La Negra Formation (Spanish: Formación La Negra) is a geologic formation of Jurassic age, composed chiefly of volcanic and volcaniclastic rocks, located in the Coast Range of northern Chile. The formation originated in marine and continental (terrestrial) conditions, and bears evidence of submarine volcanism as well as large explosive eruptions. The volcanism of La Negra Formation is thought to have lasted for about five million years.[1]
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.[5][6][7] 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.[3] In relation to present-day geography La Negra Arc went from latitude 18°S to 26°S[8] and its orientation was roughly north-south similar to today's disposition of La Negra Formations outcrops.[1]
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.[9]
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.[13] A nearby gabbro body, the Coloso Coastal Gabbro, is thought to share the same origin as the volcanic rocks of La Negra Formation.[11]
Inside the intrusions that are hosted within La Negra Formation there are several vertical or near-vertical veins bearing chalcopyrite, magnetite and actinolite. Some more rare veins are made up of some of the following associations: silver-gold, iron-copper, iron and nickel-cobalt. All of these veins have similar ages as the larger intrusions.[4]
^ abGoguitchaichvili, Avto; Alva-Valdivia, Luis M.; Urrutia-Fucugauchi, Jaime (2003). "Paleomagnetism and Rock Magnetism of the Jurassic La Negra Formation, Northern Chile: Implications for Tectonics and Volcanic Stratigraphy". International Geology Review. 45 (6): 563–573. Bibcode:2003IGRv...45..563G. doi:10.2747/0020-6814.45.6.563. S2CID129653059.
^ abcCharrier, Reynaldo; Pinto, Luisa; Rodríguez, María Pía (2006). "3. Tectonostratigraphic evolution of the Andean Orogen in Chile". In Moreno, Teresa; Gibbons, Wes (eds.). Geology of Chile. Geological Society of London. pp. 47–48. ISBN9781862392199.
^Grocott, J.; Taylor, G. K. (2002). "Magmatic arc fault systems, deformation partitioning and emplacement of granitic complexes in the Coastal Cordillera, north Chilean Andes (25 30'S to 27 00'S)". Journal of the Geological Society. 159 (4): 425–443. doi:10.1144/0016-764901-124. S2CID55083388.
^ abcWolf, F.B.; Fontboté, L.; Amstutz, C.G. (1990). "The Susana Copper (-Silver) Deposit in Northern Chile Hydrothermal Mineralization Associated with a Jurassic Volcanic Arc". In Fontboté, L.; Amstutz, G.C.; Cardozo, M.; Cedillo, E.; Frustos, J. (eds.). Stratabound Ore Deposits in the Andes. Springer-Verlag. pp. 319–338.
^ abPichowiak, Siegfried (1994). "Early Jurassic to Early Cretaceous Magmatism in the Coastal Cordillera and the Central Depression of North Chile". In Reutter, K.-J.; Scheuber, E.; Wigger, P.J. (eds.). Tectonics of the Southern Central Andes: Structure and Evolution of an Active Continental Margin. Springer. pp. 203–217. ISBN978-3-642-77355-6.
^Scheuber, Ekkehard; Bogdanic, Tomislav; Jensen, Arturo; Reutter, Klaus-J. (1994). "Tectonic Development of the North Chilean Andes in Relation to Plate Convergence and Magmatism Since the Jurassic". In Reutter, K.-J.; Scheuber, E.; Wigger, P.J. (eds.). Tectonics of the Southern Central Andes: Structure and Evolution of an Active Continental Margin. Springer. pp. 121–139. ISBN978-3-642-77355-6.
