Geology and geochronology of Cerros San Juan formation (department of Colonia, Uruguay), Río de la Plata Craton

The Cerros San Juan Formation is located in the southwest of the Colonia Department, in Uruguay. It is represented by a metavolcanosedimentary sequence in green schist facies, locally reaching amphibolite facies. This formation outcrops poorly into two separated blocks by a dextral N30°W fault. These blocks, named East and West, present N30° - 50°E and N50° - 70°E general trends, respectively. The unit is largely covered by Cenozoic deposits and soil cover and its main exposure occurs in the San Juan Hill. The metasedimentary sequence is represented by marbles with tremolite, schists with talc, schists with chlorite, schists with quartz and epidote, metaconglomerates, quartzites rich in hornblende and quartzites sensu stricto. The metavolcanic sequence comprises metatuffs, metarhyolites, metarhyodacites, metaandesites and metabasalts. Associated granophyric rocks occur locally. The whole rock chemical analysis of the metavolcanic unit shows that the acidic metalavas have calc-alkaline and peraluminous affinity, while the basic metalavas have tholeiitic to transitional and metaluminous affinity. The metalavas of the San José Belt, developed to the east of Cerros San Juan Formation, show similar geochemical behavior in general terms. The isotopic data obtained by zircon U–Pb SHRIMP (ca. 2.1 Ga), together with the petrological and geochemical data, suggest that the sequence could be related to a convergent or orogenic environment.