U–Pb age data from the Sunsas region of Eastern Bolivia, evidence for the allochthonous origin of the Paragua Block

Abstract

Ion microprobe U–Pb dating of detrital zircons from the Chiquitania and San Ignacio groups of Eastern Bolivia show that these rocks were derived mostly from a 1765 Ma source and were deposited after 1690 Ma. The emplacement of the Lomas Maneches granitoid suit, dated at 1690–1660 Ma, accompanied the deposition of these metasediments. These rocks, which collectively define the basement of Eastern Bolivia, were subsequently deformed and metamorphosed during two discrete periods of orogenesis. The oldest of these events, the San Ignacio Orogeny, deformed the terrane through three successive phases of deformation and was accompanied by voluminous syn-tectonic granite intrusion and migmatisation of the 1690 Ma sediments and intrusions. U–Pb age data from igneous and metamorphic zircon constrain the San Ignacio Orogeny to between 1340 and 1320 Ma, while the orientation of structures suggests deformation was the result of W–NW directed shortening. A 300 million years hiatus, during which the overlying Sunsas Group was deposited, separated the San Ignacio Orogeny from the Sunsas Orogeny. Shortening during Sunsas orogenesis resulted in an intense NW trending structural overprint in the southern and western extremes of the study area. The intensity of deformation decreased toward the northeast such that in the San Ignacio region, Sunsas tectonism resulted in distinct dome and basin fold interference patterns. Late syn- to post-tectonic granites were emplaced in the region of highest Sunsas strain. One of these intrusions, the Taperas granite, yielded an age of ca. 1075 Ma. When taken collectively, these new data highlight a number of important points: (1) The deposition of the Chiquitania and San Ignacio groups, and the synchronous emplacement of the Lomas Maneches granitoid suite (1690–1660 Ma) have no temporal equivalents in the either the Rondônia and Mato Grosso regions of Brazil to the north or east, or the Proterozoic pre-Andean inliers to the west; (2) the trend of the structures attributed to the San Ignacio Orogeny are oriented at a high-angle to the NW trending pre-Sunsas margin of Amazonia and; (3) the Sunsas Orogeny represents the only geological event with a common age and direction of crustal shortening recorded in both the Amazon Craton and the rocks of Eastern Bolivia. These data are interpreted to suggest that the rocks of Eastern Bolivia represent a geologically distinct microcontinent (the Paragua Craton) that was accreted to the southern margin of Amazon Craton during the Sunsas Orogeny.