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 150m, 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.

"Using results from geological mapping, detailed stratigraphy, mineral chemistry, and geochemistry, the authors have developed a model to explain characteristics of Dominica not seen on many island arcs worldwide. The model suggests that during the Pleistocene partial melting of the island-arc crust generated a single magma body of batholithic proportions beneath the island"--Provided by publisher.

Petrochronology is a rapidly emerging branch of Earth science that links time (ages or rates) with specific rock-forming processes and their physical conditions. It is founded in petrology and geochemistry, which define a petrogenetic context or delimit a specific process, to which chronometric data are then linked. This combination informs Earth’s petrogenetic processes better than petrology or geochronology alone. This volume and the accompanying short courses address three broad categories of inquiry. Conceptual approaches chapters include petrologic modeling of multi-component chemical and mineralogic systems, and development of methods that include diffusive alteration of mineral chemistry. Methods chapters address four main analytical techniques, specifically EPMA, LA-ICP-MS, SIMS and TIMS. Mineral-specific chapters explore applications to a wide range of minerals, including zircon (metamorphic, igneous, and detrital/Hadean), baddeleyite, REE minerals (monazite, allanite, xenotime and apatite), titanite, rutile, garnet, and major igneous minerals (olivine, plagioclase and pyroxenes). These applications mainly focus on metamorphic, igneous, or tectonic processes, but additionally elucidate fundamental transdisciplinary progress in addressing mechanisms of crystal growth, the chemical consequences of mineral growth kinetics, and how chemical transport and deformation affect chemically complex mineral composites. Most chapters further recommend areas of future research.

Most of the world's great volcanic massive sulphide ore deposits, most of its porphyry copper ores, and many of its richest gold deposits occur in association with basalt-andesite-dactite-rhyolite laval suites that have been erupted from the volcanoes of ancient volcanic island arcs and their Precambrian equivalents. The volcanic arc and related aquatic-volcanic environments are now recognized as critical to an understanding of the evolution of the earth's crust, to the unravelling of the petrogenesis of many important ore types, and to the planning of much mineral exploration. This book is concerned with the behaviour of the `ore elements' - copper, zinc, lead, cobalt, nickel, barium, and other - in young, unmetamorphosed volcanic arc suites ranging from high-magnesium basalts to dactites and rhyolites. Using principally the Late Tertiary to Recent lavas of the Solomon Islands Younger Volcanic Suite, the author considers the patterns of abundance of these elements bothin the `whole rocks' and their constituent minerals, and the way in which the metals may be enriched or impoverished in the residual melt as an arc lava series evolves from basalt to rhyolite.

Approximately 6,100 m of metamorphosed volcanic and sedimentary rocks are exposed near Mision Calamajue (latitude 29° 25' longitude 114° 15'). Lower greenschist metamorphism and shear cleavage parallel to axial plane folation is pervasive in the area. The lowermost 1,000 m represent a coarsening-upward sequence of thin-bedded to massive silty claystone interlayered with limestone, pebble conglomerate and bedded chert and capped by a coarse-grained chert litharenite. Conodonts recovered from near the top of this underlying unit include Gnathodus bilineatus (Roundy) of Early Chesterian age. The overlying 2,300 m consist of intercalated basalt, hornblende andesite and dacite submarine flows, overlain by 400 m of dacite to rhyolite tuff, volcaniclastic sediments, pebbly mudstone, pebble conglomerate and metaquartzite boulder conglomerate. These rocks represent an island-arc system. Overlying the volcanic sequence is 2,000 m of very thinly-bedded clayey siltstone containing widely spaced intervals of limestone. Conodont fragments recovered from this overlying unit limit the upper age of the volcanic strata to Carboniferous. A Pb/Pb minimum age of 262 Ma was provided by the analysis of U/Pb from one discordant zircon subfraction. This is consistent with the Carboniferous age provided by the conodonts. A highly sheared, 400 m thick basic volcanic or hypabyssal unit interfingers with this 2,000 m thick thin-bedded clayey siltstone and represents a mylonitized zone. The study area of Mision Calamajue may be a southern extension of the Carboniferous inner-arc basin between the Antler Orogenic Highland and the Klamath-Northern Sierra Nevada island-arc. Palinspastic reconstruction along the Mojave-Sonora Megashear(s) may juxtapose the Mision Calamajue area with other island-arc sequences in the western United States.