Comparison of Mesozoic magmatic evolution and iron oxide (-copper-gold) ('IOCG') mineralization, Central Andes and western North America.

摘要

Comparison of Mesozoic magmatism along the western margins of North and South America reveals similar episodic patterns of activity, but differences in tectonic setting, in composition, in peak magnitudes of magma fluxes, and in the development of IOCG mineralization. Peak magmatism in the coastal central Andes was emplaced in an extensional setting, was dominantly dioritic, and characterized by uniformly depleted mantle-like Nd isotopes. Contrasting North American arcs had peak magmatic outputs during or closely following contractional periods, reworked considerable older crust as evidenced by Nd isotopes, and were dominantly granitic-granodioritic. Maxima in Andean Coastal Batholith magmatism may reflect rapid reworking of juvenile material generated in response to back arc extension operating on a thinning crust whereas maxima in other regions may reflect fundamentally different processes.;Episodic magmatism and magmatic "flare-ups" can be generated in both extensional and contractional settings. There is no unique model to explain these phenomena as they reflect the thermal state of the lower crust and its ability to melt and assimilate into arc derived basalts. Heterogeneities in lower crustal composition, crustal thickness, and tectonic setting all contribute to the observed chemical evolutions of North and South American Cordilleran batholiths. Because the same pattern of episodicity and magnitudes of peak magmatic flux are observed in both contractional and extensional settings, the nature of episodic magmatism appears to be influenced by far-field global tectonic processes and thermal limitations to heating or cooling of the lower crust.;The Jurassic arc of the central Mojave Desert, California, and Jura-Cretaceous arc of the central Andes are regions where IOCG mineralization is prevalent, however they are not equally endowed with Cu+/-Au. Comparison of these regions highlights key genetic processes for IOCG deposits. Igneous rocks provide the heat to circulate saline (evaporitic) fluids, resulting in the generation of abundant, typically metal-depleted Na+/-Ca alteration, Fe-oxide-rich mineralization, and Cu+/-Au only when there is an available sulfur source to trap chalcophile elements. Comparison of the tectonic setting, magmatic evolution, and IOCG systems of North and South America indicates that rather ordinary arc-derived magmas are associated with their formation. Differences in magmatic compositions, magma fluxes, and the Cu+/-Au endowment of IOCG's can be explained in, broadly, the context of the tectonic evolution of North and South America which are out of phase with each other in regard to the timing of continental margin extension and contraction (and orogenesis).;The out of phase timing of contraction and extension along the western margins of North and South America contributes to both the styles of Cordilleran magmatism and mineralization. Early phases of arc evolution are characterized by neutral-extensional tectonics, magmatism correlated with extension in the arc, and IOCG mineralization where saline fluids such as evaporative meteoric or marine waters can be circulated via heat from arc intrusions. Late phases of arc evolution are typified by orogenesis wherein the crust is thickened, felsic and hydrous magmas are derived from greater depths and considerably rework lower crust, and IOCG mineralization is sparse because the required saline surface fluids are not widely present in the emergent arc setting.