THE LIFE AND TIMES OF AN ACCRETED CRETACEOUS EXTENSIONAL ARC (ROSARIO SEGMENT OF THE ALISITOS ARC); AN OUTSTANDING ANALOG TO ACTIVE WESTERN PACIFIC EXTENSIONAL ARCS

The Rosario segment of the Cretaceous Alisitos oceanic arc provides outstanding 3-D exposures of an extensional arc, where crustal generation processes are recorded in a structurally intact upper- to middle crustal section. Field and geochemical linkages between exposed crustal levels provide an analog for extensional arc systems such as the Izu-Bonin-Mariana (IBM) Arc.

Upper-crustal units comprise a 3-5 km thick volcanic-volcaniclastic stratigraphy, including rhyolitic ignimbrites and basaltic lava flows, intruded by sills and dikes. Coarse-grained plutonic rocks intrude these units over a transition of <500m (100% plutonic to 100% volcanic), where rafted volcanic blocks were progressively engulfed by the pluton. There is striking compositional overlap in whole-rock and mineral chemistry between the plutonic and volcanic units (basalt to rhyolite), supporting a co-magmatic source. Units are predominantly low K with flat REE patterns, and show LILE enrichment and HFSE depletion. Initial isotope ratios (Sr, Nd, Pb) overlap for all units, and the limited range in ENd (5.7–7.1) implies no continental craton involvement. This agrees with low Sr/Y ratios of all rock types, indicative of thin, immature island arc crust that is estimated to be <20 km thick.

Our new geochemical data match that of the extensional zone immediately behind the Izu arc front, and is different from the arc front and rear arc, consistent with geologic relations. New single-crystal zircon analysis by TIMs supports the interpretation that the entire upper-middle crustal section generated in ca. <4 myr. Differentiation occurred in a closed system (i.e., fractional crystallization/self-melting with back mixing), producing the entire crustal section in a relatively short period of time.

As a result of this study, we are developing a publically accessible, 3D oceanic arc crustal model, with geologic maps draped on Google Earth images, and GPS-located outcrop information linked to new geochemical, geochronological and petrographic data, with the goal of detailing the relationships between plutonic, hypabyssal, and volcanic rocks.