ABSTRACT A U-Pb SHRIMP zircon geochronological study was carried out in the Nico Pérez Terrane in the central-eastern portion of Uruguay with the aim of constraining its geological evolution and its cratonic affinity. Nico Pérez Terrane is made up by a mosaic of tectonic blocks with diferente lithologies and sizes. This terrane is limited to the west by the Piedra Alta Terrane through the Sarandí del Yí shear zone, and to the east, is in tectonic contact through the Retamosa thrust with the Dom Feliciano Belt (Brasiliano orogenic cycle). Lithologically, the Nico Pérez Terrane is composed by medium to high grade metamorphic rock contained in three tectonic blocks (Pavas, Valentines and Rivera blocks) represented mainly by granitoids, as well as ortho and parametamorphic rocks, such as amphibolites, metapyroxenites, BIFs, schists, quartzites with fuchsite, among others. Archean inheritance, Rhyacian, Statherian, and Neoproterozoic magmatic ages are reported here. Similar ages, which show Archaean inheritance and Neoproterozoic imprint, reported in units of the Piedra Alta Terrane modify the idea that the Río de La Plata Craton only corresponds to a juvenile Paleoproterozoic tectonic unit. This, together with recently published geophysical information, supports the cratonic affinity of the Nico Pérez Terrane with the Río de La Plata Craton.

RESUMEN Un estudio geocronológico U-Pb SHRIMP en circón fue realizado en rocas del Terreno Nico Pérez, con el objetivo de circunscribir la evolución geológica y su afinidad cratónica. Este terreno, localizado en la porción central del Uruguay, está constituído por un mosaico de tres bloques tectónicos y se encuentra limitado al oeste por el Terreno Piedra Alta a través de la zona de cizallamiento Sarandí del Yí. Al este, se encuentra en contacto tectónico a través del corrimiento Retamosa con el Cinturón Dom Feliciano (Ciclo Orogénico Brasiliano). Litológicamente, el Terreno Nico Pérez está compuesto por rocas metamórficas de grado medio a alto (bloques Pavas, Valentines y Rivera) representados principalmente por granitoides, así como por rocas orto- y parametamórficas, tales como anfibolitas, metapiroxenitas, BIFs, esquistos, cuarcitas con fucsita, entre otras. Se presenta en este trabajo un conjunto de edades magmáticas que poseen herencia arqueana, y edades riacianas, estaterianas y neoproterozoicas. Edades similares a estas, que muestran herencia arqueana e impronta neoproterozoica, reportadas en unidades del Terreno Piedra Alta, modifican la idea de que el Cratón del Río de La Plata corresponde a una unidad tectónica paleoproterozoica juvenil. Esto, junto con la información geofísica publicada recientemente, respalda la afinidad cratónica del Terreno Nico Pérez con el Cratón del Río de La Plata.

RESUMEN El presente trabajo describe a los diques máficos de afinidad lamprofídica presentes en la costa de la ciudad de Montevideo. Estos diques presentan tendencias estructurales N75°-85° con buzamientos verticales a subverticales y recortan a unidades metamórficas Paleoproterozoicas de 2.1 Ga del cratón del Rio de la Plata. Presentan texturas principalmente porfiríticas con macrocristales de flogopita y clinopiroxeno en una matriz compuesta por carbonatos, flogopita, augita y feldespatoides. Se observa, además, la presencia de estructuras ocelares rellenas por leucita, carbonatos y anfíbol fibroso alcalino. La textura y contenido mineralógico presente en estos diques permiten clasificarlos como lamprófidos (minettes), pero de acuerdo a su naturaleza química, pueden ser clasificados como lamprófidos alcalinos. Fue obtenida una edad de cristalización por método Ar-Ar (en biotita/flogopita) de 1.42 Ga.

ABSTRACT Mafic dykes of lamprophyric affinity cropping out along the coastal area of Montevideo city are described. These dykes trend N75°-85° and crosscut 2.1 Ga Paleoproterozoic metamorphic units of the Rio de la Plata craton. They show mainly porphyritic textures with phlogopite and clinopyroxene macrocrysts in a groundmass composed of carbonates, phlogopite, augite, and feldspathoids. Ocellar structures filled with leucite, carbonates, and fibrous alkaline amphibole are present. The mineralogical assembly allows their classification as lamprophyres (minettes), but according to their chemical nature, they can be classified as alkaline lamprophyres. A crystallization age of 1.42 Ga by Ar-Ar method (on biotite/phlogopite) was obtained.

Abstract The aim of this work is to propose a new stratigraphic scheme of the Barriga Negra Formation, in order to solve the controversies in one of the most discussed units of Uruguay. Here, we introduce a new map (unmodified since 1984) and amend the stratigraphic profile for the Barriga Negra Formation, establishing the correct sequence, order and thickness, dividing it in four members (Volcaniclastic, Arkosic Conglomerate, Calcareous Conglomerate and Petromictic Conglomerate Member). We describe by first time the presence of basal volcanic and volcanoclastic rocks that implies a tectonic active environment. Volcaniclastic rock dating yield an U-Pb concordia age of 633 ± 3.4 Ma. Moreover, we show that this unit has no stratigraphic continuity with other units that previously were used to constrain its stratigraphic position, age and paleoenvironment. We confirm unequivocally that the unit is as a continental conglomeratic sequence deposited in arid and active tectonic conditions, and not transitional passive margin deposits as some works claim. Finally, we revisit the metamorphism, deformation, stratigraphy, structural relationships of this unit and identify new basement units. As consequence of this new evidence, we reexamine its correlation with other regional units. This work is also a key to understand the chaotic stratigraphic proposals of Uruguay..

RESUMO: Este trabalho resulta de um esforço de vários anos tentando usar a geologia de campo para descrever litologias, estabelecer relações de contato e perfazer a evolução tectônica das sucessões metassedimentares, de idades meso a neoproterozoicas, que foram deformadas dentro da Faixa de Xistos do Cinturão Dom Feliciano. Essas rochas constituem uma cobertura de baixo grau que repousa em cima do embasamento de alto grau metamórfico dos complexos La China e Las Tetas. Esse embasamento é de idade paleoproterozoica-arqueana. A Faixa de Xistos é sotoposta em discordância angular pela formação Barriga Negra. O Complexo Lavalleja e a formação Barriga Negra se deformaram junto durante o evento D2 (~ 570 - 540 Ma), mas a formação Barriga Negra registrou apenas parcialmente o evento transpressivo D2, enquanto o Complexo Lavalleja foi afetado pelos dois eventos (D1 e D2). O evento D1 teria desenvolvido uma dobra - nappe com vergencia para o sul. Essa hipótese é sustentada por diferentes estruturas: (i) dobras recumbentes e dobras retas orientadas E-W, (ii) Foliação subhorizontal milonítica em mármores (calco-xistos); (iii) lineações de estiramento mergulhando para SW em metaconglomerados do Complexo Las Tetas, e (iv) retrabalhamento da foliação subhorizontal, paralelamente à zona de cisalhamento transcorrente Sarandí del Yí.

ABSTRACT: This work is the result of a multiyear effort to use field geology to describe lithologies, to establish contact relationships and to create a sketch of the tectonic evolution of the Meso- to Neoproterozoic metasedimentary successions within the Schist Belt of the Dom Feliciano Belt. This low-grade metamorphic cover rests on the high-grade metamorphic basement of the La China and Las Tetas complexes. This basement is Archean-Paleoproterozoic in age. The Schist Belt is overlapped unconformably by the Barriga Negra formation. The Lavalleja complex and the Barriga Negra formation both deformed together during the D2 deformation event (~ 570 -540 Ma), but the Barriga Negra only partially recorded the D2 transpressive event, whereas the Lavalleja complex was affected by both the D1 tangential event and the D2 event. Event D1 would have developed a fold nappe with vergence to the south. This hypothesis is supported by different structures: (i) recumbent and upright folds oriented E-W, (ii) subhorizontal mylonitic foliation in marbles (calc-schists), (iii) stretching lineations plunging towards the SW in metaconglomerates of the Las Tetas Complex, and (iv) a reworking of the subhorizontal foliation parallel to the Sarandí del Yí strike-slip shear zone.

Este trabajo presenta nuevos resultados geológico-estructurales y geoquímicos relacionados con el magmatismo Mesozoico del extremo Este de Uruguay (Sierra de San Miguel, Departamento de Rocha), asociados a la apertura del océano Atlántico Sur. Fueron reconocidas para la región estudiada tres unidades volcánicas/subvolcánicas. La asociación volcánica félsica está compuesta por derrames riolíticos - dacíticos, con texturas dominantemente porfiríticas y naturaleza química subalcalina. Asociadas a estas riolitas ocurren niveles de rocas piroclásticas. La asociación subvolcánica félsica está compuesta por granófiros con una extensión superficial de aproximadamente 25 km2, cortados por diques máficos y félsicos. Fue identificada también una asociación básica compuesta por diques y un cuerpo de gabro. Todas estas unidades son de edad Mesozoico (130 - 127 Ma) y de acuerdo a su carácter químico corresponden a magmas de naturaleza subalcalina a débilmente peralcalina.

This work presents new results of a detailed geological and structural investigation focusing the easternmost Uruguayan Mesozoic magmatic occurrences related to the south Atlantic opening. Lithological descriptions, their stratigraphic relationships and complimentary lithochemical characterizations carried out in the San Miguel region (East Uruguay) are presented. Three volcanic/sub-volcanic units have been recognized. The fel-sic volcanic association is composed by rhyolitic - dacitic flows, mainly with porphyritic textures and sub-alkaline nature and related pyroclastic rocks. The felsic sub-volcanic association is characterized by granophyres of about 25 km2 of exposed area, cross- cut by mafic and felsic dykes. Finally, a mafic association has been identified characterized by dykes and a small intrusion of gabbroic composition. All these units are Mesozoic in age (130- 127 Ma) and according to their chemical nature they correspond to sub-alkaline to weak peralkaline magmas.