Assembly Of Gondwana Research Articles

U–Pb detrital zircon dating applied to metavolcano-sedimentary complexes of the São Gabriel Terrane: New constraints on the evolution of the Dom Feliciano Belt

The Tonian to Cryogenian São Gabriel Terrane located at the northwestern portion of Dom Feliciano Belt, in southern Brazil, provides sedimentation and magmatism records associated with the Rodinia supercontinent and its subsequent deformation and metamorphism during the West Gondwana assembly. It is composed by a succession of arc-related associations including metavolcano-sedimentary and orthogneisses complexes, accretionary prism associations with mafic-ultramafic complexes (ophiolites) and passive margin metasedimentary associations. New U–Pb analyses by LA-ICPMS on detrital zircon from the supracrustal metamorphic complexes in its type-area indicate two distinct times of deposition, sources and tectonic settings. Arc-related complexes occur as parallel bodies interleaved with the metaplutonic associations- of the Passinho (0.9–0.85 Ga) and São Gabriel (0.78–0.72 Ga) arcs. These complexes have detrital zircon data ranging between 0.94 and 0.70 Ga and depositional age constrained at 0.68 Ga. Passive margin complexes constitute a long and continuous belt located at the eastern and southern borders of the São Gabriel Terrane and were deposited over the Paleoproterozoic basement. These complexes show a wide range of detrital zircon ages, between 3.6–1.62 Ga, and a maximum depositional age of 1.58 Ga. The regional mixed sources and the geological context are consistent with accumulation in a passive margin basin situated on the border of an ancient continental plate represented by the Rio de la Plata Craton. Metamorphic zircon overgrowths indicate that the arc-related complexes were accreted during a Cryogenian (0.71–0.70 Ga) collisional event. The contact between the São Gabriel Terrane and the Paleoproterozoic basement units located to the east and south is tectonic. The distinct geological and geophysical features of this boundary characterize it as a suture zone and emphasize its allochthonous and exotic character.

Tectonometamorphic evolution and U–Pb dating of the high-grade Hammar Domain (Southern Ethiopian Shield); implications for the East-African Orogeny

Based on new U–Pb zircon data, field structural analysis and a detailed petrological study of the plutonic and high-grade metamorphic rocks of the Hammar Domain (Southern Ethiopian Shield) overall geodynamic scenario was inferred, bringing broad implications for the East-African Orogeny. The structural evolution of the Hammar Domain can be summarized into four phases, D1 to D4, resulting in (a) relict compositional banding (S1), (b) flat-lying migmatite foliation (S2) defining the primary contacts of granulites and migmatites, (c) superimposed steeply dipping N–S trending compressional foliation (S3) due to regional ~E–W oriented compression and (d) later ~NW–SE trending left-lateral transpressive fabric (S4). New geochronological data point to long-lasting orogenic convergence forming the East-African Orogeny which resulted in two main geodynamic events: (a) Late Tonian to late Cryogenian episode (ca. 770 to 650 Ma) where large volcanic arc construction as the source of rock photoliths (dated at ca. 770 Ma) was followed by crustal accretion and flat-lying fabrics origin (D2 stage), intense migmatization and HT-MP metamorphism (T: 700–850 °C and P: 0.7–0.9 GPa) at depths of ~25–35 km (dated at ca. 720 and 715 Ma). (b) Late Cryogenian to early Ediacaran episode (ca. 650 to 620 Ma) as the key era of continental collision leading to the Greater Gondwana assembly. An early ~E–W oriented compression (D3 stage) resulted in ~N–S trending fabrics that have been continuously changed to the left-lateral transpression (D4 stage) forming ~NW–SE oriented foliations. The time-scale of D3 and D4 events is inferred by syn-tectonic granitoid intrusions yielded at ca. 648 Ma and ca. 630 Ma respectively. Furthermore, the syn- to post-tectonic leucogranite dike, dated at ca. 630 Ma, marks the upper limit for the ductile or brittle-ductile deformation and regional metamorphic events.

888–444 Ma Global Plate Tectonic Reconstruction With Emphasis on the Formation of Gondwana

The formation of Gondwana results from a complex history, which can be linked to many orogenic sutures. The sutures have often been gathered in the literature under broad orogenies — in particular the Eastern and Western Pan-African Orogenies — although their ages may vary a lot within those wide belts. ThePanalesismodel is a plate tectonic model, which aims at reconstructing 100% of the Earth’s surface, and proposes a geologically, geometrically, kinematically, and geodynamically coherent solution for the evolution of the Earth from 888 to 444 Ma. Although the model confirms that the assembly of Gondwana can be considered complete after the Damara and Kuunga orogenies, it shows above all that the detachment and amalgamation of “terranes” is a roughly continuous process, which even persisted after the Early Cambrian. By using the wealth of Plate Tectonics, thePanalesismodel makes it possible to derive numerous additional data and maps, such as the age of the sea-floor everywhere on the planet at every time slice, for instance. The evolution of accretion rates at mid-oceanic ridges and subduction rates at trenches are shown here, and yields results consistent with previous estimates. Understanding the variation of the global tectonic activity of our planet through time is key to link plate tectonic modeling with other disciplines of Earth sciences.

Monazite petrochronology of polymetamorphic granulite‐facies rocks of the Larsemann Hills, Prydz Bay, East Antarctica

AbstractThe Prydz Bay coast, including the Larsemann Hills, features relatively extensive bedrock exposures of interest because of the proximity to a hypothesized suture associated with Gondwana assembly. Critical units are the basement Søstrene Orthogneiss (1,126 ± 11 Ma protolith) and cover Brattstrand Paragneiss (maximum depositional age 1,023 ± 19 Ma). The two units share a polymetamorphic history with events at ~900 Ma (D1) and ~530 Ma (D2‐4). Here we present electron microprobe dates of monazite growth zones and Perple_X pseudosection models of granulite‐facies rocks from the Søstrene Orthogneiss, Brattstrand Paragneiss, and D2‐4 pegmatites of the Larsemann Hills. We propose a scenario for Cambrian metamorphism involving a peak stage at 6–7.5 kbar and 800–860°C (D2 convergence), melt crystallization and garnet breakdown during decompression to early retrograde conditions of 3–4.5 kbar and 700–750°C (D2 convergence, D3 extension), and a late retrograde stage with decompression and cooling to 3–3.5 kbar and 550–650°C (D4). We combine monazite chemistry with phase assemblages predicted by pseudosection modelling to link specific monazite growth domains to individual tectonic stages. Monazite domains containing moderate Th and low to moderate Y are interpreted to be preserved from the prograde path when garnet was stable, and constrain the timing of prograde metamorphism at 536 ± 4 Ma. High‐Th, low‐Y domains, dated at 527 ± 2 Ma, represent the earliest stages of post‐peak melt crystallization. Monazite domains with elevated Y and low‐moderate Th are interpreted to represent monazite growth during garnet breakdown at 514 ± 2 Ma. Our monazite ages, combined with published biotite Ar–Ar cooling ages, yield a two‐stage history of cooling at 3–8°C/Myr from ~530 Ma to ~510 Ma followed by cooling at 18–25°C/Myr from ~510 Ma to ~490 Ma, corresponding to 0.2–0.6 mm/yr of exhumation. This duration of granulite‐facies metamorphism in the Larsemann Hills is consistent with estimates for Precambrian granulite facies metamorphic complexes elsewhere.

New high-quality paleomagnetic data from the Borborema Province (NE Brazil): Refinement of the APW path of Gondwana in the Early Cambrian

The Neoproterozoic-Paleozoic transition (~541 Ma) was a turning point in Earth’s history resulting in great biological changes between the microbial Precambrian life and the Ediacaran biotic revolution with the occupation of the sedimentary substrate, the dawn of biomineralization and the appearance of the earliest multicellular organisms. In parallel, this period is marked by a large plate reorganization leading to the assembly of Gondwana and by major climatic changes (extreme glacial events). Due in part to a poor paleomagnetic database for the different cratons in the Ediacarian-Cambrian times, the global paleogeography at that time remains controversial. In this study we present a new high-quality paleomagnetic pole (R = 7) for the Monteiro dyke swarm in the Borborema Province (NE Brazil) located at 18.2°S and 344.9°E (A95 = 11.7° K = 9.3). They are fine-grained hornblende dolerite dated by U-Pb on zircon at ~538 Ma. Rock magnetic data indicate that magnetite and pyrrhotite are the main remanence carriers. Positive baked-contact tests support the primary remanence obtained for these dykes (19 sites). A positive reversal test (classified C) was also obtained from the 14 sites with negative inclination and the 5 sites with positive inclination, indicating that the paleosecular variation was eliminated. Our new key pole is not consistent with the classical apparent polar wander path of the Gondwana which consists of a long track from a southern polar position at ~590 Ma to an equatorial position at ~520 Ma, and suggests instead rapid and small oscillations of the APW, after the end of the large IITPW at ca. 560 Ma. These TPWs are supposedly caused by changes in the inertia tensor of the Earth due to internal mass redistribution, related to rapid changes in subduction velocity. Links of these rapid oscillations and the timing of the Cambrian radiation could be crucial to understand the early history of animal life

Late Neoproterozoic to early Paleozoic paleogeographic position of the Yangtze block and the change of tectonic setting in its northwestern margin: Evidence from detrital zircon U-Pb ages and Hf isotopes of sedimentary rocks

AbstractThe crustal evolution of the Yangtze block and its tectonic affinity to other continents of Rodinia and subsequent Gondwana have not been well constrained. Here, we present new U-Pb ages and Hf isotopes of detrital zircons from the late Neoproterozoic to early Paleozoic sedimentary rocks in the northwestern margin of the Yangtze block to provide critical constraints on their provenance and tectonic settings. The detrital zircons of two late Neoproterozoic samples have a small range of ages (0.87–0.67 Ga) with a dominant age peak at 0.73 Ga, which were likely derived from the Hannan-Micangshan arc in the northwestern margin of the Yangtze block. In addition, the cumulative distribution curves from the difference between the depositional age and the crystalline age (CA–DA) together with the mostly positive εHf(t) values of these zircon crystals (−6.8 to +10.7, ~90% zircon grains with εHf[t] > 0) suggest these samples were deposited in a convergent setting during the late Neoproterozoic. In contrast, the Cambrian–Silurian sediments share a similar detrital zircon age spectrum that is dominated by Grenvillian ages (1.11–0.72 Ga), with minor late Paleoproterozoic (ca. 2.31–1.71 Ga), Mesoarchean to Neoarchean (3.16–2.69 Ga), and latest Archean to early Paleoproterozoic (2.57–2.38 Ga) populations, suggesting a significant change in the sedimentary provenance and tectonic setting from a convergent setting after the breakup of Rodinia to an extensional setting during the assembly of Gondwana. However, the presence of abundant Grenvillian and Neoarchean ages, along with their moderately to highly rounded shapes, indicates a possible sedimentary provenance from exotic continental terrane(s). Considering the potential source areas around the Yangtze block when it was a part of Rodinia or Gondwana, we suggest that the source of these early Paleozoic sediments had typical Gondwana affinities, such as the Himalaya, north India, and Tarim, which is also supported by their stratigraphic similarity, newly published paleomagnetic data, and tectono-thermal events in the northern fragments of Gondwana. This implies that after prolonged subduction in the Neoproterozoic, the northwestern margin of the Yangtze block began to be incorporated into the assembly of Gondwana and then accept sediments from the northern margin of Gondwanaland in a passive continental margin setting.

Recovering P–T–t paths from ultra-high temperature (UHT) felsic orthogneiss: An example from the Southern Brasília Orogen, Brazil

Mineral assemblages that are diagnostic of ultrahigh-temperature (>900 °C) metamorphism are generally restricted to Mg- and Al-rich rocks. However, in many metamorphic terrains such compositions do not occur, and the rocks are dominated by orthogneisses that typically contain high-variance mineral assemblages that are not diagnostic of particular pressure–temperature (P–T) conditions. Here we use whole-rock and mineral chemistry, thermodynamic phase equilibrium modelling, and zircon U–Pb–Hf isotopic data to constrain the P–T–time history of granulites from the Socorro-Guaxupé Nappe of the Southern Brasília Orogen. These rocks occur as part of a stack of Neoproterozoic nappes that were assembled during the formation of West Gondwana. Based on phase equilibrium modelling, the peak assemblage in a garnet-bearing felsic granulite (ortho- and clinopyroxene, garnet, K-feldspar, plagioclase, quartz, ilmenite, and silicate melt) records pressures of around 11.5 kbar and temperatures of ~1000 °C, consistent with the high-Al content of orthopyroxene. The retrograde path records a small decrease in pressure to 9 kbar marked by orthopyroxene replacing garnet and the exsolution of magnetite from ilmenite followed by near-isobaric cooling that led to the development of coronas of hornblende around pyroxene due to crystallization of the last vestiges of melt. Zircon cores yielding U–Pb ages of ca. 625 Ma are interpreted as dating close-to-peak conditions during high-temperature metamorphism. In one sample, zircon rims and ‘soccer ball’ zircon neoblasts both have high U contents and yield ages of ca. 615 Ma, interpreted to mark the onset of melt crystallization. In two other samples, low-U rims and ‘soccer ball’ zircons grains yield ages of ca. 600–595 Ma, interpreted to date the end of melt crystallization. These data reveal a protracted regional metamorphic event in which the lower crust remained at granulite-facies conditions for at least 30 Myr during collision and assembly of West Gondwana.

Two-stage crustal growth in the Arabian-Nubian shield: Initial arc accretion followed by plume-induced crustal reworking

Island-arc accretion during the assembly of Gondwana has been widely regarded as the main mechanism for Neoproterozoic crustal growth in the Arabian-Nubian Shield (ANS). However, processes involved to transform the newly accreted juvenile terranes into a typical continental crust remain unclear. Here, we present geochemical, isotopic, and U-Pb geochronological data from the El-Shadli volcanic province (80 km × 35 km and > 10 km thick) in the Eastern Desert of Egypt, which lies in the north-western part of the ANS, and overlies strongly deformed, previously accreted arc terranes. The El-Shadli volcanic province consists mainly of a mafic-felsic bimodal suite and subordinate intermediate rocks that intrude the mafic rocks of the suite. The bimodal suite rocks are tholeiitic, whereas the intermediate rocks have a calc-alkaline affinity. The bimodal suite and the intermediate rocks both yield U-Pb zircon ages of ~ 700 Ma implying they are coeval. High zircon εHf(t) values for the bimodal suite (average εHf(t) = +11.46) as well as the intermediate rocks (average εHf(t) = +9.76) indicate they were either magmatic extractions derived directly from a depleted mantle source, or the products of remelting of juvenile crust. Oxygen isotope data for zircon yield similar δ18O values for both the bimodal suite (average δ18O = 4.94‰) and intermediate rocks (average δ18O = 4.79‰). These are lower than typical mantle values indicating the parental magma in both cases interacted with hydrothermal fluids. Based on the petrological, geochemical, and isotopic data, we suggest that the El-Shadli bimodal suite and the intermediate rocks were produced by reworking of MORB-like and arc-like oceanic lithosphere, respectively, most likely driven by a mantle plume during the break-up of Rodinia. The recognition of the El-Shadli volcanic province as a likely mantle plume-induced post-kinematic magmatism provides a mechanism for the transformation of newly accreted juvenile crustal terranes into a chemically stratified normal continental crust. In addition, such plume events may result in new mantle extractions that are converted into new continental crust.

Remote sensing and structural studies of the Central Asir Shear Zone, Western Arabian Shield: Implications for the late Neoproterozoic E-W Gondwana assembly

The Central Asir Shear Zone (CASZ) is a N- to NNE-oriented inhomogeneous strain zone parallel to the Nabitah shear belt in southern Saudi Arabia. It is ~200 km long and 20 km wide, and traverses the Asir terrane. The deformed terrains in the vicinity of the CASZ have experienced a prolonged history involving three successive phases of deformation: arc-collision, dextral transpression and sinistral strike-slip faulting. Remote-sensing data analysis and field investigation provide insights into the geological and structural features along the CASZ. Image transformation and enhancement reveal faults and folds, and discriminate between granitoids, metavolcanics, gabbros and younger (Ablah Group) sediments. The obtained evidence points to polyphase deformation of the area, characterized by narrow zones of high strain, folding and shearing enclosing slices of less deformed rocks. Lineament analysis shows predominantly NNE, NNW, N and NE trends of folds and shear zones, and NW, NE and E fracture trends. The CASZ has a >100 m.y. long deformation history, including a change from dextral to sinistral shearing. Both shearing senses are found on the older N-S and younger NW-SE fault systems. N-S oriented shearing was accompanied by intrusion of granitoids and localized hydrothermal alteration along N to NNE trending shear zones. Strain analysis shows low strain in the Ablah basin sediments, consistent with Ablah deposition after the tectonic climax in the region.

Reconstructing the metamorphic evolution of the Araçuaí orogen (SE Brazil) using in situ U–Pb garnet dating and P–T modelling

AbstractThe Araçuaí orogen of SE Brazil consists of a suite of deformed, metamorphosed, and partly migmatitic sedimentary rocks and granitoid batholiths that predominantly formed during collision of the São Francisco and Congo cratons during West Gondwana assembly in the late Neoproterozoic. While widespread anatexis and a prolonged magmatic record are well‐established, insufficient information exists about the extensive exposure of metamorphic rocks along the Araçuaí orogen. Combining information from a wide range of these metamorphic rocks is essential to reconstruct mountain‐building processes and heat sources that operate in convergent tectonic settings. New petrographic observations, mineral chemistry, and phase equilibria modelling were used to constrain peak metamorphic conditions of amphibolite facies metasedimentary rocks from the central domain of the orogen at 580–620°C and 7–9 kbar. We applied in situ low‐U (<1 µg/g) garnet U–Pb geochronology, which yielded ages in the range of 590–565 Ma, interpreted as the timing of bulk garnet growth during prograde metamorphism, and consistent with previously established monazite ages for the peak metamorphic event. Metamorphic zircon ages of c. 630 Ma are related to a cryptic terrane accretion event prior to the orogeny, whereas garnet U–Pb ages record the main orogenic event as revealed by disequilibrium rare earth element partitioning between the two minerals. A compilation of our results with previously reported metamorphic conditions defines a clockwise P–T evolution for the Araçuaí orogeny, characterized by slow burial to depths of 26–30 km, followed by nearly isothermal decompression from ~10 to 6 kbar. Substantial differences of peak temperatures in various domains of the orogen are consistent with the presence of different protoliths in the crustal sections. The higher concentration of heat‐producing elements (HPEs) in lithologies from the anatectic domains may have triggered partial melting of metasedimentary rocks. This suggests that HPEs were a significant heat source for crustal reworking during West Gondwana convergent tectonics.

So close and yet so far: U–Pb geochronological constraints of the Jaibaras Rift Basin and the intracratonic Parnaíba Basin in SW Gondwana

AbstractSeveral sag-type basins apparently developed from rift systems, but there is no consensus about how and if these grabens influenced the sedimentation of the post-rift thermal subsidence phase. The Ediacaran Jaibaras Rift Basin is one of the best-exposed sedimentary records among the NE Brazil late Precambrian – early Cambrian rift system, cropping out at the eastern margin of the intracratonic Parnaíba Basin and extending below it towards the west. Here we present detrital zircon U–Pb ages of rocks from the Jaibaras (Aprazível Formation) and Parnaíba (Ipu Formation) basins, in order to understand the provenance patterns, maximum depositional ages (MDA) and age relationship between these units. The MDA for the Aprazível Formation (c. 499 ± 5 Ma) indicates a Cambrian age for the upper part of the Jaibaras Basin. The bulk U–Pb data indicate that the Ipu Formation started to deposit during late Cambrian and/or Early Ordovician time, despite its MDA (c. 528 ± 11 Ma) being older than that of the Aprazível Formation. Detrital zircon provenance suggests that the primary source areas for the early deposits of the Parnaíba Basin were mountains related to the Brasiliano Orogeny to the south and SE (e.g. Rio Preto and Riacho do Pontal metamorphic belts). Finally, our data emphasize the key change in source areas from the rift to the initial deposition of the intracratonic phase, indicating major depositional style changes between both basins after the Gondwana assembly.

Biogeochemical status of the Paleo-Pacific Ocean: clues from the early Cambrian of South Australia

The Ediacaran–Cambrian transition was a time of profound reorganisation of the biosphere, coinciding with the assembly of Gondwana and increasing atmospheric and oceanic oxygenation. The lower Cambrian marine sediments of the Stansbury Basin, South Australia, were deposited at low northern paleolatitudes on the western margin of the emerging Pacific Ocean. Here we report results of a multi-pronged investigation employing trace and rare earth element (REE) abundances, total organic carbon (TOC) contents and pyritic sulfur isotopic compositions (δ34Spy) in carbonaceous shales from three formations within the Normanville and Kanmantoo groups (sequences Є1.2 to Є2.2). TOC ranges from ≤0.5 wt% in the Emu Bay Shale, through 0.4–1.9 wt% in the Talisker Formation, to 0.2–2.6 wt% in the Heatherdale Shale. Covariance is demonstrated between trace elements and organic matter, with comparative uranium enrichment in the Heatherdale Shale likely linked to increased primary productivity. Heavy REEs and yttrium are typically more depleted than light REEs. Provenance appears to evolve through the upper Normanville sediments into the Kanmantoo Group, corroborating published detrital zircon interpretations that suggest an increasing dominance of southerly derived sources. The prevailing paleoredox regime for the Heatherdale Shale and the Talisker Formation was dysoxic, evolving into progressively more reducing conditions up section, the latter exhibiting a secular decline in δ34Spy (from +10 to −11‰ VCDT). Conversely, redox proxies concur in demonstrating that the basal fossiliferous Emu Bay Shale accumulated beneath an oxic water column. Comparison of trace element and REE distributions with those of slightly older sequences on the Yangtze Platform, South China, reveals striking similarities, implying that the trace element chemistry of the Paleo-Pacific and Asian oceans was homogeneous. KEY POINTS Trace and rare earth element, total organic carbon and sulfur isotopic data provide a multi-faceted profile of shales from three formations in the Stansbury Basin. The prevailing paleoredox regime for the Heatherdale Shale and Talisker Formation was dysoxic, whereas the basal Emu Bay Shale accumulated beneath an oxic water column. The strong covariance between TOC and normalised Mo, V and U in these shales highlights the evolving bioproductivity and oxygenation of the early Cambrian ocean. Similar element signatures in lower Cambrian black shales, South China, reflect the homogeneous biogeochemistry of the Paleo-Pacific and Asian oceans.

Long-lived intracontinental deformation associated with high geothermal gradients in the Seridó Belt (Borborema Province, Brazil)

The Borborema Province (NE-Brazil) experienced widespread intracontinental deformation associated with low-pressure metamorphism during the Neoproterozoic-Cambrian assembly of West Gondwana. The widespread intracontinental deformation was driven by tectonic stresses derived from two continental collisions reported in the literature at ca. 630–610 Ma and includes the development of the continental-scale Borborema Shear Zone System. In this study, we investigate metasedimentary rocks from the Seridó Belt and eastern Patos Shear Zone, which occur in an intracontinental deformation zone at several hundred kilometers from the recognized collision zones. Using field data, quantitative compositional mapping, thermodynamic modeling, and monazite petrochronology, we constrained the metamorphic conditions and duration of intracontinental deformation in the inner Borborema Province. Monazite from leucosome-rich migmatite portions in the eastern Patos Shear Zone display 206Pb/238U dates spreading from 580 ± 13 to 512 ± 11 Ma (n = 125) with a cluster around ca. 570–550 Ma (n = 70). Staurolite-bearing schists from the western Seridó Belt display a large spread of 206Pb/238U dates between 626 ± 13 and 519 ± 12 Ma (n = 50), with a well-defined cluster between ca. 570 and 550 Ma (n = 37). Garnet-andalusite-cordierite schists from the inner and eastern Seridó Belt display monazite 206Pb/238U dates ranging from ca. 550 to 500 Ma (n = 126). Schists from the NNE-SSW striking Picuí Shear Zone in the eastern Seridó Belt exhibit Crd + And-rich nodules wrapped by the foliation, attesting the persistence of deformation after their generation. Monazite inclusions in cordierite and andalusite have REE patterns similar to matrix monazite and together they yield weighted average ages between ca. 530–525 Ma, interpreted as the age of metamorphism within the Picuí Shear Zone. Iterative thermodynamic modeling using the local bulk composition of a Crd + And-rich nodule yielded optimal P-T conditions of ca. 590 °C and 3.8 kbar. The presented data indicate that intracontinental deformation in the inner Borborema Province was long-lived and lasted until at least 530–525 Ma. The deformation was associated with high geothermal gradient conditions that reduced lithospheric strength allowing the development and maintenance of the Borborema Shear Zone System.

Peninsular Malaysia transitional geodynamic process from Gondwana to Pangaea: New constraints from 500 to 200 Ma magmatic zircon U-Pb ages and Hf isotopic compositions

The geodynamic process from the evolution of supercontinent has distinct isotope characteristics explorable using zircon Hf isotopic composition. Since Peninsular Malaysia associates with Gondwana dispersal and Pangaea formation, analyzing the U-Pb and Hf-isotopic content of its 500–200 Ma magmatic zircon could reveal the signal left by the transitional geodynamic process between the supercontinents. We collected two groups of magmatic rocks from West Malaya: Ordovician meta-volcanics (n = 8), and Triassic Main Range granitoid province (MRGP) (n = 6); and three groups from East Malaya: Carboniferous meta-volcanics (n = 2), Permian-Triassic Eastern granite province (EGP) (n = 6), and Permian-Triassic EGP volcanics (n = 8). Difference in magmatic zircon Hf isotopic crustal model ages uphold the previous rationale which separates Peninsular into two blocks: West Malaya (part of Sibumasu terrane) magmatic zircon Hf isotopic crustal model ages (Average TDM2: 1.3 Ga) are older than East Malaya (part of Chanthaburi-Sukhothai-Lincang arc of Indochina terrane) (Average TDM2: 0.9 Ga). During the final assembly of Gondwana from 500 to 450 Ma, West Malaya and East Malaya were at the outboard of Gondwana Proto-Tethys margin. The shift of East Malaya zircon Hf array towards higher εHf(t) (external orogenic system) after ca.370 Ma may infers Paleo-Tethys ocean broadening and East Malaya separation from Gondwana. The 370–350 Ma juvenile zircon Hf isotopic composition in East Malaya is a significant improvement over radiolarian age to show the broadening and subduction of the Paleo-Tethys ocean between the two terranes. After ca.280 Ma, East Malaya zircon Hf array shifted towards lower εHf(t) (internal orogenic system). Coinciding with the Indosinian collision at ca.230 Ma, crustal reworking signal increases in both blocks, signifying the end in Peninsular Malaysia Gondwana to Pangaea transitional geodynamic process. As the Paleo-Tethys segment was completely subducted after 230 Ma, the peninsular crustal thickening starts from 230 to 218 Ma. The post-collision phase would begin at ca.215 Ma.

Geochemistry and provenance of the metasedimentary rocks surrounding the Santa Quitéria magmatic arc, NE Brazil: Tectonic and paleogeographic implications for the assembly of West Gondwana

Western Gondwana resulted from the closure of the Pharusian-Goianides Ocean, a process that culminated in the formation of magmatic arcs, high and ultra-high-pressure (HP-UHP) belts, and sedimentary basins. These tectonic units occur in the NW part of the Borborema Province (Northeastern Brazil), which encloses the Neoproterozoic Santa Quitéria magmatic arc (SQMA). Metasedimentary rocks (sillimanite-, garnet- and biotite-bearing gneiss, quartzite, and schist) that constitute the Canindé Complex and the Independência Unit surround the western and eastern margins of this arc. According to A/CNK and N/K ratios and the Chemical Index of Alteration calculated for each rock type, the protolith of these rocks correspond to moderately weathered pelites, greywacke, and lithic-arenite. Trace element distributions (LILE, HFSE and REE) indicate that the Canindé Complex rocks are depleted in Sc, V, Cr, Co, Ni and LILE and are formed by mature and stable minerals such as quartz, muscovite and K-feldspar, similarly to passive margin sediments. In contrast, the Independência rocks present higher concentrations of transition trace elements, constituted by unstable-system minerals (iron-magnesium minerals) and immature sediments, typical of active margin settings. In general, these rocks present felsic-source components, with ample fractionation between LREE over HREE (LaN/YbN = 26.2 to 4.16) and with a slight to strong negative Eu anomaly (Eu/Eu* = 1.26 to 0.33). Isotopic Hf data identified a mixture of crustal- and mantle-derived sources. According to tectonic setting discrimination diagrams (Co-Th-Zr/10 and Sc-Th-Zr/10) and the concentrations of characteristic trace elements and ratios (e.g., Th, V, Co, La, Ce, LaN/YbN), these rocks were interpreted as being deposited in basins formed in active and passive margin settings associated with continental and intraoceanic arcs. U-Pb zircon data indicates source rocks from the Archean (3.0 to 2.6 Ga) to the Neoproterozoic (820 to 541 Ma). According to our results, the Canindé and Independência supracrustal rocks were deposited in basins affected by different tectonic regimes, controlled by the installation of the Santa Quitéria Magmatic Arc (SQMA), starting with a passive margin or intracratonic basins that evolved into an active margin setting on the western SQMA margin (Canindé Complex) and a predominantly passive margin setting on the eastern SQMA margin (Independência Unit), leading to the formation of forearc and backarc basins.

Tectonically-induced strontium isotope changes in ancient restricted seas: The case of the Ediacaran-Cambrian Bambuí foreland basin system, east Brazil

The Bambuí Group is a marine sedimentary record of an intracratonic foreland basin developed at the terminal Ediacaran and early Cambrian during the assembly of West Gondwana. Here we present a basin-scale high-resolution Sr isotope stratigraphy for the basal Bambuí Group, aiming to understand the spatial and temporal variations of the 87Sr/86Sr ratios and to explore the controls over the Sr isotope system in intracontinental marine environments. Assessment of the stratigraphic evolution of both Sr concentrations and Sr isotopes shows a major increase in Sr/Ca ratios (up to 0.004) and a decrease in the 87Sr/86Sr ratios from 0.7086 to 0.7076 in the high stand system tract of the basal 2nd-order sequence. These changes precede a large positive δ13C excursion typically found across the basin in the middle Bambuí Group. The high variability of both 87Sr/86Sr and Sr/Ca ratios was not caused by globally uniform changes in isotopic compositions of seawater, but rather likely reflect marine restriction and paleogeographic changes of the depositional environments at basin scale. This would result from the tectonic uplift of Neoproterozoic orogenic belts around the São Francisco craton, which generated an isolated foreland marine basin. Compared to the global ocean, such a smaller intracontinental reservoir would be more sensitive to the Sr isotope composition from the different rock sources. We suggest that changes on the balance between carbonate production and accommodation associated with tectonically-related flexural subsidence progressively modified the continental drainage patterns, sedimentary sources and the chemical weathering regimes, altering the strontium influxes and isotopic compositions of the seawater in the early Bambuí basin cycle. Similar anomalies in the strontium isotope record are also recorded in coeval marine basins across West Gondwana and suggest that tectonics might have played an important role on seawater chemistry at the Neoproterozoic-Paleozoic transition.

Cambrian magmatic flare-up, central Tibet: Magma mixing in proto-Tethyan arc along north Gondwanan margin

AbstractAccompanying Gondwana assembly, widespread but diachronous Ediacaran–early Paleozoic magmatism of uncertain origin occurred along the supercontinent's proto-Tethyan margin. We report new geochemical, isotopic, and geochronological data for Cambrian magmatic rocks (ca. 500 Ma) from the Gondwana-derived North Lhasa terrane, located in the present-day central Tibetan Plateau. The magmatic rocks are composed of basalts, gabbros, quartz monzonites, granitoids (with mafic microgranular enclaves), and rhyolites. Nd-Hf isotopic and whole-rock geochemical data indicate that these rocks were probably generated by mixing of mantle-derived mafic and crust-derived felsic melts. The mantle end-member volumes of mafic, intermediate, and felsic rocks are ∼75%–100%, 50%–60%, and 0–30%, respectively. Integration of our new data with previous studies suggests that the North Lhasa terrane experienced long-term magmatism through the Ediacaran to Ordovician (ca. 572–483 Ma), with a magmatic flare-up at ca. 500 Ma. This magmatism, in combination with other Ediacaran–early Paleozoic magmatism along the proto-Tethyan margin, was related to an Andean-type arc, with the magmatic flare-up event related to detachment of the oceanic slab following collisional accretion of Asian microcontinental fragments to northern Gondwana. Diachroneity of the proto-Tethyan arc system along the northern Gondwanan margin (ca. 581–531 Ma along the Arabian margin and ca. 512–429 Ma along the Indian-Australian margin) may have been linked to orogenesis within Gondwana. The North Lhasa terrane was probably involved in both Arabian and Indian-Australian proto-Tethyan Andean-type orogens, based on its paleogeographic location at the northern end of the East African orogen.

Neoproterozoic copper-gold mineralization in the Amani area, southwestern Tanzania

The Amani area in southwestern Tanzania contains several prospects where primary Cu-Au mineralization occurs at surface, the most noteworthy of these being the Amani Hills and Mpanga Hill prospects. At the Amani Hills prospect, structurally-controlled gold mineralization is hosted in quartz-muscovite schists of the Amani Group and minor high-grade gneisses belonging to the Kabelege Group. Here, quartz-carbonate veins contain pyrite-pyrrhotite-chalcopyrite, with free-milling gold occurring predominantly in quartz and along sulphide grain boundaries. These veins are associated with sericite-epidote-albite alteration of the wallrocks. At the Mpanga Hill prospect, Cu ± Au mineralization is hosted within impure micaceous marbles of the Rafiki Group. At this prospect, both the sulphidised carbonate host rocks and quartz-carbonate veins contain chalcopyrite, bornite and pyrite assemblages, with accessory phases of Au ± Pb ± Zn ± Ag ± Bi ± Se. Fluid inclusion microthermometry indicate that auriferous veins at both prospects record a bimodal occurrence of early low-temperature and low-salinity (≤23.2 wt% NaCl eq.) fluids that are overprinted by later, high temperature and hypersaline fluids (32.8 – 54 wt% NaCl eq.). These ore-forming fluids contain H2O-NaCl ± CO2 ± CH4 compositions and were trapped at near-lithostatic to supralithostatic conditions of 330 – 400 °C and 1.7 – 6.9 kbar. These hydrothermal fluids were likely variably sourced and were focussed along kilometre-scale brittle-ductile reverse shear zones in the Amani area, where fluid-rock interactions led to Cu-Au mineralization. 40Ar/39Ar dating of muscovite constrained the timing of copper – gold mineralization in the Amani area to between c. 550 – 600 Ma. Mineralization is therefore related to the Pan-African aged Kuunga orogeny, which resulted from the collision between the Tanzania Craton and Bangweulu Block during the assembly of supercontinent Gondwana. The new findings presented in this study highlight the exploration potential of copper – gold mineralization along large-scale Proterozoic shear zones in the underexplored and overlooked Upangwa terrane of southwestern Tanzania.

Fluid flow and syntectonic veining in an Ediacaran-Cambrian foreland fold–thrust zone, western margin of the São Francisco Craton, Brazil

The western margin of the São Francisco Craton, central Brazil presents a 1300 km long foreland fold–thrust belt where Ediacaran-Cambrian (560–520 Ma) metasedimentary rocks from the Bambuí Group were subsequently deformed during post-collisional stages (520–495 Ma) related to Gondwana assembly. This scenario provides an opportunity to quantify fluid flow regimes and fault-related processes that were active in exhumed foreland fold–thrust zones, which were estimated based on structural, microstructural and fluid inclusion studies of syntectonic veins and host rocks. Kaolinite-bearing synkinematic mineral assemblages from metasedimentary rocks, thermodynamic models and grain-scale deformation accommodated by dissolution–precipitation creep and intracrystalline deformation indicate metamorphic and deformational conditions of 250–270°C. Subhorizontal extensional veins formed under subhorizontal shortening and subvertical extension, supporting vein development under a fold–thrust regime that formed regional NW–SE-trending thrust fault zones and megafolds with NW–SE-trending axes. Orientation and growth microstructures indicate that NW–SE-trending subvertical cleavage-parallel veins formed under subhorizontal NE–SW extension, compatible with those inferred to produce mapped kilometre-scale gentle folds with NE–SW-trending traces. Two primary aqueous fluid inclusion assemblages (FIA) are distinguished by salinity variation: 2–21 wt% NaCl eq. in subhorizontal veins and 6–0 wt% NaCl eq. in cleavage-parallel subvertical veins. Fluid inclusion thermometry and microstructural analysis suggest that veins crystallized between 250 and 270°C under fluid pressure fluctuating within a range of 50–500 MPa (subhorizontal veins) and 80–320 MPa (cleavage-parallel subvertical veins), evidencing fault-valve behaviour. Trends of coupled decreases in salinity and homogenization temperatures in both FIA indicate downward mixing of meteoric fluids, which was more effective in subvertical veins and was in both cases enhanced by fault-valve behaviour. Dominance of moderate salinity and absence of CO 2 and CH 4 indicate that the fluids are dominated by formation waters. The salinity signature is similar to those of formation waters and metamorphic fluids derived from rocks of shallow marine environments worldwide. Supplementary material: Details of samples and analytical data are available at https://doi.org/10.6084/m9.figshare.c.5275031

The fast exhumation pattern of a Neoproterozoic nappe system built during West Gondwana amalgamation: Insights from thermochronology

In this contribution we investigate the exhumation T-t path of a nappe system located in the southernmost edge of the Brasília Orogen, southeast Brazil, developed during the West Gondwana assembly. The allochthons represent an inverted metamorphic pile of nappes and were deformed during the collision between the Paranapanema (active margin) and São Francisco (passive margin) paleocontinents. The nappe system comprises UHT rocks of a magmatic arc root (upper Socorro-Guaxupé Nappe), the accretionary wedge – foreland units metamorphosed under high-pressure conditions (intermediate Andrelândia Nappe System) and the lower passive margin metasedimentary sequences (the high-pressure Carrancas Nappes System and the Lima Duarte Nappe). New U-Th-PbT in monazite and 40Ar/39Ar in hornblende, biotite and muscovite ages combined with previously published data indicate different patterns of cooling for each allochthon. The upper nappes register a collision to exhumation/cooling path, from 630–625 Ma to 590–580 Ma, which indicates that the geological process active during the West Gondwana amalgamation involved fast exhumation tectonics. Cross-sections along the main transport direction of the nappes indicate a progressive decrease of metamorphic age peaks (630–625 Ma to 590–570 Ma) and 40Ar/39Ar cooling ages (600 Ma to 540 Ma) from the internal regions (SW) to the front (NE) of the nappe system, which indicate that the propagation of the nappe pile advanced progressively from the upper to the lower nappes.