Extreme positive Ce-anomalies in a 3.0 Ga submarine volcanic sequence,Murchison Province: Oxygenated marine bottom waters

Systematic positive anomalies of Ce, where Ce/Ce* spans 2.1 to 11.4, are present in basalts and rhyolites of a 2.96 Ga submarine volcanic sequence of the Murchison Province, Western Australia. This volcanic sequence is host to a stratabound Cu–Zn deposit that formed on the seafloor from a seawater hydrothermal system. These are true Ce anomalies as Pr/Pr* < 1. In modern oxygenated marine water Ce is sequestered by Mn-oxides and hydroxides, which coprecipitate with Fe-oxides and hydroxides as nodules and crusts on the ocean floor, as well as Fe–Mn chemical sediments from hydrothermal systems at ocean spreading centers. Fe–Mn sediments have positive Ce anomalies and marine water complementary negative anomalies. Such Ce anomalies have not formerly been reported for Archean hydrothermally altered volcanic rocks. These extreme anomalies are attributed to Mn-transport in shallow-circulating oxygenated marine bottom waters peripheral to the deeper, hotter, hydrothermal system from which the Cu–Zn deposit formed, and record an oxygenated marine environment ~ 500 Ma before the so-called great oxidation event at ~ 2.4 Ga. Results for positive Ce anomalies in the Golden Grove volcanic sequence are complementary to negative anomalies in Archean BIF, collectively stemming from particulate scavenging of Ce^+ 3 in an oxic water column.     

Precambrian iron formations from the Cauvery Suture Zone,Southern India: Implications for sub-marine hydrothermal origin in Neoarchean and Neoproterozoic convergent margin settings

Thick horizons of iron formations including Banded Iron Formations (BIFs) and Banded Silicate Formations (BSFs) occur as E–W trending bands in the eastern part of Cauvery Suture Zone (CSZ) in the Sothern Granulite Terrane of India. Some of these occur in close association with the Neoarchean-Neoproterozoic suprasubduction zone complexes, where as some others are associated with metamorphosed accretionary sequences including pyroxene granulites and other high grade rocks. The iron formations are highly deformed and metamorphosed under amphibolite to granulite facies conditions and are composed of quartz–magnetite–hematite–goethite–garnet–pyrite together with grunerite and pyroxene. Here we report the geochemical characteristics of twenty representative samples from the iron formations that reveal a widely varying composition with Fe₂O₃^(t) (22–65 wt.% as total iron) total- Fe₂O₃/TiO₂ (205–6532), MnO/TiO₂ (0.25–12.66) and SiO₂ (33–85 wt.%), broadly representing the two types of iron formations. These formations also show very low Al/(Al + Fe + Mn) ratio (0.001–0.01), Al₂O₃ (0.07–0.76 wt.%), Al₂O₃/TiO₂ ratio (2.7–21), MgO (0.01–4.41 wt.%), CaO (0.1–1.24 wt.%), Na₂O (0.01–0.05 wt.%) and K₂O (0.01 wt.%) together with low total REE (3.38–31.63 ppm). The trace and REE elemental distributions show wide variation with high Ni (274 ppm), and Zn contents (up to 87 ppm) when compared to mafic volcanics of the adjoining areas. Tectonic discrimination plots indicate that the iron formations of the Cauvery Suture Zone are of hydrothermal origin. Their chondrite normalized patterns show slight positive Eu anomaly (Eu/Eu* = up to 1.77) and relatively less fractionation of REE with slight LREE enrichment compared to HREE. However, the PAAS (Post Archean Average of Australian Sediments) normalized REE patterns display significant positive Eu anomaly (Eu/Eu* up to 2.32) with well represented negative Ce anomalies (Ce/Ce* = 0.66–1.28). The above results together with petrological characteristics and available geochronology of the associated lithologies suggest that the iron formations can be correlated to Algoma-type. The Fe and Si were largely supplied by medium to high temperature sub-marine hydrothermal systems in Neoarchean and Neoproterozoic convergent margin settings.     

The U–Pb age and Lu–Hf isotope composition of detrital zircon from metasedimentary rocks of the Onot greenstone belt (Sharyzhalgay uplift,southern Siberian craton)

We present data on the composition of metasedimentary rocks from the greenstone belt of the Onot terrane (Sharyzhalgay uplift) and results of U–Pb dating (SHRIMP II) and Lu–Hf isotope study of detrital zircon from garnet–staurolite schists. The metasedimentary rocks of the Onot greenstone belt are dominated by garnet- and staurolite-bearing schists alternating with amphibolites (metabasalts) in the upper part of the section. Compositionally the protoliths of garnet–staurolite schists correspond to sedimentary rocks, ranging from siltstone to pelitic mudstone. The trace-element characteristics of the garnet–staurolite schists indicate that the terrigenous material was derived from three different rock types, such as tonalite–trondhjemite plagiogneisses (elevated Gd/Yb ratios), mafic rocks (elevated Cr/Th ratios and reduced Th/Sc ratios), and felsic igneous rocks formed by crustal melting (the presence of a Eu minimum), which agrees with the set of potential source rocks from the Onot terrane. The age of predominant detrital zircon reflects the erosion of mainly Neoarchean igneous rocks; this fact, combined with the poor rounding of zircon and tectonically active sedimentation conditions accompanied by mafic volcanism, suggests that the probably depositional age is ca. 2.7 Ga. Older source rocks (2.80–3.35 Ga) contributed to the sediment deposition along with the Neoarchean ones. According to the Hf isotope composition of detrital zircon from the garnet–staurolite schists, the source provenances had different crustal prehistories. The source provenances include Paleoarchean and juvenile Neoarchean crust and rocks formed by the mixing of melts from ancient and juvenile crustal sources.     

Magma sources and petrogenesis of the early–middle Paleozoic backarc granitoids from the central part of the Qilian block,NW China

The petrology, geochemistry, geochronology, and Sr–Nd–Hf isotopes of the backarc granitoids from the central part of the Qilian block are studied in the present work. Both S- and I-type granitoids are present. In petrographic classification, they are granite, alkali feldspar granite, felsic granite, diorite, quartz diorite, granodiorite, and albite syenite. The SHRIMP ages are 402–447 Ma for the S-type and 419–451 Ma for the I-type granitoids. They are mostly high-K calc-alkaline granitoids. The S-type granitoids are weakly to strongly peraluminous and are characterized by negative Eu anomalies (Eu/Eu* = 0.18–0.79). The I-type granitoids are metaluminous to weakly peraluminous and are characterized mostly by small negative to small positive Eu anomalies (Eu/Eu* = 0.71–1.16). The initial (⁸⁷Sr/⁸⁶Sr) values are 0.708848–0.713651 for the S-type and 0.704230–0.718108 for the I-type granitoids. The ε_Nd(450 Ma) values are − 8.9–−4.1 and − 9.7–+ 1.9 for the S-type and I-type granitoids, respectively. The T_DM values are 1.5–2.4 Ga for the S-type and 1.0–2.3 Ga for the I-type granitoids. For the Qilian block, the backarc granitoid magmatism took place approximately 60 million years after the onset of the southward subduction of the north Qilian oceanic lithosphere and lasted approximately 50 million years. Partial melting of the source rocks consisting of the Neoproterozoic metasedimentary rocks of the Huangyuan Group and the intruding lower Paleozoic basaltic rocks could produce the S-type granitoid magmas. Partial melting of basaltic rocks mixed with lower continental crustal materials could produce the I-type granitoid magmas. Major crustal growth occurred in the late Archean and Meso-Paleoproterozoic time for the Qilian block. The magma generation was primarily remelting of the crustal rocks with only little addition of the mantle materials after 1.0 Ga for the Qilian block.     

Geochemistry of Archean Radioactive Quartz Pebble Conglomerates and Quartzites from western margin of Singhbhum-Orissa Craton,eastern India: Implications on Paleo-weathering,provenance and tectonic setting

Lenses of radioactive Quartz Pebble Conglomerates (QPC) and associated quartzites are exposed along western margin of Archean Bonai Granite in Singhbhum-Orissa Craton, eastern India intermittently spreading over a strike length of 8–10 km. QPCs are radioactive while quartzites are mostly non-radioactive in nature. The purpose of the research is to investigate and characterize the radioactive QPC and quartzites geochemically to decipher their paleo-weathering conditions, provenance characteristics and possible tectonic setting of deposition. Geochemical data suggest moderate to high chemical weathering conditions in the provenance areas of QPC and quartzites. Major, trace and REE data indicate predominantly felsic to partly mafic-ultramafic sources for the deposition of radioactive Quartz Pebble Conglomerates from the surrounding Archean terrain.Elevated values of Th, U, Pb, La, Ce, Y and low Sc with high critical trace elemental ratios of Th/Sc, La/Sc, Th/Cr and Zr/Sc in radioactive QPC indicate their derivation from felsic igneous source. Low concentration of Th and Sc in quartzites compared to QPC and their variable Th/Sc ratios indicate both felsic and mafic sources for quartzites, albeit their preferential felsic affiliation. Higher Cr/Th ratios in quartzites (18.4), moderate Cr/Th in QPC (5.42), low to moderate Y/Ni in QPC (0.36–12.4) and quartzite (0.29–1.88), along with Au ranging from 30 to 1527 ppb, Pt up to 188 ppb and 682 ppb in QPC and quartzites respectively point towards some contribution from mafic-ultramafic source as well. REE patterns and in particular negative Eu anomalies for both QPC and quartzites further support their derivation from felsic rocks and could possibly linked to some of the phases of Archean Singhbhum Granite and Bonai Granite. Granitic to pegmatitic source for QPC is also revealed by the presence of rounded to sub-rounded monazite, zircon and thorian-uraninite grains in their matrix. Study indicates that QPC and quartzites were deposited in a passive margin tectonic setting developed during Archean between a span of 3.3 and 3.16 Ga along the western margin of Bonai Granite when the reducing condition was prevalent as indicated by their low Th/U ratios (<4.0) and presence of detrital grains of uraninite and pyrite in QPC. Radioactive QPC from western margin of Archean Singhbhum-Orissa Craton bears broad resemblance with QPC from Witwatersrand basin of South Africa and Elliot Lake, Canada and thus appears to be ideal sites for exploring QPC hosted U (+Au-PGE) mineralization in the analogous areas.     

In-situ LA-ICPMS trace elements and U–Pb analysis of titanite from the Mesozoic Ruanjiawan W–Cu–Mo skarn deposit,Daye district,China

In this paper, we present U–Pb ages and trace element compositions of titanite from the Ruanjiawan W–Cu–Mo skarn deposit in the Daye district, eastern China to constrain the magmatic and hydrothermal history in this deposit and provide a better understanding of the U–Pb geochronology and trace element geochemistry of titanite that have been subjected to post-crystallization hydrothermal alteration. Titanite from the mineralized skarn, the ore-related quartz diorite stock, and a diabase dike intruding this stock were analyzed using laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). Titanite grains from the quartz diorite and diabase dike typically coexist with hydrothermal minerals such as epidote, sericite, chlorite, pyrite, and calcite, and display irregular or patchy zoning. These grains have low LREE/HREE and high Th/U and Lu/Hf ratios, coupled with negative Eu and positive Ce anomalies. The textural and compositional data indicate that titanite from the quartz diorite has been overprinted by hydrothermal fluids after being crystallized from magmas. Titanite grains from the mineralized skarn are texturally equilibrated with retrograde skarn minerals including actinolite, quartz, calcite, and epidote, demonstrating that these grains were formed directly from hydrothermal fluids responsible for the mineralization. Compared to the varieties from the quartz diorite stock and diabase dike, titanite grains from the mineralized skarn have much lower REE contents and LREE/HREE, Th/U, and Lu/Hf ratios. They have a weighted mean ²⁰⁶Pb/²³⁸U age of 142 ± 2 Ma (MSWD = 0.7, 2σ), in agreement with a zircon U–Pb age of 144 ± 1 Ma (MSWD = 0.3, 2σ) of the quartz diorite and thus interpreted as formation age of the Ruanjiawan W–Cu–Mo deposit. Titanite grains from the ore-related quartz diorite have a concordant U–Pb age of 132 ± 2 Ma (MSWD = 0.5, 2σ), which is 10–12 Ma younger than the zircon U–Pb age of the same sample and thus interpreted as the time of a hydrothermal overprint after their crystallization. This hydrothermal overprint was most likely related to the emplacement of the diabase dike that has a zircon U–Pb age of 133 ± 1 Ma and a titanite U–Pb age of 131 ± 2 Ma. The geochronological results thus reveal two hydrothermal events in the Ruanjiawan deposit: an early one forming the Wu–Cu–Mo ores related to the emplacement of the quartz diorite stock and a later one causing alteration of the quartz diorite and its titanite due to emplacement of diabase dike. It is suggested that titanite is much more susceptible to hydrothermal alteration than zircon. Results from this study also highlight the utilization of trace element compositions in discriminating titanite of magmatic and hydrothermal origins, facilitating a more reasonable interpretation of the titanite U–Pb ages.     

Formation and evolution of Precambrian continental lithosphere in South China

An overview is presented for the formation and evolution of Precambrian continental lithosphere in South China. This is primarily based on an integrated study of zircon U–Pb ages and Lu–Hf isotopes in crustal rocks, with additional constraints from Re–Os isotopes in mantle-derived rocks. Available Re–Os isotope data on xenolith peridotites suggest that the oldest subcontinental lithospheric mantle beneath South China is primarily of Paleoproterozoic age. The zircon U–Pb ages and Lu–Hf isotope studies reveal growth and reworking of the juvenile crust at different ages. Both the Yangtze and Cathaysia terranes contain crustal materials of Archean U–Pb ages. Nevertheless, zircon U–Pb ages exhibit two peaks at 2.9–3.0 Ga and ~ 2.5 Ga in Yangtze but only one peak at ~ 2.5 Ga in Cathaysia. Both massive rocks and crustal remnants (i.e., zircon) of Archean U–Pb ages occur in Yangtze, but only crustal remnants of Archean U–Pb ages occur in Cathaysia. Zircon U–Pb and Lu–Hf isotopes in the Kongling complex of Yangtze suggest the earliest episode of crustal growth in the Paleoarchean and two episodes of crustal reworking at 3.1–3.3 Ga and 2.8–3.0 Ga. Both negative and positive ε_Hf(t) values are associated with Archean U–Pb ages of zircon in South China, indicating both the growth of juvenile crust and the reworking of ancient crust in the Archean. Paleoproterozoic rocks in Yangtze exhibit four groups of U–Pb ages at 2.1 Ga, 1.9–2.0 Ga, ~ 1.85 Ga and ~ 1.7 Ga, respectively. They are associated not only with reworking of the ancient Archean crust in the interior of Yangtze, but also with the growth of the contemporaneous juvenile crust in the periphery of Yangtze. In contrast, Paleoproterozoic rocks in Cathaysia were primarily derived from reworking of Archean crust at 1.8–1.9 Ga. The exposure of Mesoproterozoic rocks are very limited in South China, but zircon Hf model ages suggest the growth of juvenile crust in this period due to island arc magmatism of the Grenvillian oceanic subduction. Magmatic rocks of middle Neoproterozoic U–Pb ages are widespread in South China, exhibiting two peaks at about 830–800 Ma and 780–740 Ma, respectively. Both negative and positive ε_Hf(t) values are associated with the middle Neoproterozoic U–Pb ages of zircon, suggesting not only growth and reworking of the juvenile Mesoproterozoic crust but also reworking of the ancient Archean and Paleoproterozoic crust in the middle Neoproterozoic. The tectonic setting for this period of magmatism would be transformed from arc–continent collision to continental rifting with reference to the plate tectonic regime in South China.     

Origin of Archean tonalite–trondhjemite–granodiorite (TTG) suites and granites in the Fiskenæsset region,southern West Greenland: Implications for continental growth

Mesoarchean to Neoarchean orthogneisses (2.95–2.79 Ga) in the Fiskenæsset region, southern West Greenland, are composed of an older suite of metamorphosed tonalites, trondhjemites, and granodiorites (TTGs), and a younger suite of high-K granites. The TTGs are characterized by high Al₂O₃ (14.2–18.6 wt.%), Na₂O (3.4–5.13 wt.%), and Sr (205–777 ppm), and low Y (0.7–17.4 ppm) contents. On chondrite- and N-MORB-normalized trace element diagrams, the TTGs have the following geochemical characteristics: (1) highly fractionated REE patterns (La/Yb_cn = 14–664; La/Sm_cn = 4.3–11.0; Gd/Yb_cn = 1.5–19.7); (2) strong positive anomalies of Sr (Sr/Sr* = 1.0–15.9) and Pb (Pb/Pb* = 1.4–34.9); and (3) large negative anomalies of Nb (Nb/Nb* = 0.01–0.34) and Ti (Ti/Ti* = 0.1–0.6). The geochemical characteristics of the TTGs and trace element modeling suggest that they were generated by partial melting of hydrous basalts (amphibolites) at the base of a thickened magmatic arc, leaving a rutile-bearing eclogite residue. Field observations suggest that spatially and temporarily associated tholeiitic basalts (now amphibolites) in the Fiskenæsset region might have been the sources of TTG melts. The high-K granites have steep REE patterns (La/Yb_cn = 3.8–506; La/Sm_cn = 2.7–18.9; Gd/Yb_cn = 0.92–12.1) and display variably negative Eu anomalies (Eu/Eu* = 0.37–0.96) and moderate Sr (84–539 ppm) contents. Four outlier granite samples have variably positive Eu (Eu/Eu* = 1.0–12) anomalies. Given that the granodiorites have higher K₂O/Na₂O than the tonalites and trondhjemites, it is suggested that the granites were derived from partial melting of the granodiorites. It is speculated that the dense eclogitic residues, left after TTG melt extraction, were foundered into the sub-arc mantle, leading to basaltic underplating beneath the lower rust. Melting of the granodiorites in response to the basaltic underplating resulted in the production of high-K granitic melts. Formation of the Fiskenæsset TTGs, the foundering of the eclogitic residues into the mantle, and the emplacement of the high-K granites led to the growth of Archean continental crust in the Fiskenæsset region.     

Age and geochemistry of contrasting peridotite types in the Dabie UHP belt,eastern China: Petrogenetic and geodynamic implications

The Dabie ultrahigh-pressure (UHP) metamorphic belt, central China, contains two contrasting types of ultramafic–mafic complex. The Bixiling peridotite in the southern Dabie terrane contains abundant garnet (21–32 vol.%) and thus has high CaO +Al₂O₃ (9.94–15.3 wt.%). The peridotite also has high REE contents with flat REE patterns, high contents of S and other incompatible trace elements, together with low-Mg^# olivine and pyroxene and low Ni and PGE contents. Zircons from this peridotite mostly have low Th/U ratios, interpreted to reflect a metamorphic origin, and give dominantly Triassic ages (ca. 210 Ma). Other zircons with high Th/U ratios give upper intercept ages of 745 Ma. Most zircons have positive ?_Hf (+- 3.6 ～ +- 8.1) values with depleted-mantle model ages (T_DM) of 0.6–1.0 Ga (mean 0.8 Ga) and crustal model ages (T_crust) of 0.8–1.4 Ga (mean 1.1 Ga). We interpret that the Bixiling complex was formed as cumulates in a Neoproterozoic asthenosphere-derived magma chamber in the continental crust, and was later carried to garnet-stable depths (ca. ～ 4.4 GPa) during the subduction of the Yangtze Craton in the Triassic. In contrast, the Raobazhai peridotite in the northern Dabie terrane was metamorphosed at lower P/T conditions (i.e. 15 Kb and 1000 °C). All zircon grains from the peridotite yield Triassic ages (ca. 212 Ma) and have negative ?_Hf values (? 16.6 ～ ? 3.2), Mesoproterozoic model ages (T_DM = 1.0–1.5 Ga) and Paleoproterozoic crustal model ages (T_crust = 1.5–2.3 Ga). The peridotite is enriched in LREE ((La/Yb)_n ≈ 3.5), has high-Mg^# olivine and pyroxene, high Ni and PGEs but low Pd/Ir (mean 3.0). It represents a highly refractory residue of partial melting (up to 18%) of primitive mantle, and is similar to the cratonic mantle xenoliths in Phanerozoic igneous rocks from the eastern North China Craton (e.g. Mengyin and Hebi areas). Negative Ce, Eu and HFSE anomalies in the peridotites suggest that their protolith was derived from the shallow part of the mantle wedge (e.g. plagioclase-stable field) of the North China Craton, and was pulled to a deeper level (e.g. spinel-stable field) during the subduction of the Yangtze Craton. The mantle wedge, like peridotitic xenoliths in the Jurassic Xinyang diatremes at the southern edge of the North China Craton, was metasomatised by fluids/melts released from the subducted continental crust. The fragments of this modified mantle wedge were incorporated into the Yangtze crust during its subduction.     

U–Pb SHRIMP monazite ages of the giant Morro Velho and Cuiabá gold deposits,Rio das Velhas greenstone belt,Quadrilátero Ferrífero,Minas Gerais,Brazil

U–Pb SHRIMP results of 2672 ± 14 Ma obtained on hydrothermal monazite crystals, from ore samples of the giant Morro Velho and Cuiabá Archean orogenic deposits, represent the first reliable and precise age of gold mineralization associated with the Rio das Velhas greenstone belt evolution, in the Quadrilátero Ferrífero, Brazil. In the basal Nova Lima Group, of the Rio das Velhas greenstone belt, felsic volcanic and volcaniclastic rocks have been dated between 2792 ± 11 and 2751 ± 9 Ma, coeval with the intrusion of syn-tectonic tonalite and granodiorite plutons, and also with the metamorphic overprint of older tonalite–trondhjemite–granodiorite crust. Since cratonization and stable-shelf sedimentation followed intrusion of Neoarchean granites at 2612 + 3/− 2 Ma, it is clear that like other granite–greenstone terranes in the world, gold mineralization is constrained to the latest stages of greenstone evolution.     

Eocene potassic and ultrapotassic volcanism in south Tibet: New constraints on mantle source characteristics and geodynamic processes

In the Yangbajing area, southern Tibet, several monogenic volcanoes were conformably superimposed on the Linzizong calc-alkaline volcanic successions. According to their petrologic and geochemical characteristics, these monogenic volcanoes are composed of three rock varieties: tephritic phonolitic plugs and shoshonitic and trachytic lavas. Their geochemical systematics reveals that low-pressure evolutionary processes in the large voluminous Linzizong calc-alkaline magmas were not responsible for the generation of these potassic–ultrapotassic rocks, but the significant change in petrologic and geochemical characteristics from the Linzizong calc-alkaline to potassic–ultrapotassic magma is likely accounted for the change of metasomatic agents in the southern Tibetan lithospheric mantle source during the Paleocene to Eocene. The tephritic phonolites containing both leucite and plagioclase show primary ultrapotassic character similar to that of Mediterranean plagioleucititic magmas. Radiogenic Sr increases with SiO₂ in the xenolith-bearing trachytes strongly suggesting significant crustal assimilation in the shoshonitic magmas. The Yangbajing ultrapotassic rocks have high K₂O and Al₂O₃, and show depletion of high field strength elements (HFSEs) with respect to large ion lithophile elements. In primitive mantle-normalized element diagrams, all samples are characterized by positive spikes at Th (U) and Pb with negative anomalies at Ba, Nb–Ta and Ti, reflecting the orogenic nature of the ultrapotassic rocks. They are characterized by highly radiogenic ⁸⁷Sr/⁸⁶Sr_(i) ratios (0.7061–0.7063) and unradiogenic ¹⁴³Nd/¹⁴⁴Nd_(i) (0.5125), and Pb isotopic compositions (²⁰⁶Pb/²⁰⁴Pb = 18.688–18.733, ²⁰⁷Pb/²⁰⁴Pb = 15.613–15.637, and ²⁰⁸Pb/²⁰⁴Pb = 38.861–38.930) similar to the global subducting sediment. Strong enrichment of incompatible trace elements and high Th fractionation from the other HFSEs (such as Nb and U) clearly indicate that the Th-enriched sedimentary component in a network veined mantle source was mainly introduced by sediment-derived melts. In addition, the ultrapotassic rocks have significant Ce (Ce/Ce* = 0.77–0.84) and Eu (Eu/Eu* = 0.72–0.75) anomalies, suggesting a subduction sediment input into the southern Tibetan lithospheric mantle source. In contrast, high U/Th (> 0.20) and Ba/Th (> 32) and low Th/La (< 0.3) in the shoshonites indicate that the Eocene potassic magma originated from partial melting of the surrounding peridotite mantle pervasively affected by slab-related fluid addition from the dehydration of either the subducting oceanic crust or the sediment. Thus, at least two different subduction-related metasomatic agents re-fertilized the upper mantle. According to the radiometric ages and spatial distribution, the Gangdese magmatic association shows a temporal succession from the Linzizong calc-alkaline to ultrapotassic magmas. This indicates a late arrival of recycled sediments within the Tibetan lithospheric mantle wedge. The most diagnostic signatures for the involvement of continent-derived materials are the super-chondritic Zr/Hf (45.5–49.2) and elevated Hf/Sm values (0.81–0.91) in the ultrapotassic rocks. Therefore, the occurrence of orogenic magmatism in the Gangdese belt likely represents the volcanic expression of the onset of the India–Asia collision, preceding the 10 Ma Neo-Tethyan slab break-off process at 42–40 Ma. The absence of residual garnet in the mantle source for the ultrapotassic volcanism seems to imply that the southern Tibetan lithosphere was not been remarkably thickened until the Eocene (～ 50 Ma).     

Zircon ages and Hf isotopic constraints on sources of clastic metasediments of the Slyudyansky high-grade complex,southeastern Siberia: Implication for continental growth and evolution of the Central Asian Orogenic Belt

We present results of combined in situ U–Pb dating of detrital zircons and zircon Hf and whole-rock Nd isotopic compositions for high-grade clastic metasedimentary rocks of the Slyudyansky Complex in eastern Siberia. This complex is located southwest of Lake Baikal and is part of an early Paleozoic metamorphic terrane in the eastern part of the Central Asian Orogenic Belt (CAOB). Our new zircon ages and Hf isotopic data as well as whole-rock Nd isotopic compositions provide important constraints on the time of deposition and provenance of early Paleozoic high-grade metasedimentary rocks as well as models of crustal growth in Central Asia. Ages of 0.49–0.90 Ga for detrital zircons from early Paleozoic high-grade clastic sediments indicate that deposition occurred in the late Neoproterozoic and early Paleozoic, between ca. 0.62–0.69 and 0.49–0.54 Ga. Hf isotopic data of 0.82–0.69 Ga zircons suggest Archean and Paleoproterozoic (ca. 2.7–2.8 and 2.2–2.3 Ga; Hfc = 2.5–3.9 Ga) sources that were affected by juvenile 0.69–0.82 Ga Neoproterozoic magmatism. An additional protolith was also identified. Its zircons yielded ages of 2.6–2.7 Ga, and showed high positive ε_Hf(t) values of +4.1 to +8.0, and Hf model ages t_Hf(DM) = t_Hfc = 2.6–2.8 Ga, which is nearly identical to the crystallization ages. These isotopic characteristics suggest that the protolith was quite juvenile. The whole-rock Nd isotopic data indicate that at least part of the Slyudyansky Complex metasediments was derived from “non-Siberian” provenances. The crustal development in the eastern CAOB was characterized by reworking of the early Precambrian continental crust in the early Neoproterozoic and the late Neoproterozoic–early Paleozoic juvenile crust formation.     

Age of the Siberian craton crust beneath the northern kimberlite fields: Insights to the craton evolution

Comprehensive studies of zircon xenocrysts from kimberlites of the Kuoika field (northeastern Siberian craton) and several kimberlite fields of the eastern Anabar shield, along with data compilation on the age of kimberlite-hosting terranes, reveal details of the evolution of the northern Siberian craton. The age distribution and trace element characteristic of zircons from the Kuoika field kimberlites (Birekte terrane) provide evidence of significant basic and alkaline–carbonatite magmatism in northern Siberia in the Paleozoic and Mesozoic periods. The abundance of 1.8–2.1 Ga zircons in both the Birekte and adjacent Hapchan terranes (the latter hosting kimberlites of the eastern Anabar shield) supports the Paleoproterozoic assembly and stabilization of these units in the Siberian craton and the supercontinent Columbia. The abundance of Archean zircons in the Hapchan terrane reflects the input of an ancient source other than the Birekte terrane and addresses the evolution of the terrane to west (Magan and Daldyn terranes of the Anabar shield). The present study has also revealed the oldest known remnant of the Anabar shield crust, whose 3.62 Ga age is similar to that of another ancient domain of Siberia, the Aldan shield. The first Hf isotope data for the Anabar shield coupled with the U–Pb systematics indicate three stages of crustal growth (Paleoproterozoic, Neoarchean and Paleoarchean) and two stages of the intensive crustal recycling in the Paleoproterozoic and Neoarchean. Intensive reworking of the existing crust at 2.5–2.8 Ga and 1.8–2.1 Ga is interpreted to provide evidence for the assembly of Columbia. The oldest Hf model age estimation provides a link to Early Eoarchean (3.7–3.95 Ga) and possibly to Hadean crust. Hence, some of the Archean cratonic segments of the Siberian craton could be remnants of the Earth's earliest continental crust.     

Geochemistry,provenance, and tectonic setting of Neoproterozoic metavolcanic and metasedimentary units,Werri area,Northern Ethiopia

Major, trace and rare earth element (REE) compositions of upper Proterozoic metavolcanic and metasedimentary rocks from the Tsaliet and Tembien Groups in the Werri district of northern Ethiopia were determined to examine their tectonic setting of eruption, provenance and source area weathering conditions. Tsaliet Group metavolcanic rocks in the Werri area have sub-alkaline chemistry characterized by low to intermediate SiO₂ contents, high Al₂O₃, low MgO and very low Cr and Ni. High field strength element (HFSE) abundances are highly variable. ∑REE abundances vary from 66.7 to 161.3 ppm, and chondrite-normalized REE patterns are moderately fractionated, with La_N/Yb_N values of between 3.1 and 9.0. Europium anomalies are variable (Eu/Eu* 0.80–1.21) but are generally positive (average Eu/Eu* 1.06). On tectonic discrimination diagrams, most samples have either volcanic-arc chemistry or fall in the overlap field with mid-oceanic ridge basalt (MORB). However, primitive mantle-normalized trace element abundances are comparable with sub-alkaline basalts from developed island arcs. ¹⁴⁷Sm/¹⁴⁴Nd ratios range from 0.1167 to 0.1269 (n = 3), yielding initial εNd_(800 Ma) of +3.8 to +4.9 and mean T_DM model age of 0.96 Ga, indicative of derivation from juvenile Neoproterozoic mantle. Metasediments from three locations (Werri1, Werri2 and Tsedia) in the Werri and Tsedia Slates have similar Al₂O₃, TiO₂ and HFSE contents but variable and low Na₂O, CaO and K₂O. Cr and Ni are slightly enriched in the Werri2 and Tsedia suites. SiO₂ is very variable, with average values of 70.75, 72.2 and 66.4 wt.% in the Werri1, Werri2 and Tsedia suites, respectively. ∑REE abundances in the metasediments (14.74–108.1) are lower than in the metavolcanics, and are slightly less fractionated, with La_N/Yb_N ratios of 0.8–5.9. Europium anomalies vary (Eu/Eu* 0.80–1.21) but are insignificant on average (Eu/Eu* 0.96). High values for the Chemical Index of Alteration (generally 70–90), and Plagioclase Index of Alteration (>75) in the Werri metasediments indicate moderate to severe chemical weathering in their source. Average major and trace element compositions of the metasediments and their REE patterns are comparable with the metavolcanics. ¹⁴⁷Sm/¹⁴⁴Nd ratios of the metasediments range from 0.1056 to 0.1398 (n = 4), with initial εNd_(800 Ma) of +3.4 to +5.0 and mean T_DM model age of 0.97 Ga, indicating derivation from juvenile Neoproterozoic crust similar to the underlying metavolcanics, with minimal (4–10%) contribution from older crust. The most sensitive tectonic setting discriminators indicate the Werri metasediments represent developed oceanic island arc sediments. The chemical similarity of the Werri metavolcanics to the nearby Adwa metavolcanics, Nakfa terrane in Eritrea, and volcanic units in central Saudi Arabia imply that juvenile Neoproterozoic Arabian Nubian Shield crust extended south at least as far as the Werri area of northern Ethiopia. The comparable geochemistry of the metasediments and their underlying lithologies attests to their derivation from this juvenile crustal material.     

In situ zircon U–Pb dating and O isotopes of the Neoarchean Hongtoushan VMS Cu–Zn deposit in the North China Craton: Implication for the ore genesis

The Hongtoushan volcanogenic massive sulfide (VMS) deposit is the largest Archean Cu–Zn deposit in China, located in the Qingyuan greenstone belt on the northern margin of the North China Craton. The Cu–Zn mineralization was stratigraphically controlled by the interbeds (~ 100 m in thickness) of mafic–felsic volcanic sets and overlain by banded iron layers. However, the relationship between VMS deposits and associated volcanics has not been examined. This study ultimately clarifies the times and sources of the volcanics and mineralization. Based on in situ zircon U–Pb and O isotope on VMS-hosting mafic, felsic volcanic rocks, banded and massive sulfide ores and postmineralization pegmatite vein, we considered that there were two main formation stages for the Qingyuan Cu–Zn deposits; one was exhalative-hydrothermal sedimentation and another was further Cu–Zn enriched by later hydrothermal processes. The timing of the first stage occurred at 2571 ± 6 Ma based on the magmatic zircons in the VMS-hosting mafic volcanic rocks, from which the inherited zircons also indicate the existence of 2.65–3.12 Ga ancient supercrustal rocks in the Qingyuan district. A modern mantle-like δ¹⁸Ozircon value of 5.5 ± 0.1‰ (2SD) for this volcanism was well preserved in the inherited core domains of ore samples. It suggests that the mafic volcanics was most likely sourced from partial melting of juvenile crust, e.g., TTG granites. A large-scale metamorphic or hydrothermal event is documented by the recrystallized zircons in sulfide ores. The timing is tightly constrained by the hydrothermal zircon U–Pb ages. They are 2508 ± 4 Ma for the banded ore, 2507 ± 4 Ma for the massive ore and 2508 ± 2 Ma for the postmineralization pegmatite vein. These indistinguishable ages indicate that the 2507 Ma hydrothermal systems played a significant role in the upgrading of the VMS Cu–Zn orebodies. The weighted δ¹⁸O values of hydrothermal zircons show a successively increasing trend from 6.0 ± 0.1‰ (2σ) for the banded ore, 6.6 ± 0.2‰ (2σ) for the massive ore to 7.3 ± 0.2‰ (2σ) for the later pegmatite vein. This variation might be induced by gradual inputting of the δ¹⁸O-rich oceanic crust and/or oceanic sediment during the hydrothermal cycling system. Considering its modern mantle-like oxygen isotope composition of 2571 Ma volcanism, a submarine volcanic hydrothermal system involving mantle plumes is a preferred setting for the Neoarchean VMS Cu–Zn deposits in the Qingyuan greenstone belt.     

Field and geochemical characteristics of Mesoarchean to Neoarchean volcanic rocks in the Storø greenstone belt,SW Greenland: Evidence for accretion of intra-oceanic volcanic arcs

The Storø greenstone belt, southern West Greenland, consists of thrust-imbricated slices of Mesoarchean (>3060 Ma) and Neoarchean (ca. 2800 Ma) mafic to ultramafic volcanic rocks, volcaniclastic sediments, and gabbro–anorthosite associations. The belt underwent polyphase metamorphism at upper amphibolite facies conditions between 2650 and 2600 Ma. The contacts between the Mesoarchean and Neoarchean volcanic rocks, and surrounding Eoarchean to Neoarchean tonalite–trondhjemite–granodiorite (TTG) gneisses are tectonic and typically bounded by high-grade mylonites. Regardless of age, the volcanic rocks are dominated by mafic amphibolites with a tholeiitic basalt composition, near-flat to slightly enriched light rare earth element (LREE) patterns (La/Sm_cn = 0.91–1.48), relatively flat to slightly depleted heavy-REE (HREE) (Gd/Yb_cn = 1.0–1.28), and pronounced negative Nb–Ta anomalies (Nb/Nb* = 0.34–0.73) on chondrite- and primitive mantle-normalized diagrams. These geochemical characteristics are consistent with subduction zone geochemical signatures and partial melting of a shallow (<80 km) mantle source free of residual garnet. There is no geochemical evidence for contamination by older continental crust. The overall field and geochemical characteristics suggest that the thrust-imbricated basaltic rocks were erupted in intra-oceanic subduction zone settings. Sedimentary rocks are represented by garnet–biotite and quartzitic gneisses. They are characterized by relatively high contents of transition metal (Ni = 10–154 ppm; Cr = 7–166 ppm) and enriched LREE patterns (La/Sm_cn = 1.38–3.79). These geochemical characteristics suggest that the sedimentary rocks were derived from erosion of felsic to mafic igneous source rocks. Collectively, the structural and lithogeochemical characteristics of the Storø greenstone belt are consistent with collision (accretion) of unrelated Archean volcanic rocks formed in supra-subduction zone geodynamic settings. Accordingly, the Mesoarchean and Neoarchean rock record of the Storø greenstone belt may well be explained in terms of modern-style plate tectonic processes.     

Geochemistry and geochronology of Carboniferous volcanic rocks in the eastern Junggar terrane,NW China: Implication for a tectonic transition

The Carboniferous tectonic setting of the Junggar terrane, northern Xinjiang, NW China, has long been a matter of debate. Voluminous Carboniferous volcanic rocks are widely distributed in the Karamaili area, the southern part of the eastern Junggar terrane. Early Carboniferous rocks comprise basalts and basaltic andesites, with enrichment of LREE and LILE and depletion of HFSE, and uniformly high ε_Nd(t) (+ 3.7 to + 4.0). Late Carboniferous rocks consist of basalts, basaltic andesites, rhyolites and minor dacites, and can be subdivided into basic and felsic groups. The basic rocks are depleted in HFSE, and show variable high ε_Nd(t) (+ 4.8 to + 6.9). They have higher Cr and Ni and lower Na₂O, U and Th contents than early Carboniferous basic rocks. The felsic rocks show A-type affinity, with typical enrichment of alkalis, LREE and HFSE and strong depletion in Ba, Sr, Eu and Ti. They have high values of ε_Nd(t) and zircon ε_Hf(t) (+ 11.6 to + 17.9). New LA-ICPMS zircon U–Pb analyses constrain their emplacement to late Carboniferous time (306.5–314.3 Ma).The Carboniferous basic rocks show negative Zr-Hf anomalies and low Th/Ce (< 0.07) and Th/La (0.06–0.16), excluding significant crustal contamination during magma evolution. They have low La/Ba (0.03–0.12), Ce/Y (< 3) and (Tb/Yb)_N (< 2) and variable Ba/Th (28–318) and Ba/La (3.1–34), suggesting that they were derived from a main spinel with minor garnet lherzolite mantle source metasomatized by slab-derived fluids. The late Carboniferous felsic rocks were produced when upwelling asthenosphere triggered partial melting of juvenile lower crust. The early Carboniferous volcanism occurred in an island-arc setting related to the southward subduction of the Paleo-Junggar Ocean plate, whereas the late Carboniferous rocks erupted in a post-collisional extensional setting. Thus, a rapid tectonic transition from arc to post-collisional extension may have occurred between early and late Carboniferous, and probably resulted from slab break-off or lithospheric delamination.     

The exhumation history of collision-related mineralizing systems in Tibet: Insights from thermal studies of the Sharang and Yaguila deposits,central Lhasa

The large, newly discovered Sharang porphyry Mo deposit and nearby Yaguila skarn Pb–Zn–Ag (–Mo) deposit reside in the central Lhasa terrane, northern Gangdese metallogenic belt, Tibet. Multiple mineral chronometers (zircon U–Pb, sericite ⁴⁰Ar–³⁹Ar, and zircon and apatite (U–Th)/He) reveal that ore-forming porphyritic intrusions experienced rapid cooling (> 100 °C/Ma) during a monotonic magmatic–hydrothermal evolution. The magmatic–hydrothermal ore-forming event at Sharang lasted ~ 6.0 Myr (~ 1.8 Myr for cooling from > 900 to 350 °C and ~ 4.0 Myr for cooling from 350 to 200 °C) whereas cooling was more prolonged during ore formation at Yaguila (~ 1.8 Myr from > 900 to 500 °C and a maximum of ~ 16 Myr from > 900 to 350 °C). All porphyritic intrusions in the ore district experienced exhumation at a rate of 0.07–0.09 mm/yr (apatite He ages between ~ 37 and 30 Ma). Combined with previous studies, this work implies that uplift of the eastern section of the Lhasa terrane expanded from central Lhasa (37–30 Ma) to southern Lhasa (15–12 Ma) at an increasing exhumation rate. All available geochronologic data reveal that magmatic–hydrothermal–exhumation activities in the Sharang–Yaguila ore district occurred within four periods of magmatism with related mineralization. Significant porphyry-type Mo mineralization was associated with Late Cretaceous–Eocene felsic porphyritic intrusions in the central Lhasa terrane, resulting from Neotethyan oceanic subduction and India–Asia continental collision.     

Geochemistry of the Apulian karst bauxites (southern Italy): Chemical fractionation and parental affinities

This study focuses on a late Cretaceous karst bauxite deposit in the Murge area of the Apulia district, southern Italy. The first analysis of the vertical distributions of a wide range of elements (including REEs and selected trace elements) throughout the deposit was shown in order to identify the processes determining element fractionation during the evolution of the bauxite.The studied karst bauxite deposit exhibits an ooidal texture, is mineralogically homogeneous, and contains higher abundances of boehmite than of hematite, kaolinite, and anatase. The major element composition of the bauxite is dominated by elevated concentrations of SiO₂, TiO₂, Al₂O₃, and Fe₂O₃, and analyses of element mobility within the bauxite indicate that all elements except for Nb and Ni, and to a lesser extent Cr, are depleted relative to the immobile element Ti. R-mode factor analysis revealed a number of key findings: (i) some low-solubility elements (e.g., Zr, Th, Ti, V, Ga) were concentrated in detrital zircon and monazite (Zr, Th), in anatase (Ti, V), and possibly in boehmite and hematite (Ga) during the later stages of bauxitisation; (ii) Fe and Cr were concentrated during wet conditions, whereas Al and Co accumulated during dry conditions; (iii) distributions of the light REEs (LREEs) and the heavy REEs (HREEs) are controlled by the same processes, suggesting that little LREE/HREE fractionation occurred during bauxite formation; and (iv) the behaviour of cerium is different from that of the other REEs, and highly variable cerium anomalies are observed across the deposit, with three characteristic Ce/Ce* maxima with values of > 2. Parisite was the only authigenic cerium mineral detected during this study. Thus, we propose a three-step model to explain the distribution of Ce: cerianite is dissolved by cerium reduction, is transported by downward-moving meteoric water (per descensum), and finally parisite is precipitated. This cycle was repeated several times in the Apulian karst bauxite in response to Eh decreases under alkaline conditions, promoted by fluctuations in the groundwater level. Finally, we used the value of the Eu anomaly to discuss the parental affinity or protolith of the bauxite. The value of the Eu anomaly (min. Eu/Eu* = 0.76, max. Eu/Eu* = 0.89) indicates that the bauxite was not derived from carbonates, but rather, that the majority of the bauxite was influenced by intermediate to mafic magmatic sources. Eu/Eu* vs Sm/Nd diagram suggests that the parental material for the bauxite was derived from a combination of a distant magmatic source and clastic material derived from a continental margin (northern Africa) to the south.     

Detrital zircon U–Pb geochronology,trace-element and Hf isotope geochemistry of the metasedimentary rocks in the Eastern Himalayan syntaxis: Tectonic and paleogeographic implications

The origin of the Greater Himalayan Sequence in the Himalaya and the paleogeographic position of the Lhasa terrane within Gondwanaland remain controversial. In the Eastern Himalayan syntaxis, the basement complexes of the northeastern Indian plate (Namche Barwa Complex) and the South Lhasa terrane (Nyingchi Complex) can be studied to explore these issues. Detrital zircons from the metasedimentary rocks in the Namche Barwa Complex and Nyingchi Complex yield similar U–Pb age spectra, with major age populations of 1.00–1.20 Ga, 1.30–1.45 Ga, 1.50–1.65 Ga and 1.70–1.80 Ga. The maximum depositional ages for their sedimentary protoliths are ~ 1.0 Ga based on the mean ages of the youngest three detrital zircons. Their minimum depositional ages are ~ 477 Ma for the Namche Barwa Complex and ~ 499 Ma for the Nyingchi Complex. Detrital zircons from the Namche Barwa Complex and Nyingchi Complex also display similar trace-element signatures and Hf isotopic composition, indicating that they were derived from common provenance. The trace-element signatures of 1.30–1.45 Ga detrital zircons indicate that the 1.3–1.5 Ga alkalic and mafic rocks belt in the southeastern India is a potential provenance. Most 1.50–1.65 Ga zircons have positive ε_Hf(t) values (+ 1.2 to + 9.0), and most 1.70–1.80 Ga zircons have negative ε_Hf(t) values (− 7.1 to − 1.9), which are compatible with those of the Paleo- to Mesoproterozoic orthogneisses in the Namche Barwa Complex. Provenance analysis indicates that the southern Indian Shield, South Lhasa terrane and probably Eastern Antarctica were the potential detrital sources. Combined with previous studies, our results suggest that: (1) the Namche Barwa Complex is the northeastern extension of the Greater Himalaya Sequence; (2) the metasedimentary rocks in the Namche Barwa Complex represent distal deposits of the northern Indian margin relative to the Lesser Himalaya; (3) the South Lhasa terrane was tectonically linked to northern India before the Cambrian.