MIDDLE MIOCENE VOLCANISM IN THE SOUTH CARIBBEAN DEFORMED BELT (SINU BELT, NORTHERN COLOMBIA): PETROTECTONIC HYPOTHESIS

The Neogene margin of northern South America has been commonly considered as an amagmatic margin characterized by extremely flat slab subduction or extremely oblique convergence. Recent field observations have identified the occurrence of several dikes and sills basalts intruding Middle Miocene sediments which have been related to the South Caribbean belt (Sinu belt). This volcanism is characterized by Ar-Ar crystallization ages of ca. 13 Ma with LILE enrichment and Nb, Ta and Ti negative anomalies resembling a subduction modified mantle wedge. Ce/Y ratios suggest that this magmatism was formed by relatively shallow melting.

Time constraints from this magmatic activity overlap with major phases of basin subsidence and possible extension in northern Colombia.

We suggest that the position of this belt near the ocean-continent plate interface (resembling a fore arc position?) and the association with extensional conditions in the margins may reflect upper plate roll-back conditions and some modifications in the slab angle. This transient condition in the margin may have allow inflow of a modified astenospheric mantle.

The associated extension on the upper continental plate in northern Colombia may be also link to the final accommodation of the Panama block against the South American margin and the tectonic escape of the composite continental margin blocks.