The Imataca Complex, NW Amazonian Craton, Venezuela: Crustal evolution and integration of geochronological and petrological cooling histories

摘要

SHRIMP U/Pb-zircon data and Nd mean crustal resi-dence ages indicate that the Imataca Complex developed from an Archean (≥3.2 Ga) continental protolith which has undergone considerable isotopic disturbance plus and juvenile accretion during late-Archean (～2.8 Ga) times. Transamazonian granulites experienced peak metamorphic conditions of 750 - 800℃, 6 - 8 kbar with associated transpressive thrusting and tectonic imbrication. Geochronology on zircon, pyroxene and garnet constrains the timing of peak metamorphism at 1.98 - 2.05 Ga. Diffusion modeling of Fe-Mg exchange between biotite inclusions and host garnet yields (near metamorphic peak) cooling rates of 50 -100℃/Ma, with petrological cooling rates being generally consistent with cooling rates determined from geochronology. Combining the retrograde P-T path with cooling rates suggests that after the metamorphic peak, large portions of the Imataca Complex were exhumed from 30 to 17 km at a rate of 7 - 2 km/Ma. After this, exhumation rates progressively decreased as the rocks approached the surface. Rapid overall uplift/erosion had ceased when the rocks passed below 600 - 550℃ at 2.01 - 1.96 Ga ago. Observed variations in mineral cooling ages are interpreted as to reflect episodic differential tectonic exhumation within major fault systems. Inferred (maximum) ages of fault re-activation generally coincide with major continental accretion events in the Amazonian Craton and reflect long-term thermal evolution of the Imataca terrane, as conditioned by variable response to continued continental convergence during the Proterozoic.