Léo Afraneo Hartmann

Juvenile accretion at 750–700 Ma in southern Brazil

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Advances in SHRIMP geochronology and their impact on understanding the tectonic and metallogenic evolution of southern Brazil

Significant improvements, both in understanding the evolution of zircons and in understanding the geotectonic and metallogenetic evolution of the complex terrain of southern Brazil, are obtained from a SHRIMP geochronology study and reviewed in this paper. The use of backscattered electron and cathodoluminescence images, prior to SHRIMP isotopic determinations, proved of enormous fundamental value for this technique. Zircon is a domainal open‐system mineral in many geological conditions; very old domains may be preserved, but the same crystal may show ages of younger tectonic events. Zircons may recrystallise inwards from the rims or outwards from the cores, and also along euhedral high‐U or metamict thin zones. Zircons also may be recrystallised during gold‐related hydrothermalism, phyllic alteration of granitic rocks. The precise dating of amphibolite dykes can be achieved by the identification and dating of magmatic zircons. Precambrian orogenies are identified along with the intervening intracratonic tectonic cycles of supercontinents in southern Brazil from 3300 to 470 Ma. Granulite protoliths were formed during the Jequié Orogeny (ca 2600 Ma), but extensive arc accretion occurred in the Palaeoproterozoic (ca 2250 Ma) Encantadas Orogeny. Late in the Transamazonian Cycle, granites were formed by crustal melting at about 2000 Ma in the Camboriú Orogeny. Both accretionary and collisional orogenies are also identified in the Neoproterozoic Brasiliano Cycle. These are the accretionary Passinho Orogeny (ca 900 Ma) and São Gabriel Orogeny (ca 700 Ma), that were succeeded by the collisional Dom Feliciano Orogeny (ca 600 Ma). Base‐metal and gold deposition occurred in juvenile island arcs and in late orogenic porphyry‐copper‐type magmatic‐hydrothermal settings during the Neoproterozoic.

Duration of the Trans-Amazonian Cycle and Its Correlation within South America Based on U-Pb SHRIMP Geochronology of the La Plata Craton, Uruguay

The La Plata craton of Uruguay consists mostly of Paleoproterozoic terranes that are ideal for the investigation of the Trans-Amazonian Cycle evolution because, unlike the terranes of southeastern Brazil, they are unaffected by the Brasiliano collisional orogenies (640-590 Ma). The U-Pb isotopic dating of zircons from four Uruguayan rocks by the Sensitive High-mass Resolution Ion Micro Probe (SHRIMP II) shows an evolution from 2224 to 2056 Ma. We also extend the discussion of the cycle to the entire South American continent to provide a broad overview of the processes and belts involved. The Trans-Amazonian Cycle of South America consists of four main orogenies, of which three are present in the Uruguayan Piedra Alta (Isla Mala, Paso Severino, and Soca units) and Nico Pérez (Rivera and Valentines samples) terranes. All four Trans-Amazonian Cycle orogenies described in the classical Trans-Amazon orogen of the type-area (São Luís cratonic fragment, Gurupi belt) and in the Amazon craton are present in the La Plata craton. In Uruguay the second (2180-2120 Ma) and the third (2080-2050 Ma) Trans-Amazonian Cycle orogenies are well developed, whereas the oldest orogeny (2260-2200 Ma) is identified by only one inherited zircon (2224 Ma, Valentines granulite) and the fourth orogeny (2020-2010 Ma) has to be detected, but it is present in neighboring southern Brazil (Itapema, Camboriú, and Santa Maria-Chico units). The Rivera meta-trondhjemite was formed at 2140 ± 6 Ma, an epoch of granitoid-greenstone formation along all the Trans-Amazonian Cycle belts of South America, and by the Paso Severino felsic volcanics, formed at 2146 ± 7 Ma. The Rivera trondhjemite was metamorphosed at 2077 ± 6 Ma, a period characterized by intrusion of post-tectonic potassic granitoids and regional high-grade metamorphism. The metamorphism of the Valentines granulite is slightly younger (2058 ± 3 Ma) and this rock contains zircon with several inherited ages, such as 2224, 2163 (second orogeny), 2535, and 2619 Ma. The Soca charnockite is the youngest known Trans-Amazonian Cycle rock in Uruguay (2056 ± 6 Ma) and represents a continent-wide period of emplacement of post-tectonic, evolved granites and charnockites, such as the Calçoene charnockite (2059 Ma) of Amapá, Brazil. The data presented improve our understanding of tectonic processes active in South America during the Rhyacian Trans-Amazonian Cycle. However, the evolution of large segments of Paleoproterozoic crust and the Trans-Amazonian Cycle subdivision remain uncertain in many regions where U-Pb data are scarce or not available. The dating and characterization of these terrains require further investigation.

Age, source, and regional stratigraphy of the Roraima Supergroup and Roraima-like outliers in northern South America based on U-Pb geochronology

New 207 such as the Serra Surucucus, Araca ´, and Neblina units, do not belong to the supergroup. Rocks of the Roraima Supergroup and post-Roraima sandstones were deposited in two separate but overlapping basins, each ;1,200,000 km2 in area. The Roraima Su- pergroup represents fill in a foreland basin that was derived mostly from the Trans- Amazon orogenic belt to the north and northeast, whereas the fill in the post- Roraima Neblina successor foreland basin was derived from both the Trans-Amazon and Tapajos-Parima orogenic belts to the east and northeast. Although most of both basins are largely flat lying or gently de- formed, some of the westernmost outliers of the post-Roraima sandstones were de- formed by the far-field effects of the Sunsas (Grenvillian) orogen at 1.33 Ga (Ar-Ar in muscovite). These results are based on U-Pb deter- minations of nine samples, Ar-Ar step- heating plateau age of one muscovite sam- ple, and a complete review of many earlier Rb-Sr and K-Ar earlier dates.

SHRIMP U/Pb Zircon Dating of Neoproterozoic Granitic Magmatism and Collision in the Pelotas Batholith, Southernmost Brazil

The introduction of robust geochronological methods for age determinations of the southernmost segment of the Neoproterozoic terranes of Brazil, namely the Dom Feliciano Belt, provides important clues for unraveling the complex evolution of the Brasiliano/Pan-African orogeny in this southwestern portion of the Gondwana supercontinent. Except for associated small schist belts and post-orogenic foreland basins, the belt is represented in this region of southeastern South America by the Pelotas Batholith. Precise SHRIMP U/Pb zircon geochronological techniques based on the study of 95 individual spots on 74 zircon crystals (three samples) and on Nd-isotopic determinations (three samples) are used to assess the late Neoproterozoic history of the belt, especially the orthogneisses interleaved with the batholithic plutons. Three petrotectonic associations were selected for detailed isotopic investigations—the Pinheiro Machado syncollisional monzogranites, the widespread Piratini gneiss tonalitic xenoliths, and t...

Predominance of high Th/U, magmatic zircon in Brazilian Shield sandstones

The strong predominance of magmatic (Th/U >0.2) zircon grains in quartz sandstones from 10 locations in the Brazilian Shield, as measured by sensitive high-resolution ion microprobe (SHRIMP), introduces a strong bias in the tectonic interpretation of source terrains, because the metamorphic sources remain undetected in the detrital population. Zircon grains from many mature, and some immature, sedimentary terrains show Th/U ratios >0.2, ratios commonly considered magmatic, complemented by the near absence of metamorphic Th/U ratios <0.1. Metamorphic rims on magmatic zircon grains from the Brazilian Shield are nearly absent from quartz sandstones, only being present in shielded young cores of a few crystals. High-U, metamorphic portions of zircon grains are more defective and liable to undergo comminution by abrasion during weathering and transport, and they do not remain in the sand fraction. Ages obtained from detrital zircon in mature sandstones reflect magmatic sources, not the full spectrum of orogenic events. Only less mature sedimentary rocks preserve a full record of orogenic rocks in the source. Mostly magmatic events were characterized in previous zircon provenance studies. Brazilian Shield detrital zircon may be a proxy for equivalent investigations in other provinces of the world.

U-Pb SHRIMP and Sm-Nd age constraints on the timing and sources of the Pan-African Cape Granite Suite, South Africa

Geochemical and isotopic UPb SHRIMP and SmNd studies in three granitic plutons from the Pan-African Neoproterozoic/Cambrian Saldania Mobile Belt, southwestern South Africa supports differences in the sources and timing of emplacement of the main plutons related to the Cape Granite Suite as established by previous researchers. The Phase I, early syn-tectonic Darling Batholith Granitoid reflects an overall peraluminous chemical signature compatible with derivation mainly from melts extracted from paraderived crustal sources (Sa1 association). The Phase II, late to post-tectonic Robertson Pluton, reflects affinities to the Australian l-type granites (1a association). Despite the good structural constraints on the syn- and post-tectonic origin, the UPb ages point to a broadly synchronous crystallisation episode at 547±6 Ma and 536±5 Ma, respectively. In addition to UPb, Nd isotopic studies were also carried out for both plutons, as well as for the Riviera Granite, another phase II (1a association) pluton. The initial eNd (550 Ma) based on a depleted mantle model range from −3.5 (Darling), to −3.1 (Robertson) and to −2.6 (Riviera). The Nd mean crustal residence ages are 1559 Ma for Darling, 1626 Ma for Robertson and 1243 Ma for Riviera. Despite the small databank, a dominant Mesoproterozoic (∼ 1600 Ma) crust may be seen as the best candidate to explain the model TDM ages obtained. All the data largely overlap with others recently obtained for other plutons within the Cape Granite Suite and cast doubts on the current correlation between Saldania and the southeastern Brazilian, Dom Feliciano Belts.

Deepest exposed crust of Brazil-SHRIMP establishes three events

The deepest exposed crust of Brazil is in the western portion of the exposed Precambrian shield of southernmost Rio Grande do Sul State and comprises a bimodal sequence of mafic garnet granulites and metatrondhjemites, intercalated with smaller volumes of metamorphosed pyroxenites, anorthosites, sillimanite gneisses, and banded iron formation. As determined by zircon U/Pb sensitive high-resolution ion microprobe (SHRIMP), this granulite terrain accreted from the mantle at the end of the Archean (ca. 2.55 Ga) and was deformed in high-pressure granulite facies metamorphic conditions in the Paleoproterozoic (ca. 2.02 Ga). A younger event, possibly ca. 900 Ma, caused local amphibolite facies retrogression in the complex. Zircons from a metabasalt and a metatrondhjemite show external morphologies typical of high-grade metamorphic rocks, whereas backscattered electron images reveal internal oscillatory zoning of their igneous precursors. Spot ages from zircon agree well with previous model Nd and Sm/Nd mineral isochron ages.

Zircon U-Pb SHRIMP dating of a Neoproterozoic overprint in Paleoproterozoic granitic-gneissic terranes, southern Brazil

Abstract The Neoproterozoic Florianópolis batholith provides the major clue for unraveling the pre- Brasiliano Cycle evolution of Santa Catarina State, southern Brazil, because it contains septa of basement units such as the Camboriú and Águas Mornas complexes reactivated during the younger Brasiliano orogeny at about 600 Ma. We establish precise U-Pb zircon SHRIMP ages and Nd isotopic evolution of four key samples from the septa. The zircon population displays complex internal structures in three of the four dated samples. The complex crystals have well-preserved magmatic cores and altered rim domains. The core populations were dated at about 2180 Ma (Águas Mornas complex), about 2160 Ma (Camboriú complex amphibolite xenolith), and 2000 Ma (Camboriú complex orthogneiss), whereas the ages of the measured rims and altered domains are about 590 Ma in all three samples. On the other hand, the Presidente Nereu tonalite has a simpler zircon population without core/rim separation, and yields magmatic ages about 2200 Ma. The Nd isotopic analyses of the three samples emphasize the polycyclic evolution of the batholith, which is much more complex than previously realized. The evolution is marked by two successive Paleoproterozoic accretionary orogenic events at about 2200-2175 Ma and by a crustal melting event at about 2000 Ma, which had an Archaean source (Sm-Nd depleted mantle model age, TDM = 2860 Ma). These orogenic events were followed 1.4 billion years later by a Neoproterozoic hydrothermal/ metamorphic overprint at about 590 Ma. This time gap permits us to identify a long intracratonic period-the Atlantica supercontinent. The unraveling of the Proterozoic evolution and the precise characterization of the basement remnants show that the Paleoproterozoic was a crustbuilding, orogenic interval and was the major source of the much later Neoproterozoic granites. The integrated use of back-scattered electron and cathodoluminescence imaging with SHRIMP spot dating and Nd isotopes provide a clear understanding of the timing of crustal generation and deformation in southern Brazil, including strong evidence for a long-lasting supercontinent cycle.

A late Proterozoic UPb age for the Bossoroca Belt, Rio Grande do Sul, Brazil

Abstract The Bossaroca Belt is a part of the Vacacai Group in the southern sector of the Precambrian Mantiqueira Province (also called the Ribeira Belt) bordering most of southeastern Brazil. A rhyolite from the Campestre Sequence of the Bossoroca Belt yielded a UPb zircon crystallization age of 753 ± 2 Ma and an inheritance age of 1034 ± 45 Ma. These data indicate that at least part of the Vacacai Group is Late Proterozoic in age and that some of the previous regional correlations may not be valid. The 753 ± 2 Ma age also indicates that volcanic activity related to the Brasiliano Orogeny ( ca. 650-450 Ma) extends farther west than previously thought. Both the age of crystallization and the age of inheritance are identical to ages obtained in the Damara Orogen in Namibia, southwestern Africa, providing further evidence for close temporal and spatial relationships between the Brasiliano and Damaran orogens.