ABSTRACT

Six new K-Ar ages were determined for three of four volcanic systems that constitute the San Pedro-Ceboruco graben of western Mexico, in order to give constraints on stratigraphic relationships and problems of magma genesis in an area characterized by the coexistence of magmas with different petrologic affinities (calc-alkaline and intra-plate type). Previous age determinations are scarce for this region. Samples were dated using the unspiked K-Ar sensitivity method with a mass-fractionation correction procedure. Our new data, along with those in literature, suggest that volcanism started at ∼2.5 Ma in the southwest part of the graben. Between ∼2.3 Ma to ∼0.1 Ma volcanic activity continued in the central part of the graben and starting from ∼0.5 Ma, volcanism also extended to the northern part. This northward shifting of the volcanic activity with time occurred along NW-SE trending lines which coincide with the main regional fault systems, and could be linked to the resumption of convergence after a period of near-cessation. The major phase of the magmatic activity is younger than 0.5 Ma, but the presence of volcanic activity strictly differentiated in magmatic affinities (orogenic and intra-plate), started earlier, around 1.1 Ma and is linked to sub-slab melts flowing in the mantle wedge. The age obtained on the dome complex (San Pedro-Cerro Grande Volcanic Complex) along with literature data suggest that the caldera collapse event occurred before ∼1.1 Ma and was followed by intracaldera and pericaldera activities. After ∼0.6 Ma, subsidence and consequent reactivation of the eastern caldera rim, can have determined the cutting of some pericaldera domes.