We assembled a petrological and geochemical database for México's Quaternary volcanic rocks as one component of an interactive CD-ROM titled Volcanoes of México. That original database was augmented to a total of 2180 records for whole-rock analyses published through May 2004 in peer-reviewed literature, supplemented by a few Ph.D. dissertations for otherwise uncovered areas. The Quaternary volcanic rocks of México can be divided geographically into three tectonic settings: the Northern Mexican Extensional Province, Pacific islands, and the Mexican Volcanic Belt. The rocks also largely fall into three magma series: (1) intraplate-type alkaline, (2) calc-alkaline, and (3) lamprophyre. Many transitional varieties also exist, but we have established compositional rules to classify all samples into these three series. Intraplate-type alkaline rocks account for 30.8% of the database. Mafic intra-plate-type rocks are particularly abundant at Northern Mexican Extensional Province and Pacific island volcanoes. They are characterized by strong enrichments in Ti-Ta-Nb, and many have nepheline in their CIPW norms (named for the four petrologists, Cross, Iddings, Pirsson and Washington, who devised it in 1931) and carry xenoliths of deep-crustal granulite and upper-mantle spinel and/or plagioclase peridotite. Available data indicate that significant compositional differences exist between intraplate-type mafic rocks from these two tectonic environments, with the Pacific island examples relatively depleted in Cs, Rb, Th, U, K, Pb, and Sr compared to Northern Mexican Extensional Province equivalents. Mafic intraplate-type rocks from the Camargo and San Quintín fields in the northern part of the Northern Mexican Extensional Province are relatively enriched in 206 Pb/ 204 Pb (19.1–19.6), indicating likely involvement of HIMU (high µ) mantle in their genesis. Differentiated intraplate-type rocks (trachytes) are common at the Pacific island volcanoes, but nearly absent at the Northern Mexican Extensional Province volcanoes. Intraplate-type mafic alkaline rocks are also found in many different parts of the Mexican Volcanic Belt; we believe that the latter occurrences reflect involvement of Northern Mexican Extensional Province–type mantle sources in magma generation beneath the Mexican Volcanic Belt, where subduction-modified mantle is the dominant source feeding calc-alkaline and minor lamprophyric magmas to the surface. The calc-alkaline series, which accounts for 62.5% of the database, ranges from basalts (and lesser trachybasalts) to rhyolites but is dominated by andesites. These rocks are most prevalent in the E-W–trending, subduction-related Mexican Volcanic Belt, but are also found in Baja California, part of the Northern Mexican Extensional Province. They are characterized by enrichments in K-Ba-Sr and depletions in Ti-Ta-Nb, the classic global-scale features of subduction-related rocks. About 8.3% of the rocks from the Mexican Volcanic Belt have corundum in their CIPW norms, evidence of a significant role for sediment involvement in their petrogenesis, through either sub-duction of seafloor clays or contamination by pelitic lithologies during ascent through the crust. Sr and Nd isotopic data for calc-alkaline rocks from the Mexican Volcanic Belt form an array that is shifted toward higher 87 Sr/ 86 Sr compared to the intraplate-type suites, consistent with incorporation of subducted marine Sr. Calc-alkaline and lamprophyric rocks from Colima volcano and nearby Cántaro mark the depleted end of the Mexican Volcanic Belt isotopic array (lowest 87 Sr/ 86 Sr and 206 Pb/ 204 Pb, highest ϵ Nd ); the enriched end is marked by various basaltic andesites to rhyolites from the east-central part of the Mexican Volcanic Belt, where México's continental crust reaches its maximum thickness of 40–50 km, a fact that favors crustal contamination during magma ascent. Lamprophyres account for only 6.7% of the database. True lamprophyres, with phlogopite or amphibole phenocrysts in the absence of feldspar phenocrysts, are found exclusively in the western part of the Mexican Volcanic Belt, but compositionally (not mineralogically) similar rocks are found in four volcanic fields in northern Baja California, where they have been termed bajaites, and likened to adakites. Lamprophyres have extreme subduction-related geochemical signatures, with strong enrichments in K-Ba-Sr, and equally strong relative depletions in Ti-Ta-Nb. We consider western Mexican lamprophyres to represent the “essence of subduction,” partial melts of phlogopite- and apatite-rich veinlets in the subarc mantle, which are usually diluted by partial melts of the surrounding depleted peridotitic wall rocks to produce “normal” calc-alkaline magmas. Lamprophyres reached the surface in the western Mexican Volcanic Belt relatively undiluted by wall-rock melts only because of the strong extension imposed on the region by the influence of nearby plate-boundary activity.

Anhydrite has been identified as a phenocrystic phase in some silicic volcanic magmas, but it is not commonly described in plutonic rocks. Anhydrite-bearing magmas tend to form in arc environments and to contain hydrous, low-temperature, oxidised mineral assemblages. Phenocrystic anhydrite coexists with sulphur-enriched apatite and sometimes with pyrrhotite, in silicate melt that contains from 50 ppm to 1 wt% S, depending on temperature and f O 2 conditions. Vapour coexisting with anhydrite- and water-saturated magma may contain from a few tenths of a mole per cent to a few mole per cent sulphur gases (SO 2 and H 2 S), with the exact composition and gas speciation depending on temperature and oxygen fugacity. Samples of one anhydrite-bearing magma, the 1991 Pinatubo dacite, have been experimentally crystallised to determine whether the magma retains its characteristic sulphur-rich mineral phases during solidification. Results show that anhydrite and sulphur-rich apatite are retained throughout crystallisation and vapour phase evolution. This suggests that anhydrite-bearing intrusive equivalents of the Pinatubo dacite should be present in arc plutonic complexes.