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.

The Southwestern Atlantic margin is characterized by several canyon systems incised on a huge contourite depositional system associated with the interaction of strong Antarctic water masses with the seafloor. So far, however, only one mega slide canyon (Cabo Polonio) has been described in the Uruguayan continental slope. In this work, sedimentary processes dominating the head of this mega slide canyon are described and interpreted based on the analysis of the composition (texture and fossils) and distribution of widespread gravel along its head and thalweg. This information is integrated with acoustic (multibeam and seismic) and hydrological data. Results suggest that the evolution of the head of the canyon presents retrogressive erosion related to debris and turbidity flows. This erosion is ongoing and/or has been active during the recent past and contourite deposits are involved in headwall erosion. The pathway of gravel along the canyon and the thalweg was reconstructed. The strong flow of the South Atlantic Central Water, and its interaction with the Antarctic Intermediate Water, dominates the modern hydrology, promoting highly energetic conditions enhancing headwall erosion. This work contributes to a better understanding of the sedimentary processes connected to an mega slide canyon in the upper slope off Uruguay and located in the northernmost distribution of the South Atlantic Contourite Depositional System.

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.

El Haz de Diques de San Gregorio de Polanco (Departamento de Tacuarembó, Uruguay) es la ocurrencia más meridional de diques pertenecientes a la Provincia Magmática Paraná en Uruguay. Fueron identificados cinco 5 diques principales con dos direcciones estructurales principales: N140º - N170º y N50º - N80º, respectivamente. Son diques de afinidad tholeítica compuestos por plagioclasa (An55), augita y augita-pigeonita, relictos de olivina y minerales opacos. Estos diques se caracterizan por el alto contenido de óxidos de Fe y Ti (titanomagnetitas), cuyas características mineralógicas y texturales fueron estudiadas con microscopio electrónico de barrido y espectrometría de energía dispersiva (SEM-EDS), incluyendo mapeos composicionales. Estas características junto con otras relaciones mineralógico-texturales presentes en estas rocas permitieron proponer la siguiente secuencia de cristalización: (i) cristalización de fenocristales de olivina, plagioclasa y piroxenos cálcicos; (ii) precipitación de una primera población de titanomagnetita, (iii) cristalización de plagioclasa y piroxenos conformando la matriz; (iv) disolución parcial de la primera población de titanomagnetitas por reacción con fluidos magmáticos; (v) cristalización de la segunda población de titanomagnetitas y finalmente, (vi) cristalización de barita intersticial.

The San Gregorio de Polanco Dike Swarm (Tacuarembó Department, Uruguay) is the southernmost set of dikes in the Paraná Magmatic Province of Uruguay. Five major dikes have been identified with two main structural trends: N140º-N170º and N50º-N80º. The dikes have tholeiitic affinities and are composed of plagioclase (An55), augite and augite-pigeonite, relicts of olivine and opaque minerals. These rocks have high contents of Fe-Ti oxides (titanomagnetites), the mineralogical and textural characteristics of which have been studied using scanning electron microscopy and energy dispersive spectrometry techniques (SEM - EDS). These features, along with other mineralogical and textural relationships, have been used to propose the following crystallization sequence for the dikes: (i) crystallization of olivine, plagioclase and Ca-rich pyroxene phenocrysts; (ii) precipitation of the first population of Ti-magnetite; (iii) crystallization of plagioclase and pyroxene in the groundmass; (iv) partial dissolution of Ti-magnetite by reaction with magmatic fluids; (v) crystallization of the second population of Ti-magnetite and finally, (vi) crystallization of interstitial barite.

The southeastern border of the Laguna Merín Basin (Rocha Department, East Uruguay) consists of Mesozoic volcanic deposits of the Puerto Gómez and Arequita formations, overlain by a sedimentary cover of Cenozoic and recent deposits. Wellpreserved lava flow and pyroclastic deposits have been identified in the Arequita Formation after detailed mapping (1.50.000) in Sierra de los Ajos region. The present study focuses on the petrographic characterization of this felsic volcanism, with special emphasis on the pyroclastic units. Differences in the structural and petrographic distribution of these volcanic units are related to the N20ºE trending India Muerta Lineament. East of this lineament, SiO2-rich rhyolites and pyroclastic deposits (ignimbrites) are present whereas to the west, the volcanic rocks are mainly represented by basic to felsic lavas. Based on petrographic features and regional distribution, explosive mechanisms have been assigned to some of the volcanic units, interpreted as pyroclastic flow and pyroclastic surge deposits.

El extremo sureste de la Cuenca Laguna Merín (Departamento de Rocha, Uruguay) está constituido por depósitos volcánicos mesozoicos, correspondientes a las Formaciones Puerto Gómez y Arequita, cubiertos por sedimentos Cenozoicos y de edad reciente. Los trabajos desarrollados en la región de Sierra de los Ajos permitieron identificar en la Formación Arequita flujos de lava y depósitos piroclásticos bien preservados, cuyas características petrográficas son presentadas en este trabajo. La distribución espacial y estructural de este volcanismo en la región de estudio está relacionada con el Lineamiento India Muerta de dirección N20ºE. Al este del mismo, se encuentran depósitos piroclásticos (ignimbritas) y derrames riolíticos ricos en SiO2 mientras que hacia el oeste la región está constituida fundamentalmente por flujos de lava básicos y ácidos. En función de las características petrográficas y de la distribución espacial que presentan los depósitos piroclásticos se interpretan a los mismos como depósitos de flujo y oleada piroclástica.

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.