The Archean Pavas Block in Uruguay: Extension and tectonic evolution based on LA-ICP-MS U–Pb ages and airborne geophysics

Abstract

The Pavas Block is an Archean/Paleoproterozoic inlier located in the central segment of Uruguay's Precambrian exposition. A new insight initiated due to the acquisition of airborne geophysical data and new U–Pb zircon dating. The correlation of Archean crustal inliers in the amalgamation of the South American Platform is one of the main objectives in the Precambrian geological investigation. Although Ediacaran lateral strike-slip shear zones controlled the Pavas Block, their ultimate boundaries are not clear. Furthermore, a pre-Neoproterozoic metasedimentary succession discordant on this block is often confused with Neoproterozoic ones. Correctly identifying these limits is a necessary task to modify wrong tectono-stratigraphic models. The airborne total count radiometric imaging and the horizontal derivative of the total magnetic field anomaly reduced to the pole at 1000 m elevation led to establishing the Pavas Block's dimensions, based on new structural interpretation. The filtered magnetic anomaly is concordant with the previous Bouguer anomaly indicating a westward virgation of the Pavas Block in the north. This inlier narrows through the Minas city and along the Sarandí del Yí shear zone. The Sierra de Sosa and Sierra Ballena shear zones were confirmed as lateral limits. The total count radiometric map highlights the presence of brittle-fault reactivation. The Archean La China Complex containing zircons from three periods (~3.3–3.1 Ga; 3.0; 2.8–2.7 Ga) is in the Pavas Block's centre. New U–Pb zircon geochronology provided six samples from different metagranitoids that yielded LA-ICPMS U–Pb zircon ages ranging between 3353 ± 9 Ma and 1839 ± 2 Ma. Its southern periphery shows more recent Archean and Orosirian metagranitoids. These rocks were later partially affected by Neoproterozoic deformation. The preservation of igneous textures, field relationships, and zircon textures suggested that younger Neoarchean intrusive granitoids emplaced within the basal granitic gneiss “La China” complex. The pre-Neoproterozoic metasedimentary cover intercalates with Neoproterozoic successions until the southern end of the Pavas Block against the Sarandí del Yí Shear Zone. Finally, one granitoid devoid of Archean inherited zircons yielded a concordant 207Pb/206Pb zircon age of 1999 ± 3 Ma (MSWD = 8.1) interpreted as magmatic crystallisation. This Orosirian K-feldspar granite is likely to set the southern border of the block in a transitional crust. The Pavas Block is limited from the Villa Serrana Block by a schist belt of Mesoproterozoic protoliths, and reports enough information to be interpreted as a suspect terrane.