Geochemical evidence for a rift-related origin of bimodal volcanism at Meseta Rio San Juan, north-central Mexican volcanic belt

This study reports new geochemical and Sr and Nd isotope data for 11 sample s of hy-normative late Miocene (similar to6.5 Ma) basalt, basaltic andesite , and rhyolitic volcanic rocks from Meseta Rio San Juan, located in the sta tes of Hidalgo and Queretaro, Mexico, in the north-central part of the Mexi can Volcanic Belt (MVB). The in situ growth-corrected initial isotopic rati os of these rocks are as follows: Sr-87/Sr-86 0.703400-0.709431 and Nd-143/ Nd-144 0.512524-0.512835. For comparison, the isotopic ratios of basaltic r ocks from this area show very narrow ranges as follows: 87Sr/86Sr 0.703400- 0.703540 and Nd-143/Nd-144 0.512794-0.512835. The available geological, geo chemical, and isotopic evidence does not support the generation of the basi c and intermediate magmas by direct (slab melting), nor by indirect (fluid transport to the mantle) participation of the subducted Cocos plate. The ba saltic magmas instead could have been generated by partial melting of the u pper mantle. The evolved basaltic andesite magmas could have originated fro m such basaltic magmas through assimilation coupled with fractional crystal lization. Rhyolitic magmas might represent partial melting of different par ts of the underlying heterogeneous crust. Their formation and eruption prob ably was facilitated by extensional tectonics and upwelling of the underlyi ng mantle. The different petrogenetic processes proposed here for basaltic and basaltic andesite magmas on one hand and rhyolitic magmas on the other might explain the bimodal nature of Meseta Rio San Juan volcanism. Finally, predictions by the author about the behavior of Sr and Nd isotopic composi tions for subduction-related magmas is confirmed by published data for the Central American Volcanic Arc (CAVA).