Oxygen Isotope Ratios in Late Cenozoic Andean Volcanics
O-isotope ratios for 127 Cenozoic volcanic rocks from the Andean Cordillera between 5°N and 4°S range from +5.2 to +14.0‰ (SMOW). Lavas from eight volcanoes in the SVZ between 36–42°S exhibit a very narrow range of δ18O values (+5.2 to +6.7‰), with O-isotope variations independent of bulk chemical composition. The highest O-isotope ratios occur in the CVZ where lavas from 16 volcanic centres between 16–26°S have a wide range of δ18O values (+6.9 to +14.0‰), with 18O/16O ratios well correlated with both bulk chemical composition and radiogenic isotope variations at some centres. In the NVZ, between 5°N and 2°S, lavas from nine volcanoes have intermediate O-isotope compositions (δ18O = +6.3 to 7.7‰), which are unrelated to chemical composition. The low 18O/16O ratios for the lavas of the SVZ are most easily explained by derivation from a mantle source, with the parent magmas rising largely unmodified through the thin (<35 km) continental crust and differentiation occurring by fractional crystallization, which did not significantly modify their O-isotope composition. The high 18O/16O ratios and large range of δ18O values documented for the lavas of the CVZ requires a large crustal component in all lavas. This was probably introduced by the extensive interaction of mantle-derived melts with lower and/or upper crust during transit through abnormally thick (>60 km) continental crust of the region. The intermediate O-isotope character for the lavas of the NVZ suggests that both mantle and crustal sources were important in the petrogenesis of lavas in the region.
KeywordsContinental Crust Basaltic Andesite Volcanic Centre Tholeiitic Basalt Andesitic Lava
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