Abstract
The tectonic evolution of the proto-Andean margin of western Gondwana has been commonly seen in terms of terrane accretion processes, requiring the existence of early Palaeozoic terrane boundaries and associated sutures. A new study of the Cordon de Lila Ordovician volcano-sedimentary successions in northern Chile reveals for the first time an arc assemblage deposited on thin crust within a continental arc system, having regional implications. Primitive basalts, rhyolites, volcanogenic wackes and siltstones are associated, bearing not only debris from mainly arc sources but also basement rocks; the latter is only accessory in the form of metamorphic lithoclasts and detrital zircons with ages around 1.0 Ga. Magmatic zircons in rhyolites reveal an eruptive age of ca. 478 Ma, concordant with Upper Arenigian to Lower Llanvirnian ages of brachiopods in overlying conglomerates. Bimodal volcanic associations, including low-K tholeiites, characterize the magmatic rocks, with evolved rhyolitic rocks showing pronounced arc-like geochemical signatures (negative anomalies in Ti, Nb and Ta). Some of the basaltic rocks are tholeiitic and display Ce/Y ratios below 1 and might point to a Moho depth of less than 10 km, hence a thin continental crust, coinciding with depletions in Zr and Hf concentrations. Associated volcaniclastic rocks display generally low Th/Sc (0.4–1), La/Sc (mostly <3.5), Zr/Sc (6–20) and high Ti/Zr (~10–60) ratios. The rock succession resembles the same geochemical and lithostratigraphical trends as retro-arc basin deposits further east in the Argentinean Puna of similar age. However, in the Cordon de Lila, the intercalated mafic rocks are less evolved, and the percentage of arc debris is higher, while the percentage of metamorphic lithoclasts and rounded quartz grains is much lower, indicating the existence of a thinned continental margin underpinning. We propose a transition in the Ordovician of the Central Andes from trench-ward fore-arc deposits to a dominantly intra-arc basin (Cordon de Lila), transitioning further eastwards towards a retro-arc basin (Puna) and foreland basin deposits of the Cordillera Oriental of northwestern Argentina. The Cordon de Lila intra-arc assemblage and associated fore-arc basin deposits therefore defined the western margin of Gondwana during the Ordovician. The absence of any terrane boundaries and sutures across strike is consistent with an evolving continental margin arc constructed on attenuated crust of the proto-Andean margin.
Published Version
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