Continental Crustal Rocks Research Articles

Comprehensive Assessment of A-Type Granite from South Sinai, Egypt: Geochemistry, Zircon Geochemistry, and Geochronology

The A-type granites are an important component of the Earth's continental crust and play a significant role in understanding crustal evolution and tectonic processes. This study presents a comprehensive assessment of A-type granites from Ataitir El Dehami granites (ADG) that located at the north-western part of Wadi Seih area in South Sinai, using geochemistry, zircon geochemistry, and geochronology characteristics. The geochemical analysis reveals that ADG are characterized by high silica content, relatively high FeO*, and high alkali concentration consistent with the A-type granites that derived from the melting of tonalite sources. Zircons from Ataitir El Dehami granites have high REE contents with Ce enrichment and depletion of Eu, which, together with the high U/Yb ratios, suggest that the studied granites were generated from melting of continental crust rocks. Zircon U-Pb geochronological data indicates that the granite emplacement occurred around 575 to 603 Ma throughout continuous exhumation pulses. The isotopic analyses also indicate that the presence of inherited zircon grains (735± 9Ma). The present data suggest that the studied granites were formed in a post-collisional setting, and represent the transition stage from convergence to extension that occurred at 600 Ma.

Geochemical and Isotopic evidence for volcanism on the Alleppey Terrace, southwest Indian continental margin

The Arabian Sea consists of prominent undersea bathymetric highs whose genesis is often attributed to volcanism either related to the 90-85 Ma Marion or the 70-65 Ma Reunion hot spot activity on the Indian Plate. The enigmatic Alleppey Terrace (AT) is one such significant bathymetric high in the southwestern continental margin of India. We studied the siliciclastic sediments deposited on the AT and the adjacent shelf region using major/trace element, and Sr-Nd isotope geochemistry to decipher their provenances and implications of the findings for the origin of the Terrace. Geochemical data suggest that the AT sediments are chemically distinct from the adjacent shelf sediments, with the former having been sourced from juvenile mafic rocks and the latter from significantly older, felsic continental crustal rocks. Although currently located at a water depth of ∼300 m, AT was likely exposed above the sea level in the past and had undergone subaerial weathering and lateritization. The εNd of AT sediments (−17.1 to −10.9), which is much radiogenic than that of the shelf sediments (−26.4 to −22.2), indicates significant presence of a juvenile basaltic component - likely derived from in situ weathering of the local seafloor/basement. The common component in both the terrace and shelf sediments appears to be terrigenous, deposited by the local rivers draining the Western Ghats. Conspicuous similarities of trace element ratios and isotopic compositions of the terrace sediments with those of the volcanic rocks temporally and spatially related to the Deccan volcanism suggest that the AT may contain traces of the end Cretaceous Reunion-Deccan plume/hotspot activity on the India subcontinent.

High-pressure Raman scattering and X-ray diffraction study of kaolinite, Al2Si2O5(OH)4

Kaolinite is formed by weathering of continental crustal rocks and is also found in marine sediments in the tropical region. Kaolinite and other layered hydrous silicate minerals are likely to play a vital role in transporting water into the Earth's interior via subducting slabs. Recent studies have experimentally documented the expansion of the interlayer region by intercalation of water molecules at high pressures i.e., pressure-induced hydration. This is counter-intuitive since the interlayer region in the layered silicates is quite compressible, so it is important to understand the underlying mechanism that causes intercalation and expansion of the interlayer region.To address this, we explore the high-pressure behavior of natural kaolinite from Keokuk, Iowa. This sample is free of anatase impurities and thus helps to examine both low-energy (0–1200 cm−1) and high-energy hydroxyl (3000–4000 cm−1) regions using Raman spectroscopy and synchrotron-based powder X-ray diffraction.Our results show that the pressure dependence of the hydroxyl modes exhibits discontinuities at ∼3 GPa and ∼ 6.5 GPa. This is related to the polytypic transformation of Kaolinite from K-1 to K-II and K-II to K-III phase. Several low-energy Raman modes' pressure dependence also exhibits similar discontinuous behavior. The synchrotron-based powder X-ray diffraction results also indicate discontinuous behavior in the pressure dependence of the unit-cell volume and lattice parameters. The analysis of the bulk and the linear compressibility reveals that kaolinite is extremely anisotropic and is likely to aid its geophysical detectability in subduction zone settings. The K-I to K-II polytypic transition is marked by the snapping of hydrogen bonds, thus at conditions relevant to the Earth's interior, water molecules intercalate in the interlayer region and stabilize the crystal structure and help form the super-hydrated kaolinite which can transport significantly more water into the Earth's interior.

Concurrent adakitic and non-adakitic Late Miocene-Quaternary magmatism at the Sahand volcano, Urumieh-Dokhtar Magmatic Arc (NW Iran)

The Sahand volcano in NW Iran is characterised by voluminous Late Miocene to Quaternary magmatic activity. Petrographic, mineral chemical, whole-rock geochemical and Sr-Nd-Pb isotope data allowed to identify two rock groups, continuously emplaced from the ancient Paleo-Sahand to the later Neo-Sahand stages. These are represented by abundant evolved rocks, with dacitic to rhyolitic compositions and less common intermediate (andesitic) rocks. Both rock types have the typical LILE-enriched and HFSE-depleted fingerprints of magmas originated from subduction-modified mantle sources, as well as radiogenic Sr- and Pb- (18.8–18.9 206Pb/204Pb, 15.6–15.7 207Pb/204Pb and 38.8–39.2 208Pb/204Pb) isotope compositions. Nevertheless, the evolved rocks have lower 87Sr/86Sr (0.70437–0.70491 vs 0.70494–0.70585) and display an evident high Sr/Y vs Y and high La/Yb vs Yb “adakitic” signature, suggesting derivation from the partial melting of lower continental crust rocks, rather than from the differentiation of the non-adakitic intermediate rocks. The latter would be the derivative melts of magmas generated by the partial melting of lithospheric mantle sources modified by metasomatic fluids released by the subducted Neotethyan oceanic lithosphere. During their ascent and differentiation, these magmas likely provided the heat necessary to induce partial melting of the lower continental crust, eventually producing adakitic evolved magmas. Quantitative models and geothermobarometric calculations suggest that differentiation processes for both rock groups occurred mainly by polybaric fractional crystallisation (in the 29–4 km depth range), possibly coupled with minor assimilation of local crustal rocks.The peculiar style of the Sahand magmatic activity (with both adakitic and non-adakitic products) and its temporal evolution (with an overall increase of the degree of evolution and a decrease of the non-adakitic melts) are somewhat different with respect to those of the coeval neighbouring districts of Saray, Sabalan and Bijar-Qorveh. These can be reconciled within a general model for NW Iran where magmatism is triggered by upwelling of the sub-lithospheric mantle as a consequence of slab break-off (or the opening of a slab tear). The progressive propagation (and subsequent exhaustion) of the thermal pulse and the increasing thickness of the continental crust played a main role in determining both the progressively later inception of active magmatism (from ∼11 to 4.5 Ma) and the change in magma types (from largely variable to adakitic only) at increasing distance from the Iran-Arabia suture.

Geochemistry of Precambrian dyke swarms in the Singhbhum craton, India: Implications for recycled crustal components in the mantle source

The Singhbhum craton, eastern India records multiple stages of emplacement of Precambrian dyke swarms with contrasting petrogenetic models proposed for their formation. In this study, we document elemental and Sr-Nd isotopic data for three major dyke swarms in the southern part of the craton, including the ca. 2.7 Ga Ghatgaon dyke swarm, the Early Proterozoic Keonjhar dyke swarm and the ca. 1.76 Ga Pipilia dyke swarm. Dyke compositions are dominated by basalt and basaltic andesite with minor andesite, showing trace element signatures typical of continental crustal rocks. Age-corrected Nd isotopic data for Ghatgaon (εNdt = −4.8 to + 4.6), Keonjhar (εNdt = −11.9 to + 3.8), and Pipilia (a single sample with εNdt = −8.8) dyke swarms display substantial variations. The lack of magma compositions that could indicate the presence of elevated mantle potential temperature among the rocks suggests melting regime was likely similar to the ambient mantle. The Dy/Yb and Dy/Dy* systematics of the rocks indicates melting occurred between spinel-stable depths and the spinel-garnet transition zone. The dominantly mafic compositions of the rocks and ubiquitous continental crustal trace element signature are best explained by peridotite source with recycled crustal components, probably in the form of pyroxenites. Our new Nd isotopic data, which argue against any simple secular evolution trend invoked in previous studies, indicate that crustal recycling was likely an episodic phenomenon rather than a discrete, single-stage process since the Archean. Geochemical modelling indicates that a sublithospheric mantle source with (10% or less) recycled crustal components satisfactorily explains the trace element variations of the dyke swarms.

Paleoproterozoic (ca. 2.1–1.9 Ga) dioritic-granitic magmatism in the Dunhuang Terrane, Northwestern China: Implications for petrogenesis and tectonic evolution

The implications of petrogenesis and tectonics of Paleoproterozoic (ca. 2.1–1.9 Ga) granitoid rocks in Dunhuang Terrane have hitherto received less attention. According to data from zircon UPb dating, the ages of dioritic gneiss, granodioritic gneiss and potassic granite are 2045 ± 44 Ma, 1998 ± 15 Ma and 1950 ± 38 Ma, respectively. The dioritic gneiss has MgO = 1.87–2.93 wt% (Mg# = 46.8–50.9), Yb ≤ 1.9 ppm and Y ≤ 18 ppm, with variable εHf(t) (−2.8 − +5.5) and εNd(t) (−5.6 − +1.3) values, indicating that the mantle wedge source above the subducted slab contributed with subduction fluids and ocean sediment melt in an arc setting. The granodioritic gneiss has relatively high MgO (3.00–1.83 wt%), variable Cr (13.5–106 ppm) and Ni (12.2–72.9 ppm), negative εHf(t) (−20.4 to −10.1) and εNd(t) (−15.2 to −12.8) values, and high P2O5 and TiO2 saturation temperatures (∼800 °C). These data indicate magma source of late Paleo- to early Mesoarchean (∼3.2–3.1 Ga) continental crust rocks. The potassic granite has high K2O (5.52–6.79 wt%) and low MgO (0.10–0.19 wt%) concentrations, low Mg# (37.7–44.6) values, and negative εHf(t) (−12.8 to −2.5) and εNd(t) (−10.6 to −4.8) values. These results suggest that pre-existing Neoarchean crustal rocks were partially melted possibly under transitional stage from syn- to post-collision. Compared with the adjacent areas, Paleoproterozoic (ca. 2.1–1.8 Ga) evolutionary period related to assembly of Columbia supercontinent in Dunhuang Terrane is comparable with that in northeastern Tarim Craton and southwestern Alxa Block, but terminated ∼100 Ma later than that in southeastern Tarim Craton (Aketashtage Terrane).

Zircon U–Pb dating and geochemistry of intrusive rocks in the Shangdan suture zone of the Qinling Orogenic Belt: petrogenesis and tectonic implications

We studied the zircon U–Pb ages, whole-rock geochemistry and Sr–Nd isotopic compositions of a Paleozoic gabbro–diabase–granite association in the Shangdan suture zone to constrain the petrogenesis of these intrusive rocks and identify their tectonic implications. These rocks have distinctive crystallization ages: the Guchenggou gabbro yielded zircon U–Pb ages of 424–423 Ma, the average age of the Yueritan diabase was 411 Ma and the Danfeng granites yielded zircon U–Pb ages of 414–411 Ma. The Guchenggou gabbro was formed by partial melting of a phlogopite-bearing garnet lherzolite modified by subduction fluids in a continental arc setting. The Yueritan diabase was generated by mixing a phlogopite-bearing garnet lherzolite modified by subduction melts with continental crustal rocks in a post-collision setting. The Danfeng granites were weakly peraluminous I-type granites formed by mixing juvenile crust with metagreywackes at high temperatures and pressures in a tectonic environment transitioning from compression to extension. We suggest that the subduction of the Shangdan Ocean induced the formation of the gabbro. The diabases and granites are related to relamination of the continental crust in response to the collision of the North Qinling Belt with the South Qinling Belt, suggesting that closure of the Shangdan Ocean may have occurred at c. 410 Ma.

A likely geological record of deep tremor and slow slip events from a subducted continental broken formation

Fluids in subduction zones play a key role in controlling seismic activity, drastically affecting the rheology of rocks, triggering mineral reactions, and lowering the effective stress. Fluctuating pore pressure is one important parameter for the switch between brittle and ductile deformation, thus impacting seismogenesis. Episodic tremor and slow slip events (ETS) have been proposed as a common feature of the geophysical signature of subduction zones. Their geological record, however, remains scanty. Only the detailed and further characterization of exhumed fossil geological settings can help fill this knowledge gap. Here we propose that fluctuating pore pressure linked to metamorphic dehydration reactions steered cyclic and ETS-related brittle and ductile deformation of continental crustal rocks in the subduction channel of the Apennines. Dilational shear veins and ductile mylonitic shear zones formed broadly coevally at minimum 1 GPa and 350 °C, corresponding to ~ 30–40 km depth in the subduction zone. We identify carpholite in Ca-poor metasediments as an important carrier of H2O to depths > 40 km in cold subduction zones. Our results suggest that the described (micro)structures and mineralogical changes can be ascribed to deep ETS and provide a useful reference for the interpretation of similar tectonic settings worldwide.

Zircon Xenocrysts from the Shaka Ridge Record Ancient Continental Crust: New U-Pb Geochronological and Oxygen Isotopic Data

Over the past two decades, a significant number of discoveries of ancient zircon xenocrysts in ocean-floor magmatic rocks have been reported. These findings provide compelling evidence for the presence of ancient continental crust within young oceanic lithosphere. Almost all finds of ancient zircon xenocrysts within oceanic crust are from the Mid-Atlantic Ridge. For other localities, however, similar data are very limited. This report presents the first age determinations (U-Pb, SHRIMP-II) and isotope-geochemical data (oxygen, trace and rare earth elements) for zircon xenocrysts from gabbro-diorites of the Shaka Ridge, in the vicinity of the Shaka fracture zone, near the western end of the Southwest Indian Ridge. The work is based on a study of bottom rock material dredged during expeditionary research on the R/V “Akademik Fedorov” (Russia) in 2016. The U-Pb isotope system of the zircon xenocrysts recorded a crystallization age of ∼2.8 Ga at an upper discordia intercept and an age of ∼600 Ma interpreted as the timing of a superimposed thermal event at a lower discordia intercept. The zircon xenocrysts show geochemical signatures of magmatic origin, i.e., fractionated REE distribution spectra with an increase in chondrite-normalized values from light to heavy REE, positive Ce anomalies and negative Eu anomalies, and high Th/U ratios (0.59–7.77). In discrimination diagrams based on a series of inter-element relationships, zircon compositions fall into the fields of zircons from rocks of continental crust, mostly granitoids. The Li content of the zircons is high (1.8 ppm–50 ppm), adding further evidence to their derivation from rocks of continental crust. During their residence within young oceanic crust, the zircon xenocrysts experienced alterations under the influence of submarine high-temperature hydrothermal fluids, which selectively affected the distribution of trace elements in the zircons and reduced the δ18O values to 1.75‰–3.15‰. The presence of obviously older zircons in Shaka gabbro-diorites clearly demonstrates the presence of ancient continental fragments and their recycling into the mantle at the western end of the Southwest Indian Ridge.

Insights into the tectonostratigraphic setting of the Southern Appalachians during the Blountian tectophase from an integrated geochemical analysis of magmatic phenocrysts in the Ordovician Deicke K-bentonite

We evaluate competing hypotheses regarding tectonic models of the early Taconic Orogeny in the Southern Appalachians during the Blountian tectophase with new geochemical data obtained from analyses of apatite and zircon phenocrysts, and melt and mineral inclusions therein, from the Ordovician Deicke K-bentonite (453.35 ± 0.10 Ma). Apatite geochemistry confirms that samples from different locations represent the same eruptive event, and zircon geochemistry and UPb geochronology (including several Grenville-age inherited detrital cores) confirm an evolved magma of continental-arc composition, with the trace-metal geochemistry of magmatic Taconic-age rims on Deicke zircons pointing to a provenance of continental crustal igneous rock melts, specifically a granitic melt with a high level of differentiation, rather than a melt that was mantle-derived, or strongly mantle-influenced. Melt inclusions within Deicke phenocrysts are dacitic (apatite) to rhyolitic (zircon), differences that imply an evolving Deicke magma during its eruption.Our data 1) indicate multiple crustal events, and 2) are tectono-chemical evidence supporting an arc-trench subduction system where subducting preexisting sedimentary rocks were subsequently incorporated into the melt during or prior to collision. Indonesia provides modern analogs: the Banda Arc system, where Australian continental crust is jamming the subduction zone in association with the development of a foreland basin and a sedimentary wedge; the New Guinea – western Melanesian region, with its orogenic highlands and associated foredeep; and the Sumatra –Toba system, where subduction-related explosive volcanism has generated enormous caldera-forming eruptions above continental crust basement. Thus, a model for closure of the Iapetus Ocean involving subduction of older passive margin sediments as part of an island-arc collision with the Laurentian passive margin is acceptable, but models based on a back-arc basin are not compatible with our data.

Polyphase deformation and ultrahigh temperature metamorphism of the deep continental crust: Implications for tectonic evolution of the northern Eastern Ghats Belt, India

We present the structural history of a suite of high-grade rocks of the Phulbani area of the Eastern Ghats Belt, India, based on structural study at meso- and micro-scales combined with published metamorphic and the geochronological data. Structural data suggest multiple generations of fabric (S1–S5) development during a punctuated deformational history (D1−D5). The S1 fabric is present only in grain-scale and developed during the prograde metamorphic stage (M1). The regionally recognizable composite S2/S3 gneissic fabric developed in the realm of ultra-high temperature metamorphism (D2/D3, M2) during ca. 987–970 Ma as a result of the contraction and closure of the continental back-arc basin. The D4 deformation folded the earlier formed composite S2/S3 gneissic fabric and developed a shear-related mylonitic foliation (S4), possibly after ca. 970 Ma. Deformation in the Phulbani area culminated through the formation of the S5 mylonitic fabric (D5, M3) along the Ranipathar Shear Zone, possibly at ca. 500 Ma, which exposed the anomalously hot deep continental crustal rocks to shallower crustal level. Development of the Ranipathar Shear Zone is interpreted to be kinematically linked with several other such crustal-scale shear zones in the area which together played a pivotal role in the final tectonic evolution of the northern Eastern Ghats Belt.

Neodymium isotope composition of Palaeoproterozoic Birimian shales from the Wa‐Lawra Belt, north‐west Ghana: Constraints on provenance

An Nd‐isotopic study was carried out on Palaeoproterozoic Birimian shales from the Wa‐Lawra volcanic belt, located in the southeastern margin of the West African Craton. The objective of this study was to assess the nature and average crustal residence ages of the source areas, and deduce the provenance of these shales. Nd‐model ages range from 2.12 to 2.58 Ga, which are within the range of Nd‐model ages reported for the Birimian volcanic rocks, suggesting that the Birimian volcanic rocks are the main sources of detritus for the Birimian shales. The εNd values of the Birimian shales, calculated at an age of 2.1 Ga, which represents the time of the deposition of the Birimian shales, ranges from −1.65 to +3.01. The range of observed εNd values (i.e., −1.65 to +3.01) suggest that the Birimian shales were predominantly derived from the juvenile Palaeoproterozoic crust, with minor contribution of older crustal material. εNd(T) versus Th/Sc, and ƒSm/Nd versus εNd(T) systematics indicate that the provenance can be explained by mixing between Palaeoproterozoic volcanic arc rocks and variable amounts of Archean continental crustal rocks. Provenance modelling shows that the Birimian shales received at least 80% detritus from a juvenile Birimian volcanic arc. The mean Archean component is 8%, suggesting that the continental crust of the Wa‐Lawra Belt is clearly dominated by material derived directly from the mantle.

Different melt source regions for the volcanics of the Bushveld large igneous province: New observations from MELTS modeling of the Palaeoproterozoic Rooiberg Group (South Africa)

The volcanic Rooiberg Group represents the extrusive phase of the Bushveld Magmatic Province in South Africa, forming the roof as well as the floor of the mafic-ultramafic 2057 Ma Rustenburg Layered Suite. Typically, the volcanic rocks of the Rooiberg Group vary from mafic compositions in the oldest unit (the Dullstroom Formation) to felsic compositions in the younger overlying units (the Damwal, Kwaggasnek and Schrikkloof formations). The lower parts of the Dullstroom Formation (including a basal rhyolitic unit) occur beneath the Rustenburg Layered Suite (RLS) in the southeastern part of the Bushveld Province, whereas the remainder of the Rooiberg Group occurs above the RLS. In this study, petrographic descriptions, whole rock geochemistry and MELTS modeling are used to show that the Dullstroom rhyolite could have evolved from fractional crystallisation of a siliceous and magnesian mafic liquid such as the so-called B1 liquid, parental to the lower parts of the Rustenburg Layered Suite. Due to its unique position at the base of the otherwise andesitic to dacitic Dullstroom Formation, the focus of this contribution is on the Dullstroom rhyolite and a comparison thereof with the rhyolites of the upper formations. Consistent with previous studies, the new data generated in this study show that a clear distinction can be made between the rhyolite in the Dullstroom, and those of the Damwal, Kwaggasnek and Schrikkloof formations. The Dullstroom rhyolite exhibits higher MgO contents (1.41–1.87 wt%) compared to the distinctly ferroan rhyolites of the Damwal, Kwaggasnek and Schrikkloof formations (0.01–0.91 wt% MgO). Similarly, immobile trace elements such as Y and Nb range from 9.72 to 12.7 ppm and 4.43–4.53 ppm, respectively, for the Dullstroom rhyolite, and are significantly different to the upper rhyolites (Y - 12.6–87.2 ppm and Nb - 12.3–24.2 ppm) suggesting likely petrogenetic differences. MELTS modeling shows that the Dullstroom rhyolite could not have evolved from the same liquids that generated the rhyolites of the Damwal, Kwaggasnek and Schrikkloof formations. The modeling suggests that the Dullstroom rhyolite formed through ~20% assimilation of upper continental crustal rocks during fractional crystallisation of the B1 composition, and not from the low-Ti basaltic andesite, as previously proposed for the overlying rhyolites. The modeling aspects of this study provide evidence for different sources and melting-fractionation pathways throughout the evolution of the Bushveld Magmatic Province, consistent with characteristics recorded by the volcanic edifice of this large igneous province.

Re-Os Isotope Systematics of Sulfides in Chromitites and Host Lherzolites of the Andaman Ophiolite, India

Laser ablation MC-ICP-MS was used to measure the Os-isotope compositions of single sulfide grains, including laurite (RuS2) and pentlandite [(Fe,Ni)9S8], from two chromitite bodies and host lherzolites from ophiolites of North Andaman (Indo-Burma-Sumatra subduction zone). The results show isotopic heterogeneity in both laurite (n = 24) and pentlandite (n = 37), similar to that observed in other chromitites and peridotites from the mantle sections of ophiolites. Rhenium-depletion model ages (TRD) of laurite and pentlandite reveal episodes of mantle magmatism and/or metasomatism in the Andaman mantle predating the formation of the ophiolite (and the host chromitites), mainly at ≈0.5, 1.2, 1.8, 2.1 and 2.5 Ga. These ages match well with the main tectonothermal events that are documented in the continental crustal rocks of South India, suggesting that the Andaman mantle (or its protolith) had a volume of lithospheric mantle once underlaying this southern Indian continental crust. As observed in other oceanic lithospheres, blocks of ancient subcontinental lithospheric mantle (SCLM) could have contributed to the development of the subduction-related Andaman–Java volcanic arc. Major- and trace-element compositions of chromite indicate crystallization from melts akin to high-Mg IAT and boninites during the initial stages of development of this intra-oceanic subduction system.

The 142Nd/144Nd variations in mantle-derived rocks provide constraints on the stirring rate of the mantle from the Hadean to the present

Early silicate differentiation events for the terrestrial planets can be traced with the short-lived 146Sm-142Nd system (∼100-My half-life). Resulting early Earth-produced 142Nd/144Nd variations are an excellent tracer of the rate of mantle mixing and thus a potential tracer of plate tectonics through time. Evidence for early silicate differentiation in the Hadean (4.6 to 4.0 Ga) has been provided by 142Nd/144Nd measurements of rocks that show both higher and lower (±20 ppm) values than the present-day mantle, demonstrating major silicate Earth differentiation within the first 100 My of solar system formation. We have obtained an external 2σ uncertainty at 1.7 ppm for 142Nd/144Nd measurements to constrain its homogeneity/heterogeneity in the mantle for the last 2 Ga. We report that most modern-day mid-ocean ridge basalt and ocean island basalt samples as well as continental crustal rocks going back to 2 Ga are within 1.7 ppm of the average Earth 142Nd/144Nd value. Considering mafic and ultramafic compositions, we use a mantle-mixing model to show that this trend is consistent with a mantle stirring time of about 400 My since the early Hadean. Such a fast mantle stirring rate supports the notion that Earth's thermal and chemical evolution is likely to have been largely regulated by plate tectonics for most of its history. Some young rocks have 142Nd/144Nd signatures marginally resolved (∼3 ppm), suggesting that the entire mantle is not equally well homogenized and that some silicate mantle signatures from an early differentiated mantle (>4.1 Ga ago) are preserved in the modern mantle.

Geochemistry of continental alkali basalts in the Sabzevar region, northern Iran: implications for the role of pyroxenite in magma genesis

A geochemical study was undertaken on basaltic lava flows intercalated with Oligocene to Miocene strata in the Sabzevar region, northern Iran, to examine their petrogenesis in a regional tectonic framework. The lavas are either aphyric or have phenocrysts and micro-phenocrysts of olivine and, to a lesser extent, clinopyroxene. Geochemically, the lavas are silica-undersaturated alkali basalts characterized by relatively high Mg# (~ 57–66) and Na2O/K2O (~ 2.0–6.7). They have distinctive trace element patterns characterized by strong rare-earth element fractionation, negative Nb–Ta and Zr-Hf anomalies and a positive Sr anomaly. Significant contamination by crustal materials either in the magma source or during ascent is ruled out on the basis of trace element compositions and Sr–Nd isotopic compositions (87Sr/86Sr = 0.7037–0.7048 and 143Nd/144Nd = 0.5128–0.5130), both of which differ markedly from continental crustal rocks. Phenocryst assemblage, analysis of multiple saturation points in lherzolite systems, and covariations of La/Yb with MgO of the studied lavas are generally consistent with an origin involving high-pressure fractionation of peridotite-derived melts. Primary magma compositions calculated by reversed fractionation of clinopyroxene and olivine for the relatively primitive samples (> 9 wt.% MgO), however, do not plot on the lherzolite multiple saturation points. Also, high-pressure fractionation predicts increasing trends of silica undersaturation and alkalinitiy with differentiation, and such trends are not indicated by the geochemical data. We suggest that the mixed trends shown by the data might be related to melt generation from both peridotite and silica-deficient pyroxenite sources, superimposed by variable degrees of high-pressure fractionation. The role of pyroxenite in magma genesis is indicated not only by the positive Sr anomaly shown by the trace element patterns, but also first-row transition element systematics of the studied lavas. The silica-deficient pyroxenites contributing to melt generation might have been transformed from mafic–ultramafic cumulates in subducted, lower oceanic crust, or might have formed in the lower crust or mantle lithosphere under continents during earlier magmatic episodes.

Multiphase magmatic overprinting in the Late Jurassic Laoniushan pluton at the SW margin of the North China Craton: Geochronological and petrogenetic constraints

Late Mesozoic granitoids are widely emplaced in the Qinling Orogen on the southern margin of North China Craton (NCC), and can provide important clues on the Late Mesozoic regional geodynamic setting. The Late Jurassic Laoniushan granitic pluton is located in the western end of the southern NCC margin. Zircon U–Pb dating for the granodiorite, quartz diorite, and biotite monzogranite of the pluton yielded 152.5 ± 1.6 Ma, 148.7 ± 1.2 Ma, and 149–146 Ma, respectively. Compilation of the single zircon ages revealed magmatic peaks at ca. 146, 152, and 163 Ma, suggesting multiphase Late Jurassic magmatism. These granitoids are high‐K calc‐alkaline, I‐type, and metaluminous to weakly peraluminous. The rocks show chondrite‐normalised LREE enrichments, HREE depletions and flat HREE patterns, together with (slightly) negative Eu, Nb, Ta, P, and Ti anomalies. The Laoniushan granodiorite and biotite monzogranite have similar REE patterns to the quartz diorite, but the former two have much smaller Eu anomalies than the latter. The negative zircon εHf(t) (−32.6 to −13.7), εNd(t) (−20.7 to −14.3) and crustal‐like whole‐rock trace‐elemental ratios (e.g., Nb/Ta, Th/U, Nb/U, and Ce/Pb), together with the low Mg (MgO = 0.18–2.02 wt%, Mg# = 20–34), Cr (7.00–39.0 ppm), and Ni (0.80–4.00 ppm) contents, suggests that the Late Jurassic granitoids were probably derived from the partial melting of ancient crustal rocks with minor mantle‐derived input. The granodiorite and quartz diorite have similar average εHf(t) (−26.5 and −26.7, respectively), εNd(t) (−20.5 and −20.2, respectively), TDM2(Hf) (2880 and 2885 Ma, respectively) and TDM2(Nd; 2607 and 2578 Ma, respectively), whereas the biotite monzogranite has higher average εHf(t) (−17.1) and εNd(t) (−15.3), and younger TDM2(Hf) (2281 Ma) and TDM2(Nd) (2176 Ma). This suggests that the biotite monzogranite has different magma sources and shallower magma formation depths compared to the granodiorite and quartz diorite. The latter two were likely formed by the partial melting of continental crustal rocks (notably the Taihua Group basement rocks), which was triggered by mantle underplating led by asthenospheric upwelling. The Late Jurassic Laoniushan granitoids may have formed in an extensional setting caused by the Palaeo‐Pacific Plate subduction beneath Eastern China. Commencement of Late Jurassic Laoniushan granitic magmatism likely marked the incipient of lithospheric thinning along the southern NCC margin. Considering also the regional geological evolution, we conclude that although the Laoniushan pluton exhibits spatial–temporal relationships with the nearby Mo (–U) and REE deposits, there is still no unambiguous metallogenic link with these deposits.

Volatile loss under a diffusion-limited regime in tektites: Evidence from tin stable isotopes

Tektites are glasses derived from near-surface continental crustal rocks that were molten and ejected from the Earth's surface during hypervelocity meteorite impacts. They are among the driest terrestrial samples, although the exact mechanism of water loss and the behaviour of other volatile species during these processes are debated. Based on the difference in magnitude of the Cu and Zn isotopic fractionations in tektites, and the difference of diffusivity between these elements, it was suggested that volatile loss was diffusion-limited. Tin is potentially well suited to testing this model, as it has a lower diffusivity in silicate melts than both Cu and Zn, but a similar volatility to Zn. Here, we analysed the Sn stable isotopic composition in a suite of seven tektites, representing three of the four known tektite strewn fields, and for which Zn and Cu isotopes were previously reported. Tin is enriched in the heavier isotopes (≥2.5‰ on the 122Sn/118Sn ratio) in tektites, correlated with the degree of Sn elemental depletion in their respective samples as well as with Cu and Zn isotope ratios, implying a common control. While the isotope fractionation of Sn, Cu and Zn is a result of volatility, the magnitude of isotope fractionation is strongly moderated by their relative rates of diffusion in the molten tektite droplets. An Australasian Muong Nong-type tektite analysed has the least Sn depletion and Sn isotope fractionation, consistent with these samples being more proximal to the source and experiencing a shorter time at high temperatures.

In situ trace element and Sm-Nd isotope analysis of accessory minerals in an Eoarchean tonalitic gneiss from Greenland: Implications for Hf and Nd isotope decoupling in Earth's ancient rocks

Some of the Earth's oldest preserved continental crustal rocks, from southern West Greenland, contain contradictory radiogenic isotope signatures, whereby Hf isotope ratios are chondritic but Nd isotope ratios are distinctly super-chondritic. Models to explain this discrepancy are speculative and variously invoke deep magma ocean crystallisation, HfNd decoupling in subduction zones, or metamorphic disturbance during younger thermal events. Determining the cause of this discrepancy is essential for understanding Eoarchean crust-mantle differentiation. We employ, for the first time, micro-analysis of REE-rich accessory minerals to shed light on the Nd isotope evolution of a key tonalitic gneiss sample from southern West Greenland that displays the apparent HfNd isotope decoupling. The results show that the Sm-Nd isotope system was homogenized during a metamorphic event at ca. 2690 Ma. We suggest that metamorphic reactions involving consumption and re-crystallisation of REE-bearing phases were accompanied by LREE element mobility and the loss of unradiogenic Nd, shifting the bulk rock composition to a more radiogenic Nd isotope value. Our study provides the first direct evidence that the anomalous Nd isotope signatures in some Eoarchean gneisses are artefacts of the disturbance of the Sm-Nd isotope system, and not due to extensive differentiation of the bulk silicate Earth by magma ocean crystallisation or continental crust formation.

Petrogenesis of Synplutonic High-Mg Porphyritic Dikes from Mafic–Granitoid Plutons of the Voronezh Crystalline Massif, East European Platform

Abstract—The paper reports data on the high-Mg porphyrite dikes (PD) in the ~2.09 Ma mafic to felsic plutons of the Elan Complex (EC). Thin (a few cm thick) synplutonic dikes are characterized by the sharp even contacts without visible chill zones, unlike thicker (up to 119 m thick) dikes with gradual transitions. The dikes are fresh, porphyritic (bronzite, Al-enstatite, and labradorite phenocrysts) with fine-grained mainly quartz–feldspar (+biotite, sulfides, accessories, ±hypersthene) groundmass. Based on chemistry, the PD are intermediate rocks (SiO2 = 58.9–60.3 wt %) ascribed to the calc-alkaline series with high magnesian (Mg# ~0.7) whole-rock composition and felsic (68.9–70.2 wt %) groundmass (Mg# ~0.5). The porphyrites have fractionated REE distribution pattern without sharp Eu anomalies. Bronzite phenocrysts of variable size show patchy zoning and contain unevenly distributed inclusions of olivine (Mg# ~0.85), clinopyroxene (Mg# ~0.88), phlogopite (Mg# up to 0.94), labradorite, chromite, graphite, and sulfide. Al-enstatite phenocrysts are almost devoid of trace elements and mineral inclusions. Geochemical features, as well as the presence of diffusion zones, reaction rims, and resorbed facets in the orthopyroxene and plagioclase phenocrysts serve as evidence for disequilibrium recrystallization and/or partial dissolution of the crystals and intratelluric nature of dike phenocrysts, the core of which was inherited from derivatives/cumulates of the EC. Mineral thermometry showed that: (1) the initial temperatures of parental magma are 1200–1400°С, (2) the crystallization temperatures of the EC are 1080–1155°С, and (3) the emplacement temperatures of the PD are 910–1070°С. Petrological model suggests that the high-temperature boninite-like EC magmas were generated from the metasomatized upper mantle. The parental melt was contaminated by continental crust rocks. The existence of an transitional evolved magma chamber is inferred in the crust. At the final stage of its evolution, the PD melt intruded yet hot EC plutons.