Abstract The recycling of supracrustal materials, and in particular hydrated rocks, has a profound impact on mantle composition and thus on the formation of continental crust, because water modifies the physical properties of lithological systems and the mechanisms of partial melting and fractional fractionation. On the modern Earth, plate tectonics offers an efficient mechanism for mass transport from the Earth's surface to its interior, but how far this mechanism dates back in the Earth's history is still uncertain. Here, we use zircon oxygen (O) isotopes to track recycling of supracrustal materials into the magma sources of early Archean igneous suites from the Kaapvaal Craton, southern Africa. The mean δ18O values of zircon from TTG (tonalite–trondhjemite–granodiorite) rocks abruptly increase at the Paleo-Mesoarchean boundary (ca. 3230 million years ago; Ma), from mantle zircon values of 5‰–6‰ to approaching 7.1‰, and this increase occurs in ≤3230 Ma rocks with elevated Dy/Yb ratios. The 18O enrichment is a unique signature of low-temperature water–rock interaction on the Earth's surface. Because the later phase was emplaced into the same crustal level as the older one and TTG magmas would derive from melting processes in the garnet stability field (>40 km depth), we suggest that this evident shift in TTG zircon O isotopic compositions records the onset of recycling of the mafic oceanic crust that underwent seawater hydrothermal alteration at low temperature. The onset of the enhanced recycling of supracrustal materials may also have developed elsewhere in other Archean cratons and reflects a significant change in the tectonic realm during craton formation and stabilization, which may be important processes for the operation of plate tectonics on early Earth.

In this study, new geological, geochronological, geochemical, and Nd-Hf isotopic data are presented for the Melange Zone within the Zavkhan terrane, Mongolia, and the terrane structure, early Neoproterozoic continental crust growth, and microcontinent formation in the north-central part of the Central Asian Orogenic Belt (CAOB) are discussed. The Melange Zone separates high-grade complexes of the northwestern part of the Zavkhan terrane and unmetamorphosed Neoproterozoic Zavkhan Formation covered by Ciyogenian-Cambrian shelf deposits of the southwestern part. Zone consist of a lower-grade association of basalts, basaltic andesites, rarely felsic volcanic rocks, trondhjemites of the Kharuul Massif and variably metamorphosed from greenschist- to upper amphibolite-facies, and high-grade metamorphic rocks including quartzite-gneisses, hornblende schists, and amphibolites with relics of eclogite and blueschist-facies metamorphism assemblages. Emplacement of trondhjemites and gabbro dykes of the Kharuul Massif occurred at about 960-930 Ma. Geochemical, Nd whole-rock, and Hf-in-zircon isotopic data indicate an oceanic island arc setting for the lower-grade association. LA-ICP-MS dating and Hf-in-zircon data for detrital zircons from the quartzite-gneisses of the Melange Zone indicate that the sources comprise Palaeoproterozoic (ca. 2.02 and 2.48 Ga), Neoarchean (ca. 2.59 and 2.67 Ga), and Mesoarchean (ca. 2.8 and 3.0 Ga) magmatic and metamorphic crustal rocks as well as 2.6-2.5 Ga juvenile complexes. High-grade amphibolites show geochemical similarity to continental arc and within-plate basalts, and were formed from enriched mantle sources. Obtained combined with published data demonstrate that the Zavkhan terrane represents composite terrane composed of ca. 960-930 Ma island arc complex, and ca. 860-800 Ma active continental margin complexes in the north and reworked at ca. 800-720 Ma early Precambrian continental crust in the south, which are separated by a Melange Zone. We speculate that formation of the latest Mesoproterozoic-early Neoproterozoic island arc and active continental margin complexes in the north-central CAOB could be related to the assembly of the Rodinia supercontinent at ca. 1.1-1.0 Ga, which initiated subduction in Mirovian Ocean and led to the development of accretionary orogens around supercontinent margins. (C) 2020 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

The Agardagh Tes-Chem complex (ATCC) in Tuva, Central Asia (50.5 degrees N, 95 degrees E) exposes a rare mafic to ultramafic crust-mantle fragment that developed within a late Neoproterozoic (similar to 570 Ma) intra-oceanic island arc system that was accreted to the Tuva-Mongolian microcontinent during the formation of the Central Asian Orogenic Belt. Residual mantle rocks (harzburgites and dunites) are highly refractory with high Cr# (0.59-0.83) and intermediate Mg# (0.46-0.52) in spinel and experienced high degrees of total melt extraction (up to 25%). In ultramafic cumulate rocks (wehrlites and pyroxenites), Cr# and Mg# in spinel are distinctly lower (0.22-0.45 and 0.34-0.37), and rare earth element (REE) abundances of clinopyroxenes range between 0.1 and 1-times chondritic with chondrite normalized La/Yb of similar to 0.04. Trace element abundances in wehrlite clinopyroxene are lower than in pyroxenite clinopyroxene, whereas all have positive Sr but pronounced negative Zr anomalies. Calculated equilibrium melts of wehrlites and clinopyroxenites suggest strongly depleted hydrous high-MgO high-SiO2 primary magmas (picrites to boninites) from which these rocks have evolved. Incompatible trace element concentrations in mafic (i.e. gabbroic to dioritic) rocks vary strongly with REE contents between similar to 0.7 and 20-times chondritic, and chondrite normalized La/Yb between 0.25 and 0.98. Initial cm varies between +4.8 and +7.1. These features, along with the broad textural and modal variations indicate that the mafic rocks developed from a single intrusive body that underwent substantial crustal assimilation. Cr and Ni contents in the mafic rocks are low and indicate their formation from already differentiated magmas. The evolution of the ATCC prior to amalgamation is explained by a two stage magmatic process. During the initial stages of melting low degree magmas evolved from a refractory mantle wedge fluxed by hydrous fluids and sediments or sediment derived melts to form enriched upper crustal shallow level intrusive and extrusive rocks. Later stage melting from a less hydrous and more depleted but SiO2-enriched source produced the parental picrites/boninites from which the lower and middle arc crustal rocks formed. These magmas differentiated within at least two distinct magma reservoirs at lowermost crustal levels, where cumulate wehrlites formed mostly independently from cumulate pyroxenites. This is evident from distinctly different trace-element compositions of these two rock types. The wehrlite derivative magmas subsequently intruded to mid-crustal levels where they crystallized the mafic rocks within a single intrusion, but accompanied by assimilation of preexisting enriched arc crust.

The Sangilen and Khan-Khuhei blocks in the southwestern part of the Tuva-Mongolian terrane (TMT) combine into a composite structure, formed in the course of the Early Paleozoic (ca. 505–495 Ma) regional low- to moderate-pressure metamorphism. Manifestations of an earlier higher pressure metamorphism are known in both blocks. The upper age limit for the Sangilen block is defined by granites with an age of 536 ± 6 Ma. The early metamorphic rocks in both blocks are intruded by the granitoids of the Orto-Adyr complex with ages of 516 ± 5 and 513 ± 4 Ma, respectively. The emplacement of this complex preceded the Early Paleozoic (505–495 Ma) low- to moderate-pressure metamorphism. The high-temperature metamorphic rocks of the Sangilen and Khan-Khuhei blocks of TMT can be considered as the fragments of the Late Ediacaran higher pressure metamorphic belt, accreted to the TMT margin at ca. 510–505 Ma and reworked in the interval of 505–495 Ma under conditions of a regional low- to moderate-pressure metamorphism. The emplacement of the granitoids of the Orto-Adyr complex marks the conversion of the passive margin of the Neoproterozoic TMT block, covered by the Ediacaran carbonate platform, into an active margin. The geochronological studies of detrital zircons from the metaterrigenous rocks of the Sangilen block demonstrated that the source rocks of these zircons were mostly the Early Neoproterozoic magmatic rocks.

The Velké Vrbno Dome crops out at the boundary between the Brunovistulian Terrane and the internal parts of the Bohemian Massif. Here, eclogite boudins occur within an Ediacaran volcano-sedimentary sequence. Strong Nb depletion (Nb/Nb* = 0.19–0.82) combined with moderately positive Nd isotopic compositions (εNd (i) = +3.89 – +5.77) is used to argue for emplacement of the eclogite protoliths in a transitional supra-subduction to continental-rift setting. Conversely, heterogeneously enriched large ion lithophile elements and highly radiogenic Sr isotopic ratios ( 87 Sr/ 86 Sr = 0.705–0.720) are interpreted to have been modified following fluid infiltration subsequent to eclogite-facies metamorphism. U–Pb laser ablation inductively coupled plasma mass spectrometry dating of magmatic zircon from the rift-type eclogite indicates Early Cambrian emplacement ( c. 535 Ma) following episodic Ediacaran volcanic arc activity. Moreover, a continental setting is emphasized by zircon dating of a mylonitic orthogneiss, revealing a fragment of Palaeoproterozoic ( c. 2000 Ma) basement, the first such finding within the Brunovistulian Terrane sensu stricto . The new data from eclogite confirm that rifting in this segment of Gondwana pre-dated the Ordovician opening of the Rheic Ocean and therefore that the suture between Brunovistulia and the rest of the Bohemian Massif likely represents the vestige of an older hyperextended basin or oceanic tract. Supplementary material: Previously unpublished single zircon evaporation ages from Ediacaran orthogneiss from the Velké Vrbno Dome (supplement A); detailed analytical methodology (supplement B); whole rock geochemical data (supplement C); and U-Pb LA-ICP-MS zircon data (supplement D) are available at https://doi.org/10.6084/m9.figshare.c.5233079

The Galápagos Archipelago is the surface expression of an active hotspot or long-lived mantle plume. The Archipelago consists of a group of 13 main islands which are located in the eastern central Pacific Ocean about 1,000 km west of the northern edge of the South American continent, east of the East Pacific Rise and south of the Galápagos spreading center. Because of the large distance to the nearest continental land mass, Galapagos can be seen as an almost isolated sedimentary system. A provenance study conducted on samples collected from seventeen beaches on eleven islands, demonstrates that mineral grains and particles were derived from weathering of predominantly basaltic rocks and were transported within the islands, between the islands or inside the coastal area around the Archipelago. The exclusion of external sources allows advanced studies about erosion processes, transport pathways of particles and the accumulation of autochthonous sediments. The combined usage of optical particle size and shape analysis with RAMAN spectroscopy allows a successful spatial delimitation of host rocks and a reconstruction of transport pathways. The analyzed samples can be subdivided into three groups: 1) Type-A sediments: fine-grained and sampled on beaches of the oldest islands in the eastern part of Galápagos. The composition of volcanic minerals corresponds to the alkaline character of the basaltic source rocks. 2) Type-B: well sorted sediments characterized by medium-grained olivine, pyroxene, plagioclase and even a small amount of quartz grains. The islands of this group are located in the central region of the Archipelago. 3) Type-C samples: olivine and pyroxene are the predominant volcanic minerals. These samples indicate bimodal, coarse-grained size distributions and large proportions of pumice and are found in Floreana in the south and the youngest islands Isabela and Fernandina in the west of Galápagos.

Sequential leaching was completed by successive interactions of ultra-pure H2O, HAc, HCl and HNO3 with silicified/carbonated Archaean whole rocks from Barberton Greenstone belt of South Africa. The purpose of this experiment was an identification of the minerals interacting with each reagent and, therefore, a detailed succession of isotopically dated tectonic-thermal episodes recorded in these minerals. The contents of the major, trace and rare-earth elemental, together with the Sr, Nd and Pb isotopic compositions of untreated, leachate and residue triplets allowed identification and analysis of various types of carbonates and sulfates mixed with insoluble silicates. The samples yield two overall geological age milestones at 2.9 +/- 0.1 and 2.1 +/- 0.1 Ga, as well as varied age values, some with large uncertainties. A further event at about 1.6 +/- 0.1 Ga is suggested probably in relation with the emplacement of the Bushveld complex or of further intrusive complexes of the Kaapvaal Craton. While the basics of the different used isotopic systems can be considered to be similar, the Rb-Sr method appears to be the best suited to detail the studied rocks, especially their sensitive soluble minerals. The Sm-Nd method is less flexible because of its inherent need of widely ranging Sm/Nd ratios that depend on the mineral assemblage. In the case of the Rb-Sr ages well constrained by isochron arrays, more similar ages were obtained by tentatively called isotrends that consist in data points slightly scattered along the arrays giving higher uncertainties, especially with the Sm-Nd method. The fact that these isotrends gave similar ages in addition to those of the isochrons comforts the historic evolution of the studied material. These scatters most probably correspond to slight changes in the chemical characteristics of minerals leached by the successive reagents, but analytical uncertainties cannot be completely excluded, especially in the case of the HNO3 leaching step. Finally, the Pb-Pb method appears to be of a more limited application, possibly because of possible metal contamination of the host rocks, either during their evolution in a region characterized by metal-rich concentrations or more recently during discrete surficial to sub-surficial alteration/weathering processes. In terms of leaching efficiency, that with H2O removed expectedly the lowest amounts of soluble components followed by HNO3, the two efficient removers being dilute HAc and HCl. In turn, the obtained geochronological ages consolidate previously published ages that concentrate on post-depositional tectonicthermal events within the Kaapvaal Craton during about 1.0 Ga. However, they also suggest some reconsideration for other ages of the literature.

Tonalite, Trondhjemite, Granodiorite (TTG) rocks in Viti Levu, Fiji islands formed through hydrous melting of gabbroic oceanic crust at low-pressure amphibolite-facies conditions caused by flat subduction of an oceanic plateau from Yavuna creek. During mid Miocene time, magmatic underplating took place and a Qtz-diorite unit was formed out of the gabbro under granulite-facies conditions. The investigated TTG´s occur as stocks and veins within the older gabbroic unit of the Yavuna Pluton. Zircon ages show the parental gabbro to be ~47.5 Ma in age, whereas the TTG´s, which can be subdivided into a tonalite and a Qtz-diorite suite, are ~37.1 Ma and ~16.5 Ma, old respectively. The average d18O value of ~4.8 in zircon selected from the parental gabbro and the tonalite suggest a very homogenous mantle source. However, about 50% of the analyzed zircons from the gabbroic and tonalitic rock samples showing lower d18O values, and these are interpreted as reflecting interaction of hydrothermally altered seafloor with the deep depleted mantle source. eHf in zircon values of ~13 in the analyzed TTG´s are interpreted as reflecting typical juvenile continental crust. PerpleX whole-rock calculations suggest that the tonalite formed by melting of the gabbro through decompression under water-saturated amphibolite-facies conditions at a temperature of ~770 °C and a pressure of ~3.8 kbar, whereas the Qtz-diorite formed at a temperature up to ~900 °C at very shallow depth close to the Earth’s surface caused by the emplacement of a magmatic underplate during the mid Miocene. Our investigation provides new evidence for episodic growth of continental crust < 0.1 Ga in the South Pacific region.

Archean supracrustal rocks of amphibolite-facies occur as enclaves within granitoids gneiss domes and belts between domes, representing collision or sagduction regimes. In order to distinguish between tectonic regimes using metamorphic patterns, systematic data on metamorphic evolution and zircon age dating are presented for the Caozhuang supracrustal sequence of eastern Hebei Province, China, which occurs as enclaves in gneisses. A garnet biotite gneiss records a P-T path involving pre-peak isobaric heating to peak conditions at 780-800 degrees C and 10-11 kbar (medium-PIT type), followed by decompression to 5-6 kbar. Two Mg/Al-rich schists from the same locality show low P/T conditions of 6-7 kbar/690-750 degrees C and 4-7 kbar/750-780 degrees C, similar to that of the decompression stage for the biotite gneiss. Zircon age dating suggests that the amphibolite-facies metamorphism occurred at similar to 2.49 Ga, coeval with TTG magmatic activities. Combined with a summary of the metamorphism from Kaapvaal and Pilbara Cratons, we concluded that Archean amphibolite-facies supracrustal rocks occurring as enclaves and belts show different metamorphic patterns. The former exhibits the metamorphic patterns the same as those recovered for the Caozhuang rocks, characteristic of pre-peak isobaric heating to 600-800 degrees C at 10-12 kbar followed by isothermal decompression with the overprinting of medium- and/or low-PIT assemblages in different enclaves. The latter shows medium- or high-P/T peak conditions mostly lower than 600-650 degrees C followed by decompression, lacking the pre-peak heating. These metamorphic patterns could be good indicators of a sagduction regime.

Tonalite, trondhjemite, granodiorite (TTG) rocks in Viti Levu, Fiji Islands formed through hydrous partial melting of gabbmic oceanic crust at low-pressure amphibolite-facies conditions caused by flat subduction of an oceanic plateau from Yavuna creek during late-Eocene time. This was followed by the formation of a quartz diorite unit by anatexis of the gabbro in the mid-Miocene. The TTGs occur as migmatitic discordant stocks and dykes within the older gabbroic unit of the Yavuna Pluton. Zircon ages show the parental gabbro to be similar to 47.5 Ma in age, whereas the TTGs, consisting of a tonalite and a quartz diorite suite, are similar to 37.1 Ma and similar to 16.5 Ma old, respectively. The average delta O-18 value of 4.8 parts per thousand in zircons from the gabbro and the tonalite suggest a homogenous mantle source. However, about 50% of the analyzed zircons from the gabbmic and tonalitic rock samples show lower delta O-18 values than this average, and are interpreted as reflecting interaction of hydrothermally altered seafloor with the deep mantle source. The zircon epsilon Hf of 13 in the analyzed TTGs is interpreted as reflecting typical juvenile continental crust. PerpleX whole-rock calculations supports that the tonalite formed by hydrous partial melting of the gabbro through decompression under water-saturated amphibolite-facies conditions at a temperature of similar to 770 degrees C at 3.8 kbar, whereas the quartz diorite formed by anatexis under granulite-facies conditions at similar to 800 degrees C at similar to 1.7 kbar at very shallow depth. Our investigation provides new evidence that the tonalite formed by hydrous partial melting and the quartz diorite by metasomatism and anatexis of an extant gabbroic rock/crust at < 0.1 Ga in the South Pacific region.

The timing of the emergence of modern-style plate tectonics on Earth is of fundamental importance in understanding the thermal and compositional history of the planet. Although the magmatism and metamorphism in the eastern Kaapvaal Craton was thought by some as geological records for mid-Archean subduction at 3.2 Ga, their petrogenesis was fiercely debated. To reveal the nature of the 3.2 Ga magmatism and metamorphism in the eastern Kaapvaal Craton, here we provide a comprehensive zircon and monazite U-Pb geochronological investigation, coupled with in situ trace element and Hf-O isotopic analyses, for the magmatic and metamorphic rocks from the Barberton granitoid-greenstone terrane in South Africa and Ancient Gneiss Complex in Swaziland. In spite of deriving from potentially different sources as revealed by different lithology, nine magmatic rocks sampled from different plutons have consistent zircon/monazite SIMS Pb-207/Pb-206 ages of 3240-3220 Ma. Distinct zircon grains, including inherited magmatic and metamorphic, are identified in the metamorphic rocks by detailed CL images and U-Pb geochronological, trace elemental and Hf-O isotopic analyses. The metamorphic zircons give consistent Pb-207/Pb-206 ages of 3240-3220 Ma. One granitic gneiss and a metapelite give monazite SIMS Pb-207/Pb-206 dates of 3230.6 +/- 2.7 Ma and 3223.1 +/- 2.4 Ma, respectively, which are consistent with the results of metamorphic zircon and interpreted as the age of metamorphism. Most metamorphic samples give Ti-in-zircon temperatures of 650-740 degrees C and one sample yields Y-in-monazite temperatures of 918-966 degrees C, which imply that the rocks of AGC were universally metamorphosed to upper amphibolite facies, and locally reaching high to ultrahigh temperature (HT-UHT) granulite facies (>900 degrees C) at 3240-3220 Ma. This study, in combination with previous results, suggests widespread distribution for the 3.2 Ga magmatism and metamorphism in the eastern Kaapvaal Craton without asymmetry in the thermal structure, which is quite different from those of the modern subduction zones. The synchronous occurrence of the metamorphism and magmatism in the eastern Kaapvaal Craton at 3.2 Ga was interpreted as the results of partial convective overturn of the crust.

The oldest magmatic rocks from the South Tianshan orogen of Kyrgyzstan (STS) are important for better understanding of the Neoproterozoic and early Palaeozoic evolution of the southwestern Central Asian Orogenic Belt. Bulk rock major and trace element and Sm-Nd isotopic composition and zircon U-Pb ages of granitoids from melange blocks reveal two previously unknown episodes of arc magmatism in the STS orogen of Kyrgyzstan, namely, the Ediacaran (ca. 624 Ma) and Early Ordovician (ca. 472 Ma) episodes. Moderately positive epsilon Nd(t) value of + 5.8 of the Ediacaran granodiorite indicates a mainly juvenile source for this rock. Negative epsilon Nd(t) value of - 9.1 for the Early Ordovician granodiorite indicates melting of the predominantly Precambrian crustal material. The latter is also supported by the inherited zircon cores with an age range of 1.04-2.7 Ga. The geochemical signature of the Silurian (late Llandovery and Wenlock) basalts suggests formation in arc settings. Arc-related origin of the Ediacaran granodiorite suggests that mature oceanic basin with subduction system existed in the western STS since the late Neoproterozoic. The arc also evolved in the Ordovician and Silurian and was inactive in the middle Cambrian to the earliest Ordovician. Ediacaran granodiorite, old zircon cores and negative epsilon Nd(t) values of the Ordovician granodiorite suggest the existence of Precambrian microcontinents in the Kyrgyz STS. These crustal fragments were likely rifted off the Tarim and / or Karakum-Tajik microcontinents, where the rocks with similar ages occur. In the Palaeozoic, they traveled north and were incorporated in the STS orogen in the late Carboniferous.

This study investigates garnet corona textures in high-grade granulite facies meta-anorthosites from south central Tanzania. Garnet coronas form due to drastic changes in P-T conditions leading to instability of certain minerals and, consequently, to a change in the mineral paragenesis for a given bulk rock chemical composition. The garnet corona textures from this study, separate an metastable magmatic core inside the corona from the surrounding matrix minerals. In contrast to previous investigations, the garnet in the matrix are formed prograde by isobaric heating, followed by isothermal compression and finally in a second stage garnet coronas are formed by isobaric cooling. The garnet corona forming reaction was triggered by an internal fluid flux released by the breakdown of scapolite during prograde metamorphism. Three types of garnet coronas could be identified; (i) garnet corona around metastable magmatic ilmenite rimmed by titanite, plagioclase and clinopyroxene, which is a product of the breakdown reaction of scapolite. (ii) garnet corona around original plagioclase and clinopyroxene, and (iii) garnet corona around metastable magmatic chalcopyrite, hematite and pyrite surrounded by retrograde formed scapolithe. Peak metamorphic conditions at ~780°C and ~12.4 kbar have been identified for the suggested anticlockwise P-T path. To get evidence of the main rock forming mineral reactions during high grade metamorphism, Laser ICPMS analyes

The geodynamic setting of the 3.44-3.46 Ga Dwalile greenstone remnant (DGR) of the Ancient Gneiss Complex (Swaziland) and its relationship to other Greenstone belts of similar age is investigated in this contribution. We present Hf-Nd isotope, major and trace element data for komatiites, komatiitic basalts, basalts, basaltic andesites as well as felsic volcanic and sedimentary rocks, all metamorphosed at amphibolite-facies grade. Major and trace element compositions of metabasalts and metakomatiites indicate crustal contamination. The best-preserved samples yield Lu-176-Hf-176 and Sm-147-Nd-143 isochron ages of ca. 3500 and 3460 Ma, respectively, in line with U-Pb zircon ages of interlayered felsic volcanic rocks of 3460 Ma. Isotope and trace element data for the metasedimentary rocks reveal a contribution of older crustal material with 3.53 Ga zircon grains. Together with >3.7 Ga inherited zircon grains in the Ngwane gneiss (NG) of the basement, this evidence supports a model of deposition of the Dwalile supracrustal rocks on or near older continental crust. Initial epsilon Hf and epsilon Nd values of the least contaminated DGR metabasalts and metakomatiites are similar to those of the contemporaneous Komati Formation komatiites and tholeiitic basalts, however, the trace element composition and slight correlations of initial epsilon Hf and epsilon Nd values with major and trace element parameters indicative for crustal contamination reveal differences in the basement of both units.

The P-T-t path of high-grade metamorphic rocks is significant for understanding vertical motions and heating of the crust. In general, garnet is used to constrain P-T paths and metamorphic zircon is used to obtain t for different metamorphic stages. However, this approach may be complicated by polyphase tectonothermal events. The Central Zone of the Limpopo Belt in South Africa is a complex Precambrian metamorphic terrane that has experienced three distinct high-grade metamorphic events at 3.22 Ga (M-1), 2.62 Ga (M-2), and 2.02 Ga (M-3), based on zircon dating. A previous investigation of a metapelite enclave in the Bulai pluton in the Central Zone showed that metamorphic zircon in the enclave recorded mainly M-2 granulite-facies metamorphism and was "protected" by the Bulai pluton. However, titanite and rutile U-Pb ages of the same sample record the M-3 event. Here we investigated garnet Sm-Nd isotopic systematics of four metapelite samples from the Evelyn enclave in the Bulai pluton. We constructed single mineral Sm-Nd isochrons for garnet using micro-samples, in order to avoid the risk of isotopic disequilibrium between different minerals or whole-rocks. Metamorphic zircon with sillimanite inclusions has yielded a granulite-facies metamorphic age of ca. 2.6 Ga. However, the Sm-Nd isotopic data for the garnet record much younger ages of ca. 2.03 Ga. These ages are consistent with the M-3 event in this area, but were rarely identified in previous zircon dating studies. This suggests that the M-3 event reached sufficiently high temperatures to totally reset the rutile and titanite U-Pb and garnet Sm-Nd systems. Moreover, the decoupling of the age of garnet formation and the date it records needs to be carefully considered when reconstructing P-T-t paths, and again highlights the usefulness of garnet in the study of polyphase metamorphic terranes.

The Paleo-Asian Ocean (PAO), evolution from ocean opening at ca. 1020 Ma to final ocean closure in the Permian and collision of the North China, Siberia and Tarim cratons with formation of an orogenic collage lasted some 800 Ma (Kröner et al., 2014), and its subduction-accretion history is an important element to study the evolution of the Central Asian Orogenic Belt (CAOB). Although much remarkable progress has been made to study the closure of the eastern Paleo-Asian Ocean, the deep structure of the crust and its evolution is less constrained, and two outstanding issues need to be resolved (Wilde, S.A., 2015). One is to define the eastern limit of the CAOB, the other controversial issue is the view that the CAOB contains numerous Precambrian microcontinental blocks of mainly Neoproterozoic age, and their tectonic relationship still remains to be studied.

We addressed when plate-tectonic processes first started on Earth by examining the ca. 2.0 Ga Limpopo orogenic belt in southern Africa. We show through palinspastic reconstruction that the Limpopo orogen originated from >600 km of west-directed thrusting, and the thrust sheet was subsequently folded by north-south compression. The common 2.7-2.6 Ga felsic plutons in the Limpopo thrust sheet and the absence of an arc immediately predating the 2.0 Ga Limpopo thrusting require the Limpopo belt to be an intracontinental structure. The similar duration (similar to 40 m.y.), slip magnitude (>600 km), slip rate (>15 mm/yr), tectonic setting (intracontinental), and widespread anatexis to those of the Himalayan orogen lead us to propose the Limpopo belt to have developed by continent-continent collision. Specifically, the combined Zimbabwe-Kaapvaal craton (ZKC, named in this study) in the west (present coordinates) was subducting eastward below an outboard craton (OC), which carried an arc equivalent to the Gangdese batholith in southern Tibet prior to the India-Asia collision. The ZKC-OC collision at ca. 2.0 Ga triggered a westward jump in the plate convergence boundary, from the initial suture zone to the Limpopo thrust within the ZKC. Subsequent thrusting accommodated >600 km of plate convergence, possibly driven by ridge push from the west side of the ZKC. As intracontinental plate convergence is a key modern plate-tectonic process, the development of the Limpopo belt implies that the operation of plate tectonics, at least at a local scale, was ongoing by ca. 2.0 Ga on Earth.

The eastern part of the Kaapvaal Craton represents a classical granitoid-greenstone terrain and contains the oldest rocks of the African continent, exhibiting about 1000 Ma of crustal evolution from 3.66 to 2.67 Ga. The granitoid rocks predominantly consist of the tonalite-trondhjemite-granodiorite (TTG) association with true granites becoming abundant at about 3 Ga. Greenstones are represented by the well-preserved and well-studied 3.54-3.2 Ga Barberton Greenstone Belt and smaller ca. 3.45 Ga greenstone belt remnants infolded in TTG gneisses around the BGB as well as in the Ancient Gneiss Complex in Swaziland. The origin of both the TTGs and greenstone units is still debated as strong deformation and medium- to high-grade metamorphism have obliterated most of the original rock relationships. This is largely due to an extensive tectono-magmato-metamorphic event at ca. 3.2 Ga that affected virtually the entire eastern part of the craton. Heat provided by mantle-derived melts during this event led to extensive intracrustal melting, with melt migration resulting in depletion of the lower crust in radioactive and other mobile elements. Long-lived extraction of granitoid magmas up to about 2.67 Ga increased the rigidity of the lower crust, causing tectonic stabilization of the Kaapvaal Craton.

Abundant late Neoarchean granitoids occur in southwestern Liaoning Province, part of the Eastern Ancient Terrane of the North China Craton. These rocks include intermediate gneiss, TTG gneisses and potassic granitoids, and we report on the geochemistry and zircon SHRIMP ages as well as Hf-in-zircon isotopes of these granitoids in order to determine their petrogenesis. Field relationships suggest that most of these granitoids experienced widespread metamorphism and deformation, associated with anatexis at some localities. The intermediate gneisses, TTG gneisses and potassic granitoids were all emplaced at the end of the Neoarchean (2.50-2.53 Ga), and CL images document widespread recrystallization in the zircons. The intermediate and TTG gneisses yielded similar Hf isotopic systematics (epsilon(Hf(t)) = -3.73 to +6.42) as the associated potassic granitoids (epsilon(Hf(t)) = -2.44 to +7.80). and both rock types yielded mean Hf crustal model ages of 2.8-2.9 Ga. Combined with the geochemistry, we propose that the formation of the intermediate and TTG gneisses was related to partial melting of mafic rocks at different depth, whereas the potassic granitoids have variable petrogenesis. The nearly coeval TTG gneisses and potassic granitoids and their widespread metamorphism, deformation and zircon recrystallization suggest that a large-scale heat source must have been present at or near the base of the crust in southwestern Liaoning Province at the end of the Neoarchean. We propose that collision and post-collisional extension is the most likely tectonic environment for generation of the above granitoids, and the formation of widespread potassic granitoids played an important role in the maturation of continental crust in the North China Craton. (C) 2019 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.