Building Arc Crust – Plutonic to Volcanic Connections in an Extensional Island Arc, the Alisitos Arc Crustal Section (Southern Rosario Segment), Baja California

The southern volcano-bounded basin of the Rosario segment of the Cretaceous Alisitos oceanic arc provides outstanding 3-D exposures of the upper 7 km of an extensional arc, where crustal generation processes are recorded in the upper crustal volcanic rocks and underlying plutonic rocks. These exceptional exposures allow for the study of the physical and chemical links between the rock units, and helps constrain the differentiation processes active during the growth and evolution of the arc. Upper crustal volcanic rocks comprise a 3-5 km thick volcanic-volcaniclastic stratigraphy with shallow sill and dike intrusions. Coarse-grained plutonic rocks intrude these units over a transition of \u3c150 m, where rafted volcanic blocks are exposed. There is striking compositional overlap in whole-rock and mineral chemistry between the plutonic and volcanic units, supporting a comagmatic source. Whole-rock geochemistry shows similar linear trends in mafic to intermediate compositions, and non-linear trends in felsic compositions. 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 imply no cratonic continental involvement. This agrees with low Sr/Y ratios of all rock types, indicative of thin, immature island arc crust. Modeling results show that closed-system fractional crystallization drove differentiation for mafic-intermediate to intermediate compositions, where open-system processes likely occurred to produce some of the felsic compositions. Differentiation occurs in a two-step fractionation process. Step 1 fractionates an anhydrous gabbroic cumulate from a hydrous basaltic andesite parental melt to produce andesite after 40% crystallization. Step 2 fractionates a hydrous amphibole cumulate from the andesite to produce rhyolite after a total of 65% crystallization. Our results can be used as a reference model for differentiation processes relating to the growth of the middle and upper crust within active extensional systems, such as the Izu Bonin Arc. Exposed mid crustal plutonic units may be analogous to the low-velocity zone (Vp=6.0-6.5 km/s) imaged within the extensional Izu Bonin Arc. Comparisons with across-arc segments of the Izu Bonin Arc implies that the entire southern-volcano bounded basin suite of rocks were built from extensional active rift type magmatism. Similar U-Pb zircon ages for plutonic and volcanic units imply that the entire crustal column (at least 435 km3) was built in less than 8 million years