Paper No. 227-2
Presentation Time: 9:00 AM-1:00 PM
CRUSTAL EVOLUTION FROM U-PB GEOCHRONOLOGY AND ISOTOPIC CONSTRAINTS OF THE BOLIVIAN PRECAMBRIAN SHIELD
We present new and compiled U-Pb zircon ages and Nd and Hf constraints for the Paraguá Microcontinent of the Precambrian Shield of Bolivia, located in the SW fringe of the Amazonian Craton. This crustal segment, compartmented into the San Diablo and Paraguá blocks, witnesses a long-lived tectonic-magmatic history, now 1941-1849 Ma, 1690-1610 Ma, and 1430-1340 Ma. Two terranes with distinct evolutionary histories built the microcontinent. The oldest San Diablo block comprises amphibolite facies gneissic rocks (1941 ± 40 Ma), intruded by plutonic bodies with documented ages between 1874 and 1849 Ma, Sm-Nd TDM ages ranging from 1.96 to 2.29 Ga, and εNd(t) values between +1.76 to -2.73. The youngest Paraguá block contains a granulitic crust as old as 1820 Ma, where the available Sm-Nd TDM model ages between 1.7 and 2.2 Ga and εNd(t) values ranging from +3.0 to -2.9 suggest the derivation from short-lived protoliths. The basement rocks are crosscut by the Yarituses Suite (1683-1610 Ma) that includes the newly dated La Cruz (1673 ± 21 Ma), San Miguel (1681 ± 13 Ma), Rosario (1678 ± 21 Ma), granites with Sm-Nd TDM model ages of 1.8 to 2.5 Ga and εNd(t) of +4.06 to -3.8. These igneous units are younger than the Chiquitania meta igneous-sedimentary unit (1750-1630 Ma) and the Ambaybo rhyolite (1691 ± 8 Ma) of the San Ignacio Schist Group. Collectively, these four units are components of a Statherian accretionary Suruquiso orogeny that built the Paraguá Microcontinent. The Pensamiento Granitoid Complex (1440-1270 Ma) comprises the San Ramón and Coronación granodiorites (1429-1423 Ma) are pre-kinematic, whereas the San Martin (1409 ± 17 Ma) granites are syn-to late kinematic. The Diamantina granite (1357 ± 19 Ma) is late- to post-kinematic, while the San Andres granite (1289-1275 Ma) is a post-kinematic pluton.