The central Andes of northern Chile and northwestern Argentina developed in a largely autochthonous, intracontinental setting during Proterozoic and Paleozoic time through a recurrent sequence of extensional and compressional tectonic regimes. The exposed pre-Andean crust consists of tectonically isolated outcrops of: (1) metamorphosed basement, (2) plutonio bodies, and (3) slightly metamorphosed volcanic and sedimentary strata resting upon and intruded by multiple plutonic units. U-Pb and Nd-Sm geochronology indicates the existence of a Precambrian foundation for the central Andes dating from at least middle Proterozoic time. Three episodes of Precambrian/Paleozoic deformation/magmatism are recognized: (1) the Panamerican at the Precambrian/Cambrian transition, (2) the Caledonian at the Ordovician/Silurian boundary, and (3) the Variscan during the Carboniferous. Intrusive units can be grouped into synorogenic (S-type) and anorogenic (A-type) rocks based on structural, petrographic, and geochemical relations. Mafic lavas of middle Proterozoic to lowerCarboniferous age likely were generated during episodes of crustal dilatation.

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Jurassic and Lower Cretaceous magmatic activity in the present-day CoastalCordillera and the Central Depression of northern Chile is marked by important plutonic and volcanic cycles. The Initial Basics unit of the Coloso Coastal gabbro complex contains the first intrusives into an ensialic marginal basin related to extension, probably due to strike-slip plate motion. These gabbros have affinities with continental tholeiitic basalts. The subsequent Jurassic Volcanic unit, ranging from basalt to andesite in composition, erupted onto the subsiding basin. Simultaneous with and after the eruption of the Volcanic unit, the Late Granitoids unit was intruded. The earliest intrusives of this Late Granitoid unit (GI of the Coastal Cordillera) follow the Initial Basics in time and have similar geochemical source indicators. Geochemical signatures of both the Volcanic unit and the Late Granitoid intrusives suggest that they are related to contemporaneous subduction. Some Late Granitoids (GII of the Central Depression) show evidence of significant crustal contamination much like that of modern Andean magmatism.

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The Precordillera of the Cordón de Lila/Sierra Almeida area south of the Salar de Atacama (25°S68°W) was the center of continuous magmatic activity from Early Ordovician to Early Permian time. Various plutonic units form a basement that is overlain by Paleozoic to Cenozoic sedimentary and volcanic sequences. A continental tholeiitic series of komatiitic and tholeiitic pillow basalts, andesitic to plagidacitic lavas, and pyroclastic flows occurs along the north-central portion of this tectonic horst and is a key example for the early Paleozoic development of the central Andes. The lavas are intercalated with hemipelatic to very shallow marine clastic sediments. The sequence is solely affected by weak deformation and burial metamorphism of its stratigraphically lowest part. Geochemical data suggest formation of parental liquids in a lherzolitic upper-mantle source, followed by low-pressure tholeiitic differentiation through combined FC and AFC processes at two distinct crustal levels. The discharge of these lavas and the accompanying flysch-type sedimentation was triggered by crustal extension during Ordovician time, which resulted in a highly segmented, horst and graben-type geodynamic environment.

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ZusammenfassungDie nordchilenisehe Kiistenkordillere wird von einem dichten Netz vorwiegend marlscher G~inge durehzogen. Es sind dies keine Lamprophyre, sondern sie stellen Zufuhrsysteme effusiver T/~tigkeit im Zeitraum Jura bis Unterkreide dar.Die Ganggesteine lassen sich petrographisch und aufgrund Haupt-und Spurenelementehemischer Charakteristika in drei Serien einteilen. Die ~ltere alkaline Serie I zeichnet sich durch erh6hte Na-, K-, Fe-, Ti-, Mn-, P-, V-sowie niedrige A1-und Sr-Gehalte aus. Die Basalte und Andesite der zweiten Serie hingegen zeigen eine normale kalkalkaline Entwicklung, die sich in den Rhyodaziten der dritten Serie fortsetzen diirfte.W~ihrend die Magmen der ersten Serie Affinit/iten zu Basaltassoziationen kontinentaler Bruchsysteme aufzeigen, sind die kalkalkalinen Zusammensetzungen fiir die Magmenentwicklung des andinen, aktiven Kontinentalrandes seit dem Jura typisch. AbstractDense swarms of predominantly marie dykes outcrop in the north Chilean Coast Range. These rocks are not lamprophyres. They mark channelways used by Jurassic-to-Lower Cretaceous extrusives.The dykes can be divided into three groups according to their petrography, major and trace element compositions, Rocks of the oldest group (series I) are essentially andesitic with high Na, K, Fe, Ti, Mn, P, V and low AI and Sr values. The rocks of series II are normal calc-alkaline basalts and andesites, whereas the rocks of series III are rhyodacites with calc-alkaline affinities.It is proposed that the series I dykes are genetically related to a continental fault system, whereas the dykes of series II and III are directly related to the post-Triassic evolution of the Andean active continental margin.