Paleoproterozoic Orogeny Research Articles

Evolução metamórfica/metassomática paleoproterozóica do Complexo Saúde, da Bacia Jacobina e de leucogranitos peraluminosos na parte norte do Cráton do São Francisco

The Saúde Complex and the Jacobina Basin, located in the northern part of the São Francisco Craton, are constituted by metasedimentary rocks, metamorphosed during the paleoproterozoic orogeny (≈ 2.08-1.88 Ga), and are associated to leucogranites. The kinzigites and the non-garnetiferous paragneisses of the Saúde Complex, the aluminous schists of the Jacobina Basin and the leucogranites contain mineral assemblages related to the orogenic collision. The garnet in the kinzigites and staurolite in the paragneisses, associated to sillimanite + quartz + biotite, constitute mineral assemblages formed during the progressive metamorphic path whereas cordierite and plagioclase crystallized during the retrometamorphism associated to the orogenic decompression. In the aluminous schists, garnet and staurolite crystallized during the metamorphic progressive path, but andaluzite and plagioclase record the retrometamorphism. The non-garnetiferous paragneisses, the andaluzite-schists and the leucogranites have fibrolite-quartz-muscovitebearing microstructures formed from the circulation of reducing fluids that lixiviated mainly base-cations of the preexisting silicates. Those microstructures characterize an event of metasomatism that occurred during and/or after the orogenic decompression with the leucogranites acting as thermodynamic triggers in the mobilization of the crustal fluids. The metasomatic event was responsible for almost isobaric inflections (≈ 3.0 kbar) in the regressive P–T path of the non-garnetiferous paragneisses and of the aluminous schists, toward temperatures near 600oC. The absence of fibrolite and muscovite in the kinzigites besides geothermometry estimations, placed between the low-granulite and the high-amphibolite facies, suggest allochthonous positioning of those rocks originated from the orogen infracrust.

Late Paleoproterozoic to mid-Neoproterozoic history of northern Laurentia: An overview of central Rodinia

At the assumed core of reconstructed Rodinia, northern Laurentia, comprising the Canadian Shield (including Greenland) and its buried western and northern extensions beneath Phanerozoic cover, is considered in two parts. On its southeast side, the Grenville Province is part of a major late Mesoproterozoic orogen inferred to be related to the assembly of the Rodinia supercontinent. The remainder of northern Laurentia experienced a long period of crustal stability beginning at ca. 1.8 Ga, recorded for the most part by intracontinental and passive-margin sedimentation along with mafic igneous activity, mainly in the mid-Mesoproterozoic, related to limited extension. To the south, this stable part of northern Laurentia is overprinted by late Paleoproterozoic orogeny (Yavapai, Mazatzal, and Central Plains orogens) and early Mesoproterozoic magmatism (granite-rhyolite provinces). Deformed and metamorphosed rocks related to these form a major part of the interior of the Grenville orogen, where mid- to late Mesoproterozoic orogenic events led to accretion of terranes to Laurentia and subsequent terminal collision. The first manifestation of Rodinia breakup began in the west at ca. 0.8 Ga, some 0.2 Ga earlier than along the southeast side; the age of initial rifting along the northern margin is not known.

A review of Archaean and Paleoproterozoic evolution of the In Ouzzal granulitic terrane (Western Hoggar, Algeria)

The In Ouzzal terrane (Western Hoggar) is an example of Archaean crust remobilized during a very-high-temperature metamorphism related to the Paleoproterozoic orogeny (2 Ga). Pan-African events (≈0.6 Ga) are localized and generally of low intensity. The In Ouzzal terrane is composed of two Archaean units, a lower crustal unit made up essentially of enderbites and charnockites, and a supracrustal unit of quartzites, banded iron formations, marbles, Al–Mg and Al–Fe granulites commonly associated with mafic (metanorites and garnet pyroxenites) and ultramafic (pyroxenites, lherzolites and harzburgites) lenses. Cordierite-bearing monzogranitic gneisses and anorthosites occur also in this unit. The continental crust represented by the granulitic unit of In Ouzzal was formed during various orogenic reworking events spread between 3200 and 2000 Ma. The formation of a continental crust made up of tonalites and trondhjemites took place between ⩾3200 and 2700 Ma. Towards 2650 Ma, extension-related alkali-granites were emplaced. The deposition of the metasedimentary protoliths between 2700 and 2650 Ma, was coeval with rifting. The metasedimentary rocks such as quartzites and Al–Mg pelites anomalously rich in Cr and Ni, are interpreted as a mixture between an immature component resulting from the erosion and hydrothermal alteration of mafic to ultramafic materials, and a granitic mature component. The youngest Archaean igneous event at 2500 Ma includes calc-alkaline granites resulting from partial melting of a predominantly tonalitic continental crust. These granites were subsequently converted into charnockitic orthogneisses. This indicates crustal thickening or heating, and probably late Archaean high-grade metamorphism coeval with the development of domes and basins. The Paleoproterozoic deformation consists essentially of a re-activation of the pre-existing Archaean structures. The structural features observed at the base of the crust argue in favour of deformation under granulite-facies. These features are compatible with homogeneous horizontal shortening of overall NW–SE trend that accentuated the vertical stretching and flattening of old structures in the form of basins and domes. This shortening was accommodated by horizontal displacements along transpressive shear corridors. Reactional textures and the development of parageneses during the Paleoproterozoic suggest a clockwise P–T path characterized by prograde evolution at high pressures (800–1050 °C at 10–11 kbar), leading to the appearance of exceptional parageneses with corundum–quartz, sapphirine–quartz and sapphirine–spinel–quartz. This was followed by an isothermal decompression (9–5 kbar). Despite the high temperatures attained, the dehydrated continental crust did not undergo any significant partial melting. The P–T path followed by the granulites is compatible with a continental collision, followed by delamination of the lithosphere and uprise of the asthenosphere. During exhumation of this chain, the shear zones controlled the emplacement of carbonatites associated with fenites.

Colagem paleoproterozóica de placas arqueanas do cráton do São Francisco na Bahia

Recent petrological, geochronological and isotopic researches has identified four important Archean crustal segments in the Sao Francisco Craton, SFC, in the state of Bahia. The oldest Gaviao Block occurs in the WSW part of the SFC is composed essentially of granitic, granodioritic and migmatitic rocks. It includes remnants of TTG suites, considered to represent the oldest rocks in the South American continent (~3,4Ga) and associated Archean greenstone belt sequences. The youngest segment, named the Itabuna-Salvador-Curaca Belt is exposed along the Atlantic Coast, from the SE part of Bahia up to Salvador and then along a NE trend. It is mainly composed of tonalite/trondhjemites, but also includes stripes of intercalated metasediments and ocean-floor/back-arc gabbros and basalts. The Jequie Block, the third segment, is exposed in the SE-SSW area, being characterized by Archean granulitic migmatites with supracrustal inclusions and several charnockitic intrusions. The Serrinha Block (fourth segment) occurs to the NE, composed of orthogneisses and migmatites, which represent the basement of Paleoproterozoic greenstone belts sequences. During the Paleoproterozoic Orogeny, these four crustal segments collided, resulting in the formation of an important mountain belt. Geochronological constrains indicate that the regional metamorphism resulting from crustal thickening associated with the collision process took place c.a. 2.0 Ga.

Paleoproterozoic orogeny in South Korea: evidence from Sm–Nd and Pb step-leaching garnet ages of Precambrian basement rocks

Abstract Geology and tectonics of the Korean Peninsula have been considered to be closely associated with those of the adjacent Chinese continent. Although Phanerozoic connections between the two areas have been reasonably well worked out, Precambrian ones are less well established. We obtained Sm–Nd and U–Pb garnet ages of 12 metamorphic rocks from widely separated parts of the Gyeonggi and Yeongnam massifs in South Korea to better understand their Precambrian history. The two massifs consist of late Archean to Paleoproterozoic basement rocks and Meso- to Neoproterozoic supracrustal rocks. Previous studies have shown that the two massifs experienced upper amphibolite to granulite facies metamorphism of medium pressure type with clockwise P – T -time paths, suggesting that the two massifs may represent remnants of a collisional orogen. Pb–Pb and U–Pb garnet ages give two distinct age groups: ten Paleoproterozoic (1989±34 to 1835±56 Ma) ages of basement rocks from the two massifs, and one late Permian (265±8 Ma) age of a supracrustal rock from the Gyeonggi massif that corroborates previous estimates of Permo-Triassic (298–245 Ma) metamorphic age for the supracrustal rocks. On the other hand, Sm–Nd garnet ages are less precise and display a wide range of ages from 2257 to 413 Ma for the basement rocks. When we compare the Sm–Nd and Pb–Pb ages from the same sample, the Sm–Nd age is similar to or younger than the Pb–Pb age, suggesting that the blocking temperature of the Sm–Nd garnet system may be lower than that of the U–Pb system (>800 °C). We interpret the Pb–Pb and U–Pb ages as representing the timing of metamorphic garnet growth, since the blocking temperature of the U–Pb garnet system is similar to or higher than the metamorphic temperature of the analyzed samples. Apparently, the younger metamorphism, possibly associated with the Permo-Triassic collision between the North and South China cratons, was not strong enough to reset the older metamorphic age. Similar Paleoproterozoic metamorphic ages for basement rocks of the two massifs suggest that they may have come from the same Precambrian terrane. The metamorphic ages of the basement rocks of South Korea agree very well with those of the Central Zone (orogenic belt) in the North China craton, suggesting a possible correlation between the two terranes.

Reavaliação e novos dados geocronológicos (Pb/Pb e K/Ar) da região do Quadrilátero Ferrífero e adjacências

Geochronological investigations have clarified the geodynamic process and ages of the Rio das Velhas Supergroup and Minas Supergroup, two of the main stratigraphic units of the Quadrilatero Ferrifero, Minas Gerais, Brazil. . The best characterized geologic event (U/Pb in zircon crystals) is related to deposition, metamorphism and magmatism of the Rio das Velhas Supergroup. The magmatic event of the Rio das Velhas Supergroup occurred between 2.78 Ga and 2.73 Ga. The volcanic rocks of the Nova Lima Group, the basal unit of the Rio das Velhas Supergroup, have an Archaean age of ca 2.77 Ga. Pb/Pb dates on carbonate rocks of the Minas Supergroup yield an age of 2.42 Ga. Intrusive granitoids in the basal units of the Minas Supergroup have yielded a U/Pb zircon date of 2.612 Ga and Pb/Pb zircon date of 2.608 Ga. Transamazonian magmatism is well represented by granite emplacement along crustal discontinuities at ca 2.120 Ga. The vergence and metamorphic polarity of the Archaean and Paleoproterozoic orogenies are still controversial. The Brasiliano cycle was responsible for isotopic rejuvenation of the K/Ar system by partial or total argon losses.

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.

The Late Paleoproterozoic Orogeny in the North China Craton

Granulite TTG and khondalite formations are the two typical types of Precambrian metamorphic basement in the North China Craton. The former belongs to the basement of the Archean craton, while the latter is a Paleoproterozoic metasedimentary rock series. The formation of the TTG rock series took place during the period of 2500 to 2450 Ma, near the end of the Archean, and it has undergone multiple times of metamorphism. The cooling rate of the TTG lower crust is quite low (less than 0.3°/Ma), developed during slow crustal cooling in a rather stable tectonic environment. The khondalite rock series underwent amphibolite- to granulite-facies metamorphism that is isotopically constrained to 2000 to 1800 Ma followed by fast crustal cooling from 2.4° to 3.4°/Ma. Based on the thermal and dynamic features of different types of metamorphic rocks, the tectonic state of the crust, and the obvious change of crustal cooling rate from Neo-Archean to Paleoproterozoic, it is clear that continental orogeny existed in the North China craton between 2000 Ma and 1800 Ma.

Sr and Nd isotopic characteristics of 1.77-1.58 Ga rift-related granites and volcanics of the Goiás tin province, central Brazil

Supracrustal rocks of the Araí Group, together with coeval A-type granites represent a ca. 1.77-1.58 Ga old continental rift in Brazil. Two granite families are identified: the older (1.77 Ga) group forms small undeformed plutons, and the younger granites (ca. 1.58 Ga) constitute larger, deformed plutons. Sr-Nd isotopic data for these rocks indicate that the magmatism is mostly product of re-melting of Paleoproterozoic sialic crust. Initial Sr ratios for both granite families are ca 0.726 and 0.720. Most TDM model ages are between 2.58 and 1.80 Ga. epsilonND(T) values are between +3.6 and -11.9. Araí volcanics are bimodal, with basalts and dacites/rhyolites interlayered with continental sediments. The felsic volcanics show Nd isotopic characteristics which are very similar to the granites, and are also interpreted as reworking of Paleoproterozoic crust. Detrital sediments of the Araí Group revealed T DM model ages between 2.4 and 2.16 Ga, indicating that they are the product of erosion of Paleoproterozoic crust. The data indicate that the Araí rift system was established on crust that had just become stable after the Paleoproterozoic orogeny.