Geological significance of Rb-Sr isotopic data of northern Chile crystalline rocks of the Andean orogen between latitudes 23° and 27° South
Total rock and mineral ages of Chilean diorite to rhyodacite igneous rocks are middle(?) to late Paleozoic near the Argentine frontier, Permian-Triassic along the Pacific coast (as well as some 125 km inland), and Jurassic to Holocene from the Pacific coast to the Argentine border. To the east, in northwestern Argentina, plutonic igneous rocks fall within two age groups, latest Precambrian to Paleozoic and Early Cretaceous. Copper mineralization in the Imilac region is radiometrically dated as late Paleozoic. This is the first evidence of metalliferous deposits of this age in northern Chile. Analyses of ten samples of quartz-plagioclase porphyry from the middle Tertiary El Salvador mine give an average Sr 87 /Sr 86 initial ratio of 0.7033 ± 0.0004 (2σ) which suggests that these copper porphyries did not originate from sialic crustal source rocks. El Salvador porphyries have uniform strontium isotopic composition and marked fractionation trends with respect to rubidium and strontium concentrations. The initial Sr 87 /Sr 86 ratios of igneous rocks with Rb/Sr ratios less than one and of pre–late Cenozoic age range between 0.703 and 0.706. The late Cenozoic volcanic rocks have Sr 87 /Sr 86 ratios of 0.7058 to 0.7069.