FORMATION AGE AND MAGMA SOURCES FOR THE ANTIOQUEÑO BATHOLITH DERIVED FROM LA-ICP-MS URANIUM–LEAD DATING AND HAFNIUM-ISOTOPE ANALYSIS OF ZIRCON GRAINS

Integrated application of U–Pb dating and Hf-isotope analysis to zircon grain populations offers a rapid means for constraining formation age and magma sources associated with the intrusion of batholithic masses. The Antioqueño Batholith (AB) is the largest igneous intrusion (~8,000 km²) in the Central Cordillera, Colombia, and was emplaced in a rigid crustal block (through intrusive contacts) and into the Cajamarca Polymetamorphic Core Complex (Pz). Unlike many cordilleran plutons, the AB has a rather homogeneous composition (98% granodiorite-tonalite) and has an almost circular outcrop (S=1, not elongated, does not follow cordilleran trend). The Ovejas Batholith (OB) is an adjacent, much smaller pluton (< 240km²) that possesses similar petrologic, lithologic and structural characteristics that have been taken to imply consanguinity with the AB. Previous geochronology shows wide ranges in age for the AB and OB fluctuating between 56 and 98 Ma. It has been postulated that the best estimate of the age of formation for the AB is a Rb/Sr isochrone yielding 98 Ma. To date no attempts to produce formation ages in the U-Pb zircon system have been made. Evolution of the parental magma has been investigated trough Rb/Sr and Sm/Nd studies in the AB while magmatic evolution has not been assessed for the OB. We performed in-situ LA-ICP-MS U–Pb and Hf-isotope analyses of 90 zircons from six samples collected at the upper and lower portions of three vertical profiles (from 700 to 2500 m) with the objective of constraining formation ages and magma evolution for both intrusive bodies. U-Pb ages show very good reproducibility varying between 77-71 Ma and show no correlation with elevation (R²=0.1). We interpret the ages as representing relatively rapid formation for both batholiths near the transition Campanian-Maastrichtian in a way that may challenge the view that most I-type plutons are assembled through incremental processes. Coherence of our dataset indicates that this is a robust estimate of the formation age. Consistent with its subduction settings, εHf shows a range from -3 to +5, which reveals that the magma source for AB and OB was a blend between sedimentary/upper crustal and mantle material corroborating previous Rb/Sr and Sm/Nd results so that a mixed source for this granodiorite suite seems incontrovertible.