DETRITAL ZIRCON U-PB GEOCHRONOLOGY OF LOW-GRADE METASEDIMENTS FROM THE BASEMENT OF THE NORTH ANDEAN “LLANOS” FORELAND BASIN - COLOMBIA: PROVENANCE SIGNATURES OF EARLY IAPETAN PASSIVE MARGIN DEPOSITS IN NORTHWESTERN AMAZONIA

Late Neoproterozoic – Early Paleozoic paleogeography of western and northwestern Amazonia is characterized by the collapse of the Grenvillean orogen, intracontinental rift development, and initiation of passive margin sedimentation around the shores of the newborn Iapetus Ocean. In contrast to the eastern Laurentian margin where alkaline and A-type magmatism provide evidence for the timing of early rift development, such information in the Amazonian counterpart is scarce because most of it lies buried beneath a thick foreland basin sequence related to the evolution of the Andean orogen. Preliminary U-Pb detrital zircon (DZ) geochronology of two samples recovered from deep exploratory wells that cored metasedimentary basement of the north Andean “Llanos” Foreland basin, provides evidence for (1) latest Neoproterozoic - early Paleozoic clastic sedimentation in the area and (2) the occurrence of a subsequent low-grade metamorphic event that marks the initiation of tectonic activity along the proto-Andean margin of Amazonia after the break-up of Rodinia. Despite the geographical proximity and petrographic similarity of the studied samples, there are significant variations in the position of some of the age peaks and their relative abundances. Both of the analyzed samples have zircons ranging from Ediacaran to mid Paleoproterozoic in age. A sample from the Castilla-1 well has peak modes at ~1330, ~1520, ~1780 and ~1980 Ma, while the sample from the Candilejas-1 well has peaks at ~608, ~629, ~644, ~656, ~1320 and ~1750. Notable is the absence of the 1.0-1.1 Ga peak of the Grenvillean orogen that characterizes the “know” portion of the Mesoproterozoic orogen in northwestern Amazonia. The position of these metasediments with respect to the active Andean belt, along with the occurrence of Ediacaran rift-related intrusives to the east, suggests that their original sedimentary protolith was deposited on a passive margin developed after the break-up of Rodinia, provides better constraints about the timing of Laurentian-Amazonian rifting, and doesn’t support the presence of the recently proposed mid- to late Neoproterozoic magmatic arc along this segment of the proto-Andean margin.