| Popis: |
Silicic volcanism in the Andean Central Volcanic Zone (CVZ) produced one of the world's largest Neogene ignimbrite provinces. The largest and best-known CVZ ignimbrites are located on the Altiplano-Puna plateau north of 24 °S. Their compositions and huge erupted volumes suggest an origin by large-scale crustal melting, and present-day geophysical anomalies in this region suggest still active zones of partial melting in the middle crust. Farther south in the CVZ, the Cerro Galan complex erupted ignimbrites in the late Miocene and Pliocene that are quite similar in volume and composition to those from north of 24 °S and they have a similar origin. However, there are a great many other, smaller ignimbrites in the southern CVZ whose compositions and geodynamic significance are poorly known. These are the subject of this paper. We present a geochemical study of 28 ignimbrite units from the southern CVZ at 25 °S to 27 °S, whose ages cover the full span of arc activity in this area, from about 25 Ma to 1 Ma. The small to medium volume ignimbrites ( The major conclusion of this study is that silicic magmas of crustal origin are rare in the CVZ south of 24 °S, and the only known example, Cerro Galan, is considerably younger than the ignimbrite “flare-up” in the Altiplano-Puna plateau region. The thermal conditions for widespread crustal melting in the Altiplano-Puna north of 24 °S are related to increased mantle heat flux caused by slab steepening and/or delamination at the base of overthickened lithosphere. These conditions have not (yet) been achieved in the southern CVZ because of the southward shallowing of slab dip and younging of compressive deformation and plateau growth. |
