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 likely one of the so much intensely studied and broadly debated themes within the earth sciences at this time. Calc-alkaline volcanic and plutonic rocks in orogenic settings express 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 zone for island-arc and continental-margin basaltic to andesitic magmas. The position of the higher continental crust can also be a question of continuous controversy. it truly is transparent that crustal infection is a standard, yet no longer common, function of damaging plate margin magmatism. no matter if this illness is brought at resource by means of subduction-related techniques or happens in the course of magma transit through bulk anatexis and magma blending, selective illness or coupled fractional crystallization-assimilation mechanisms is important to most modern discussions of andesite petrogenesis. This e-book offers a sequence of papers which without delay deal with 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 essentially 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 suggestions, principles and speculations on 'andesitic' magmatism at harmful plate margins.

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A similar relationship is observed between Sierra Nevada (38°30'S) and its younger neighbour Llaima (38°45'S). Due to their greater degree of alteration, the Miocene volcanic rocks are more difficult to characterize chemically. Between 33-34°S, the volcanics of the Longitudinal Depression are similar chemically to Province I PlioceneQuaternary rocks. Pyroxene 'andesites' (Si0 2 =49 - 53 wt. %) are more basic than the hornblende 'andesites' (Si0 2 = 55 - 59 wt. %).

With respect to major element chemistry (Table 1 and Figure 2), one of the significant features of the samples collected from throughout the entire A VZ is the limited range of Si02 content, which varies only from 60 to 67 wt. %; neither basalts nor rhyolites have been encountered in the volcanic centres of the A VZ. In this respect, these volcanic centres differ markedly both from those centres in the SVZ north of the Chile Rise-Trench triple junction which are formed predominantly of basalt, and from the volcanic centres in the CVZ in which rhyolite is common.

Although not depicted, the data of TIlton (1979) and James (this volume) fall within the range of the data shown here. Sources of lead data: Barreiro (1982a), Barreiro and Stem (1982), Harmon et al. 65). The constant 207Pbj204Pb places constraints on the relative age difference between northern and southern sources. Because ancient boosts in U/Pb ratios would result in increased 207Pb/204 Pb as well as 206Pbj204Pb ratios, any increase in U/Pb ratio in a northern source relative to the southern source must have occurred within the past few hundred million years or less.

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