Plutonic and metasedimentary rocks from the Coastal Range of northern Chile: Rb[sbnd]Sr and U[sbnd]Pb isotopic systematics

Abstract

More than 50% of the surface in the Coastal Range of northern Chile between the towns of Taltal and Chan˜aral consists of granitoid rocks. Age determinations by means of UPb isotopic analyses on sized zircons and by means of RbSr isotopic measurements, in conjuction with other geochemical studies, reveal a complex pattern for the development of the plutonites. Though closely associated, two groups of intrusive rocks can be distinguished on the basis of isotopic and geochemical data:In a west-east transect southeast of Chan˜aral, several calc-alkaline plutonites with basic to acid members intruded mainly into Paleozoic metasedimentary rocks. The UPb data of the zircons from the plutonites yielded concordant ages ranging from Lower Jurassic to Lower Cretaceous. UPb ages are identical with RbSr biotite ages within the error limits and almost coincide with the KAr mineral ages. Both the ages and the initial87Sr86Sr ratios decrease continuously from west to east. The low initial Sr isotopic ratios (0.703–0.705) and other geochemical parameters indicate I-type characteristics.In the west-east segment northeast of Chan˜aral, mainly alkaline plutonic rocks intruded in an interval from Upper Permian to Upper Triassic. The UPb data of zircons are slightly discordant and indicate a small portion of inherited crustal material> 1 Ga old. RbSr biotite ages were reset by thermal pulses due to the emplacement of younger granitoid rocks. The initial 87Sr86Sr ratios of < 0.710 point to an origin of these rocks by anatexis of crustal rocks (S-type).The oldest rocks in the area of Chan˜aral consist of thick sequences of metasediments. The material for these rocks was derived from a source> 2.5 Ga old as indicated by the UPb data of the zircons. The lower intercept age of the zircons points to metamorphism of the source area at about 600 Ma. Due to uplift in the Early Paleozoic the source area supplied the material for the sediments which were deposited in the Devonian. Contact metamorphism in the Triassic transformed the sediments into metasedimentary rocks as demonstrated by RbSr whole-rock data. This event was caused by the emplacement of the S-type granitoid rocks which probably originated, however, from a deeper level of the crust.