GEOCHEMICAL FEATURES OF ALKALINE BASALTS IN THE TEPIC-ZACOALCO RIFT, WESTERN MEXICAN VOLCANIC BELT

Na-alkaline basalts with geochemical features similar to those of intraplate Ocean Islands have been emplaced along the main trace of the Tepic-Zacoalco rift (TZR), a unique tectonic structure in which an extensional tectonic regime coexists with the Trans-Mexican Volcanic Belt. We report new geochemical data from two representative alkaline volcanic series from the TZR: 1) the Plan de Barrancas-Santa Rosa plateau basalts (PB-SR), located near the eastern edge of TZR and 2) the Sangangüey alkaline basalts (SBalk) close to the Pacific coast. Both series are mostly olivine phyric hawaiites and mugearites with overlapping major elements contents, but rocks belonging to the PB-SR are slightly more evolved.

Our new geochemical data show that the SBalk display the HFSE enrichments that are typical of some OIB (Ba/Nb= 3-12). On the other hand, the PB-SR series have slightly higher Ba/Nb (12-50) ratios, indicating slab or crustal contributions. Sr-Nd isotopic compositions mostly overlap between the series, but Pb isotopes of SBalk form a distinctive mixing trend bracketed between the Pacific MORB and an enriched mantle component that is indistinguishable to that found in some Mexican Pacific Islands, like Socorro and Isabel. In contrast the isotopic compositions of rocks from the PB-SR series form a sub-parallel array bracketed between the MORB and the Pacific plate sediments, indicating that small crustal Pb contributions can easily obscure the pristine isotopic features of the background mantle wedge. Melting sensitive geochemical proxies like Nb/Zr ratios are consistent with a gradual decrease in the extent of mantle melting towards the west. In contrast, more vigorous magmatic activity is promoted by a small amount of water in the source region of the PB-SR. The overall geochemical evidence thus indicates that the background mantle wedge in western Mexico is as heterogeneous as that below the Pacific basin. Extension-driven mantle upwelling in a continental setting can only melt a dry upper mantle to its lowest extents, and therefore sample its most enriched and easily fusible components. Yet even a small amount of slab-derived water will promote larger extents of melting and create magmas with compositions that are more typical of a volcanic arc setting.