Estudios isotópicos de Hf en zircones de granitoides pérmicos en el NW de México: Evidencia de mezcla de magmas generados a partir de la fusión de múltiples fuentes corticales
This study presents new Lu-Hf isotopic data for magmatic zircons from Permian granitoids interpreted to be associated with the early stages of subduction in NW Mexico. In-situ microanalyses by LA-MC-ICPMS technique indicate large variations in Hf isotopic compositions of zircons with the same age and from the same granitic rock sample (up to 15 epsilon Hf units). The range of initial epsilon Hf values for these Permian zircons is between -9.0 and -24.9, suggesting a mixture of magmas derived from melting of different crustal sources to generate the precursor magmas that formed the granitoids. The Hf crustal model ages for zircons vary in a range between 1.59–2.39 Ga, with most of the values between 1.6–1.8 Ga, indicating that Proterozoic basement rocks, similar to those present in NW Mexico today, were the source of melting to form the predecessor magmas for the Permian granitoids. The Hf isotopic data presented in this study indicates a complex process of crustal magma formation that cannot be revealed from just conventional whole rock geochemical and isotopic studies (e.g., major and trace elements and Sm-Nd isotopes).
Our hypothesis about the evolution of crust/mantle input in magma genesis in NW Mexico from Proterozoic to the present proposes that the mantle played an important role in the initial stages of crustal formation (magmatism) during the Paleoproterozoic. Subsequently, from Mesoproterozoic to Permian time, the crust had a preponderant role in the generation of magmas. During the Jurassic and Cretaceous periods, the crustal input in magma formation starts to be significantly reduced in these subduction related magmas, to the point that Cenozoic felsic volcanic rocks have a large mantle component, but being this more extreme during the Quaternary, as basaltic lavas have an asthenospheric mantle signature with no evidence of crustal input (e.g., Pinacate Volcanic Field).