Abstract:

The distribution, petrology, and geochemistry of mid-late Tertiary magmatic rocks in the Main Andean Cordillera over the modern zone of shallow subduction ("flatslab"- 28 to 33°S) correlate with the shallowing of the subducted plate and the thickening of the crust in central Chile and Argentina. The evolving characteristics of these "flat-slab" magmatic rocks suggest that crustal thickening occurred earlier in the central (near 30.5°) and northern (near 28°S) regions than in the southern region (near 33°S). Crustal thicknesses approximated by comparing the chemical characteristics (particularly the La/Yb ratios) of the "flat-slab" magmatic rocks with those of similar rocks in the modern Southern Volcanic Zone indicate that the crust thickened from ∼35 to 40 km in the late Oligocene to ≈55 to 65 km in the late Miocene in the northern and central "flat-slab." As the region was under compression, ductile deformation in the lower crust accompanying magmatism was probably responsible for these relatively rapid crustal thickness increases. The mineralogy of crustal residues calculated from basaltic composition parents for the "flat-slab" volcanic rocks changes from a hydrous amphibolegarnet- plagioclase assemblage to an almost anhydrous plagioclase-poor garnet granulite assemblage as the crust thickens. Geochemically, these changes are reflected in the melts by increasing La/Yb ratios and Sr contents associated with decreasing Eu anomalies. A limit to crustal thickening may be the attainment of mantle density by lower crustal rocks as garnet and Al-rich pyroxene replace plagioclase. Early Miocene (≈20 Ma) back-arc alkaline basalts at 31°S have relatively low 87Sr/86Sr ratios (≈0.7036) and high εNd (+4.5) compared to "flat-slab" calc-alkaline magmatic rocks. This fact combined with evidence for increasing crustal thickness suggests that progressively higher 87Sr/86Sr ratios and lower εNd (87Sr/86Sr = 0.7046 to 0.7064; εNd = +1.2 to -3.5) in the "flat-slab" volcanic rocks correlate with an increase in crustal contamination through time. The crustal contaminant could contain an important component of underplated basalts and residue from crustal melting associated with the formation of the extensive late Paleozoic-early Mesozoic Choiyoi granite-rhyolite complex that outcrops in this region. An additional component derived from subducted sediment or by sub-continental erosion associated with progressive shallowing of the subduction zone cannot be precluded.