Rhyolite Ignimbrite Generation in the Northern Andes

Effects of crustal assimilation on 238U-230Th disequilibria in continental arc settings

“…87 Sr/ 86 Sr, Nd, and Hf range from 0.7044-0.7057, -0.6 -+3.6, and +0.47 -+15.27, respectively. Although 87 Sr/ 86 Sr and Nd values reported here are within the range observed in previous studies, 87 Sr/ 86 Sr measurements are relatively low while Nd is relatively high compared to previously reported values for Eastern Cordillera basement samples (Hammersley et al, 2022). Additionally, the ranges in 87 Sr/ 86 Sr and Nd suggest that the Sub-Andean basement is heterogeneous, containing some isotopically primitive rocks, which are compositionally similar to the Western Cordillera basement, in addition to more evolved rocks.…”

“…% SiO2, they have lower levels of LREE/HREE enrichment compared to younger samples (Figure 4A and B). Hammersley et al, 2022;Garrison et al, 2018). Excluding 207 Pb/ 204 Pb, the isotopic data form approximately linear trends in Sr-Nd-Pb-Hf isotope space (Figure 6).…”

“…Relative to the Reventador lavas, the assimilant must have high 87 Sr/ 86 Sr and εNd = -10. These values were used to model evolution at Chalupas (Hammersley et al 2022) and are comparable to the most evolved basement samples analyzed as part of that study as well as model crust compositions from previous studies (Figure 5; Bryant et al, 2006;Hidalgo et al, 2012). As with the parental basalts, we estimate εHf of the assimilant by projecting its εNd onto the εHf-εNd terrestrial array (Vervoort et al, 2001).…”

“…In contrast, Eastern Cordillera lavas have 87 Sr/ 86 Sr = 0.7041-0.7047 and 143 Nd/ 144 Nd = 0.51255-0.51285, or εNd = -2.59 -+3.26. Here, magmas assimilate Paleozoic and Mesozoic metamorphic and intrusive rocks with evolved isotopic compositions (Ancellin et al, 2017, and references therein;Hammersley et al, 2022). Back-arc lavas have 87 Sr/ 86 Sr and εNd values similar to Western Cordillera and Inter-Andean Valley lavas due to minimal crustal assimilation by ascending lavas (Garrison et al, 2018).…”

“…The concentrations of the assimilant, Ca, are similarly hard to define. This is due to the lack of exposure, and also because the metamorphic basement in the NVZ is lithologically and geochemically diverse (Hammersley et al, 2022).…”