La Negra Formation

Together with the Arica Group and the Camaraca, Los Tarros and Oficina Viz formations, it constitutes the remains of the La Negra Arc, an ancient volcanic arc that existed in Jurassic times. Albeit early, this is not the first recorded instance of Andean volcanism in northern Chile as the Triassic and Jurassic formations of Cifuncho, Agua Chica, Pan de Azúcar and Posada de los Hidalgos that underlie La Negra Formation do also bear evidence of volcanism. and its orientation was roughly north-south similar to today's disposition of La Negra Formations outcrops. Near Taltal the volcanism of La Negra Arc developed largely in marine conditions with most volcanic eruptions being effusive, yet volcanism was punctuated by large explosive eruptions. ==Lithology==

The formation is made up of volcaniclastic rocks, The non-volcaniclastic rocks correspond variously to lava flows with autobrecciated parts, lava domes, cryptodomes and pillow lavas. The volcaniclastic rocks of the formation include hyaloclastite breccias, peperites and felsic ignimbrites. This last rock type forms at some locations the lowermost parts of the formation. In addition, the formation contains limestone, marl, conglomerate and grey breccias. ==Geochemistry and petrogenesis==

The volcanic rocks of La Negra Formation show little diversity in their geochemistry. Fractional crystallization of olivine, plagioclase and pyroxene appear to be the cause of the igneous differentiation that produced basaltic andesites and andesites. The rocks belong to the calc-alkaline magma series and high-K calc-alkaline magma series. The magma series and other geochemical characteristics found in the volcanic rocks of La Negra Formation are characteristic of subduction zone volcanism. The magmas originated in a parcel of Earth's mantle that was metasomatized by fluid derived from the tectonic plate that was subducting beneath what is now Chile. A nearby gabbro body, the Coloso Coastal Gabbro, is thought to share the same origin as the volcanic rocks of La Negra Formation. ==Metamorphism and mineralization==

The formation has been subject to mild low-grade metamorphism as evidenced by the occurrence of an epidote-chlorite-calcite-chalcedony-zeolite-prehnite-pumpellyite association. Metamorphism is more pronounced in the brecciated parts of lava flows and near Antofagasta if compared with La Negra Formation near Tocopilla. ==See also==