K-Ar AND GEOLOGIC DATA BEARING ON THE AGE AND EVOLUTION OF THE TRANS-MEXICAN VOLCANIC BELT

New and published K-Ar dates and geologic mapping in the western and central parts of the Trans-Mexican Volcanic Belt (TMVB) provide a basis for examining the evolution of Late Cenozoic volcanism in Central Mexico. The modem volcanic arc, composed principally of calc-alkaline andesite and dacite, was formed during the Quaternary and its development is intimately connected with the contrasting subduction regimes of the Rivera and northern Cocos plates. Major andesitic centers in the western TMVB that lie above the subducted portion of the Rivera plate began to evolve between 0.6 and 0.2 Ma. However, in the central and eastern parts of the arc where subduction of the Cocos plate is taking place, the construction of andesite-dacife cones started considerably earlier at approximately 1.7 Ma. The width of active arc above the subducted Cocos plate has decreased appreciably during the Quaternary such that andesitic volcanism is currently focused nearest the trench. The early extinction of volcanism in the backarc region explains why historically active volcanoes are commonly found at the volcanic front and why they generally provide a more complete record of Quaternary volcanism. Their large volumes are a direct consequence of their longevity. The average eruption rates for major Quaternary cones are estimated at 0.2 - 0.3 km³/ka and do not appear particularly sensitive to differences in the rate of subduction. The development of an E-W - trending calc-alkaline arc system in Central Mexico probably dates back to Late Miocene or Early Pliocene. Arc volcanism since that time has migrated trenchward arriving at its present position by Early Quaternary time. Exposures of Early Pliocene to Recent K-rich lamprophyres in the Colima graben exhibit space-time patterns correlative with those deduced for coeval calc-alkaline volcanism and point to subduction as the common causative mechanism. Early Pliocene rifting and lamprophyric activity in the Colima graben are linked to the early stages of formation of a transform boundary separating the Cocos and Rivera plates. Today, this seismically active transform continues to be subducted beneath the Colima graben. Sodic alkaline suites in the westernmost part of the TMVB may represent backarc volcanism or be related to poorly understood phenomena occurring at the termination of subduction zones.