Geochronology of Precambrian rocks from the northern part of the Guiana Shield, State of Roraima, Brazil

Abstract

Analyses of U-Pb in zircons, and Rb-Sr and Sm-Nd of whole rock samples have been made for a group of rock units from Roraima State in the northern part of the Brasilian Guiana Shield. The zircon data represent the first U-Pb zircon ages for rocks of the State of Roraima. The rocks analyzed crop out in two general areas; south and west of Boa Vista in the area of the Mucajaí and Apiaú Rivers, and north of Boa Vista in the Pacaraima Mountains, and occur at the suggested boundary between the Central Amazonia Province and the Maroni-Itacaiunas Belt. Three samples of granitoid gneisses associated with charnockitic granulites of the Kanuku Complex yield zircon U-Pb upper intercept ages on Concordia of 1943 ± 7 Ma., 1921 ± 15 Ma., and 1911 ± 13 Ma. Zircons from the Apiaú Gneiss give an upper intercept age of 1883 ± 46 Ma. Detrital zircons from the garnet-rich Taiano paragneiss yield an upper intercept age of 2235 ± 19 Ma. suggesting a Trans-Amazonian or older source for this rock. Zircon U-Pb, Rb-Sr, and Sm-Nd results from the Kanuku charnockitic granulites are scattered. A Rb-Sr “errorchron” gives an age of 2019 ± 71 Ma. Neodymium model ages relative to Depleted Mantle ranged from 1107 to 1963 Ma., while the zircon data are highly discordant. Three of the zircon analyses yield an age of 1827 ± 19 Ma. Detrital zircons from the Arai Formation of the Roraima Supergroup in the northeastern part of the study area give an age of 2171 ± 16 Ma. suggesting a Trans-Amazonian source for these sediments. The Mucajaí rapakivi granite yields a zircon age of 1544 ± 42 Ma., correlating with ages of other rapakivi granites in the Guiana Shield. Our results suggest the lack of any extensive Archean rocks in this area of the Guiana Shield, as well as limit the early Trans-Amazonian signature (circa 2100 Ma.) to the northern part of Roraima state. The granitoid gneisses appear to be syntectonic intrusives into the charnockitic granulites of the Kanuku Complex. The ages obtained in this study correlate with those found to the east in Guyana, and to the northwest in Venezuela.