb compositions similar to those of intraplate magmas. The distribution pattern of volcanism recognized in the SOB is consistent with different degrees of mantle melting produced by variable contributions from the oceanic plate. In particular, decreasing Ba-La/Nb and Zr/Nb 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. The geochemical features of the high-K suite indicate that the metasomatic agent involved in the petrogenesis is a partial melt of the eclogite-facies subducting oceanic plate. Specifically, strong HREE depletions and fractionations, high Nb/Ta ratios and negative Zr-Hf anomalies in these rocks provide evidence for the presence of residual garnet and rutile in the slab; whereas preferential breakdown of white mica and allanite/monazite may account for the marked LILE-LREE enrichments. The contribution of this high-pressure subduction agent is consistent with the steep geometry and great depth of the Cocos plate beneath the SOB. In lower proportions, this slab melt component also appears to contribute to the genesis of the typical calc-alkaline rocks that coexist with the high-K suite at the arc front; and it becomes progressively more diluted in the transitional products emplaced along the central and northern sectors of the basin. On the other hand, the high-Nb suite was likely produced by low degrees of decompression melting of a drier mantle, with negligible contributions from the deeply subducting slab. Work supported by PAPIIT-UNAM IB100912-2 grant.