Geoquímica, dataciones U-Pb SHRIMP sobre circón e isótopos de Hf del magmatismo gondwánico en el NW de Argentina: petrogénesis e implicancias geodinámicas.

Abstract

ABSTRACT.We have carried out zircon U-Pb SHRIMP dating and Hf isotope determinations as well as geochemical analyses on three plutonic units of Gondwanan magmatism that crop out in NW Argentina. Two episodes of different age and genesis have been identified. The older one includes gabbros and diorites (Río Grande Unit) of 267±3 Ma and granitoids (belonging to the Llullaillaco Unit) of 263±1 Ma (late Permian, Guadalupian); the parent magmas were generated in an intraplate environment and derived from an enriched mantle but were subsequently contaminated by crustal components. The younger rocks are granodiorites with arc signature (Chuculaqui Unit) and an age of 247±2 Ma (middle Triassic-Anisian). Hf isotope signature of the units indicates mantle sources as well as crustal components. Hf model ages obtained are consistent with the presence of crustal Mesoproterozoic (mainly Ectasian to Calymnian (T DM(c) =1.24 to 1.44 Ga-negative Hf (T) ) and juvenile Cryogenian sources (T DM =0.65 to 0.79 Ga-positive ɛ Hf (T) ), supporting the idea of a continuous, mostly Mesoproterozoic, basement under the Central Andes, as an extension of the ArequipaAntofalla massif. The tectonic setting and age of the Gondwanan magmatism in NW Argentina allow to differentiate: a. Permian intra-plate magmatism developed under similar conditions to the upper section of the Choiyoi magmatism exposed in the Frontal Cordillera and San Rafael Block, Argentina; b. Triassic magmatism belonging to a poorly known subduction-related magmatic arc segment of mostly NS trend with evidence of porphyry type mineralization in Chile, allowing to extend this metallotect into Argentina.

Geochemistry of Permian to Triassic igneous rocks from northern Chile (28º-30º15’S): Implications on the dynamics of the proto-Andean margin.

“…The magmatic sources would have changed significantly, decreasing the crustal component, between the Late Permian and the Early Jurassic. This change, which occurred during the Triassic, is explained by some authors as the response to a shift from subduction in a compressive setting to an extensional continental rift-related tectonic setting due to Pangea breakup (Mpodozis and Kay, 1992;Hervé et al, 2014), or as extensional deformation in the overriding plate, in a continuous subduction setting (Vásquez et al, 2011;Poma et al, 2014;Del Rey et al, 2016).…”

Detrital zircon record of Phanerozoic magmatism in the southern Central Andes

“…Triassic (238-208 Ma) zircons record positive mean εHf values and a systematic increase in Th/U ratios to 0.9, suggestive of increased depleted components related to asthenospheric upwelling triggered by extensional thinning. The petrography, geochemistry, and isotopic composition of Triassic volcanic rocks show progressive changes over time yet indicate dominant subduction-related characteristics since the Carboniferous (Vásquez et al, 2011;Poma et al, 2014;del Rey et al, 2016;Coloma et al, 2017;Oliveros et al, 2018;González et al, 2018). In this manner, a combination of postorogenic collapse and backarc extension during slab rollback marked the onset of a principally extensional to neutral tectonic regime that characterized much of western South America during the Mesozoic (Fig.…”