Constraints on Plateau Architecture and Assembly From Deep Crustal Xenoliths, Northern Altiplano (Southern Peru)

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This award will support a petrologic and geochemical study of the portions of the crust that were picked up and embedded in volcanic rocks (xenoliths) once used by the Incas to build the city of Cuzco, in Peru. The geologic history of this part of the Andes is still poorly understood, specifically the reason for the high altitude that characterizes the region called the Altiplano. The rocks to be analyzed sampled the deepest portions of the crust in the Altiplano that are not exposed anywhere else in the region. It is expected that the results will provide us with a new understanding of how the deepest portions of continents evolve in mountainous areas. This study will also foster scientific collaborations with Peruvian and Chilean experts on local and regional Andean geology. Results of this project will be incorporated into the teaching curriculum at the University of Arizona, and will serve as the basis for the training of a M.Sc. student and also that of an undergraduate student. Because the volcanic rocks were the main building materials used by the Incas in Cuzco, knowledge derived from this project may impact our understanding of archeology in this famous heritage site. It is proposed to conduct a petrographic, thermobarometric, geochemical, isotopic and geochronologic study of newly discovered lower crustal xenoliths in Quaternary volcanic rocks from the northernmost Altiplano Plateau, Peru. At least eight individual lava flows contain xenoliths. Together, they are the second known occurrence of crustal xenoliths from the Altiplano and can provide invaluable information about the plateau's origin and development. This award will support a study of the crustal xenoliths aimed at (a) resolving the timing of crustal thickening and metamorphism initiation under the northern Plateau, (b) understanding whether crustal shortening was primarily driven by material underplated from the west (arc-side) or east (fold and thrust belt side), and finally (c) quantifying the role of magmatism to crustal thickening/loading. These xenoliths fill an important gap in our convergent margin rock record. The research plan comprises a field-sampling component followed by a petrographic and petrologic study of xenoliths. Sm-Nd and Lu-Hf garnet geochronology and monazite U-Th-Pb geochronology will be used to determine the age of metamorphism in conjunction with a thermobarometric study of the garnet-bearing assemblages. Igneous and detrital zircon geochronology will be employed to determine the age of magmatic intrusions in the Altiplano crust, and to constrain the origin of metasedimentary rocks, respectively. Elemental and radiogenic isotopic geochemistry will further illuminate the origin and crustal age of the assemblages studied here. The main deliverable after this project is a manuscript that will be submitted to a scholarly journal. Samples collected from this study will be made available to other interested members of the community.