CARACTERIZACIóN PETROGRáFICA, GEOQUíMICA Y EDAD DE LA SIENITA NEFELíNICA DE SAN JOSé DEL GUAVIARE

the san josé del guaviare nepheline syenite includes two rock bodies that crop out 20 km sw of the town with the same name. the first of these bodies, which had already been reported by vesga and castillo (1972), is exposed in the la pizarra area. the second body, which had not been previously reported, occurs in the las delicias area. petrographic characterization shows that rocks correspond to nepheline syenite, nepheline monzosyenite, and feldspar + nepheline granofels. the main minerals in the rocks are microcline + nepheline + plagioclase ± biotite ± arfvedsonite, and accessory phases include fluorite, sphene, apatite, garnet, zircon, epidote and calcite. structures and textures are chiefly igneous in the la pizarra body and mainly metamorphic (restitic) in the las delicias body. the geochemical character of the rocks is alkaline, and they correspond to within- the field of rocks formed in intraplate environment. the above features suggest that the san josé del guaviare syenite was formed by anatexis in a continental environment. u/pb dating of zircon and 40ar/39ar (hs) dating of biotite indicates age 577.8 ± 6.3 - 9 ma (ediacaran) crystallization and 494±5 ma (late cambrian) cooling.