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A synthesis of the geological record of Earth's ten remaining oldest surviving gneiss complexes, each containing >3.6 Ga rocks, reveals a common history. We propose that the simplest scenario compatible with all observations is that of formation of an ancient continental mass, here named Ithaca, by 3.66 Ga from amalgamation of earlier quartzofeldspathic crust, followed by initiation of continental break-up at 3.53 Ga by rifting. Evidence for this is reconstructed from the remaining oldest rock record (only ca. 10,000 km2 globally). Dominating the surviving fragments of the proposed Itsaqia continent are 3.9 to 3.66 Ga tonalites that represent juvenile crustal additions with whole-rock initial εNd >+1 and zircon initial εHf ≈ 0. Their trace element chemistry shows that they were derived by ca. 30 percent partial melting of garnetiferous, mostly eclogitized basic rocks, leaving behind a subcrustal garnet-rich restite. The tonalites contain inclusions of mafic rocks with chemical signatures diagnostic of mantle wedge fluxing, such as enrichment in the light rare earths and depletion of Nb and Ti. We interpret that this juvenile crust formed repeatedly in arc-like constructs at convergent plate boundaries. The Acasta Gneiss of Canada is the only undisputed surviving rock record of the proposed Itsaqia continent where crust formation extends back to the Hadean. Before ca. 3.66 Ga, individual gneiss complexes show distinct chronologies of crust formation, yet despite their present-day isolation, they underwent identical 3.66 to 3.6 Ga high temperature orogenic events (Isukasian orogeny) - which we contend indicates that from 3.66 Ga these complexes had amalgamated into a single continental mass. Rare surviving 3.66 Ga high-pressure granulite rocks that underwent rapid decompression indicate tectonic crustal thickening then collapse during amalgamation. This was followed by almost 50 million years of high heat flow and lower pressure metamorphism, most probably in an extensional setting. Starting from ca. 3.53 Ga, we propose that komatiite and basalt eruption and dike emplacement marked the start of Itsaqia's dismemberment by rifting. We further speculate that the deep mantle upwelling responsible for this plume-related magmatism was triggered by either the cascade of pre-3.66 Ga sub-Itsaqia high density garnet-rich restitic subduction graveyards into the lower mantle or the thermal insulation effect of Itsaqia. This resembles the mechanisms of supercontinent breakup throughout Earth's history. Hence we propose that Wilson Cycles of continent amalgamation and breakup were already initiated by the Eoarchean, near the start of the rock record. Australia's East Pilbara region was over the top of the plume, where the thermal impact destroyed Itsaqia by melting to give rise to felsic igneous rocks coeval with komatiites. Greenland's Itsaq Gneiss Complex was peripheral to the plume, and hence was heavily diked at ca. 3.5 Ga, but was not melted. [ABSTRACT FROM AUTHOR]

[1] The world's most extensive, well preserved, early Archaean (>3500 Ma) structures occur in the northern end of the 3850-3560 Ma Itsaq Gneiss Complex, southern west Greenland, an area which includes the 3800-3700 Ma Isua supracrustal belt. These provide a unique opportunity to study early tectonics. The area is dominated by ~3800-3650 Ma orthogneisses containing migmatized inclusions of >3800 Ma metavolcanic, sedimentary, ultramafic, and gabbroic rocks. The thickest and most extensive of these inclusions forms the southern side of the Isua supracrustal belt. Also present are panels of less migmatized 3710-3690 Ma volcanic and sedimentary rocks, in the northern side of the Isua supracrustal belt, and probably also in the Tussaap supracrustal belt in the south of the area. Combined mapping and geochronology has identified mylonites which were active before regional early Archaean folding and 3580-3560 Ma metamorphism. Sensitive high-resolution ion microprobe dating of premylonitization, synmylonitization, and postmylonitization granite sheets indicates some mylonites were active between 3650 and 3600 Ma. During this period it is proposed that large horizontal motions and interleaving occurred, results of which included juxtaposition of supracrustal sequences in the Isua supracrustal belt with different depositional ages. The tectonic interleaving was most likely due to horizontal motions in the vicinity of plate margins. The new combined field observations and geochronology support hypotheses that some form of plate tectonics at least contributed to early Archaean sialic crustal development. INDEX TERMS: 8157 Evolution of the Earth: Plate motions-past (3040); 3040 Marine Geology and Geophysics: Plate tectonics (8150, 8155, 8157, 8158); KEYWORDS: Greenland, early Archaean, tectonic interleaving

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From the 3000 km² Eoarchean Itsaq Gneiss Complex (IGC) of Greenland, zircon U-Pb dating of numerous meta-granitoid and orthogneiss samples is integrated with geologic observations, whole rock geochemistry and a strategic subset of zircon Hf and whole rock Nd isotopic measurements. This shows that there are multiple episodes of TTG suite formation from ~3890 to 3660 Ma, characterized by zircon initial εHf≈0 and whole rock initial εNd of > +2. These rocks mostly ha geochemical signatures of partial melting of eclogitized mafic sources, with a subset of high magnesian, low silica rocks indicating fusion by fluid fluxing of upper mantle sources. The TTG suites are accompanied by slightly older gabbros, basalts and andesites, which have geochemical signatures pointing to magmas originating from fluid fluxing of upper mantle sources. The data show the formation of juvenile crust domains in several discrete events from ~3900 to 3660 Ma, probably at convergent plate boundaries in an environment analogous, but not identical to, modern island arcs. In the Isua area, a northern ~3700 Ma terrane formed distal from a predominantly ~3800 Ma terrane. These terranes were juxtaposed between ~3680 and 3660 Ma--respectively the age of the youngest rocks unique to the northern terrane and the lithologically distinctive ultramafic-granitic Inaluk dykes common to both terranes. This shows the assembly of different domains of juvenile rocks to form a more expansive domain of "continental" crust. A rare occurrence of high-pressure granulite is dated at ~3660 Ma, demonstrating that assembly involved tectonic crustal thickening. This continental crust was then reworked in the 3660 to 3600 Ma Isukasian orogeny. In the northern part of the Isua area, 3660 to 3600 Ma granites were emplaced into ~3700 Ma tonalites. The earliest granites are nebulous, and sigmoidal schlieric inclusions within them demonstrate ductile extension. Younger granite sheets were emplaced into extensional ductile-brittle fractures. These granite-tonalite relationships are overprinted by widespread development of late Eoarchean (pre-3500 Ma Ameralik dyke) brittle-ductile extensional cataclastic textures, together demonstrating that extension was polybaric. The southern part of the Isua area largely escaped 3660 to 3600 Ma high temperature processes and has sparse granite sheets commonly focused into coeval shear zones. In the rest of the complex, deeper crustal levels during the Isukasian orogeny are widely preserved. These experienced upper amphibolite to granulite facies moderate- to low-pressure syn-kinematic metamorphism, forming complex migmatites rich in granitic-trondhjemitic neosome. The migmatites were intruded by composite ferrogabbro and granite bodies, in which syn-magmatic extensional features are locally preserved. Thus 3660 to 3600 Ma crustal recycling involved elevated crustal thermal gradients in an extensional regime. Crustal melts formed in the Isukasian orogeny have zircon initial εHf <0 and whole rock initial εHd of ≤0 showing incorporation of slightly older Eoarchean juvenile crust. A Phanerozoic example of collisional orogeny followed by crustal thinning is explored as an analog for the Isukasian orogeny. [ABSTRACT FROM AUTHOR]

[Display omitted] • Deciphering the Eoarchean geological record must come from observation, not modelling. • Tectonic interleaving (including mantle over crustal rocks) shows crustal shortening. • Greenland's Eoarchean mafic rocks formed by fluid-fluxing of depleted upper mantle. • Formation of Greenland's Eoarchean crust matches convergent plate boundary processes. • Greenland's Eoarchean geology is inconsistent with stagnant-lid/heat-pipe models. It is 50 years since the landmark paper where Black et al. (1971) presented whole-rock Pb-Pb and Rb-Sr isotopic evidence for some rocks in Greenland surviving from Earth's first billion years; the ≥ 3700 Ma Amîtsoq gneisses. This overturned ideas prevalent at that time that the young Earth was far too violent for such ancient rocks to survive. In the following few years it emerged how 'normal' this early Earth appeared to be, with a retained hydrosphere (oceans) by 3700 Ma and 'continental' crust dominated by rather normal granitic sensu lat o rocks. By several decades ago this led to a vision of Eoarchean lithosphere development being controlled by a geodynamic regime with definite similarities to that in the Phanerozoic, particularly with evidence for lateral lithosphere movement (mobile lid) such as exemplified by arc-like magmatism and Barrovian metamorphism (the latter requiring tectonic crustal thickening). However, the literature in recent years is increasingly emphasising model-driven non-uniformitarian stagnant lid geodynamic scenarios to explain the Eoarchean geological record. This paper reviews the broad range of field and laboratory evidence extracted from the Eoarchean geological record in Greenland. We argue that the non-uniformitarian sagduction within a stagnant lid regime interpretation does not fit Eoarchean facts. [ABSTRACT FROM AUTHOR]

The oldest rocks-3.85 billion years old-from southwest Greenland have coupled neodymium-142 excesses (from decay of now-extinct samarium-146; half-life, 103 million years) and neodymium-143 excesses (from decay of samarium-147; half-life, 106 billion years), relative to chondritic meteorites, that directly date the formation of chemically distinct silicate reservoirs in the first 30 million to 75 million years of Earth history. The differences in 142Nd signatures of coeval rocks from the two most extensive crustal relicts more than 3.6 billion years old, in Western Australia and southwest Greenland, reveal early- formed large-scale chemical heterogeneities in Earth's mantle that persisted for at least the first billion years of Earth history. Temporal variations in 142Nd signatures track the subsequent incomplete remixing of very-early-formed mantle chemical domains. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The article discusses various reports published within the issue, including one which focuses on minerals that may have been present on Earth during the Hadean, another which investigates the ancient zircons of the Napier Complex in Antarctica, and another which investigates a number of issues relevant to the early Earth.

• 50 years post-discovery the Isua supracrustal belt remains the best Eoarchean archive. • Rare relicts of volcanic and sedimentary structures are unique for the Eoarchean. • The belt provides the most robust and diverse evidence for early life (3800–3700 Ma) • The belt and adjacent orthogneisses are tectonically-complex with multiple events. • The Isua area's geology is inconsistent with stagnant-lid/heat-pipe models. In the early 1970s, Vic McGregor and Stephen Moorbath demonstrated that the Isua supracrustal belt (Greenland) and its surrounding orthogneisses formed in the Eoarchean (Moorbath et al., 1972 , 1973). Fifty years later, these rocks are a key resource globally for understanding the Eoarchean Earth. This is because there is an extensive area (∼200 km2) of abundant outcrops where superimposed Neoarchean strain is modest, and Neoarchean metamorphism was only at lowest amphibolite facies; these are low tectonothermal conditions compared with other Eoarchean crustal remnants globally. Therefore, this region can provide the clearest window onto early Earth geodynamics. Within the Isua supracrustal belt there are rare areas where Eoarchean strain is also low, and these areas provide unambiguous recognition of diverse lithologies including sedimentary carbonates with the oldest proposed but contested stromatolites, pillow lavas, volcano-sedimentary lithologies and ultramafic rocks derived from layered intrusions (accepted) and slivers of mantle-derived meta-serpentinite that underwent Eoarchean ultra-high-pressure metamorphism (contested). As explored in previous papers (e.g., Polat et al., 2002), the geochemical signatures of the mafic igneous rocks, which include a boninite-like suite, indicate their magmas were derived by fluid fluxing of peridotites. Radiogenic isotopic signatures define discrete pulses of Eoarchean juvenile crustal growth, until a tectonic event at ∼3660 Ma with high pressure metamorphism that was followed by a switch to crustal recycling with the production of granites sensu stricto. Our appraisal of the integrated evidence points to complex superimposed Eoarchean events (∼3820–3600 Ma) driven by lateral lithosphere movements in convergent boundary settings, rather than a single crustal overturn such as within a model heat-pipe scenario. [ABSTRACT FROM AUTHOR]

The Sanandaj-Sirjan Zone is a basement culmination northeast of the Neo-Tethys suture in Iran. In this zone near Azna, granite has a magmatic zircon U-Pb age of 568 ± 11 Ma, with 900-800, ca. 2400, and ca. 3600 Ma inherited cores. The ca. 3600 Ma inherited zircon is the oldest crustal component yet detected in Iran. Near Chadegan, orthogneiss has a magmatic zircon U-Pb age of 637 ± 15 Ma, and carries ca. 1000 and 2000 Ma inherited zircons. Inherited 900-1000 Ma zircons have juvenile initial εHf values of ca. +8 to +9, whereas the younger 630 and 568 Ma magmatic zircons show lower initial εHf values; however, the 3600 Ma core has initial εHf = 0.0. A Neoproterozoic rim on the inherited 3600 Ma core has the most extreme initial εHf value of −18. The Hf isotopic data indicates generation of the magmatic protoliths from a mixture of juvenile Neoproterozoic and Archean sources. Previous studies showed that in Turkey the Central Anatolian Crystalline Complex is underlain by Neo-Eoarchean rocks, the Menderes Massif contains Neoproterozoic granitoids, and that central Iran's basement and the northern Sanandaj-Sirjan Zone contain Neoproterozoic granitic rocks. This basement terrane is from Gondwana, and was transferred across Paleo-Tethys to dock against Eurasia's southern margin. Occurrence in Iran and Turkey of Eoarchean crust raises the possibility of sinistral migration of this terrane in the closure of Tethys because the nearest known early Archean crust occurs in northeast India. [ABSTRACT FROM AUTHOR]

Presents evidence for rhenium enrichment in the mantle wedge from submarine arc-like volcanic glasses in Papua New Guinea. Concentrations of rhenium in subaerial arc-type volcanic rocks; Loss of rhenium during magma degassing.

In order to solve the fundamental problem as to whether the extinct radioactive isotope 244Pu was present in the early Earth, we have analysed xenon in magmatic zircons from three early Archaean meta-granitoids from Greenland. Two samples, a granite (sample G97/111) and a ferrogabbro/ferrodiorite (sample G97/112), are from the same ca 3.64 Ga composite intrusion at the mouth of Ameralik fjord. The third sample (sample G97/018) is a ca 3.81 Ga tonalite from a locality south of the Isua supracrustal belt. The 3.81 Ga tonalite (sample G97/018) and possibly the 3.64 Ga granite (sample G97/111) magmatic zircons appear to have small excesses of fission xenon different from the 238U-derived spontaneous fission xenon. This excess fission xenon is consistent with that expected for 244Pu-derived spontaneous fission, and it may indicate that these zircons incorporated 244Pu when they crystallised. If this is true, it suggests the existence of 244Pu early in the Earth’s history. Furthermore, our preliminary investigation supports views that the Earth’s primitive material had a near-chondritic 244Pu/238U abundance ratio and that fissiogenic xenon in the atmosphere is of plutonic origin. [Copyright &y& Elsevier]

[Display omitted] • The variolitic texture in mafic pillow lavas of the Mount Ada Basalt of the Pilbara Supergroup formed due to liquid immiscibility. • Clinopyroxene element mapping indicate that grains hosted in both variole and host regions preserve shared magmatic histories. • The liquid immiscibility reaction occurred due to H 2 O saturation in the precursor magmas. • Variole bearing lavas have εNd i −0.3 ± 0.4, ~1ε unit lower than contemporary tholeiites. • The fourth volcanic cycle in the Pilbara Supergroup, the Mount Ada Basalt and the Duffer Formation, preserves distinctly different volcanic types that indicate significant changes in magma genesis during a 10 Ma timeframe. Archean greenstone belts contain low grade metamorphic volcanic sequences with localised examples of preserved original volcanic textures and relict magmatic minerals, which are invaluable for inferring volcanic processes and by extension magma genesis. A feature observed in many Archean greenstone belts but rare in modern volcanic rocks is variolitic texture with millimetre- to centimetre-sized leucocratic globular structures (varioles), in mafic lavas. Models proposed to explain this texture include rapid undercooling, magma mingling and liquid immiscibility. Here, we investigate samples of Paleoarchean variole-bearing pillow lavas from the ca. 3469 Ma Mount Ada Basalt of the Pilbara Supergroup, Western Australia. The samples contain relict magmatic clinopyroxene and spinel and preserve sharp contacts between varioles and host. No plagioclase is observed in the samples. The clinopyroxene mineral composition is indistinguishable between in the varioles and host. This is despite the variole material having lower MgO and Fe 2 O 3 (total) and higher SiO 2 and Al 2 O 3 concentrations. However, the variole and host materials have indistinguishable TiO 2 , Cr 2 O 3 , NiO 2 and Sm-Nd isotope systematics. These observations are consistent with an immiscibility reaction from a parental melt to a hydrous fluid phase and conjugate mafic liquid, where minerals that formed before the reaction become incorporated into both endmembers where they continued to grow by diffusive exchange between the hydrous fluid and silicate melt. Two lines of evidence indicate that hydrous flux exsolution occurred due to H 2 O saturation in the precursor magmas, including: (1) the lack of plagioclase, for which crystallisation is supressed at high H 2 O content; and (2) Nb/Th < 7, which indicates a hydrated mantle source. The variole-bearing pillow lavas are part of the 10-Myr-duration Coongan Subgroup volcanic cycle and occur interspersed with tholeiite basalts and minor komatiites of the Mount Ada Basalt and mafic to felsic volcanic rocks of the Duffer Formation. These distinctly different volcanic rock types, which have εNd i −0.3 ± 0.4, ~1ε unit lower than contemporary tholeiites, indicate that there was variation in magma genesis, which included hydrous fluxed melting likely genetically related to delamination, during the 10 Myr formation of the Coongan Subgroup volcanic cycle, as part of the Paleoarchean East Pilbara Terrane formation. [ABSTRACT FROM AUTHOR]

We present a new method for precisely determining the U-Pb isotopic composition of cassiterite by isotope dilution-thermal ionisation mass spectrometry (ID-TIMS). Complete sample dissolution and spike-sample equilibration is accomplished in a pressure vessel with HBr. Isolation of small quantities of U and Pb from the tin-rich matrix is based on a novel TRU-spec column separation with total procedural blanks of <9 pg of Pb and <0.09 pg of U. The method was applied to measure the U-Pb isotopic compositions of cassiterite from the Yankee deposit of the Mole Granite in eastern Australia. A Total-Pb/U isochron age (238U/206Pb, 207Pb/206Pb and 204Pb/206Pb) produces the most precise age estimate of this cassiterite at 246.48 ± 0.51 Ma, which is within error of a previously published ID-TIMS U-Pb age of hydrothermal xenotime from the same location. Initial 206Pb/204Pb (21.28 ± 0.4) and 207Pb/204Pb (15.759 ± 0.034) ratios of the Yankee cassiterite, determined from the isochron regression, are evolved relative to those expected for average crust and measured in Mole Granite feldspars, suggesting a non-magmatic, U-rich component in the mineralising fluids. We also present new U-Pb isotopic compositions for the AY-4 cassiterite from the Furong deposit, South China, which has been used as a U-Pb calibration reference material in several previous studies. A Total-Pb/U isochron age derived from three measured aliquots yields a crystallisation age of 151.9 ± 2.2 Ma for the AY-4 cassiterite, younger than obtained previously by ID-TIMS dating (Yuan et al., 2011). • New method for acid digestion of cassiterite and chromatographic purification of U and Pb isotopes for ID-TIMS analysis. • Precise U-Pb compositions are presented for two reference cassiterite samples. • Results provide new high-precision geochronology of the Mole Granite Sn-W system, and insights into processes involved in the formation of a widely known Sn deposit. [ABSTRACT FROM AUTHOR]

The rare preservation of a stratigraphy in the northwest of the Eoarchean Isua supracrustal belt (Greenland) demonstrates the world's earliest-recognised marine transgression. Stratigraphically lowest is the ~3710 Ma Solvang Volcanic Formation of picrites, basalts and basaltic-andesites with arc-like geochemical signatures. Close to its erosional top, this formation's volcanic rocks show increase in K 2 O with depletion of Na 2 O, depletion in Sr with no alteration in Eu abundance and δ18O VSMOW increase to +16‰, interpreted as subaerial weathering at ~3700 Ma. The sedimentary Snowpatch Formation was deposited unconformably on the Solvang Volcanic Formation. Locally, the unconformity is marked by the laterally discontinuous Conglomerate Member dominated by rounded quartz clasts, but also with clasts of basalt and andesite up to >10 cm across, layered chert and ultramafic rocks. The Conglomerate Member carries sparse detrital zircons, the youngest of which are ~3710 Ma. The conglomerates are succeeded by the Sandstone and Dolostone Member of layered quartz and dolomite clastic sandstones that locally preserve relict shallow marine cross-bedding with scoured layers, tempestite breccia beds and very rare stromatolite horizons. The Sandstone and Dolostone Member is succeeded by the ~3695 Ma BIF Member. The Snowpatch Formation quartz-dolomite sandstones and interlayered stromatolites have seawater-like REE + Y trace element patterns variably modified by a negligible to significant influx of a terrigenous component. Trace element signatures such as Ti/V ratios and normalising REE + Y data to a 3710 Ma weathered volcanic rock (rather than Post Archean Average Shale) suggest this component was derived from a juvenile arc source, resembling the basement Solvang Volcanic Formation. The detailed field and geochemical evidence such as positive correlation between Fe/Mg and Pr/Yb* in the Snowpatch Formation dolostones and BIF (banded iron formation) indicates that the dolomitic lithologies developed in a shallow water setting with clastic sedimentation, during a marine transgression over a weathered ~3710 Ma volcanic arc basement, followed by BIF deposition in deeper water. Thus by 3700 Ma, shallow-water, exposed, settings had already been established, which were the site of stromatolite development. Unlabelled Image • The ≥3695 Ma Snowpatch Fm. is unconformable on the ~3710 Ma Solvang Volcanic Fm. • The Solvang Volcanic Fm. comprises variably-weathered arc-like basalts-andesites. • The Snowpatch Fm. is transgressive: conglomerates, calc-arenites, BIF and chert. • ≥3695 Ma stromatolites occur with cross-bedded rocks and tempestite units. [ABSTRACT FROM AUTHOR]

The southern part of the early Archaean Itsaq Gneiss Complex (southern West Greenland) on Akilia and adjacent islands consists of polyphase dioritic–tonalitic–granitic injection components with inclusions of metavolcanic amphibolites, chemical sediments such as banded iron formation (BIF), gabbros and ultramafic rocks. Incipient in situ partial melting and strong deformation during several Archaean tectonothermal events strongly modified these injection components, so that they are now mostly banded, schlieric migmatites with neosome produced during several events. The margins of many inclusions have been loci for either segregation of neosome and/or higher strain—obliterating the relationship between the inclusions and the older components of the migmatites. An added complication is that none of the inclusion lithologies in the southern part of the complex contain protolith zircon, which would provide precise, direct dates. Instead, minimum ages of inclusions are obtained by dating invasive components of the granitic (sensu lato) migmatites. Previous age determinations of the inclusions were centred on a ∼200 m long body of amphibolites, BIF and ultramafic rocks on the southwestern corner of Akilia. Geochronology of this locality has been controversial, with our proposed age of &ges;3850 Ma making the inclusion the world''s oldest-dated sedimentary and mafic rocks. We continue the debate on the age of the inclusion on southwest of Akilia, and we broaden it by presenting mapping and zircon dating studies of inclusions on islands with 20 km of Akilia. This (1) addresses the contentious ages of these rocks, (2) examines the broader issue of how the complex field relationships in migmatites can lead to geochronological controversies, and (3) addresses the problem of precise dating of rocks in gneiss complexes which do not carry zircon from their protoliths. On Qilanngaarsuit (island), a gneiss sheet cutting a metaperidotite inclusion has an age of ∼3850 Ma. On an islet ∼2 km west of Akilia, tonalite and quartz-diorite which intrude and envelop two mafic amphibolite and hornblendite inclusions have dates of ∼3850 Ma also. With our proposed ages of &ges;3850 Ma, these inclusions are older than similar rocks in the 3700–3800 Ma Isua supracrustal belt. At other investigated localities ∼3850 Ma zircon dates have not been obtained from the orthogneisses enveloping or intruding inclusions. At two localities invasive leucogranite neosome and a discordant diorite sheet cutting inclusions have ages of 3600–3660 Ma—a period of known tectonothermal events including crustal melting and intrusion of gabbros and diorites. At three localities, banded-schlieric orthogneisses adjacent to supracrustal, gabbroic and ultramafic rocks contain 3800–3700 Ma igneous components. In these cases, the inclusions could still be &ges;3850 Ma and be equivalent in age to similar rocks on Akilia, but this has been masked by the local tectonothermal history and greater amount of neosome development. An alternative explanation is that some younger (<3850 Ma, but still &ges;3600 Ma) supracrustal and mafic/ultramafic inclusions (as pre-migmatisation tectonic slivers between older rocks?) might also be present within the migmatites. Thus different minimum zircon dates can be obtained for the inclusions by dating the varied components of orthogneisses. This study shows the difficulty of dating rock types in migmatite complexes that do not carry zircon from their protolith, and the importance of full integration of field observations and geochronology in these studies. [Copyright &y& Elsevier]

• Isua mantle peridotite 3.7 Ga UHP assemblages resemble modern subduction rocks. • UHP assemblages overprinted variably-serpentinised abyssal peridotites. • Incompletely serpentinised rocks preserve relict Fo 91-92 mantle olivine. • Total serpentinisation gave early serpentine + magnetite assemblages. • <4.0 δ18O of the Fo 97-98 olivine demonstrates recycling of volatiles into the deep Earth. The ~ 3700 Ma Inner Arc Group of the Isua supracrustal belt (Greenland) contains a 10 km long strip of ultramafic schists with two ≤ 1 km long meta-dunite lenses, preserving relict olivine + antigorite + titano-clinohumite and titano-chondrodite ~ 2.6 GPa ultra-high-pressure (UHP) assemblages. There are two distinct relict meta-peridotite variants: The southern lens 'A' variant is dominated by an aggregate of Fo 91-92 olivine with δ18O VSMOW of + 5.4 ± 0.1‰ (95% confidence) in which are rare small clinopyroxene inclusions, plus rare interstitial accessory chrome spinel partly altered to magnetite. The olivine grains are bounded by serpentine domains, which are intergrown with the olivine margins with a marginally more iron-rich composition of Fo 90.5. The northern lens 'B' contains coarser-grained Fo 96-98 olivine which encloses magnetite as clusters and trails, some arranged with 60°-120° conjugate intersections. The Fo 96-98 olivine has δ18O VSMOW of + 3.2 ± 0.2‰ and is in equilibrium with high-Al antigorite and the accessory UHP Ti-rich minerals. We interpret that the Fo 90-92 versus the Fo 96-98 olivine assemblage variants are caused by a varying degree of fluid influx during ca. 3700 Ma serpentinisation, prior to peak UHP metamorphism. The Fo 90-92 variant was a rock-dominated system with a postulated early serpentine + ferroan brucite alteration assemblage, with abundant relict mantle-like δ18O VSMOW + 5.4‰ olivine. The Fo 96-98 variant was a fluid-dominated system, where obliteration of the protolith olivine formed serpentine + magnetite + brucite ± magnesite. During prograde UHP metamorphism consumption of brucite by reaction with serpentine gave rise to a lower δ18O VSMOW Fo 96-98 olivine + serpentine assemblage. Both varieties have similar bulk chemistry, interpreted as depleted mantle that was enriched in LILE and LREE by a melt or fluid, prior to varying degrees of serpentinisation and superimposed UHP metamorphism. In Phanerozoic abyssal peridotites similar textural, chemical and isotopic variations are the result of rock versus fluid-dominated serpentinisation and variable alteration temperatures. Analogously, the Fo 96-98 Isua peridotites are interpreted as mantle peridotite entirely serpentinised by high temperature seawater in a fluid-dominated environment and then metamorphosed under UHP conditions at a convergent plate boundary. This indicates lateral lithosphere motions facilitating hydrosphere - mantle communication early in Earth's history. [ABSTRACT FROM AUTHOR]

• The granitoids are of higher in potassium than sodium. • SIMS zircon dating gives Neoarchean-early Paleoproterozoic protoliths ages (2.45–2.56 Ga). • Multiple metamorphic events at ~2.50 Ga, 1.8–1.9 Ga, ~160 Ma were identified. • The magmatism corresponds to incorporating juvenile crustal components and reworking of older Archean crust. • The Archean – Paleoproterozoic multiple magmatism and metamorphism of the RM are similar with eastern NCC. The geology of the Korean Peninsula's northern part is little known to the international community. Even though this area is a critical link between various eastern Asia terranes, its tectonic architecture and geochronological framework are still gaps in knowledge. This study reports integrated field, geochemical and geochronological characteristics of the Precambrian gneissic rocks from this region, focusing on the Rangnim Massif. The studied samples are divisible into three groups according to their field occurrence as granitic gneiss, fine-grained biotite gneiss and leuco-veins. The essential mineralogy of all these rocks is biotite (Bt), muscovite (Mu), plagioclase (Pl), K-feldspar (Kf) and quartz. The analyzed samples show high SiO 2 (65.92–77.63 wt%) and Al 2 O 3 (10.23 to 17.56 wt%) contents, low MgO and CaO. Their K 2 O contents is mostly higher than Na 2 O. SIMS (Secondary Ion Mass Spectrometry) zircon U-Pb dating of the eleven samples yields magmatic protolith ages of 2454–2567 Ma and multiple metamorphic overprintings during the earliest Paleoproterozoic (~2.45 Ga), Paleoproterozoic (1.87–1.93 Ga) and Jurassic (160 Ma). The presence of these potassic granitoids in different locations indicate the high extent maturation of Neoarchean continental crust in the Rangnim Massif. Zircon Lu-Hf isotopic results of these samples show large variations, mostly plotting between the Depleted mantle linear model line and Chondritic Uniform Reservoir (CHUR) line, but some have negative ε Hf (t) values. Considering that their protoliths are all crust melting products, it can be concluded that the Neoarchean magmatism incorporated both juvenile crustal components as well as a reworking of older crust. This is supported by the detection of rare early Archean xenocrysts. Based on Juvenile crust having ε Hf values of ~0 before 3500–3600 Ma, it is concluded that the reworked components were Eoarchean in age, not Hadean. Magmatic as well as Precambrian metamorphic pulses of the Rangnim Massif show broad similarities with the eastern North China Craton, Gyeonggi and Yeongnam Massifs and southwest Japan, which seem to suggest that they might share a coherent Precambrian basement and evolutionary history, at least before the end of late Paleoproterozoic. [ABSTRACT FROM AUTHOR]