COMBINED U-PB AND LU-HF DETRITAL ZIRCON SIGNATURES OF NEOPROTEROZOIC THROUGH PALEOZOIC BASINS IN NORTHWESTERN SOUTH AMERICA

The time interval between the Tonian and the Ordovician was a period of intense tectonic activity in western and northern Gondwana, characterized by the breakup of the Rodinia supercontinent, a rift-to-drift transition in sedimentation along the subsiding continental margins, and finally the switch from post-Rodinian passive-margin sedimentation to proto-Andean and Avalonic magmatic arc activity. The development of these arc complexes is coeval with the detachment of ribbon microcontinents and terranes from northwestern Gondwana, giving birth to the Rheic Ocean behind them while closing the Iapetus/Tornquist system on their way to collide with Laurentia and Baltica, respectively. These microcontinental fragments have been identified in the so-called “Gondwanan realms” of the Appalachian Orogen and as fragments included within the Variscan and Caledonian orogenic tracts. Considering this global tectonic framework, the Neoproterozoic through Paleozoic basins located along the northern portion of the Amazon Craton in Colombia and Venezuela hold a key record for unraveling the timing of these events, as well as for potentially linking peri-Gondwanan terranes of proposed Amazonian ancestry to their source region. However, the Neoproterozoic and Paleozoic basins in northern South America are extensively covered by younger Andean-related sedimentary successions, making the availability of outcrops extremely limited and hindering our understanding of their stratigraphy, chronology and regional tectonic significance. In this contribution, we present a preliminary dataset from an ongoing project that seeks to provide first-order constraints on many of these issues by means of U-Pb and Lu-Hf isotopic analyses on detrital zircons obtained from these sequences. To overcome the exposure problem, a large portion of our sampling focused on deep exploratory wells that have pierced pre-Cretaceous (meta)-sedimentary units beneath the thick cover of the Andean Llanos foreland basin. Our new results show that dominant zircon age populations occur at ~1.75, ~1.55, ~1.50, ~1.30 and ~0.6 Ga, with initial ¹⁷⁶Hf/¹⁷⁷Hf compositions that straddle the CHUR line and define arrays compatible with provenance from the SW Guyana Shield and continuous mixing between crustal and juvenile components.