TERTIARY INTRAPLATE MAFIC MAGMATISM IN THE EASTERN MEXICAN ALKALINE PROVINCE: VILLA ALDAMA VOLCANIC COMPLEX

The Pliocene to Pleistocene Villa Aldama Complex (VAVC) is the only explosive volcanic locality of the Eastern Alkaline Province (EAP) in NE Mexico. It is located 70 km NNW of Tampico and represents the last stage of the magmatism in the eastern margin of the Sierra de Tamaulipas. Volcanic gasses triggered the formation of the the cenotes of Sistema Zacatón, the deepest underwater shafts. The VAVC lithology ranges from alkali basalts to olivine trachytes, corresponding to a cogenetic evolution according to major and trace elements correlations. Some alkaline and subalkaline basalts show geochemical characteristics nearly of primary magmas generated in subcontinental mantle zones. Primitive mantle-normalized multi-element diagrams for VAVC basaltic rocks can be correlated to extension related volcanics, which are characterized by enrichment in highly incompatible elements (e.g. Rb, Sr, Ba), HFSE (e.g. Nb, Ta, P, Ti), and LREE ([La/Lu]N = 8.31-25.43). The LILEE/LREEE (0.7-1.5) and LILEE/HFSEE (1.2-1.9) ratios for the nearly primary magmas correspond with values for extension-related magmas. The geotectonic setting is confirmed by discrimination diagrams. The Ol-Jd+CaTs-Qz system using molar normative compositions, indicates that the primitive alkali basalts were in equilibrium with the mantle at pressures from 15 to 20 kbar, corresponding with a maximal depth of ~55 km. Trace-element concentration data for nearly primary magmas from de VAVC are used to develop a partial melting inversion model. The source composition calculated by this method shows an enrichment in highly incompatible elements (e.g. [CLa/CYb]N >1), without a decoupling between LILE, REE, and HFSE, reflecting a relationship typical for extension-related areas. These results indicate that the VAVC mafic magmatism correspond to a partial melting of an enriched lithospheric mantle source, with a typical spinel peridotite mineral assemblage (ol + opx + cpx + sp), precluding the presence of the garnet in the mantle source. Adiabatic melting accompanying reactivation of deep cortical faults (e.g. Tamaulipas-Chiapas) during post Laramide regional extension through the middle and late Tertiary can be made responsible for the magma generation in this complex of the EAP.