Depleted Mantle Source Research Articles

Geological Significance of Late Permian Magmatic Rocks in the Middle Section of the Ailaoshan Orogenic Belt, SW China: Constraints from Petrology, Geochemistry and Geochronology

The Ailaoshan orogenic belt, located in the SE margin of the Qinghai–Tibet Plateau, is an important Paleo-Tethys suture zone in the eastern margin of the Sanjiang Tethys tectonic domain. The areas of Mojiang and Zhenyuan, located in the middle part of the Ailaoshan orogenic belt, are the key parts of the Ailaoshan Paleo-Tethys Ocean closure and collision orogeny. The rhyolites outcropped in the Mojiang area, and the granite porphyries outcropped in Zhenyuan area, are systematically studied for petrology, isotope geochemistry and geochronology. The Zircon U-Pb geochronology of rhyolites and granite porphyries give weighted average ages of 253.4 ± 4.2 Ma and 253.3 ± 2.0 Ma, respectively, both of which were formed in the late Permian period. The rhyolites belong to potassic calc-alkaline to subalkaline series. The patterns of the rare earth elements (REE) show a right-inclined seagull-type distribution, and the trace elements plot is right-inclined. The granite porphyries are high potassic calc-alkaline to subalkaline. The REE patterns show a right-inclined distribution, and the trace elements plot is right-inclined, which is consistent with the typical patterns observed in the crust. The peraluminous, highly differentiated and high ASI values suggest that rhyolites and granite porphyries are S-type granites. The zircon εHf(t) of the rhyolites range from −7.22 to −0.72, and two-stage Hf zircon model ages are (TDMC) 1771–2352 Ma, indicating that the magma source area is mainly crust-derived. The zircon εHf(t) of the granite porphyries range from −0.97 to 4.08, and two-stage Hf zircon model ages are (TDMC) 1336–1795 Ma, indicating that the magma is derived from a depleted mantle source and the partial melting of ancient crustal materials. The rhyolites and granite porphyries were possibly formed in the syn-collisional tectonic setting during the late Permian, and their ages limited the time of the final closure of the Ailaoshan Ocean and the initiation of collisional orogeny.

New insights into the mantle source of a large igneous province from highly siderophile element and Sr-Nd-Os isotope compositions of carbonate-rich ultramafic lamprophyres

Despite being volumetrically minor components, carbonate-rich ultramafic magmas like aillikites represent good candidates to investigate the compositional variations in plume and/or lithospheric mantle sources because they represent low-degree melts which preferentially sample highly fusible components including recycled crustal material. To gain new insights into the composition of the plume-related magmas and, more broadly, the petrogenesis of ultramafic lamprophyres, we have undertaken the first comprehensive study of bulk rock and mineral (olivine and Ti-magnetite) highly siderophile element (HSE) abundances and Re-Os isotopes combined with in situ major-, trace-element and Sr-Nd isotope analyses of apatite and perovskite from the Permian Wajilitag aillikites of the Tarim large igneous province, China. The Wajilitag aillikites have high PPGE (Pt and Pd) contents relative to IPGE (Os, Ir and Ru), which can be ascribed to low-degree partial melting and/or fractionation of olivine and laurite. Measured 187Os/188Os ratios are moderately to highly radiogenic (0.186–0.313) with age-corrected γOs values up to +113. In situ Sr and Nd isotope analyses of apatite phenocrysts (87Sr/86Sr(i) = 0.70349–0.70384; εNd(i) = +1.3 to +4.9) and fresh perovskite grains (87Sr/86Sr(i) = 0.70340–0.70390; εNd(i) = +1.3 to +3.8) exhibit limited variability both within and across samples from different aillikite dykes and the only volcanic pipe in the area. These Nd isotopic values resemble those from bulk-rock samples (εNd(i) = +1.9 to +5.2), whereas Sr in apatite and perovskite extends to marginally less radiogenic values than the bulk-rock compositions (87Sr/86Sr(i) = 0.70362–0.70432). The moderately depleted Sr-Nd isotope compositions of magmatic apatite and perovskite, and the previously reported mantle-like C isotope values of these samples suggest that the aillikites and their carbon probably derived from a sub-lithospheric (plume) source with minimal contribution of deeply subducted material. Conversely, the radiogenic Os isotope compositions of the Tarim aillikites and separated minerals require some contribution from recycled crustal material in the plume source. Mass balance calculations suggest that the radiogenic Os isotopes and moderately depleted Sr-Nd isotopes can be reproduced by less than one third of eclogite component addition to a moderately depleted mantle source. We conclude that the combination of complementary isotopic systems can enlighten contributions from different components to mantle-derived magmas and, in this case, clarifies the occurrence of carbon-free subducted oceanic crust in the Tarim plume.

Mantle Xenoliths from Huanul Volcano (Central-West Argentina): A Poorly Depleted Mantle Source under Southern Payenia

Huanul is a shield volcano with several lava flows hosting mantle xenoliths erupted during the Pleistocene (0.84 ± 0.05 Ma). It is located in the southern part of the Payenia Volcanic Province, which is among the largest Neogene-Quaternary volcanic provinces of South America. The volcanism here has been ascribed as the northernmost expression of the back-arc volcanism of the Andean Southern Volcanic Zone. We present the first petrographic and mineral chemistry study of mantle xenoliths collected from Huanul lavas with the aim of reconstructing directly the mantle source of the Payenia Volcanic Province. Xenoliths are commonly small (<5 cm in radius) but scarcely crossed by basaltic veins. All xenoliths have a fertile lherzolitic modal composition and are equilibrated in the spinel-facies. Most of them exhibit an almost primitive-mantle geochemical affinity, characterized by slightly depleted clinopyroxene REE patterns reproducible by partial melting degrees between 0 and 4% of a PM source. Geothermobarometric P-T estimates of clinopyroxene-orthopyroxene couples form a linear trend between 10 and 24 kbar with constant increase of T from 814 to 1170 °C along a 50–60 mW/m2 geotherm. Evidences of interaction with the host basalts occur as spongy textures in clinopyroxene and reacted spinel, which tend to became more restitic in composition and show chromatographic or complete overprinting of the trace element compositions. The presence of plagioclase and calculated P-T values constrain this melt/rock reaction process between 6 and 14 kbar, during magma ascent, and fit the mantle adiabat model. Calculated melts in equilibrium with the primary clinopyroxenes do not fit the composition of the host basalt and, together with the geothermobarometric estimations, point to an asthenospheric mantle source for the magmatism in southern Payenia. The PM geochemical affinity of the xenoliths of Huanul is an extremely rare finding in the South America lithospheric mantle, which is commonly extensively refertilized by subduction-derived melts.

Geochemical constraints on source nature and recycled oceanic crust in the mantle of the Celebes Sea

Indian-type mantle is extensively distributed in the marginal basins of the western Pacific; however, the origin of this mantle remains unclear. The Celebes Sea is located between the Pacific and Indian oceans, and its igneous crustal rocks provide crucial constraints on if and on how this anomalous mantle developed in the region. However, limited knowledge has been obtained on the origin of mantle source of the Celebes Sea because only normal-type mid-ocean ridge basalts (N-MORBs) at ODP Site U770 in this basin have been examined. In this study, both N-MORBs and enriched (E)-MORBs were identified at Site U770, which were analyzed for whole-rock major and trace elements, Sr–Nd–Pb–Hf isotopes, and in situ olivine geochemistry. Site U770 basalt samples are Indian-type (i.e., plot above the Northern Hemisphere reference line [NHRL] on plots of 208Pb/204Pb and 207Pb/204Pb versus 206Pb/204Pb) MORBs, based on their Sr–Nd–Pb–Hf isotopic compositions. An enriched mantle end-member exists in the MORB-type depleted mantle source, as evidenced by correlations between ɛNd and SmN/YbN, Sr/Sr*, and Eu/Eu*. Recycled igneous oceanic crust that experienced plagioclase fractionation can account for the geochemically enriched end-member in the mantle source, based on the positive correlation of ɛNd vs. Sr/Sr* and Eu/Eu*. The recycled igneous oceanic crust plus minor amounts of pelagic sediments can also explain the origins of Indian-type mantle in the Celebes Sea. The E-MORBs have higher Zn/Mn and Zn/Fe ratios, and similar olivine Ni, Ca, and Mn contents as compared to global MORBs, which is also consistent with a recycled oceanic crust-derived, eclogite-rich, mantle source. This suggests recycled components contribute to the lithological and geochemical heterogeneity in the mantle beneath the Celebes Sea.

Petrogenesis and tectonic implications of the late Neoproterozoic mafic dykes in the South Qinling Belt, China

The mafic dykes exposed in the Suizhou area are crucial for understanding the late Neoproterozoic tectonic evolution of the South Qinling Belt. However, it remains unclear whether these rocks were formed in a continental rift or a back-arc tectonic regime. In this contribution, we present a series of new results of zircon U–Pb geochronology, whole-rock geochemistry, and Sr–Nd–Pb isotopes on mafic dykes in the Suizhou area. Three mafic dyke samples give zircon U–Pb ages of 635 ± 2 Ma, 631 ± 3 Ma and 630 ± 2 Ma, respectively, indicating that they were emplaced during the late Neoproterozoic. The mafic dykes are tholeiitic in composition and have low concentrations of SiO2 (44.26–49.08 wt%), varying TiO2 (0.50–1.71 wt%) and MgO (6.43–14.24 wt%) with Mg# values of 53 to 74. They are enriched in LREEs with (La/Yb)N ratios of 2.10–5.20 and display E-MORB-like trace element patterns. Isotopically, the mafic dykes show negative εNd(t) values of −5.09 to −7.15 and low ratios of 206Pb/204Pb (16.77 to 16.92), 207Pb/204Pb (15.39 to 15.41), and 208Pb/204Pb (37.23 to 37.51). These characteristics illustrate that the mafic dykes were mainly derived from the enriched lithospheric mantle, since the negligible crustal contamination has been assessed by the energy-constrained assimilation and fractional crystallization simulation of Sr-Nd isotopes. In addition, the mafic dykes have ΔNb values of −0.46 to + 0.15 with zircon εHf(t) values of −1.5 to + 10.0 previously reported, indicating that the depleted mantle source had participated in their formation. Furthermore, the non-modal batch melting calculation infer that, a mixing of melts from the asthenospheric mantle (18% extent of partial melting) at garnet-stable depth with peridotites from the lithospheric mantle could fit well with geochemical composition of the mafic dykes. The correlation of incompatible elements reveals that these rocks are akin to the intra-continental mafic rocks. Combined with the coeval magmatism in the study area, we conclude that the Suizhou mafic dykes were formed in a rifting continental margin during the late Neoproterozoic.

Compositional signatures of ophiolitic rocks from the Dongco massif: Novel insights into the evolution of the central Tibetan Meso-Tethyan oceanic plateau

The role of the Meso-Tethyan oceanic plateau in the growth of the Tibetan Plateau is debated. In this paper, we report coexistence of large-scale mid-ocean ridge basalt (MORB)-like and enormous ocean island basalt (OIB)-type mafic rocks from the Dongco ophiolitic rocks of the Bangong–Nujiang Suture Zone. The MORB-like rocks host tholeiitic geochemical characterizes, and have supra-subduction zone affinity such as enrichments in large ion lithophile elements (LILEs), depletions in high field strength elements, high εNd(t) values (+6.5 to +9.5) and low initial 87Sr/86Sr ratios (0.7033–0.7050). The OIB-type rocks have an alkaline affinity and are characterized by enrichments in LILEs and light rare earth elements, strongly negative Sr anomalies, variable εNd(t) values of −3.7 to +7.3, and low initial 87Sr/86Sr ratios ranging from 0.7034 to 0.7050. Metamorphic zircons from amphibolites in the sole of the ophiolitic rocks yielded UPb ages of 104–100 Ma. Detrital zircons from the upper Cretaceous sandstone of the Abushan Formation (K2ab) yielded young UPb ages of 100–86 Ma. Considering the regional geology, we suggest that the Dongco MORB-like rocks were derived from a depleted mantle source and formed from an intra-oceanic subduction-related setting, representing remnants of normal oceanic crust, whereas the OIB-type rocks were derived from an enriched mantle source representing remnants of a Meso-Tethyan oceanic plateau. Our preferred hypothesis is that the Meso-Tethyan oceanic plateau experienced a phase of accretion at 104–100 Ma, which probably increased local subduction erosion of the upper plate and providing provenance for the deposition of the K2ab sandstone. The maximum depositional ages of K2ab sandstone constraint the minimum ages for closure of the Bangong–Nujiang Tethys Ocean most likely at 100–86 Ma.

Early Jurassic intra-oceanic subduction initiation along the Bangong Meso-Tethys: Geochemical and geochronological evidence from the Shiquanhe ophiolitic complex, western Tibet

Due to the unclear tectonic setting of the Jurassic ophiolites within the Bangong-Nujiang suture, how and when intra-oceanic subduction initiated within the Bangong Meso-Tethys remain controversial. Based on the new petrological, geochemical, and geochronological data reported in this paper, ∼190–180 Ma MORB-like ophiolites and OIB-like mafic rocks were identified from the Shiquanhe ophiolitic complex in the western Bangong-Nujiang suture. The MORB-like gabbros exhibit strong light rare earth elements (LREEs) and heavy rare earth elements (HFSEs) depletion and were derived from partial melting of a highly refractory depleted mantle source with limited slab-derived fluids. The OIB-like mafic rocks are enriched in LREEs and HFSEs and were originated from the melting of upwelling asthenospheric mantle without subduction additions. The data for the MORB-like ophiolites reported in this paper and previously published data demonstrate the formation of Early Jurassic fore-arc ophiolites in response to the Early Jurassic intra-oceanic subduction initiation within the Bangong-Nujiang suture. The Early Jurassic OIB-like rocks formed in the extensional tectonic setting, together with regional geological evidence, suggest that the Early Jurassic initial subduction event within the Bangong Meso-Tethys resulted in broad upper plate extension. Our study not only reveals the intra-oceanic subduction initiation age of the Bangong Meso-Tethys but also implies that subduction initiation can cause strong extension in a wider region of the upper plate in the Tethys oceanic system, not just in the fore-arc region.

A Mesoarchean Large Igneous Province on the Eastern Kaapvaal Craton (Southern Africa) Confirmed by Metavolcanic Rocks from Kubuta, Eswatini

Abstract Mesoarchean magmatism is widespread on the eastern margin of the Kaapvaal Craton, but its origin is still poorly understood, mainly because geochemical data is rare. To shed some light on the source of this Mesoarchean magmatism and to relate different Mesoarchean volcanic sequences to each other, we provide major and trace element data as well as Hf-Nd isotope compositions of amphibolites sampled close to the Kubuta Ranch in south-central Eswatini. These amphibolites, so far, were of unknown correlation to any volcanic sequence in Eswatini or South Africa. Hence, the aim of our study is to characterize the mantle source composition of these volcanic rocks and, furthermore, to constrain their genetic relation to other volcanic sequences in Eswatini and South Africa. Our findings reveal that, based on coherent trace element patterns and similar Nd isotope characteristics, the Kubuta volcanic rocks can be genetically linked to the ca. 3.0 Ga Usushwana Igneous Complex in West-Central Eswatini and the ca. 2.9 Ga Hlagothi Complex located in the KwaZulu-Natal province. In contrast, the coeval ca. 3.0 Ga Nsuze and ca. 2.9 Ga Mozaan Groups (Pongola Supergroup) of south-central Eswatini and northern KwaZulu-Natal province have slightly enriched compositions compared to the newly sampled Kubuta volcanic rocks. Our results suggest that the Nsuze and Mozaan Groups were sourced from a primitive mantle reservoir, whereas the Usushwana, Hlagothi, and Kubuta mafic rocks were derived by melting of a more depleted mantle source comparable to that of modern depleted MORB. Furthermore, our assimilation-fractional crystallization (AFC) calculations and Nd isotope constraints reveal that some samples were contaminated by the crust and that the crustal contaminants possibly represent felsic rocks related to the ca. 3.5 Ga crust-forming event in the Ancient Gneiss Complex. Alternatively, melting of a metasomatized mantle or plume-lithospheric mantle interaction may also produce the trace element and isotopic compositions observed in the samples. From a synthesis of our geochemical observations and age data from the literature, we propose a refined petrogenetic model, for a continental flood basalt setting in a Mesoarchean large igneous province on the eastern Kaapvaal Craton. Our petrogenetic model envisages two magma pulses sourced from a primitive mantle reservoir that led to the formation of the Nsuze (first) and Mozaan (second) lavas. Conductive heating of ambient depleted mantle by the mantle plumes caused partial melting that led to the formation of the Usushwana Igneous Complex associated with the first magmatic event (Nsuze) and the Hlagothi Igneous Complex associated with the second magmatic event (Mozaan). However, due to lacking age data of sufficient resolution, it is not possible to affiliate the Kubuta lavas to either the first or the second magmatic event.

Petrogenesis of the late Paleoproterozoic Gleibat Lafhouda dolomite carbonatite (West African Craton Margin, Moroccan Sahara) and its relevance to the onset of fragmentation of the Columbia supercontinent

The Gleibat Lafhouda dolomite carbonatites of the Moroccan Sahara occur as three separate cone-shaped plugs intruding an autochthonous succession of Archean supracrustal basement rocks. Geochemically, the Gleibat Lafhouda dolomite carbonatites are characterized by a compositional range of 11.3–27.1 wt% MgO, 3.1–29.7 wt% CaO, 3.5–38.0 wt% FeOtot and < 0.1–7.5 wt% SiO2, and enrichment in large-ion lithophile elements (LILE), particularly Sr (2173–11,310 ppm), Ba (174–4537 ppm), U (0.1–296 ppm) and light REEs (LREEs) (131–1295 ppm), but not in the heavy REE (HREEs) and high-field strength elements (HFSE) such as Ti, Zr, and Hf. Nb and Ta show, however, much higher concentrations ranging from 0.5 ppm to 1.0 wt%, and < 0.0 to 199 ppm, respectively, which set them apart from naturally occurring carbonatites and the experimentally derived carbonated melts. The combined stable (δ13CV-PDB = −2.5 to −6.6‰, δ18OV-SMOW = 6.0 to 20.7‰) and radiogenic 87Sr/86Srin (0.7032–0.7046), 143Nd/144Ndin (0.5105–0.5106) or εNd(t) (+ 3 to +6), and 206Pb/204Pb (19.06–49.05), 207Pb/204Pb (15.90–18.87), and 208Pb/204Pb (37.87–38.50) isotope compositions are consistent with low degree partial melting, at convecting upper mantle conditions, of a predominantly depleted mantle source in a rift-related environment. Based on these geochemical features, it is suggested that the Gleibat Lafhouda dolomite carbonatites represent the earliest manifestation of rifting processes related to the fragmentation of the Columbia supercontinent at 1.85 Ga. Accordingly, we propose that these carbonatitic rocks represent the initial Mg-rich melt in the mantle plume head that derived from decompressional adiabatic melting of a depleted mantle source.

Early Cretaceous back‐arc basin basalt‐type gabbros in the southeastern Tibetan Plateau: Implications for Neo‐Tethyan oceanic slab subduction

Although progress has been made in defining and explaining the temporal and spatial distributions of Neo‐Tethyan magmatism in the Tibet–Himalaya region, the correlation of magmatic suites in the SE Tibetan Plateau remains poorly constrained. This paper reports zircon U–Pb dating, geochemistry, and Sr–Nd isotope compositions of a concealed gabbroic unit (coverage area of &gt;6.0 km2) in the southern Lancangjiang tectonic zone, western Yunnan, SW China. Zircon 206Pb/238U ages of 143–133 Ma indicate that the unit formed during the Early Cretaceous (ca. 139.7 Ma). The gabbro samples have variable MgO (7.88–16.1 wt%; Mg# = 58–74) and relatively low TiO2 (0.76–1.24 wt%) and total‐alkali (K2O + Na2O = 2.61–4.06 wt%) contents. The samples exhibit weakly fractionated rare earth element (REE) patterns with slight depletion in light REEs and negligible Eu anomalies. In a primitive‐mantle‐normalized element‐variation diagram, the gabbros, display variable enrichment in large‐ion lithophile elements and relatively flat patterns of high‐field‐strength elements. Initial 87Sr/86Sr ratios of 0.7079 to 0.7142 and relatively depleted εNd(t) values of +3.96 to +4.84 are similar to those of the mantle source of back‐arc basin basalts (BABBs) in the central Lhasa Block and mid‐ocean ridge basalt in the Indus–Tsangpo Suture Zone. These various characteristics suggest that the gabbroic unit was derived from a shallow and depleted mantle source by relatively high‐degree partial melting in the spinel stability field. The parental magma underwent slight crustal contamination, as well as fractional crystallization of clinopyroxene. The studied gabbros have distinct BABB‐type geochemical affinities similar to those of the Okinawa BABB. Combining our data with the coeval occurrence of a continental rift basin (the Simao–Lanping Basin) leads us to suggest the existence of a back‐arc rifting setting in the SE Tibetan Plateau during the Jurassic–Cretaceous. This setting can also be geochronologically and genetically correlated to the back‐arc basin developed within the Lhasa Block. These settings and constituent characteristics formed as a result of Neo‐Tethyan oceanic slab subduction during the late Mesozoic.

Evolution of the 3.65–2.58 Ga Mairi Gneiss Complex, Brazil: Implications for growth of the continental crust in the São Francisco Craton

The composition and formation of the Earth’s primitive continental crust and mantle differentiation are key issues to understand and reconstruct the geodynamic terrestrial evolution, especially during the Archean. However, the scarcity of exposure to these rocks, the complexity of lithological relationships, and the high degree of superimposed deformation, especially with long-lived magmatism, make it difficult to study ancient rocks. Despite this complexity, exposures of the Archean Mairi Gneiss Complex basement unit in the São Francisco Craton offer important information about the evolution of South America’s primitive crust. Therefore, here we present field relationships, LA-ICP-SFMS zircon U-Pb ages, and LA-ICP-MCMS Lu-Hf isotope data for the recently identified Eoarchean to Neoarchean gneisses of the Mairi Complex. The Complex is composed of massive and banded gneisses with mafic members ranging from dioritic to tonalitic, and felsic members ranging from TTG (Tonalite-Trondhjemite-Granodiorite) to granitic composition. Our new data point to several magmatic episodes in the formation of the Mairi Gneiss Complex: Eoarchean (ca. 3.65–3.60 Ga), early Paleoarchean (ca. 3.55–3.52 Ga), middle-late Paleoarchean (ca. 3.49–3.33 Ga) and Neoarchean (ca. 2.74–2.58 Ga), with no records of Mesoarchean rocks. Lu-Hf data unveiled a progressive evolution of mantle differentiation and crustal recycling over time. In the Eoarchean, rocks are probably formed by the interaction between the pre-existing crust and juvenile contribution from chondritic to weakly depleted mantle sources, whereas mantle depletion played a role in the Paleoarchean, followed by greater differentiation of the crust with thickening and recycling in the middle–late Paleoarchean. A different stage of crustal growth and recycling dominated the Neoarchean, probably owing to the thickening of the continental crust by collision, continental arc growth, and mantle differentiation.

Neogene Alkali Basalts from Central Slovakia (Ostrá Lúka Lava Complex); Mineralogy and Geochemistry

Ostrá Lúka basalts are a product of the final phase of Neogene volcanism in Central Slovakia. Their major and trace elements composition is alkaline, a feature confirmed by light rare earth elements (LREE) relative enrichment and some incompatible trace elements ratios. The basalts contain rare surrounded gabbro cumulates. Their peculiarity is a strong zonation of olivines and clinopyroxene. The significant zonation probably indicates a short stop of the melt at shallow depth. The Sr, Nd and Pb isotope compositions indicate an origin from a moderately depleted mantle source. The mineral and chemical composition of the basalts is similar to the Pannonian basin alkali basalts and the Western and Central Europe alkali basalts.

Early Precambrian Crustal Evolution in the Irkut Block (Sharyzhalgai Uplift, Southwestern Siberian Craton): Synthesis of U–Pb, Lu–Hf and Sm–Nd Isotope Data

Abstract —The paper presents a synthesis of zircon U–Pb and Lu–Hf and whole-rock Sm–Nd isotope data from main early Precambrian (3.40 to 1.85 Ga) metamorphic and magmatic units of the Irkut block (Sharyzhalgai uplift, southwestern Siberian craton). The Archean complexes consist of relict Paleoarchean (3.4 Ga) melanocratic granulites and predominant Neoarchean mafic and felsic granulites (2.70–2.66 Ga), paragneisses (≤2.75 Ga), and gneissic granites (2.54 Ga). The Paleoproterozoic complexes include paragneisses (1.95–1.85 Ga), granitoids and charnockites (1.86–1.84 Ga), as well as mafic intrusions and dikes (1.86 Ga). Few detrital zircons with Hf model ages of ≥3.6 Ga mark the Eoarchean onset of crustal growth in the Irkut block. Isotopic data record two major stages of crustal growth in early Precambrian evolution of the Irkut block: Paleoarchean (3.6–3.4 Ga) and Neoarchean (~2.7–2.66 Ga). The Paleoarchean crustal growth was most likely associated with plume magmatism fed from depleted and primitive mantle sources. The spatial distribution of Paleoarchean crust is traceable in isotopic signatures of magmatic and detrital zircons from most of Mesoarchean to Paleoproterozoic units. The Neoarchean crustal growth from a depleted mantle source was due to subduction magmatism. Moderate crustal growth occurred in the Paleoproterozoic from 2.30 to 1.85 Ga. At the turn of 1.86–1.85 Ga, mafic magmas and products of their fractionation formed from both depleted and enriched sources under postcollisional extension; the latter sources were the subcontinental lithospheric mantle formed during Neoarchean subduction. Three major stages of crustal recycling have been established: Mesoarchean (~3.0 Ga), Neoarchean (~2.55 Ga), and Paleoproterozoic (1.86–1.85 Ga), which are characterized by near-coeval intracrustal melting and metamorphism. The recycling during the ~2.55 Ga and 1.86–1.85 Ga events apparently occurred in a collisional setting. The 2.7 Ga subduction-related felsic magmas also formed through the recycling of the Paleo-Mesoarchean crust. The hypothesized scenario for the geological evolution of the Irkut block is the dominant vertical growth and crustal recycling for about two billion years. Available isotope data record similar major crustal growth in the Paleoarchean and growth combined with recycling during the Neoarchean and Paleoproterozoic events in both the southwestern and northern and central parts of the Siberian craton. The Irkut block in the southwest differed in a long and continuous recycling during the Mesoarchean and pronounced Neoarchean crustal growth.

Geochemical constraints on mantle source nature and recycling of subducted sediments in the Sulu Sea

The Sulu Sea is a marginal spreading basin in the triple junction of Euro-Asia/Indian/Pacific plates. The genesis of seafloor basalts is crucial for understanding the nature and evolution history of the underlying mantle in this region. Nevertheless, the mantle source of the Sulu Sea remains unclear due to limited geochemical data. In this study, we report a complete set of whole-rock major and trace elements and Sr-Nd-Pb-Hf isotopes for the volcanic rocks at Ocean Drilling Program Site U768 in the Southeast Sulu Sea. These volcanic rock samples are characterized by transitional trace element compositions between mid-ocean ridge basalts (MORBs) and island arc basalts (IABs). A subduction modified depleted mantle source is indicated by their higher Th/Yb ratios than the MORB-OIB array and trace element patterns. We propose that the pelagic sediments contributed to the mantle source of the Southeast Sulu Sea based on the plots of 206Pb/204Pb vs. Ce/Pb and Th/Nb. Based on the decoupled Hf–Nd isotopes and the variation of Pb isotopes for Site U768 volcanic rocks, we suggest that subducted pelagic sediments contributed to the origin of the Indian-type mantle of the Southeast Sulu Sea.

Anatomy of a Neoarchean continental arc-backarc system in the Cross Lake-Pipestone Lake region, northwestern Superior Province, Canada

The Neoarchean Cross Lake-Pipestone Lake terrain, northwestern Superior Province, consists of the Cross Lake greenstone belt, the Pipestone Lake anorthosite complex, and multiple generations of granitoids. The Cross Lake greenstone belt includes an assemblage of basalt, picrite, gabbro, and sedimentary rocks (Pipestone Lake Group) that formed between 2790 Ma and 2760 Ma, and an assemblage of polymictic conglomerate, sandstone, and siltstone (Cross Lake Group) that were deposited between 2704 Ma and 2691 Ma. The Pipestone Lake anorthosite complex is composed of gabbro, megacrystic anorthosite and leucogabbro, and mafic dykes. The Pipestone Lake Group and the Pipestone Lake anorthosite complex are intruded by ca 2760 Ma and younger granitoids. The trace element systematics of the granitoids suggest that they originated in a volcanic-arc to syn-collisional tectonic setting, whereas the trace element signatures of the basalt, picrite, gabbro, anorthosite and leucogabbro association are consistent with a back-arc setting. Whole-rock initial εNd values, zircon initial εHf values and 207Pb/206Pb zircon ages for the granitoids and the anorthosite complex are consistent with long-term depleted mantle sources and variable degrees of crustal contamination during their interaction with Mesoarchean to Paleoarchean continental crust. Zircon trace element data indicate that the granodiorite and the anorthosite complex formed in a continental crust to oceanic crust transition zone at an arc-related setting. On the basis of the field relationships, and trace element and isotopic data, we suggest that the Pipestone Lake anorthosite complex and the spatially and temporally associated Pipestone Lake Group formed in a continental back-arc setting. During and after the closure of the back-arc basin, both the Pipestone Lake Group and the anorthosite complex were intruded by arc-derived granitoids. Accordingly, we interpret the association of the Pipestone Lake Group and Pipestone Lake anorthosite complex as a Neoarchean suture zone. Following the collision and closure of the back-arc basin, the region was dissected by strike-slip faults resulting in the formation of pull-apart basins and deposition of the Cross Lake Group as Timiskaming-type sedimentary rocks.

Nd-Sr Isotopic Study of Magmatic Rocks and 40Ar/39Ar Dating of the Mafic Dike of the Proterozoic Ulan-Sar’dag Ophiolite Mélange (Southern Siberia, East Sayan, Middle Belt, Russia)

We report the first radiogenic Nd-Sr isotope data in the magmatic rocks island-arc ophiolite assemblage from the middle branch of the East Sayan ophiolite complexes to better constrain geodynamic processes in this segment of the CAOB in southern central Siberia. The magmatic rocks belong to the following geochemical types: (1) Ensimatic island-arc boninites; (2) island-arc assemblage; (3) enriched basalts of mid-ocean ridges; and (4) oceanic island-like basalts. The boninites have a positive value εNd (T), which is generated from a depleted mantle source (N-MORB). The island-arc assemblage has negative or slightly positive εNd (T) and was formed from an enriched mantle source due to the subduction of terrigenous rocks. The source of the terrigenous material was most likely the rocks of the Archean TTG (Trondhjemite Tonalite Granodiorite) complex of the Gargan block. Isotopic ratios for E-MOR and OIB-like basalts are characterized by positive or slightly negative values of εNd (T). The mafic dike, which crosscut ophiolite rocks, corresponds to OIB-like basalts. The values of εNd (T), measured 87Sr/86Sr and I (Sr), in the mafic dike correspond to the EM I mantle source. The E-MOR and OIB-like basalts appear to be formed in late-stage asthenospheric mantle melting via the decompression melting processes. The obtained isotope geochemical data for the E-MOR and OIB-like basalts probably indicate the mixing of island-arc melts with asthenospheric melts. We undertook 40Ar/39Ar dating of the mafic dike, which crosscut the ophiolite unit. The mafic dike has a whole-rock 40Ar/39Ar weighted mean plateau age of 799 ± 11 Ma. The dating constrains the minimum age of the ophiolite and island-arc magmatism in the region.

Preserved ancient oceanic lithosphere within the Buem structural unit at the eastern margin of the West African Craton

This work presents new major and trace element geochemical data and the first SmNd and LuHf isotopic data for the mafic suite of the Buem structural unit (BSU) in Ghana in order to infer its origin and geodynamic setting, relationship to the Pharusian suture zone and significance in the Dahomeyide belt. The BSU, which occurs about 50 km to the west of the Pharusian suture zone, comprises well-preserved, weakly metamorphosed, mafic volcanic and mafic-ultramafic plutonic rocks. Two lava types can be distinguished: Type I and Type II lavas that are spatially separated and geochemically distinct. The Type I lavas are subalkaline and show trace element patterns akin to mid-ocean ridge basalt (MORB). Their εNd800Ma values of +5.3 to +6.2 and εHf800Ma values of +11.1 to +17.2 are indications of juvenile magma derived from a depleted mantle source in the spinel stability field. The Type II lavas have alkaline signatures with trace element patterns that resemble ocean island basalts. They formed by smaller degrees of disequilibrium partial melting at depth in which garnet likely played a role as a residual phase having εHf800Ma and εNd800Ma values of +3.2 to +5.1 and + 4.1 to +6.3 respectively. The gabbros show similar trace elements concentration and isotopic ratios as the BSU lavas, an indication of a similar mantle source and environment of formation. Because the geochemical features of the BSU mafic suites are comparable to the MORB-type high-pressure rocks of the Pharusian suture zone, both represent the remnant of the Pharusian oceanic crust, formed during the breakup of Rodinia. The BSU corresponds to the true oceanic crust of the West Gondwana Orogen continental margin-type ophiolite, sharing similar lithological and geochemical features to other continental margin-type ophiolites in the world.

Intrusive Complexes of the Late Neoproterozoic Island Arc Structure of the Lake Zone (Mongolia): Isotope Systematics and Sources of Melts

Abstract –We present data on the geochemical and Sr–Nd isotope compositions of rocks and on the Lu–Hf isotope composition of magmatic and xenogenic zircons from granitoids and gabbroids of the late Neoproterozoic island arc structure of the Lake Zone. Plagiogranitoids, gabbroids, and quartz diorites (559–542 Ma) formed at the late Neoproterozoic subduction stage of magmatism, and two-feldspathic granites (~483 Ma) mark Cambrian–Ordovician accretion–collision processes. We have established that the volcanic rocks of the late Neoproterozoic island arc and/or its oceanic base, which formed from the depleted mantle, were the mafic source of plagiogranitoids. This is proved by the overlapping positive εNd values of plagiogranitoids and the host volcanic rocks and by the commensurate εHf values of magmatic zircons from the plagiogranitoids and depleted mantle. The lower εNd values of gabbro and quartz diorites from the Tavan Hayrhan and Shuthuyn plutons, the lower εHf values of zircons from these rocks, and the high (87Sr/86Sr)0 ratios and K2O, Rb, and Th contents point to the generation of these rocks from a less depleted mantle source, namely, mantle wedge peridotites. The isotope composition of the latter changed at the previous subduction stage under the impact of fluids and with the contribution of subducted sediments. The least radiogenic Hf isotope composition of magmatic and xenogenic zircons from Ordovician accretion–collisional two-feldspathic granites of the Ih Zamiin pluton suggests their formation through the melting of the late Neoproterozoic–Cambrian island arc crust with the contribution of more differentiated crustal sources enriched in Th, Nb, and LREE and characterized by low εNd values. The age of xenogenic zircons (≤716 Ma) in the studied granitoids and gabbroids and their similarity in Hf isotope composition to magmatic zircons from the same rocks confirm the formation of the late Neoproterozoic island arc of the Lake Zone in an intraoceanic setting far from ancient continental sources similar to the Dzavhan microcontinent.

Petrogenesis and tectonic implications of Tonian island arc volcanic rocks from the Gabgaba Terrane in the Arabian-Nubian Shield (NE Sudan)

• The volcanic rocks in the Gabgaba Terrane were erupted during Tonian. • The volcanic rocks have positive εHf(t) values, indicative of derivation from a juvenile arc magmatism . • The depleted mantle source was metasomatized by slab-dehydration fluids. • The primitive magma was subjected to crustal assimilation prior to eruption. • The volcanic rocks were erupted during different stage of arc maturity. An integrated study of zircon U-Pb geochronology and geochemistry together with Lu-Hf isotopes, has been carried out on basaltic andesite and andesite from the Haweit area (Gabgaba Terrane, NE Sudan) to indicate their magmatic evolution. In-situ U-Pb zircon dating yielded a weighted mean age of 752 ± 11 Ma for andesite and 773 ± 17 Ma for basaltic andesite, suggesting they were generated during Tonian. The Haweit volcanic rocks are generally sub-alkaline and tholeiitic lavas. They are characterized by low Mg# (29–44), low TiO 2 (0.70–1.41 wt%), and relatively high Al 2 O 3 (14.60–18.30 wt%) indicating that they are not a primitive melt. The nearly flat REEs pattern of the basaltic andesites indicate that they were formed in an immature island arc setting. While andesites are slightly enriched in LREE and LILEs, suggesting more mature island arc setting. The calculated Hf-T DM C (0.87–1.6 Ga) ages of zircons preclude the possibility of interaction of the parental magma with a pre-Neoproterozoic crustal source. The low (La/Yb) N (0.73–9.56) and high Zr/Nb (33–62) ratios, along with positive εHf(t) values (+1.33 to +12.5) indicate that the Haweit volcanic rocks were originated from depleted mantle with little crustal contamination. The geochemical data suggest that the parent magma of the Haweit volcanic rocks could be formed by low degree partial melting of spinel‐bearing Iherzolite depleted mantle metasomatized by slab‐derived fluids. The Haweit andesitic melts were later subjected to crustal assimilation before the final eruption of lavas during two stages of arc evolution.

The voluminous 2.81–2.71 Ga Goldfields Tholeiitic Super Event: Implications for basin architecture in the Yilgarn Craton and global correlations

The time-interval between 2.8 and 2.7 Ga is associated with the worldwide appearance of thick tholeiite-komatiite successions. However, their origin and affiliation, if any, is poorly understood. The Eastern Goldfields Superterrane of the Yilgarn Craton, Western Australia, is dominated by the 2.7 Ga Kalgoorlie and Kurnalpi terranes, both of which consist of well-preserved mafic-ultramafic volcano-stratigraphy. On a local scale, these supracrustal sequences permit reconstruction of volcano-sedimentary environments, which provides important constraints to the local conditions of magmatism and/or deposition. Regionally, these observations can be used to test correlations between greenstone belts, constrain basin architecture and the extent of volcanism. Studies of this nature provide important insights to global, Archean volcano-sedimentary processes.Here we present an integrated field, geochemical and U-Pb geochronological study from the well-preserved West Laverton Greenstone Belt in the Kurnalpi Terrane and compare it to the relatively well constrained Kalgoorlie Terrane. The sequence comprises three volcanic ‘events’ consisting dominantly of low-Th tholeiitic basalt and komatiite derived from depleted mantle sources. Magmatism commenced after 2854 ± 4 Ma and ceased by 2717 ± 4 Ma. Prominent banded iron formations separate volcanic cycles, indicating an overall quiescent subaqueous setting.The West Laverton Sequence is correlated with volcanic events in the neighbouring Kalgoorlie Terrane, where pre-existing basement with remarkably similar character is documented near Norseman, over 360 km away. Both terranes contain basal paragneiss (2.96–2.93 Ga) and epiclastic sediments (∼2.93–2.85 Ga) that are overlain by three variably preserved and widespread tholeiite-dominated volcanic events, here defined as the 2.81–2.71 Ga ‘Goldfields Tholeiitic Super Event’. These correlations support an autochthonous setting for the Kalgoorlie and Kurnalpi terranes with the eruption of volcanic tholeiites onto submerged continental crust.Voluminous tholeiitic volcanism at 2.8–2.7 Ga was a global event. The Zimbabwe and Slave cratons preserve remarkably similar basement overlain by thick basaltic units, that are compositionally similar to the ‘Goldfields Tholeiitic Super Event’. This correlation indicates that a globally significant rifting event at 2.8–2.7 Ga affected much of the continental crust extant.