Evolution of the Peruvian subduction margin at 9°S : evidence from geochemistry, experimental pterology and melt inclusion studies on adakite-like ignimbrites
The Miocene Yungay and Fortaleza Ignimbrites of the Peruvian Cordillera Blanca (9° S) are characterised by restricted geochemical vari~tions, with Si02 >70 wt %, Na20 >4 wt%, Mg numbers from 4 to 30, and strong depletions in Y, Yb' and Lu. La/Lus, and Sr/Y ratios are high, at >20 and >65 respectively. These geochemical indicators are typical of Phanerozoic adakite-like rocks and Archaean Trondhjemite- Tonalite-Granodiortite (TTG) suites commonly interpreted as partial melts derived from subducted oceanic slab. The Ignimbrite data differ significantly from spatially-related Cretaceous and Eocene lavas in western Peru, which. show classic LILE-enriched calc-alkaline arc signatures with Mg numbers of 25 to 45. 40 ArP9 Ar dating on Yungay feldspars yields an age range of 4.1 to 7.5 Ma, while K-Ar dating on Fortaleza biotites places them at 4.9 to 5.84 Ma. Both Ignimbrites were erupted during major tectonic modifications to the Peruvian subduction margin, in which the continental crust beneath the Western Cordillera almost doubled in thickness, to >50 km. This occurred together with shallowing subduction angle of the Nazca Plate from c. 30° to c. 5°, onset of subduction of the Nazca Ridge, and rapid crustal uplift and exhumation. It is against this complex tectonic backdrop, effectively marking the end of the current 200 Ma Andean Cycle, that the adakite-like Yungay and Fortaleza magmas were generated in rapid succession. Tracing the magma source region has profound implications for the geochemical evolution of mature continental arcs and the fact that the final magmatic products resemble Archaean TTGs needs careful explanation. There are only two possible source regions for the Peruvian Ignimbrite melts: thick underplated lower crust or the downgoing oceanic slab. Resolving between them is critical to a full and fmal understanding of the magmatic evolution of the Peruvian margin. The Ignimbrites and the adjacent Miocene Cordillera Blanca Batholith are found on either side of the Cordillera Blanca Fault. Both appear to have exploited this deep crustal lineament, implying a common structural link between their source regions. In an attempt to constrain the source material for the Yungay and Fortaleza Ignimbrites, experimental petrology and melt inclusion studies were combined with geochemical analysis. Radiogenic isotope compositions point to a juvenile crustal source (87Sr/86Sr = 0.70547 to 0.70631, l43Nd/14~d = 0.51245 to 0.51257, eHf = -3.2 to 0.88, 206PbP04Pb = 18.77 to 18.78). Limited stable oxygen isotopes (8180 Bt•= 5.92 to 7.15 %0) suggest that the Peruvian magmas have not interacted significantly with mature continental crust, unlike volcanic rocks of similar ages from the central Andes which were also intruded through thickened lithosphere. Simple mixmg models using a range of possible sources and contaminants show that magmas could have been derived from the lower crust, and may have subsequently been contaminated by Palaeozioc igneous continental material. Direct observation of source mineralogy can be achieved through experimental petrology, which mimics the partial melting process. However, this requires identification of suitable starting matèriaI. Although a major challenge (no suitable xenolithic material has been found in any of the Peruvian magmas), previous studies of the Cretaceous Coastal Batholith to the west of the study region suggest that mafic-intermediate synplutonic dykes provide the best available proxies for the true composition of lower arc crust beneath the Western Cordillera. This idea was tested for the Miocene Yungay and Fortaleza Ignimbrites, using a basaltic-andesite dyke cutting the Cordillera Blanca Batholith as a start material for a series of piston cylinder partial melting experiments. The experimental results show that to produce adakite-like melts in equilibrium with gamet, and therefore similar in composition to the Miocene Ignimbrites, pressure conditions of >2.2 GPa and temperatures in excess of 1025 "C are needed. Near-liquidus experiments on Fortaleza Ignimbrites indicate initial melts at source were in equilibrium with a fluid-absent, hydrous source mineralogy consisting of quartz, garnet and clinopyroxene (with minor zoisite). Micro-thermometric, micro-Raman and LA-ICP MS studies of quartz-hosted melt inclusions show that Yungay and Fortaleza Ignimbrite magmas were volatile-rich, and suggest the magmas experienced a short period of crystallisation (weeks or months) at intracrustal «40 km) depth. Rapid-crystallising quartz phenocrysts trapped melts with low SrN «30), in direct contrast to whole-rock high SrN characteristics. The trapped melts are not H20-saturated, which supports geochemical evidence that melting of the subducting Nazca slab has not formed the Ignimbrite adakite-like melts. In summary, the Yungay and Fortaleza Ignimbrites represent the final melts of deep juvenile mafic to intermediate lower continental crust created by progressive shallowing of the Nazca Plate, after the break-up of the Farallon Plate at 27 Ma. They were erupted immediately prior to orogenic collapse, facilitated by rapid movements on the extensional Cordillera Blanca Fault. Present-day crustal thickness measures 55 km, arguing for crustal delamination at or just after the emplacement of the Ignimbrites. This is supported by rapid uplift and exhumation in this sector of the Andes. Finally, adakite-like melts may simply reflect a natural progression in arc geochemistry as a mature subduction system evolves and crust is thickened via tectonic and magmatic events during the final stages of a magmatic cycle.