A North-South Transect across the Central Mexican Volcanic Belt at 100 W: Spatial Distribution, Petrological, Geochemical, and Isotopic Characteristics of Quaternary Volcanism

Abstract

Within the Zita¤ cuaro^Valle de Bravo (ZVB) region of the central Mexican Volcanic Belt (MVB), three lava series have erupted during the Quaternary: (1) high-K2O basaltic andesites and andesites; (2) medium-K2O basaltic andesites, andesites and dacites; (3) high-TiO2 basalts and basaltic andesites.The dominant feature of the first two groups is the lack of plagioclase accompanying the various ferromagnesian phenocrysts (olivine, orthopyroxene, augite, and hornblende) in all but the dacites. This absence of plagioclase in the phenocryst assemblages of the high-K2O and medium-K2O intermediate lavas is significant because it indicates high water contents during the stage of phenocryst equilibration. In contrast, the high-TiO2 group is characterized by phenocrysts of plagioclase and olivine.The spatial distribution of these three lava series is systematic.The southern section of the ZVB transect, 280^330 km from the Middle AmericaTrench (MAT), is characterized by high-K2Omelts that are relatively enriched in fluidmobile elements and have the highest Sr/Sr ratios. Medium-K2O basaltic andesite and andesite lavas are present throughout the transect, but those closest to theMATareMgO-rich (3 5^9 4 wt%) and have phenocryst assemblages indicative of high magmatic water contents (3 5^6 5 wt % water) and relatively low temperatures (950^10008C). In marked contrast, the northern section of the ZVB transect (380^480 km from the MAT) has high-TiO2, high field strength element (HFSE)-enriched magmas that have comparatively dry (51 5 wt % magmatic water) and hot (1100^12008C) phenocryst equilibration conditions. The central section of the ZVB transect (330^380 km from the MAT) is a transition zone and produces moderately light rare earth element (LREE) and large ion lithophile element (LILE)-enriched, medium-K2O lavas with phenocryst assemblages indicative of intermediate (1 5^3 5 wt %) water contents and temperatures. The high-K2O series compositions are the most enriched in LILE and LREE, with a narrow range of radiogenic Sr/Sr from 0 704245 to 0 704507, Nd/Nd values ranging from 0 512857 to 0 512927 (eNd1⁄4 4 27^5 63), and Pb/Pb values from 38 248 to 38 442, Pb/Pb values from 15 563 to 15 585, and Pb/Pb values from 18 598 to 18 688. The medium-K2O series compositions are only moderately enriched in the LILE and LREE, with a broader range of Sr/Sr, but similar Nd/Nd and Pb/Pb values to those of the high-K2O series. In contrast, the high-TiO2 series compositions have little enrichment in LILE or LREE and instead are enriched in the HFSE and heavy rare earth elements (HREE). The high-TiO2 lavas are isotopically distinct in their lower and narrower range of Nd/Nd. The isotopic variations are believed to reflect the upper mantle magma source regions as the low content of phenocrysts in most lavas precludes significant upper crustal assimilation or magma mixing, other than that represented by the presence of quartz xenocrysts (52 vol. %) with rhyolitic glass inclusions, which are found in many of these lavas.The systematic spatial variation in composition of the three lava series is a reflection of the underlying subduction-modified mantle and its evolution.