Abstract
The origin of the large diversity of rock types erupted along the subduction-related Trans-Mexican Volcanic Belt (TMVB) remains highly debated. In particular, several hypotheses have been proposed to explain the contemporary eruption of calc-alkaline and alkaline magmas along the belt. The Michoacán-Guanajuato Volcanic Field (MGVF) is an atypical, vast region of monogenetic activity located in the western-central part of the TMVB. Here we present new petrographic, geochemical, and isotopic (Sr–Nd–Pb–Os) data on recent volcanics in the Jorullo-Tacámbaro area that is the closest to the oceanic trench. TMVB-related volcanics in this area are Plio-Quaternary (<5 Ma) and mainly form a calc-alkaline series from basalts to dacites, with rare (<5 vol. %) alkaline rocks that range from trachybasalts to trachydacites, and transitional samples. Crystal textures are consistent with rapid crystallization at shallow depth and processes of mixing of similar magma batches (magma recharge). All of the samples exhibit an arc-type trace element pattern. Alkaline and transitional magmas have higher Na2O and K2O, lower Al2O3, and higher concentrations in incompatible elements (e.g. Sr, K, Ba, Th, Ce, P) compared to calc-alkaline rocks. Calc-alkaline rocks are similar isotopically to transitional and alkaline samples, except for a few low 87Sr/86Sr samples. Sr, Nd and Pb isotopes do not correlate with MgO or 187Os/188Os, indicating that they were not significantly influenced by crustal contamination. Isotopic and trace-element systematics suggest that the Tacámbaro magmas are produced by melting of a mantle wedge fluxed by fluids derived from a mixture of subducted sediments and altered oceanic crust. Alkaline and transitional magmas can be derived from a lower degree of partial melting of a similar source to that of the calc-alkaline rocks, whereas the few low 87Sr/86Sr calc-alkaline rocks require a lower proportion of fluid derived from oceanic sediments and crust. Volcanism at the trenchward edge of the MGVF was thus driven purely by subduction during the last 5 Ma, hence discarding slab rollback in this sector of the TMVB.
CONTEMPORANEOUS ERUPTIONS OF CALC-ALKALINE AND ALKALINE MAGMAS ALONG THE VOLCANIC FRONT OF THE MEXICAN VOLCANIC BELT
