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© 2020 Closure of Neo-Tethys between the Late Cretaceous and Miocene caused obduction of intra-Neo-Tethys volcanic arc allochthons onto the northern margin of the Arabian-Nubian Shield. This loading led to downwarping of the shield forming a flexural basin in which the Tanjero Formation and later Red Beds accumulated. Analysis of the detrital zircon populations in these sedimentary units provides additional evidence supporting the petrographic data for the presence and unroofing of the allochthons. Both the Tanjero Formation and Red Beds show evidence of initial derivation of detritus from the Nubian-Arabian Shield together with minor Paleozoic detritus from Egypt, Turkey and/or Iran. The youngest detrital zircon grains in the Tanjero Formation are of Paleocene age and probably represent northern derivation from the Sanandaj-Sirjan Zone. In contrast, the upper Miocene Red Beds contain detrital zircon of Eocene age that confirms the petrographic evidence for derivation from, and hence emplacement of, the lower allochthon of intra-Neo-Tethys Walash-Naopurdan rocks.

© 2020 Elsevier B.V. Greenland's Itsaq Gneiss Complex (IGC) shows Eoarchean (>3600 Ma) 250–400 °C/GPa (low T/P – high pressure) and ≥1000 °C/GPa (high T/P) metamorphic regimes, demonstrating a similarity of contrasting metamorphic T/P regimes from the Phanerozoic back to the start of Earth's rock record. Low T/P metamorphism produced: (i) Deep crustal eclogitised mafic rocks which upon partial melting formed the tonalites dominating the IGC; (ii) ~550 °C ≥ 2.6 GPa conditions (≤250 °C/GPa) demonstrated by an olivine + antigorite + titano-chondrodite/titano-clinohumite relict assemblage within mantle slivers showing geochemical and crystallographic features of a suprasubduction environment, that were exhumed into the crust by 3712 Ma; (iii) rare vestiges of 3658 Ma high-pressure (garnet + clinopyroxene) granulite; and (iv) Barrovian-style kyanite + staurolite assemblages. High T/P metamorphism is shown by 3669 Ma crustal melts equilibrated with orthopyroxene. This was coeval to the youngest juvenile tonalitic crust in the complex (latter derived by anatexis under low T/P conditions), and a 3670–3570 Ma history of deep crust migmatisation under low pressure, garnet-free conditions. Structural geology of the IGC indicates its low T/P regimes coincide with crustal imbrication by compression of arc-like tholeiites, boninite-like lavas, andesites, felsic-intermediate volcano-sedimentary rocks and chemical sedimentary rocks, whereas post-3660 Ma high T/P metamorphism was marked by late-orogenic extension/exhumation and deep crustal flow with mafic underplating and partial melting generating granites. Thus the diversity of Earth's earliest-recorded geodynamic settings resembles more those of modern geodynamics, than the lithological and structural relationships expected from theoretical non-uniformitarian scenarios like drip tectonics in a stagnant lid regime. The recognition of an ultra-high-pressure ≤250 °C/GPa metamorphic regime at >3700 Ma in the IGC removes the last arg

2020 International Association for Gondwana Research The Ordovician intra-oceanic Macquarie Arc of eastern Australia collided with the eastern Gondwanan margin at ~440 Ma. However, the deep crustal architecture resulting from this assembly is poorly known. This is addressed here by a zircon U-Pb-Hf study of the post-assembly Silurian Browns Creek Intrusive Complex and Davies Creek Granite dykes that intrude into the arc, and not adjacent Gondwanan sedimentary sequences. Zircon U[sbnd]Pb dating integrated with CL imagery indicate two igneous phases at 430-437 Ma and 420-426 Ma and a zircon recrystallisation phase at 395-396 Ma attributed to a late thermal event. The magmatic zircon initial ɛHf values vary from −5.1 to +4.7. This signature indicates the source of these granitic rocks is strongly influenced by typical pre-Silurian Gondwanan material. Granitic rock and zircon compositions demonstrate that at the likely temperature of the Silurian granitic magma, especially the Davies Creek Granite dykes, inherited source zircons were mostly dissolved, explaining the absence of pre-Ordovician xenocrysts within the zircon population. The unradiogenic Hf isotopic signatures preserved in the Silurian magmatic zircons demonstrate the contribution of Gondwanan crustal material to the magma source region. These results support the interpretation of the Macquarie Arc as an intra-Panthalassa ocean allochthon, emplaced and resting over the edge of Gondwanan crystalline basement, possibly including the continent-derived sedimentary rocks of the Adaminaby Group.

Prior to 1970 grey gneiss complexes were interpreted as partially-melted sedimentary sequences. Once it was recognised from the Nuuk region that they comprised calc-alkaline igneous complexes, it was understood that such complexes world-wide were dominated by TTG (trondhjemite-tonalite-granodiorite) initially found to have juvenile Sr, Nd and, subsequently, Hf isotopic signatures. Between 1970 and 1985 the Nuuk region gneiss complex was interpreted by the non-uniformitarian 'super-event' model of crust formation which proposed occasional but extensive crust formation, with craton-wide correlation of granulite facies metamorphism and deformational phases. The igneous rocks formed in a late- Meso- to early Neoarchaean super-event engulfed crust formed in an Eoarchaean super-event. Mapping and reinterpretation at Færingehavn showed there are three TTG gneiss domains, each with different early accretionary, metamorphic and tectonic histories, separated by folded meta-mylonites. This established the key feature of the tectono-stratigraphic terrane model; that each terrane has an early intra-terrane history of crust formation, deformation and metamorphism, upon which is superimposed a later deformation and metamorphic history common to several terranes after they were juxtaposed. Remapping and >250 U-Pb zircon age determinations have refined the geological evolution of the entire Nuuk region, and has confirmed at least four main crust formation events and two collisional orogenies with associated transient high pressure metamorphism within clockwise P-T-t loops. Via independent corroborative studies the tectono-stratigraphic terrane model has been accepted for the Nuuk region and, through the discovery of similar relations across other gneiss complexes, its mode of evolution is found to be applicable to Archaean high-grade gneiss complexes worldwide. The TTG and mafic components that dominate each terrane have geochemistry interpreted to indicate subduction-related magmat

The Ordovician intraoceanic Macquarie Arc terrane is faulted against coeval, quartz-rich turbidites of the Adaminaby Group within the Lachlan Orogen of eastern Australia. Debates exist concerning the polarity of subduction beneath the Macquarie Arc and the nature of its emplacement, given it is juxtaposed against the Adaminaby Group to both the west and east. We present new provenance and zircon analyses of the Triangle Formation, which consists of interleaved quartz-rich passive margin sandstones and island arc volcaniclastic rocks. In contrast, the structurally underlying Adaminaby Group contains no volcaniclastic detritus and displays a strong passive margin affinity. One sample from the Triangle Formation yielded a youngest zircon age of 456 ± 16 Ma indicating a subtle Macquarie Arc signature among an overwhelmingly Neoproterozoic and older Gondwanan provenance. The Adaminaby Group yielded a youngest zircon age of 481 ± 6 Ma and a strong Gondwanan zircon signature. We compared these results with volcaniclastic rocks from the Weemalla Formation stratigraphically higher in the Macquarie Arc, which yielded a distinctly unimodal zircon age of 451 ± 8 Ma, which is indistinguishable from the youngest zircon in the Triangle Formation. We suggest the Triangle Formation represents trench fill material sourced predominantly from the Gondwana margin but including some younger Macquarie Arc detritus. This constrains the initiation of this arc-continent collision to between 448 and 462 Ma (Late Ordovician).

Serpentine in modern seafloor and ophiolitic environments incorporates and often retains high concentrations of atmospheric noble gases and seawater-derived halogens. Ancient serpentinites therefore provide the potential to trace the composition of early surface environments. Antigorite-serpentinites locally carbonated to talc-magnesite schist outcropping in a low strain zone within the Eoarchean Isua supracrustal belt (Greenland) are investigated here, to test the retention of paleo-atmospheric noble gases and Eoarchean seawater halogens, and to further determine the genetic setting and metamorphic history of some of Earth's oldest serpentinites. Based on field relationships, whole rock major and trace element geochemistry, and mineral chemistry, the investigated serpentinites are shown to represent hydrated and variously carbonated magmatic olivine ± orthopyroxene + Cr-spinel cumulates emplaced at the base of a lava flow of boninitic affinity pillowed in its upper portion. In addition, rare zircons extracted from one of the serpentinised cumulates have distinct magmatic trace element signatures and a U-Pb age of 3721 ± 27 Ma indicating the pillowed lava flow erupted on the Eoarchean seafloor. The serpentinites have high concentrations of noble gases, but the presence of parentless radiogenic 'excess' 40Ar, introduced by crustal-derived metamorphic fluids, obscures the 40Ar/36Ar ratio of Eoarchean seawater. Local carbonation of the serpentinites also caused halogen loss and fractionation. However, the least carbonated antigorite serpentinites preserve Br/Cl and I/Cl ratios within the range of modern seafloor serpentinites, which is interpreted as indicating Archaean serpentinising fluids were similar in composition to modern seawater-derived fluids. Importantly, the lowest measured I/Cl ratio of 29 (±2) x 10−6, taken as a maximum value for the Eoarchean ocean, is an order of magnitude lower than estimates for the primitive mantle I/Cl value. Iodine has a low concen

The Eoarchean (>3600 Ma, or millions of years ago) folded and metamorphosed Isua supracrustal belt and the adjacent orthogneiss exposures of Greenland contain rare low deformation lenses that display some uniquely-preserved components of Earth's oldest rock record. These include world's oldest (but contested) stromatolites in dolomitic carbonates, conglomerates, pillow basalts demonstrating submarine eruption, slivers of upper mantle rocks, formation of earliest continental crust by multistage tonalite + diorite emplacement followed by intracrustal granite production. All these diverse occurrences are keys to establish early Earth's processes at the start of the geological record. Although some of these features are preserved at several localities, other critical ones are exposed on only a few m2 of rock at single localities or are of historical significance. None of these sites are currently protected, and there is a reliance on responsible sampling to keep them intact for future generations. Given the high interest in the Archean Eon, combined with the increased ease of fieldwork in remote localities, many significant 'Deep Time' localities in not only Greenland but worldwide are in danger of eradication. Here, five key Isua area geological sites are presented, with an explanation of their significance and worthiness for initially reliance on already-collected samples, but hopefully ultimately government protection. This highlights an increasing problem of destruction of in situ evidence of Earth's unique early geological heritage and the need for collaboration in protecting and archiving of these key scientific resources.

The Dras Arc in NW India Himalaya is a belt of basaltic-andesites intercalated with arkose-dominated volcaniclastic rocks of the Nindam Formation situated along the Indus Suture between India and Eurasia. Debates exist as to whether these rocks developed in a forearc basin to the Eurasian margin or as part of an intraoceanic island arc system that collided with either India or Eurasia before final continental collision. Detrital zircons from the Nindam Formation yield U-Pb age spectra with dominant youngest age populations of ~84-125 Ma, corresponding with arc magmatism. Sandstone provenance analysis from the Nindam Formation indicates that the Dras Arc evolved from an undissected arc to dissected arc over a period of ~41 Myr. Slightly older, smaller populations occur at ~135-185 Ma, corresponding with reported ages of Neotethyan ophiolites (e.g., Spongtang). The basal section of the Nindam Formation reveals the presence of arc-derived basaltic-andesite and tonalite clasts, plus ophiolitic components sourced from an adjacent accretionary complex. There is a distinct absence of quartz or felsic granitic clasts, suggesting that the Nindam Formation did not develop as a forearc basin to the Ladakh Batholith of southern Eurasia but rather as separate intraoceanic island arc. A distinct "Gondwanan" signature occurs in all samples, with zircon age peaks at ~514-988, ~1000-1588, ~1627-2444, and ~2500 Ma. We suggest that the Dras and Spong arcs are the same intraoceanic island arc system that developed as a result of subduction initiation along NNE-SSW transform faults perpendicular to the Indian and Eurasia continents.

The world's largest ongoing collisional orogeny is the Europe Alps-Himalayan-SE Asian belt and is a natural laboratory to understand many processes that have shaped the continents. Due to political instability and conflict throughout this millennium, the Iraq (Kurdish) sector of the Zagros mountain chain is the least studied part of this orogenic system. In Iraq, the Zagros contains the suture between the Arabian subcontinent to the south and west and the Iranian edge of the Eurasian continent to the north and east. The suture zone is marked by several allochthons of Neotethyan ophiolitic and volcanic arc assemblages that were obducted onto the Arabian margin. New geochronological data, including SHRIMP U-Pb zircon, integrated with whole rock geochemistry, indicates that both Cretaceous (˜96 Ma) and Cenozoic (˜40 Ma) assemblages are present. The relationships between these units are complicated, thus some Cretaceous arc rocks were intruded by Cenozoic arc rocks, and out-of-sequence thrusting has interleaved and juxtaposed assemblages of different ages. Ongoing wrench faulting since continental collision at ˜14 Ma has further complicated the pattern of lithotectonic units, particularly those that were obducted out of the Neotethyan realm. The new data indicate that the Iraqi sector of Neotethys was not 'quiet' in the Cretaceous, but contains fragments of arcs of that age, contiguous with those along strike in Turkey, Iran and the Himalayas.

The ∼3700 Ma and 3800 Ma meta-volcanic and -sedimentary rocks in the Isua supracrustal belt (Greenland) were affected by heterogeneous ductile deformation under amphibolite facies conditions (∼500-650 °C), and variably modified by secondary silica and carbonate mineralisation deposited from diagenetic and metasomatic fluids. Rare low-deformation areas preserve original volcanic features - submarine basaltic pillows and sedimentary features - including bedding. These are best-preserved in two dimensions on flat- to moderately-inclined outcrop surfaces, but invariably are tectonically-stretched along a steeply-plunging third dimension, through stretching in the direction of fold axes; a style of deformation found throughout Earth's history. There is a debate about whether rare relicts of ∼3700 Ma stromatolites preserved in metadolomites that formed in a shallow marine setting (Nutman et al., 2016) represent bona fide biogenic primary structures fortuitously preserved in low deformation, or whether these structures are manifestations of deformation combined with non-biogenic deposition of secondary carbonate (Allwood et al., 2018). Here, we critically test the primary nature of the sedimentary rocks hosting the proposed stromatolites and also the veracity of the proposed stromatolites, by addressing the following questions: (i) Are the rocks an in situ outcrop of known age, or displaced blocks of unknown age or origin?; (ii) How much of the carbonate is of an originally sedimentary versus a secondary (i.e., metasomatic - introduced) origin?; (iii) Is the seawater-like REE + Y (rare earth element and yttrium) trace element signature carried definitely by carbonate minerals and therefore diagnostic of a cool, surficial sedimentary system?; (iv) Are the proposed stromatolites consistent with biogenicity in terms of their geometry and fine-scale layering, or could they be the product of soft sediment or structural deformation (compression in folding)? The answers to these

The Manning Group is characterised by rapidly filled strike-slip basins that developed during the early Permian along the Peel--Manning Fault System in the southern New England Orogen. Typically, the Manning Group has been difficult to date owing to the lack of fossiliferous units or igneous rocks. Thus, the timing of transition from an accretionary convergent margin in the late Carboniferous to dominantly strike-slip tectonic regimes that involved development and emplacement of the Great Serpentinite Belt (Weraerai terrane) is not well constrained. One exception are rhyolites of the Ramleh Volcanics that were erupted into the Echo Hills Formation. These developed along the dextral Monkey Creek Fault splay east of the Peel--Manning Fault System. Zircons extracted from the Ramleh Volcanics yield a U-Pb (SHRIMP) age of 295.6 ± 4.6 Ma that constrains the minimum age of deposition in this basin to earliest Permian. Whole-rock geochemistry indicates these are peraluminous felsic melts enriched in LREE and incompatible elements with strong depletions in U, Nb, Sr and Ti. These are similar in age and composition to the nearby S-type Bundarra and Hillgrove plutonic supersuites. We suggest that extensive movement along the east-dipping Peel--Manning Fault System was responsible, not only for strike-slip basin development at the surface (Manning Group), but was also the locus for crustal melting that was responsible for generating S-type felsic melts that utilised hanging-wall fault splays as conduits to the surface or to coalesce in the crust as batholiths exclusively to the east of the Peel--Manning Fault System.

Ash fall layers and vitroclastic-carrying sediments distributed throughout the entire Permian stratigraphic range of the Paraná Basin (Brazil and Uruguay) occur in the Tubarão Supergroup (Rio Bonito Formation) and the Passa Dois Group (Irati, Estrada Nova/Teresina, Corumbataí, and Rio do Rasto Formations), which constitute the Gondwana 1 Super-sequence. U-Pb zircon ages, acquired by SHRIMP and isotope-dissolution thermal ionization mass spectrometer (ID-TIMS) from tuffs within the Mangrullo and Yaguari Formations of Uruguay, are compatible with a correlation with the Irati and parts of the Teresina and Rio do Rasto Formations, respectively, of Brazil. U-Pb zircon ages suggest maximum depositional ages for the samples: (1) Rio Bonito Formation: ages ranging from 295:8 5 3:1 to 304:0 5 5:6 Ma (Asselian, lowermost Permian), consistent with the age range of the Protohaploxypinus goraiensis subzone; (2) Irati Formation: ages ranging from 279:9 5 4:8 to 280:0 5 3:0 Ma (Artinskian, middle Permian), consistent with the occurrence of species of the Lueckisporites virkkiae zone; (3) Rio do Rasto Formation: ages ranging from 266:7 5 5:4 to 274:6 5 6:3 Ma (Wordian to Roadian, middle Permian). All the SHRIMP U-Pb zircon ages are consistent with their superimposition order in the stratigraphy, the latest revisions to the Permian timescale (International Commission of Stratigraphy, 2018 version), and the most recent appraisals of biostratigraphic data. The ID-TIMS U-Pb zircon ages from the Corumbataí Formation suggest that U-Pb ages may be 110% younger than interpreted biostratigraphic ages.

The Gubaoquan eclogite occurs in the Paleozoic Beishan Orogen of NW China. Previously it has been interpreted as a fragment of subducted oceanic crust that was emplaced as a mélange within continental rocks. Contrary to this, we demonstrate that the Gubaoquan eclogite protolith was a Neoproterozoic basic dyke/sill which intruded into Proterozoic continental rocks. The SHRIMP U-Pb zircon dating of the metamorphic rims of the Gubaoquan eclogite yields an age 466 ± 27 Ma. Subdued heavy rare earth element abundances and lack of negative Eu anomalies of the metamorphic zircon domains confirm that this age represents eclogite facies metamorphism. The host augen orthogneiss has a U-Pb zircon age of 920 ± 14 Ma, representing the timing of crystallization of the granitic protolith. A leucogranitic vein which intrudes the eclogite has a U-Pb zircon age of 424 ± 8.6 Ma. This granitic vein marks the end of high-grade metamorphism in this area. The overcomplication of tectonic history of the Beishan Orogen is partially caused by inconsistent classifications and nomenclature of the same rock units and arbitrary subdivisions of Precambrian blocks as individual microcontinents. In an attempt to resolve this, we propose a simpler model that involves the partial subduction of the northern passive margin of the Dunhuang Block beneath the active continental margin developing on the Mazongshan-Hanshan Block to the north. Ocean closure and continental collision during the Late Ordovician resulted in continental thickening and eclogite facies metamorphism recorded by the mafic dykes/sills (now the Gubaoquan eclogite). In the light of the new data, the tectonothermal evolution of the Beishan Orogen is reviewed and integrated with the evolution of the Central Asian Orogenic Belt.

The Pushtashan suprasubduction zone assemblage of volcanic rocks, gabbros, norites and peridotites occurs in the Zagros suture zone, Kurdistan region, northeastern Iraq. Volcanic rocks are dominant in the assemblage and consist mainly of basalt and basaltic andesite flows with interlayered red shale and limestone horizons. Earlier lavas tend to be MORB-like, whereas later lavas display island arc tholeiite to boninitic geochemical characteristics. Tholeiitic gabbros intrude the norites and display fractionation trends typical of crystallisation under low-pressure conditions, whereas the norites display calc-alkaline traits, suggesting their source included mantle metasomatised by fluids released from subducted oceanic crust. Enrichment of Rb, Ba, Sr, Th and the presence of negative Nb anomalies indicate generation in a suprasubduction zone setting. Trondhjemite and granodiorite intrusions are present in the volcanic rocks, gabbros and norites. SHRIMP U-Pb dating of magmatic zircons from a granodiorite yields a mean 206Pb/238U age of 96.0 ± 2.0 Ma (Cenomanian). The initial e(open) Hf value for the zircons show a narrow range from +12.8 to +15.6, with a weighted mean of +13.90 ± 0.96. This initial value is within error of model depleted mantle at 96 Ma or slightly below that, in the field of arc rocks with minimal contamination by older continental crust. The compositional bimodality of the Pushtashan suprasubduction sequence suggests seafloor spreading during the initiation of subduction, with a lava stratigraphy from early-erupted MORB transitioning into calc-alkaline lavas and finally by 96 Ma intrusion of granodioritic and trondhjemitic bodies with juvenile crustal isotopic signatures. The results confirm another Cretaceous arc remnant preserved as an allochthon within the Iraqi segment of the Cenozoic Zagros suture zone. Implications for the closure of Neo-Tethys are discussed.

The Hutuo Group, as one of the classic examples of Paleoproterozoic strata, plays an important role in establishing evolutionary processes in the Trans-North China Craton. In this contribution, we present petrologic, detrital zircon U-Pb ages and Lu-Hf isotopes from three subgroups of the Hutuo Group. Sandstones in the Doucun and Dongye Subgroups are dominated by Q (monocrystalline and polycrystalline quartz) and F (K-feldspar and plagioclase) with minor lithic fragments, suggesting that the detrital components were mainly derived from both the continental block and recycled orogen. In contrast, clastic components in the Guojiazhai Subgroup are mainly Q and lithic fragments, indicating they were derived predominantly from the recycled orogen. The ages of detrital zircons from sandstones in the Doucun and Dongye Subgroups are mainly concentrated at ca. 2.5 Ga and 2.2-2.1 Ga with minor 2.7 Ga zircons also present which indicates they were dominantly sourced from 2.5 Ga Wutai, Fuping and Zanhuang Complexes, and Paleoproterozoic intrusives. The Guojiazhai Subgroup displays a different zircon age population of ca.2.4 Ga, 2.2-2.1 Ga and 2.0-1.9 Ga, which indicates likely derivation from Paleoproterozoic intrusives in the Wutai, Lüliang and Hengshan areas. Based on the observation that sandstone clastic components in the Doucun and Dongye Subgroups are dominantly quartz, feldspar and sedimentary lithic fragments, but not volcanic lithics, we propose that they were deposited in a rift-related setting. Zircons from the lower sequence of the Hutuo Group yielded a young population of 2140 Ma. Considering the volcanic rocks of 2140 ± 14 Ma at the base of the group and 2.2-2.0 Ga magmatism along the TNCO, we propose that Doucun and Dongye Subgroups formed at 2.2-2.0 Ga. The youngest, ca. 1.9 Ga, zircons in the Guojiazhai Subgroup indicate this Group was deposited during closure of the rift at 1.9-1.8 Ga. The two stage model ages of the detrital zircons mainly range from 2.6 to 2

The folded, amphibolite facies Isua supracrustal belt of the North Atlantic Craton (Greenland) contains rare low-strain lacunae that display the world's oldest sedimentary structures in dolomitic carbonates, banded iron formations, volcaniclastic sedimentary rocks, and very rare sandstones and conglomerates. U-Pb zircon geochronology shows that Isua sedimentary rocks were deposited as two unrelated assemblages of (1) ∼3810-3800 plus ∼3750 Ma rocks and (2) 3710-3695 Ma rocks that were tectonically juxtaposed in the Eoarchean. Felsic-mafic volcaniclastic sedimentary rocks are chemically immature, and were derived from volcanic sources, not from significantly older continental regions. An inferred ∼3700 Ma unconformity is marked by a basal conglomeratic lag deposit on underlying 3710-3720 Ma altered (subaerially weathered?) volcanic rocks and is succeeded by ∼3695 Ma chemical sedimentary rocks. Holistic appraisal of the Isua sedimentary units within the context of their associated volcanic sequences indicates they formed over a 100-million-year period in suprasubduction zone settings.

Biological activity is a major factor in Earth's chemical cycles, including facilitating CO2 sequestration and providing climate feedbacks. Thus a key question in Earth's evolution is when did life arise and impact hydrosphere-atmosphere-lithosphere chemical cycles? Until now, evidence for the oldest life on Earth focused on debated stable isotopic signatures of 3,800-3,700 million year (Myr)-old metamorphosed sedimentary rocks and minerals1,2 from the Isua supracrustal belt (ISB), southwest Greenland3. Here we report evidence for ancient life from a newly exposed outcrop of 3,700-Myr-old metacarbonate rocks in the ISB that contain 1-4-cm-high stromatolites-macroscopically layered structures produced by microbial communities. The ISB stromatolites grew in a shallow marine environment, as indicated by seawater-like rare-earth element plus yttrium trace element signatures of the metacarbonates, and by interlayered detrital sedimentary rocks with cross-lamination and storm-wave generated breccias. The ISB stromatolites predate by 220 Myr the previous most convincing and generally accepted multidisciplinary evidence for oldest life remains in the 3,480-Myr-old Dresser Formation of the Pilbara Craton, Australia4,5. The presence of the ISB stromatolites demonstrates the establishment of shallow marine carbonate production with biotic CO2 sequestration by 3,700 million years ago (Ma), near the start of Earth's sedimentary record. A sophistication of life by 3,700 Ma is in accord with genetic molecular clock studies placing life's origin in the Hadean eon (>4,000 Ma)6.

The Lajeado Group in the Ribeira Belt, Southeastern Brazil, corresponds to an open-sea carbonate platform, comprised of seven overlapping siliciclastic and carbonatic formations, intruded in its upper portion by the Apiaí Gabbro. These rocks have a Neoproterozoic tectonometamorphic overprint related to arc magmatism and the Brasiliano collisional orogeny. Geochronological constraints are given by new U-Pb SHRIMP and LA-ICP-MS data for Lajeado Group detrital zircons and for magmatic zircons from the Apiaí Gabbro. The youngest detrital zircons in the Lajeado Group are 1400-1200 Ma, and constrain its maximum age of deposition to beMa, whereas the 877 ± 8 Ma age for magmatic zircons in the Apiaí Gabbro give the minimum age. Detritus source areas are mainly Paleoproterozoic (2200-1800 Ma) with some Archean and Mesoproterozoic contribution (1500-1200 Ma), with distal or tectonic stable cratonic character. The Lajeado Group should be a Stenian-Tonian carbonate platform passive margin of a continent at this time, namely the Columbia/Nuna or the Rodinia. The Apiaí Gabbro displays similar age to other intrusive basic rocks in the Lajeado and Itaiacoca groups and represents tholeiitic MORB-like magmatism that we relate to the initial break-up of a Mesoproterozoic continent and the formation of the Brasiliano oceans.

The extension of subduction processes into the Eoarchaean era (4.0-3.6 Ga) is controversial. The oldest reported terrestrial olivine, from two dunite lenses within the ~3,720 Ma Isua supracrustal belt in Greenland, record a shape-preferred orientation of olivine crystals defining a weak foliation and a well-defined lattice-preferred orientation (LPO). [001] parallel to the maximum finite elongation direction and (010) perpendicular to the foliation plane define a B-type LPO. In the modern Earth such fabrics are associated with deformation of mantle rocks in the hanging wall of subduction systems; an interpretation supported by experiments. Here we show that the presence of B-type fabrics in the studied Isua dunites is consistent with a mantle origin and a supra-subduction mantle wedge setting, the latter supported by compositional data from nearby mafic rocks. Our results provide independent microstructural data consistent with the operation of Eoarchaean subduction and indicate that microstructural analyses of ancient ultramafic rocks provide a valuable record of Archaean geodynamics.

In NE Iraq, the eastern edge of the Arabian plate is overlain by arc rock allochthons whose genesis and tectonic emplacement were related to the consumption and closure of the Neotethys Ocean. This paper demonstrates the occurrence of unrelated Paleogene arc rocks in two adjacent allochthons. The Bulfat Igneous Complex at Wadi Rashid (NE Iraq) is an intrusion within the Upper Allochthon Albian-Cenomanian Gimo-Qandil sequence suprasubduction zone assemblage. A thrust separates this allochthon from the underlying Lower Allochthon of the Eocene-Oligocene Walash-Naopurdan volcanic-sedimentary arc rocks. The Bulfat Igneous Complex at Wadi Rashid consists of gabbro and granitic composite intrusions in which components mingle down to a small scale. Textural relationships in the Bulfat Igneous Complex rocks indicate emplacement at high crustal levels with rapid cooling, which is consistent with amphibole geobarometry indicating crystallisation pressures between ~ 250 and 300 Mpa. Ti-rich igneous pargasite and Ti-rich igneous Fe-biotite from gabbroic and granitic components yielded 40Ar/39Ar ages of 39.23 ± 0.21 and 38.87 ± 0.24 Ma respectively. These ages agree within analytical error and suggest coeval emplacement and rapid cooling of mafic and felsic magmas in the Eocene, in an event that was distinct and much younger than the host Albian-Cenomanian rocks. This igneous event was unrelated to formation of Cenozoic rocks in the underlying, tectonically separate, lower allochthon. The trace element signatures of the Wadi Rashi volcanic rocks show volcanic-arc characteristics for the granites and the gabbroic rocks resemble E type MORB. The presence of Eocene arc-related rocks in two allochthons suggests complexity in Paleogene subduction systems, with possibly two subduction zones operating at that time.

The Zagros Orogen, marking the closure of the Neo-Tethyan Ocean, formed by continental collision beginning in the late Eocene to early Miocene. Collision was preceded by a complicated tectonic history involving Pan-African orogenesis, Late Palaeozoic rifting forming Neo-Tethys, followed by Mesozoic convergence on the ocean's northern margin and ophiolite obduction on its southern margin. The Sanandaj-Sirjan Zone is a metamorphic belt in the Zagros Orogen of Gondwanan provenance. Zircon ages have established Pan-African basement igneous and metamorphic complexes in addition to uncommon late Palaeozoic plutons and abundant Jurassic plutonic rocks. We have determined zircon ages from units in the northwestern Sanandaj-Sirjan Zone (Golpaygan region). A sample of quartzite from the June Complex has detrital zircons with U-Pb ages mainly in 800-1050 Ma with a maximum depositional age of 547 ± 32 Ma (latest Neoproterozoic¿earliest Cambrian). A SHRIMP U-Pb zircon age of 336 ± 9 Ma from gabbro in the June Complex indicates a Carboniferous plutonic event that is also recorded in the far northwestern Sanandaj-Sirjan Zone. Together with the Permian Hasanrobat Granite near Golpaygan, they all are considered related to rifting marking formation of Neo-Tethys. Scarce detrital zircons from an extensive package of metasedimentary rocks (Hamadan Phyllite) have ages consistent with the Triassic to Early Jurassic age previously determined from fossils. These ages confirm that an orogenic episode affected the Sanandaj-Sirjan Zone in the Early to Middle Jurassic (Cimmerian Orogeny). Although the Cimmerian Orogeny in northern Iran reflects late Triassic to Jurassic collision of the Turan platform (southern Eurasia) and the Cimmerian microcontinent, we consider that in the Sanandaj-Sirjan Zone a tectonothermal event coeval with the Cimmerian Orogeny resulted from initiation of subduction and closure of rift basins along the northern margin of Neo-Tethys.

The Hutuo Group, located in the Wutai Mountains area, is characteristic of Paleoproterozoic strata in the North China Craton, and plays an important role in establishing evolutionary processes in the Trans-North China Orogen. We present petrological observations along with geochemical and Nd isotopic data for basalts in the Qingshicun and Hebiancun Formations of the Hutuo Group. These basalts are enriched in TFeO (11.97−18.01 wt.%), consistent with occurrences of iron-rich basalt world-wide. They also display relatively high MgO (4.95−12.25 wt.%), Ni (79−121 ppm) and Cr (37−101 ppm) and low in SiO2 (41.37−51.95 wt.%). Therefore, the iron-rich character of the Hutuo Group basalts originated with their parental magma rather than via shallow crustal fractionation at low oxygen fugacity, as inferred for many high iron basalts. The Hutuo Group basalts exhibit weak to moderate differentiation of light REE over heavy REE ((La/Lu)cn = 2.61−6.79) with minor or slight Eu anomalies (Eu/Eu* = 0.77−1.41), and also show the Nb and Ti troughs without obvious Zr and Hf anomalies. The ɛNd(t) values of the basalts range from −1.23 to +0.81 with TDM of 2489−2664 Ma. The basalts most plausibly originated from the fertile subcontinental lithosphere mantle and, in the case of the Qingshicun Formation basalts, also underwent the crustal contamination. Basalts in the Hutuo Group also have the high Zr/Hf ratios, and present high Ti/V values of 24−55 and Zr/Y ratios ranging from 4.6 to 13.6, suggesting that they were emplaced in a within-plate setting. Considering the sedimentary associations and detrital fragments in the Hutuo and Gantaohe Groups, and bimodal magmatism along the Trans-North China Orogen, we propose that 2.2−2.0 Ga geological events within the orogen were rift-related, but did not result from subduction or island arc processes.

The Mesoarchaean Kapisilik and Eoarchaean Isukasia terranes in the Nuuk region of southern West Greenland were tectonically juxtaposed in the Archaean. The north of the Isukasia terrane is distal from the Kapisilik terrane and has only rare growth of ~2690Ma metamorphic zircon and no 2980-2950Ma metamorphic zircon. The southern part of the Isukasia terrane lies between two ~2690Ma shear zones, and has locally preserved high pressure granulite facies assemblages and widespread growth of 2980-2950Ma metamorphic zircon and also sporadic growth of ~2690Ma metamorphic zircon. Within this southern part of the Isukasia terrane there is a folded klippe of mylonitised Mesoarchaean detrital meta-sedimentary rocks (carrying >3600 and ~3070Ma detrital zircons), mafic and ultramafic rocks, with ~2970Ma metamorphic zircon overgrowths. South of the Isukasia terrane is the Kapisilik terrane, containing ~3070Ma arc-related volcanic rocks, gabbro-anorthosites and meta-tonalites, intruded by 2970-2960Ma granites. Zircons of an Ivisârtoq supracrustal belt ~3075Ma intermediate volcanic rock have initial e{open}Hf values of +2 to +5 thus are juvenile crustal additions. ~3070Ma tonalites along the northern edge of the Kapisilik terrane have whole rock positive initial e{open}Nd values and thus are also juvenile crustal additions. In contrast, igneous zircons in 2960Ma granites intruded into juvenile ~3075Ma supracrustal rocks of the Kapisilik terrane have initial e{open}Hf values of -5 to -10, and must have involved the partial melting of >3600Ma Isukasia terrane rocks.The integrated structural and zircon U-Th-Pb-Hf isotopic data show that at 2980-2950. Ma the Kapisilik terrane juvenile arc components collided with, and over-rid, the Isukasia terrane. The southern edge of the Isukasia terrane came to lie in the deep crust under the Ivisârtoq supracrustal belt and melted at 2970-2960. Ma to produce granites. These granites derived from ancient crust rose into the upper crust, where they in

This paper presents new SHRIMP U-Pb zircon and 40Ar-39Ar hornblende ages as well as elemental and Sr-Nd isotopic data for greenschist to amphibolite facies meta-mafic rocks from the Votuverava Group (Ribeira Belt, Brazil). These mafic rocks appear to be mostly subvolcanic and volcanic rocks and, in part, intrusions coeval with the host sedimentary succession. Magmatic zircons have a U-Pb age of 1488 ± 4 Ma. An 40Ar-39Ar hornblende age of 1390 ± 20 Ma suggests cooling after a thermal event. The mafic rocks have chemical compositions of basalt and basaltic andesite (SiO2 of 45-55 wt.%) of the tholeiitic series. Chemical data indicate the coexistence of rocks with BABB and E-MORB signatures. BABB-like rocks with a Ti/V of 9-20 show depletion in HFSE relative to MORB values and slightly fractionated REE patterns (Lacn/Ybcn of 0.8-1.8), while those with a Ti/V of 22-30 have HFSE contents similar to MORB and slightly more fractionated REE patterns (Lacn/Ybcn of 1.1-2.8). E-MORB-like rocks display a Ti/V of 27-41 and moderately fractionated REE patterns (Lacn/Ybcn of 3.6-5.3). The ɛNd(t) values ranged from −3.02 to +3.55. Whole rock Sm-Nd isotopic data yielded a regression with a slope of 1225 ± 68 Ma, which can be interpreted as a mixing line of no age significance that is related to different source components of the metabasic rocks. Elemental and Nd isotopic signatures suggest that the Votuverava metabasites originated from a MORB-type mantle source heterogeneously metasomatized by subduction processes. The data are consistent with magmatic activity in a back-arc basin environment. The 1490 Ma mafic magmatism is associated with a 500-km-long and 60-km-wide Calymmian belt interpreted as an accretionary orogen. Available thermochronological data suggest that this accretionary orogen could have evolved into a collisional orogen in the period 1280-1100 Ma, possibly related to the assembly of Rodinia. During the Brasiliano-Pan African cycle (650-550 Ma), related to amalgamat

A diverse assemblage of accretionary complex, island-arc, ophiolitic and high-pressure, low-temperature metamorphic rocks occurs within the serpentinite mélange at Port Macquarie on the eastern extremity of the New England Orogen of eastern Australia. New field observations, U-Pb zircon dating, petrography and geochemistry presented here establish a more robust chronology and interpretation of these rocks. Previously, all basalt, chert and volcaniclastic sandstones at Port Macquarie were grouped into the Watonga Formation. Ordovician to middle Devonian radiolarians and conodonts from 'Watonga' chert-basalt associations shows that they are older than, and unrelated to, 'Watonga' volcaniclastic rocks like those at Green Mound which contain volcanic/detrital zircons as young as 335 Ma that were derived from a Carboniferous arc. Volcanic detritus with pillow lava forming a block within the serpentinite mélange yielded 452 ± 10 Ma igneous zircons, indicating an Ordovician age. The Tacking Point Gabbro has an age of 390 ± 7 Ma (Devonian) and geochemical affinities with intra-oceanic arc igneous suites. It was intruded into deformed cherts of the Watonga Formation giving a spatial link between an Ordovician-Devonian? Accretionary complex and adjacent Devonian island-arc. The MORB-like basalt-chert association of the Watonga Formation and the Devonian Tacking Point gabbro represents a mid-Paleozoic assemblage allochthonous to Gondwana, which possibly correlates with the Djungati and Gamilaroi terranes respectively located further west in the New England Orogen. Zircon dating shows that post-serpentinite mafic-felsic dykes were emplaced into the Port Macquarie serpentinite at 247 ± 20 Ma and further disrupted. Therefore, tectonism affecting the serpentinite continued into the Early Triassic, with final movement during the Hunter-Bowen Orogeny. Our results from Port Macquarie are compatible with a tectonic model for the New England Orogen that involves episodic island-arc col

The Archaean gneisses of the Nuuk area (southern West Greenland) are partitioned into tectonostratigraphic terranes - blocks of arc-like crust that evolved independently until they coalesced by collisional orogeny. To 'map' terranes to the east under the Inland Ice, sand samples were taken from rivers issuing from the edge of the Icecap; three from the Isua area and one from ∼20 km to the south. Bedrock along this part of the ice front consists of ∼3820-3600 Ma amphibolite facies rocks. 40 km south of Isua (Kapisilik terrane in the Ivisaartoq area) and also from ∼10 km to the north there are Mesoarchaean amphibolite facies gneisses (3070-2950 Ma) exposed at the ice front. In the moraine fields in the Isua area there are erratic blocks of granulite facies gneisses. These were sourced from a hidden terrane to the east under the ice because no such rocks are exposed in the Isua area. The majority of the zircons from the sand samples yielded close to concordant U-Pb ages. Apart from one 2414 Ma grain, all are Archaean. The Isua sands show ∼2695, 2710 and 2730 Ma; then 2790, 2805 and 2840 Ma clusters with a few grains back to 2950 Ma and then a complex 3440-3960 Ma spectrum. Less than 1% of the grains have ages between 3000 and 3400 Ma. In the Isua sands, Neoarchaean and late Mesoarchaean zircons form >50% of the population. The sand 20 km to the south shows a similar span of zircon ages, but there is a 2960 Ma peak not seen in the Isua sands. As the Eoarchaean and late Mesoarchaean-Neoarchaean cycles progress, the spread of Th/U and ɛHf values seen in each zircon age population increases. This is interpreted as repeated addition of juvenile material to the crust, but also with increasing amounts of high temperature tectonothermal recycling of older materials within a package. However, there is no isotopic evidence for any contribution of Eoarchaean crust to the later ≥2750 Ma Archaean cycle(s), showing evolution in disparate terranes. Furthermore, no evidence for Hadean

Gneiss complexes are the world’s most horrendously complexrocks to understand (see cover of this issue), yet they form a largeamount of the continents, albeit they are normally hidden fromview by sedimentary sequences. The last half century has seenenormous advances in understanding these rocks, which form notonly a large amount of the continental crust, but also hold much ofEarth’s oldest rock record – the Archaean.

The Camboriú Complex is the only gneissic-migmatitic inlier within the Neoproterozoic Brusque Group supracrustal rocks, in the northernmost part of the Dom Feliciano Belt, southern Brazil. It comprises the Morro do Boi migmatites and the diatexitic Ponta do Cabeço Granite. Zircon U-Pb dating of migmatites and associated granitic neosomes shows that crustal evolution started in the Paleo- Mesoarchean (3.3-3.0 Ga), continued with events through the Neoarchean and Paleoproterozoic and ended in the Neoproterozoic (0.64-0.61 Ga). Integration of zircon Hf isotopic data and U-Pb ages indicate that juvenile crustal accretion was restricted to the Archean and that afterwards intracrustal reworking predominated. The exception to this is the ca. 1.56 Ga xenoliths (basic dike remnants?), whose magmatic zircons have juvenile Hf isotopic signatures. This basic magmatism marks extension of the earlier Precambrian complex. Although the Camboriú Complex is dominated by early Precambrian crustal additions, it was so strongly reworked in the Neoproterozoic that melts derived from it intruded the adjacent Neoproterozoic Brusque Group supracrustal rocks. Because of this strong overprint, we regard the Camboriu Complex as a Neoproterozoic (Ediacaran) geotectonic unit. In terms of its history, the Camboriú Complex most closely matches the Atuba Complex, the basement of the Curitiba Microplate that occurs further to the north, close to the Ribeira Belt, another Neoproterozoic orogen of southern Brazil.

Eoarchaean juvenile crust formed as ‘proto-arcs’. The northern side of the Isua supracrustal belt is an archetypal proto-arc, with ≥3720 Ma boninites, c. 3720 Ma basalts and gabbros, 3720–3710 Ma andesites, diorites and mafic tonalites, 3710–3700 Ma intermediate-felsic volcanic and sedimentary rocks and 3700–3690 Ma chemical sedimentary rocks. On its northern side there is an extensive body of 3700–3690 Ma tonalite. During its evolution, the c. 3700 Ma Isua volcanic–sedimentary assemblage was partitioned into tectonic slices, with intercalation of mantle dunites with pillow basalts, prior to intrusion of c. 3710 Ma quartz diorites. Partitioning also occurred at 3690–3660 Ma, when the 30–20 million years life of the c. 3700 Ma Isua proto-arc was terminated by juxtaposition with the c. 3800 Ma terrane that occurs along the south of the Isua supracrustal belt. The trace element chemistry for all the ≥3720–3700 Ma mafic to intermediate volcanic rocks indicates fluid-fluxing mantle melting. The c. 3690 Ma tonalites have signatures showing melting of garnet-bearing mafic (eclogite) sources. The Isua c. 3700 Ma assemblage developed at an intra-oceanic convergent plate boundary, and it has a life-cycle broadly analogous to (but not identical to) an oceanic island arc eventually accreted against older crust.

A diverse suite of Archaean gneisses at Huangbaiyu village in the North China Craton, includes rare fuchsite-bearing (Cr-muscovite) siliceous rocks - known as the Caozhuang quartzite. The Caozhuang quartzite is strongly deformed and locally mylonitic, with silica penetration and pegmatite veining common. It contains abundant 3880-3600 Ma and some Palaeoarchaean zircons. Because of its siliceous nature, the presence of fuchsite and its complex zircon age distribution, it has until now been accepted as a (mature) quartzite. However, the Caozhuang quartzite sample studied here is feldspathic. The shape and cathodoluminescence petrography of the Caozhuang quartzite zircons show they resemble those found in immature detrital sedimentary rocks of local provenance or in Eoarchaean polyphase orthogneisses, and not those in mature quartzites. The Caozhuang quartzite intra-zircon mineral inclusions are dominated by quartz, with lesser biotite, apatite (7%) and alkali-feldspar, and most inclusions are morphologically simple. A Neoarchaean orthogneiss from near Huangbaiyu displays morphologically simple inclusions with much more apatite (73%), as is typical for fresh calc-alkaline granitoids elsewhere. Zircons were also examined from a mature conglomerate quartzite clast and an immature feldspathic sandstone of the overlying weakly metamorphosed Mesoproterozoic Changcheng System. These zircons have oscillatory zoning, showing they were sourced from igneous rocks. The quartzite clast zircons contain only rare apatite inclusions (

Chemical sedimentary rocks such as banded iron formation (BIF) and pillow basalts are persistent features of the oldest volcanic and sedimentary record by 3.8-3.7 Ga, and are direct evidence for oceans by the start of the Archean. However, their presence does not dictate an equitable 3.8-3.7 Ga terrestrial climate. This is because they could have formed in oceans below global pack ice on a frigid Earth. The oldest known depositional structures occur as locally preserved features in ca 3.7 Ga deformed, amphibolite facies rocks of the Isua supracrustal belt (Greenland). These include units up to *1 m thick, in which there are stacked or jumbled clasts of chert. Detailed structural analysis shows that these rocks are not tectonic breccias. Also, their reported location in chemical sedimentary units capping slightly older volcanic rocks shows they are unlikely to be mass-flow deposits in a deep basin. In both composition and structure, these units resemble edgewise breccias observed on later Precambrian and Phanerozoic chemical sediment platforms, which formed when laminated sediments are disrupted by storm waves. Hence, the wave origin shows oceans were not ice covered, because in that case atmospheric storms would not generate waves. The Isua supracrustal belt also contains 3.72-3.70 Ga felsic and pelitic sedimentary rocks, derived from juvenile volcanic arc sources. These sedimentary rocks have chemical weathering indices that deviate from those of both fresh Eoarchean and modern igneous rocks. Furthermore, their weathering indices are congruent with rare examples of weathered (not hydrothermally altered) Isua volcanic rocks we have identified. Although no doubt the chemistry of Eoarchean weathering processes was different from those now, this nonetheless shows that these sedimentary rocks contain large contributions from highly weathered source materials. Rapid advanced weathering (these rocks consist of materials shed from an arc) is most feasible with an equitable

We present field relationships, major and trace element geochemistry and U-Pb SHRIMP and ID-TIMS geochronology of the A-type Ordovician Quintas pluton located in the Ceará Central Domain of the Borborema Province, in northeastern Brazil. This pluton presents a concentric geometry and is composed mainly of syenogranite, monzogranite, quartz syenite to quartz monzodiorite, monzogabbro and diorite. Its geochemical characteristics [SiO2 (52-70%), Na2O/K2O (1.55-0.65), Fe2O3/MgO (2.2-7.3), metaluminous to sligthly alkaline affinity, post-collisional type in (Y + Nb) x Rb diagram, and A-type affinity (Ga > 22 ppm, Nb > 20 ppm, Zn > 60 ppm), REE fractioned pattern with negative Eu anomaly] are coherent with post-collisional A2-type granitoids. However, the emplacement of this pluton is to some extent temporally associated with the deposition of the first strata of the Parnaíba intracratonic basin, attesting also to a purely anorogenic character (A1-type granitoid). The emplacement of this pluton is preceded by one of the largest known orogenesis of the planet (Neoproterozoic Pan-African/Brasiliano) and, if it is classified as an A2-type granitoid, it provides interesting constraints about how long can last A2-type magmatic activity after a major collisional episode, arguably triggered by disturbance of the underlying mantle, a topic extensively debated in the geoscience community.

We present four SHRIMP U–Pb zircon ages for the Choiyoi igneous province from the San Rafael Block, central–western Argentina. Dated samples come from the Yacimiento Los Reyunos Formation (281.4 ± 2.5 Ma) of the Cochicó Group (Lower Choiyoi section: andesitic breccias, dacitic to rhyolitic ignimbrites and continental conglomerates), Agua de los Burros Formation (264.8 ± 2.3 Ma and 264.5 ± 3.0 Ma) and Cerro Carrizalito Formation (251.9 ± 2.7 Ma Upper Choiyoi section: rhyolitic ignimbrites and pyroclastic flows) spanning the entire Permian succession of the Choiyoi igneous province. A single zircon from the El Imperial Formation, that is overlain unconformably by the Choiyoi succession, yielded an early Permian age (297.2 ± 5.3 Ma), while the main detrital zircon population indicated an Ordovician age (453.7 ± 8.1 Ma). The new data establishes a more precise Permian age (Artinskian–Lopingian) for the section studied spanning 30 Ma of volcanic activity. Volcanological observations for the Choiyoi succession support the occurrence of explosive eruptions of plinian to ultraplinian magnitudes, capable of injecting enormous volumes of tephra in the troposphere–stratosphere. The new SHRIMP ages indicate contemporaneity between the Choyoi succession and the upper part of the Paraná Basin late Paleozoic section, from the Irati up to the Rio do Rasto formations, encompassing about 24 Ma. Geochemical data show a general congruence in compositional and tectonic settings between the volcanics and Paraná Basin Permian ash fall derived layers of bentonites. Thickness and granulometry of ash fall layers broadly fit into the depletion curve versus distance from the remote source vent of ultraplinian eruptions. Thus, we consider that the Choiyoi igneous province was the source of ash fall deposits in the upper Permian section of the Paraná Basin. Data presented here allow a more consistent correlation between tectono-volcanic Permian events along the paleo-Pacific margin of southweste

The Ikkattoq gneisses of the Archaean gneiss complex in the Nuuk region, southern West Greenland, are the orthogneiss component within the amphibolite facies Tre Brodre terrane. They have mostly granodioritic compositions, with a small amount of quartz diorite. Sm-Nd isotopic data for a quartz diorite and five granodiorite Ikkattoq gneiss samples from within 5 km of the Ikkattoq (fjord) type locality yielded a regression with a slope equivalent to 2005 +/- 52 Ma (MSWD = 0.72). Regardless of the low MSWD, this cannot be the true age of the Ikkattoq gneisses, because all Ikkattoq gneisses yield U-Pb zircon dates of c. 2825 Ma and they are cut by the undeformed 2560 Ma Qorqut granite complex. This anomalously young regression 'age' resulted instead from mixing of different Nd components, indicating that the Ikkattoq gneisses are derived from mixed source materials. Taking the true age of the Ikkattoq gneisses as 2825 Ma from U-Pb zircon dating, the range of initial epsilon(Nd) in the Ikkattoq gneisses is -7.1 to -1.8. The negative initial epsilon(Nd) values mean that older, light rare earth enriched, sialic crust contributed to the igneous precursors of the Ikkattoq gneisses. This Nd evidence for contribution of older sialic crust is supported by positive epsilon(Sr) values for the Ikkattoq gneisses. With epsilon(Nd) values as low as -7.1 this older crustal component has to be Eoarchaean. The presence of scarce quartz diorites (low SiO(2), high MgO) suggests that ultramafic rocks (upper mantle?), metasomatised by the passage of fluids or silicic melts, were another contributing source. The Ikkattoq gneisses are proposed as a complex suite incorporating material derived from melting of much older sialic crust and probably upper mantle. The intercalation of tectonostratigraphic terranes during collisional orogeny at c. 2720 Ma destroyed the architecture of this 2825 Ma magmatic system, and the Ikkattoq gneisses now form a slice tectonically isolated from their source reg

Modern chemical sediments display a distinctive rare earth element + yttrium (REE + Y) pattern involving depleted LREE, positive La/La*(SN), Eu/Eu*(SN), and Y(SN) anomalies (SN = shale normalised) that is related to precipitation from circumneutral to high pH waters with solution complexation of the REEs dominated by carbonate ions. This is often interpreted as reflecting precipitation from surface waters (usually marine). The oldest broadly accepted chemical sediments are c. 3,700 Ma amphibolite facies banded iron-formation (BIF) units in the Isua supracrustal belt, Greenland. Isua BIFs, including the BIF international reference material IF-G are generally considered to be seawater precipitates, and display these REE + Y patterns (Bolhar et al. in Earth Planet Sci Lett 222:43-60, 2004). Greenland Eoarchaean BIF metamorphosed up to granulite facies from several localities in the vicinity of Akilia (island), display REE + Y patterns identical to Isua BIF, consistent with an origin by chemical sedimentation from seawater and a paucity of clastic input. Furthermore, the much-debated magnetite-bearing siliceous unit of "earliest life" rocks (sample G91/26) from Akilia has the same REE + Y pattern. This suggests that sample G91/26 is also a chemical sediment, contrary to previous assertions (Bolhar et al. in Earth Planet Sci Lett 222:43-60, 2004), and including suggestions that the Akilia unit containing G91/26 consists entirely of silica-penetrated, metasomatised, mafic rock (Fedo and Whitehouse 2002a). Integration of our trace element data with those of Bolhar et al. (Earth Planet Sci Lett 222:43-60, 2004) demonstrates that Eoarchaean siliceous rocks in Greenland, with ages from 3.6 to 3.85 Ga, have diverse trace element signatures. There are now geographically-dispersed, widespread examples with Isua BIF-like REE + Y signatures, that are interpreted as chemically unaltered, albeit metamorphosed, chemical sediments. Other samples retain remnants of LREE depletion but a

The Precambrian shield of northern Greenland has been investigated by SHRIMP U-Pb zircon dating of 14 orthogneisses and granitoids plus 5 metasediments, integrated with mapping by the Geological Survey of Denmark and Greenland and whole-rock Nd isotopic studies. The Inglefield Mobile Belt is a tract of Palaeoproterozoic sedimentation, plutonism, polyphase deformation and high-grade metamorphism that underlies Inglefield Land and northern Prudhoe Land. In the southern part of the belt at 78 degrees 30'N, the E-W-trending Sunrise Pynt Straight Belt is a high-grade, but structurally late, shear zone with contrasts in the geology on either side. South of the Sunrise Pynt Straight Belt, ca. 1980 Ma diorites and tonalites were emplaced into older orthogneisses and metasediments. Detrital zircons from two metaquartzites (deposited on Archaean basement?) yielded complex age spectra from ca. 3250 Ma to 2350 Ma, with 2600-2450 Ma grains dominant. In associated mica schist, low Th/U, 1923 +/- 8 Ma zircons date high-grade metamorphism. The most southern orthogneiss investigated (77 degrees 45'N) is Neoarchaean (ca. 2600 Ma), in agreement with previously published isotopic data. North of the Sunrise Pynt Straight Belt to 79 degrees 10'N an amphibolite-granulite-facies complex with extensive pelitic to psammitic paragneisses are the oldest rocks recognised. Two psammitic paragneisses yielded unrounded zircons with a unimodal detrital age population centred on 2000-1980 Ma. Their source could be ca. 1980Ma orthogneisses from south of the Sunrise Pynt Straight Belt, or from basement inliers in the North-East Greenland Caledonian fold belt. The metasediments were intruded by tonalites and diorites with dates of 1949 +/- 13 Ma and 1943 +/- 11 Ma, and then by granitoids (free of zircon inherited from older rocks) with ages of 1924 +/- 29 Ma to 1915 +/- 19 Ma. The metasediments show development of low Th/U zircon overgrowths at ca. 1920 Ma, coeval with the granitoids. Finally, other gr

The Ribeira belt in SE Brazil is a Neoproterozoic to Early Palaeozoic orogen, whose architecture and history is not yet fully understood. The depositional age of many of the sedimentary sequences in the Ribeira Belt remains unconstrained, and with debate concerning their depositional environment and tectonic setting. In this paper we present SHRIMP zircon U/Pb age constraints for one such problematic unit in the Ribeira Belt – the Iporanga Formation – and discuss the significance of this age with regards to the timing of Neoproterozoic glacial events in southeast Brazil. Using a felsic volcanic unit immediately under the Iporanga Formation and granite cobbles from breccias in its basal parts a reconnaissance SHRIMP U/Pb zircon maximum depositional age of 580 Ma is assigned for the base of this unit. This age is marginally younger than the 625–605 Ma ages for intrusions into the Lajeado and Ribeira subgroups, with which the Iporanga Formation is in tectonic contact. This indicates that the Lajeado and Ribeira subgroups are not stratigraphically equivalent to the Iporanga Formation, as thought previously by some workers. The maximum depositional age of 580 Ma also places a maximum time constraint on the tectonic juxtaposition of the Iporanga Formation with other supracrustal units, and on the greenschist facies metamorphism and isoclinal folding that affected it. The potential glacial origin for the Iporanga Formation, if correct, would place it in the late Ediacaran — provisionally equivalent to the Gaskiers glaciation.

Ti-in-zircon thermometry with SHRIMP II multi-collector has been applied to two well-documented Archean igneous and metamorphic samples from southern West Greenland. Zircons from 2.71 Ga partial melt segregation G03/38 formed in a small (<1 >m(3)), closed system within a mafic rock under high pressure granulite facies conditions. Results of 14 Ti analyses present a mean apparent zircon crystallization temperature of 679 +/- 11 degrees C, underestimating independent garnet-clinopyroxene thermometry by 20-50 degrees C but consistent with reduced a(TiO2) in this system. 36 spot analysis on 15 zircons from 3.81 Ga meta-tonalite G97/18, with an estimated magmatic temperature >1000 degrees C, yield a low-temperature focused normal distribution with a mean of 683 +/- 32 degrees C, further demonstrated by high resolution Ti mapping of two individual grains. This distribution is interpreted to represent the temperature of the residual magma at zircon saturation, late in the crystallization history of the tonalite. Hypothetically, Ti-in-zircon thermometry on Eoarchaean detrital zircons sourced from such a high temperature tonalite would present a low-temperature biased image of the host magma, which could be misconstrued as being a minimum melt granite. Multiple analyses from individual zircons can yield complex Ti distributions and associated apparent temperature patterns, reflecting cooling history and local chemical environments in large magma chambers. In addition to inclusions and crystal imperfections, which can yield apparent high temperature anomalies, zircon surfaces can also record extreme (>1000 degrees C) apparent Ti temperatures. In our studies these were traced to (49)Ti (or a molecular isobaric interference) contamination derived from the double sided adhesive tape used in sample preparation, and should not be assigned geological significance. (C) 2007 Elsevier B.V. All rights reserved. Document Type: Article

The 200 km wide, east-west trending Paleoproterozoic mobile belt of the Ammassalik region of South-East Greenland contains a diverse assemblage of Paleoproterozoic and Archean rocks, variably affected by Paleoproterozoic deformations and high-grade low or high pressure metamorphism. By using previous field and geochemical data combined with new zircon dating and zircon trace element geochemistry, this mobile belt is confirmed as a 1870 to 1840 Ma collisional orogen, which contains one or more 1900 to 1880 Ma magmatic suites and northern and southern Archean basement terranes. The most studied 1900 to 1880 Ma magmatic suite is the Ammassalik Intrusive Complex, which is dominated by diorites (with arc-like geochemical signatures and with Paleoproterozoic Nd depleted mantle model ages), which was intruded into sedimentary rocks, (with predominantly Paleoproterozoic detrital zircons). Both these associated rock types show 1900 to 1880 Ma moderate pressure granulite facies metamorphism (7 kbar, >= 850 degrees C). Paleoproterozoic mylonites separate the Ammassalik Intrusive Complex from Archean orthogneisses containing 1870 Ma eclogite (11 kbar, 650-700 degrees C) and high-pressure granulite facies assemblages in Palaeoproterozoic diabase dike remnants. Associated with these Archean gneisses are pelitic metasediments, marbles and orthoquartzites (with Archean detrital zircon with complex 1870 to 1740 Ma metamorphic rims) that contain kyanite, thereby also showing high-pressure metamorphism. In the Ammassalik area we propose that one or more Paleoproterozoic magmatic arcs with syn-magmatic moderate pressure, high temperature metamorphism were emplaced over the edge of southern Archean continental crust. This resulted in at least doubling of crustal thickness, causing the transient 1870 Ma eclogite to high-pressure granulite facies metamorphic conditions in the buried southern Archean terrane. Archean orthogneisses in the north of the mobile belt preserve low-pressure Arche