By R. S. Thorpe (auth.), R. S. Harmon, B. A. Barreiro (eds.)
Destructive plate margin magmagenesis is among the such a lot intensely studied and extensively debated themes within the earth sciences at the present. Calc-alkaline volcanic and plutonic rocks in orogenic settings show any such 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 sector for island-arc and continental-margin basaltic to andesitic magmas. The function of the higher continental crust is usually an issue of continuous controversy. it truly is transparent that crustal illness is a standard, yet no longer common, characteristic of harmful plate margin magmatism. even if this infection is brought at resource by way of subduction-related approaches or happens in the course of magma transit via bulk anatexis and magma blending, selective infection or coupled fractional crystallization-assimilation mechanisms is valuable to most modern discussions of andesite petrogenesis. This publication offers a sequence 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 US. even though it is aimed essentially at postgraduate scholars and researchers conversant in the Andes, it's also an invaluable basic reference for staff in different fields who desire to achieve an perception into present suggestions, rules and speculations on 'andesitic' magmatism at harmful plate margins.
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Additional info for Andean Magmatism: Chemical and Isotopic Constraints
62) as most northern and southern Andean samples. 8 206 Pb /204 Pb 19D Figure 3 Lead isotope compositions of Late Cenozoic Central Andean volcanics from Chile, Argentina and Bolivia (solid circles) compared to data from the northern and southern Andes. Data: Barreiro (1982a) and Harmon et 01. Q a.. • 0 N ...... a.. Q .... 0 Figure 4 Lead isotope compositions of Late Cenozoic Central Andean volcanics, including those from southern Peru, (open circles, Tilton and Barreiro, 1980, Barreiro, 1982a).
The lavas from Mt. Burney have Sr, Nd and Pb isotope compositions similar to the basalts from the SVZ, but because Mt. Burney andesites have significantly higher Si02 and lower MgO contents than the SVZ basalts, but similar K20, Rb and Ba contents (see Figure 3) and higher Sr contents (Figure 6), their origin is inconsistent with reasonable fractionation models from a basaltic parental composition. The low LREE abundance of the samples from Mt. Burney suggest that they may have been derived from a parental composition that formed by slightly greater degrees of partial melting than estimated for the Cook Island andesites, which would also account for their low Zr and Ti contents.
Similar characteristics are observed in the Miocene volcanism which occurs along the High Andean Cordillera. The pyroxene andesites contain phenocrysts of plagioclase (mostly labradorite), clinopyroxene (augite), orthopyroxene (hypersthene), olivine and Fe-Ti oxides. The hornblende andesites contain phenocrysts of plagioclase (mostly andesine), hornblende and biotite. Pyroxenes are rare in the hornblende andesites. , 1976). Hornblende is sometimes present and secondary quartz is quite common. Major element geochemistry I Province I rocks (33-37°S) are similar in major element composition to those from the CVZ with equivalent Si0 2 content and petrography.