Garnet can be found mainly in metamorphic environments and in granite pegmatite rocks. In both, they have the potential to record the conditions and timing of its growth. For example, LA-ICP-MS U-Pb ages from andradite common in skarn type deposits can be directly related to hydrothermal activity. East of Colombia in Guainía region, outcrops of the Proterozoic basement from the Amazonian Craton northwest part, at the Nabuquen river and in Barranquilla site near Cuiarí river are characterized by the presence of red to orange and purple crystalline garnets up to 20 mm long, discovered in their host rocks and in adjacent sedimentary deposits. The geochemical composition of major oxides acquired by means EPMA revealed nonappreciable variation regarding the (37 wt%) SiO2, (22 wt%) Al2O3, (25.5 wt%) MnO, (15 wt%) FeO and (0.5 wt%) CaO typically from spessartine endmember. Measurements in single garnet crystals from the two localities sampled yielded enough U, Pb and Th to conduct LA-ICP-MS dating. Mali garnet reference material was used. Nabuquen garnets were found in micaceous tourmaline schist in the vicinity of pegmatitic dikes of quartz, microcline, muscovite, and in gravel-sandy deposits along with tourmaline, monazite, zircon, apatite, and quartz crystals. The SEM-EDS imagery revealed monazite inclusions, whereas LA-ICP-MS data acquired yields an intercept age of 1775 ± 22.3 Ma (MSWD= 1.5; n= 32). On the other hand, the Barranquilla garnets were found in K-feldspar pegmatites and in colluvial deposits with heavy minerals such as columbite-tantalite, (Nb,Ta)-rutile, monazite, xenotime, tourmaline, and REE-Zircon crystals. The SEM-EDS images show graphite micro-inclusions, whereas LA-ICP-MS data acquired yields an intercept age of 1789 ± 33.3 Ma (MSWD= 0.99; n= 20). The calculated ages presented here are interpreted as crystallization ages and are coherent with reported ages for the oldest Mitú Complex rocks of 1752 ± 21 Ma in zircons from a monzogranite of the Nabuquen river and ages of 1783 ± 8 Ma in zircons from granites of Cuiarí river. The data acquired brackets an age span of 1800 -1750 Ma for the rocks of this part of the Amazonian Craton and may indicate either anatexis, metasomatic locally related intrusions, or ultimately UHP metamorphism. However, further studies are required to give insight the P-T conditions under which these Paleoproterozoic garnets were formed. Finally, the euhedral shapes, size, translucent colour, crystallinity, of these garnet crystals make them suitable as a quality-gem stone.
- Jose Alejandro. Franco1-2, Albrecht Von Quadt3; Alejandro Piraquive2-4, Thomas Cramer1-2, Amed Bonilla2, David Andrei Contreras4, Zeze Amaya2