LATE CENOZOIC CENTRAL ANDEAN GEODYNAMICS FROM PAIRED U-PB AND LU-HF ZIRCON PETROCHRONOLOGY

Abstract

Geodynamic processes responsible for the formation of high topography in the Central Andean Plateau (CAP) are debated. Specifically, the relationship between crustal thickening and surface uplift in the CAP, and in Cordilleran orogens in general, is poorly understood. End member hypotheses include (1) coupled pure shear crustal thickening and concomitant slow (i.e., km/10s Myr) surface uplift; and (2) partially decoupled crustal thickening and rapid (i.e., km/Myr) surface uplift driven by foundering of mantle lithosphere. These hypotheses are addressed with new laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) paired U-Pb and Lu-Hf isotopic analysis of Oligocene-Miocene zircons from ignimbrite deposits in the Western Cordillera of southern Peru, collected and interpreted in the context of Oligocene – Miocene hinterland basin stratigraphy and published records of igneous whole rock geochemistry. Lu-Hf results calculated for the timing of crystallization, εHft, range from -0.4 to 16.8, and are consistent with magma generation in thickened continental crust. Results show systematic increases in εHft from 22–17 Ma and 12–5 Ma, followed by a return to median preshift εHft values of ~-8. Increases in εHft between -10 and 0 are interpreted as juvenile magmatic pulses into existing, more evolved crustal melt. The timing of pulsed juvenile magmatism is roughly coeval with a transition in hinterland basin deposition from dominantly fluvial to evaporitic lacustrine, and with recently reported rapid surface uplift between 22 and 10 Ma; decreases in εHft do not correspond to episodes of surface uplift. Results tentatively support a partial decoupling between crustal thickening and surface uplift in the northern CAP during the late Cenozoic due to the formation and subsequent foundering of dense lower lithosphere via Rayleigh-Taylor instability, after protracted crustal thickening