Punta Del Este Terrane Research Articles

Pb isotopic constraints on the evolution of the South Adamastor paleo-oceanic remnants, SW gondwana paleocontinent

This research presents the first whole-rock Pb isotope data for Tonian rocks from the Brazilian sector of the Punta del Este Terrane (amphibolites, magnesian schists and quartz-syenitic leucosomes of the Arroio Grande Ophiolite, and amphibolite and metagabbro xenoliths within granitoids), Dom Feliciano Belt. These rocks represent metamorphic remnants of the South Adamastor paleo-ocean. The new Pb isotope data suggest that these rocks are derived from a crustally-contaminated, depleted MORB mantle source, corroborating previously published hypotheses based on elemental geochemistry and Sr–Nd isotope data. The comparison with their Uruguayan (amphibolites of the Paso del Dragón Complex, Punta del Este Terrane) and Namibian (metagabbros and metabasalts of the Chameis Subterrane, Marmora Terrane) counterparts suggests that these rocks represent: (i) distinct regions (e.g. mid-ocean ridge, seamounts, trench) of the South Adamastor back-arc paleo-oceanic lithosphere, which were juxtaposed together during the closure of the basin; (ii) and/or, along the back-arc ridge, the magmatism was chemically heterogeneous.

The U–Pb and Lu–Hf zircon record of the Neoproterozoic tectonomagmatic evolution of the Ventania System basement, central-eastern Argentina: The southernmost exposure of the Dom Feliciano Belt?

In the Ventania System of central-eastern Argentina, Neoproterozoic igneous rocks of the Sauce Chico Complex (SCC) crop out. They consist of low-grade mylonitized granites and rhyolitic rocks. Metasedimentary rocks are represented by metapelite and metacarbonate xenoliths hosted in Neoproterozoic granites. This work presents lithogeochemical, and coupled LA-MC-ICP-MS U–Pb and Lu–Hf zircon data of Neoproterozoic rocks of the SCC in order to reinforce the already proposed link between these rocks and the Brasiliano/Pan-African Dom Feliciano Belt of southern Brazil and eastern Uruguay. Novel U–Pb results yielded crystallization ages of 623 ± 3 Ma and 608 ± 4 Ma in granite samples, and 545 ± 3 Ma in a tuff sample. Considering these new U–Pb ages and those published in previous works, Neoproterozoic rocks of the SCC can be grouped in Tonian (ca. 784–777 Ma) and Ediacaran (ca. 623–607 Ma) granites, and Ediacaran (ca. 577–544 Ma) volcanic and pyroclastic rocks. Xenocrystic/inherited ages span from the Ediacaran (ca. 555 Ma) to the late Neoarchean (ca. 2535 Ma). Ninety-four spots on zircon grains gave the following U–Pb and Lu–Hf results: ca. 2040–1700 Ma (n = 6, εHf(t) = -14.6/+6.3, TDM = 3.5–2.1 Ga); ca. 1500–850 Ma (n = 9, εHf(t) = -26.3/+4.5, TDM = 3.3–1.9 Ga); ca. 830–700 Ma (n = 26, εHf(t) = -40.5/+10.7, TDM = 4.1–0.9 Ga); and ca. 650–530 Ma (n = 53, εHf(t) = -38.8/+2.9, TDM = 3.9–1.3 Ga). When the Neoproterozoic rocks of the SCC are compared with those of the Punta del Este Terrane of the Dom Feliciano Belt, lithological, geochemical, geochronological, and isotopic similarities are evident. Both regions have Tonian orthoderived rocks (ca. 800–760 Ma) intruded by Ediacaran granites (ca. 630–570 Ma), and late Ediacaran plutonic and volcano-sedimentary rocks (<ca. 580 Ma). Tonian rocks of the Punta Mogotes Formation (Mar del Plata city area, southeasternmost tip of the Tandilia System) might represent the connection between the Punta del Este Terrane and the Ventania System basement. Additionally, the presence of Brasiliano/Pan-African rocks along the southern border of the Río de la Plata Craton effectively eliminates the possibility of the craton extending further south/southwest of the system.

Insights into the evolution of the southeastern Dom Feliciano Belt and its connection to the Pan-African Orogeny based on new U-Pb and Lu-Hf zircon data

The Neoproterozoic supracrustal units of the Dom Feliciano Belt in southern Brazil and eastern Uruguay provide a record of the tectonic evolution of the Brasiliano/Pan-African cycle in terms of sedimentation, deformation, and metamorphism during the western Gondwana supercontinent assembly. From the northern Punta del Este Terrane (Jaguarão Domain) and neighboring Herval Complex were acquired new LA-ICPMS U-Pb and Lu-Hf zircon data from significant samples of metasedimentary rocks and intrusive granites. The central units of the domain are the Arroio Telho and Arroio Grande metasedimentary complexes, which have detrital zircon data ranging in age from 3.2 to 0.7 Ga, with prominent age peaks at 2.2–1.7 Ga, 1.5–1.0 Ga, and 1.0–0.7 Ga. The Herval Complex samples revealed a similar provenance of detrital zircon ages, with two main intervals of 1.5–1.1 Ga and 1.0–0.7 Ga, and also minor Paleoproterozoic (2.0–1.7 Ga) and Archean (2.8–2.4 Ga) ages. The maximum depositional age of the supracrustal complexes is constrained to age populations between 700 and 650 Ma. The mylonitic intrusive peraluminous granites yielded crystallization ages of ca. ∼570 Ma. The Neoproterozoic metamorphic zircon overgrowths range from 680 to 570 Ma. The Lu-Hf data from the meso- to Paleoproterozoic zircons have εHf(t) values ranging from −10 to +10, with the dominance of slightly positive results, and Hf(t) model ages from Paleoproterozoic to Archean. The εHf(t) values in Neoproterozoic zircons are dominantly negative (−15 to +5) with meso- to Paleoproterozoic Hf(t) model ages. These results demonstrate that sedimentary deposition ceased and underwent metamorphism during the development of the Cryogenian/Ediacaran arc-related magmatism of the Dom Feliciano belt. Given the scarcity of Mesoproterozoic rocks in the Mantiqueira Province and nearby cratonic areas, the obtained data suggest that these detrital populations originated in southwestern African rocks. The Congo/Kalahari cratons (Africa) are the most likely source area for Paleoproterozoic and Archean detrital zircons. The geological context and isotopic results of the investigated supracrustal complexes point to a prior extensional tectonic setting during the Tonian (∼800 Ma), with regional deformation and metamorphism linked with the Dom Feliciano Belt accretionary and collisional stages (660–570 Ma).

Reassessing the polyphase Neoproterozoic evolution of the Punta del Este Terrane, Dom Feliciano Belt, Uruguay

Some recent models challenge the position and extension of the assumed oceanic basins formed through the break-up of Rodinia, and the tectonic processes involved in the Gondwana assembly, making the investigation of the Early Neoproterozoic record of great relevance. Within the South-American Atlantic margin, the Punta del Este Terrane (PET) of the Dom Feliciano Belt (DFB) comprises a unique Tonian to Ediacaran record, and has a strategic position to reconstruct spatio-temporal relationships with the southern African orogenic belts. Novel zircon U–Pb and Lu–Hf data from the PET basement orthogneisses display Tonian magmatic ages (805–760 Ma) and Hf isotopic signatures indicative of mainly crustal/metasedimentary sources, (Nd TDM ages: 2.2–1.9 Ga, and εHf(t): − 12 to − 4). The basement paragneisses yielded late Paleoproterozoic to Neoproterozoic U–Pb ages, but dominantly positive εHf(t) values. The presented results confirm the correlation of the PET with the Coastal Terrane of the Kaoko Belt, and discard the idea of the Nico Pérez Terrane as a source. Detrital zircon U–Pb and Lu–Hf data from the Rocha Formation yielded a main peak at ca. 660 Ma, with the Neoproterozoic grains showing a εHf(t) between + 1 and + 14. The deposition age of the Rocha Formation is constrained by the youngest detrital zircon age peak (660 Ma), and the beginning of the deposition of the Sierra de Aguirre Formation (580 Ma). The data indicate common sources with the Marmora Terrane, and it is thus proposed that the Rocha Formation belongs to the Gariep Belt, and it was juxtaposed during the Ediacaran to the DFB.

Petrographic and geochemical constraints on the evolution of the Matarazzo Sequence, Arroio Grande Ophiolite, Brazil: Evidence from migmatites and marbles

This study comprises the amphibolites, marbles, granitoids, metadiabases, and skarns of the Matarazzo Sequence (Arroio Grande Ophiolite, Dom Feliciano Belt, Brazil), which are here, for the first time, discussed from a migmatite perspective. The contribution presents novel petrographic and elemental geochemical data for the Sequence. Petrographic evidence demonstrates that high degrees of partial melting of the paleosomes (amphibolites) led to the development of millimeter-to meter-sized networks of mesocratic neosome veins (diorite to quartz-monzodiorite). The partial melting of the neosomes, in turn, led to the formation of micrometer-to meter-sized veins and pods of leucosomes (quartz-syenite to monzogranite). Garnet- and epidote-rich skarn layers were generated by the marble/melt interaction. The synkinematic nature of the partial melting event is evidenced by macrostructures such as rotated amphibolite schollen, neosome/leucosome veins concordant to the NE-SW-trending regional shear structures, and accumulation of melt in the hinge of folds. The minimum crystallization temperature of the neosomes and leucosomes are constrained, respectively, at 709 ± 31 and 732 ± 63 °C (Zr saturation thermometry). The geochemical data show that the trace element behavior of the paleosomes, neosomes and leucosomes are similar, strengthening the hypothesis of a genetic relationship and the interpretation that the metadiabases and amphibolites are the protoliths of the diatexites. The geochemical signature of the metadiabases and amphibolites suggest magmatic protoliths developed in an oceanic back-arc setting. The composition of the associated marbles is also consistent with this tectonic setting, reinforcing previous hypotheses that suggest that the studied ophiolite represents metamorphic remnants of an oceanic back-arc basin (South Adamastor paleo-ocean). The minimum metamorphic and obduction age of the Arroio Grande Ophiolite is, so far, marked by the U–Pb crystallization age of the studied leucosomes at 640 Ma. Since the same age is recorded in migmatitic orthogneisses from the southernmost sector of the Punta del Este Terrane in Uruguay, we suggest that the Punta del Este Terrane reached, synchronically, partial melting conditions at 640 Ma.

Arcabouço magnetométrico-geológico do setor sudeste do Cinturão Dom Feliciano, Rio Grande do Sul, Brasil

No setor sudeste do Cinturão Dom Feliciano, Rio Grande do Sul, está localizado o limite entre os Terrenos Pelotas e Punta del Este, região que contém diversos granitoides e rochas metamórficas neoproterozoicas e vulcânicas mesozoicas. Três domínios magnéticos foram individualizados com base em diferentes orientações dos lineamentos observados: Domínio I, II e III, com lineamentos dominantes, respectivamente, N040–90ºE, N060–90ºE, e N070–90ºE. Modelagens magnéticas 2.5D foram realizadas em dois perfis geológico-geofísicos orientados N-S, com 6 km de extensão, distantes 3 km entre si. O perfil A-A’ secciona o Terreno Pelotas até a porção Norte do Terreno Punta del Este, enquanto o perfil B-B’ secciona apenas esse último terreno. No perfil A-A’, em uma análise qualitativa do modelo, observam-se três patamares de susceptibilidade que refletem o contraste litológico da região: rochas básicas aflorantes (Gabro Desvio Herval); granitoides associados a estruturas tardias (Suíte Dom Feliciano); e rochas granitoides/metamórficas associadas ao Complexo Pinheiro Machado e Gnaisse Arroio Pedrado. No perfil B-B’, também se observa três patamares de susceptibilidade que refletem as diferenteslitologias da região: corpos básicos tabulares anastomosados não aflorantes; granitoides associados a estruturas de cisalhamento (Granitos Chasqueiro e Três Figueiras), granitoides associados a estruturas tardias (Granito Bretanha) e rochas metamórficas associadas aos Complexos Arroio Telho e Arroio Grande. A caracterização magnetométrica das rochas da região, em conjunto com os dados geológicos disponíveis, possibilitam um avanço no entendimento do setor sudeste do Cinturão Dom Feliciano. Sugere-se, assim, como limite entre os Terrenos Pelotas e Punta del Este a Zona de Cisalhamento Ayrosa Galvão, bem como a identificação de corpos em profundidades e não aflorantes, sincinemáticos às zonas de cisalhamento e rochas magmáticas básicas

Provenance of late Paleozoic glacial/post-glacial deposits in the eastern Chaco-Paraná Basin, Uruguay and southernmost Paraná Basin, Brazil

The Paraná Basin, Brazil and the Chaco-Paraná Basin, Uruguay both contain sedimentary records that are critical to reconstructing late Paleozoic ice centers in central Gondwana. The orientations of subglacial landforms and glaciotectonic structures suggest that late Paleozoic glacial deposits in the eastern Chaco-Paraná Basin and the southernmost Paraná Basin are genetically related, as they were likely glaciated by the same ice center. However, the location and extent of the ice center responsible for depositing these sediments are unclear. Furthermore, changes in sediment dispersal patterns between glacial, inter-glacial, and post-glacial intervals are not understood for this region of Gondwana. Therefore, this study utilized U–Pb detrital zircon geochronology to assess the provenance of glacial and post-glacial sediments from the eastern Chaco-Paraná Basin (San Gregorio, Cerro Pelado, Tres Islas Formations) and the southernmost Paraná Basin (Itararé Group). Results show dominant age peaks at ~520–555 Ma, ~625 Ma, 750–780 Ma, and 900–1000 Ma in all samples from the eastern Chaco-Paraná Basin. These zircons are interpreted to have been derived from sources in the Cuchilla Dionisio Terrane and Punta del Este Terrane in southeastern Uruguay, and possibly the Namaqua Belt in southern Namibia. Another important source was likely Devonian sedimentary rocks of the Durazno Group in central/eastern Uruguay. Meanwhile, a sample of the glaciogenic Itararé Group from the southernmost Paraná Basin contains a different detrital zircon signature with peaks at ~580 Ma, ~780 Ma, ~2110 Ma, and ~2500 Ma that closely resembles underlying sedimentary and meta-sedimentary rocks of the Precambrian/Cambrian Camaquã Basin. Detrital zircon ages in the glacial and post-glacial sediments indicate that local sources were dominant. In contrast, zircon ages from relatively ice-distal glaciomarine intervals in the Chaco-Paraná Basin reflect more distal sources to the east and southeast, which indicates a larger drainage catchment opened when glaciers retreated and/or the zone of maximum subglacial erosion shifted. Although most zircon ages in the Chaco-Paraná Basin can be attributed to Uruguayan sources, results support the hypothesis that glaciers emanated from southern Namibia and southeast Uruguay into the Chaco-Paraná Basin. From there, ice flowed northwest into the Paraná Basin and then receded back towards Africa as the paleoclimate warmed. The detrital zircon inventory in our study region is distinct from the eastern Paraná Basin, suggesting at least two unique African source regions for glaciers that deposited sediments in the Paraná and Chaco-Paraná Basins.

Sm–Nd systematics of metaultramafic-mafic rocks from the Arroio Grande Ophiolite (Brazil): Insights on the evolution of the South Adamastor paleo-ocean

In this contribution we present new insights on the evolution of the Dom Feliciano Belt, southernmost Mantiqueira Province, integrating new whole-rock Sm–Nd isotopic data for the Arroio Grande Ophiolite (Punta del Este Terrane, Brazil/Uruguay border) with previously published bulk-rock and isotope geochemistry of the South Adamastor paleo-ocean metamafic rocks located in Uruguay (Paso del Dragón Complex, Punta del Este Terrane) and Namibia (Chameis Subterrane, Marmora Terrane, Gariep Belt). For the regional geology, the new data corroborate previous hypothesis and demonstrate the depleted mantle features of the amphibolites and metagabbros of the studied ophiolite. The Arroio Grande Ophiolite rocks are compared with its Uruguayan and Namibian counterparts, demonstrating their isotopic and geochemical similarities and differences, and the back-arc affinity of the South Adamastor paleo-ocean. The MORB-affinity amphibolites from the Arroio Grande Ophiolite-Paso del Dragón Complex are, so far, the most juvenile rocks in the eastern sector of the Dom Feliciano Belt, yielding εNd(640 ​Ma) between +7.3 and ​+​9, and high 147Sm/144Nd (>0.169) and 143Nd/144Nd(640 ​Ma) ratios (0.51219–0.51229). The South Adamastor is interpreted in this paper as an internal back-arc ocean, with limited lateral extension, opened at around 750–650 ​Ma as the result of the closure of the older Charrua-Goianide paleo-ocean during the Brasiliano/Pan-African orogenic cycle and final configuration of the West Gondwana paleocontinent.

Two distinct crustal sources for Late Neoproterozoic granitic magmatism across the Sierra Ballena Shear Zone, Dom Feliciano Belt, Uruguay: Whole-rock geochemistry, zircon geochronology and Sr-Nd-Hf isotope evidence

Granite plutons and small stocks of similar Late Neoproterozoic age are widespread in the southernmost section of the Dom Feliciano Belt of Uruguay on both sides of the Sierra Ballena Shear Zone (SBSZ): Nico Pérez Terrane (e.g. western domain) and Punta del Este Terrane (e.g. eastern domain). Most granites exposed at both sides of the SBSZ largely show a field paucity of associated mafic rocks and similar characteristics to those derived from the partial melting of I-type metaigneous sources. However, granites with signatures akin to S- and A-type sources do also occur. Western domain granites display crystallization ages between 596 and 610 Ma, Archaean to Neoproterozoic inheritance ages (3072–640 Ma) and strongly evolved isotopic signatures (87Sr/86Sri of 0.708352; εNd of −14 to −20; Nd TDM2 ages of 2.3–2.8 Ga; εHf(t) of −21 to −27 and zircon Hf model ages as old as 3.6 Ga). Granites from the eastern domain, while showing similar but slightly broader range of crystallization ages (567–617 Ma), reveal contrastingly younger inheritance (775–589 Ma) and less evolved isotope characteristics (87Sr/86Sri of 0.707959–0.711420; εNd of −2.0 to −7.7, Nd TDM2 of 1.4–1.8 Ga; εHf(t) of −4.2 to −9.2 and zircon Hf model ages as old as 2.1 Ga). Combined field evidence and the contrast in ages of inherited zircon populations and in zircon and whole-rock isotopic geochemical compositions indicate that granites were generated by the reworking of old crustal blocks of different ages. Granitic melts were produced from partial melting of metaigneous and metasedimentary sources at lower to intermediate crustal levels with a possible contribution of a minor mantle component. A regional geological and isotopic resemblance between different geographic sections of the Pan-African/Brasiliano orogenic belts of Brazil and Uruguay might be significant and indicative of a certain commonality of sources in the production of granitic melts on a regional scale.

Chafalote Metamorphic Suite (Uruguay): Reflections on the evolution of the Punta del Este Terrane

The Chafalote Metamorphic Suite, located in the Uruguayan sector of the Punta del Este Terrane, southeasternmost Dom Feliciano Belt, comprises semipelitic, migmatitic metapelites, mafic granulites, and calc-silicate/amphibolitic gneisses. These supracrustal rocks occur as roof pendants in a granodiorite belonging to the Cerro Olivo Complex. The main structure of the Chafalote Metamorphic Suite is a composite S0/S1 banding, related to the main deformation event (D1) developed under upper-amphibolite/granulite facies conditions. This work presents an integrated study on the latter rocks, involving different geochronological methods (monazite U-Pb LA-ICP-MS and zircon U-Pb SHRIMP) and whole rock Sm-Nd isotope data in order to better understand the metamorphic evolution of the Chafalote Metamorphic Suite during the amalgamation of the SW Gondwana paleocontinent in the Neoproterozoic. The studied rocks record a peak metamorphic event (M2) occurred at around 660 Ma (monazite U-Pb ages), which was followed by decompression and partial melting (M3 event) at around 640 Ma (zircon U-Pb ages), probably during the late stages of the D1 event. The retrograde methamorphic event (M4) is related to the transition between the D1 and the development of NE-SW striking, low-grade sinistral shear zones (D2 event) and intrusion of syn-to post-orogenic Brasiliano granitoids of the Aiguá Batholith at around 600 Ma. The metamorphic conditions, together with geochronological and isotopic similarities, allow the correlation between the studied high-grade metasedimentary rocks and those from the Paso del Dragón Complex in Uruguay, and from the Várzea do Capivarita, Arroio Grande, and Telho Complex in Brazil. The latter rocks possibly share similar source areas and represent different portions of the Neoproterozoic fore-arc and/or back-arc basins developed in the Dom Feliciano Belt during the Brasiliano/Pan-African orogenic cycle.

The Neoproterozoic basement of the Sauce Chico Inlier (Ventania System): Geochemistry and U–Pb geochronology of igneous rocks with African lineage in central-eastern Argentina

This study describes the geology, geochemistry, and LA-ICP-MS U–Pb geochronology of igneous rocks that crop out in the Sauce Chico Inlier (SCI) and constitute the Neoproterozoic basement of the Ventania System, Argentina. Magmatism registered in the SCI has developed in two phases. The first phase, of Tonian age, is represented by rift-related calc-alkaline and alkaline granites with ages of 783.8 ± 3.7 Ma (εNd(t) = −7.20) and 776.5 ± 4.7 Ma (εNd(t) = +1.65), respectively. This phase would be related to the break-up of the Rodinia supercontinent and subsequent opening of the Adamastor Ocean. In the Dom Feliciano Belt of southern Brazil and eastern Uruguay, the Tonian magmatism is represented in the Cerro Olivo Complex of the Punta del Este Terrane and in basement inliers of the Pelotas Batholith. The second phase, of Ediacaran age, started with the intrusion of syn-orogenic calc-alkaline granites (620.8 ± 5.8 Ma and 620.3 ± 2.5 Ma; εNd(t) = −9.94/−9.18) and continued with the extrusion of post-orogenic alkaline (577.3 ± 3.9 Ma; εNd(t) = −6.29) and calc-alkaline (543.6 ± 4.0 Ma; εNd(t) = −3.38) acid volcanic rocks. This phase would be related to the closure of the Adamastor Ocean, generation of a magmatic arc along the western margin of the Kalahari Craton, and collision between this and the Río de la Plata Craton. A post-collisional magmatism would have developed due to orogenic collapse (< 580 Ma). With the exception of the Tonian alkaline granite, in the remaining SCI igneous rocks, the juvenile component becomes more important as the crystallization age decreases. The available Nd model ages are between 1.80 and 1.14 Ga and suggest a mixing of older crust with juvenile material. The tectonic evolution of the Ventania System basement is consistent with that observed in the Dom Feliciano Belt and its African counterparts. We present here the first evidence of Tonian magmatism in the Ventania System basement. The results presented here also confirm those obtained previously by other authors. Zircon cores from the SCI igneous rocks have U–Pb inherited ages between ca. 1200 and 900 Ma that could be indicative of a lineage with the Gariep Belt and its Namaqua basement in southwestern Africa.

Plagioclase-hornblende geothermobarometry of metamafites from the Arroio Grande Ophiolite, Dom Feliciano Belt, southernmost Brazil

This paper reports the first geothermobarometry data for the Brazilian sector of the Punta del Este Terrane (southeasternmost Dom Feliciano Belt), based on petrographic features (optical and electron microscopy) and mineral chemistry (electron probe microanalysis) of metamafites from the Arroio Grande Ophiolite, located near the Brazil/Uruguay border. Microtextures found in the studied amphibolite and metagabbro samples suggest that the southern portion of the ophiolite underwent two main metamorphic events, named M1 and M2. Based on field relations and radiometric ages from the literature, the minimum age for the M1 is constrained at ca. 640 Ma (migmatization event in the northern portion of the ophiolite). The M2 is constrained at ca. 610-585 Ma (syn-kinematic granite intrusion and formation of blackwall reaction zones in its southern portion). The P-T conditions of both events are estimated from plagioclase-hornblende geothermobarometry, constraining the M1 between 706 and 750 ± 40 °C and 5.5–6.3 ± 1.5 kbar, and the M2 between 595 and 698 ± 40 °C and 3.4–5.3 ± 1.5 kbar. It suggests that, as early as 640 Ma, fragments of the ophiolite were buried at depths of ca. 20–23 ± 5 km and, at around 610-585 Ma, these rocks were affected by a shear zone system at depths of ca. 15–19 ± 5 km. The P-T estimates presented in this paper, together with the suggested ages for the M1 and M2 events, are in good agreement with data reported for the southern (Cerro Olivo Complex) and northern (Paso del Dragón Complex) sectors of the Punta del Este Terrane in Uruguay. Thus, the present contribution further demonstrates that high-T/low-P conditions, up to the lower-granulite facies, together with partial melting events, are ubiquitous in the southeasternmost sector of the Dom Feliciano Belt during the Late Cryogenian events related to the Brasiliano/Pan-African orogenic cycle and the amalgamation of the Southwest Gondwana paleocontinent.

Insights on the evolution of the Arroio Grande Ophiolite (Dom Feliciano Belt, Brazil) from Rb-Sr and SHRIMP U-Pb isotopic geochemistry

This paper presents the first Rb-Sr isotopic geochemistry study and SHRIMP zircon U-Pb dating in the Arroio Grande Ophiolite (southeasternmost Dom Feliciano Belt in Brazil), an extension of the Uruguayan Punta del Este Terrane. The metamafic units of the ophiolite show 87Sr/86Sr ratios ranging from 0.7036 to 0.7070 (recalculated at 630 Ma), which suggest a MORB magmatic source for the amphibolite protoliths and a volcanic magmatic arc source (with possible crustal contamination) for the metagabbro protoliths. Together, these features suggest a supra-subduction zone origin for these rocks, corroborating previous studies based on bulk-rock chemistry. In the metaultramafics, the 87Sr/86Sr630 ratio of the chloritite sample (0.7152) is comparable to that of the Três Figueiras granite (0.7139), a syn-kinematic peraluminous granite related to the Ayrosa Galvão-Arroio Grande Shear Zone (which also affected the southern portion of the ophiolite, where the studied samples are located). The emplacement of the latter granite could be responsible for the metasomatism of the former serpentinites, generating talc- and chlorite-rich reaction zones. The SHRIMP U-Pb analysis of twelve chloritite zircons strengthens that assumption, as zircons with ca. 610-585 Ma (the age of the granite) were found in association with metasomatic microtextures. Zircon ages ranging from ca. 2000 to 660 Ma were also found in the chloritite. These latter ages are analogous to those found in the metasiliciclastic rocks that envelop the metaultramafic rocks of the ophiolite, so that the chloritite probably represents a blackwall reaction zone between (i) a former serpentinite, (ii) the Três Figueiras granite, and (iii) metasiliciclastic rocks. Additionally, we perform a SHRIMP U-Pb dating in a quartz-syenite melt, which are related to a tonalitic intrusion within the metasedimentary unit (the Matarazzo Marbles) of the ophiolite. The obtained concordia age of 640 Ma constrains the minimum age of ophiolitic mélange emplacement, as the tonalitic intrusion is related to continental arc magmatism (Pinheiro Machado Intrusive Suite).

Geology and geochronology of Paso del Dragón Complex (northeastern Uruguay): Implications on the evolution of the Dom Feliciano Belt (Western Gondwana)

New geologic and geochronologic data of Paso del Dragón Complex (PDC) located at Northeastern Uruguay is analyzed. Two lithodemic units are described and characterized, La Micaela Schists and Cerro La Tuna Serpentinites. La Micaela Schists represent 80% of PDC and it comprises a meta-volcano-sedimentary succession poly-deformed, composed by muscovite-quartz-feldspathic schists, quartzites, and felsic meta-volcanic rocks. Cerro La Tuna Serpentinites represents 20% of PDC and it comprises mafic-ultramafic rocks, also deformed and metamorphosed, composed by serpentinites, varied magnesian schists, hosted by La Micaela Schists. The complex is hosted to the north and west by the Granitic belt of the Dom Feliciano Belt that includes poly-intrusive plutons known from south to north as Aiguá, Cuchilla Dionisio and Sierra de los Ríos batholiths classified as high-K calc-alkaline pre-to post-collisional granitoids. Relevant U–Pb LA-ICP-MS analyses of metamorphic/detrital zircon grains from migmatitic-gneissic basement suggest an age of ca. 668 Ma for the metamorphic event associated with the migmatization process. The deposition and metamorphism age for La Micaela Schists lithodeme are ca. 590 Ma and 570 Ma respectively. The Complex is interpreted as remains of sedimentary and magmatic units generated in an arc-related environment tectonically emplaced within the granites. Their generation, deformation and metamorphism, mainly of Ediacaran age, are associated to the Western Gondwana assembly. This thrust sheet could be regarded as a dismembered ophiolite as previously suggested by other authors. Finally, it is proposed the correlation between Cerro Olivo Complex of Punta del Este Terrane and the migmatitic-gneissic basement of Paso del Dragón Complex based on their similar zircon U-Pb age patterns. Also, it is suggested that Cerro Olivo Complex of Punta del Este Terrane and the Granitic belt might be the main detrital sources for La Micaela Schist.

Did the circum-Rodinia subduction trigger the Neoproterozoic rifting along the Congo–Kalahari Craton margin?

Early Neoproterozoic metaigneous rocks occur in the central part of the Kaoko–Dom Feliciano–Gariep orogenic system along the coasts of the southern Atlantic Ocean. In the Coastal Terrane (Kaoko Belt, Namibia), the bimodal character of the ca. 820–785 Ma magmatic suite and associated sedimentation sourced in the neighbouring pre-Neoproterozoic crust are taken as evidence that the Coastal Terrane formed as the shallow part of a developing back arc/rift. The arc-like chemistry of the bimodal magmas is interpreted as inherited from crustal and/or lithospheric mantle sources that have retained geochemical signature acquired during an older (Mesoproterozoic) subduction-related episode. In contrast, the mantle contribution was small in ca. 800–770 Ma plutonic suites in the Punta del Este Terrane (Dom Feliciano Belt, Uruguay) and in southern Brazil; still, the arc-like geochemistry of the prevalent felsic rocks seems inherited from their crustal sources. The within-plate geochemistry of a subsequent, ca. 740–710 Ma syn-sedimentary volcanism reflects the ongoing crustal stretching and sedimentation on top of the Congo and Kalahari cratons. The Punta del Este–Coastal Terrane is interpreted as an axial part of a Neoproterozoic “Adamastor Rift”. Its opening started in a back-arc position of a long-lasting subduction system at the edge of a continent that fragmented into the Nico Perez–Luis Alves Terrane and the Congo and Kalahari cratons. The continent had to be facing an open ocean and consequently could not be located in the interior of the Rodinia. Nevertheless, the early opening of the Adamastor Rift coincided with the lifetime of the circum-Rodinia subduction system.

Chapter 16 The Chameis Gate Member, Chameis Group, Marmora Terrane, Namibia

Abstract The Chameis Gate Member is a poorly exposed and poorly investigated diamictite in the Chameis Subterrane of the Marmora Terrane, which forms the western, completely allochthonous part of the Pan-African Gariep Belt (southwestern Namibia). Its significance lies in its position in an entirely oceanic unit, the Dernburg Formation, which is dominated by mafic volcanic rocks. The diamictite contains exotic dropstones in a mafic volcaniclastic matrix, thus providing evidence for transport by ice away from the continental margin into an oceanic environment. No direct age data are available and stratigraphic relationships are obscured by limited outcrop and intense syn-orogogenic deformation. Preliminary chemostratigraphic data obtained on carbonate rocks below and above the diamictite, imprecise Pb–Pb age data on the largely volcaniclastic silicate fraction within associated stromatolitic reef carbonates, and imprecise Ar–Ar data on early hornblende related to sea-floor metamorphism of the associated volcanic rocks all point to an age loosely constrained between 640 and 580 Ma. Based on a comparison between the tectono-stratigraphic units of the Marmora Terrane with the continental Port Nolloth Group on the one side and the Rocha Group of the Punta del Este Terrane in Uruguay on the other side of the terrane, it is suggested that the diamictite was deposited in a back-arc basin that developed in response to the 640–590 Ma volcanic arc of the Dom Feliciano Belt in southeastern Brazil and eastern Uruguay.

The basement of the Punta del Este Terrane (Uruguay): an African Mesoproterozoic fragment at the eastern border of the South American Río de La Plata craton

The Punta del Este Terrane (eastern Uruguay) lies in a complex Neoproterozoic (Brasiliano/Pan-African) orogenic zone considered to contain a suture between South American terranes to the west of Major Gercino–Sierra Ballena Suture Zone and eastern African affinities terranes. Zircon cores from Punta del Este Terrane basement orthogneisses have U–Pb ages of ca. 1,000 Ma, which indicate an lineage with the Namaqua Belt in Southwestern Africa. U–Pb zircon ages also provide the following information on the Punta del Este terrane: the orthogneisses containing the ca. 1,000 Ma inheritance formed at ca. 750 Ma; in contrast to the related terranes now in Africa, reworking of the Punta del Este Terrane during Brasiliano/Pan-African orogenesis was very intense, reaching granulite facies at ca. 640 Ma. The termination of the Brasiliano/Pan-African orogeny is marked by formation of acid volcanic and volcanoclastic rocks at ca. 570 Ma (Sierra de Aguirre Formation), formation of late sedimentary basins (San Carlos Formation) and then intrusion at ca. 535 Ma of post-tectonic granitoids (Santa Teresa and Jose Ignacio batholiths). The Punta del Este Terrane and unrelated western terranes represented by the Dom Feliciano Belt and the Rio de La Plata Craton were in their present positions by ca. 535 Ma.

The transpressional connection between Dom Feliciano and Kaoko Belts at 580–550 Ma

A new U–Pb SHRIMP age of 551 ± 4 Ma on a mylonitic porphyry that intruded into the Sierra Ballena Shear Zone (Southernmost Dom Feliciano Belt, Uruguay) and a review of relevant published data make possible a more refined correlation and reconstruction of Brasiliano/Pan-African transpressional events. Paleogeographic reconstruction, kinematics and timing of events indicate a connection between the shear systems of the Dom Feliciano and Kaoko Belts at 580–550 Ma. Sinistral transpression recorded in shear zones accommodates deformation subsequent to collision between the Congo and Río de la Plata Cratons. The correlation is strengthened by the similarity of magmatic and metamorphic ages in the Coastal Terrane of the Kaoko Belt and the Punta del Este Terrane of the Dom Feliciano Belt. This post-collisional sinistral transpression brought these units near to their final position in Gondwana and explains the different evolution at 550–530 Ma. While in the Kaoko Belt, an extensional episode resulted in exhumation as a consequence of collision in the Damara Belt, in the Dom Feliciano Belt, sinistral transpression occurred associated with the closure of the southern Adamastor Ocean due to Kalahari-Río de la Plata collision.

Neoproterozoic geodynamic evolution of SW-Gondwana: a southern African perspective

Our current understanding of the tectonic history of the principal Pan-African orogenic belts in southwestern Africa, reaching from the West Congo Belt in the north to the Lufilian/Zambezi, Kaoko, Damara, Gariep and finally the Saldania Belt in the south, is briefly summarized. On that basis, possible links with tectono-stratigraphic units and major structures on the eastern side of the Rio de la Plata Craton are suggested, and a revised geodynamic model for the amalgamation of SW-Gondwana is proposed. The Rio de la Plata and Kalahari Cratons are considered to have become juxtaposed already by the end of the Mesoproterozoic. Early Neoproterozoic rifting led to the fragmentation of the northwestern (in today’s coordinates) Kalahari Craton and the splitting off of several small cratonic blocks. The largest of these ex-Kalahari cratonic fragments is probably the Angola Block. Smaller fragments include the Luis Alves and Curitiba microplates in eastern Brazil, several basement inliers within the Damara Belt, and an elongate fragment off the western margin, named Arachania. The main suture between the Kalahari and the Congo-Sao Francisco Cratons is suspected to be hidden beneath younger cover between the West Congo Belt and the Lufilian/Zambezi Belts and probably continues westwards via the Cabo Frio Terrane into the Goias magmatic arc along the Brasilia Belt. Many of the rift grabens that separated the various former Kalahari cratonic fragments did not evolve into oceanic basins, such as the Northern Nosib Rift in the Damara Belt and the Gariep rift basin. Following latest Cryogenian/early Ediacaran closure of the Brazilides Ocean between the Rio de la Plata Craton and the westernmost fragment of the Kalahari Craton, the latter, Arachania, became the locus of a more than 1,000-km-long continental magmatic arc, the Cuchilla Dionisio-Pelotas Arc. A correspondingly long back-arc basin (Marmora Basin) on the eastern flank of that arc is recognized, remnants of which are found in the Marmora Terrane—the largest accumulation of oceanic crustal material known from any of the Pan-African orogenic belts in the region. Corresponding foredeep deposits that emerged from the late Ediacaran closure of this back-arc basin are well preserved in the southern areas, i.e. the Punta del Este Terrane, the Marmora Terrane and the Tygerberg Terrane. Further to the north, present erosion levels correspond with much deeper crustal sections and comparable deposits are not preserved anymore. Closure of the Brazilides Ocean, and in consequence of the Marmora back-arc basin, resulted from a change in the Rio de la Plata plate motion when the Iapetus Ocean opened between the latter and Laurentia towards the end of the Ediacaran. Later break-up of Gondwana and opening of the modern South Atlantic would have followed largely along the axis of the Marmora back-arc basin and not along major continental sutures.

Geochronological constraints on the evolution of the southern Dom Feliciano Belt (Uruguay)

Abstract: New U–Pb sensitive high-resolution ion microprobe, K–Ar and Ar–Ar data from the southernmost Dom Feliciano Belt allow the identification of four major events. Orthogneisses from the Punta del Este Terrane indicate a magmatic episode at c . 770 Ma and high-grade metamorphism at 641 ± 17 Ma. Granitoid emplacement at 627 ± 23 Ma was roughly coeval with peak metamorphism. Volcaniclastic rocks of the Las Ventanas Formation dated at 573 ± 11 Ma can be correlated with the peripheral foreland basin (571 ± 8 Ma). Transpression and coeval high-K calc-alkaline magmatism is recorded in the Maldonado granite dated at 564 ± 7 Ma. The following events are postulated: (1) magmatism at 850–750 Ma related to rifting; (2) metamorphism and granite emplacement at 650–600 Ma; (3) molasse sequences and foreland basins at c . 573 Ma; (4) late magmatism at 580–560 Ma associated with transpression. The data indicate that (1) the Punta del Este Terrane could be a portion of the Coastal Terrane of the Kaoko Belt, (2) granitoid emplacement at 650–600 Ma in the Punta del Este and Nico Pérez terranes favours westward subduction, and (3) widespread post-collisional synkinematic magmatism occurred in the Dom Feliciano and Kaoko belts between 580 and 550 Ma. Supplementary material: Analytical methods and data are available at http://www.geolsoc.org.uk/SUP18369 .