Sr, Nd and Pb isotope and geochemical data from the Quaternary Nevado de Toluca volcano, a source of recent adakitic magmatism, and the Tenango Volcanic Field, Mexico

Abstract

Volcanic activity at Nevado de Toluca (NT) volcano began 2.6 Ma ago with the emission of andesitic lavas, but over the past 40 ka, eruptions have produced mainly lava flows and pyroclastic deposits of predominantly orthopyroxene–hornblende dacitic composition. In the nearby Tenango Volcanic Field (TVF) pyroclastic products and lava flows ranging in composition from basaltic andesite to andesite were erupted at most of 40 monogenetic volcanic centers and were coeval with the last stages of NT. All volcanic rocks in the study area are characterized by a calc-alkaline affinity that is consistent with a subduction setting. Relatively high concentrations of Sr (>460 ppm) coupled with low Y (<21 ppm), along with relatively low HREE contents and Pb isotopic values similar to MORB-EPR, suggest a possible geochemical adakitic signature for the majority of the volcanic rocks of NT. The HFS- and LIL-element patterns for most rocks of the TVF suggest a depleted source in the subcontinental lithosphere modified by subduction fluids, similar to most rocks from the Trans-Mexican Volcanic Belt (TMVB). The isotopic compositions are similar for volcanic rocks of NT and TVF regions ( 87Sr/ 86Sr: 0.703853–0.704226 and 0.703713–0.704481; ɛNd: +4.23–+5.34 and +2.24–+6.85; 206Pb/ 204Pb: 18.55–18.68 and 18.58–18.69; 207Pb/ 204Pb: 15.54–15.62 and 15.56–15.61; 208Pb/ 204Pb: 38.19–38.47 and 38.28–38.50, respectively), suggesting a MORB-like source with low crustal contamination. Metamorphic xenoliths from deeper continental crust beneath NT volcano show isotopic patterns similar to those of Grenvillian rocks of north-central Mexico ( 87Sr/ 86Sr: 0.715653–0.721984, ɛNd: –3.8 to –7.2, 206Pb/ 204Pb: 18.98–19.10, 207Pb/ 204Pb: 15.68–15.69, 208Pb/ 204Pb: 39.16–39.26 and Nd model age (T DM) of 1.2–1.3 Ga). In spite of a thick continental crust (>45 km) that underlies the volcanoes of the study area, the geochemical and isotopic patterns of these rocks indicate low interaction with this crust. NT volcano was constructed at the intersection of three fault systems, and it seems that the Plio–Quaternary E–W system played an important role in the ascent and storage of magmas during the recent volcanic activity in the two regions. Chemical and textural features of orthopyroxene, amphibole and Fe–Ti oxides from NT suggest that crystallization of magmas occurred at polybaric conditions, confirming the rapid upwelling of magmas.