Andean Magmatism and Its Tectonic Setting

Recent lavas from the Andean volcanic front (33 to 42°S); Interpretations of along-arc compositional variations
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Published:January 01, 1991
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CiteCitation
Daniel R. Tormey, Rosemary Hickey-Vargas, Frederick A. Frey, Leopoldo López-Escobar, 1991. "Recent lavas from the Andean volcanic front (33 to 42°S); Interpretations of along-arc compositional variations", Andean Magmatism and Its Tectonic Setting, Russell S. Harmon, Carlos W. Rapela
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Along the volcanic front of the Southern Volcanic Zone of the Andes between 33°S and 42°S the continental crust increases in thickness from south to north, and erupted lavas define several along-arc geochemical trends. These geochemical trends are the integrated effects of processes occurring in the subducted oceanic crust, the overlying mantle, and the continental crust. Major- and trace-element abundances and isotopic ratios are used to distinguish the effects of crustal and mantle processes. In the relatively thin crust region south of 37°S, lavas evolve dominantly by low-pressure crystallization and incorporate insignificant amounts of continental crust. Abundance ratios of most incompatible elements and isotopic ratios in these lavas reflect components derived from the mantle and subducted oceanic crust. Compared to basalts from north of 37°S, basalts from 37 to 42°S have higher CaO but lower Na2O and incompatible element contents. These differences are consistent with a north to south increase in degree of mantle melting, perhaps controlled by variable fluxing from the subducted slab (Rb/Cs ratios decrease from north to south), or variations in the thickness of the mantle column that undergoes melting (thicker crust in the north leads to a thinner mantle column and lower degree of melting). In the thicker crust region north of 37°S crustal contamination exerts a greater control on lava compositions. In the interval from andesite to rhyolite, upper crustal contamination causes Rb, Cs, and Th enrichment and isotopic variability. The evolution from basalt to basaltic andesite in this region occurs in the lower crust and crustal contamination causes enrichment in Rb, Cs, and Th and increased La/Yb. The most probable lower crustal protolith for the contaminant is a young, arc-derived garnet granulite.
- Andes
- andesites
- assimilation
- basalts
- chemical composition
- composition
- continental crust
- crust
- crystallization
- differentiation
- granulites
- high pressure
- igneous rocks
- isotopes
- lava
- low pressure
- major elements
- metamorphic rocks
- oceanic crust
- petrology
- pollution
- pressure
- rhyolites
- South America
- subduction
- thickness
- trace elements
- volcanic rocks