Assembly Of Gondwana Research Articles

Evolution of Neoproterozoic Shillong Basin, Meghalaya, NE India: implications of supercontinent break-up and amalgamation

AbstractFragmentation and amalgamation of supercontinents play an important role in shaping our planet. The break-up of such a widely studied supercontinent, Rodinia, has been well documented from several parts of India, especially the northwestern and eastern sector. Interestingly, being located very close to the Proterozoic tectonic margin, northeastern India is expected to have had a significant role in Neoproterozoic geodynamics, but this aspect has still not been thoroughly studied. We therefore investigate a poorly studied NE–SW-trending Shillong Basin of Meghalaya from NE India, which preserves the stratigraphic record and structural evolution spanning the Neoproterozoic Era. The low-grade metasedimentary rocks of Shillong Basin unconformably overlie the high-grade Archean–Proterozoic basement and comprise a c. 4000-m-thick platform sedimentary rock succession. In this study, we divide this succession into three formations: lower Tarso, middle Ingsaw and upper Umlapher. A NW–SE-aligned compression event later caused the thrusting of these sedimentary rocks over the basement with a tectonic contact in the western margin, resulting in NE–SW-trending fold belts. The rift-controlled Shillong Basin shows a comparable Neoproterozoic evolution with the equivalent basins of peninsular India and eastern Gondwana. The recorded Neoproterozoic rift tectonics are likely associated with Rodinia’s break-up and continent dispersion, which finally ended with the oblique collision of India with Australia and the intrusion of Cambrian granitoids during the Pan-African Orogeny, contributing to the assembly of Gondwana. This contribution is the first to present a complete litho-structural evolution of the Shillong Basin in relation to regional and global geodynamic settings.

Neoproterozoic Earth-life system

The Earth-life system witnessed dramatic changes over the Neoproterozoic (1000–541 Ma). It saw supercontinent reconfigurations from the breakup of Rodinia to the assembly of Gondwana; climatically, the deepest global freeze in Earth history; geochemically, some of the most extreme perturbations on record; and biologically, the prominent rise of complex multicellular eukaryotes. However, the potential mechanisms that lie behind the convergence of such geological and biological revolutions remain elusive. The 27 articles in this special issue represent frontiers in disentangling the potential links between geological and biological changes in the Neoproterozoic and will inspire future interdisciplinary studies toward a better understanding of the Neoproterozoic Earth-life system.

Three stages of early Paleozoic magmatism in the Tibetan-Himalayan orogen: New insights into the final Gondwana assembly

Earth’s evolution involves deep, hot rocks rising upward by convection to near-surface environments, as well as the unique scenarios such as the formation and breakup of Gondwana and Pangea . These processes have been well recorded by the widely distributed magmatism . To understand the tectonic evolution and Gondwana assembly, we compiled early Paleozoic zircon U–Pb ages (n = 67), in-situ zircon Hf isotopes (n = 1011), and bulk-rock elemental compositions (n = 293) and Sr–Nd isotopes of magmatic rocks in the Tibetan–Himalayan orogen. Three stages of Paleozoic magmatism were identified here, including early (>490 Ma) to middle (490–470 Ma) to late (<470 Ma) stages, in response to the final assembly of the Gondwana supercontinent . Early-stage magmatic rocks are characterized by highly variable SiO 2 (48.0–80.0 wt%) and MgO (0.02–9.65 wt%) contents with Mg# range from 5.4 to 78, K 2 O/Na 2 O ratios ranging from 0.14 to 2.81, and whole-rock Sr–Nd ( 87 Sr/ 86 Sr(i) (0.7035–0.7340), ε Nd (t) (-9.5–+1.0)) and zircon Hf (ε Hf (t) = –15 to + 8.0) isotopic compositions with significant mantle contributions; they could be generated in an Andean-type arc setting along the active northern continental Gondwana margin. Middle-stage magmas were dominated by fertile continental crustal signatures (SiO 2 > 70 wt%, A/CNK > 1.10, MgO < 2.69 wt%) in a crustal thickening setting. Late-stage magmas also have highly variable Si, Mg and isotopic components, with coeval mantle-derived magmas developing in an extensive setting. Importantly, the widespread presence of early Cambrian to late Ordovician peraluminous high-K calc-alkaline magmatism in the Tibetan–Himalayan orogen indicates that the reworking of ancient continental crustal materials played a key role in reconstructing and stabilizing the final Gondwana assembly.

New SIMS zircon U-Pb ages and oxygen isotope data for ophiolite nappes in the Eastern Desert of Egypt: Implications for Gondwana assembly

Ophiolite nappes, encompassing dismembered oceanic lithosphere sections, in the Eastern Desert of Egypt are mainly confined to Tonian-Cryogenian inter-terrane sutures. A cohesive geodynamic model of these ophiolites remains elusive in light of the variably obliterated field relationships and lack of ample geochronological information. Available geochemical data of the well-studied ophiolites lead to different interpretations of mid-ocean ridge, back-arc and/or fore-arc affinities. Relevant geochronological data are likewise heterogeneous in terms of methods and the dated lithologic units. Here, we attempt to constrain the timing and genesis of some Eastern Desert ophiolites by using new SIMS zircon U-Pb and oxygen isotope data of gabbros from four different ophiolites.The new zircon U-Pb ages and oxygen isotope values integrated with available geochemical data constrain ages of 737 ± 6 Ma and 720 ± 6 Ma and fore-arc origin of the Wadi El-Sid and Gabal Abu Dahr-Abu Siayil ophiolites, respectively. The δ18O(Zrn) values of these fore-arc ophiolites (+2.5 to +8.5‰) deviate significantly from primitive mantle values, suggestive of substantial degrees of subduction-induced metasomatism. The Gabal El-Rubshi and Wadi Ghadir back-arc ophiolites returned younger and mutually similar ages (702 ± 5 Ma and 698 ± 4 Ma, respectively) and less heterogeneous δ18O(Zrn) values (+3.1 to +6.1‰). Excluding outliers, most measured δ18O(Zrn) values are indistinguishable from the unaltered MORB values and the restricted mantle-like δ18O range.Results of the present study suggest multiple arc-fore-arc accretion events and an extensive late Tonian back-arc basin opening event in the evolution of the Eastern Desert accretionary belt. Subduction tectonics continued to stabilize the immature arc crust ∼750–720 Ma and a tapered back-arc spreading ridge engendered at ∼700 Ma. The final terrane accretion and suturing could have been associated with subduction-related melts that circulated and locally metasomatized back-arc ophiolites. This model is in broad agreement with the recently advocated ∼720 Ma plate reorganization in the closure of the Mozambique Ocean during the course of Gondwana assembly.

South Tarim tied to north India on the periphery of Rodinia and Gondwana and implications for the evolution of two supercontinents

Abstract Constraining the positions of, and interrelationships between, Earth's major continental blocks has played a major role in validating the concept of the supercontinent cycle. Minor continental fragments can provide additional key constraints on modes of supercontinent assembly and dispersal. The Tarim craton has been placed both at the core of Rodinia or on its periphery, and differentiating between the two scenarios has widespread implications for the breakup of Rodinia and subsequent assembly of Gondwana. In the South Tarim terrane, detrital zircon grains from Neoproterozoic–Silurian strata display two dominant populations at 950–750 and 550–450 Ma. Similarly, two main peaks at 1000–800 and 600–490 Ma characterize Neoproterozoic–Ordovician strata in northern India. Moreover, the two dominant peaks of South Tarim and north India lag two global peaks at 1200–1000 and 650–500 Ma, which reflect Rodinia and Gondwana assembly, arguing against a position within the heart of the two supercontinents. Ages and Hf isotopes of Tarim's detrital zircons argue for a position on the margin of both supercontinents adjacent to north India with periodic dispersal through opening and closing of small ocean basins (e.g., the Proto-Tethys). Alternating tectonic transitions between advancing and retreating subduction in North Tarim coincide with periodic drift of South Tarim from north India in Rodinia and Gondwana, emphasizing the importance of retreating subduction in supercontinent dispersal. Moreover, the Rodinia-related orogenic belts spatially overlap the Gondwana-related orogenic belts in the two blocks, indicating no significant relative rotation of India and Tarim during the evolution from Rodinia to Gondwana.

Granitic rocks from Rwanda: Vital clues to the tectonic evolution of the Karagwe–Ankole Belt

Petrological, geochemical and geochronological studies of granitic rocks from the Karagwe–Ankole Belt (KAB) have helped to establish the tectonic setting of the orogenic belt. Four categories of granitic rocks have been recognized, providing constraints on the geodynamics and evolution of the KAB. Anorogenic (A-type) garnet-biotite granite was emplaced at 1372 ± 5 Ma along deep, weak crustal structures. Its major, trace and rare earth element composition, TDM model age of 2.33–1.96 Ga, high initial 87Sr/86Sr (0.71193–0.72159) and negative ƐNd (−1.9 to −4.0), indicates derivation from a much older mafic source, probably originated from lower-crustal granulitic residue or lower continental lithospheric mantle contaminated by crustal material. Ascent of the small-batch A-type magma was aided by a localized crustal-scale zone of weakness within Archaean crust underlying both the Eastern and Western Domains of the KAB. The rest of the granites have S-type orogenic characteristics. Large batholiths of two-mica granite emplaced at 1369 ± 5 Ma have major, trace and rare earth element compositions, high initial 87Sr/86Sr (0.70808–0.71795), strongly negative ƐNd (−9.4 to −10.1) and a TDM model age of 2.82–2.40 Ga, indicative of derivation by partial melting of a pelitic source containing Palaeoproterozoic and Archaean components, possibly with a minor contribution from magma of lower crustal derivation. This magmatism occurred during the amalgamation of the Congo and Tanzania cratons at an early stage of Rodinia formation. Muscovite granite and leucogranite, derived from fluid-fluxed partial melting of metapelites, were emplaced at 1011 ± 18 to 976 ± 11 Ma in a late to post-collisional setting during final Rodinia formation. Their high initial 87Sr/86Sr (0.72893–0.74456) and strongly negative ƐNd (−6.8 to −12.4) indicate a supracrustal source. One younger S-type leucogranite (614 ± 9 Ma) was emplaced in a volcanic-arc setting during the assembly of West Gondwana. The combined available evidence points to tectonic evolution of the Mesoproterozoic KAB during accretion-collisional tectonism.

Understanding pre- and syn-orogenic tectonic evolution in western Himalaya through age and petrogenesis of Palaeozoic and Cenozoic granites from upper structural levels of Bhagirathi Valley, NW India

AbstractThe Himalaya is characterized by the presence of both pre-Himalayan Palaeozoic and syn-Himalayan Cenozoic granitic bodies, which can help unravel the pre- to syn-collisional geodynamics of this orogen. In the Bhagirathi Valley of Western Himalaya, such granites and the Tethyan Himalayan Sequence (THS) hosting them are bound to the south by the top-to-the-N extensional Jhala Normal Fault (JNF) and low-grade metapelite of the THS to its north. The THS is intruded by a set of leucocratic dykes concordant to the JNF. Zircon U–Pb laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) geochronology of the THS and one leucocratic dyke reveals that the two rocks have a strikingly similar age distribution, with a common and most prominent age peak at ~1000 Ma. To the north of the THS lies Bhaironghati Granite, a Palaeozoic two-mica granite, which shows a crystallization age of 512.28 ± 1.58 Ma. Our geochemical analysis indicates that it is a product of pre-Himalayan Palaeozoic magmatism owing to extensional tectonics in a back-arc or rift setting following the assembly of Gondwana (500–530 Ma). The Cenozoic Gangotri Leucogranite lies to the north of Bhaironghati Granite, and U–Pb dating of zircon from this leucogranite gives a crystallization age of 21.73 ± 0.11 Ma. Our geochemical studies suggest that the Gangotri Leucogranite is a product of muscovite-dehydration melting of the lower crust owing to flexural bending in relation to steepening of the subducted Indian plate. The leucocratic dykes are highly refracted parts of the Gangotri Leucogranite that migrated and emplaced along extensional fault zones related to the JNF and scavenged zircon from the host THS during crystallization.

The Capiru Group: From passive margin to syn-orogenic units, implications for the evolution of the northern margin of the Curitiba microplate, Western Gondwana

The Capiru Group (Neoproterozoic) is a 4 km-thick succession deposited on the northern margin of the Curitiba microplate, Southern Ribeira Belt, Brazil. The Capiru Group successions are deformed within the Brasiliano - Pan African Ribeira Belt, which culminated in the amalgamation of Western Gondwana. Even though the rocks are metamorphosed and deformed, preserved sedimentary structures and outcrop exposures allowed the interpretation of depositional environments and stratigraphic analysis. The Capiru Group metasedimentary rocks were studied by field mapping, measured sections and bulk-rock chemical compositions. Six formations were identified, termed from oldest to youngest as: Santana (proximal to distal marine shelf), Juruqui (proximal to distal deltaic), Rio Branco (carbonate platform), Morro Azul (lagoonal to shallow marine), Morro Grande (proximal to distal deltaic/marine) and Bocaina formations (estuarine to shallow-marine). Two main basin stages were identified: a passive continental margin stage, with increasing tectonic quiescence followed by a marine regression, and a syn-orogenic stage at the Curitiba microplate margin.Chemical and mineralogical Fe- and Al-rich compositions suggest continental denudation during the passive continental margin stage, with low sediment input rates from a relatively low-gradient relief, highly weathered continental source, and stability since Mesoproterozoic time. Sedimentological and stratigraphic similarities were found among Capiru Group passive margin successions and units deposited on the Río de Plata and Southern Paranapanema margins.The paleoenvironmental interpretations, such as greenhouse conditions and high weathering rates alternated with depositional gaps, indicate climatic changes during the Capiru Group deposition, with far-field or Phantom Glacial influence related to the regional discordances. These features are important for the paleogeographic reconstructions that must consider a tropical or equatorial position ∼650 - 635 Ma for the Curitiba microplate.During deposition of the Bocaina Formation, the arkosic composition of sandstones and lithic conglomerate lags suggest source rejuvenation during the syn-orogenic stage, prior to the collisional thrusting that led to the Western Gondwana assembly.

Thermochronology in South America

The Capiru Group (Neoproterozoic) is a 4 km-thick succession deposited on the northern margin of the Curitiba microplate, Southern Ribeira Belt, Brazil. The Capiru Group successions are deformed within the Brasiliano - Pan African Ribeira Belt, which culminated in the amalgamation of Western Gondwana. Even though the rocks are metamorphosed and deformed, preserved sedimentary structures and outcrop exposures allowed the interpretation of depositional environments and stratigraphic analysis. The Capiru Group metasedimentary rocks were studied by field mapping, measured sections and bulk-rock chemical compositions. Six formations were identified, termed from oldest to youngest as: Santana (proximal to distal marine shelf), Juruqui (proximal to distal deltaic), Rio Branco (carbonate platform), Morro Azul (lagoonal to shallow marine), Morro Grande (proximal to distal deltaic/marine) and Bocaina formations (estuarine to shallow-marine). Two main basin stages were identified: a passive continental margin stage, with increasing tectonic quiescence followed by a marine regression, and a syn-orogenic stage at the Curitiba microplate margin.Chemical and mineralogical Fe- and Al-rich compositions suggest continental denudation during the passive continental margin stage, with low sediment input rates from a relatively low-gradient relief, highly weathered continental source, and stability since Mesoproterozoic time. Sedimentological and stratigraphic similarities were found among Capiru Group passive margin successions and units deposited on the Río de Plata and Southern Paranapanema margins.The paleoenvironmental interpretations, such as greenhouse conditions and high weathering rates alternated with depositional gaps, indicate climatic changes during the Capiru Group deposition, with far-field or Phantom Glacial influence related to the regional discordances. These features are important for the paleogeographic reconstructions that must consider a tropical or equatorial position ∼650 - 635 Ma for the Curitiba microplate.During deposition of the Bocaina Formation, the arkosic composition of sandstones and lithic conglomerate lags suggest source rejuvenation during the syn-orogenic stage, prior to the collisional thrusting that led to the Western Gondwana assembly.

Deciphering early Neoproterozoic and Cambrian high-grade metamorphic events in the Archean/Mesoproterozoic Rauer Group, East Antarctica

A U–Pb geochronological and rare earth element (REE) geochemical study of zircon, monazite and garnet was carried out on rocks of Mesoproterozoic and Archean crustal domains in the Rauer Group of East Antarctica. The zircon and monazite U–Pb age spectra define concordia intercepts mainly at ca 1200, 990–910, and 530–500 Ma, suggesting that the Mesoproterozoic crustal domain is a significant part of the Rayner Complex that also underwent early Neoproterozoic and Cambrian high-grade metamorphism. The age data, mineral inclusion assemblages in zircon, and REE features for zircon and garnet indicate that all the granulite facies mineral assemblages in this domain might have formed during early Neoproterozoic metamorphism. Some zircon and monazite grains or domains have experienced complete U–Pb isotopic resetting during Cambrian reworking, which did not result in new zircon and monazite growth. The Archean crustal domain consists mainly of Paleo–Mesoarchean orthogneisses interleaving with Neoproterozoic paragneisses that contain inherited metamorphic zircon domains with ages of ca 1330 and 970 Ma. The mineral assemblages in these gneisses formed during a single Cambrian granulite facies metamorphic event. Garnet-bearing and -free rocks cooled to solidus temperatures at ca 527 and 517 Ma, respectively, whereas the isotopic system of early-crystallized zircon was completely reset during the growth of new zircon. As such, all the zircon domains in the same sample could have the same concordant or weighted mean age. The 511 Ma monazite and 506 Ma zircon overgrowths in a paragneiss have REE contents in equilibrium with garnet, implying that later modification and isotopic resetting of zircon and monazite might have resulted in younger U–Th–Pb ages and, in this case, establishing the age–mineral assemblage relationship based on REE partition coefficients between zircon/monazite and garnet may be invalid. Overall, the available data support the notion that different crustal components of the Rauer Group were juxtaposed in the Cambrian as a consequence of the Gondwana assembly.

An extensive record of orogenesis recorded in a Madagascar granulite

AbstractWe present a comprehensive petrological and geochronological study of a single granulite sample from the lithosphere‐scale Beraketa shear zone in southern Madagascar to constrain the orogenic history of Gondwana assembly in this region. The studied sample provides a panoply of data constraining the prograde, retrograde, and late metasomatic history of the region via the application of Ti‐in‐quartz, Ti‐in‐zircon, Zr‐in‐rutile, and Al‐in‐orthopyroxene thermobarometry; phase‐equilibrium modelling; U–Pb monazite, zircon, and rutile petrochronology; and trace element diffusion chronometry in rutile. Our results reveal five stages of metamorphism along a narrow clockwise P–T path that may have begun as early as 620–600 Ma and certainly by 580–560 Ma, based on the oldest concordant zircon dates. The rock was heated to &gt;725°C at less than 7.5 kbar (Stage 1) before burial to ~8 kbar (Stage 2). By c. 540 Ma, the rock had heated to ~970°C at ~9 kbar, and lost approximately 12% melt (Stage 3), before decompressing and cooling to the solidus at ~860°C and 6.5 kbar within 10 Ma (Stage 4). The vast majority of monazite and zircon dates record Stage 4 cooling and exhumation. Monazite and zircon rim dates as young as c. 510 Ma record subsolidus cooling (Stage 5) and associated symplectite formation around garnet. U–Pb rutile dates record partial resetting at c. 460 Ma; Zr‐ and Nb‐in‐rutile diffusion chronometry link these dates to a metasomatic event that lasted &lt;1 Ma at ~600°C. In addition to chronicling a near‐complete cycle of metamorphism in southern Madagascar, this study constrains the rates of heating and cooling. We estimate that heating (7–14°C/Ma) outpaced reasonable radiogenic heating rates with modest mantle heat conduction. Therefore, we conclude that elevated mantle heat conduction or injection of mantle‐derived magmas likely contributed to regional ultrahigh‐temperature metamorphism (UHTM). Exhumation and cooling from peak metamorphic conditions to the solidus occurred at rates greater than 0.45 km/Ma and 14°C/Ma.

Petrochronological constraints and tectonic implications of Tonian metamorphism in the Embu Complex, Ribeira Belt, Brazil

The Embu Complex of the Ribeira Belt, southeast Brazil, provides records of orogenic processes within the time period between Rodinia break-up and West Gondwana assembly. Recent studies have questioned the accretionary setting for the belt, suggesting an intracontinental setting. In this paper, we present new petrological, U–Pb and Rb–Sr data from metasedimentary rocks of the Embu Complex, discussing the implications for the tectonic evolution from Rodinia to West Gondwana in the Ribeira Belt. Sillimanite, sillimanite–K-feldspar and cordierite zones represent the high-grade metamorphic zones for the metamorphic event identified in the southern portion of the Embu Complex. Petrological and thermodynamic modelling data indicate that the high-grade metamorphic rocks of the complex attained conditions between 750–805 °C and 3.6–9.2 kbar. Monazite and zircon U–Pb data indicate that the high-grade metamorphic event occurred between 810–760 Ma, concomitant with igneous activity recorded during this period. In addition, zircon and apatite U–Pb ages, as well as, biotite and muscovite Rb–Sr ages, between 675–550 Ma record a slow cooling rate of 4.4 °C/Ma for the period between 675–640 Ma, and a late stage of shear zones development. These results reinforce the accretion of distinct units within the Ribeira Belt during the Neoproterozoic Era.

Mafic intrusions in southwestern Australia related to supercontinent assembly or breakup?

Variably oriented dolerite intrusions outcrop in the Albany–Fraser Orogen along the south coast of Western Australia with previously unknown ages but where previous studies interpreted Mesoproterozoic to Cretaceous emplacement. Here, we place temporal constraints on seven mafic intrusions across ∼150 km of coast using zircon U–Pb, apatite U–Pb, and plagioclase 40Ar/39Ar geochronology, coupled with whole-rock major and trace-element geochemistry, that reveal late Mesoproterozoic to potentially Early Cretaceous crystallisation ages. Three intrusions metamorphosed to greenschist facies are likely associated with either the emplacement of the ca 1210 Ma Marnda Moorn large igneous province or Stage II Albany–Fraser Orogeny, both of which were associated with the assembly of Rodinia. Three unmetamorphosed dykes have (probable) Neoproterozoic to lower Cambrian emplacement ages, likely associated with the ca 550–500 Ma Kuunga Orogeny during Gondwana assembly. The final sill, also unmetamorphosed, strikes perpendicular to the other six intrusions, shows unusual Pb anomalies and contains inherited zircon that has been reset by a Permian or younger event, pointing towards magmatism in southwestern Australia during the breakup of Gondwana. The new results provide hitherto unrecognised mafic intrusive evidence for modification of Proterozoic crust, potentially associated with Rodinia assembly, Gondwana assembly and Gondwana breakup in southwestern Australia. KEY POINTS Variably oriented mafic dykes in southwest Australia are dated by zircon U–Pb, apatite U–Pb and plagioclase 40Ar/39Ar methods. The dykes are related to Rodinia assembly (ca 1200 Ma), Gondwana assembly (ca 550 Ma) and, probably, Gondwana breakup (ca 135 Ma). These new ages provide evidence for mafic activity clearly linked to the supercontinent cycle.

Neoproterozoic high magmatic addition rate episode building a composite batholith in northeastern Brazil, and implications for the western Gondwana assembly

The composite Santana do Ipanema batholith is made up of several Cryogenian-Ediacaran granitic plutons in the Pernambuco-Alagoas (PEAL) Domain of the Borborema province, northeastern Brazil. The individual plutons in this batholith are high-K calc-alkalic to shoshonitic, metaluminous to peraluminous quartz syenites to granites that belong to magnetite-series and magnesian Cordilleran-type granitoids. They carry comagmatic microgranular mafic enclaves with evidence of magma mixing. Their chemical compositions overlap, and they exhibit enrichment in some incompatible and depletion in HFS elements, with variable initial 87Sr/86Sr ratios that range from 0.70511 to 0.71071. The εNd(0.6Ga) values cluster into two groups, in which plutons with ages ≥ 625 Ma exhibit slightly negative to slightly positive εNd values (average −1.6), in contrast with the younger plutons that have more negative ones (average −8.7), pointing out that mantle contribution to the source decreased with time, while recycled ancient crust augmented. Geochronological data from the literature indicate that the batholith has been constructed by continuous amalgamations of successive magma pulses via diking for ~30 m.y., with no large crystallization age gap among the plutons. A magma supply rate of 0.003 m3/year is estimated considering this batholith together with two other nearby composite batholiths of similar age in the PEAL Domain, and low volcanic-plutonic ratio. This rather high magmatic addition rate episode has been probably triggered by extensive and repeated basaltic underplating stalled in the lower crust in the magmatic arc formed during the convergence of the São Francisco craton and the Pernambuco-Alagoas block in the course of the western Gondwana amalgamation.

Disclosing Rhyacian/Orosirian orogenic magmatism within the Guanhães basement inlier, Araçuaí Orogen, Brazil: A new piece on the assembly of the São Francisco-Congo paleocontinent

The Guanhães block is one of the basement units present in the context of the Neoproterozoic West Gondwana assembly. It was reworked as a basement inlier within the Araçuaí orogen, in the margins of the São Francisco-Congo (SF-C) paleocontinent. The Guanhães block’s Rhyacian-Orosirian record is revealed for the first time by the Felício and the Mercês granitic plutons. Geophysical, petrographic, lithogeochemical and U-Pb (LA-ICPMS) analyses define and characterize these plutons, and link their genesis to the major Paleoproterozoic orogenic event that is thought to have amalgamated the SF-C paleocontinent. The Felício pluton is composed of mesocratic fine-grained epidote-biotite-granodiorite, the Mercês pluton comprises biotite-monzogranite and hornblende-syenogranite. These two plutons show different geochemical features; the Felício rocks are low-K, magnesian, calcic to calc-alkalic, metaluminous, and the Mercês rocks are rich in K, ferroan, alkali-calcic to alkalic, peraluminous, their shared common features are enrichment in LILE, LREE and HFSE and negative anomalies of Ti, Ta, Nb and P. Zircons collected from the Felício pluton granodiorite yielded a U-Pb concordia age of 2151 ± 12 Ma, interpreted as the pluton’s crystallization age. The Mercês pluton monzogranite yielded a discordia with upper intercept at 2014 ± 11 Ma and lower intercept at 539 ± 13 Ma, interpreted as the Mercês pluton’s cristallization age and the Brasiliano metamorphic overprint, respectively. Trace elements and REE signatures suggest that the magma that originated both plutons shares a common source, compatible with the metasomatization of a mantle wedge by dehydration of a subducted slab. Based on the petrographic, geochemical and geochronological data, the following tectonic scenario is suggested: subduction and accretion of the Mantiqueira arc and emplacement of the Felício pluton at c. 2.15 Ga, followed by collision, gravitational collapse of the orogen, post-collisional magmatism and emplacement of the Mercês pluton at c. 2.01 Ga. This scenario is reinforced by the regional geochemical correlations among the many coeval terranes in the eastern margin of the São Francisco craton, along the Minas-Bahia orogen. At a broader scale, these correlations can be expanded to the African counterpart, giving insights into the SF-C paleocontinent framework in the Precambrian.

Evidence for Neoproterozoic terrane accretion in the central Borborema Province, West Gondwana deduced by isotopic and geophysical data compilation

ABSTRACT The Borborema Province in NE Brazil is part of a large Neoproterozoic orogenic system related to the assembly of West Gondwana. Its evolution is contentious, due to the superposition of events that overprinted former records. We present a compilation of geological, isotopic (U-Pb detrital zircon and whole-rock Sm-Nd), and geophysical data that support a terrane accretion model for the Transversal subprovince (central Borborema Province). Bounded by thrust- and strike-slip shear zones of regional extent, the terranes vary in nature and age, and magmatic-arc related rocks, retro-eclogites, and dismembered ophiolites provide evidence of accretion. In addition, detrital zircon compilation suggests variable sources, including Palaeoproterozoic detritus related to the Rio Capibaribe and Alto Moxotó terranes and Neoproterozoic sources attributed to the Alto Pajeú and Piancó-Alto Brígida blocks. Accordingly, Archaean and Palaeoproterozoic Nd TDM model ages are concentrated within the Alto Moxotó and Rio Capibaribe terranes, whereas early- to late Mesoproterozoic isotopic sources dominate in the Alto Pajeú and Piancó-Alto Brígida. A compilation of robust geophysical data shows contrasts in magnetic, gamma-spectrometric, and gravity signatures between the terranes, whereas modelled depths and density values are variable, coinciding with the proposed terrane boundaries. Our analysis suggests that now eroded oceanic lithospheric branches existed within the Transversal subprovince, later converted into dry suture zones. Terrane accretion is a plausible mechanism to rearrange the crustal pieces of West Gondwana, including those of the Borborema Province, and should be considered in other orogenic belts of South America.

Mariana-type ophiolites constrain the establishment of modern plate tectonic regime during Gondwana assembly

Initiation of Mariana-type oceanic subduction zones requires rheologically strong oceanic lithosphere, which developed through secular cooling of Earth’s mantle. Here, we report a 518 Ma Mariana-type subduction initiation ophiolite from northern Tibet, which, along with compilation of similar ophiolites through Earth history, argues for the establishment of the modern plate tectonic regime by the early Cambrian. The ophiolite was formed during the subduction initiation of the Proto-Tethys Ocean that coincided with slab roll-back along the southern and western Gondwana margins at ca. 530-520 Ma. This global tectonic re-organization and the establishment of modern plate tectonic regime was likely controlled by secular cooling of the Earth, and facilitated by enhanced lubrication of subduction zones by sediments derived from widespread surface erosion of the extensive mountain ranges formed during Gondwana assembly. This time also corresponds to extreme events recorded in climate and surface proxies that herald formation of the contemporary Earth.

Banded Charnockite: The Result of Crustal Magma Generation, Piecemeal Emplacement, and Fluid-Driven Mineral Replacement in High-Grade Crust (Central Dronning Maud Land, Antarctica)

Our study of a banded charnockite complex of the Mühlig-Hofmannfjella in Dronning Maud Land, Antarctica, illustrates how the combination of high-temperature (re-)crystallization processes, melts, and volatile fluids leads to complex intrusive, metasomatic, and structural relationships. The igneous complex consists of gently dipping sets of charnockite interlayered with dolerite and leucogranite. The banded complex formed primarily by magmatic processes, but with superimposed modifications by metasomatism. The charnockite has a ferroan composition and contains both orthopyroxene (Fs80–84) and olivine (Fa94–96). Zircon U-Pb dates the emplacement of charnockite at 515 Ma, and inherited zircon cores and negative εNd values of −3 to −5 indicate that the age of the source of the magma was about 1100 Ma. Neodymium isotopes were not homogenized during the Cambrian magmatic event, which suggests that the generation and emplacement of the magma took place in separate batches during construction of the banded complex. By contrast, the Rb-Sr system in the charnockite was extensively homogenized, likely because of the superimposed late-magmatic fluid activity, which also produced the bands and networks of leucogranites. These events occurred during the late stages of the assembly of Gondwana, with postcollisional extension and mantle upwelling maintaining a high heat flow.

Paleoproterozoic emplacement and Cambrian ultrahigh-temperature metamorphism of a layered magmatic intrusion from the Central Madurai Block, southern India: From Columbia to Gondwana

The Madurai Block in the Southern Granulite Terrane (SGT) of Peninsular India is one of the largest crustal blocks within the Neoproterozoic Gondwana assembly. This block is composed of three sub-blocks: the Neoarchean Northern Madurai block, Paleoproterozoic Central Madurai block and the dominantly Neoproterozoic Southern Madurai Block. The margins of these blocks are well-known for the occurrence of ultrahigh-temperature (UHT) granulite facies rocks mostly represented by Mg-Al metasediments. Here we report a dismembered layered mafic–ultramafic intrusion occurring in association with Mg-Al granulites from the classic locality of Ganguvarpatti in the Central Madurai Block. The major rock types of the layered intrusion include spinel orthopyroxenite, garnet-bearing gabbro, gabbro and gabbroic anorthosite showing rhythmic stratification and cumulate texture. The orthopyroxene-cordierite granulite from the associated Mg-Al layer is composed of spinel, cordierite and orthopyroxene. The pyroxene in both rock units is high-Al orthopyroxene formed under UHT metamorphic conditions. Conventional thermobarometry yields near-peak metamorphic conditions of 9.5–10 kbar pressure and a minimum temperature of 980 °C. We computed P–T pseudosections and contoured for the compositional as well as modal isopleths of the major mineral phases, which yield temperature above 1000 °C. FMAS petrogenetic grid, Al-in-orthopyroxene isopleth, conventional thermobarometry and calculated pseudosection reveal a clockwise pressure–temperature (P–T) path and near isothermal decompression. The U–Pb data on zircon grains from the layered magmatic suite indicate emplacement of the protolith at ca. 2.0 Ga and the metamorphic overgrowths yield weighted 206Pb/238U mean ages ca. 520 Ma. Monazite from the garnet-bearing gabbro and Opx-Crd granulite yielded 206Pb/238U weighted mean ages of ca. 532 Ma and 523 Ma marking the timing of metamorphism. We correlate the layered intrusion to a Paleoproterozoic suprasubduction zone setting, defining the Ganguvarpatti area as part of a collisional suture assembling the Northern and Central Madurai Blocks. The Paleoproterozoic magmatism and late Neoproterozoic-Cambrian UHT metamorphism can be linked to the tectonics of the Columbia and Gondwana supercontinents.

Combined use of Sm–Nd isotopes and lithogeochemistry in the sedimentary provenance of the southern Ediacaran-Cambrian Bambuí foreland basin system, Brazil

Sedimentary provenance analysis based on lithogeochemistry and isotope data has been extensively applied in the investigation of ancient sedimentary successions. This approach contributes to understand the tectonic setting in which these strata were deposited and to recognize major provenance patterns in time and space. The Ediacaran to Cambrian Bambuí Group is exposed in the São Francisco craton (SE Brazil) and represents the sedimentary record of a foreland basin system developed during the assembly of the West Gondwana. Different lines of evidence indicate that this basin system evolved into a confined setting, due to the overloads imposed on the western and eastern margins of the São Francisco paleoplate by the diachronic Brasília and Araçuaí orogenic systems, respectively. New whole-rock lithogeochemistry and Sm–Nd isotopic data were applied to evaluate the overall sedimentary provenance patterns of the Bambuí strata exposed in the southern portion of the São Francisco craton (i.e., within the Sete Lagoas basement high) and to identify its possible sources. Coupled (La/Yb)n and Th/Sc ratios with 147Sm/144Nd and εNd isotopes allowed the recognition of three contrasting source patterns associated with different 2nd order sequences. The lower sequence comprises carbonate ramp deposits and shows highly heterogeneous and more negative εNd ranging from −10.5 to −4.5, as well as Th/Sc and (La/Yb)n ratios between 0.6 and 0.7 and 0.54 and 0.76, respectively. The provenance patterns point toward intermediate-dominated and isotopically evolved crustal sources, with mean TDM model ages between 1.5 and 2.0 Ga. The overlying 2nd-order sequence is composed of fine-to medium-grained siliciclastics grading upward into platformal oolitic calcarenites and calcilutites. This succession exhibits homogeneous and less negative εNd(t) values between −6.8 and −5.2, with low Th/Sc (0.6–0.8) and (La/Yb)n (0.60–0.84) ratios. The uppermost Bambuí 2nd-order sequence, marked by greenish siltstones and arkoses grading upward into storm-bedded arkoses and lithic sandstones, shows homogeneous εNd(t) ranging from −8.5 to −6.1, slightly more negative than the underlying successions, and relatively higher Th/Sc and (La/Yb)n ratios ranging from 0.7 to 1.3 and 0.85 to 1.28, respectively. The Sm–Nd isotopic signature of the upper deposits indicates felsic-dominated and more isotopically juvenile crustal sources with TDM model ages of 1.5–1.7 Ga, characteristics compatible with Ediacaran arc-related rocks and post-tectonic granites exposed in the Araçuaí orogen. Our data indicate that the Proterozoic cratonic covers, their correlatives in the surrounding orogens, and magmatic rocks from the Araçuaí orogen acted as major sedimentary sources during the first evolutionary stages of the Bambuí basin-cycle in the Sete Lagoas basement high, which behaved as a forebulge depocenter. During the deposition of the overlying sequences, the Araçuaí orogen rocks became increasingly important in sourcing the Ediacaran- Cambrian foreland system in this depocenter. Besides revealing the major provenance patterns through the evolution of an ancient and partially confined foreland basin system, our data also demonstrates that even minor changes in the Sm–Nd system might be useful as reliable elements on the provenance analysis of sedimentary systems elsewhere.