TIME CONSTRAINTS FOR MAGMATIC AND METAMORPHIC /HYDROTHERMAL PROCESSES DURING THE JURASSIC AND EARLY-CRETACEOUS IN THE COASTAL CODILLERA, NORTHERN CHILE

The La Negra Formation and its equivalents crop out as a 7 km thick homoclinal sequence of intermediate volcanic and sedimentary Jurassic rocks along the Coastal Cordillera of northern Chile between 18º30' and 23º30'. Huge plutons, intermediate to acidic in composition, intrude this sequence together with numerous small basic to acidic dykes and stocks. These geologic units represent a Jurassic–Early Cretaceous volcanic arc generated under a transtensional regime due to oblique plate and correspond to the first stages of Andean subduction. All along the Coastal Cordillera, a very low to low-grade alteration can be found affecting both plutonic and volcanic rocks (regional scale alteration process). Stratabound and breccia–style volcanic–hosted copper deposits are main economic deposits in the region. Local–scale hydrothermal alteration processes accompanied the mineralization events. 40Ar-39Ar analyses were carried out on primary (plagioclase, amphibole, biotite) and secondary minerals (sericite in situ in plagioclase, k-feldspar and actinolite) from volcanic and plutonic rocks. Ages for volcanic rocks range between 150.0 ± 0.7 and 175.8 ± 3.4 Ma (2 sigma confidence level). Analyzed plutonic rocks yielded ages between 137 ± 1.0 and 159 ± 0.9 Ma but previous data indicate a much wide range of 100 Ma for plutonic activity during Jurassic and Early Cretaceous. The relatively short-lived volcanism, when compared to the whole activity of the arc, could have been probably the result of particular tectonic setting due to changes in the subduction conditions. The analyzed secondary minerals yielded ages between 163.6 ± 1.8 and 101.6 ± 5.0 but several alteration “events” can be recognized along this period. Thus, the so-called “regional alteration” event would rather correspond to the combination of four distinct phenomena that occurred through the development of the magmatic arc: burial–type metamorphism, high thermal gradients related to the intrusion of huge plutons, seawater/rock interaction and the circulation of non magmatic fluids related to the activity of a regional scale fault system. Copper mineralization events would have occurred between 160 and 155 Ma and 140-141 Ma and seem chronologically linked to magmatic activity.