João Orestes Schneider Santos

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.

Two Paleoproterozoic Orogenies in the Evolution of the Tandilia Belt, Buenos Aires, as Evidenced by Zircon U-Pb SHRIMP Geochronology

The superposition of two orogenies during the evolution of the Paleoproterozoic Tandilia Belt, Argentina, produced complex geological structures in the granitic-gneissic-migmatitic Buenos Aires Complex. Ion microprobe (SHRIMP II) dating of 61 spots in 56 zircon crystals from 10 rock samples shows tectonic activity related to the accretionary Encantadas orogeny of 2.25-2.12 Ga, although a major overprint of the collisional Camboriú orogeny of 2.10-2.08 Ga is also recognized. Only one zircon age is Neoarchean. Two age peaks for the Tandilia Belt are ~2.16 Ga and ~2.08 Ga, which occur along most of the Brazilian shield, both in granite-greenstone and in granulite-facies terrains. All zircon crystals show complex internal structures, but magmatic and metamorphic ages are restricted to the Trans-Amazonian Cycle between 2234 Ga and 2065 Ma. The Trans-Amazonian Cycle is a dominantly juvenile event followed shortly thereafter by crustal recycling. The Tandilia Belt exhibits a comparable petrotectonic evolution to the Piedra Alta Terrane of Uruguay and to the basement of the Dom Feliciano Belt in southern Brazil. These results require a re-evaluation and integration of the models proposed for the evolution of the southern Brazilian shield in relation to western Gondwana and other Precambrian supercontinents. Although only one Archean age is registered in the dated zircon crystals, Nd isotopic data on 13 samples show that the Tandilia Belt has Neoarchean crust formation ages, similar to other rocks from the southern Brazilian shield.

Paleoproterozoic Magmatic Provenance of Detrital Zircons, Porongos Complex Quartzites, Southern Brazilian Shield

Provenance investigation using detrital zircon U-Pb SHRIMP (sensitive high-resolution ion microprobe) age dating for six quartzite samples (167 spot determinations on 166 grains) indicates that metasedimentary rocks of the Porongos Complex from the southern Brazilian shield were derived almost entirely from Paleoproterozoic sources. Intensive study of igneous and metamorphic rocks in this portion of southwestern Gondwana employing SHRIMP geochronology over the past five years provides important evidence for provenance investigation of zircon grains. Ages of magmatic sources of the zircon detritus in the deformed quartz sandstones show an eight-peak distribution (known equivalent rocks in parenthesis): 2470 Ma (Neto Rodrigues Gneiss), 2350 Ma (Santa Maria Chico Complex Granodiorite), 2200 Ma (Encantadas Complex Tonalite), 2140 Ma (Paso Severino Dacite, Miséria Mylonite), 2100 Ma (Sierra Azul Granite), 2080 Ma (Chacofy Tonalite, Villa Monica Monzogranite), and 2040 and 2020 Ma (Itapema Monzogranite). Ages between 2260 and 2000 Ma correspond to the Trans-Amazonian Cycle, and were already known in the basement of the region, but the 2470 Ma and 2350 Ma ages are a new contribution regarding the tectonic evolution of the southern Brazilian shield. Tectonic scenarios for the Porongos basin fill are restricted to two different environments: (1) cratonic cover, if the fill is near 1500 Ma; or (2) passive margin if the age of fill is near 1000 Ma (this explanation requires additional studies). We have thus elucidated a major problem in the provenance of detrital zircon in the southern Brazilian shield, because we now know that the protolith sediments of the Porongos Complex were deposited under stable tectonic conditions and derived from Paleoproterozoic sources. This occurred after cratonization of the crust and formation of supercontinent Columbia following the end of the Trans-Amazonian Cycle. Nearly all zircon analyses have Th/U ratios higher than 0.2. Thus, only magmatic crystals survived the sedimentary processes. We show that detailed study of detrital zircon is a powerful tool for the understanding of the provenance of sandstones.

Protolith age of Santa Maria Chico granulites dated on zircons from an associated amphibolite-facies granodiorite in southernmost Brazil

U-Pb dating of zircon was undertaken with the Beijing SHRIMP II (sensitive high resolution ion microprobe) on anamphibolite facies granodiorite and an almandine-albite granulite from the Santa Maria Chico Granulitic Complex, southern Brazilian Shield. This work was also done to unravel protolith ages which are often hidden in the array of partly reset data. The obtained metamorphic ages of the granodiorite gneiss and the granulite are 2035 +/- 9 Ma and 2006 +/- 3 Ma, respectively. These data are within the range of metamorphic ages determined in previous studies (2022 +/- 18 Ma and 2031 +/- 40 Ma). However, protolith ages for the granodiorite (2366 +/- 8 Ma) and the granulite (2489 +/- 6 Ma) were obtained which are outside the previously recognized range (> 2510-2555 Ma). The magmatic protolith age of the granodiorite refers to a previously little known magmatic event in the shield. Further investigations may demonstrate that amphibolite facies zircon crystals are useful as a window into geological events in associated granulites, because zircon ages are blurred in the studied granulites.

Metamorphic evolution and U-Pb zircon SHRIMP geochronology of the Belizário ultramafic amphibolite, Encantadas Complex, southernmost Brazil

The integrated investigation of metamorphism and zircon U-Pb SHRIMP geochronology of the Belizario ultramafic amphibolite from southernmost Brazil leads to a better understanding of the processes involved in the generation of the Encantadas Complex. Magmatic evidence of the magnesian basalt or pyroxenite protolith is only preserved in cores of zircon crystals, which are dated at 2257 ± 12 Ma. Amphibolite facies metamorphism M1 formed voluminous hornblende in the investigated rock possibly at 1989 ± 21 Ma. This ultramafic rock was re-metamorphosed at 702 ± 21 Ma during a greenschist facies event M2; the assemblage actinolite + oligoclase + microcline + epidote + titanite + monazite formed by alteration of hornblende. The metamorphic events are probably related to the Encantadas Orogeny (2257 ± 12 Ma) and Camboriu Orogeny (∼ 1989 Ma) of the Trans-Amazonian Cycle, followed by an orogenic event (702 ± 21 Ma) of the Brasiliano Cycle. The intervening cratonic period (2000-700 Ma) corresponds to the existence of the Supercontinent Atlantica, known regionally as the Rio de la Plata Craton.

Detrital Zircon U-Pb Age Data, and Precambrian Provenance of the Paleozoic Guaritas Formation, Southern Brazilian Shield

Ion microprobe age investigation of 73 detrital zircon crystals contained in a sandstone sample from the Paleozoic Guaritas Formation, southern Brazil, provides insights into the tectonic evolution of the southern Brazilian shield. The zircon U-Pb ages reflect three major orogenic cycles—Neoarchean, Paleoproterozoic (2.26-2.00 Ga), and Neoproterozoic (880-535 Ma), including both juvenile and crustal reworking events. Zircon age peaks indicate that each orogeny had several pulses of granite injection. We conclude that the zircon crystals were derived from rocks from the southern Brazilian shield; no African connection is required to explain the age distribution. The shield was part of supercontinent Columbia, because few ages are observed between 2.0 and 0.9 Ga as a consequence of lack of orogenic activity. Final cratonization of the southern Brazilian shield occurred at 560 Ma, but some granitic activity extended to 535 Ma. Thus, detrital zircon ages provide a reliable proxy for the chronostratigraphy and orogenic history of extensive tracts of the southern Brazilian shield.

Ion microprobe (SHRIMP) dates complex granulite from Santa Catarina, southern Brazil

Complex polymetamorphic granulites have been dated in the Santa Catarina granulite complex of southern Brazil through SHRIMP study of zircon. This complex is dominated by intermediate-acid plutonic rocks and contains small volumes of mafic and ultramafic rocks, and minor quartzite and banded iron formation. Porphyroblasts of orthopyroxene, clinopyroxene and plagioclase in mafic and acid rocks are interpreted as magmatic remnants in a volumetrically dominant granoblastic aggregate (M1) of the same minerals and hornblende. Hornblende formed during a later M2 metamorphic event constitutes rims around pyroxene, but the hornblende is also rimmed by granoblastic simplectites of orthopyroxene, clinopyroxene, hornblende and plagioclase in a second granulite facies event (M3). Chlorite and epidote occur in shear zones (M4). This granulite terrain is part of a Neoproterozoic craton, because it was little affected by the Brasiliano Cycle. The two granulite-facies events (M1 and M3) are dated by U/Pb zircon SHRIMP at about 2.68 and 2.17 Ga, while the magmatic protoliths formed at about 2.72 Ga. The amphibolite facies event (M2) probably occurred close to the 2.17 Ga granulitic metamorphism.

Reconstructing the Cryogenian–Ediacaran evolution of the Porongos fold and thrust belt, Southern Brasiliano Orogen, based on Zircon U–Pb–Hf–O isotopes

ABSTRACT The Neoproterozoic geotectonic triad of the Brasiliano Orogen is reconstructed in southern Brazil from studies focused on the Porongos fold and thrust belt. We integrate field geology with isotopic studies of zircon U–Pb SHRIMP and Lu–Hf–O laser determinations in seven metasedimentary and three metavolcanic rock samples. The results indicate that the Porongos palaeo-basin was derived from mixed sources (3200–550 Ma), with major contributions from Rhyacian (2170 Ma) and Ediacaran (608 Ma) sources. Minor contributions from Archaean to Tonian sources are also registered. The maximum depositional age of the Porongos palaeo-basin is established by the age range of 650–550 Ma with TDM model ages between 2.5 and 1.3 Ga. The reworked signature (εHf values = −34 to −4) and the characteristic crustal magma reservoirs (δ18O ≥5.3 ‰) indicate that these sediments are equivalent to Neoproterozoic granites of the Dom Feliciano Belt. The episodic depositional history started in the Cryogenian (650 Ma) and lasted until the Ediacaran (most likely 570 Ma). A magmatic event of Tonian age is recorded in rhyodacite samples interleaved with the metasedimentary rocks and dated at 773, 801, and 809 Ma. The crustal evolution of the Sul-Riograndense Shield included mountain building, folding and thrusting and flexural subsidence in the foreland. An orogenic triad is revealed as the Pelotas Batholith, the Porongos fold and thrust belt and the Camaquã Basin, all part of the Dom Feliciano Belt.

Tectonic evolution of the southern margin of the Amazonian craton in the late Mesoproterozoic based on field relationships and zircon U-Pb geochronology

New U-Pb zircon geochronological data integrated with field relationships and an airborne geophysical survey suggest that the Nova Brasilândia and Aguapei belts are part of the same monocyclic, metaigneous and metasedimentary belt formed in the late Mesoproterozoic (1150 Ma-1110 Ma). This geological history is very similar to the within-plate origin of the Sunsas belt, in eastern Bolivia. Thus, we propose that the Nova Brasilândia, Aguapei and Sunsas belts represent a unique geotectonic unit (here termed the Western Amazon belt) that became amalgamated at the end of the Mesoproterozoic and originated through the reactivation of a paleo-suture (Guapore suture zone) in an intracontinental rift environment. Therefore, its geological history involves a short, complete Wilson cycle of ca. 40 Ma. Globally, this tectonic evolution may be related with the final breakup of the supercontinent Columbia. Mafic rocks and trondhjemites in the northernmost portion of the belt yielded U-Pb zircon ages ca. 1110 Ma, which dates the high-grade metamorphism and the closure of the rift. This indicates that the breakup of supercontinent Columbia was followed in short sequence by the assembly of supercontinent Rodinia at ca. 1.1-1.0 Ga and that the Western Amazon belt was formed during the accretion of the ArequipaAntofalla basement to the Amazonian craton.

Zircon and Titanite U–Pb SHRIMP dating of unexposed basement units of the Buenos Aires region, southeastern Río de La Plata Craton, Argentina

ABSTRACT We present U–Pb Sensitive High Mass Resolution Ion MicroProbe (SHRIMP) data of unexposed igneous-metamorphic basement rocks from two areas of the southeastern Río de la Plata craton (RPC; Buenos Aires city) located within the Buenos Aires–Piedra Alta (BAPA) terrane – and the Tapalqué area (Buenos Aires province) – located within the Tandilia terrane, and discuss the tectonic evolution of that portion of the craton based on both the new data and previous work. The newly obtained geochronological data of drill cores indicate that: (a) arc magmatism occurred at 2164–2186 Ma corresponding to early ‘Trans-Amazon’ (early Rhyacian) arc magmatism; (b) the age of collision between the BAPA and Tandilia terranes is inferred to have commenced at ca. 2110 Ma; (c) peak metamorphism occurred at ca. 2069 Ma; and (d) the presence of rocks related to the RPC is confirmed under Cenozoic sediments in a large area between Martin Garcia Island and Tandil. We envisage an early Rhyacian divergent double subduction scheme between the BAPA and Tandilia terranes – i.e. N-dipping towards BAPA and S-dipping towards Tandilia – prior to the late Rhyacian collision between these two terranes.