Assembly Of Gondwana Research Articles

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.

Formation of juvenile continental crust in northern Nubian Shield: New evidence from granitic zircon U-Pb-Hf-O isotopes

The Neoproterozoic Arabian-Nubian Shield (ANS) consists of continental crust formed prior to and during the collision between East and West Gondwana. The Eastern Desert of Egypt (i.e., northern Nubian Shield) constitutes the north-western part of the ANS. Neoproterozoic magmatism in the Eastern Desert region occurred between ∼ 800 Ma and ∼ 550 Ma. Although there is a broad consensus that ∼ 800–650 Ma pre-collisional igneous rocks comprise the main magmatic stage contributing to Eastern Desert crustal evolution, whether this crust is purely of a juvenile composition, or was generated by recycling/reworking of much older continental crust, remains debated. Here, we present new whole-rock geochemical and Sr-Nd isotopic data and an integrated zircon U-Pb-Hf-O-trace element dataset for two granitoids (Um Khariga and Genina Gharbia) from the Eastern Desert. Samples from the Um Khariga granitoids yield zircon U-Pb ages of 746 ± 4 and 753 ± 4 Ma that overlap within error, and the Genina Gharbia granitoids have an age of ∼ 690 Ma, indicating that both granitoids belong to different phases of pre-collisional magmatism. Zircon hafnium isotopic data of both granitoids yield weighted mean εHf(t) values ranging from + 8.01 ± 0.23 to + 10.52 ± 0.13, indicating that both granitoid suites were derived by melting of a juvenile source. SIMS oxygen isotope data for zircon show that the magmatic zircon has mantle-like δ18O values with weighted means ranging from 4.73 ± 0.02 to 5.04 ± 0.08 ‰. Based on previously published zircon U-Pb-Hf-O-trace element data for granitoids and volcanic rocks of the Eastern Desert, plus our new isotopic datasets obtained from the Um Khariga and Genina Gharbia granitoids, we show that the ∼ 800–650 Ma magmatism is characterized by high zircon εHf(t) (+8 to + 15), and mantle-like zircon δ18O values (∼+5‰), implying that the continental crust of the Eastern Desert was either extracted directly from a depleted mantle and/or reworked from accreted juvenile oceanic crust or oceanic arc with no evidence for the input of old continental material. The overall zircon εHf(t) and δ18O evolution trends for the northern Nubian Shield show that juvenile mantle input dominated the early phases (ca. 800–670 Ma) of crustal growth and evolution in this part of the ANS, coinciding with the break-up stage of the supercontinent Rodinia. The involvement of continental crustal recycling only started to play a significant role since the ca. 670 Ma regional collisional event during the early stage of Gondwana assembly.

Multi‐stage metamorphism of ultrahigh‐temperature Mg‐Al granulites during Gondwana assembly: Evidence from southern India

The Southern Granulite Terrane (SGT) in India hosts granulite facies rocks metamorphosed at ultra‐high temperature (UHT) conditions in the various crustal blocks, as well as within the Palghat‐Cauvery Suture Zone (PCSZ), that is considered as a trace of the Late Neoproterozoic—Cambrian Gondwana suture. Here we investigate UHT granulites from the northern margin of the Madurai Block adjacent to the PCSZ where Mg‐Al‐rich granulites are exposed. We identify sodic gedrite + kyanite in these rocks as the high‐pressure prograde stage assemblage, followed by sillimanite‐garnet‐orthopyroxene that formed during pressure decrease and temperature increase. The rare remnant gedrite is also stable at the near‐peak UHT metamorphism until it was replaced by sapphirine. The rocks subsequently underwent decompression that formed sapphirine + cordierite and sapphirine + plagioclase symplectite around sillimanite. Dehydration during decompression generated orthopyroxene‐sillimanite‐quartz assemblage with the appearance of sapphirine, defining the diagnostic mineral assemblage indicative of peak UHT metamorphism (T > 900°C) at relatively high‐pressure (P > 9 kbar). The UHT peak metamorphism in this region is consistent with the results of P–T calculations using conventional geothermometers and phase equilibrium modelling (T up to 1,050°C, P over12 kbar). Zircon and monazite geochronology on the UHT metapelites indicate distinct stages. Detrital zircon grains in the metasediments indicate protolith from ca. 2.5 Ga igneous source and the metamorphic overgrowths yield 206Pb/238U mean ages concentrated at ca. 550–520 Ma. Monazite ages define another younger group 206Pb/238U mean ages at ca. 450 Ma. The prograde high‐pressure granulite‐facies metamorphism and following UHT event correlate with the subduction‐collision tectonics at 550–500 Ma associated with the final stage of amalgamation of the Gondwana supercontinent, while the 420–460 Ma monazite age records a later hydration at the post‐orogenic stage, possibly associated with deep shearing and fluid influx.

The latest Neoproterozoic basaltic and siliciclastic rocks and tectonic implications in the Tarim Craton, NW China

The Tarim Craton records protracted late Neoproterozoic rifting that overlaps with the breakup of Rodinia and assembly of Gondwana. However, the timing and interactions of these two tectonic processes remain contested. To constrain the tectonic significance of this prolonged rifting event, we present LA-ICP-MS zircon U–Pb ages and geochemical data from mafic and (meta)sedimentary rocks at the top of the craton’s Ediacaran succession. Two basalt samples from the northwestern Tarim Craton contain inherited zircon grains with a wide range of U–Pb ages, the youngest at ca. 620 ± 11 providing a maximum age for the magmatism. The youngest detrital zircon U–Pb ages from the southwestern and northern Tarim Craton are ca. 607 Ma and 617 Ma, respectively. Integrated with compiled magmatic and detrital zircon U–Pb ages, our study suggests that an episode of latest Neoproterozoic (ca. 620–600 Ma) magmatism occurred throughout the Tarim Craton. The Ediacaran mafic rocks form a transitional eruptive series between alkali and tholeiitic basalts derived from lithospheric mantle with slight crustal contamination and formed in an intra-continental rifting environment through mantle reactivation during post-subduction extension. Positioned at the outer margin of Rodinia, it is considered unlikely that the magmatism records prolonged plume-related breakup. Instead, it is thought to reflect rifting of an active continental margin in a fashion analogous to many of the peri-Gondwanan terranes, possibly related to the subduction mechanism of supercontinental breakup.

Phase equilibrium modelling and zircon-monazite geochronology of HT-UHT granulites from Kambam ultrahigh-temperature belt, south India

ABSTRACT The Madurai block, the largest crustal block in the Precambrian Southern Granulite Terrane of south India, preserves rare assemblages of high- to ultrahigh-temperature metamorphic rocks. These rocks bear exclusive evidence for the high thermal regime prevalent during their formation and can be constrained from accessory phases such as zircon, monazite and garnet. Here, we present the P-T-t evolutionary history of garnet-cordierite-spinel granulites from different localities in the Madurai block. Combined petrography, mineral reaction, geothermobarometry and pseudosection modelling of the samples record HT to near-UHT metamorphic conditions with clockwise P-T trajectories. LA-(MC)-ICPMS U-Pb/Hf isotopic studies on zircons point to Paleoproterozoic high-grade metamorphism in the area with juvenile magmatic signatures. LA-ICPMS/EPMA monazite dating constrains the timing of HT to near-UHT metamorphism at ~580 and ~550 Ma. The heat source required for the Paleoproterozoic high-T event is correlated with the synchronous igneous emplacements reported in the region, whereas the heat source responsible for the Neoproterozoic HT to near-UHT event can be related to the processes associated with crustal thickening and synchronous mafic emplacement. The clockwise P-T trajectories of these granulites are interpreted as the signature of collisional orogeny prevalent in the region during the final stages of Gondwana assembly.

Detrital zircon U–Pb and Lu–Hf constraints on the age, provenance and tectonic setting of arc-related high-grade units of the transition zone of the Araçuaí and Ribeira orogens (SE Brazil)

Sedimentary basins with epiclastic and volcanoclastic components are important geotectonic components for understanding the complex evolution of orogenic systems. The age, provenance and tectonic setting of ancient basins are, however, often hard to constrain, especially in the high-grade core of ancient mountain belts where original sedimentary and stratigraphic relations have been largely obliterated by deformation and metamorphism. In this case, analytical tools such as whole-rock geochemistry and U–Pb and Lu–Hf analyses on detrital zircon grains can be deployed to unravel the provenance and tectonic setting of high-grade metasedimentary and metavolcaniclastic units. Here, we apply these analytical tools on samples from gneissic paleosomes of migmatites found in the eastern transition zone between the Neoproterozoic–Eopaleozoic Araçuaí and Ribeira orogens (southeastern Brazil). The studied paleosomes comprise peraluminous paragneiss rich in biotite, garnet and sillimanite, metaluminous hornblende-bearing gneiss free of peraluminous silicates, and intermediate terms between these compositions, all of them with geochemical characteristics pointing to sources located in magmatic arc systems. Overall, U–Pb data from detrital zircon grains bracket a maximum sedimentation age for all samples between 600 and 627 Ma, while εHf(t): +5.7 to −18.7 and Hf TDM model ages of 1.0–2.2 Ga suggest moderately juvenile to evolved sources for the gneiss protoliths. However, despite the similarity in age range, the analyzed samples show contrasting isotope data indicating significant variability in the source areas. Sample 7 only has grains with negative εHf (−5.9 to −18.7), most of them ranging in age from 708 Ma to 758 Ma, with very few older (1021–1143 Ma) and younger (642–651 Ma) zircons grains, indicating evolved crustal sources of distinct ages (e.g., the Late Tonian – Early Cryogenian South Bahia Alkaline Province, the Early Cryogenian magmatism found in the West Congo Belt, the Stenian to Early Tonian Espinhaço and Matadi-Noqui igneous rocks, and magmatic rocks of the early Rio Doce arc). Conversely, sample 13 has twenty grains with positive εHf values and ages between 602 and 758 Ma, suggesting sourcing from (meta)igneous rocks of the Rio Negro arc and their remelting products, and forty grains with negative εHf values with ages between 591 and 646 Ma suggesting magmatic sources in the Rio Doce arc. Additionally, the two youngest spot ages (553 Ma and 571 Ma) have highly negative εHf values (−11.1 and −16.4) suggesting their origin from collisional anatexis. Similarly, sample 12 shows detrital grains within the consistent time interval (ca. 596 Ma – 798 Ma) of the Rio Negro arc and Rio Doce arcs, reinforcing that the fragments of these arcs, or zircons that were inherited from these terranes, are potential sources to the sedimentary protoliths of the paragneisses. The regional scenario together with our field and analytical data, compared with data available in the literature, suggest the sedimentary protoliths of the studied samples filled orogenic basins related to the Rio Doce magmatic arc. Most detrital zircon ages are close to the maximum depositional age, a classical signature of orogenic basins related to magmatic arcs. The studied paragneiss unit correlates with the Nova Venécia Complex, located on the intra-arc to back-arc zones of the Araçuaí orogen, and with the orogenic succession of the distal Andrelândia Group, shared by both the Araçuaí and Ribeira orogens. Furthermore, our data also suggest the juvenile Rio Negro - Serra da Prata arc system was either amalgamated with the Rio Doce arc or that fragments of these arcs were incorporated as unmelted xenoliths and xenocrystals within the Rio Doce arc rocks before ca. 614 Ma, providing key hints to unravel the evolution of the Araçuaí - Ribeira orogenic system (AROS) and its role in the Gondwana assembly.

Provenance and depositional setting of the Buem structural unit (Ghana): Implications for the paleogeographic reconstruction of the West African and Amazonian cratons in Rodinia

We present new field, petrological and geochemical data, combined with U-Pb zircon ages and Lu-Hf isotope compositions for the sandstones of the Neoproterozoic Buem structural unit (BSU) of the Dahomeyide belt, and whole-rock geochemical data of BSU shale to investigate their provenance and depositional setting. The BSU contains siliciclastic sequences and fragments of oceanic lithosphere (pillow lavas, gabbro, and peridotite) that have archived the entire evolution of the Ediacaran West Gondwana Orogen (WGO), from early accretion to final amalgamation of the West African Craton (WAC) with the Benino-Nigerian Shield. Three broad groups of samples exist within the BSU in Ghana: those with dominantly older age fractions of 2300 – 1800 Ma, represented by samples from the base of the BSU; those with prominent 1700 – 1100 Ma zircons, occupying the middle part of the BSU; and those with significant 1000 – 970 Ma age fractions, forming the uppermost part of the BSU. Results from this study, together with published data on the BSU, and adjacent Togo structural unit and Voltaian Supergroup, reveal two main sedimentary sequences in the Dahomeyide belt, i.e., passive margin and foreland basin sequences with three potential provenances: Amazonian Craton, Benino-Nigerian Shield, ± WAC. This interpretation resembles that for the sedimentary rocks of the Borborema Province, NE Brazil, which implies similar evolution along the WGO. Thus, a long, >2500 km passive margin basin developed at 1000 – 700 Ma, which was subsequently inverted during the Braziliano/Pan-African plate convergence and collision, and the concomitant formation of the foreland basin during the assembly of the supercontinent Gondwana. Because a greater portion of the detritus in the BSU is probably from the Amazonian Craton, we propose that the WAC and the Amazonian Craton adjoined each other from the Paleoproterozoic onward within the supercontinents Rodinia and Gondwana, until the opening of the Atlantic Ocean.

Tonian and Cryogenian – Early Cambrian sedimentation in NW India: Implications on the transition from Rodinia to Gondwana

The Tonian Punagarh-Sindreth and Cryogenian – early Cambrian Marwar sequences in NW India encompass a geological record that could provide vital information for working out the tectonic evolution of this region and a better understanding of the Rodinia to Gondwana transition. The Tonian Punagarh and Sindreth sub-basins received a dominantly felsic detritus with negligible sorting of heavy minerals and mild to weak chemical weathering conditions. The clastic compositions and geochemical characteristics of the Punagarh and Sindreth groups suggest a back-arc basin setting and the detritus sourced from a volcanic arc and cratonic interior region. The Punagarh basin was proximal to the craton while the Sindreth basin was close to an arc system. In contrast, the Cryogenian – early Cambrian Marwar Supergroup clastic sequence contains mature quartz arenite whose high CIA (77.8–94.8) and low ICV (0.18–1.00) values suggest a strong chemical weathering in the source region and noteworthy physical sorting during transportation and sedimentation. An abundance of quartz grains (87–94%), enrichment of Zr, and detritus predominantly derived from the cratonic basement indicate deposition of Marwar sediments in a tectonically stable basin, temporally coinciding with the Gondwana assembly. Collectively, the ∼ 760 Ma active continental margin in NW India, documented by the provenance and tectonic setting of the Punagarh and Sindreth basins, is in agreement with the subduction of the peripherical Rodinia supercontinent during its breakup. On the other hand, the passive continental margin sedimentation in Marwar Supergroup points toward an open sea between NW India and western Gondwana during the Gondwana assembly.

Locating Lhasa terrane in the Rodinia and Gondwana supercontinents: A key piece of the reconstruction puzzle

Abstract The debate over global continental reorganization from Rodinia to Gondwana likely stems from some key Neoproterozoic units being overlooked in previous reconstructions. Here we provide a self-consistent set of evidence, based on magmatism and metamorphism, that the Lhasa terrane is the “lost” part of NW India and retains crucial archives, including 760–730 Ma ophiolitic-arc magmatic rocks and evidence of a distinctive metamorphic event at ca. 660 Ma. Hence, the Lhasa terrane together with Madagascar and Seychelles formed in a late Neoproterozoic juvenile arc system laying either along the periphery of rifting Rodinia or outboard of the supercontinent. Subsequently, these terranes made up the northern East African Orogen (EAO) during the initial assembly of Gondwana. As a result of lateral-escape tectonics after collision and dextral strike-slip along the northern margin of Gondwana, the Lhasa terrane escaped from the EAO and had migrated to the northern margin of Australia by the Middle Cambrian.

Subduction initiation of the western Paleo-Asian Ocean linked to global tectonic reorganization: Insights from Cambrian island-arc magmatism within the West Junggar, NW China

Abstract The subduction initiation associated with the beginning of accretionary orogens has been thought to be related to global plate reorganization. To characterize the initial subduction within the western Central Asian Orogenic Belt, this integrated study focuses on Cambrian tholeiitic to calc-alkaline plutons in the Barleik-Mayile-Saleinuohai area of West Junggar, NW China. Zircon U-Pb results of felsic plutons reveal a wide range (511–488 Ma) of ages with older ages up to 514–511 Ma. The felsic rocks exhibit variable SiO2 (53.0–77.4 wt%) and K2O (0.05– 2.24 wt%) contents and can be classified as diorite, granodiorite, trondhjemite, and tonalite. On the basis of their low TiO2 (0.12– 0.71 wt%) contents and characteristic trace element trends as well as high zircon εHf(t) (+10.5 to +14.5) and mantle-like zircon δ18O (5.0 ± 0.48‰ to 5.4 ± 0.43‰, two standard deviations) values, we interpret that the Cambrian felsic rocks have diverse origins, involving differentiation of arc basalts and partial melting of subducted oceanic crust, arc mafic crust, and metasomatized mantle wedge. The Saleinuohai gabbroic pluton shows zircon δ18O ratios from 4.2 to 4.7‰, which are lower than those of igneous zircons in equilibrium with mantle and thus reflect modification of their mantle source by hydrothermal fluids with seawater-like oxygen isotopes at high temperature. Combined with regional data, we propose that the West Junggar arc represents the extending of the Boshchekul-Chingiz arc in the Early Cambrian, defining a long (>1000 km) E-W–trending subduction zone. The earliest island-arc tholeiitic felsic plutons in the West Junggar took place at ca. 514–511 Ma, which, coupled with other early subduction records (e.g., 530 Ma SSZ-type Kopu-relisay ophiolites) in the western Paleo-Asian Ocean, indicates that initial stages of subduction of the western Paleo-Asian Ocean probably occurred in the Early Cambrian. The simultaneity between the initial subduction of the western Paleo-Asian Ocean, Gondwana assembly, and Laurasia breakup suggests a causal link between the three, collectively correlated to a global plate adjustment event.

Implications of the dominant LP–HT deformation in the Guanhães Block for the Araçuaí West-Congo Orogen evolution

The Araçuaí-West Congo Orogen is the result of convergence between the São Francisco and Congo cratons during the assembly of West Gondwana. Within this orogen, the Araçuaí Belt in SE Brazil reflects the western external domain. The Araçuaí Belt is characterized by major east-dipping structures and high- to medium pressure metamorphic rocks developed during crustal thickening at ca. 585–560 Ma and are juxtaposed along crustal-scale structures to the crystalline core of the orogen.The Guanhães Block in the southern Araçuaí Belt is characterized by higher metamorphic grade and dominantly shallowly west-dipping structures compared to the neighboring tectonic domains. While several studies have focused on the evolution of the axial crystalline core of the Araçuaí orogen and on the early collisional stages in the Araçuaí Belt, little attention has been given to the structural and metamorphic history of the poorly studied Guanhães Block.The supracrustal rocks in the central Guanhães Block record two tectono-metamorphic stages: D2/M2 and D3 /M3, with potential D1/M1 relicts rarely preserved within D2/M2 low-strain domains. The D2/M2 overprinting stage is associated with the development of a dominant, shallow axial planar foliation (S2) of isoclinal F2 folds under low-pressure and high-temperature (LP–HT) conditions (ca. 6 kbar and 750 °C). During this D2/M2 stage continuous segregation of syn-kinematic partial melting and generation of granitoid rocks occurred. The 530 Ma, syn- D2/M2 granitoid has negative εHf(i) at ca. −32 which indicates the source of magma from melting of older, isotopically evolved, Archaean or Paleoproterozoic crust.U–Pb geochronology on monazite, titanite and hydrothermal zircon constrain the retrograde D3/M3 event at ca. 510 Ma. The Guanhães Block uniquely records dominant deformation under LP–HT metamorphic conditions during the late-orogenic stages and its evolution could have important tectonic implications for the concurrent models suggested for the late Neoproterozoic Araçuaí-West Congo Orogen.

The tectonic context of hafnium isotopes in zircon

The assembly and dispersion of continental crust are first-order controls on paleogeography and geochemical cycles. The associated reworking of Earth's crust can be tracked with zircon initial hafnium (εHfT) through space and time. Here we apply a new method of quantitative analysis using εHfT density estimates based on a compilation of 155,329 εHfT values. Investigation of the global database reveals significant geographic and temporal bias in the εHfT record associated with sampling and regional tectonic events. Recent research has attempted to address global εHfT bias using resampling methods to augment gaps of low εHfT data density, which in turn obfuscates tectonic signals and artificially weights outliers. Instead, we evaluate εHfT density patterns for both igneous and detrital zircon on eight continental zones demarcated by Paleozoic sutures: Africa, Antarctica, Asia, Australia, Baltica, North America, Peri-Gondwana, and South America. Pairwise two-dimensional quantitative comparison highlights similarity in timing and εHfT values between zones, all of which can be linked to documented shared regional tectonism. Integration of all pairwise comparisons reveals that peak similarity corresponds to the timing of supercontinent amalgamation, and that the associated εHfT differs depending on the style of supercontinent amalgamation, particularly internal versus external orogenesis. The three most recent supercontinents produced distinctive εHfT signals, shared by the constituent continental zones. The supercontinents Rodinia and Pangea were constructed through collisions of marginal arc terranes, peripheral to ancient crust, and did not produce highly enriched εHfT values. In contrast, Ediacaran to Cambrian formation of the Gondwana supercontinent was largely the product of internal Pan-African orogens that formed directly after Neoproterozoic Rodinia rifting and arc accretion forming the Arabian Shield. The final assembly of Gondwana was dominated by continent-continent collisions of old radiogenic crust without establishment and accretion of extensive intervening depleted arc terranes, resulting in a more enriched distribution of εHfT values compared to prior and subsequent supercontinent formation. The secular εHfT record is the product of spatiotemporally biased sampling and preservation of specific orogenic belts with predictable εHfT data arrays, modulated by the amalgamation, tenure, and breakup of supercontinents through time.

Deformation, thermochronology and tectonic significance of the crustal-scale Cubatão Shear Zone, Ribeira Belt, Brazil

Crustal-scale shear zones are key structures in orogenic belts linked to the assembly of West Gondwana. The Cubatão Shear Zone (CSZ) represents a significant segment of a crustal-scale transcurrent shear zone system that cuts Paleoproterozoic to Neoproterozoic units in the Ribeira Belt (SE Brazil). Despite its tectonic importance, the relationship between transcurrent deformation and orogenesis is still controversial. This work aims to understand this relationship by integrating the transcurrent deformation history of the CSZ with regional orogenic processes through structural analyses, phase-equilibria modelling, and zircon and apatite U-Pb-REE analyses. The results indicate that the CSZ developed within a transpressional setting with a dominant subhorizontal stretching lineation at medium-temperature conditions of 460–520 °C and 4.5–9.5 kbar, consistent with quartz recrystallisation microstructures indicative of subgrain rotation recrystallisation. Igneous-type apatite from the mylonites of the CSZ record ages of ca. 610–570 Ma, which are interpreted as reset ages due to ductile deformation at medium-temperature conditions. Zircon U-Pb geochronology indicates that mylonites are sourced from the Juquiá Granite and the Atuba Complex. The CSZ activity is coeval with the main period of activity of other ductile shear zones from the Ribeira Belt, as indicated by available geochronological data. This age interval coincides with the main period of other regional shear zones activity, voluminous granitic magmatism, regional metamorphism and convergence of the cratons surrounding the Ribeira Belt, indicating active participation of the transcurrent shear zones in the orogenic processes that built up West Gondwana.

A trans-Iapetus transform fault control for the evolution of the Rheic Ocean: Implications for an early Paleozoic transition of accretionary tectonics

AbstractThe Paleozoic era begins with the final assembly of Gondwana and ends with the amalgamation of the supercontinent Pangea. Although this tectonic progression is generally well documented, one fundamental but under-studied phenomenon during this era is the transition from two-way to one-way (northward) migration of peripheral terranes between Gondwana and Laurentia-Baltica from Cambrian to Ordovician time. The two-way terrane accretion was likely initiated during the opening of the Rheic Ocean at ca. 510 Ma when several Gondwana-derived terranes comprising Carolinia, Ganderia, Avalonia, and Meguma sequentially drifted from the northern margin of Gondwana and eventually collided with Laurentia or Baltica. Over the same time interval, the Laurentia-derived Cuyania terrane (a.k.a., the “Precordillera terrane” is commonly believed to have accreted to the proto-Andean margin of Gondwana, whereas the peri-Laurentian/Dashwoods ribbon continent separated from, then re-accreted to, the Laurentian margin after being trapped during the collision of the Taconic arc. Alternatively, the Cuyania terrane is suggested to have remained attached to the Ouachita Embayment throughout the Cambrian–Ordovician, and then passed onto Gondwana during the collision between the proto-Andean margin and the hypothesized Texas Plateau at the leading edge of Cuyania. Here we explain the enigmatic, pene-contemporaneous migration of these peripheral terranes by a trans-Iapetus transform fault that was likely active between 510 and 450 Ma and extended from the proto-Appalachian margin of Laurentia to the proto-Andean margin of Gondwana. The trans-Iapetus transform fault terminated after the respective accretion of Carolinia and Ganderia to the proto-Appalachian margin and of Cuyania to the proto-Andean margin. We interpret the development of the trans-Iapetus transform during the Cambrian–Ordovician to be a consequence of the global geodynamic transition from the break-up of Rodinia (continents/terranes drifting away from Laurentia) to Pangea amalgamation (continents/terranes drifting toward assembling Laurussia).

Cryogenian accretion of the Northern Arabian-Nubian shield: Integrated evidence from central Eastern Desert Egypt

Accretionary geology of the Arabian-Nubian shield (ANS) is critical in deciphering the Neoproterozoic global tectonics. Multidiscipline investigation of ophiolites, sediments, and igneous units of central Eastern Desert Egypt results in a revisited tectono-magmatic scenario. The first baddeleyite U-Pb age (ca. 687 Ma) of the ANS ophiolites is obtained from the layered gabbro of the Wadi Ghadir ophiolite, which limits the back-arc spreading age. Significant Archean-Paleoproterozoic detrital zircon age spectrum, similar to those of the Saharan-Congo craton, is identified from a conglomerate of the Atud turbidites, indicating the existence of craton basement in the shield. Significant ca. 840 Ma and ca. 750 Ma detrital zircon ages with a mixing of juvenile and ancient Hf isotope features approve the provenances to be arc(s) with craton basement. Two episodes of calc-alkaline units represented by the Umm Rus complex (URC) and the El Sibai complex (ESC) are dated at ca. 680 Ma and ca. 660 Ma, respectively. They were both originated from juvenile sources (in situ zircon ɛHft = +6.1 ∼ +12.2) with depleted high-strengthen field elements (e.g., Nb, Ta, Zr, Hf) but enriched large ion lithophile (e.g., Ba, Sr) and radiogenic elements (e.g., Th, U). The Ti-in-zircon temperature of the ESC granitoids (696 ± 16 °C) is lower than that of the older URC (749 ± 21 °C), as well as the inferred ca. 840 Ma and ca. 750 Ma arcs; and this reconciles an increased hydrous melting during the final Gondwana assembly. The older plutons represented by the URC show an E-W elongated distribution consistent with the alignment of the regional imbricated thrusting; whereas the elongation of the younger generation represented by the ESC is N-NE to S-SW and is coherent with the axis of the core complexes and the major strike-slip faults of the region. This possibly coincides a direction-change of paleo-subduction. In summary, the Northern ANS likely represents a superimposed arc accreted at 680–660 Ma during the closure of the Mozambique ocean.

Carbon isotopic chemostratigraphy of the Ediacaran-Cambrian successions in the northwestern Tarim Craton, NW China: Correlations with Gondwana supercontinent

Among the three cratons (North China, Yangtze, and Tarim) in China, the Tarim Craton during Precambrian is the least known for its evolution due to its broad Phanerozoic cover. The affinity of the Tarim Craton with Gondwana supercontinent during the Cryogenian-Ediacaran interval has long been debated. In this contribution, we report carbon and oxygen isotopes of the Ediacaran-Cambrian siliciclastic‑carbonate from successions in the northwestern Tarim Craton. The results exhibit one pronounced negative and one positive carbon isotopic excursions (CIE) in the Ediacaran sections (N3 and P1), and a distinct negative CIE at the base of the Cambrian (N4). With the constraints of geochronological data, we proposed that the Ediacaran-lower Cambrian chemostratigraphy of the northwestern Tarim basin correlates well with the lower Ediacaran and lower Cambrian from Yangtze, Oman, Australia, Namibia, and Canada basins. N3 in the Tarim Craton possibly represents a new contribution to the Shuram-Wonoka anomaly globally. With these data, together with previously published tectonostratigraphic and paleogeographic reconstructions, we propose that the Tarim Craton may have been involved in the Pan-African Orogeny during the assembly of Gondwana, and was positioned close to the Yangtze, Oman, and Namibia cratons along the northern periphery of the Gondwana supercontinent

Ediacaran-Cambrian microbialites of the Southern Amazon Craton: relation with the metazoan rise, sea-level changes, and global tectonics

Microbialites are the most abundant life evidence in Precambrian sedimentary rocks. They are produced by microbial interaction activity and sedimentary processes reflecting paleoenvironmental conditions. The Ediacaran-Cambrian carbonate and siliciclastic successions in the Southern Amazon Craton in Central Brazil, provide a key opportunity to understand how the metazoan life coexisted with the microbial communities. The spatial and temporal distribution of microbialites as well as morphological and paleoenvironmental changes have been assessed, reinterpreting previous works and including new data from the Araras-Alto Paraguai and Corumbá basins. The deposition was controlled by subsidence and sea-level changes that affected these basins, considered extensions of epicontinental seas during the Gondwana assembly. The stromatolites are restricted to coastal deposits and experienced thriving flourishment intervals after the Marinoan Glaciation (635 Ma). Post-glacial transgression was marked by microbial colonization in shallow platforms represented by stratiform and giant domical stromatolites in the Araras-Alto Paraguai Basin. The continuity of the transgression generated a moderately deep aragonite sea at about 622 Ma. A progressive sea-level fall caused the implantation of coastal environments under greenhouse conditions with tidal flat and sabkha settings colonized by centimetric-scale stromatolites. The sea retreat was accompanied by progressive uplift, causing a moderate inversion of the basin and erosion of the succession until ~560 Ma with the deposition of the last preserved tidal flat deposits with the occurrence of thrombolites. The subsiding Corumbá Basin was the site of microbially-induced deposition of carbonates in a shallow platform connected to an offshore setting with the proliferation of metazoan straddling the Ediacaran-Cambrian boundary. Microbial communities were restricted to lagoon deposits during the Lower Cambrian transgression in the Araras-Alto Paraguai Basin and the last phase refers to the sea retreat towards southeast, developing a fluvial system connected with the arid and evaporitic tidal flats colonized by microbialites that lasted until the upper Cambrian. Except for the post-glacial stromatolites, the columnar and domal microbialite indicate that the coastal settings dominated the Ediacaran-Cambrian transition. The preservation of microbialites in the post-glacial intervals can be associated with the Mg-Ca-CO3 oversaturation in dolomitic platforms. The rapid calcification and ability to resist the dissolution and replacement have increased the stromatolites’ preservation potential reported here, where its well-preserved occurrence in tidal flats and sabkha occurs due to intense early diagenetic silicification. The change from carbonate accumulation to siliciclastic-rich environments contributed to the demise of microbially-induced strata. In general, the scarce coexistence between coastal stromatolite and metazoan-bearing marine deposits makes it challenging to establish a competitive relationship between these organisms, as previously postulated.

Paleoproterozoic Metagabbro Xenolith in the Southeastern Dom Feliciano Belt, South Brazil: A New Piece in the West Gondwana Assembly

Paleoproterozoic Metagabbro Xenolith in the Southeastern Dom Feliciano Belt, South Brazil: A New Piece in the West Gondwana Assembly

A suture related accretionary wedge in the Gondwana assembly: Insights from serpentinites in the Hoggar shield, Algeria

The assembly of West Gondwana supercontinent involved several complex processes that led to the formation of the Hoggar shield during the Panafrican-Brasiliano event. We present the first petro-geochemical, geochronological and field data of serpentinite lenses exposed along the In-Tedeini domain, within deformed talc-schist and magmatic-sedimentary formations. The serpentinites preserved the main characteristics of their parent peridotites (Al/Si: 0.004–0.03; Mg/Si: 1.14–1.62; Al2O3: 0.15–1.37 wt%; Mg#: 85.3–94; Ti: 9.34–120 ppm; Nb: 0.007–0.46 ppm), attesting to a highly depleted mantle wedge protolith involved in a subduction zone. This is in agreement with the high Cr# (0.55–0.6), low to moderate Mg# (0.36–0.65) and low TiO2 contents (<0.1 wt%) of their constitutive Cr-spinels. Geochemical modelling suggest that both the North and South In-Tedeini serpentinite units have experienced intense and similar fluid-induced dynamic melting episodes. The evolution of these two units has diverged, with the Southern In-Tedeini unit being refertilized by a small volume of island-arc basaltic melts generated in the mantle wedge. Serpentinization of these rocks probably occurred under static conditions at high temperature corresponding essentially to amphibolite-facies conditions. Field relations suggest that the exhumation of the massive serpentinites occurred along major sinistral shear zones steeply dipping eastward, assisted by talc-schists that highly localized transpressive deformation. First U-Pb zircon ages obtained from a metasomatic chloritite in North In-Tedeini serpentinites; they may have recorded the age (770 ± 5 Ma) of the subduction related Panafrican island arcs and the emplacement (631 ± 10 Ma) of In-Tedeini serpentinites within the crust; or they may rather correspond to the serpentinization events endured by the rocks. All together, the reported results support the presence of a major suture zone, oriented NNW-SSE. This suture is outlined by mantle serpentinite lenses exhumed in a collisional accretionary wedge, which connects the western and the central Hoggar, following a Panafrican east-dipping subduction. This tectonic system would have contributed to the closure of the Goiás-Pharusian ocean.

A Gondwanan micro-fragment adjacent to southern granulite terrane of India: Evidence from satellite gravity studies

The Southern Granulite Terrane (SGT) of Peninsular India is a complex collage of Mesoarchean to Late Neoproterozoic – Cambrian tectonic units, which were part of Earth's major supercontinents such as Ur, Columbia, Rodinia, Gondwana and Pangea. We analyzed high-resolution satellite gravity field with a view to demarcate the underlying crustal and lithospheric mantle structure of the SGT. The western part of SGT is characterized by a high-order elongated negative residual gravity anomaly of −80 mGal together with a thicker Moho (43 km) and the lithosphere (~130 km). In comparison, in the central and eastern parts of SGT, the gravity anomalies are distinctly positive, Moho is shallow (~36 km) and the lithosphere is thin (~100 km). An important finding of the present study is the delineation of a fragmented micro-continental block of the SGT, submerged under Indian Ocean, and located about 30 km west of the rifted continental margin of India, which is characterized by moderate crustal seismicity, and high-order gravity low of −80 mGal, identical to the features of western SGT. This distinct micro-block might represent a vestige of the rifting between India and Madagascar at around 90 Ma due to upwelling of the Marion mantle plume and marking the final breakup of the Gondwana assembly.