Abstract The eruption of the c. 10 km 3 rhyolitic Las Derrumbadas twin domes about 2000 years ago generated a wide range of volcano-sedimentary deposits in the Serdán-Oriental lacustrine basin, Trans-Mexican Volcanic Belt. Some of these deposits have been quarried, creating excellent exposures. In this paper we describe the domes and related products and interpret their mode of formation, reconstructing the main phases of the eruption as well as syn- and post-eruptive erosional processes. After an initial phreatomagmatic phase that built a tuff ring, the domes grew as an upheaved plug, lifting a thick sedimentary pile from the basin floor. During uplift, the domes collapsed repeatedly to form a first generation of hetero-lithologic hummocky debris avalanche deposits. Subsequent dome growth produced a thick talus and pyroclastic density currents. Later, the hydrothermally altered over-steepened dome peaks fell to generate second-generation, mono-lithologic avalanches. Subsequently, small domes grew in the collapse scars. From the end of the main eruptive episode onwards, heavy rains remobilized parts of the dome carapaces and talus, depositing lahar aprons. The Las Derrumbadas domes are still an important source of sediments in the basin, and ongoing mass-wasting processes are associated with hazards that should be assessed, given their potential impact on nearby populations.

ABSTRACT The Oaxacan Complex represents the largest outcrop of Grenvillian basement in Mexico. Broadly, it consists of pelitic gneisses, quartzofeldspathic gneisses, metasomatic calc-silicates, orthoamphibolites, and marbles, all intruded by anorthosites, orthocharnockites, and orthogneisses. The entire assemblage underwent granulite-facies metamorphism ca. 1 Ga. We studied for the first time the ultramafic rocks of the Oaxacan Complex, represented by six different samples, all corresponding to ultramafic granulites. Their igneous equivalents are orthopyroxenites, websterites, and clinopyroxenites, and they occur as metric-scale lenses or centimetric layers in paragneisses, or in mingling textures with anatectic marbles. We studied their petrography, geochemistry, geochronology, and geothermobarometry to elucidate their genesis and tectonic implications. Our samples have enriched mid-ocean-ridge basalt and oceanic-island-arc affinities, both tholeiitic and calc-alkaline. Rare earth element patterns normalized to chondritic uniform reservoir from whole rock or single minerals define two or three main groups related to their origin and metamorphic history. Based on their protoliths, these rocks can be divided into: (1) ortho-derived pyroxenites (pre–Grenvillian orogeny), the origin of which was a magmatic cumulate or mafic melt or a mantle rock that had undergone metasomatism; and (2) para-derived pyroxenites (syn- or post-Grenvillian orogeny), the origin of which was a calc-silicate rock undergoing pervasive anatectic and metasomatic processes. The geothermobarometry revealed different stages in the syn- and post-Grenvillian granulitic metamorphic history of the Oaxacan Complex. The high temperature calculated from one sample (~945 °C), in the ultrahigh-temperature metamorphic field, is probably closer to the granulitic metamorphism peak than those obtained in previous studies, although a relict igneous temperature cannot be ruled out with the present data.