Heterogeneous nature of the pre-subduction mantle wedge and their transformation to pyroxene-rich lithologies by slab-derived chemical agents: Tepic-Zacoalco rift, western Trans-Mexican Volcanic Belt

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) in western Mexico. Comprehensive geochemical and petrologic data on volcanic rocks along the rift indicates that the pre-subduction background mantle wedge in western Mexico is as heterogeneous as that below the Pacific basin, and includes a recycled, high-μ component (HIMU; μ =238U/204Pb) in their mantle source. Olivines contained within these samples have NiO and CaO contents similar to olivines from MORB, suggesting that the source of enrichment must be entirely hosted in peridotite. More evolved rocks within the TZR have a stronger subduction signatures and water contents, and display a distinctive Pb isotopic array that suggest slab additions. Olivine phenocrysts from these rocks have lower Fo but extend higher NiO and lower CaO contents than those from more mafic magmas, suggesting derivation from a secondary pyroxenite source. It thus appears that the heterogeneous nature of the pristine pre-subduction mantle wedge in western Mexico can only be sampled when extension-driven mantle upwelling induces low extents of melting of a dry peridotitic mantle that preferentially sample its most enriched and easily fusible components. Yet even a small amount of slab-derived silica promotes a secondary petrologic transformation to pyroxene-rich lithologies that upon remelting create magmas with compositions that are similar to arc volcanoes. Metasomatic silica addition is ultimately related to the convergent margin, but deep subduction of Rivera plate below the TZR preclude a direct derivation from the slab surface. The participation of hybrid subduction mélanges that detach from the downgoing slab and melt in the hot core of the mantle wedge can provide a viable alternative explanation.