Deep crustal melting in the Peruvian Andes: Felsic magma generation during delamination and uplift

The Miocene-aged Yungay and Fortaleza ignimbrites (YFI), 9° S, Cordillera Blanca, Peru, share geochemical affinities typical of Phanerozoic adakite-like rocks and Archaean tonalite–trondhjemite–granodiorite (TTG) suites. In this contribution, we will investigate the melt source region(s) of the adakitic ignimbrites and their relationship to the dynamic tectonic regime in Peru at the time of eruption. The geochemistry of the YFI differs significantly from spatially related lavas in western Peru, which are characterised by classic calc-alkaline arc signatures. The YFI were erupted along crustal-scale normal faults at the culmination of major tectonic changes affecting the western Peruvian margin, where they represent the last volcanic activity recorded in the area. These regional changes included crustal thickening, shallowing of the Nazca Plate from c. 30° to c. 5°, the arrival and subduction of the Nazca Ridge and rapid crustal uplift and exhumation. The previously unstudied adakitic YFI are examined, then a series of high-pressure piston-cylinder experiments are detailed. Synplutonic mafic-intermediate dyke material is used as an analogous source, and direct experiments are performed on ignimbrite samples. These show that the geochemical signature and mineralogy of the YFI can be produced at pressures N2.2 GPa and temperatures N1025 °C. These data support an origin for both ignimbrites as partial melts of juvenile, garnet-bearing, hydrous, mafic lower crust (granulite to eclogite), modified by ~20% contamination by igneous crust. A slab source is considered highly improbable. Eclogite and granulite at N2 GPa (N65 km depth) are absent within the modern lithospheric architecture of the Peruvian margin, requiring removal of this source region syn- or post-eruption. A likely mechanism for removal is crustal delamination in an east-relative motion, associated with shortening caused by progressive shallowing of the Nazca Plate subducting slab. Due to the restricted mantle flow through the thinned mantle wedge, there have been no further magmatic episodes since the eruption of the Yungay and Fortaleza ignimbrites.