Mineralogical and geochemical constraints for the origin and evolution of magmas in Sierra Chichinautzin, Central Mexican Volcanic Belt

Abstract

The Sierra Chichinautzin Volcanic Field is a key area for understanding the origin of the controversial Mexican Volcanic Belt (MVB). This is due to its recent volcanic activity, which is located at the front of the Central MVB. Although a direct genetic relationship between the volcanism of the MVB and the subduction of the Cocos plate is accepted by most authors, geological, geophysical and geochemical key features of the MVB depart from those of typical magmatic arcs. This fact has promoted a debate on the nature of the complex mantle beneath the MVB: (1) slab-induced convection in the mantle wedge that causes advection of asthenospheric mantle and (2) anomalously hot mantle related to the eastward channeling of plume-related material. This is a petrological study of the Sierra Chichinautzin volcanism, using mineralogical data from selected representative samples, together with their geochemistry, to refine the existing models of magmatic processes. The integrated petrological analyses have been used to define four main groups of rocks in Sierra Chichinautzin: (1) Ol-mafic rocks, (2) Opx-intermediate rocks, (3) Opx-felsic rocks, and (4) Qtz-intermediate/felsic rocks. Moreover, two different types of mantle-derived primitive mafic magmas have been described based on their mineralogical and geochemical features: (1) OIB-like mafic magmas (presence of olivine+plagioclase phenocrysts; higher incompatible element contents with no HFSE/LILE negative anomalies), and (2) mafic magmas derived from a metasomatized mantle (no plagioclase phenocrysts and more forsteritic olivine; HFSE/LILE negative anomalies). We prove that the diversity of magmas (basalts to dacites) in Sierra Chichinautzin is un-related to fractional crystallization processes from the mafic magmas. Rather, magma mixing phenomena between mafic magmas and two different crustal-derived felsic magma types (Opx-dacite and Qtz+Bt+Amp+Pl+Cpx+Opx-dacite) explain the textural, mineralogical, and geochemical signatures of most of the Sierra Chichinautzin rocks. There is a strong genetic relationship between the mafic and felsic end members of each mixing trend. The relatively anhydrous OIB-like mafic magmas promote partial melting of a basaltic lower crust under low water fugacity conditions, generating Opx-bearing felsic rocks. On the contrary, hydrous mafic magmas derived from a metasomatized mantle, promote basaltic crustal partial melting under high water fugacities, yielding magmas with a complex hydrous phenocryst assemblage. Several important geodynamical implications for the origin of the MVB magmatism can be extracted from this petrological study: (1) the existence of two different primitive mafic magmas implies the presence of an heterogeneous mantle source beneath Sierra Chichinautzin; (2) present data do not offer enough evidence to constrain the nature of the metasomatizing fluids beneath Sierra Chichinautzin; and (3) the negative Nb anomaly of most of the Sierra Chichinautzin rocks (andesites and dacites) is due to magma mixing, and therefore is not a signature of the role of fluids from the subducting Cocos plate. These geochemical features, together with the geophysical and tectonic scenario of the Sierra Chichinautzin, indicate that volcanism in this area is mainly related to the upwelling of an anomalously hot OIB-like mantle, and cast further doubts on a significant role of the subducting Cocos plate in the origin of this magmatism.