Andean Geology ISSN: 0718-7092 [email protected] Servicio Nacional de Geología y Minería Chile Oliveros, Verónica; Morata, Diego; Aguirre, Luis; Féraud, Gilbert; Fornari, Michel Jurassic to Early Cretaceous subduction-related magmatism in the Coastal Cordillera of northern Chile (18°30'-24°S): geochemistry and petrogenesis Andean Geology, vol. 34, núm. 2, julio, 2007, pp. 209-232 Servicio Nacional de Geología y Minería Santiago, Chile Available in: http://www.redalyc.org/articulo.oa?id=173918417003 How to cite Complete issue Scientific Information System More information about this article Network of Scientific Journals from Latin America, the Caribbean, Spain and Portugal Journal's homepage in redalyc.org Non-profit academic project, developed under the open access initiative Jurassic to Early Cretaceous subduction-related magmatism in the Coastal Cordillera of northern Chile (18°30’-24°S): geochemistry and petrogenesis Verónica Oliveros Departamento de Geología, Universidad de Chile, Casilla 13518, Correo 21, Santiago, Chile Diego Morata [email protected] Luis Aguirre [email protected] [email protected] Gilbert Féraud UMR 6526 Géosciences Azur, IRD-CNRS-UNSA, Université de Nice- Sophia Antipolis, Michel Fornari 06108 Nice cedex 02, France [email protected] [email protected] ABSTRACT Jurassic to Early Cretaceous magmatism in the Coastal Cordillera of northern Chile is represented by thick sequences of mostly basaltic-andesitic to andesitic lava flows and minor sedimentary rocks. The volcanic sucession was intruded by large plutonic bodies and smaller stocks and dikes. New geochemical data, including major and trace elements for a suite of Middle to Upper Jurassic volcanic and plutonic rocks from six localities in the Coastal Cordillera (18°30'-24°S), are presented here. The volcanic rocks are characterized by their petrological and chemical homogeneity; they are highly porphyritic basaltic-andesites and andesites with calc-alkaline to high-K calc-alkaline affinities, higher LILE than HFSE abundances, negative Nb and Ti anomalies, and LREE/HREE fractionation, which are the typical compositional features of subduction-related igneous rocks. No significant differences are observed in rocks from different areas or ages, but the plutonic rocks show subparallel, less and more enriched patterns respectively compared to volcanic rocks. The evolution and differentiation of the parental magmas is mainly due to fractional crystallization dominated by plagioclase, olivine and clinopyroxene. Assimilation of the continental crust was not important, although Th and La contents would indicate increasing sediment contribution or crustal contamination of the magmas with time. The magma source is likely to be a depleted mantle metasomatized by fluids, which originated from dehydration of the subducted oceanic crust. No evidence of slab melting was found in the studied rocks. The extensional tectonic setting that dominated the evolution of the Jurassic-Early Cretaceous arc in northern Chile would have favoured the extrusion of huge amounts of volcanic rocks during a relatively short period of time, avoiding thus a mayor interaction with the continental crust. Key words: Geochemistry, Subduction, Jurassic, Chile. RESUMEN Magmatismo asociado a subducción del Jurásico a Cretácico Inferior en la Cordillera de la Costa del norte de Chile (18°30'-24°S): geoquímica y petrogénesis. En la Cordillera de la Costa del norte de Chile, el magmatismo del Jurásico y Cretácico Inferior está representado por potentes secuencias de rocas volcánicas y volcanoclásticas, principalmente de composición intermedia, y rocas sedimentarias. Ellas están intruidas por grandes plutones, además de diques e intrusivos menores. En este trabajo se presentan nuevos datos geoquímicos, de elementos mayores y traza, correspondientes a un grupo de rocas volcánicas y plutónicas del Jurásico Medio a Superior recogidas en seis localidades en la Cordillera de la Costa del norte de Chile (18°30'-24°S). Las rocas volcánicas Revista Geológica de Chile, Vol. 34, No. 2, p. 209-232, 10 Figs., 4 tables, July 2007. 210 JURASSIC TO E ARLY C RETACEOUS S UBDUCTION-RELATED M AGMATISM I N T HE C OASTAL C ORDILLERA ... son bastante homogéneas en sus características petrográficas y geoquímicas y corresponden a lavas porfiríticas de composición basáltico-andesítica a andesítica, de afinidad calcoalcalina a calcoalcalina de alto-K, presentando mayor abundancia de elementos LIL con respecto a elementos HFS y anomalías negativas de Nb y Ti. Todos estos rasgos geoquímicos son típicos de rocas generadas en ambientes de subducción. No se observan diferencias significativas entre rocas volcánicas de distintas regiones o edades. Sin embargo, las rocas plutónicas presentan patrones de REE más planos, levemente más enriquecidos o empobrecidos, con respecto a los de las rocas volcánicas. La evolución y diferenciación de los magmas parentales se debió principalmente a la cristalización fraccionada de plagioclasa, olivino y clinopiroxeno sin una asimilación importante de corteza continental. Sin embargo, los contenidos de La y Th indicarían un aumento de la contribución de los sedimentos subductados en la génesis de los magmas o un aumento del grado de contaminación cortical con el tiempo. La fuente magmática fue posiblemente un manto deprimido, posteriormente metasomatizado por fluidos provenientes de la deshidratación de la placa subducida. No se encontraron evidencias geoquímicas de fusión de la corteza oceánica en las rocas estudiadas. El régimen tectónico extensional que dominó la evolución del arco magmático del Jurásico y Cretácico Inferior en el norte de Chile habría favorecido la extrusión de grandes volúmenes de rocas volcánicas en un período de tiempo relativamente corto, evitándose así la interacción de los magmas con la corteza continental. Palabras claves: Geoquímica, Subducción, Jurásico, Chile. INTRODUCTION Huge amounts of volcanic and plutonic rocks without subduction-processes involved (Lucassen from the Coastal Cordillera of northern Chile are et al., 1996) and magmatism controlled by pull- related to the Jurassic-Lower Cretaceous magmatic apart structures linked to oblique subduction arc which is thought to represent the first stages of the (Pichowiak, 1994). All these models were estab- Andean subduction. The Jurassic-Lower Cretaceous lished mainly by geochemical and petrological arc would have been developed in an extensional evidence from the Coastal Cordillera between 22° to transtensional tectonic regime due to a variably and 25°S. An arc/back-arc setting was proposed in oblique subduction of the Phoenix plate under the the Arica and Iquique areas (18°30’-21°30); a South American Plate at the western margin of westward migration of the arc and slab melt Gondwana (Jaillard et al., 1990; Scheuber and contribution to the magmas at the beginning in the González, 1999; Grocott and Taylor, 2002). The Middle Jurassic and at the end in the Late Jurassic development of the arc is characterized by a long of the magmatic activity as a result of changes in the lasting (~100 Ma) plutonic activity and a relatively tectonic regime and subduction rate was proposed short (15 to 25 Ma) volcanic activity (Oliveros et al., for this area (Kramer et al., 2005). 2006), the latter is represented by the extrusion of These models are supported by variable thick sequences of mainly intermediate lava flows. amounts of geological, geochemical and geo- The end of magmatic activity in the Coastal Cordillera physical data but rely on scarce age constraints, would be the result of an eastward migration of the since the age of the volcanic units from the Coastal arc during the Cretaceous and Cenozoic until it Cordillera has been largely unknown, with the reached its present day position in the Andean exception of the Iquique region where the strat- Cordillera (Mpodozis and Ramos, 1989). igraphic record is important. In this paper we present Although many authors agree that the volcanic new geochemical data (major and trace elements) and plutonic rocks cropping out in the Coastal of recently dated volcanic and plutonic rocks from Cordillera were originated from likely depleted the Coastal Cordillera of northern Chile, between mantle-derived magmas, the geological setting of 18°30’ and 24°S (Oliveros et al., 2006; Oliveros et the magmas has been interpreted in various ways, al., in press a, b), in order to 1) establish similarities e.g., an island-arc environment (Palacios, 1978), a and differences between the volcanic units along back-arc setting in an active continental margin the Coastal Cordillera, and 2) constrain the magma (Buchelt and Tellez, 1988; Rogers and sources of the igneous rocks, the tectonic setting for Hawkesworth, 1989), magmatism triggered from their emplacement, and the possible evolution of decompressional melting of the lithospheric mantle the magmas with time. V. Oliveros, D. Morata, L. Aguirre, G. Féraud, M. Fornari 211 GEOLOGIC AND TECTONIC SETTING The Coastal Cordillera of northern Chile, Jurassic. Nevertheless, recent 40Ar/39Ar dating between 18°30’ and 23°30’ (700 km) is a major indicate that volcanism took place mainly between geomorphologic unit mainly consisting of Mesozoic 165 and 150 Ma (Oxfordian-Kimmeridgian, ISC volcanic, plutonic and sedimentary rocks (Fig. 1). 2004) (Oliveros et al., 2006), with a probable older During the Jurassic and Early Cretaceous a sub- episode at 170-175 Ma (Aaleninan-Early Bajocian; duction-related magmatic arc system developed ISC, 2004)