Andean Magmatism: Chemical and Isotopic Constraints by R. S. Thorpe (auth.), R. S. Harmon, B. A. Barreiro (eds.)

By R. S. Thorpe (auth.), R. S. Harmon, B. A. Barreiro (eds.)

Destructive plate margin magmagenesis is among the so much intensely studied and extensively debated themes within the earth sciences at the moment. Calc-alkaline volcanic and plutonic rocks in orogenic settings convey this type of variety of composition and personality that the subducted oceanic lithosphere, the overlying 'enriched' lithospheric mantle and the decrease continental crust all were recommended lately because the basic resource quarter for island-arc and continental-margin basaltic to andesitic magmas. The function of the higher continental crust is usually a question of constant controversy. it's transparent that crustal illness is a standard, yet no longer common, function of harmful plate margin magmatism. even if this infection is brought at resource via subduction-related procedures or happens in the course of magma transit through bulk anatexis and magma blending, selective illness or coupled fractional crystallization-assimilation mechanisms is principal to most present discussions of andesite petrogenesis. This e-book offers a chain of papers which without delay handle those and different vital geological and geochemical difficulties in the context of the Mesozoic-Cenozoic calc-alkaline magmatism attribute of the Andean Cordillera of western South the United States. even though it is aimed basically at postgraduate scholars and researchers acquainted with the Andes, it's also an invaluable basic reference for staff in different fields who desire to achieve an perception into present recommendations, rules and speculations on 'andesitic' magmatism at damaging plate margins.

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0 Figure 5 Lead isotope composition of Andean igneous rocks near Toquepala, south Peru. Squares (right, circled field) represent data from Jurassic to Eocene plutonic and volcanic rocks. Circles (centre) are data from Miocene to Recent volcanics which show evidence of crustal interaction with the basement gneisses. Only one Palaeogene high-silica ash flow tuff falls within the centre field of 'more contaminated' rocks Andes mountains in this region and the inferred crustal over-thickening. In their view, the exceptionally thick crust beneath southernmost Peru encouraged protracted fractional crystallization and increased the proportion of crustal material incorporated into the magmas.

Not only are these basalts isotopically distinct from the Cook Island andesites (see Figure 5), but they have higher Rb and lower Sr contents which would be increased and decreased respectively by near-surface crystalliquid fractionation. The isotopic composition of the Cook Island andesites J'ules out any significant component of continental crust, subducted sediments or highly altered oceanic crust in their genesis. Taken together, the isotope and trace element data indicate that the Cook Island andesites are derived, without significant near-surface fractionation or crustal contamination, from subducted mid-oceanic ridge basalt or from subcontinental mantle with MORB source-type geochemical characteristics.

Also, the volcanic centres of the AVZ are characterized by little trace element variation within each centre, but marked systematic south-to-north intravolcano variations. Andesites from Cook Island have very low concentrations of the highly incompatible large-ion-lithophile elements such as Cs, Rb and Ba (but not La) which are chemically similar to K (Table 2). ), similar to mid-ocean ridge basalts as well as the high-Mg andesites from the Aleutians (Figure 4). As a result the Cook Island andesites have very high KlRh ratios.

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