TWO-STAGE CRUSTAL GROWTH OF THE NORTHERN CENTRAL ANDEAN PLATEAU

The timing of crustal growth in the northern Central Andean Plateau (CAP) and its relationship to surface elevation and regional climate is debated. The imperfect correlation between elevation and crustal thickness in the modern continental lithosphere illustrates why this issue is unresolved. Zircon qualitatively records crustal thickness as its trace element chemistry is controlled by the growth of cogenetic minerals and their relative uptake of light versus heavy Rare Earth Elements (LREE and HREE). We present new U-Pb and trace element data collected via single-collector laser ablation ICP-MS for Jurassic to Neogene zircons extracted from volcanic rocks, sandstones, and modern river sediments. Results show two distinct increases in ∑LREE/∑HREE, Eu/Eu*_N, Ti-in-zircon crystallization temperature, Th/U, La/Yb, Sm/Yb, Ce/Y, and Dy/Yb at 80–60 Ma and 40–20 Ma. These time periods are interpreted as phases of increased magmatic differentiation resulting from pulsed crustal growth, as they temporally correspond to the timing of high-flux magmatism and documented episodes of crustal shortening. Results point to a two-stage history of crustal growth in the northern CAP starting in the late Cretaceous. The first stage may correspond to an early phase of surface uplift, although no proxy elevation estimates exist before latest Oligocene time in the northern CAP; proxy records in the southern CAP suggest high elevation as early as late Eocene time. The second, more substantial phase of crustal growth, and lithospheric removal, resulted in the establishment of near modern elevations and arid conditions along the CAP Pacific coast no later than middle to late Miocene time.