Extensional lower cretaceous volcanism in the Coastal Range (29 degrees 20 '-30 degrees S), Chile: geochemistry-and petrogenesis

Lower Cretaceous volcanism in the Coastal Range (29degrees20'-30degreesS) of Chile is mainly represented by highly porphyritic (20-30% phenocrysts) lavas with unzoned Ca-rich plagioclase (An(57-54)Ab(40-42)Or(3-4)), clinopyroxene (Wo(40)En(43)Fs(17)), magnetite, and minor idiomorphic, altered olivines. Geochemically, these lavas are characterized by a relative homogeneity with high Al2O3 and low MgO contents, and classified as high-K to shoshonitic basaltic andesites to andesites generated in an intra-arc extensional setting due to oblique subduction. Their isotopic geochemistry is characterized by highly homogeneous low initial Sr ratios ((Sr-87/Sr-86)(0) similar to 0.7036) and positive epsilon(Nd) values (epsilon(Nd) = +2.9 to +4.7 (Nd-143/Nd-144)(0) similar to 0.5127) that are very different from those proposed as representative of `Andean-type' magmatism. A non-Andean modern setting dominated by subduction associated with intra-arc extension is proposed. On a (Sr-87/Sr-86)(0) versus epsilon(Nd) diagram, these lavas fit a model mixing curve for which the end members are Pacific MORB and Jurassic plutonic rocks from the Coastal Range. Coeval granitoids from the Coastal Range and lavas from the High Andes plot on the same field. Isotopically depleted mafic magmas could be metasomatized by the subducted sediments, which would increase their LILE content, and then partially contaminated by Jurassic plutonic rocks. The genesis of this magmatism may be related to a global low-spreading rate of 5 cm yr(-1) in the southeast Pacific during 125-110 Ma. (C) 2003 Elsevier Ltd. All rights reserved.