La Primavera is a large Quaternary comenditic volcano situated at the western end of the dominantly andesitic Mexican Volcanic Belt. There is no central vent or cone but a multitude of low hills of rhyolite scattered over 220 km2 stand above an ignimbrite apron. The rhyolite hills consist of lavas and pyroclastic rocks and appear to lie along inner and outer rings; within the inner ring there are sedimentary rocks which were apparently deposited in a caldera lake. A general geological account, including a detailed stratigraphical history substantiated by numerous age determinations, will be found in Mahood (1980). Clough et al. (1982) describe the morphology and dimensions of the younger lavas, while Walker et al. (1981) have investigated the pyroclastic geology and distinguished more than 30 fall deposits totalling some 90 km3. The lavas are mildly peralkaline, those of the inner ring generally being porphyritic, those of the outer aphyric. Phenocrysts are sanidine, quartz, ferrohedenbergite, fayalite, ilmenite and rare titanomagnetite, zircon, and apatite (Mahood, 1981, p. 130). Major and minor element analyses of rocks and microprobe analyses of minerals will be found in Mahood (1981). The Guadalajara ignimbrite outcrops north and northeast of La Primavera, and consists of nearly equal proportions of aphyric and sparsely porphyritic peralkaline rhyolite (Mahood et al., 1985).