Geologia USP. Série Científica (Jun 2009)
Geochemistry and Nd-Sr Isotopic Signatures of the Pensamiento Granitoid Complex, Rondonian-San Ignacio Province, Eastern Precambrian Shield of Bolivia: Petrogenetic Constraints for a Mesoproterozoic Magmatic Arc Setting
Abstract
The Pensamiento Granitoid Complex (PGC), located in the northern part of the eastern Precambrian shield of Bolivia, istectonically assigned to the Rondonian-San Ignacio Province (1.55 - 1.30 Ga) of the Amazonian Craton that is made up byArchean and Proterozoic provinces. The Proterozoic ones result from accretionary orogens that become successively youngersouthwestwards, such as the Rondonian/San Ignacio (1.37 - 1.32 Ga) and the Sunsás orogenies (1.20 - 1.00 Ga). The PGCcrops out mainly on the “Paragua craton” bounded to the south by the Sunsás belt, and composed of granites and subvolcanicterms, and subordinately of syenites, granodiorites, tonalites, trondhjemites and diorites as orogenic representatives of theRondonian/San Ignacio Orogeny, intrusive into the Lomas Maneches (ca. 1.68 Ga) and Chiquitania (ca. 1.7 Ga) complexes.Thirteen whole rock chemical analyses for major, trace and REE elements were performed for the La Junta, San Martín, Diamantina,Porvernir, San Cristobal, Piso Firme plutons of the PGC. The negative trends of MgO, Al2O3 and CaO contents withincreasing SiO2 suggest that fractional crystallization played an important role in the petrogenesis of the investigated rocks.The data also indicate a mainly peraluminous, sub-alkaline to high-K calc-alkaline composition, and fractionated LREE/HREE patterns are consistent with a magmatic arc character for these plutons. SHRIMP U-Pb zircon ages of the La Junta andSan Martín syn- to late-kinematic plutons are 1347 ± 21 Ma and 1373 ± 20 Ma respectively, and the Sm-Nd TDM model agesare between 1.9 to 2.0 Ga, while εNd(1330) values range from +1.8 to -4.3, respectively. In addition, the late- to post-kinematicDiamantina pluton yields SHRIMP U-Pb zircon age of 1340 ± 20 Ma, and variable Sm-Nd TDM model ages (1.6 to 1.9 Ga) andεNd(1330) values (+0.4 to -1.2) that are comparable with previous results found for other coeval plutons. The Porvenir, San Cristobaland Piso Firme plutons show εNd(1330) signatures varying from +1.5 to +2.7, in agreement with a plutonic arc setting as issuggested for the Diamantina pluton. Integrated interpretation of the geochemical and isotopic data coupled with new geologiccorrelations of the PGC with contemporary units in the Brazilian counterpart establishes one Mesoproterozoic magmatic arcin the evolution of the Rondonian-San Ignacio province.
