Distinguishing crustal recycling and juvenile additions at active continental margins: the Paleozoic to recent compositional evolution of the Chilean Pacific margin (36–41°S)

This paper presents new insights on the evolution of continental crust in a segment of the southern Central Andes (Chile–Argentina; ca. 36–41 °S) from compositional and isotopic age data of metamorphic and magmatic rocks. The crustal reservoir of this long-standing active continental margin is characterized by the isotopic composition of Paleozoic (meta)sedimentary rocks which is typical of Proterozoic continental crust (Nd model ages 1.6–2.1 Ga; isotopic composition at ca. 300 Ma, the time of formation of the late Paleozoic granites: 87Sr/86Sr ca. 0.71–0.72; 143Nd/144Nd ca. 0.5117–0.5120; 207Pb/204Pb ca. 15.64–15.70, 206Pb/204Pb ca. 18.2–18.6, 208Pb/204Pb ca. 37.2–38.5). The late Paleozoic and late Triassic granitoids (ca. 220 Ma) contain 30–80% of this material assuming a mantle endmember with the isotopic composition of Pacific MORB. The proposed mechanism of magma generation in the late Paleozoic and Triassic granitoids is assimilation of crustal material by mantle derived melts, i.e. mixing between crustal and mantle melts. The isotopic composition of the Paleozoic rocks in the southern Central Andes indicates a coherent evolution of the western Gondwana margin (21–41°S) during the Paleozoic mainly recycling Proterozoic material by sedimentation, metamorphism and magmatism. gmatism of the Andean cycle represents important additions of juvenile material to the crust. The Mesozoic to Tertiary intrusions have a relatively uniform isotopic composition (87Sr/86Sri ca. 0.7036–0.7066; 143Nd/144Ndi ca. 0.5125–0.5129; Pbi 207Pb/204Pb ca. 15.60–15.65, 206Pb/204Pb ca. 18.0–18.5, 208Pb/204Pb ca. 37.2–38.4) indicating only small contributions of the crustal reservoir (ca. 5–10%) to mantle derived magma with presumed MORB isotope signature. The proposed main mechanism of crustal contribution in these rocks is mixing of continent-derived sediments into the mantle wedge.