La Franja de Maricunga: sintesis de la evolucion del Frente Volcanico Oligoceno-Mioceno de la zona sur de los Andes Centrales

Abstract

RESUMEN. La Franja de Maricunga, de 200 km de largo, portadora de mineralizacion de metales preciosos, se extiende en el borde occidental del Altiplano de Copiapo (26-28°S) y representa el frente volcanico Oligoceno-Mioceno de la zona sur de los Andes Centrales. La actividad volcanica se organiza en cinco eventos. El mas antiguo (26-21 Ma) dio origen al complejo de estratovolcanes de Cerros Bravos-Barros Negros, y a los grupos de domos multiples asociados a mineralizacion de Esperanza y La Coipa (26°30'-27°S) que hicieron erupcion a traves de una corteza de ~45 km de espesor. En la zona surde la franja (27-28°S) la actividad fue mas reducida y asociada a pequenos complejos de domos multiples, con mineralizacion de oro y plata (Pantanillo, Refugio y La Pepa) emplazados a traves de una corteza mas delgada (~35-40 km). El segundo episodio (20-17 Ma) se asocia a un evento de deformacion compresiva, engrosamiento cortical y disminucion de la actividad volcanica. Entre los 16-12 Ma, el volcanismo se reanudo con vigor. Los magmas asociados a los centros mas antiguos del ciclo (Ojos de Maricunga, Santa Rosa, Jotabeche Norte; 16-15 Ma) evolucionaron en niveles corticales profundos, en equilibrio con granate. Al final del periodo (13-12 Ma) se emplazaron complejos volcanicos (Pastillos) y 'porfidos auriferos' (Lobo y Marte) en regimen tectonico extensional; tal como entre los 26-21 Ma, estos derivan de magmas hidratados que dejaron anfibola como fase residual estable. Un retomo a condiciones de corteza mas gruesa y regimen compresivo se detecta entre los 11 y 7 Ma cuando la actividad se redujo al Complejo Volcanico Copiapo (27°15'S). El evento final (6-5 Ma) ocurrio en la zona del Nevado de Jotabeche (27°40'S), una asociacion bimodal de riodacitas (Caldera Jotabeche) y andesitas maficas vitreas (Pircas Negras) eruptadas a traves de fallas en el extremo sur del Altiplano, donde la corteza alcanzaba mas de 60 km de espesor. Poseen valores extremadamente elevados de La/Yb y altas concentraciones de Na2O y Sr. Se interpretan como derivadas de magmas que evolucionaron en presencia de un manto litosferico frio, sobre una zona de Benioff de baja inclinacion, y en niveles profundos de la corteza tectonicamente engrosada, en facies de granulitas granatiferas o eclogitas. La Franja de Maricunga se sobreimpone al corredor de fallas noroeste Valle Ancho-Potrerillos, rasgo estructural que podria coincidir con ellrmite norte del terreno de Chilenia, acrecionado contra el margen de Gondwana, en el Paleozoico superior. ABSTRACT. The Maricunga Belt: evolution of the Oligocene-Miocene Magmatic Front of the Southern Central Andes. The formation of epithermal gold-silver deposits in the Maricunga mineral belt (28-26"S) in Northern Chile is related to Late Oligocene-Middle Miocene volcanic activity along the magmatic front of the South Central Andes. The Maricunga belt magmatism occurred in live discrete magmatic-tectonic episodes. During the first, Late Oligocene to Early Miocene (26-21 Ma) episode, volcanic complexes erupted over through a ~45 km thick crust in the north and a ~35 to 40 km thick crust in the southern part of the belt. Centres included the large andesitic to dacitic Cerros Bravos stratovolcano and the mineralized Esperanza and La Coipa dacitic dome clusters in the north (26°30'-27°S) and small dome complexes (La Pepa, Pantanillo and Refugio) hosting gold-silver epithermal mineralization in the south. A virtual volcanic lull occurred during the Early Miocene (20-17 Ma) associated with a period of compressional deformation and crustal thickening. The third period in the Middle Miocene (16-12 Ma) was marked by the eruption of voluminous stratovolcanic comp4exes (Dona Ines, Ojo de Maricunga and Cadillal) and ended at 13-12 Ma with the emplacement of mineralized 'gold porphyries' (Lobo, Marte) and the small-volume Pastillos stratovolcano. The mafic residual mineral assemblage associated with the Middle Miocene 'gold porphyries' was dominated by amphibole, as was also the case with the domes hosting the 26-21 Ma mineral deposits. During the fourth period in the Early to Middle Late Miocene (11-7 Ma), the activity became concentrated at the dacitic Copiapo Volcanic Complex (27°15'S). The magmatic chemistry at this time is consistent with renewed crustal thickening and gamet-bearing residual mineral assemblages. Small volumes of activity at the southern end of the belt occurred during the last magmatic period in the Late Miocene to Pliocene (6-5 Ma), Including rhyodacites from the Jotabeche Caldera (27°40'S) and glassy mafic andesitic Flows (Pircas Negras) erupted along fault zones at the southern end of the Altiplano. These magmas have extreme La/Yb ratios and high Na2O and Sr concentrations that are unique in the Central Andes. Geochemical signatures of these lavas are interpreted as the result of magmas equilibrating with a high-pressure garnet-bearing residual assemblage in a thickened garnet-granulitic to eclogitic facies crust, and a cooling mantle wedge over a shallowing subduction zone. Crustal thickness reached more than 60 km in the Jotabeche region. The Maricunga belt is cut by the WNW striking Valle Ancho-Potrerillos fault zone which could mark the northern limit of the Chilenia terrane, which was accreted to Gondwana in the Late Paleozoic.