Abstract:
In order to investigate the role that mafic-silicic magma interactions play in the origin, evolution, and eruption of shallow crustal magma chambers, a three-part study was undertaken of both effusive (Mt. Dutton volcano, Alaska) and explosive (Volcan Ceboruco, Mexico) eruptions, as well as associated volcanic (Unalaska Formation) and plutonic (Captain's Bay pluton) suites. Major- and trace-element variations suggest that the eruptive products (both andesite and dacite) of Mt. Dutton are not simply a result of fractional crystallization, but instead are affected to varying degrees by two-component mixing of distinct and separate magmas. In this case, petrologic and geochemical evidence, as well as eruptive stratigraphy, suggests the evolution of shallow, silicic magmatic systems inferred to exist beneath small stratovolcanoes can be modeled as resulting from repeated intrusion of mantle-derived mafic magmas into shallow, silicic, crystal-rich, crustal magma chambers. Volcan Ceboruco, Mexico, erupted ~1000 years ago, producing the Jala Pumice and forming a ~4 km wide caldera. During that eruption, 2.8 to 3.5 km3 of rhyodacite magma and 0.2 to 0.5 km 3 of mixed dacite magma were tapped and deposited as the Jala Pumice. Subsequently, the caldera was partially filled by extrusion of the Dos Equis Dome, a low-silica dacite dome with a volume of ~1.3 km3. In this case, petrographic evidence indicates that the Jala and Dos Equis dacites originated largely through the mixing of three end-member magmas: (1) rhyodacite magma, (2) dacite magma, and (3) mafic magma. Study of the Captain's Bay pluton and Unalaska Formation volcanics from Unalaska Island, Alaska, indicates that whole-rock compositions between the two suites span a similar range and particular plutonic units correspond chemically to specific volcanic products. Plagioclase phenocrysts from these chemically similar units also display comparable textures and compositional zoning patterns. Most strikingly, magmatic enclaves found within the pluton show a chemical affinity to andesite lavas from the volcanic suite. In this case, mixing of melts and extrusion of hybrid lava may be a prompt response to recharge, whereas the enclaves may represent "leftovers" that thermally equilibrated with the reservoir as a whole.
