Arc magmatic evolution and the construction of continental crust at the Central American Volcanic Arc system

摘要

Whether or not magmatic arcs evolve compositionally with time and the processes responsible remain controversial. Resolution of this question requires the reconstruction of arc geochemical evolution at the level of a discrete arc system. Here, we address this problem using the well-studied Central American Volcanic Arc System (CAVAS) as an example. Geochemical and isotopic data were compiled for 1031 samples of lavas and intrusive rocks from the similar to 1100km-long segment of oceanic CAVAS (Panama, Costa Rica, Nicaragua) built on thickened oceanic crust over its 75million year lifespan. We used available age constraints to subdivide this data set into six magmatic phases: 75-39Ma (Phase I or PI); 35-16Ma (PII); 16-6Ma (PIII); 6-3Ma (PIV); 5.9-0.01Ma (PVa arc alkaline and PVb adakitic); and 2.6-0Ma (PVI, Quaternary to modern magmatism, predominantly 1Ma). To correct for magmatic fractionation, selected major and trace element abundances were linearly regressed to 55wt.% SiO2. The most striking observation is the overall evolution of the CAVAS to more incompatible element enriched and ultimately continental-like compositions with time, although magmatic evolution took on a more regional character in the youngest rocks, with magmatic rocks of Nicaragua becoming increasingly distinguishable from those of Costa Rica and Panama with time. Models entailing progressive arc magmatic enrichment are generally supported by the CAVAS record. Progressive enrichment of the oceanic CAVAS with time reflects changes in mantle wedge composition and decreased melting due to arc crust thickening, which was kick-started by the involvement of enriched plume mantle in the formation of the CAVAS. Progressive crustal thickening and associated changes in the sub-arc thermal regime resulted in decreasing degrees of partial melting over time, which allowed for progressive enrichment of the CAVAS and ultimately the production of continental-like crust in Panama and Costa Rica by similar to 16-10Ma.