U-PB AND HF ZIRCON ISOTOPES FROM LEUCOGRANITES OF THE SIERRA NEVADA DE SANTA MARTA, COLOMBIA: A RECORD OF EARLY FORE-ARC SHORTENING DURING SUBDUCTION INITIATION IN THE SOUTHERN CARIBBEAN

Subduction initiation models built mostly on island arc systems predicts that the fore-arc experienced an initial shortening of the margin followed by the inception of the mantle wedge to produce extensive fore-arc magmatism. We present field relations together with new and U-Pb and Hf zircon from a leugranitic plutonic suite in the Sierra Nevada de Santa Marta that seems to represent an unique subduction initiation record in the South American continent. The leucogranite suite include small plutons and sills that are also related to pyroxene-amphibole rocks. U-Pb crystallyzation ages of ca. 61-63 Ma, Ɛ_Hf values between 2-4 and the presence of inherited Pre-Mesozoic zircons suggest that the origin of this magmatism was related to the mix of a Caribbean oceanic crust and South American related (meta) sediments similar to those exposed in the fore-arc. We suggest that this magmatism is related to partial melting of a thickened fore-arc during subduction inception. This event was followed by more extensive I-type magmatism that intrude the fore-arc crust including the leucogranitic suite and may also be a major record of the advance of the mantle wedge as the new subduction slab advance into the mantle. This arc did not continue its growth as oblique convergence and relatively slow convergence rate limits magmatic evolution.