The Triassic magmatism in southwestern Gondwana: An example of arc batholith construction in a retreating margin

Abstract

The Chollay-Piuquenes batholith (CPB) represents voluminous Lower-Middle Triassic magmatism on the western margin of Gondwana. It crops out in the Chilean Frontal Cordillera (28°30′S–30°30′S), covering ∼2,400 km2. It is composed of the Chollay and Piuquenes plutonic complexes, that were emplaced over a 16 Myr interval, with magma production rates ranging from 6 to 7.5 km3/Myr km−1. The batholith lithologies vary from diorites to syenogranites, with a predominance of monzogranites and granodiorites. It was previously interpreted as an anorogenic, post-collisional magmatism, originated from crustal anatexis in a rifting continental margin (Pre-Andean Cycle). This field and geochemical study proposes that CPB is likely a subduction-related batholith constructed in a convergent retreating margin. This interpretation is consistent with the Triassic geotectonic context proposed for western Gondwana. Moreover, the extensional context for the CPB emplacement is inferred from the contemporaneous development of forearc and back-arc basins, and from geochemical signals indicating Mesozoic crustal thinning along the margin. The CPB rocks exhibit subalkaline, meta- to peraluminous, calc-alkaline to alkaline-calcic affinities, enrichment in LILE relative to HFSE, depletion in Nb-Ta, Ti, Sr, and P, and Pb enrichment. The rocks display flat REE patterns (LaN/YbN: 3.40–13.78) and Al-in-Hbl barometer calculations (1.7–1.8 ± 0.6 kbar) suggest an epizonal emplacement. The Sr-Nd-Pb isotopic signature suggests a mixture of depleted mantle and continental crust, and not only crustal reworking, as the main magma-generating process. A comparative analysis of CPB samples with other well-known examples of both retreating and advancing margin batholiths allows the establishment of criteria to distinguish each tectonic context.