The Amazonas region of southeastern Colombia is underlain by the western part of the Guiana Shield. Isotopic age measurements are reported on granites and gneisses of the shield basement, mafic intrusives, and a sequence of rhyodacitic lavas overlying the

Abstract

The Amazonas region of southeastern Colombia is underlain by the western part of the Guiana Shield. Isotopic age measurements are reported on granites and gneisses of the shield basement, mafic intrusives, and a sequence of rhyodacitic lavas overlying the shield. Rb-Sr whole-rock analysis of 46 granites and gneisses and U-Pb analysis of two suites of zircons and a monazite reveal that during its development the Guiana Shield passed through at least two major orogenic episodes. The present basement was essentially formed during the Parguazan tectonomagmatic episode by large-scale granitic plutonism and metamorphic reconstitution of older crustal material, about 1560-1450 Ma ago. Most of the older isotopic record was obliterated during the Parguazan reworking, but some Rb-Sr whole-rock and U-Pb zircon systems indicate relict ages of at least 1850-1800 Ma, suggesting that the pre-Parguazan crust may be related to the Trans-Amazonian Orogenic Cycle. Rb-Sr and K-Ar analyses of 37 micas from basement rocks which are widely distributed over the area display ages cluster between about 1350 Ma and 1250 Ma; they are attributed to a general resetting of the isotopic systems by the Nickerie Metamorphic Episode about 1300 Ma ago. Evaluation of the Rb-Sr whole-rock data from five mafic intrusives and a suite of six samples from the rhyodacitic lavas suggest ages of about 1200 Ma and 920 Ma, respectively.