Crustal contribution in the genesis of the bimodal Triassic volcanism from the Coastal Range, central Chile

Abstract

Triassic volcanism in the Coastal Range of central Chile is represented by the Pichidangui Formation (31°54' to 32°15'S) composed of acid and basic lava flows and sub-volcanic intrusives with some sedimentary intercalations. Both rock types have been affected by non-deformational very low- to low-grade metamorphism. Their geochemical characteristics indicate a tholeiitic affinity, with Nb/Y ratios ranging from 0.20-0.17. Low [mg] values in the basic rocks are indicative of non-primitive liquids. Differences in immobile trace element and REE (LREE/HREE, Eu/Eu*) between basic and acid rocks are consistent with their isotopic signature, with initial ratios of (⁸⁷Sr/⁸⁶Sr)_o and (¹⁴³Nd/¹⁴⁴Nd)_o ranging from 0.70397 to 0.70597 and from 0.512515 to 0.512608 respectively in the basic rocks, and from 0.70514-0.71161 to 0.512108-0.512526 respectively in the acid rocks. These chemical and isotopic differences between both types of magmatism are interpreted as a consequence of different degrees of crustal contamination from an enriched mantle source with a lithospheric component in an extensional intracontinental geodynamic setting followed by crystal fractionation to generate acid and basic melts. Geochemical and isotopic similarities between the Triassic basic rocks and the Lower Jurassic gabbros from the Limarí Complex in the Coastal Range in central Chile are interpreted as a consequence of a cogenetic event, related to the Mesozoic evolution of this portion of the Gondwana pacific margin.