The Mesoarchean emergence of modern-style subduction

Well-preserved volcanic sequences span the Paleoarchean to Neoarchean evolution of the Pilbara Craton, in northwestern Australia. This region provides the best physical evidence bearing on the stage of Earth's history when modern-style tectonic processes began. Paleoarchean assemblages in the eastern nucleus of the craton (the 3.51–3.24 Ga Pilbara Supergroup) show few features that can reasonably be interpreted as evidence for modern-style subduction processes. Incompatible trace element-enriched felsic volcanic horizons show geochemical evidence for the interaction between mafic magmas and crust, but this evidence, on its own, can equally well be interpreted in terms of either a subduction-enriched mantle source or local and limited assimilation of felsic crust into the voluminous tholeiitic magmas that dominate the Pilbara Supergroup. Viewed in context within the thick autochthonous and consistently upward-younging Pilbara Supergroup, these felsic units are most likely related to the same plume-dominated processes that formed the basalts that dominate the supergroup. It is very unlikely that modern-style plate tectonic processes played any role in the Paleoarchean evolution of the Pilbara Craton, although some form of non-uniformitarian (e.g. flat) subduction process may have operated.In stark contrast, the Mesoarchean units of the West Pilbara Terrane and the late-tectonic basins that cover that boundary between the West and East Pilbara Terranes, show clear evidence for modern-style convergent margin processes. Igneous rocks in this belt, which flanks the old eastern cratonic nuclei, have enriched geochemical signatures that cannot be accounted for by crustal contamination. This region is also characterised by a linear magmatic and structural fabric, by the presence of lithologically and geochronologically exotic belts, and by the presence of a broad belt of isotopically more juvenile crust. The collective strength of these arguments provides compelling evidence that a modern-style oceanic arc fringed the East Pilbara Terrane at 3.12 Ga and accreted to that terrane by 2.97 Ga. These assemblages mark the minimum age for the birth of modern-style plate subduction process.     

The Hadean–Archaean transition at 4 Ga: From magma trapping in the mantle to volcanic resurfacing of the Earth

The Hadean–Archaean transition is poorly known because of the dearth of Hadean rocks. A new conceptual model is presented based on variations in mantle potential temperature (T_p) with time. The critical issue is the depth of melting with respect to a negatively buoyant magma sink between 410 and 330 km (14–11 GPa). Hadean plume magmatism begins below the magma sink, leading to generation of a refractory upper mantle reservoir and the minor production of boninite‐like magmas near the surface. With cooling, the onset of melting migrates above the magma sink, a situation likely occurring since 3.9 Ga and corresponding to T_ps of ~1870°C or less. Therefore, a burst of mafic to ultramafic volcanism was produced at 3.9–3.8 Ga. This extensive volcanism may have triggered gravitational instabilities and favoured the recycling of the Hadean crust into the mantle. Results of this model are discussed in the light of existing isotopic data.     

Early Palaeozoic rift-related magmatism in Variscan Europe: fragmentation of the Armorican Terrane Assemblage

Early Palaeozoic bimodal rift-related magmatism is widespread throughout much of the Variscides of Europe. It is traceable from the Polish Sudetes to NW Iberia. Granitic plutonism generally predates Cambro–Ordovician bimodal magmatism. In the N Bohemian Massif this early Palaeozoic granitic plutonism was generated by partial melting of Cadomian basement, whereas contemporaneous alkali granites with a mantle component are typical of the NW Iberian Massif. Silurian-Devonian mafic magmatism in the N Bohemian Massif, Massif Central and NW Iberian Massif is partly preserved as obducted ophiolites. Compositional diversity displayed by Cambro-Ordovician mafic magmatism can be accounted for by interaction between a spreading centre and an upwelling mantle plume. This indicates that combined tensional forces and mantle plume convection assisted the early Palaeozoic dispersal of terranes from the N Gondwana margin. Continued fragmentation resulted in development of an archipelago of related terranes separated by a network of seaways and formation of oceanic crust.     

Eastern Ancient Terrane of the North China Craton

Based on the spatial distribution of ancient rocks and zircons, three ancient terranes older than ca. 2.6 Ga have recently been identified in the North China Craton, namely the Eastern, Southern, and Central Ancient Terranes. The Eastern Ancient Terrane is the best studied and understood of the three ancient terranes. It has a long geological history back to ca. 3.8 Ga ago and includes the areas of Anshan-Benxi, eastern Hebei, eastern Shandong and western Shandong. In Anshan-Benxi, several different types of 3.8 Ga rocks were discovered together with 3.1-3.7 Ga rocks, whereas 2.9-3.0 Ga K-rich granites and 2.5 Ga syenogranite occur on larger scales. In eastern Hebei, 3.0-3.4 Ga rocks and older detrital and xenocrystic zircons were identified. In eastern Shandong, there are a large volumes of 2.7 Ga and 2.9 Ga rocks. In western Shandong, early Neoarchean(2.6-2.7 Ga) intrusive and supracrustal rocks are widely distributed. Whole-rock Nd and zircon Hf isotope data suggest that both mantle additions and crustal recycling played important roles within the Eastern Ancient Terrane during almost every tectono-magmatic event. Most BIFs in the North China Craton are late Neoarchean in age and are distributed on continental crust along the western margin of the Eastern Ancient Terrane, probably suggesting that a stable environment was one of the key factors for the formation of large-scale BIFs.     

A general model for the intrusion and evolution of 'mantle' garnet peridotites in high-pressure and ultra-high-pressure metamorphic terranes

Garnet‐bearing peridotite lenses are minor but significant components of most metamorphic terranes characterized by high‐temperature eclogite facies assemblages. Most peridotite intrudes when slabs of continental crust are subducted deeply (60–120 km) into the mantle, usually by following oceanic lithosphere down an established subduction zone. Peridotite is transferred from the resulting mantle wedge into the crustal footwall through brittle and/or ductile mechanisms. These ‘mantle’ peridotites vary petrographically, chemically, isotopically, chronologically and thermobarometrically from orogen to orogen, within orogens and even within individual terranes. The variations reflect: (1) derivation from different mantle sources (oceanic or continental lithosphere, asthenosphere); (2) perturbations while the mantle wedges were above subducting oceanic lithosphere; and (3) changes within the host crustal slabs during intrusion, subduction and exhumation. Peridotite caught within mantle wedges above oceanic subduction zones will tend to recrystallize and be contaminated by fluids derived from the subducting oceanic crust. These ‘subduction zone peridotites’ intrude during the subsequent subduction of continental crust. Low‐pressure protoliths introduced at shallow (serpentinite, plagioclase peridotite) and intermediate (spinel peridotite) mantle depths (20–50 km) may be carried to deeper levels within the host slab and undergo high‐pressure metamorphism along with the enclosing rocks. If subducted deeply enough, the peridotites will develop garnet‐bearing assemblages that are isofacial with, and give the same recrystallization ages as, the eclogite facies country rocks. Peridotites introduced at deeper levels (50–120 km) may already contain garnet when they intrude and will not necessarily be isofacial or isochronous with the enclosing crustal rocks. Some garnet peridotites recrystallize from spinel peridotite precursors at very high temperatures (c. 1200 °C) and may derive ultimately from the asthenosphere. Other peridotites are from old (>1 Ga), cold (c. 850 °C), subcontinental mantle (‘relict peridotites’) and seem to require the development of major intra‐cratonic faults to effect their intrusion.     

Age of younger tonalitic magmatism and granulitic metamorphism in the South Indian transition zone (Krishnagiri area); comparison with older Peninsular gneisses from the Gorur–Hassan area

Abstract A major episode of continental crust formation, associated with granulite facies metamorphism, occurred at 2.55–2.51 Ga and was related to accretional processes of juvenile crust. Dating of tonalitic–trondhjemitic, granitic gneisses and charnockites from the Krishnagiri area of South India indicates that magmatic protoliths are 2550–2530 ± 5 Ma, as shown by both U–Pb and ²⁰⁷Pb/²⁰⁶Pb single zircon methods. Monazite ages indicate high temperatures of cooling corresponding to conditions close to granulite facies metamorphism at 2510 ± 10 Ma. These data provide precise time constraints and Sr–Nd isotopes confirm the existence of late tonalitic–granodioritic juvenile gneisses at 2550 Ma. Pb single zircon ages from the older Peninsular gneisses (Gorur–Hassan area) are in agreement with some previous Sr ages and range between 3200 ± 20 and 3328 ± 10 Ma. These gneisses were derived from a 3.3–3.5-Ga mantle source as indicated from Nd isotopes. They did not participate significantly in the genesis of the 2.55-Ga juvenile magmas. All these data, together with previous work, suggest that the 2.51-Ga granulite facies metamorphism occurred near the contact of the ancient Peninsular gneisses and the 2.55–2.52-Ga ‘juvenile’tonalitic–trondhjemitic terranes during synaccretional processes (subduction, mantle plume?). Rb–Sr biotite ages between 2060 and 2340 Ma indicate late cooling probably related to the dextral major east–west shearing which displaced the 2.5-Ga juvenile terranes toward the west.     

Australian island arcs through time: Geodynamic implications for the Archean and Proterozoic

The operation and extent of modern-style plate tectonics in the Archean and Paleoproterozoic are controversial, although subduction and terrane accretion models have been proposed for most Archean cratons in the world, including both the Yilgarn and Pilbara Cratons of Western Australia. The recognition of ancient island arcs can be used to infer convergent plate margin processes, and in this paper we present evidence for the existence of several intraoceanic island arcs now preserved in Australia. Beginning in the Archean, Australia evolved to its present configuration through the accretion and assembly of several continental blocks, by convergent plate margin processes. In Australia, possibly the best example of an Archean island arc (or primitive continental arc) is preserved within the Mesoarchean (ca. 3130–3112 Ma) Whundo Group in the Sholl Terrane of the West Pilbara Superterrane. Two younger, Neoarchean, island arc terranes, and associated accretion, have also been proposed for the Yilgarn Craton: the Saddleback island arc (ca. 2714–2665 Ma) in the southwest Yilgarn Craton and the Kurnalpi island arc (ca. 2719–2672 Ma) in the eastern Yilgarn Craton. In the early Proterozoic, in the Central Zone of the Halls Creek Orogen, northern Western Australia, the Tickalara Metamorphics (ca. 1865–1850 Ma) have been interpreted to represent an island arc. In the southwest Gawler Craton in South Australia, the St Peter Suite (ca. 1631–1608 Ma), of juvenile I-type calcalkaline tonalite to granodiorite, possibly represents an island arc. In the Musgrave Province in central Australia, age and geochemical constraints are poor due to later overprinting tectonic events, but felsic orthogneisses (ca. 1607–1565 Ma) possibly represent juvenile felsic crust which was emplaced though subduction-related processes into an oceanic island arc. The arcs are volumetrically insignificant, but important, in that they separate much larger tracts of, usually older, continental crust, often of different composition and geological history. The arcs were sutured to continental crust during arc–continent collisional events, which eventually resulted in the assembly of much of present-day Australia. The arcs, thus, indicate lost oceanic crust. The recognition of island arcs in the ancient rock record indicates that subduction processes, similar in many ways to modern day processes at convergent plate margins, were operating on Earth by at least 3100 Ma ago.     

Isotopic characteristics of river sediments on the Tibetan Plateau

We systematically collected 40 modern clastic sediment samples from rivers in different tectonic units of the Tibetan Plateau and measured their Sr–Nd isotopic compositions. The isotopic characteristics provide insight into the controversial paleo-tectonic affinity of terranes of the Tibetan Plateau and the provenance of Songpan–Ganzi flysch complex. The Qilian Terrane and Himalaya Terrane have more negative ε_Nd(0) values (from ? 14.3 to ? 11.8 and from ? 20.64 to ? 13.26, respectively) and high ⁸⁷Sr/⁸⁶Sr values (from 0.719674 to 0.738818 and from 0.721020 to 0.824959, respectively), reflecting old and mature continental crust origin of these two terranes. The southern Lhasa Terrane is more radiogenic in ε_Nd(0) values (from ? 8.82 to ? 3.8) and low in ⁸⁷Sr/⁸⁶Sr values (from 0.711504 to 0.719489), implying the combined impact of the Neo-Tethys mantle and Himalaya old continental crust. Sr–Nd isotopic compositions of the Qilian Terrane are similar to those in the Yangtze Craton, indicating that the Qilian Terrane was probably separated from the Yangtze Craton. Sr–Nd isotopic characteristics of the Songpan–Ganzi Terrane are similar to the Yangtze Craton and are remarkably different to those in the North China Craton, eastern Kunlun–Qaidam and the central Qiangtang metamorphic belt, implying that the widely distributed flysch complex of the Songpan–Ganzi Terrane was sourced from the Yangtze Craton.     

High-pressure granulites: formation, recovery of peak conditions and implications for tectonics

High‐pressure granulites are characterised by the key associations garnet‐clinopyroxene‐plagioclase‐quartz (in basic rocks) and kyanite‐K‐feldspar (metapelites and felsic rocks) and are typically orthopyroxene‐free in both basic and felsic bulk compositions. In regional metamorphic areas, two essential varieties exist: a high‐ to ultrahigh‐temperature group and a group representing overprinted eclogites. The high‐ to ultrahigh‐temperature type formerly contained high‐temperature ternary feldspar (now mesoperthite) coexisting with kyanite, is associated with garnet peridotites, and formed at conditions above 900 °C and 1.5 GPa. Clinopyroxene in subordinate basic rocks is Al‐rich and textural evidence points to a high‐pressure–high‐temperature melting history. The second variety contains symplectite‐like or poikilitic clinopyroxene‐plagioclase intergrowths indicating former plagioclase‐free, i.e. eclogite facies assemblages. This type of rock formed at conditions straddling the high‐pressure amphibolite/high‐pressure granulite field at around 700–850 °C, 1.0–1.4 GPa. Importantly, in the majority of high‐pressure granulites, orthopyroxene is secondary and is a product of reactions at pressures lower than the peak recorded pressure. In contrast to low‐ and medium‐pressure granulites, which form at conditions attainable in the mid to lower levels of normal continental crust, high‐pressure granulites (of nonxenolith origin) mostly represent rocks formed as a result of short‐lived tectonic events that led to crustal thickening or subduction of the crust into the mantle. Short times at high‐temperature conditions are reflected in the preservation of prograde zoning in garnet and pyroxene. High‐pressure granulites of both regional types, although rare, are known from both old and young metamorphic terranes (e.g. c. 45 Ma, Namche Barwa, E Himalaya; 400–340 Ma, European Variscides; 1.8 Ga Hengshan, China; 1.9 Ga, Snowbird, Saskatchewan and 2.5 Ga Jianping, China). This spread of ages supports proposals suggesting that thermal and tectonic processes in the lithosphere have not changed significantly since at least the end of the Archean.     

Tectonic setting, evolution and orogenic gold potential of the late Mesoarchaean Mosquito Creek Basin, North Pilbara Craton, Western Australia

The geology, evolution, and metallogenic potential of the Mesoarchaean Mosquito Creek Basin remains poorly understood, despite the presence of several orogenic gold deposits. The basin is dominated by medium- to coarse-grained, poorly sorted and chemically immature sandstone and conglomerates, characterised by very high Cr/Th, high Th/Sc, and low Zr/Sc relative to average continental crust. These features are consistent with the presence of significant mafic rocks in the source terrain(s), a limited role for sediment recycling, and deposition in an increasingly distal passive margin setting on the southeastern edge of the Palaeo- to Mesoarchaean East Pilbara Terrane.New U–Pb SHRIMP data on 358 detrital zircons indicate a conservative maximum depositional age of 2972 + 14/−37 Ma (robust median; 96.1% confidence). Zircon provenance spectra from conglomeratic rocks near the base of the unit are consistent with substantial derivation from the East Pilbara Terrane, but finer-grained sandstones higher in the stratigraphy appear to have been sourced elsewhere, as their zircon age spectra are not well matched by any of the exposed Pilbara terranes.The Mosquito Creek Basin was deformed before and during collision with the northern edge of the Mesoarchaean Kurrana Terrane, which resulted in the development of macroscopic north-verging folds, thrust faulting, and widespread sub-greenschist to greenschist facies metamorphism. This collisional event probably took place at ca. 2900 Ma, based on two identical Pb–Pb model ages of 2905 ± 9 Ma from epigenetic galena associated with vein-hosted gold–antimony mineralization. The metallogenic potential of the Mosquito Creek Basin remains largely unevaluated; however, the possibility of a passive margin setting and continental basement points to relatively limited potential for the formation of major orogenic gold deposits.     

Palaeoproterozoic tectonothermal evolution and deep crustal processes in the Jiao‐Liao‐Ji Belt,North China Craton: a review

The Palaeoproterozoic Jiao‐Liao‐Ji Belt is located in the eastern margin of the Eastern Block of the North China Craton. In this paper, we synthesize the tectonothermal evolution and deep crustal processes in the Jiao‐Liao‐Ji Belt based on recent information. A mantle plume‐related underplating from 2.53 to 2.36 Ga is envisaged which led to the emplacement of the 2.47–2.33 Ga alkali granite plutons and mafic dyke swarms, followed by the development of the Jiao‐Liao‐Ji Rift and bi‐modal volcanism. The underplating resulted not only in different sedimentary environments in the upper crust, but also in a differentiation of the initial thermal structure in the rift. This controlled the metamorphism and style of P‐T‐t paths in the different parts of the rift. Subsequent underplating resulted in the emplacement of the A‐type Liaoji granites of ca. 2.17 Ga in the lower crust, and the formation of associated pegmatites of 2.2 and 2.0 Ga, together with the development of a bedding‐parallel extension. However, the main orogeny occurred between 1.93 and 1.88 Ga with closing of the rift, compressional deformation and high‐pressure granulite metamorphism in the southern part of the orogen. Subsequently, lithospheric blocks were possibly delaminated at ∼1.85 Ga; anorogenic magmatic rocks such as rapakivi granite, alkaline granites and syenite were intruded, and pegmatite veins and mafic dyke swarms were emplaced cross‐cutting all the earlier structural traces. We identify that the underplating styles, collision processes and delamination types in the deep lithosphere controlled the tectonothermal evolution of the crust in the Jiao‐Liao‐Ji region. Copyright © 2011 John Wiley & Sons, Ltd.     

华北克拉通古老大陆地壳组成及演化

对鞍本、冀东、鲁西、阴山等早前寒武纪典型地区和深部物质进行了深入研究,总结了华北克拉通早期地壳形成演化历史。揭示出鄂尔多斯地块本身强烈卷入了古元古代晚期构造热事件。首次在华北克拉通划分出3个>2.6 Ga古陆块。     

A new cache of Eoarchaean detrital zircons from the Singhbhum craton,eastern India and constraints on early Earth geodynamics

The dominant geodynamic processes that underpin the formation and evolution of Earth’s early crust remain enigmatic calling for new information from less studied ancient cratonic nuclei.Here,we present U-Pb ages and Hf isotopic compositions of detrital zircon grains from^2.9 Ga old quartzites and magmatic zircon from a 3.505 Ga old dacite from the Iron Ore Group of the Singhbhum craton,eastern India.The detrital zircon grains range in age between 3.95 Ga and 2.91 Ga.Together with the recently reported Hadean,Eoarchean xenocrystic(up to 4.24 Ga)and modem detritus zircon grains from the Singhbhum craton,our results suggest that the Eoarchean detrital zircons represent crust generated by recycling of Hadean felsic crust formed at^4.3-4.2 Ga and^3.95 Ga.We observe a prominent shift in Hf isotope compositions at^3.6-3.5 Ga towards super-chondritic values,which signify an increased role for depleted mantle and the relevance of plate tectonics.The Paleo-,Mesoarchean zircon Hf isotopic record in the craton indicates crust generation involving the role of both depleted and enriched mantle sources.We infer a short-lived suprasubduction setting around^3.6-3.5 Ga followed by mantle plume activity during the Paleo-,Mesoarchean crust formation in the Singhbhum craton.The Singhbhum craton provides an additional repository for Earth’s oldest materials.     

Early Paleozoic Granite in the Talate Mining District,Chinese Altay,and its Geological Significance for the Altay Orogenic Belt

Zircon dating, geochemical and Nd-Sr isotopic analyses have been determined for samples from two granitic intrusions in the Talate mining district, Chinese Altay. Our data suggest that these intrusions were emplaced from 462.5 Ma to 457.8 Ma. These rocks have strong affinity to peralumious S-type granite and are characterized by prominent negative Eu anomalies(δEu=0.20–0.35), strong depletion in Ba, Sr, P, Ti, Nb, Ta and positive anomalies in Rb, Th, U, K, La, Nd, Zr, Hf. Nd-Sr isotopic compositions of the whole rock show negative εNd(t) values(-1.21 to-0.08) and Mesoproterozoic Nd model ages(T2 DM=1.20–1.30 Ga). Their precursor magmas were likely derived from the partial dehydration melting of Mesoproterozoic mica-rich pelitic sources and mixed with minor mantle-derived components, under relatively low P(≤1 kbar) and high T(746–796°C) conditions. A ridge subduction model may account for the early Paleozoic geodynamic process with mantle-derived magmas caused by Ordovician ridge subduction and the opening of a slab window underplated and/or intraplated in the middle–upper crust, which triggered extensive partial melting of the shallow crust to generate diverse igneous rocks, and provided the heat for the crustal melting and juvenile materials for crustal growth.     

Comparison of Proterozoic and Phanerozoic rift-related basaltic-granitic magmatism

This paper compares the 1.67–1.47 Ga rapakivi granites of Finland and vicinity to the 1.70–1.68 Ga rapakivi granites of the Beijing area in China, the anorogenic 130 Ma granites of western Namibia, and the 20–15 Ma granites of the Colorado River extensional corridor in the Basin and Range Province of southern Nevada. In Finland and China, the tectonic setting was incipient, aborted rifting of Paleoproterozoic or Archean continental crust, in Namibia it was continental rifting and mantle plume activity that led to the opening of southern Atlantic at 130 Ma. The 20–15 Ma granites of southern Nevada were related to rifting that followed the Triassic–Paleogene subduction of the Farallon plate beneath the southwestern United States. In all cases, extension-related magmatism was bimodal and accompanied by swarms of diabase and rhyolite–quartz latite dikes. Rapakivi texture with plagioclase-mantled alkali feldspar megacrysts occurs in varying amounts in the granites, and the latest intrusive phases are commonly topaz-bearing granites or rhyolites that may host tin, tungsten, and beryllium mineralization. The granites are typically ferroan alkali-calcic metaluminous to slightly peraluminous rocks with A-type and within-plate geochemical and mineralogical characteristics. Isotope studies (Nd, Sr) suggest dominant crustal sources for the granites. The preferred genetic model is magmatic underplating involving dehydration melting of intermediate-felsic deep crust. Juvenile mafic magma was incorporated either via magma mingling and mixing, or by remelting of newly hybridized lower crust. In Namibia, partial melting of subcontinental lithospheric mantle was caused by the Tristan mantle plume, in the other cases the origin of the mantle magmatism is controversial. For the Fennoscandian suites, extensive long-time mantle upwelling associated with periodic, migrating melting of the subcontinental lithospheric mantle, governed by heat flow and deep crustal structures, is suggested.     

Microcontinents among the accretionary complexes of the Central Asia Orogenic Belt: In situ Re–Os evidence

Major elements and Re–Os isotope ratios were analysed in situ on individual sulfide grains in spinel peridotite xenoliths hosted by Quaternary intraplate basalts from the Tariat volcanic field, Central Mongolia. The sulfides are dominantly high-temperature (>900 °C) Fe-rich monosulfide solid solution (MSS). Some sulfides with low Ni contents may be residual MSS, whereas other sulfides defining a negative Ni–Cu correlation may be crystallization products of fractionated sulfide melts. The subchondritic ¹⁸⁷Re/¹⁸⁸Os and ¹⁸⁷Os/¹⁸⁸Os of some sulfides also indicate they are residual MSS. Os isotope compositions of sulfides reveal the presence of Archean to Proterozoic lithospheric mantle beneath the region. The sulfides have T_RD model ages ranging from 3.0 to 0.2 Ga, with peaks at 1.5–1.3, 1 and 0.7–0.5 Ga. The peak ages are indicative of significant events in the lithospheric mantle at those times. The timing of these events is remarkably consistent with those of the major crust-building events within the Tarvagatay Terrane where the Tariat volcanic field is located. The similarity in the ranges of crustal U–Pb ages and Nd model ages, and our sulfide Os model ages, suggests that the sulfide ages may date metasomatic events in the underlying lithospheric mantle, which were related to tectonothermal events that affected the overlying crust. Radiometric ages from the Tarvagatay Terrane appear to correspond to the Archean model ages from its SCLM counterpart. The last two events (1.1 and 0.7–0.5 Ga) recorded in the Tarvagatay Terrane suggest involvement of the “CAOB mantle” and development of significant juvenile crustal growth in the orogeny.     

Magmatic systems of large continental igneous provinces

Large igneous provinces (LIPs) formed by mantle superplume events have irreversibly changed their composition in the geological evolution of the Earth from high-Mg melts (during Archean and early Paleoproterozoic) to Phanerozoic-type geochemically enriched Fe-Ti basalts and picrites at 2.3 Ga. We propose that this upheaval could be related to the change in the source and nature of the mantle superplumes of different generations. The first generation plumes were derived from the depleted mantle, whereas the second generation (thermochemical) originated from the core-mantle boundary (CMB). This study mainly focuses on the second (Phanerozoic) type of LIPs, as exemplified by the mid-Paleoproterozoic Jatulian–Ludicovian LIP in the Fennoscandian Shield, the Permian–Triassic Siberian LIP, and the late Cenozoic flood basalts of Syria. The latter LIP contains mantle xenoliths represented by green and black series. These xenoliths are fragments of cooled upper margins of the mantle plume heads, above zones of adiabatic melting, and provide information about composition of the plume material and processes in the plume head. Based on the previous studies on the composition of the mantle xenoliths in within-plate basalts around the world, it is inferred that the heads of the mantle (thermochemical) plumes are made up of moderately depleted spinel peridotites (mainly lherzolites) and geochemically-enriched intergranular fluid/melt. Further, it is presumed that the plume heads intrude the mafic lower crust and reach up to the bottom of the upper crust at depths ～20 km. The generation of two major types of mantle-derived magmas (alkali and tholeiitic basalts) was previously attributed to the processes related to different PT-parameters in the adiabatic melting zone whereas this study relates to the fluid regime in the plume heads. It is also suggested that a newly-formed melt can occur on different sides of a critical plane of silica undersaturation and can acquire either alkalic or tholeiitic composition depending on the concentration and composition of the fluids. The presence of melt-pockets in the peridotite matrix indicates fluid migration to the rocks of cooled upper margin of the plume head from the lower portion. This process causes secondary melting in this zone and the generation of melts of the black series and differentiated trachytic magmas.     

Guandishan Granitoids of the Paleoproterozoic Lüliang Metamorphic Complex in the Trans-North China Orogen: SHRIMP Zircon Ages, Petrogenesis and Tectonic Implications

Abstract: The Paleoproterozoic Lüliang Metamorphic Complex (PLMC) is situated in the middle segment of the western margin of the Trans-North China Orogen (TNCO), North China Craton (NCC). As the most important lithological assemblages in the southern part of the PLMC, Guandishan granitoids consist of early gneissic tonalities, granodiorites and gneissic monzogranites, and younger gneissic to massive monzogranites. Petrochemical features reveal that the early gneissic tonalities and granodiorites belong to the medium-K calc-alkaline series; the early gneissic monzogranites are transitional from high-K calc-alkaline to the shoshonite series; the younger gneissic to massive monzogranites belong to the high-k calc-alkaline series, and all rocks are characterized by right-declined REE patterns and negative Nb, Ta, Sr, P, and Ti anomalies in the primitive mantle normalized spidergrams. SHRIMP zircon U–Pb isotopic dating reveals that the early gneissic tonalities and granodiorites formed at ~2.17 Ga, the early gneissic monzogranites at ~2.06 Ga, and the younger gneissic to massive monzogranites at ~1.84 Ga. Sm–Nd isotopic data show that the early gneissic tonalities and granodiorites have εNd(t) values of +0.48 to ?3.19 with Nd-depleted mantle model ages (TDM) of 2.76–2.47 Ga, and early gneissic monzogranites have εNd(t) values of ?0.53 to ?2.51 with TDM of 2.61–2.43 Ga, and the younger gneissic monzogranites have εNd(t) values of ?6.41 to ?2.78 with a TDM of 2.69–2.52 Ga.These geochemical and isotopic data indicate that the early gneissic tonalities, granodiorites, and monzogranites were derived from the partial melting of metamorphosed basaltic and pelitic rocks, respectively, in a continental arc setting. The younger gneissic to massive monzogranites were derived by partial melting of metamorphosed greywackes within the continental crust. Combined with previously regional data, we suggest that the Paleoproterozoic granitoid magmatism in the Guandishan granitoids of the PLMC may provide the best geological signature for the complete spectrum of Paleoproterozoic geodynamic processes in the Trans-North China Orogen from oceanic subduction, through collisional orogenesis, to post-orogenic extension and uplift.     

Five hundred million years of punctuated addition of juvenile crust during extension in the Goochland Terrane,central Appalachian Piedmont Province

ABSTRACT

The Goochland Terrane is an enigmatic crustal block in the Appalachian Piedmont Province of central Virginia, USA. Sparse exposures of terminal Mesoproterozoic and late Neoproterozoic igneous rocks in the central Goochland Terrane offer the opportunity to investigate both the continental affinity of the terrane during the Proterozoic Eon and the timing and mechanisms of crustal growth. We apply multiple geochemical tools to these rocks: tectonic discrimination using whole-rock major and trace element abundances; whole-rock Sm-Nd isotopes; O, U-Pb, and Lu-Hf isotope analyses of spots in zircon; and measurement of O isotopes in multi-grain quartz separates. Eruption of the Sabot Amphibolite protolith is difficult to date, but we tentatively assign an age of 552 ± 11 Ma. Goochland Terrane continental crust first separated from the mantle prior to ca. 1050–1010 Ma intrusion of the Montpelier Anorthosite and the State Farm Gneiss protolith. The granitic magma that became the State Farm Gneiss protolith could have been derived entirely from partial melting of this initial Goochland Terrane crust. In contrast, the magmas that became the Montpelier Anorthosite, Neoproterozoic granitoid, and the Sabot Amphibolite were mixtures of mantle melt and preexisting Goochland Terrane crust. This production of juvenile continental crust occurred during continental extension and, eventually, rifting. The timing and compositions of terminal Mesoproterozoic magmatism in the Goochland Terrane closely match those in the nearby Blue Ridge Province. Although the compositions of the Neoproterozoic magmas in the two regions are similar, intrusion and possibly eruption occurred about 10 M.y. later in the Goochland Terrane.     

Sm—Nd and Zircon U—Pb Isotopic Constraints on the Age of Formation of the Precambrian Crust in Southeast China

Nd model ages(TDM) of the Pre-Mesozoic crustal rock samples from Southeast China range from 1.2 to 3.5Ga.Two age peaks of 1.4Ga and 1.8 Ga are observed in the histogram of TDM model ages.Available U-Pb zircon inheritance ages are concentrated around 1.2-1.4Ga,1.8Ga and 2.5Ga,respectively.The combined use of Sm-Nd and U-Pb zircon inheritance ages suggests that the formation of the Precambrian curst is of episodic character.The oldest crustal nucleus may have been formed during the Late Archean(2.5Ga or older?).A rapid production of the crust took place 1.8 Ga ago,consistent with the global crust formation event at 1.7-1.9Ga.Another important episode of the addition of juvenile crustal material from the mantle in Southeast China took place 1.2-1.4Ga ago,during which the pre-existing crust was strongly reworked and/or remelted.