Zircon U-Pb geochronology of the mesozoic to lower Cenozoic rocks of the Coastal Cordillera in the Antofagasta region (22°30‧-23°00‧ S): Insights to the Andean tectono-magmatic evolution
Résumé
This work reports on new zircon U-Pb ages (U-Pb LA-ICPMS Geochronology) obtained from volcanic, intrusive and sedimentary rocks which have been exposed in the Coastal Cordillera of the Antofagasta region (22°30‧-23°00‧S). A frequency diagram was performed with all the obtained ages. The diagram showed peaks representing emplacement or crystallization, depositional and inherited zircon ages. The zircons showed to be 300 Ma and younger form four major peaks. These peaks are consistent with previously dated emplacement and depositional ages, as well as, with the ages of the rock units recognized in this work. The oldest cluster ranges between 250 and 230 Ma and represents the first volcanic episode in the study area that corresponds to the Middle Triassic Sierra Miranda-Cerro Camaleón Rhyolites which yielded a zircon age of 240 ± 1.2 Ma. A second age cluster, between 220 and 130 Ma, can be associated with the Lower Jurassic volcanic and plutonic rocks, to Middle-Upper Jurassic plutonic rocks and Lower Cretaceous subvolcanic rocks. The Lower volcanic rocks correspond to La Negra Formation, which yielded ages that range between ca. 196 and 177 Ma. These ages are older than the Middle-Upper Jurassic ages commonly reported for this formation. The La Negra Formation and the interbedded marine Rencoret Beds are in stratigraphic continuity with the Middle Triassic Sierra Miranda-Cerro Camaleón Rhyolites. The ages that range between 170 and 150 Ma can be associated with the larger intrusive rock bodies from the Middle-Upper Jurassic Coastal Batholite of Northern Chile and Southern Perú. Lower Cretaceous ages (ca. 140 Ma) corresponds to subvolcanic rocks related to the metallogenic belt of the easternmost Coastal Cordillera of the Antofagasta region. A zircon ages gap is observed in the Lower Cretaceous (between 130 and 100 Ma). This gap indicates that either the study area did not undergo magmatic and volcanic activity at that time, or that the Peruvian tectonic phase allowed for the erosion of the geological record of such activity. An Upper Cretaceous cluster occurs at ca. 100-80 Ma and is correlated with the Cerro Empexa, Quebrada Mala, and Hornitos volcanic formations. The last volcanic activity occurred at about ca. 70-60 Ma. Lower Paleocene (ca. 63 Ma) volcanic unit indicates that the products of the Paleocene volcanism reached as far west as the Coastal Cordillera at the latitude of Antofagasta. Even the presence of lava indicates the proximity of effusive volcanoes, which, in turn, suggests that the Paleocene volcanic arc extended west away from the Domeyko Cordillera. Both Upper Cretaceous and Lower Paleocene volcanic units are restricted to the west by the Atacama Fault System.