Asthenospheric Mantle Source Research Articles

New Caledonia Ophiolite, Marginal Rifting to Fore‐arc Evolution

AbstractThe New Caledonia Ophiolite (Peridotite Nappe), represents about one third of the island's surface (i.e. 5 500 km2). The ophiolite is composed of harzburgites, dunites, lherzolites, minor mafic‐ultramafic cumulates, and various dykes and sills. The mantle section underwent a polyphase evolution, which involved prominent depletion and re‐fertilization.The oldest events are probably recorded by abyssal‐type lherzolites of the northern massifs, which bear traces of moderate partial melting. Plagioclase lherzolites were formed by shallow entrapment of highly depleted MORB melt in residual spinel lherzolites. Nd isotope compositions are consistent with derivation from an asthenospheric mantle source that experienced a recent MORB‐producing depletion. This evolution was most likely accomplished during the late Cretaceous breakup of the eastern Australian margin.The harzburgite‐dunite association, which forms the bulk of Peridotite Nappe was probably formed through a multistage magma‐producing process. Harzburgites composition may have be obtained by a first phase of ∼15% dry fractional melting, followed by 15%–18% hydrous melting in a supra‐subduction zone setting. Variable ∊Nd negatively correlate with 87Sr/86Sr, while Pb isotopes cover a wide range, trending from depleted mantle towards enriched, sediment‐like, compositions. Such signatures likely reflect the evolution of a highly depleted fore‐arc mantle wedge variably modified by different fluid and melt inputs during Eocene subduction.The harzburgite‐dunite set is overlain by a dunite transition zone ∼300m thick, in turn discontinuously covered by cumulate lenses consisting of layered pyroxenites, dunites, and wherlites at the base and gabbronorites/websterites on top. The mafic cumulates crystallized from primitive, ultra‐depleted melts in the nascent lower fore‐arc crust. In particular, FME enrichments and Nd‐Pb isotopes support an origin from a refractory mantle source modified by slab fluids for the gabbronorite‐forming melts.The Peridotite Nappe has been extensively serpentinized (40% to 100%) with extremely scarce occurrences of unserpentinized rocks. Lizardite, brucite, magnetite and minor chrysotile developed from joints and intra‐granular cooling cracks in a near‐static environment. Serpentine‐coated joints and peridotite foliation have been thereafter reopened and injected by various felsic, mafic and ultramafic supra‐subduction melts emplaced within a narrow time interval (55–50 Ma), immediately after subduction inception at 56 Ma, i.e. the age of granulite‐facies metamorphic sole. The youngest magmatic event is represented by island‐arc tholeiite dykes dated at 50 Ma.A widespread set of antigorite and tremolite‐bearing veins crosscut all previous structures in a progressively cooling fore‐arc environment. The former are synkinematic crack seals, which display highly radiogenic, sediment‐like 87Sr/86Sr ratios suggesting direct derivation of fluids from the subduction zone, while the latter bear mantle‐like isotopic signatures and probably originated from the interaction of wall rocks with Ca‐rich fluids released by Eocene dykes or fluids that leached them.Finally, continental subduction and obduction occurred during the 44–34 Ma interval and were accompanied by the development of the HP‐LT metamorphic belt of northern New Caledonia, which constrains the polarity of subduction.

Slab roll-back triggered back-arc extension south of the Paleo-Asian Ocean: Insights from Devonian MORB-like diabase dykes from the Chinese Altai

To better understand the origin of mid-ocean ridge basalt (MORB)-like mafic magmas and its geodynamic implications for the subduction zone system, we present an integrated study of petrology, mineralogy, geochronology and geochemistry of newly-discovered diabase dykes from the Chinese Altai, southwestern Central Asian Orogenic Belt. The diabases have zircon UPb ages of ~386 Ma, and are mainly composed of clinopyroxene (Wo43–55En17–27Fs26–34) and albite (An0–1Ab92–99Or0–7). Although the clinopyroxenes have consistent Mg# values (up to 75) that appear to be in equilibrium with bulk compositions, they can be divided into two groups based on their differences in other geochemical variables: type ǀ has high SiO2 and CaO but low TiO2 and Al2O3 contents while type ǁ displays low SiO2 and CaO but high TiO2 and Al2O3 contents. This indicates that the two types of clinopyroxenes may share a common source but crystallized at different depths under different P-T conditions (e.g., type ǀ: 6.3 kbar (~20 km), 1227 °C; type ǁ: 15.5 kbar (~49 km), 1332 °C). The dykes have low SiO2 (44.5–48.2 wt%) and K2O (0.17–0.33 wt%), but high (Fe2O3)T (11.2–13.6 wt%) and MgO (7.16–9.90 wt%) contents, placing them within the low-K tholeiitic series. With both N-MORB- and arc-like geochemical characteristics, including weakly fractionated rare earth element (REE) patterns ((La/Yb)N 1.43–1.62), enrichment in large-ion lithophile elements (LILE) (e.g., Pb) and slightly depletion of high field strength elements (HFSE) (e.g., Nb, Ta and Ti), and highly depleted SrNd isotopic compositions (87Sr/86Sri 0.7039–0.7042, ɛNd(t) +8.3 to +8.6), the diabase dykes were probably derived by partial melting of a N-MORB-like asthenospheric mantle source metasomatized by subduction-related fluids under spinel-facies conditions. Melting temperatures and pressures for the primary magmas were estimated at 13711394 °C and 2.22.4 GPa, respectively. The dykes underwent insignificant crustal contamination, but trace-element modeling indicates that minor subcontinental lithospheric mantle (SCLM) components could have been involved in their source region. We suggest that the MORB-like magmatism was triggered by asthenospheric upwelling in a back-arc extensional setting, in response to the roll-back subduction of the Paleo-Asian oceanic slab. Combined with the coeval arc magmatism in the study area, we envisage that an active arc–back-arc basin system developed in the Chinese Altai during Devonian time, linked to the northward subduction of the Paleo-Asian Ocean. This study emphasizes that MORB-like mafic rocks in paleo-subduction zones could be an important tectonomagmatic process-indicator (e.g., back-arc extension), which offers a new perspective in reconstructing past tectonic regimes.

Whole-Rock Elemental and Sr-Nd Isotope Geochemistry and Petrogenesis of the Miocene Elmadağ Volcanic Complex, Central Anatolia (Ankara, Turkey)

This study presented whole-rock elemental and Sr-Nd isotope geochemistry data with the purpose to decipher the origin and evolution of the Miocene Elmadağ Volcanic Complex, Central Anatolia (Ankara, Turkey). Volcanic products spanned in composition from mildly alkaline basaltic (47–52 wt% SiO2) and medium- to high-K calc-alkaline intermediate (54–62 wt% SiO2; andesite to trachyandesite) to felsic (64–74 wt% SiO2; dacite to rhyolite) units. Despite a homogeneous major element composition, basaltic rocks were characterized by two distinct trace element and isotopic signatures, which have been correlated with different mantle sources. The first group of basaltic rocks was similar to those of oceanic island basalts (OIB) and was derived from asthenospheric mantle source. The second group had geochemical characteristics of orogenic basalts derived from subduction-modified lithospheric mantle source and represented parental magma of the intermediate to felsic rocks. By coupling geochemical and textural analyses of the rocks from the Elmadağ Volcanic Complex, I suggest that crystallization of olivine + clinopyroxene + apatite played an important role in the evolution of basaltic rocks, while plagioclase + amphibole + apatite + Fe-Ti oxides ± zircon crystallization was major process involved in the evolution of intermediate to felsic rocks. The EVC basaltic rocks were associated with the post-collisional extensional tectonic regime in the Central Anatolia, but the coexistence of the OIB-like volcanism implies variations in the extension dynamics during Miocene.

Neo‐Tethyanslab tearing constrained by PalaeoceneN‐MORB‐like magmatism in southern Tibet

Southern Tibet is a key region for investigating the magmatism that occurredduring the collision between India and Asia. The break‐off of the Neo‐Tethyan slab, after the onset of continental collision at ca. 65–60 Ma, affected the lithospheric thermal regime and led to a series of magmatic and tectonic events. Many magmatic rocks generated by slab break‐off have been reported in southern Tibet; however, which of these record the initial slab break‐off (i.e., slab tearing) is still an open question. The Lianxiang gabbros, which intrude an ophiolite in central‐southern Tibet, provide an ideal opportunity to investigate the footprint of the initial slab break‐off. Zircon U–Pb ages show that the Lianxiang gabbros crystallized at 57–53 Ma, coeval with other mafic dikes in southern Tibet. The Lianxiang gabbros are characterized by high Na2O (2.13–5.42 wt%), Al2O3(13.86–17.13 wt%), and V (188–346 ppm) contents and moderate Mg# (40.8–65.9). The gabbrosare slightly depleted in light rare earth elements [LREE; (La/Yb)N= 0.45–0.76], similar to normal mid‐ocean ridge basalt (N‐MORB). They also have low87Sr/86Sriratios (0.703538–0.705698) and highεNd(t) values (9.49–10.68), indicating that they were derived mainly from a shallow asthenospheric mantle source. Combining our results with data from other magmatism at ~57 Ma in the southern Lhasa Terrane, we propose that the N‐MORB‐type rocks reflect the tearing of the Neo‐Tethyan slab. The process of slab break‐off differed between western and eastern southern Tibet, possibly as a result of different lithospheric structures and compositions.

Zircon U–Pb–Hf isotopes and whole rock geochemistry of magmatic rocks from the Posht-e-Badam Block: A key to tectonomagmatic evolution of Central Iran

The Posht-e-Badam Block in Central Iran likely formed part of the Neoproterozoic Arabian-Nubian Shield (ANS). However, its Phanerozoic history is not well constrained. Zircon UPb ages, Hf isotopic compositions and whole-rock geochemistry of igneous rocks from the Bafq district were determined to constrain their source and tectonomagmatic evolution. Two magmatic cycles are identified; early Paleozoic events associated with the Cadomian Orogeny resulting from Proto-Tethys subduction beneath the northern margin of Gondwana, and Cenozoic volcanism related to the Alpine-Himalayan Orogeny and closure of Neo-Tethys. The main plutonic and volcanic rocks record early Cambrian ages. The Zarigan, Narigan and Chahcholeh granitoids, volcanic rocks of the lower Cambrian volcano-sedimentary unit (CVSU) and the Zarigan gabbro have remarkably similar ages of 536 to 528 Ma. The Zarigan and Chahcholeh granitoids and volcanic rocks of the CVSU have positive εHf(t) values of 1.4 to 9.3, with Hf model ages (Hf-TDMc) of 715–1034 Ma. They are mostly A2-types, although some samples show affinity to A1- and I-types, suggesting an asthenospheric mantle source, modified by various amounts of crustal assimilation. The Narigan granite records εHf(t) values of −3.8 - 3.1, with Hf-TDMc of 970 to 1255 Ma and displays I-type characteristics, formed through mixing of mantle-derived melts with predominantly Neoproterozoic crust. All rocks were generated in a post-collisional setting. The Esfordy syenite and mafic dikes are younger and essentially coeval, with weighted mean ages of 457 ± 5 Ma and 451–448 Ma, respectively. The Esfordy syenite has A1-type characteristics, indicating possible derivation in an intraplate rift environment and was likely plume-related. The mafic dikes have OIB-like characteristics. Mafic rocks related to closure of Neo-Tethys include the ca. 23 Ma Bahabad diorite that records εHf(t) values of −16.8 to 10.1 (Hf-TDMc = 241–283 Ma) and an OIB-like signature, indicating assimilation of crustal material by asthenospheric mantle-derived melt.

Geochronological and geochemical constraints on the petrogenesis of late Mesoproterozoic mafic and granitic rocks in the southwestern Yangtze Block

Late Mesoproterozoic igneous rocks in the SW Yangtze Block are important for understanding the role of it in reconstruction of the Rodinia supercontinent. In the present study, we report new geochronological, geochemical, and Nd–Hf isotopic data for the Cuoke plagioclase amphibolites and granites in the SW Yangtze Block. Geochronological results show that the plagioclase amphibolites and granites have similar late Mesoproterozoic zircon U–Pb ages of 1168–1162 ​Ma, constituting a bimodal igneous assemblage. The plagioclase amphibolites have high and variable TiO2 contents (1.15–4.30 ​wt.%) and Mg# (34–66) values, similar to the tholeiitic series. They are characterized by enrichment in LREEs and LILEs, and have OIB-like affinities with positive Nb and Ta anomalies. The plagioclase amphibolites have positive whole-rock εNd(t) (+3.2 to +4.3) and zircon εHf(t) (+4.3 to +10.7) values, indicating that they were derived from an OIB-like asthenospheric mantle source. The granites belong to the reduced peralkaline A-type series and have negative εNd(t) value of −6.0 and εHf(t) values of −5.8 to −13.8, indicating a derivation from the partial melting of ancient mafic lower crust. In combination with the ∼1.05–1.02 ​Ga bimodal igneous assemblage in the SW Yangtze Block, we propose that the Cuoke 1168–1162 ​Ma igneous rocks were likely formed in a continental rift basin and argue against the existance of Grenvillian Orogen in the SW Yangtze Block during the late Mesoproterozoic.

Late Cretaceous-Cenozoic intraplate extension and tectonic transitions in eastern China: Implications for intraplate geodynamic origin

Abstract The geodynamic origin of intraplate extension remains poorly understood. For the case of the Late Cretaceous-Cenozoic intraplate extension in eastern China, its driving mechanism remains highly controversial due to the absence of a comprehensive understanding of significant tectonic transitions. This study uses the well-documented Cretaceous tectonic evolution as a background and integrates extensive multidisciplinary data in the Jianghan Basin, Bohai Bay Basin and Zhejiang area, to develop an integrated view of the Late Cretaceous-Cenozoic geodynamic origin for the first time. During the Late Cretaceous, rifting and depocentres were distributed in the Jianghan Basin and were located in the central Bohai Bay Basin. During the Cenozoic, rifting and depocentres were initially focused in the western basins and then progressively migrated eastward, rather than continuing migrating eastward. The rifting and volcanism during the Middle Cenozoic (50-23 Ma) significantly increased, and all rift basins in eastern China were abandoned at the end of the Middle Cenozoic (~26-23 Ma). The volcanism in the Zhejiang area was markedly enhanced during the Late Cenozoic (23-0 Ma). The asthenospheric mantle sources beneath the two rift basins were isotopically more depleted with an increased extent of melting from the Late Cretaceous (100-65 Ma) to Early Cenozoic (65-50 Ma); however, they became isotopically more enriched and heterogenous with a decreased melting extent (significantly reduced volatile content) from the Early Cenozoic to Middle Cenozoic. The mantle source beneath the Zhejiang area during the Late Cenozoic was isotopically more depleted and heterogeneous than during the Middle Cenozoic, with an increased extent of melting. These three-phase tectonic transitions together with tectonic correlations directly constrain the causal relationships between the intraplate extension in eastern China and the motions of the adjacent plates. Consequently, we propose that the Late Cretaceous-Cenozoic intraplate extension and tectonic transitions in eastern China were successively driven by the Paleo-Pacific slab rollback during the Late Cretaceous, the India-Asia collision during the Early-Middle Cenozoic and by the combination of the subduction of the Pacific Plate and India-Asia collision during the Late Cenozoic. Our study provides an integrated perspective for unraveling the geodynamic origin of intraplate extension.

Origin of the early Cretaceous Liucheng OIB-type dolerite sill in Zhejiang Province, coastal region of SE China: Implications for recycling of the Paleo-Pacific slab

Both early Cretaceous ocean island basalt (OIB)-type and subduction-related arc-type mafic magmatism occurred in coastal region of SE China. However, whether there existed a genetic relationship between the OIB-type magmatism and contemporary subduction of the Paleo-Pacific Ocean remains unclear. Here we present a comprehensive geochemical dataset of the Liucheng dolerite sill (~108 Ma) from coastal SE China, with aims to investigate the possible genetic link between them. The dolerite displays OIB-like trace element features and contains moderately radiogenic Sr-Nd-Pb-Hf and light Ca isotopic compositions. The nearly identical Sr-Pb isotopes between plagioclase and bulk rock indicate a minor role of crustal assimilation during magmatic evolution. The remarkable positive Ba anomalies, similar Sr and Pb isotopic compositions to the contemporary arc-type mafic rocks and light Ca isotopes suggest a source modified by recycled sediments. Additionally, the low La/Nb and Zr/Nb ratios indicate the involvement of an oceanic crust component that experienced dehydration and metamorphism to retain rutile. Further element-isotopic modeling suggests derivation of the parental magma from the asthenospheric mantle source that had been metasomatized by melts from both sediments and mafic oceanic crust. Considering that SE China was mainly influenced by subduction of the Paleo-Pacific Ocean during late Mesozoic, the subducting Paleo-Pacific slab was therefore the most likely candidate for such recycled components. Interaction of the melts from the recycled oceanic slab with the asthenospheric mantle, possibly as a consequence of rollback and tearing of the subducting Paleo-Pacific slab, formed the OIB-type source of the Liucheng dolerite sill.

Early Cretaceous Wulong intermediate-mafic dike swarms in the Liaodong Peninsula: Implications for rapid lithospheric delamination of the North China Craton

The North China Craton (NCC) underwent a dramatic change in the lithospheric architecture, and more than 100 km of the ancient and refractory lithospheric mantle was removed. However, the timing and mechanism of the lithospheric thinning remain controversial. New zircon UPb ages and whole-rock/in-situ geochemical and isotopic data of the Early Cretaceous intermediate-mafic dike swarms in the Liaodong Peninsula have been carried out to decipher the timing and mechanism of lithospheric thinning of the NCC. The dike swarms are chemically subdivided into the ca. 126 Ma diorite porphyry and coeval (ca. 119 Ma) low-Ti (TiO2 < 1.20 wt%, Ti/Y < 375) and high-Ti (TiO2 > 1.90 wt%, Ti/Y > 580) diabase dikes. The diorite porphyries exhibit a moderate silica (56.71–61.13 wt%) and high-K calc-alkalic signature, with low TiO2 and total Fe2O3 and high Cr and Ni compositions. They are enriched in large ion lithophile elements (LILEs) (e.g., Rb, K, Pb, and Sr) and depleted in high field strength elements (HFSEs) (e.g., Nb, Ta, Ti, and P). They have high radiogenic Sr isotopes (whole-rock (87Sr/86Sr)i = 0.711553–0.714284, in-situ (87Sr/86Sr)i of plagioclase = 0.71392), low εNd(t) values (−19.9 to −14.5), and old Nd model ages (1.81–2.25 Ga). Meanwhile, some plagioclase phenocrysts have reverse zonings with Na-rich cores (An: 35–48) and Ca-rich rims (An: 48–57). These features imply that the diorite porphyries were generated by mixing between magmas from the Archean–Paleoproterozoic lower crust and the lithospheric mantle. The low-Ti diabase dikes are characterized by medium to high-K calc-alkaline series with high Al2O3 but low TiO2 contents. They show arc-like trace element signatures, such as enrichment in LILEs (Rb, K, Pb, and Sr) and depletion in HFSEs (Nb, Ta, Zr, Hf, Ti, and P). They also have relatively lower initial 87Sr/86Sr ratios (0.706331–0.708315 and 0.70837 for the whole-rock and in-situ Sr isotopes, respectively) compared to the porphyries and negative εNd(t) values (−13.6 to −5.0), indicating that they were derived from partial melting of an enriched lithospheric mantle. Their variable Th/Yb and Th/Nb ratios with constant Sr/Nd and U/Th values further suggest that the mantle source might have been previously modified by the subduction-related melt. In contrast, the high-Ti diabase dikes belong to the high-K calc-alkaline to shoshonitic series, with higher TiO2 and total Fe2O3 but lower Al2O3 contents. They have OIB-like trace elements (e.g., slight enrichments of Nb and Ta with no HFSE depletion, high Nb/U values (42 ± 5)) and SrNd isotopic compositions (e.g., positive whole-rock εNd(t) values (+3.1 to +4.3)), suggesting that they were generated from an asthenospheric mantle source. The coexisting Wulong low-Ti and high-Ti mafic dikes with similar formation ages (ca. 119 Ma) may indicate a transition from lithospheric to asthenospheric mantle source. In combination with previous studies on the Mesozoic–Cenozoic mafic rocks in the Eastern Block of the NCC, we propose that the lithospheric thinning beneath the Liaodong Peninsula initiated at ca. 119 Ma and this process beneath the NCC lasted for a narrow duration (ca. 13 Ma). The unsteady mantle flows due to the rapid subduction and rollback of the Paleo-Pacific plate might trigger the thickened, modified lithosphere to lose its gravitational stability, eventually leading to lithospheric delamination and thinning of the NCC.

Unraveling the Effects of Melt–Mantle Interactions on the Gold Fertility of Magmas

The oxidation state of the Earth’s mantle and its partial melting products exerts a fundamental control on the behavior and distribution of sulphur and chalcophile and siderophile elements between the mantle and crust, underpinning models of ore deposit formation. Whether the oxidized nature of magmas in inherited from the asthenospheric mantle source or acquired during ascent and differentiation is vigorously debated, limiting our understanding of the mechanisms of extraction of sulphur and metals from the mantle. Here, we focused on the redox-sensitive behavior of sulphur in apatite crystallized from quenched alkaline basaltic melts preserved within a peridotite xenolith from the El Deseado Massif auriferous province in southern Patagonia. We took advantage of this unique setting to elucidate the redox evolution of melts during their ascent through the subcontinental lithospheric mantle, and grasp the inner workings of the Earth’s mantle during gold metallogenesis. Our data reveal that an initially reduced silicate melt (ΔFMQ -2.2 to -1.2) was oxidized to ΔFMQ between 0 and 1.2 during percolation and interaction with the surrounding peridotite wall-rock (ΔFMQ 0 to +0.8). This process triggered changes in sulphur speciation and solubility in the silicate melt, boosting the potential of the melt to scavenge ore metals such as gold. We suggest that large redox gradients resulting from the interaction between ascending melts and the surrounding mantle can potentially modify the oxidation state of primitive melts and enhance their metallogenic fertility. Among other factors including an enriched metal source and favorable geodynamic conditions, redox gradients in the mantle may exert a first-order control on the global-scale localization of crustal provinces endowed with gold deposits.

Geology, geochronology and geochemistry of the Miocene Sulutas volcanic complex, Konya-Central Anatolia: genesis of orogenic and anorogenic rock associations in an extensional geodynamic setting

ABSTRACT Subduction-related environments are complex geological sites where various magmas occur in close spatial and temporal association. To better understand geological processes led to the generation of compositionally distinct magmas, we need to know the origin and evolution of magmas formed in these settings. Our purpose here is to determine the genesis of the Sulutas Volcanic Complex (SVC), located near Konya (Central Anatolia, Turkey) and to understand its relation to the subduction-related processes. Based on a detailed field study of the SVC, we present here Ar-Ar geochronology, mineral chemistry, whole-rock major, trace element and Sr-Nd-Pb isotope data. The SVC can be subdivided into three distinct chemical groups. The oldest group (~16 Ma) characterized by bimodal association is composed of Na-alkaline basaltic rocks and adakitic dacite. The Na-alkaline basaltic rocks were derived from an OIB-like enriched asthenospheric mantle source, but the adakitic dacites from a mafic lower crust. Geochemistry of potassic lamprophyres (~13.5 to 12.5 Ma), the second group, can be best explained by melting of a depleted lithospheric mantle source that was metasomatized by slab-derived sediment melts. This metasomatic component generated phlogopite-rich veins in their source. The youngest group (~13 to 11 Ma), high-K calc-alkaline rocks (HKCA) is composed of basalt, andesite and mafic enclave-bearing dacite. Parental magmas to the HKCA are consistent with the derivation from a lithospheric mantle source metasomatized by fluid-like subduction components. Geochemical variations in the HKCA can be explained by magmatic processes (e.g. assimilation-fractional crystallization for basalt and dacite, fractional crystallization for andesite) from a common parental magma similar to the mafic enclave. The SVC composed of orogenic and anorogenic rocks occurred in a graben-like extensional basin in the Central Anatolia, and this extensional tectonics is related to the retreating subduction zone along the Cyprus arc within the convergence system of the African-Eurasian plates during Miocene.

Ca. 780 Ma OIB-like mafic dykes in the Western Jiangnan orogenic Belt, South China: evidence for large-scale upwelling of asthenosphere beneath a post-orogenic setting

ABSTRACT Post-collisional alkaline mafic rocks record significant information regarding crust–mantle interaction in the subduction channel and provide an excellent opportunity to investigate the mechanism of transition from pre-collisional to post-collisional magmas. In this contribution, we present whole-rock major and trace element and radiogenic isotope data obtained from alkaline mafic dykes in the Guzhang-Anjiang areas in the southwestern Jiangnan orogenic belt (JOB), South China. LA–ICP–MS U–Pb zircon dating of four representative samples yields a consistent age of ~780 Ma. These mafic dykes are characterized by large variations in SiO2 (42.4–54.1 wt.%) and MgO (2.2–15.0 wt.%), alkalis (K2O+Na2O = 1.96–11.19 wt.%), Nb (17.55–49.62 ppm) and Ta (1.15–3.26 ppm). They exhibit ocean island basalt (OIB)-like geochemical features, which are characterized by enrichment in light rare earth elements (LREEs) and large ion lithophile elements (LILEs) relative to heavy rare earth elements (HREEs) and by positive zircon εHf(t) (+1.86 to +5.14) and whole-rock εNd(t) (+0.31 to +1.80) values. All these geochemical characteristics suggest these dykes were derived from a small degree (2–5%) of partial melting of an asthenospheric mantle source that had previously been modified by earlier subduction processes. Based on the previous reports and present study, we infer that three periods of Neoproterozoic basaltic magmatism occurred in the western Jiangnan Orogen, and constrain the time of the tectonic regime transformation at ~830, and at ~780 Ma. The source of these basaltic rocks varied in composition spatially and evolved with time. The ~780 Ma OIB-like rocks in this study serve as an important indicator of large-scale upwelling of the asthenospheric mantle and the transition from orogenic to post-orogenic tectonic setting.

The transformation of the lithospheric mantle beneath South China Block (SCB): constraints from petrological and geochemical studies of Daoxian and Ningyuan basalts and their melt inclusions

ABSTRACTThe lithospheric mantle beneath the South China Block (SCB) underwent a dramatic transformation from depleted to enriched during late Mesozoic. With a view to deeply understand this process, here we investigate the Mesozoic basalts and their melt inclusions from the Daoxian and Ningyuan regions within the central SCB. The geochemical features of the melt inclusions in these basalts suggest that these rocks originated from the lithospheric mantle enriched through interaction with K-rich aqueous fluids released from subducted Palaeo-Pacific oceanic sediments, whereas the Ningyuan basalts were mainly derived from the asthenospheric mantle source. Geochemical modelling indicates that the Daoxian basalts were generated from 15%-25% of partial melting of garnet lherzolite, whereas the Ningyuan basalts originated from 10%-20% of partial melting of garnet pyroxenites. Our data, combined with those from other Jurassic basalts suggest a temporal evolution of the SCB mantle sources during the Late Mesozoic. Diverse crust–mantle interactions through mixing of the asthenospheric melts with variable proportions of subducted Palaeo-Pacific oceanic sediments might account for the spatial heterogeneity of mantle sources observed beneath the SCB. The transition from Tethyan tectonic realm to the Palaeo-Pacific tectonic regime might have played a significant role in the transformation of the lithospheric mantle beneath the SCB.

Eruption history and petrogenesis of rocks from Nyos volcano (NW Cameroon): Evidence from lithostratigraphy and geochemistry

Even though ca 1746 people and over 3000 cattle were killed in 1986 by sudden release of about 800 million m3 of CO2 from Lake Nyos volcano in northwest Cameroon, the lake's formation history is not known. Here we report results of a lithostratigraphic and petrogenetic study of Nyos volcano that allows us to confirm its phreatomagmatic (maar) origin. Eruptive products are divided into 2 units, the phases of each of which are closely related in time. Unit A comprises 4 phases, the last (A-4) represents the cataclysmic phreatomagmatic formation of Nyos maar and may have been triggered by collapse of the vent system of the previous (A-1 to A-3) phases, and/or reactivation of basement faults. After a repose period, activity shifted NE of the maar to form Unit B products. The B-1 phase of this unit deposited the scoria now covering most of the Lake Nyos area. Unit B-2 constructed the Fon's cone. A small lava flow (Unit B-3) represents the latest phase of volcanic activity of Nyos volcano. Based on area-thickness estimation methods, a total of ca 0.8 km3 of eruptive material was produced from the volcano. Geochemical data suggest that magma of the 2 units probably formed by melting of the same garnet-bearing OIB-like asthenospheric mantle source, and evolved mainly by crustal contamination and fractional crystallization in independent magma batches that erupted without mixing. Our results provide a basis for advice on general hazard mitigation in the Lake Nyos area.

Neoproterozoic magmatism in the northern margin of the Yangtze Block, China: Implications for slab rollback in a subduction-related setting

The South Qinling Belt was located in the northern part of the Yangtze Block during the Neoproterozoic. Here we investigate two major magmatic intrusions in the northern and southern part of the belt, the Douling plutons (DLP) and the Fenghuangshan plutons (FHSP), to gain insights into the Neoproterozoic tectonic processes along the northern segment of the Yangtze Block. Zircon grains in the DLP yield weighted mean 206Pb/238U ages of 750–709 Ma, whereas those from the FHSP show ages of 749–728 Ma. The new results, together with the published data, reveal that these plutons were emplaced at ca. 759–685 Ma and ca. 802–704 Ma, respectively. Geochemically, the intermediate-felsic rocks from the DLP and FHSP show enrichment of light rare earth elements (LREE) and large-ion lithophile elements (LILE) (Rb, Ba and K), depletion of high field strength elements (HFSE) (Nb, Ta, P and Ti), and low Sr/Y and (La/Yb)N values, which are typical features of arc magmatic rocks. The gabbros from the FHSP have high Mg#, and display LREE and LILE (Rb, Ba, K, and Sr) enrichment and depletion of HFSE (Th, U, Nb, Ta, and Ti), suggesting that they were probably formed in a mantle wedge setting. Both DLP and FHSP show arc-related geochemistry modified by slab-derived fluids, and were formed at relatively high-temperature and low-pressure conditions. However, they display distinct Sr-Nd-Hf isotopic compositions. Zircon grains of two samples from the DLP respectively show mean εHf(t) values of −2.6 ± 0.3 and 0.9 ± 0.3, whereas zircon grains of four samples from the FHSP show mean εHf(t) values ranging from 7.5 ± 0.5 to 8.7 ± 0.4. The DLP shows dominantly negative εNd(t) values and inconsistent initial 87Sr/86Sr ratios, whereas FHSP shows relatively constant and positive εNd(t) values (3.69–3.74) but variable initial 87Sr/86Sr ratios. The Sr-Nd-Hf isotopic and geochemical features suggest that the enriched mantle and crustal materials were involved in the magma evolution of the DLP, whereas the FHSP is characterized by significant juvenile components with a rising asthenospheric mantle source playing a key role. We propose a subduction-related setting for these intrusions, and envisage slab rollback during the Neoproterozoic.

Geochronology, geochemistry, and Sr–Nd–Hf isotopic compositions of mafic–ultramafic intrusions in the Niubiziliang Ni–(Cu) sulfide deposit, North Qaidam Orogenic Belt, NW China: Implications for magmatic source, geodynamic setting, and petrogenesis

Whole-rock geochemical and SrNd isotopic data, and zircon U–Pb–Hf isotopic data are reported for mafic–ultramafic intrusions in the Niubiziliang Ni–(Cu) sulfide deposit, located in the westernmost segment of the North Qaidam Orogenic Belt (NQOB), NW China. NiCu sulfide ores of economic interest are found within these intrusions, with the Niubiziliang III and IV intrusions hosting most of the Ni–(Cu) mineralization. The Niubiziiang mafic–ultramafic intrusions intrude biotite–plagioclase gneiss of the Paleoproterozoic Dakendaban Group. Zircons separated from the gabbro, plagioclase-bearing clinopyroxenite, and olivine websterite yield UPb ages of 389 ± 2, 373 ± 2, and 380 ± 2 Ma, respectively, indicating a genetic linkage with Middle–Late Devonian regional magmatism (390–370 Ma). Gabbro zircons have εHf(t) values of 4.2–10.9 with older TDM1 ages (0.86–0.59 Ga). The Niubiziliang intrusions contain many xenoliths of Paleoproterozoic biotite–plagioclase gneiss, and the intrusions have similar trace element patterns with strong enrichment in LILEs (e.g., K, Rb, and Th) and moderate depletion in HFSEs (e.g., Nb, Ta, P, and Ti). The major and trace element characteristics, and Sr–Nd–Hf isotopic compositions indicate the parental magmas originated from a metasomatized, asthenospheric mantle source that had previously been modified by subduction-related fluids. The magmas experienced significant crustal contamination both in the magma chamber and during ascent, triggering sulfur over-saturation that resulted in the deposition and enrichment of sulfides. Considering the tectonic evolution of this region, we infer that the Niubiziliang mafic–ultramafic intrusions formed in a post-collisional extensional setting related to delamination of the subcontinental lithosphere.

Geochronology and petrogenesis of Jurassic intraplate alkali basalts in the Junggar terrane, NW China: Implication for low-volume basaltic volcanism

High-volume volcanic fields which are observed at continental intraplate setting are generally known as the large igneous provinces (LIPs), fed by hot and active plumes upwelling from the deep mantle. However, the source and mechanism of low-volume effusive volcanism within continental interiors remains poorly unknown. Here, we present a combined study of 40Ar/39Ar geochronology, mineral chemistry, whole-rock major and trace elements as well as Sr-Nd isotopes of Jurassic basalts from the Karamay area, Junggar terrane (NW China), aiming to determine their formation ages, constrain the petrogenesis and reveal their tectonic implications.New whole-rock 40Ar/39Ar dating yields consistent ages of 189.4–193.3 Ma for magma emplacement. The basalts exhibit a columnar joint structure and porphyritic texture with phenocryst minerals of olivine (Fo63–79), clinopyroxene (Wo39–46En38–47Fs12–22) and plagioclase (An25–54). They have SiO2 contents ranging from 45.0–51.8 wt% and belong to alkaline series (δ2.7–13.3, average of 5.0). The basalts are characterized by oceanic island basalt (OIB)-like trace element distribution patterns with enrichment in L-MREE, HFSE (e.g., Nb and Ta) and LILE (e.g., K, Sr and Ba), and slight depletion in HREE, relative to normal mid-ocean ridge basalt (N-MORB). Positive εNd(t) (+3.0), low to moderate 87Sr/86Sri (0.7048–0.7049) isotopic compositions, and trace element ratios (e.g., high Nb/U > 49.0 and Ce/Pb > 13.9) suggest that crust contamination was insignificant in the formation process. P–T estimates of major phenocryst minerals in these basalts reflect high crystallization temperatures (>1100 °C) but low pressures (<9.8 kbar). Geochemical features and REE modeling indicate that the basaltic magmas were likely derived by low degrees of partial melting (~5% – 10%) of an asthenospheric mantle source in the garnet stability field. Calculated lithospheric thickness and initial melting depths for the studied basalts are about 120 km and 180–200 km, respectively. Based on the available data and regional secular geological evolution, we suggest that decompression melting of upwelling asthenosphere triggered by small-scale removal of thickened lithospheric root could be a possible mechanism to explain Jurassic low-volume intraplate volcanism in the Junggar terrane.

Geochemistry and apatite U–Pb geochronology of alkaline gabbros from the Nodoushan plutonic complex, Sanandaj–Sirjan Zone, Central Iran: Evidence for Early Palaeozoic rifting of northern Gondwana

The Zagros Orogen formed during the Cenozoic collision of Arabia with Eurasia and resulted in the closure of the Neo‐Tethys Ocean. Collision was preceded by a complicated tectonic history involving Pan‐African orogenesis, Late Palaeozoic rifting and the formation of Neo‐Tethys, and subsequent Mesozoic convergence on the northern margin of the ocean contemporaneous with ophiolite obduction on its southern margin. The Sanandaj–Sirjan Zone (SaSZ) is a metamorphic belt within the Zagros Orogen of Gondwanan provenance. U–Pb zircon geochronology has revealed the presence of Pan‐African igneous and metamorphic basement complexes, in addition to minor late Palaeozoic plutons and abundant Jurassic plutonic rocks. This study presents a LA‐ICP‐MS U–Pb apatite age of 440.7 ± 3.7 Ma from a gabbroic body on the central north‐eastern margin of the SaSZ (Nodoushan region). Based on petrography and geochemistry, en route fractional crystallization of ascending magma was an important process in the evolution of the intrusive rocks. Geochemical characteristics imply that the south‐west Nodoushan alkaline gabbroic magma was originated from the asthenospheric mantle source, whereas the high ratios of (La/Yb)N and (Dy/Yb)N are related to the low degree of partial melting from the garnet‐bearing mantle source. Enrichment pattern of Nb, Ta, and Ti and depletion of Rb, Th, and Y are similar to the OIB pattern and intraplate alkaline magmatic rocks. High εNd(t) values (+1.29 to +1.50), low initial Sr isotopic ratios (0.703679–0.704098), and trace element ratios (e.g., Nb/La, Ce/Pb, Ba/Nb, and Th/Nb) indicate that crustal contamination was insignificant in its petrogenesis. The petrogenesis of the Nodoushan alkaline gabbroic rocks could be related to the presence of extensional phase, upwelling and decompressional melting of asthenospheric mantle in the rift basin which was associated with the opening of the Paleo‐Tethys Ocean in Late Ordovician to Silurian times.

Timing of subduction initiation in the Proto-Tethys Ocean: Evidence from the Cambrian gabbros from the NE Pamir Plateau

In order to determine the timing of subduction initiation for the Proto-Tethys Ocean and to better understand the assembly of the Precambrian terranes in east Asia to Eastern Gondwana, we report ages and geochemical data for newly identified gabbro sheets/stocks emplaced in the lower part of the early Paleozoic accretionary wedge at the NE Pamir, the western section of the West Kunlun Orogenic Belt (WKOB). The gabbros from the NE Pamir consist of clinopyroxene and plagioclase with minor olivine, orthopyroxene, Ti-Fe oxides, and accessory minerals apatite and zircon. SHRIMP U-Pb dating of co-magmatic zircons yield weighted mean 206Pb/238U ages of ca. 530 Ma. Whole-rock and mineral chemistry indicates that these gabbros crystallized from an evolved tholeiitic mafic magma. Isotopically, whole-rock gabbros exhibit positive εNd(t) values (~4.0). The zircons show positive εHf(t) values (+5 to +8), and have elevated δ18O values (average δ18O = 6.52 ± 0.22) relative to mantle zircons. Our isotopic and chemical data suggest that the Cambrian gabbros in the NE Pamir were derived from a depleted asthenospheric mantle source in a fore-arc setting. The Cambrian gabbros in the NE Pamir and the coeval gabbros in the Tianshuihai terrane at the eastern section of WKOB reveal that the southward subduction of the Proto-Tethys Ocean initiated at ~530 Ma. This long-term southward subduction led to the formation of a massive early Paleozoic accretionary wedge in WKOB and the final closure of the Proto-Tethys Ocean at ca. 440 Ma. As a result, the terranes along the northern margin of the Tibetan Plateau were docked along the northern fringe of the Eastern Gondwana at ca. 440 Ma.

Petrology and geochemistry of the Tasse mantle xenoliths of the Canadian Cordillera: A record of Archean to Quaternary mantle growth, metasomatism, removal, and melting

Mantle xenoliths hosted by the Quaternary Tasse alkaline basalts in the Canadian Cordillera, southeastern British Columbia, are mostly spinel lherzolite originating from subcontinental lithospheric mantle. The xenoliths contain abundant feldspar veins, melt pockets and spongy clinopyroxene, recording extensive alkaline metasomatism and partial melting. Feldspar occurs as veins and interstitial crystal in melt pockets. Melt pockets occur mainly at triple junctions, along grain boundaries, and consist mainly of olivine, cpx, opx and spinel surrounded by interstitial feldspar.The Nd, Sr and Pb isotopic compositions of the xenoliths indicate that their sources are characterized by variable mixtures of depleted MORB mantle and EM1 and EM2 mantle components. Large variations in εNd values (−8.2 to +9.6) and Nd depleted mantle model ages (TDM = 66 to 3380 Ma) are consistent with multiple sources and melt extraction events, and long-term (>3300 Ma) isolation of some source regions from the convecting mantle. Samples with Archean and Paleoproterozoic Nd model ages are interpreted as either have been derived from relict Laurentian mantle pieces beneath the Cordillera or have been eroded from the root of the Laurentian craton to the east and transported to the base of the Cordilleran lithosphere by edge-driven convection currents.The oxygen isotope compositions of the xenoliths (average δ18O = +5.1 ± 0.5‰) are similar to those of depleted mantle. The average δ18O values of olivine (+5.0 ± 0.2‰), opx (+5.9 ± 0.6‰), cpx (+6.0 ± 0.6‰) and spinel (+4.5 ± 0.2‰) are similar to mantle values. Large fractionations for olivine-opx, olivine-cpx and opx-cpx pairs, however, reflect disequilibrium stemming from metasomatism and partial melting.Whole-rock trace element, Nd, Sr, Pb and O isotope compositions of the xenoliths and host alkaline basalts indicate different mantle sources for these two suites of rocks. The xenoliths were derived from shallow lithospheric sources, whereas the alkaline basalts originated from a deeper asthenospheric mantle source.