Lithospheric evolution and structure of the Pre- and Early Andean margin as inferred from the Carboniferous to Jurassic igneous record (Chile-Argentina, 20°-40°S).

The pre-Andean and Early-Andean evolution of the southwestern Gondwana margin comprises three stages: the Gondwana cycle (Late Carboniferous-Early Permian), the Pre-Andean stage (Middle Permian-Late Triassic) and the Early Andean cycle (Early Jurassic-Early Cretaceous). These stages have been traditionally interpreted as the upper crustal response to major changes in the tectonic setting: Cordilleran-style continental arc (Gondwana cycle), followed by 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). However, recent geological, geochronological, geochemical and petrological data for the Carboniferous to Jurassic igneous rocks 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 Sr-Nd-Pb isotopes suggest that magma sources are a mixture of depleted mantle and variable contribution from the continental crust. Chemical evolution in the magmatic record points to a decline in the contribution of crustal or lithospheric sources to the magmatism with time. Thus, SiO₂, La/Yb and ⁸⁷Sr/⁸⁶Sr₀ exhibit a systematic decrease from the middle Permian to the Jurassic, whereas the εNd₀ parameter increases in the same period.

The geological record suggest that these changes were accompanied by the shift from dominant compressional (Carboniferous-Early Permian) to transtensional deformation (Middle Permian-Jurassic) in the upper crust with the convergent margin transitioning from advancing to retreating.  Slab roll-back during the Late Permian to Triassic may have induced extension in the upper crust and lithospheric loss as a consequence of delamination or thermal erosion.

At ca. 215-210 Ma, the magmatic loci advanced trenchward at least 150 km, establishing the new arc front in the present-day Coastal Cordillera, where it stayed during the Jurassic and Early Cretaceous. This trenchward shift in the arc location that marks the initiation of the Early Andean arc in Chile, may have been a continental-scale process but its causes remain unknown. The margin did not witness significant changes in the tectonic stress, maintaining its transtensional character from Late Triassic to Jurassic. However, the composition of the Pre-Andean intrusions differs from the Early Andean granitic rocks, with tonalite-granodiorite-monzogranite (M<15) in the former and dominant quartz diorite-tonalite-granodiorite lithologies (M>15) in the latter, perhaps due to distinct thickness or composition of the crust in with each arc emplaced.

Present-day Pb isotopic ratios suggest that the Gondwana and Pre-Andean magmatism were emplaced within a heterogeneous continental crust, formed by  basement blocks with contrasting isotopic nature (pre-Gondwana cycle terranes). In spite of such heterogeneity, the Gondwana stage (Carboniferous to Early Permian) may have been a period of crustal changes along the margin because magmatism from this period is consistently more radiogenic than its basement.  In contrast, the magmatism from the  Pre-Andean stage and Early Andean cycle would have had a larger contribution from a juvenile, mantle-dominated, source, given its less radiogenic character.