The jurassic la negra formation in the area of antofagasta, Northern Chile (lithology, petrography, geochemistry)

Abstract

Based on their lithology and their geographical position, the rocks of the La Negra Formation in the investigated area could be subdivided into the following three groups: First, the Cuevitas-group, which overlies marine sediments of the Hettangian and consists of HK-calc-alkaline basaltic to andesitic volcanics (partly submarine), pyroclastics and intercalations of marine and terrestrial sediments. Second, the La Chimba-group, composed of calc-alkaline basalts to basaltic andesites (in the lower parts submarine) intercalated by terrestrial sediments. Third, the La Negra-group, which represents a widespread distribution of co-existing lava successions with thicknesses up to 1400m.

Magmagenetically, the volcanics have a polymagmatic origin. Although most of the rocks show a basic character, true basalts are not present. The predominant series of calc-alkaline to HK-calc-alkaline basalts, basaltic andesites and andesites are enriched by the LIL-elements Rb, K, Ba and Th, probably originating from the dehydrating subducted oceanic crust. The concentrations of the HFS-elements Ta, Nd, P, Hf and Zr show enrichments which are developed in a back-arc volcanism with affinities to within-plate basalts.

Since the Lower (?) Sinemurian the volcanic arc has migrated 40–50km trenchward to the West. Geochemical parameters point to a change from a more alkaline to a calcalkaline trend, indicating a more evolved stage of the back-arc volcanism. The most intense volcanic activity in the "retreating" back-arc at an active continental margin was during the Middle- to Late Jurassic in the present coast area.