TECTONIC CONTROLS OF EARLY JURASSIC ARC MAGMATISM IN THE COLOMBIAN ANDES

Jurassic magmatism is widely exposed as various belts of differentially exhumed plutonic and volcanic rocks in the central and eastern Colombian Andes. Their spatial, temporal and compositional characteristics record both magmatic processes and the tectonic evolution of the convergent margin of western South America contemporaneous with different stages of Pangea break-up.

Stratigraphic, geochemical and zircon U-Pb and Hf constraints from the Early to Middle Jurassic magmatic and sedimentary record in central and southern Colombia are used to evaluate the magmatic evolution and relate it with major changes in upper plate evolution during this time interval. The sedimentary record shows a Late Triassic transition from a phase of tectonic quiescence associated the formation of a marine carbonate to regressive conditions in which continental fluvial environments with a clastic record dominate by red beds took place by ca. 203 Ma associated to the installation of volcanic activity. Such record overlies or intrudes the former Early Triassic extensional magmatic and sedimentary record.

The Early Jurassic Magmatic record until ~178 Ma includes andesite to dacites volcanics and granodiorites to tonalites plutons, characterized by their enriched High-K calc-alkaline character, strong Pb and Th anomalies and well-defined Nb and Ti anomalies. Zircon Hf isotopes are characterized by predominantly negative εHf values until -6, whereas La/Yb derived mohometry is also compatible with crustal thickening.

We suggest that the high continental lithosphere input in the Jurassic arc magmas for this time interval as well as the wide regional expression is link to a major switch to faster plate convergence velocity and more frontal subduction of the pacific Farallon plate during this time interval that also promote significant crustal thickening and inversion of the former Early Triassic extension. After 178 Ma Jurassic arc magmatism change to more mantle derived sources probably reflecting both a new change to extensional conditions and the addition of mantle derived magmas in the base of the crust that replace the older continental crust as the major source of magmatic assimilation or melting.