Geology and petrology of Cerro Azul volcano, Isabela Island, Galapagos Archipelago

Cerro Azul is a 1,640 m active basaltic shield volcano forming the southwestern end of Isabela Island in the western Galapagos Archipelago. The Galapagos are one of the most active volcanic fields in the world, with at least eight historically-active volcanoes resulting from a hotspot centered today under the westernmost Island of Fernandina. Ten documented eruptions have occurred at Cerro Azul between 1932 and 1998, an average of one eruption approximately every 6.5 years. This investigation was initiated as part of a systematic study of the western Galapagos volcanoes.;Part A. Pioneering studies in experimental petrology predicted that alkaline magmas could be derived from tholeiitic magmas by high pressure crystal fractionation. It has been demonstrated in many volcanic provinces, however, that alkaline basalts are derived from different mantle sources, from greater depths, or by lower extents of melting than tholeiitic basalts erupted from the same volcano. A suite of contemporaneous alkaline and tholeiitic basalts was recognized at Cerro Azul that are related by high-pressure fractional crystallization in the lower crust and upper mantle. The higher pressures enhance augite crystallization, which depletes the residual liquids in silica while enriching them in K and Na, just as predicted by the phase equilibria experiments.;Part B. Observations support the contention that the steep upper slopes were constructed by the buildup of short lava flows rather than by the structural deformation of shallow dipping primary slopes. In addition to the large eruption rates, a subdued lower flank geometry is promoted by deposition of lava deltas onto the shallow Galapagos platform. 40Ar/39Ar geochronology and volume estimates show that despite their morphologic differences, the growth of the western Galapagos shields has been nearly synchronous. The wide variations in elevation, volume, area, and the distribution of flank slope angles among the western volcanoes can be linked instead to the long-term eruption rate and, to a lesser degree, the position of each volcano relative to the edge of the Galapagos platform. This suggests that the morphology and location of Galapagos volcanism is controlled by a combination of plume flux and lithospheric structure above the Galapagos plume.;Part C. Major and trace element modeling show that the source for Cerro Azul and Sierra Negra is identical, while the source for Alcedo's basalts is slightly more isotopically depleted. Modeling also reveals that all three volcanoes share a similar depth of melting in the garnet stability field, and there are small (2--3%) systematic differences in the extent of partial melting between the volcanoes that can be related to their distance from the proposed plume center below the westernmost island of Fernandina. (Abstract shortened by UMI.)...