Hawaiite and mugearite, alkali-olivine basalt, and basaltic andesite have erupted from three shield volcanoes in the Atenguillo graben, Jalisco, Mexico. The youngest shield volcano, La Laja (0.66 Ma), is the focus of this study. Basal pyroclastic surge deposits of basaltic ash, lapilli, and blocks are overlain by seven to eight, 5- to 10-m-thick hawaiite flows. Pillow lavas along a 4-km stretch on the southeast flank of the volcano and lake-bed sediments define a shoreline and lake bottom southward to the town of Atenguillo. Eruption of La Laja apparently dammed the northward-flowing Atenguillo River and formed a lake of as much as 100-m depth. Subsequent fluvial erosion has destroyed the lava dam, exposing the shield succession. Discharge rates for the Atenguillo River and rainfall estimates for the Atenguillo basin both suggest that this lake formed within 20 to 40 yr. Eruption of nearly 10 km3 of material within 10 to 100 yr indicates an output rate of 0.1 to 1 km3/yr, which is larger than any rate calculated for volcanoes in the Mexican Volcanic Belt (MVB), but similar in magnitude to that of Mauna Loa and Kilauea volcanoes (Hawaii). The older volcanoes, El Vigia (2.7 Ma) and Tin Cleto (3.6 Ma), are composed of basaltic-andesite and alkali-olivine basalt, respectively.

The composition of these lavas is unusual for a continental-arc setting. Total FeO, TiO2, and alkali concentrations are higher than those of basic calc-alkaline lavas from the western MVB. Alkaline earth/light-rare-earth and light-rare-earth/high-field-strength element ratios in these lavas are low relative to those of calc-alkaline lavas but similar to those of oceanic-island basalt values. Such compositional traits coupled with a high magma-output rate are characteristic of oceanic-island volcanoes. Both hydrous, fluid-enriched, sub-arc mantle, and oceanic-island-type mantle components are present beneath western Mexico; contemporaneous subduction and extension of this region of the western MVB allow tapping of both sources and thus the eruption of compositionally diverse lavas.

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