Petrography and geochemistry of successions from northwest Bolivia

Abstract

A complete section of Paleozoic sedimentary rocks has been sampled in northwest Bolivia to investigate compositional changes based on petrographic but mainly geochemical data. Studies of Paleozoic succession in southern Peru, northern Chile and northwest Argentina allowed to decipher the paleotectonic history of the region for the Paleozoic (e.g. compilations in Bahlburg et al., 2009). However, a few is only known about the area geographically in between. We focus here on an area in the vicinity of the capital La Paz, where the Paleozoic belongs paleogeographically to the so-called ‘altiplano’ and the ‘Cordillera Oriental del Norte’ and tectonically to the ‘Faja de Huarina del Norte’. The objective of the study is, hence, to establish a first complete stratigraphic insight of the Paleozoic to enable further studies on key formations or horizons. While the abovementioned areas to the north and south do reflect in the sedimentary record the emergence of a continental arc during the Ordovician, followed by a significant tectonic quiescence during the late Silurian and Devonian and succeeded by the evolution of a second active margin with a continental arc during the late Paleozoic, these events are not visible in the Bolivian samples which is sandwiched between the westerly Arequipa Massif and the eastern Sunsas belt, both composed of mainly metamorphic rocks.The Paleozoic section in northwest Bolivia shows highly recycled quartz-arenites of Ordovician age followed by Silurian and Devonian successions mainly composed of subangular to surrounded detritus, moderately sorted, devoid of significant typical metamorphic detritus. During the late Paleozoic, the rocks tend to a larger variability, partly even poorly sorted, higher amount of angular grains and the occurrence of volcaniclastic debris. Geochemically the same trend can be observed, with strongly recycled successions during the Ordovician (Zr/Sc 20–200) followed by moderately recycled rocks (mainly Zr/Sc 10–20100) with typical trace element composition for unrecycled upper continental crust (UCC). During the Permian, sediments which are less weathered and slightly less fractionated in their overall geochemical composition, have been deposited. The geochemistry may point to the evolution of late Paleozoic volcanism, recorded in overlying, but however, Triassic rocks. Significant increase or even typical UCC values of compatible elements or ratios are absent besides Cr and Ni increase in Ordovician rocks. The absence of any significant input of arc related detritus can be explained by not effective sediment dispersal system to transport arc related detritus into the depositional area. The low compositional and low to moderate textural maturity throughout the entire post-Ordovician stratigraphy implies relatively proximal sources and insignificant intra-basinal recycling. This together with the absence of metamorphic detritus, allows in proposing thick homogeneous sedimentary successions with low or no metamorphic overprint as major sources. Those covered possibly the today exposed metamorphic successions of the aforementioned basin surroundings. Sedimentary or any other lithologies older than the Ordovician and younger than Mesoproterozoic are nearly unknown in the region, besides one exception (Chilla beds; Bahlburg et al., 2020). Those hypothetical deposits would be at this stage of knowledge the best explanation.