Subsequent Magmatism Research Articles

Cenozoic plate kinematics, subduction and magmatism: South American Andes

Revised plate reconstructions and enlarged isotopic age data sets strongly support three mechanisms for controlling the configuration and associated volcanism and tectonism of the Andean subduction zone. These include age of subducted plate, convergence rate, and interaction with hotspot traces. From 50 to 25 Ma, in the Andes, the age of oceanic plate subducted progressively decreased, apparently resulting in a'gradual decrease in subduction angle and consequent eastward shift in the loci of magmatic activity. A major reorganization of plate motions at 25 Ma resulted in an increased convergence rate normal to the Chilean Andes and was accompanied by subsequent broadening of the locus of magmatic activity into adjacent portions of Bolivia and Argentina. Convergence rates changed only slightly across the Peruvian margin and no clear effect on subsequent magmatism is indicated. Interaction of the Andean subduction zone with the Juan Fernandez and Easter-Nazca hotspot traces apparently resulted in lower angle subduction and consequent gaps in magmatic activity as predicted continuations of the traces correspond with regions lacking dated volcanic rocks. The current gaps in igneous activity correspond with contemporary subduction of the two traces, and this association apparently occurred in the past as well.

Water contents of silicic to intermediate magmas

Silicic to intermediate magmas have water contents that range from < 1 wt. % to around 7 wt. % depending on the character and depth of the magma source, type of melting reaction and the degree of differentiation. The condition of water saturation is rare, implying that crustal magma genesis generally takes place under fluid-absent conditions or that any fluid present must contain volatile species other than H 2O. The source and quantity of water present during partial melting profoundly affect the types of melting reactions, residues and magmas as well as subsequent magma evolution. For S-type granitic magmas, melting reactions involving muscovite breakdown in pelites yield low-temperature, “wet” melts that form veins, pockets and small plutons in regional metamorphic terranes. Biotite breakdown in pelites near the amphibolite-granulite facies transition yields hotter magmas with variable water contents. These are emplaced at medium to high levels. Under granulite facies conditions, biotite breakdown in quartzofeld-spathic rocks yields high-temperature, “dry” magmas emplaced as subvolcanic batholiths and volcanics.