Depleted Mantle Source Research Articles

Zircon ages, mineralogy, and geochemistry of ophiolitic mafic and island-arc rocks from central Cuba: Implications for Cretaceous tectonics in the Caribbean region

Ophiolite-related rocks and volcanic-arc rocks are the main geological components in Cuba; deciphering their origin and evolution is fundamental in interpreting and reconstructing the tectonic processes for the Caribbean region. In this paper, we report new zircon U Pb chronology, mineralogy, and geochemistry of ophiolitic metadiabase dikes and island-arc rocks in central Cuba, and discuss their origin and petrogenesis. The metadiabase dikes have a zircon U Pb age of 135.0 ± 1.7 Ma; they are low-K tholeiites , and have geochemical characteristics intermediate between those of typical normal mid-ocean ridge basalt (N-MORB) and island-arc tholeiites (IAT), suggesting that they were formed in a back-arc basin. The investigated island-arc rocks, including amphibolite from the Mabujina amphibolite complex (MAC), plutonic diorite and their mafic enclaves, and volcanic dacite, have zircon U Pb age of 93.53 Ma, 92.31 Ma, 94.35 Ma and 83.60 Ma, respectively, recording different phases of island-arc magmatism. The amphibolite samples have geochemical compositions suggesting that the protolith of the amphibolite was basaltic igneous rocks that originated from a depleted mantle source mildly modified by fluids from a subducting slab, and that the mantle source might be deep and garnet-bearing. Microstructures and elemental geochemical data reveal that the mafic enclaves and diorites represent distinct island-arc magma pulses that originated from partial melting of depleted mantle which had been previously modified by subduction-related mobile elements. The dacites are evolved island-arc volcanic rocks that formed through fractional crystallization of hydrous basaltic magmas that originated from a strongly depleted mantle source. Above data support such a tectonic model, that is, during the Early Cretaceous (from ~135 Ma or even earlier), a typical trench–arc–basin system developed in the Caribbean region as a result of southwest-directed subduction of the Proto-Caribbean plate beneath the Caribbean plate; after that time, uninterrupted Proto-Caribbean plate subduction occurred, generating quasi-continuous arc magmatism from before ~135 to 81 Ma, and gradually consuming the Proto-Caribbean Ocean. • A diabase dike in the ophiolite in central Cuba has a zircon U Pb age of 135.0 ± 1.7 Ma. • A typical trench–arc–basin system developed in the Caribbean region during the Early Cretaceous. • The arc rocks have highly diverse major and trace elemental and isotopic compositions.

Protolith and metamorphic age of the Sieggraben Eclogites: Implications for the Permian to Cretaceous Wilson cycle in the Austroalpine unit

The Austroalpine eclogite-bearing Sieggraben Complex, exposed at the eastern margin of the Eastern Alps, is considered to represent a subducted and then exhumed fragment of the continental rift, which led to the formation of the Meliata oceanic basin. Combined zircon U-Pb dating, whole rock geochemistry and Hf isotope analysis of the Sieggraben Complex revealed that the eclogite sample shows a protolith age of 242.3 ± 2.6 Ma (Middle Triassic) and displays tholeiitic basalt/N-MORB geochemical characteristics. Associated ultramafic rocks as part of oceanic or subcontinental mantle lithosphere suggest a depleted mantle source and a deep subduction environment. Two zircon grains of eclogites with low Th/U ratios yield ages of 113 ± 2 Ma and 86 ± 4 Ma and represent the approximate age of eclogite metamorphism during Cretaceous. A trondhjemite dike cutting through eclogite gives a crystallization age of 82.19 ± 0.4 Ma and formed by partial melting during decompression. The host metasedimentary rocks are interpreted as old continental crust close to the margin of the Meliata basin were affected by Permian migmatitic metamorphism. The results of this study combined with previous results from the Sieggraben Complex are used to present an updated model for the tectonic evolution of the distal Austroalpine unit associated with the oceanic Meliata relics The Austroalpine Sieggraben Complex represents a location on the distal continental margin during Permian rifting. This piece of continental crust of an inferred paleotectonic position adjacent to the Meliata oceanic lithosphere subducted during Early Cretaceous times to mantle depth. The subducted continental crust was then exhumed incorporating even ultramafic mantle rocks. During exhumation and decompression of mafic rocks, partial melting took place forming the trondhjemite dike in Late Cretaceous times. • A Middle Triassic ptotolith age was found for eclogites of the Sieggraben Complex. • A Cretaceous-aged trondhjemite dike report formed by decompressional melting after eclogite metamorphism. • Paragneiss hosts a Panafrican detrital zircon memory and was affected by Permian migmatization. • A new tectonic model for the distal Austroalpine unit relates with the Meliata Ocean in a Wilson cycle.

Petrogenesis of mantle peridotites in Neo‐Tethyan ophiolites from the Eastern Himalaya and Indo‐Myanmar Orogenic Belt in the geo‐tectonic framework of Southeast Asia

The Neo‐Tethyan ophiolites of the Tuting‐Tidding Suture Zone (TTSZ), Eastern Himalaya (viz., Tidding Ophiolite Complex and Mayodia Ophiolite Complex) and Indo‐Myanmar Orogenic Belt (IMOB, i.e., Nagaland Ophiolite Complex and Manipur Ophiolite Complex) which lie along the southern extension of the Indus‐Tsangpo Suture Zone have been collectively investigated through the mantle‐derived peridotite sequence. The peridotites (harzburgite and dunites) in the Eastern Himalaya ophiolites are refractory, constrained by the high Fo olivine content (~95), high Cr# [Cr/(Cr + Al)] (0.90–0.99), as well as by the parental melt composition of Cr‐spinels (Al2O3 melt = 3.05–7.55 wt%, FeO/MgO = 0.69–6.46). They have slightly “U‐shaped” chondrite‐normalized Rare Earth Elements (REE) patterns with slight enrichment in Light Rare Earth Elements (LREE) relative to the patterns expected for residues of partial melting, thereby indicating a reaction with the LREE‐enriched melt. These peridotites represent residual portions of a depleted/enriched mantle that underwent partial melting up to 23% in the nascent forearc of an intra‐oceanic subduction zone, and later metasomatized by high‐temperature silicate melts and low‐temperature hydrous fluids. They are composed of a very refractory olivine‐spinel assemblage (Fo: 91.68–96.44; Cr#: 0.90–0.99), corroborating a boninitic parentage, with influence from melt‐rock interactions. In the case of the IMOB ophiolites, a wide range of chemical compositions is observed in the mantle sequence. Lherzolites display low Cr# (0.12–0.26) and TiO2 (<0.11) associated with high Mg# [Mg/(Mg + Fet)] (0.69–0.76) in the Cr‐spinels present in them. They represent the residual product of a fertile mantle that underwent low‐degree partial melting (2%–10%) in a divergent mid‐ocean ridge (MOR) tectonic setting. On the other hand, the harzburgites and dunites of the IMOB have high Cr# (0.84–0.90) and low TiO2 (<0.06 wt%) Cr‐spinels and exhibit slightly U‐shaped REE distributions indicating their derivation from a highly depleted mantle source, which experienced higher degree partial melting (17%–24%) similar to those of supra‐subduction zone (SSZ) harzburgites and dunites of the TTSZ peridotites. The occurrence of both MOR and SSZ types of melting regimes indicates that the peridotites in the IMOB ophiolites formed at two major different stages of the pre‐subduction and subduction events, respectively. However, the peridotites of Eastern Himalaya ophiolites were formed only during subduction tectonics. Thus, we argue that the mantle peridotites in the ophiolites of northeast India evolved from lherzolite through clinopyroxene‐harzburgite and harzburgite then to highly refractory dunite, supporting multistage melting and melt‐rock reaction processes during their generation.

Isotopic constraints on Davis bank, Vitória-Trindade Ridge: A Revised Petrogenetic Model

Davis Bank is the largest and most voluminous volcanic mount in the Vitória-Trindade Ridge (VTR), which comprehends a notable alignment of volcanic features on the South Atlantic Ocean, located at ca. 20°S. The VTR has been interpreted by some authors as the track of the Trindade hotspot impriented on the South American Plate, despite the evident importance of structural control on the ridge. This study presents the first integrated analysis of Sr-Nd-Pb isotopic compositions of Davis Bank to provide new constraints on its magmatic processess. We also present a new whole-rock 40Ar/39Ar mini-plateau age of 21.42 ± 0.13 Ma (P = 0.49) for one dredged sample from Davis Bank and new geochemical data. The Sr-Nd-Pb isotopic signature of Davis Bank rocks, along with isotopic signatures observed on the Vitória Seamount, are more enriched than the other magmatic manifestations of the VTR. This precludes the derivation of Davis Bank via the exclusive melting of a depleted mantle source, requiring the involvement of one or several enriched components homogenized before eruption. We propose that one of these components is related to the recycled subducted oceanic crust and evolved in an environment with time-integrated low Sm/Nd and slightly high Rb/Sr, U/Pb, and Th/Pb ratios. The enriched component is probably represented by a HIMU-type pyroxenite (hybrid component). Our modeling of the isotopic data indicates that Davis Bank isotopic composition is achieved by a dominant asthenospheric component (DMM) hybridized by the addition of EMI (< 24% in the mixing) and HIMU (up to 20% in the mixture) melts. We further argue that a deep-mantle origin associated with the Trindade Plume hypothesis lacks convincing evidence such as robust geochronological data to attest to the age progression along the VTR. We believe that the Vitória-Trindade Ridge is the surface manifestation of upwelling flow attributed to anomalous fertile shallow sources within a depleted mantle matrix with ubiquitous heterogeneities. Important contributions of detached SCLM fragments and subducted slabs from the Brasiliano Orogeny contaminating the mantle underneath the South Atlantic Ocean are suggested. The presence of such materials in the convective mantle may account for the significant shallow seismic anomalies that have been observed beneath many intraplate volcanic regions in the South Atlantic.

A Highly Depleted and Subduction‐Modified Mantle Beneath the Slow‐Spreading Mohns Ridge

AbstractThe Mohns Ridge is a very slow‐spreading ridge that, together with the Knipovich Ridge, marks the boundary between the North American and Eurasian plates in the Norwegian‐Greenland Sea. In this study, we report the major and trace element composition of spatially associated basalts and peridotites from a gabbro‐peridotite complex ∼20 km west of the Mohns Ridge rift flank. Formation of the ∼4–5 Myr crustal section involved accretion of normal mid‐ocean ridge basalts with Na‐content suggesting derivation from a depleted mantle source. This is consistent with the degree of partial melting estimated for clinopyroxene poor harzburgites using the Cr‐number of spinel (14%–18%) and rare earth element modeling of orthopyroxene (16%–24%) and reconstructed whole‐rock composition (14%–20%). If all the melting took place beneath the paleo‐Mohns Ridge, a crustal thickness of ∼7–8 km is expected, which is nearly double the observed thickness. Orthopyroxene trace elements are not consistent with typical fractional melting expected for mid‐ocean ridges but rather resemble that seen in supra‐subduction zone peridotites. The geochemistry of both the basalts and the peridotites suggests that a water‐rich slab flux in the past has influenced the mantle source. In turn, this caused hydrous melting which increased the depletion of the pyroxene components, leading to a highly depleted mantle that is now underlying much of the Arctic Mid‐Ocean Ridges and represents the source for the spreading related magmatism.

Relics of 2.6–2.5 Ga oceanic crust from the ultramafic-mafic complex of Goa, western India: Magmatic response to a progressive subduction system

Subduction initiation in the Early Earth and the onset of plate tectonics are topics of wide interest in understanding the formation and evolution of continents and supercontinents on our planet. Here we report results from integrated petrological, geochemical and geochronological (Zircon U Pb ages) studies on an ultramafic-mafic complex from the Bondla-Usgao-Durigini (BUD) sectors of Goa to address its petro-tectonic evolution and correlation with Neoarchean crustal growth processes and cratonization events. The ultramafic-mafic cumulate sequence from BUD sectors of Goa comprising dunite, peridotite, troctolite, olivine gabbro and gabbro represents a dismembered crustal section of oceanic lithosphere formed either in a MOR or SSZ fore-arc extensional regime. The geochemical attributes marked by boninitic-IAT trend, depleted to enriched mantle compositions, selective enrichment in fluid-mobile LILE and LREE over HFSE enunciate a multistage petrogenetic evolution associated with a SSZ fore-arc tectonic setting. The ocean-continent transition featured by the geochemical signatures can be translated in terms of oceanic crust formation in a juvenile extensional fore-arc in a SSZ setting, its maturation and accretion at active continental margin during Neoarchean ocean basic closure and arc-continent collision. The studied ultramafic-mafic suite fingerprints the transition of depleted mantle wedge to a LILE-LREE enriched metasomatized one through influx of subduction components and hydrous arc magmatism typical of SSZ oceanic lithosphere formed above a nascent subduction zone. The magmatic precursors of the studied lithologies were derived from a transitional depleted to enriched mantle regime marked by hydration and metasomatism of a depleted MOR-type mantle by the influx of subduction-derived fluids. Zircon grains in the peridotite sample yield positive εHf(t) of +3.2 to +7.6, together with T DM C of 2610–2877 Ma suggesting depleted mantle source. The gabbro sample shows positive εHf(t) values of +3.7 to +6.3 and T DM C age of 2731–2868 Ma also attesting to depleted mantle source. Zircon U Pb ages of 2602.42 ± 0.82 Ma for peridotite and 2479–2684 Ma for gabbro represent the magma emplacement time and Neoarchean-early Paleoproterozoic melt activity. The highly variable ages from zircon grains in the studied samples might suggest multiple stages of metasomatism and melt-rock interaction in a long-lived sub-continental lithospheric mantle. The tectonic evolution can be equated with Neoarchean global subduction-accretion-collision and arc-continent amalgamation events. This study deciphers a 2.6–2.5 Ga Neoarchean oceanic crust formation and magmatic response to subduction initiation, arc maturation and accretion processes correlatable with global crustal growth processes and amalgamation of western Dharwar Craton (WDC) and Antongil-Masora blocks into the Greater Dharwar Craton (GDC). • Ultramafic-mafic cumulate sequence from Goa represents dismembered SSZ oceanic crust. • Depleted to enriched melt composition reflects mantle heterogeneity. • Paleo-ocean closure and arc accretion culminated to GDC formation. • 2.6–2.5 Ga age coincides with Neoarchean global subduction-accretion event.

Detrital zircon LA-ICPMS U-Pb and Lu-Hf signature from the Mesoarchean Keonjhar Quartzite: Implications for the nature of Archean continental crust and geodynamics

• The Keonjhar Quartzite represents records of Mesoarchean sedimentation. • The detrital zircon LAICPMS Lu-Hf reveals superchondritic Hf isotopic compositions. • Superchondritic Hf composition suggests juvenile crustal components at source. • A stagnant lid to low angle subduction geodynamic regime for origin of granitoids. The ∼715 m thick matured siliciclastics of the Keonjhar Quartzite, North of the homonymous town, Singhbhum Craton, eastern India, represents one of the best-preserved examples of sedimentation during Mesoarchean time. The LA-ICPMS U-Pb ages with peaks at 3.2 Ga, 3.1 Ga, 3.0 Ga and 2.87 Ga of the detrital zircon grains from the upper part of the Keonjhar Quartzite reveal youngest age population of ∼2.87 Ga. The Hf isotopic compositions have mean initial 176 Hf/ 177 Hf ratio of 0.280897 and low Lu/Hf ratio of 0.01. Most of the grains of the zircon population show positive εHf (+8.75 to +0.26) values with only two negative values of -0.05 and -3.27. The superchondritic Hf isotopic compositions suggest depleted mantle source, juvenile crustal components and possible role of juvenile crust formation presumably in a stagnant lid to low angle subduction geodynamic regime for the origin of granitoids that sourced the detrital zircon grains.

Early differentiation processes on Mars inferred from silicon isotopes

Accretion of terrestrial planets involved partial or global melting events such as magma oceans or magma ponds. Mars experienced large-scale differentiation very early in its history, as shown by its 146Sm-142Nd and 182Hf-182W record. The broad variations in ε142Nd and ε142W of SNC meteorites highlight the presence of mantle sources that must have remained isolated, at least partly, after the crystallization of a global magma ocean. In this study, we have investigated whether the crystallization of the martian magma ocean could have generated mantle reservoirs characterized by different silicon isotope signatures, as the fractionation of Si isotopes between minerals and melts is known to depend on pressure. Thus, the goal of this study was to investigate whether there were any relationships between magma ocean crystallisation and possible variations in the Si isotope record of SNC meteorites. High resolution silicon isotope measurements were performed on twelve meteorites from the Shergottite, Nakhlite and Chassignite groups using a Neptune Plus MC-ICP-MS in dry plasma mode. The δ30Si values are in good agreement with previous studies but display a narrower range of variations with a mean value at −0.46 ‰ ± 0.07 (2SD). A magma ocean crystallization model shows that the range of δ30Si in SNCs is consistent with that generated by magma ocean crystallisation. In particular, there is a correlation between calculated 147Sm/144Nd for the moderately depleted mantle sources with δ30Si values; this correlation is consistent with the crystallization model if one includes trapped melt in the cumulates. In contrast, enriched shergottites displayed a very homogenous composition in Sm/Nd ratios, despite significant variability in δ30Si. This observation could be related to either fluid-rock interactions or redox effect during magma differentiation. Altogether, silicon isotope compositions of SNC provide new constraints about magma ocean crystallization processes in Mars.

ANALYSIS OF MANTLE SOURCES FOR MAFIC ASSOCIATIONS ON THE BASIS OF GEOCHEMICAL AND Nd ISOTOPIC DATA

The (La/Nb) pm , (Th/La) pm , (La/Sm) pm , (NbY) pm ratios &lt;1 in mafic rocks correspond to a depleted mantle source, while ≥1 corresponds to a undepleted/enriched source. (La/Nb) pm does not change during melting of mantle peridotites, so the Nd minimum is a characterictic feature of the lithospheric mantle metasomatized by fluid/melt. According to the isotopic Nd composition, there are two mantle sources: (1) depleted (+e Nd ), which corresponds to the asthenospheric and/or lithospheric mantle of restite origin, (2) enriched (-e Nd ) lithospheric, having decreased Sm/Nd due to metasomatism. Estimates of the character of the mantle sources of mafic rocks in terms of their trace element and isotopic composition can either correlate with each other or differ.

Lithostratigraphy, Origin, and Geodynamic Setting of Iron Formations and Host Rocks of the Anyouzok Region, Congo Craton, Southwestern Cameroon

In Cameroon, most of the iron formation occurrences reported are found within the Nyong and Ntem Complexes. The Anyouzok iron deposit is located in the Nyong Complex greenstone belts, which represent the NW margin of this Congo craton. The main lithological units comprise the iron formations (IFs) unit, consisting of banded IFs (BIFs) and sheared BIFs (SBIFs), and the associated metavolcanic rocks unit consisting of mafic granulite, garnet amphibolite, and biotite gneiss. Within the Anyouzok area, BIFs are rare, while SBIFs are ubiquitous. This study reports the petrography, mineralogy, and whole rock geochemistry of IFs and interbedded metavolcanic rocks of the Anyouzok iron deposit. The abundance of cavities, higher Fe contents (49.60–55.20 wt%), and strong Eu anomalies (Eu/Eu* = 2.14–3.17) within the SBIFs compared to the BIFs suggest that SBIFs were upgraded through post-depositional hydrothermal alteration activities. REE signatures indicate the contribution of both seawater and hydrothermal fluids during BIFs precipitation. Mafic granulite and garnet amphibolite protoliths were derived from the partial melting of a metasomatized spinel lherzolite depleted mantle source. The overall compositional variations of the Anyouzok IFs and interbedded metavolcanic rocks endorse an Algoma-type formation deposited in the back-arc basin under suboxic to anoxic conditions.

Compositional variation of mafic calc-alkaline lavas at the submarine Pausanias Volcanic Field, western South Aegean Volcanic Arc: Implications for magma formation and ascent

The calc-alkaline lavas of the submarine Pausanias Volcanic Field and the neighboring Methana peninsula represent the largest volcanic edifices of the western South Aegean Volcanic Arc (SAVA) in the Mediterranean Sea and erupted on 30–35 km thick continental crust. We report the first whole rock major and trace elements together with Sr-Nd-Pb isotope ratios for basaltic to andesitic lavas from the submarine Pausanias Volcanic Field consisting of six smaller, volcanic structures. The mafic lavas have the most primitive compositions of the SAVA with MgO of up to 9 wt% and Ni concentrations >100 ppm. The incompatible trace element abundance patterns of the Pausanias and Methana lavas overlap and have typical island arc patterns, but Pausanias lavas display a larger geochemical heterogeneity on a smaller spatial scale compared to those from Methana. The Pausanias lavas resemble those from Methana in Sr, Nd, and Pb isotopes reflecting higher sediment subduction in the western SAVA than in the central arc at Santorini. The variability of incompatible element concentrations at relatively constant isotope compositions suggests variable degrees of partial melting of a peridotite-sediment mélange diapir in the mantle wedge. The Pausanias magmas formed from a less depleted mantle source compared to the central SAVA and their ascent in an extensional basin supported relatively rapid ascent without the extensive stagnation in the crust observed in the Methana magmas. The compositional differences between Pausanias and Methana imply differences in their composition and/or transport in the mantle wedge and the crust, allowing highly variable melts to reach the surface within a spatial distance of few kilometers. • Small scale mantle heterogeneity recorded at the Pausanias Volcanic Field. • Melting of sediment-rich mélange diapirs contribute to Pausanias lavas. • At Pausanias, extension allows melts to ascend more direct through the crust compared to Methana.

Geodynamic evolution of lithospheric removal of the North China Craton: Constraints from late Mesozoic volcanic rocks in the Bohai Bay Basin, East China

The subduction history of Paleo-Pacific Plate (PPP) related to lithospheric removal in the North China Craton (NCC) still poorly constrained. To address this issue, we report a detailed investigation of LA-ICP–MS zircon U–Pb chronology, geochemistry, and Sr-Nd isotope compositions of a set of late Mesozoic volcanic rocks collected from boreholes in the Bohai Bay Basin. Late Jurassic (150.6 Ma) adakitic rocks are trachyandesites with high MgO (1.41–5.78 wt%), Mg# (46–64), Cr (25.1–110 ppm) and Ni (12.4–34.0 ppm) contents similar to those of high-Mg adakitic rocks. These samples were probably the result of partial melting of a delaminated region of thickened lower continental crust. Early Cretaceous samples are basalt and trachyandesite. Trachyandesites (ca. 115 Ma) exhibit low εNd(t) values (-15.9 to −12.9) and older TDM2 ages (2.19–1.96 Ga), indicating that they originated in the ancient lower crust. Hornblende may have been the main residual phase in the source, suggesting that these samples were likely formed by partial melting of the lower crust at depths of 30–40 km. The coeval basalts are OIB-like series with depleted Sr-Nd isotopes, depleted Pb and no depleted Nb, implying that they were derived from a depleted mantle source metasomatized by recycled oceanic crust. Combined with those of previous studies, our findings show that subducted PPP beneath the NCC experienced local slab rollback and fast slab rollback during late Mesozoic, resulting in lithospheric removal and destruction of the NCC. Finally, the magmatic activity that marked the large-scale lithosphere removal ended after ca. 115 Ma.

Recycled Crustal Components of the Iceland Plume Centre: Ultra-Dehydrated Crust and Oxidised Water-Bearing Fertile Mantle

AbstractPrimitive basalts erupted mainly above the proposed centre of the Iceland plume have been analysed geochemically. The data shed light on the distribution of plume heterogeneities and constrain their nature and origin. Low-Ca (down to 1420 ppm) olivine macrocrysts and chromite inclusions in Snæfell rocks of the eastern flank zone indicate that the magmas were hydrous (c. 2 wt% H2O) and relatively oxidised (fO2 = c. ΔQMF+1). Al-in-olivine crystallisation temperatures indicate that magmas from central Iceland magmas are not hotter than from other parts of Iceland. Some Gæsafjöll rocks with high Nb/U = 110–120 from the northern rift zone (NRZ) represent magmas widespread in Iceland that are also characterised by low Ba/La and high La/U, La/Th, Nb/Th and Ce/Pb. They reflect a mantle component with Nb/U ≈ 200 present in many basalts of all rift zones. This mantle apparently consists of peridotite refertilised by ultra-dehydrated recycled oceanic crust (ROC). This component is in clear contrast to the main source of the Snæfell magmas, which is modelled to be peridotite strongly refertilised by enriched ROC more hydrous than Mid-Ocean Ridge Basalt. In contrast to more-depleted rift zone melts, which are extracted from depths just below the thin rifted lithosphere, some rift zone basalts are extracted deeper and produced by intermediate degrees of melting of the two types of refertilised mantle. Snæfell-type melts are common in the NRZ and eastern rift zones (ERZ) and most strongly present at Upptyppingar. Mixing of magmas is indicated at crustal levels. Mantle melting may be modelled for homogeneous sources of peridotite or olivine websterite. Double-spike Pb isotope analyses suggest that the depleted mantle sources of the ERZ basalts together with Snæfell and high-Nb/U sources, both with 206Pb/204Pb = 18.3–18.6, Δ207Pb ≈ 0 and Δ208Pb = 20–30, explain both ERZ and NRZ magmas, except for Theistareykir. They are distinct from magmas of the western rift zone, which is characterised by melt contributions from a differently enriched mantle with more radiogenic lead, negative Δ207Pb, Δ208Pb ≈ 0. This difference in type of enriched mantle suggests that the Iceland mantle plume is laterally zoned.

Evidence for Protracted Intracrustal Reworking of Palaeoarchaean Crust in the Pilbara Craton (Mount Edgar Dome, Western Australia)

Abstract ~3.5-2.8 Ga granitoids from the Pilbara Craton in Western Australia are one of the most ancient and best-preserved records of early processes of continental crust generation. A number of recent studies have focused on the nature of the mantle source from which Pilbara granitoids derived, yet no consensus has been reached on whether the mantle was chondritic or depleted in the Eo/Palaeoarchaean. Here we present integrated whole-rock (major and trace elements) and zircon (U-Pb and Lu-Hf isotopes) data for 10 granitoids sampled across the Mount Edgar Dome, which recorded four main magmatic events between 3.47 and 3.23 Ga. Whole-rock major and trace element analyses suggest that the samples belong to two distinct petrogenetic groups. The first group is akin to the tonalite-trondhjemite-granodiorite (TTG) suite, representing highly fractionated magmas initially formed by partial melting of a basaltic crust. The second group, here classified as granites, is best interpreted by the remelting of a basaltic crust and the addition of more evolved material, and it is striking that TTG-like and granitic magmas occurred coevally in time and space. Overall, both groups were formed through intense intracrustal differentiation processes that lead to the loss of significant geochemical information about their original sources. High-precision Lu-Hf analyses in zircon allow to obtain such information and to trace back the isotopic composition of the Palaeoarchaean mantle. A clear change from superchondritic to subchondritic Hf isotope compositions is observed between 3.47 and 3.23 Ga. The superchondritic Hf isotope composition of the 3.47 Ga old granitoids substantiates derivation from a depleted mantle source that separated from the chondritic mantle prior to 3.8 Ga. The presence of ca. 3.5 Ga old inherited zircons in younger magmas suggests that crustal remelting processes were involved in their generation. We propose that all granitoids investigated in this study had their crustal sources originated from a single mantle–crust differentiation event that occurred at 3.50 Ga. This event resulted in the differentiation, from the same original mantle, of two distinct crustal reservoirs, i.e., a mafic reservoir with a 176Lu/177Hf ratio of 0.023, and a reservoir of intermediate/felsic composition ( 176Lu/ 177Hf=0.013). 3.32-3.31 Ga-old granitoids were produced by remelting of the mafic reservoir, whereas 3.43 and 3.23 Ga granitoids derived from the intermediate/felsic reservoir. Overall, our data suggest that protracted intracrustal remelting processes and differentiation have played a key role in the formation, evolution, and maturation of the building blocks of continents during the Palaeoarchaean.

The Neoarchean and Paleoproterozoic crustal evolution of the Clearwater block, northwestern Laurentia: Implications for the assembly of supercontinents

We report new coupled zircon U-Pb age and Hf isotope data, along with whole-rock Hf-Nd isotope data for 20 samples from the Priest River and Clearwater complexes—Neoarchean and Paleoproterozoic basement exposures in northern Idaho, northwest Laurentia. The new zircon data reveal two well-defined periods of magmatism at ∼ 2.66 and ∼ 1.86 Ga in each complex, with no evidence of any intervening or older magmatic events. Positive zircon initial εHf values of the Neoarchean rocks (+1 to +3) indicate juvenile magmas derived from a depleted mantle source. The Paleoproterozoic rocks, in both complexes, share a much wider range of initial zircon εHf values (−7 to +6) which suggests magmas derived from a depleted mantle source intruded and assimilated pre-existing Neoarchean continental crust. The shared age and isotope compositions in the Priest River and Clearwater complexes suggest that the two complexes likely belong to a single crustal block—the Clearwater block.Based on these new zircon U-Pb ages and Hf isotopic data of the Neoarchean and Paleoproterozoic rocks, the Clearwater block is shown to be distinct from the nearby Wyoming Province, the Medicine Hat block, and components of the Great Falls tectonic zone. We also report new zircon Hf isotope data for the ∼ 1.58 Ga Laclede orthogneiss in the Clearwater block, which along with published U-Pb and Hf isotope constraints, support a paleogeographic connection between the Gawler craton, East Antarctica, and northwestern Laurentia during ∼ 1.9 Ga to ∼ 1.6 Ga. Comparison with other examples of co-existing Neoarchean and Paleoproterozoic igneous rocks present in other continents suggests that the global record of 2.7–2.5 Ga magmatism often represents new crustal additions from a depleted mantle reservoir. The record of 2.0–1.7 Ga rocks also represent additions of juvenile magma, but with significantly larger contributions from pre-existing continental crust. In many cases the 2.0–1.7 Ga crust preserved on Earth today was added during the assembly of the supercontinent Nuna.

Persistent mildly supra-chondritic initial Hf in the Lewisian Complex, NW Scotland: Implications for Neoarchean crust-mantle differentiation

The composition of the mantle source from which continental crust is ultimately derived informs both the timing and rate of continental growth. This composition is, however, poorly constrained during the first billion or so years of Earth's evolution. Typically in zircon Hf isotope studies, a linearly evolving depleted mantle back-calculated from modern MORB-source mantle to chondrite at 4.57 Ga has been used to calculate model ‘crust formation’ ages, which are themselves used in crustal growth models. Yet, an increasing number of studies of Eo- to Paleoarchean continental crust suggest crust extraction from a relatively undifferentiated mantle. Zircon Hf isotope studies of ca. 2.9 Ga gneisses from the Neoarchean Lewisian Gneiss Complex of Scotland have previously revealed initial Hf isotope compositions that suggest derivation from such an undepleted mantle source. Here, we present combined zircon U-Pb and Lu-Hf isotope data from seventeen representative grey gneiss samples from across the Lewisian of the Scottish mainland and the Outer Hebridean archipelago, ranging in age from 3.1 to 2.7 Ga. Regardless of location and exact age, initial Hf isotope signatures are predominantly near- to slightly supra-chondritic. Several models are investigated to explain this observation, our favoured one involving episodic extraction of the grey gneiss precursors from a mildly depleted mantle that began to diverge from chondritic composition at ca. 3.5 Ga. Samples from the southern Outer Hebrides record slightly more radiogenic initial Hf signatures, consistent with a possible terrane boundary along the Paleoproterozoic South Harris shear zone. This study provides further confirmation that domains of Hadean to Paleoarchean, and possibly even locally Mesoarchean, mantle remained chondritic with respect to Hf isotopes. Initiation of a depleted mantle source at this later time and its limited divergence from chondritic mantle, even into the Neoarchean as suggested here, has implications for crustal growth models based on detrital zircon Hf model ages. On a regional basis, the signatures recorded in our data are consistent with contemporaneous gneisses across Fennoscandia and East Greenland and the mildly depleted mantle source of continental crust furthermore persists into the Mesoproterozoic.

The Paleo-Mesoarchaean Gondpipri Mafic-Ultramafic Intrusions, Western Bastar Archaean Craton, Central India: Insights from Bulk-Rock Geochemistry and Sm-Nd and S Isotope Studies on the Formation of Ni-Cu-PGE Mineralization

Abstract Magmatic Ni-Cu-platinum group element (PGE)-Te mineralization in the Gondpipri mafic-ultramafic layered intrusion of ca. 3323 ± 74 Ma age, western Bastar craton, central India, is one of the most prospective exploration targets for magmatic sulfides in India. The Gondpipri layered intrusion is divided into two distinct groups of rocks based on their mineralization potential, which include (1) mineralized layered gabbro and pyroxenite and (2) a barren olivine gabbro intrusion. The host rocks show Cu + Ni concentrations up to 5,000 ppm with a Cu/Ni ratio &amp;lt;1 and all PGE values between 0.1 and 1.1 ppm. Mineralization occurs in two modes: type I mineralization occurring as blebs, specks, and dissemination and type II mineralization occurring as stringers and minor veins. The geochemical data suggest that the parental magma of the host rock was generated at depths between spinel and garnet peridotite mantle source regions and subsequently modified by assimilation fractional crystallization (AFC) of the continental crust. High large ion lithophile elements, Th/Yb ratios of the studied rocks, and Sm-Nd isotope studies are consistent with a depleted mantle source. The geochemical proxies such as Th versus Ba/Th and (Ta/La)PM versus (Hf/Sm)PM and higher Sr/Nd (2.21–82.58) ratios indicate involvement of fluid-related subduction metasomatism and enrichment processes in an island-arc tectonic setting. Mineral assemblages and textural relationships between platinum group minerals (PGMs) and base metal sulfides suggest that sulfide-silicate liquid immiscibility was brought about by the precipitation of magnetite/Cr magnetite resulting in sulfide saturation in the melt by decreasing S solubility. Sulfur isotope compositions (δ34S: 1.61–3.30‰) and Sm-Nd geochemistry suggest that the sulfur was added in the tholeiitic magma by magmatic process. Crustal contamination played a significant role in sulfide saturation and in bringing about PGE and Te, As, Bi, Sb, Se (TABS) mineralization. PGM-NiTeBi developed at relatively low temperatures, whereas moncheite (PtPd)Te2 and merenskyite (PdTe) were formed at 650°C. The identification of Ni-Cu-PGM-Te in the margin of the western Bastar craton boosts deeper subsurface exploration.

Early Ediacaran and Middle Ediacaran Granitoids in the Provenances of Early Cambrian Coarse-Grained Rocks of the Bayan-Kol Formation of the Systyg-Khem Depression (Tuva)

Abstract —The Tuva segment of the Central Asian Orogenic Belt is characterized by the ubiquitous presence of conglomerates few tens of meters to a kilometer in thickness in early Paleozoic volcanosedimentary sequences. We present the first results of geochemical, isotopegeochemical (Sm–Nd and Rb–Sr), and U–Pb geochronological studies of granitoid boulders and pebbles from the conglomerate sequence of the early Cambrian Bayan-Kol Formation of the Systyg-Khem depression. These studies made it possible to establish several sources of clastic material as a result of the destruction of granitoids of different ages and isotope-geochemical compositions. At least two complexes of granitoids were denuded in the pre-Ediacaran tectonic block in the early Cambrian: (1) middle Ediacaran (~590 Ma) and (2) early Ediacaran (~630 Ma); the latter resulted from the melting of pre-Ediacaran island arc crust formed from a depleted mantle source (εNd(T) = +8.0 to +8.6). At present, no granitoids of this age and with such isotope-geochemical characteristics have been found within the Tuva segment. Probably, the granitoid complexes reconstructed from the results of study of clastic conglomerates are eroded or buried beneath younger deposits and do not expose. Thus, the study of clastic conglomerates from the Bayan-Kol Formation provided the first information about the Precambrian history of the tectonic block whose destruction led to the accumulation of this terrigenous sequence.

Evolution of the Sumdo Paleo-Tethyan Ocean: Constraints from Permian Luobadui Formation in Lhasa terrane, South Tibet

Reconstructing the evolution of the Sumdo Paleo-Tethyan Ocean (SPTO) is important for understanding the transition from the Paleo-Tethyan system to the Neo-Tethyan system. The existence of the SPTO is inferred from the presence of an (ultra-)high pressure metamorphic belt and dismembered ophiolite bodies that separate the southern Lhasa and northern Lhasa terrane. This paper presents new geochronological and geochemical data for the Luobadui Formation in the Lhasa terrane. The geochemical difference between the Lower Luobadui volcanics and the Upper Luobadui volcanics suggests a source transition from a depleted mantle source to an enriched mantle source, which indicates continental-arc subduction. The integrated geological evidence suggests that the SPTO initially opened during the late Carboniferous (ca. 304 Ma), as verified by the earliest record of oceanic crust. Our new data for the Luobadui volcanic rocks, together with the metamorphic and sedimentary records, support northward subduction of the SPTO lithosphere beneath North Lhasa, most likely starting during the Early Permian (ca. 275 Ma) and lasting until the Middle Permian (ca. 260 Ma). The main closure of the SPTO occurred during Middle–Late Triassic (ca. 240–220 Ma), as recorded by medium-pressure metamorphism and syn-collisional magmatism. The Wilson cycle of the SPTO is hereby reconstructed, providing significant insights into the amalgamation of the Tibetan Plateau.

New constraints on volcanism during Ordovician-Silurian transition: Insights from marine bentonites in northern Yili Block (NW China)

The Yili Block is located at the Chinese western Tianshan and is a constituent of the Kazakhstan microcontinent. It played an important role in the evolution of the SW Central Asian Orogenic Belt (CAOB). The Yili Block underwent the Paleozoic orogeny during the subduction and closure of the Junggar Ocean. Late Paleozoic arc-type magmatic rocks have been widely recorded in the Yili Block; however, it remains poorly constrained when and how the Junggar Ocean started to subduct beneath the Yili Block. In this study, we report new results of zircon LA-ICP-MS UPb dating and LuHf isotopic analysis on a series of bentonite beds in tightly folded upper Ordovician siliceous black shales of the Guozigou Section in the northern Yili Block. The occurrence of bentonite layers and associated sulfides within graptolite-bearing siliceous black shales indicates multiple marine volcanic eruptions. Magmatic zircons separated from seven representative bentonite samples yielded consistent UPb ages ranging from 449.5 ± 2.4 Ma to 444.8 ± 2.0 Ma (Katian). In situ zircon LuHf isotopic analyses show mostly positive εHf(t) values (−0.01 and 1.31 to 11.7) and Neoproterozoic single-stage Hf model ages (TDM1 = 0.6– 1.0 Ga). A linear correlation between the εHf(t) and TDM1 suggests that these zircons likely crystallized from a common magma originated from a depleted mantle source with variable involvement of Precambrian continental crust. Our new zircon ages and Hf isotopic data are comparable with those of the Middle-Late Ordovician continental arc-type magmatic rocks in the nearby Wenquan area, both indicating that the subduction of the Junggar Ocean beneath the northern Yili Block started in the Middle-Late Ordovician, and are therefore important for early Paleozoic paleogeographic and tectonic reconstruction of the Paleo-Asian Ocean domain. In addition, the Late Ordovician marine bentonites and black shales coincide with the worldwide intensive volcanic activities and related climate/environment changes during late Katian to Hirnantian. Thus, these new data also provide further arguments for possible causes of the global climate change and mass extinction during the Ordovician-Silurian transition.