Track D Social Science, Human Rights and Political Science

Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/138414/1/jia218442.pd

Igneous garnet and amphibole fractionation in the roots of island arcs: experimental constraints on andesitic liquids

To evaluate the role of garnet and amphibole fractionation at conditions relevant for the crystallization of magmas in the roots of island arcs, a series of experiments were performed on a synthetic andesite at conditions ranging from 0.8 to 1.2 GPa, 800-1,000 degrees C and variable H2O contents. At water undersaturated conditions and fO(2) established around QFM, garnet has a wide stability field. At 1.2 GPa garnet ? amphibole are the high-temperature liquidus phases followed by plagioclase at lower temperature. Clinopyroxene reaches its maximal stability at H2O-contents <= 9 wt% at 950 degrees C and is replaced by amphibole at lower temperature. The slopes of the plagioclase-in boundaries are moderately negative in T-XH2O space. At 0.8 GPa, garnet is stable at magmatic H2O contents exceeding 8 wt% and is replaced by spinel at decreasing dissolved H2O. The liquids formed by crystallization evolve through continuous silica increase from andesite to dacite and rhyolite for the 1.2 GPa series, but show substantial enrichment in FeO/MgO for the 0.8 GPa series related to the contrasting roles of garnet and amphibole in fractionating Fe-Mg in derivative liquids. Our experiments indicate that the stability of igneous garnet increases with increasing dissolved H2O in silicate liquids and is thus likely to affect trace element compositions of H2O-rich derivative arc volcanic rocks by fractionation. Garnet-controlled trace element ratios cannot be used as a prox

Late Cenozoic To Modern- Day Volcanism in The Northern Andes: A Geochronological

The Northern Andean Block is the result of complex tectonic interaction between the Farallon-Nazca, South American, and Caribbean Plates. Abundant late Cenozoic volcanism (and associated hypabyssal porphyritic plutonism), beginning in the mid- to late Mi

Adakite-like volcanism of Ecuador: lower crust magmatic evolution and recycling

In the Northern Andes of Ecuador, a broad Quaternary volcanic arc with significant across-arc geo- chemical changes sits upon continental crust consisting of accreted oceanic and continental terranes. Quaternary volcanic centers occur, from west to east, along the Western Cordillera (frontal arc), in the Inter-Andean Depression and along the Eastern Cordillera (main arc), and in the Sub-Andean Zone (back-arc). The adakite-like signatures of the frontal and main arc volcanoes have been interpreted either as the result of slab melting plus subsequent slab melt–mantle interactions or of lower crustal melting, fractional crystallization, and assimilation processes. In this paper, we present petrographic, geo- chemical, and isotopic (Sr, Nd, Pb) data on dominantly andesitic to dacitic volcanic rocks as well as crustal xenolith and cumulate samples from five volcanic centers (Pululagua, Pichincha, Ilalo, Chacana, Sumaco) forming a NW–SE transect at about 0° latitude and encompassing the frontal (Pululagua, Pichincha), main (Ilalo, Chacana), andback-arc (Sumaco) chains. All rocks display typical sub- duction-related geochemical signatures, such as Nb and Ta negative anomalies and LILE enrichment. They show a relative depletion of fluid-mobile elements and a general increase in incompatible elements from the front to the back-arc suggesting derivation from progressively lower degrees of partial melting of the mantle wedge induced by decreasing amounts of fluids released from the slab. We observe widespread petrographic evidence of interaction of primary melts with mafic xenoliths as well as with clino- pyroxene- and/or amphibole-bearing cumulates and of magma mixing at all frontal and main arc volcanic centers. Within each volcanic center, rocks display correlations between evolution indices and radiogenic isotopes, although absolute variations of radiogenic isotopes are small and their values are overall rather primitive (e.g., eNd = ?1.5 to ?6, 87Sr/86Sr = 0.7040–0.70435). Rare earth element patterns are characterized by variably frac- tionated light to heavy REE (La/YbN = 5.7–34) and by the absence of Eu negative anomalies suggesting evolution of these rocks with limited plagioclase fractionation. We interpret the petrographic, geochemical, and isotopic data as indicating open-system evolution at all volcanic centers characterized by fractional crystallization and magma mixing processes at different lower- to mid-crustal levels as well as by assimilation of mafic lower crust and/or its partial melts. Thus, we propose that the adakite-like sig- natures of Ecuadorian rocks (e.g., high Sr/Y and La/Yb values) are primarily the result of lower- to mid-crustal processing of mantle-derived melts, rather than of slab melts and slab melt–mantle interactions. The isotopic sig- natures of the least evolved adakite-like rocks of the active and recent volcanoes are the same as those of Tertiary ''normal'' calc-alkaline magmatic rocks of Ecuador sug- gesting that the source of the magma did not changethrough time. What changed was the depth of magmatic evolution, probably as a consequence of increased com- pression induced by the stronger coupling between the subducting and overriding plates associated with subduc- tion of the aseismic Carnegie Ridge