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The proto-Andean and Early Andean evolution of the southwestern Gondwana margin comprises three stages that differ in their magmatic evolution and deformational style: the Gondwana cycle (~330–280 Ma), the Pre-Andean stage (~280–210 Ma) and the Early Andean Cycle (210–100 Ma). These stages have been traditionally interpreted as the upper crustal response to changes in the tectonic setting which include: Cordilleran-style continental arc (Gondwana cycle), orogenic collapse and possibly slab break-off that led to continental rifting and extensive crustal melting (Pre-Andean stage), and subsequent subduction re-initiation in oceanic arc-style context (Early Andean cycle). The petrological and geochemical characteristics of Carboniferous to Jurassic igneous rocks from this region however do not support the described model. Elemental and Sr-Nd-Pb isotopic data of 86 samples, along with a compilation of ~1230 samples from the literature suggest that subduction was the most likely process by which the magmatic record was generated. Sub-alkaline affinities, LILE enrichment over HFSE, Nb Ta troughs, porphyritic textures and hornblende- and biotite-bearing lithologies are present in all studied units, whereas isotopes suggest that magma sources are a mixture of depleted mantle and variable contribution from the continental crust. Even though the aforementioned features are common to all igneous rocks, some changes point to a decline in the contribution of crustal/lithospheric sources to the magmatism with time. Thus, SiO2, LaN/YbN and 87Sr/86Srinitial exhibit a systematic decrease from ~285 to 150 Ma, whereas the eNdinitial parameter increases in the same period. These changes were accompanied by the shift from dominant compressional (Carboniferous-Early Permian) to transtentional (Middle Permian-Jurassic) stresses in the upper crust, suggesting that the margin went from advancing to retreating due to Pangea reorganization and break-up. Following a potential flat slab event, slab roll-back may have induced extension in the upper crust and lithospheric loss as a consequence of delamination or thermal erosion. |
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