Melting Of Asthenospheric Mantle Research Articles

Gabbroic–dioritic dykes from the Sanandaj–Sirjan Zone: windows on Jurassic and Eocene geodynamic processes in the Zagros Orogen, western Iran

The Sanandaj–Sirjan Zone (SaSiZ) is a magmatic terrane within the Zagros Orogen, western Iran, marking the Tethyan suture zone between the Afro-Arabian Plate and the Central Iran Micro-Continent. Mafic–intermediate dyke swarms with Middle Jurassic (Group 1: hornblende gabbro and diorite) and Late Eocene (Group 2: hornblende–pyroxene gabbro) ages are recognized in the Malayer–Boroujerd Plutonic Complex of the northern SaSiZ. Group 1 dykes have elemental and isotopic signatures consistent with melting of a mantle source modified during Neo-Tethyan subduction. Some Group 1 magmas evolved to intermediate compositions through assimilation and fractional crystallization. Group 2 dykes have within-plate trace element geochemical signatures, modelled as deriving from low-degree melting of asthenospheric mantle without a subduction influence. Published models postulate either a Cretaceous–Eocene Neo-Tethyan flat-slab scenario or a Latest Cretaceous–Paleogene Neo-Tethyan break-off event beneath the SaSiZ. Such models do not reconcile with the Late Eocene presence of within-plate magmatism in westernmost Iran, very close to the Zagros Suture. We argue that a period of flat-slab subduction concluded with sub-parallel subduction of a Neo-Tethyan ridge to the trench. The resulting slab break-off event in the Late Eocene is responsible for generation of the distinct Mesopotamia and Zagros slabs in mantle tomography models. Break-off was followed by small-volume within-plate type magmatism before short-lived re-establishment of Tethyan subduction prior to the final Arabia–Eurasia collision. Supplementary material: Field photographs, photomicrographs, additional geochemical plots, descriptions of analytical methods and tables of geochemical modelling parameters are available at https://doi.org/10.6084/m9.figshare.c.4126196

Late Cenozoic basaltic lavas from the Changbaishan-Baoqing Volcanic Belt, NE China: Products of lithosphere-asthenosphere interaction induced by subduction of the Pacific plate

Cenozoic intraplate basalts are low in volume but widespread in eastern China. They are predominantly alkaline and have oceanic island basalt (OIB)-like trace element compositions. Despite numerous studies, the origin of Cenozoic basalts in eastern China remains elusive. Possible roles of lithosphere thickness, subduction of Pacific plate and lithosphere-asthenosphere interaction in triggering spatial geochemical variations have not yet been clarified. Here, we have carried out mineral chemistry, major and trace element and SrNdHf isotope analyses of the late Cenozoic (<20 Ma) basaltic rocks from the Changbaishan-Baoqing Volcanic Belt (CVB), NE China, which revealed clear spatial compositional variations. The North CVB is dominated by basanites and alkali basalts with OIB-like trace element patterns and depleted SrNdHf isotopic compositions (87Sr/86Sr = 0.7039–0.7047, εNd = 3–5.6; εHf = 6.7–12), which may have been derived from low degree partial melting of a depleted source from asthenospheric mantle beneath a thick lithosphere. On the other hand, the South CVB consists of both alkali and sub-alkali lavas (alkali basalts, tholeiites and basaltic andesites) that display generally higher SiO2, Sm/Nd, Ba/Nb, Th/U, and lower Nb/Th, La/Sm and more enriched SrNdHf isotopic compositions (87Sr/86Sr = 0.7038–0.7056, εNd = −2.4 – +3.2; εHf = 3–8.2). These rocks may have been produced by larger degrees of partial melting of asthenospheric mantle beneath a relatively thin lithosphere. Ancient metasomatized lithospheric mantle might also have contributed to their genesis. In addition, the similar ranges of Mn, Ni and Fe/Mn for olivine phenocrysts from both the North and the South CVB suggest that they may have been derived from hybrid mantle sources containing similar proportions of peridotite and pyroxenite/eclogite components. We propose that decompression melting of upwelling asthenosphere and mechanical-chemical erosion of basal lithosphere related to lithosphere-asthenosphere interaction responsible for the genesis of the CVB magmas were likely associated with upper mantle convection and back-arc extension induced by deep subduction of the Pacific plate and its stagnancy in the transition zone.

A new approach to reconstructing the composition and evolution of kimberlite melts: A case study of the archetypal Bultfontein kimberlite (Kimberley, South Africa)

The compositions of kimberlite melts at depth and upon emplacement in the upper crust remain elusive. This can be attributed to the unquantified effects of multiple processes, such as alteration, assimilation, xenocryst contamination, and fractional crystallisation. The inability to accurately constrain the composition and physical properties of kimberlite melts prevents a comprehensive understanding of their petrogenesis.To improve constraints on the compositions of kimberlite melts, we have combined modal analysis including the discrimination of xenocrystic from magmatic phases, with mineral chemistry determinations to reconstruct a whole-rock composition. We apply this approach to a sample of “fresh” macrocrystic hypabyssal kimberlite (sample BK-1) from the Bultfontein mine (Kimberley, South Africa). The accuracy of this whole-rock reconstruction method is validated by the similarity between reconstructed and measured whole-rock compositions. A series of corrections are then applied to account for the effects of post-emplacement serpentinisation, pre-emplacement olivine crystallisation, and the inclusion and assimilation of mantle material. This approach permits discernment of melt compositions at different stages of kimberlite evolution.The primitive melt parental to the Bultfontein kimberlite is estimated to contain 17.4–19.0 wt% SiO2, 20.2–22.8 wt% MgO, 20.9–21.9 wt% CaO, 2.1–2.3 wt% P2O5, 1.2–1.4 wt% TiO2, 0.9–1.1 wt% Al2O3, and 0.6–0.7 wt% K2O, and has a Mg# of 83.4–84.4. Primary volatile contents (i.e., after an attempt to account for volatile loss) are tentatively estimated at ~2.1–2.2 wt% H2O and ~22.9–25.4 wt% CO2. This composition is deficient in SiO2, MgO and H2O, but enriched in CaO and CO2 compared with most previous estimates of primitive kimberlite melts. We suggest that the primitive melt parental to the Bultfontein kimberlite was a transitional silicate-carbonate melt, which was progressively enriched in SiO2, MgO, Al2O3, Cr2O3, and Na2O through the assimilation of lithospheric mantle material. Comparisons with experimentally produced low-degree melts of carbonated lherzolite indicate that the Bultfontein kimberlite could have formed by ~0.5% melting of asthenospheric mantle at ~6.0–8.6 GPa (i.e., ~190–285 km) and ~1400–1500 °C. The low calculated Na2O contents (<0.2 wt%), which are inconsistent with derivation from low-degree melting of lherzolite, suggest that an alkali-bearing, volatile-rich fluid was exsolved during ascent or released after emplacement, and subsequently removed.

Petrogenesis of Triassic Mafic Complexes with MORB/OIB Affinities from the Western Garzě‐Litang Ophiolitic Mélange, Central Tibetan Plateau

There is a general consensus that most ophiolites formed above subduction zones (Pearce, 2003), particularly during forearc extension at subduction initiation (Shervais, 2001; Stern, 2004; Whattam and Stern, 2011). “Supra‐Subduction zone” (SSZ) ophiolites such as the well‐studied Tethyan ophiolites, generally display a characteristic sequential evolution from mid‐oceanic ridge basalts (MORBs) to island arc tholeiities (IATs) or bonites (BONs) (Pearce, 2003; Dilek and Furnes, 2009, 2011), which were generated in sequence from the decompression melting of asthenospheric mantle and partial melting of subduction‐metasomatized depleted mantle (Stern and Bloomer, 1992; Dilek and Furnes, 2009; Whattam and Stern, 2011). However, ophiolites with MORB and/or oceanic‐island basalt (OIB) affinities are rare, and their origin and tectonic nature are poorly understood (Boedo et al., 2013; Saccani et al., 2013). It is interesting that the composition of these ophiolites from the central Tibetan Plateau (CTP) is dominated by MORBs and minor OIBs and a distinct lack of IATs and BONs, which is inconsistent with most ophiolites worldwide (Robinson and Zhou, 2008; Zhang et al., 2008). But the generation and tectonic nature of these ophiolites are still controversial.In this study, we present new geochronological, mineralogical and Sr‐Nd isotopic data for the Chayong and Xiewu mafic complexes in the western Garzě‐Litang suture zone (GLS), a typical Paleo‐Tethyan suture crossing the CTP (Fig. 1). The Triassic ophiolite in the western GLS has been described by Li et al. (2009), who found that it mainly consists of gabbros, diabases, pillow basalts and a few metamorphic peridotites. The ophiolite has been tectonically dismembered and crops out in Triassic clastic rocks and limestones as tectonic blocks. The Chayong and Xiewu mafic complexes are generally regarded as important fragments of the Triassic ophiolites (e.g., Jin, 2006; Li et al., 2009). Zircon LA‐ICP‐MS U‐Pb ages of 234±3 Ma and 236±2 Ma can be interpreted as formation times of the Chayong and Xiewu mafic complexes, respectively. The basalts and gabbros of the Chayong complex exhibit enriched MORB (E‐MORB) compositional affinities except for a weak depletion of Nb, Ta and Ti relative to the primitive mantle, whereas the basalts and gabbros of the Xiewu complex display distinct E‐MORB and OIB affinities. The geochemical features suggest a probable fractionation of olivine ± clinopyroxene ± plagioclase as well as insignificant crustal contamination.The geochemical and Sr‐Nd isotopic data reveal that the Chayong mafic rocks may have been derived from depleted MORB‐type mantle metasomatized by crustal components and Xiewu mafic rocks from enriched lithospheric mantle metasomatized by OIB‐like components. The ratios of Zn/Fet, La/Yb and Sm/Yb indicate that these mafic melts were produced by the partial melting of garnet + minor spinel‐bearing peridotite or spinel ± minor garnet‐bearing peridotite. We propose that back‐arc basin spreading associated with OIB/seamount recycling had occurred in the western GLS at least since the Middle Triassic times, and the decompression melting of the depleted MORB‐type asthenosphere mantle and partial melting of sub‐continental lithosphere were metasomatized by plume‐related melts, such as OIBs, which led to the generation of the Chayong and Xiewu mafic melts.

Geochemistry and zircon-apatite U-Pb geochronology of mafic dykes in the Shuangxiwu area: Constraints on the initiation of Neoproterozoic rifting in South China

The Shuangxiwu area of northwestern Zhejiang Province preserves the most complete magmatic records of the Neoproterozoic orogenic cycle at the eastern segment of the Jiangnan Orogen, South China. However, the transition from orogenesis to post-orogenic extension or rifting still remains debated and this led to great confusion on the breakup of Rodinia supercontinent. In this study, we carried out LA-ICP-MS zircon and apatite U-Pb dating and whole-rock geochemical analyses for the mafic dykes that intruded the Shuangxiwu arc volcanic rocks and the Heshangzhen Group in northwestern Zhejiang Province. The U-Pb dating results indicate that the mafic dykes formed at ∼760Ma, rather than ca. 860–850Ma as previously suggested. All the rocks are tholeiitic in compositions and can be divided into three subgroups based on their geochemistry. Group-1 has the lowest TiO2 contents (0.7–1.0wt%) and intermediate Mg# values (40–47), and they show enrichments in large ion lithophile elements and depletions in high field strength elements, resembling typical continental arc basalts. Group-2 has the highest TiO2 contents (1.5–3.6wt%) and Mg# values (50–55), and they do not show anomalies in P, Zr and Hf, similar to ocean island basalts. Group-3 has elevated SiO2 of 58.6–61.2wt%, intermediate TiO2 contents of 1.2–1.5wt%, the lowest Mg# values of 21–30, and negative anomalies in Nb, Ta, Ti and Y. The synthetic studies of Sr-Nd isotopes indicate two end-members for the formations of the three types of mafic dykes: depleted asthenospheric mantle and metasomatized lithospheric mantle. The upwelling of asthenosphere at ca. 760Ma led to partial melting of the pre-existed metasomatized lithospheric mantle to generate the Group-1 rocks with arc-like geochemical features. Continued extension resulted in low-degree partial melting of asthenospheric mantle and subsequent interaction with Group-1 magmas form the Group-2 rocks. The Group-3 rocks were resulted from AFC processes of the Group-2 magmas. Based on our new data and the available U-Pb ages of Neoproterozoic magmatic rocks, the rifting of South China from Rodinia should have not happened before ca. 820Ma.

Cretaceous alkaline volcanism in south Marzanabad, northern central Alborz, Iran: Geochemistry and petrogenesis

The alkali-basalt and basaltic trachy-andesites volcanic rocks of south Marzanabad were erupted during Cretaceous in central Alborz, which is regarded as the northern part of the Alpine-Himalayan orogenic belt. Based on petrography and geochemistry, en route fractional crystallization of ascending magma was an important process in the evolution of the volcanic rocks. Geochemical characteristics imply that the south Marzanabad alkaline basaltic 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 and depletion of Rb, K and Y, are similar to the OIB pattern and intraplate alkaline magmatic rocks. The K/Nb and Zr/Nb ratios of volcanic rocks range from 62 to 588 and from 4.27 to 9 respectively, that are some higher in more evolved samples which may reflect minor crustal contamination. The isotopic ratios of Sr and Nd respectively vary from 0.70370 to 0.704387 and from 0.51266 to 0.51281 that suggest the depleted mantle as a magma source. The development of south Marzanabad volcanic rocks could be related to the presence of extensional phase, upwelling and decompressional melting of asthenospheric mantle in the rift basin which made the alkaline magmatism in Cretaceous, in northern central Alborz of Iran.

Cretaceous crust–mantle interaction and tectonic evolution of Cathaysia Block in South China: Evidence from pulsed mafic rocks and related magmatism

Cretaceous tectono-magmatic evolution of the Cathaysia Block in South China is important but their mechanism and geodynamics remain highly disputed. In this study we carried out a detailed geochemical study on the recently found Kuokeng mafic dikes in the western Fujian Province (the Interior Cathaysia Block) to reveal the petrogenesis and geodynamics of the Cretaceous magmatism. Kuokeng mafic dikes were emplaced in three principal episodes: ~129Ma (monzogabbro), ~107Ma (monzodiorite), and ~97Ma (gabbro). Geochemical characteristics indicate that the monzogabbros were derived from the unmodified mantle source, while gabbros were likely derived from metasomatized mantle by subducted slab (fluids and sediments). Sr–Nd isotope compositions indicate that the parental magmas of the monzodiorites were generated by mixing of enriched, mantle-derived, mafic magmas and felsic melts produced by partial melting of crustal materials. Until the Early Cretaceous (~123Ma), the dominant ancient Interior Cathaysia lithospheric mantle exhibited insignificant subduction signature, indicating the melting of asthenospheric mantle and the consequent back-arc extension, producing large-scale partial melting of the crustal materials under the forward subduction regime of the paleo-Pacific plate. The monzodiorites and gabbros appear to be associated with northwestward subduction of Pacific plate under an enhanced lithospheric extensional setting, accompanying with mantle modification, which triggered shallower subduction-related metasomatically enriched lithospheric mantle to melt partially. After ca. 110Ma, the coastal magmatic belts formed due to a retreat and rollback of the subducting Pacific Plate underneath SE China in the continental margin arc system.

Mantle podiform chromitites do not form beneath mid-ocean ridges: A case study from the Moho transition zone of the Oman ophiolite

Podiform chromitites from the mantle transition zone at Maqsad in the Oman ophiolite have MORB-like cr# (0.48–0.59). However they show other features which are not in keeping with a MORB-origin: some parental melts have TiO2 values lower than typical MORB, they have a higher Fe3+/ΣFe ratio and a wider range of Fe3+/ΣFe ratios than might be expected in MORB and their parental melts have a higher fO2 than expected for MORB. The chromitites also contain silicate melt inclusions which are hydrous and atypical of MORB and inclusions of higher cr# chromites (cr#=0.55–0.71). These inclusions are derived from a more depleted harzburgite source than that of the harzburgites which now host the Maqsad chromitites. In addition associated MORB-like lavas display geochemical traits which are atypical of MORB. It is proposed therefore that the Maqsad chromitites were derived by mixing from parental melts which were produced by the interaction of a MORB-like melt and depleted mantle. The MORB-like melt was produced by the hydrous melting of asthenospheric mantle above a subducting slab. The trigger for chromite to appear on the liquidus of the melt was the reaction between the primary melt and depleted harzburgite, illustrating the more general relationship in which chromite appears on the liquidus in mafic and ultramafic melts as a consequence of some form of mixing process.Our preferred tectonic setting for the genesis of the Maqsad chromitites is in the context of subduction initiation in a forearc environment. We would argue on the basis of their geochemistry that they have not formed beneath a spreading ridge, despite the superficial similarity between their cr# and those of MORBs. In the absence of podiform chromitites in modern ridge environments and from ‘truly MORB ophiolites’ we propose that the very occurrence of podiform chromitites is a tectonic indicator of a subduction-related environment.

SIMS zircon U–Pb and mica K–Ar geochronology, and Sr–Nd isotope geochemistry of Neoproterozoic granitoids and their bearing on the evolution of the north Eastern Desert, Egypt

Granitic rocks are commonly used as means to study chemical evolution of continental crust, particularly, their isotopic compositions, which reflect the relative contributions of mantle and crustal components in their genesis. New SIMS and K–Ar geochronology, isotope, geochemical, and mineral chemistry data are presented for the granitoid rocks located in and around Gabal Dara in the Northern Eastern Desert of Egypt. The granitoid suite comprises quartz diorites, Muscovite (Mus) trondhjemites, and granodiorites intruded by biotite-hornblende (BH) granites and alkali feldspar (AF) granites. Mus trondhjemite, granodiorite and BH granite exhibit I-type calc alkaline affinities. Mus trondhjemite and granodiorite show medium-K calc-alkaline and metaluminous/mildy peraluminous affinities, whereas BH granites have high-K calc-alkaline and metaluminous character. Concordant 206Pb/238U weighted mean ages together with geochemical peculiarities suggest that Mus trondhjemites (741Ma) followed by granodiorites (720Ma) are genetically unrelated, and formed in subduction-related regime by partial melting of lower oceanic crust together with a significant proportion of mantle melt. The genesis of Mus trondhjemites is correlated with the main event in the evolution of the Eastern Desert, called “~750Ma crust forming event”.The field and geochemical criteria together with age data assign the high-K calc-alkaline BH granites (608–590Ma) and alkaline AF granites (600–592Ma) as post-collisional granites. The differences in geochemical traits, e.g. high-K calc-alkaline versus alkaline/peralkaline affinities respectively, suggest that BH granites and AF granites are genetically unrelated. The age overlap indicating coeval generation of calc-alkaline and alkaline melts, which in turn suggests that magma genesis was controlled by local composition of the source. The high-K calc-alkaline BH granites are most likely generated from lithospheric mantle melt which have been hybridized by crustal melts produced by underplating process. AF granites exhibit enrichment in K2O, Rb, Nb, Y, and Th, and depletion in Al2O3, TiO2, MgO, CaO, FeO, P2O5, Sr, and Ba as well as alkaline/peralkaline affinity. These geochemical criteria combined with the moderately fractionated rare earth elements pattern (LaN/YbN=9–14) suggest that AF granite magma might have been generated by partial melting of Arabian–Nubian Shield (ANS) arc crust in response of upwelling of hot asthenospheric mantle melts, which became in direct contact with lower ANS continental crust material due to delamination. Furthermore, a minor role of crystal fractionation of plagioclase, amphibole, biotite, zircon, and titanomagnetite in the evolution of AF granites is also suggested. The low initial 87Sr/86Sr ratios (0.7033–0.7037) and positive εNd(T) values (+2.32 to +4.71) clearly reflect a significant involvement of depleted mantle source in the generation of the post-collision granites and a juvenile nature for the ANS.

Early Middle Triassic mafic dikes from the Baoshan subterrane, western Yunnan: implications for the tectonic evolution of the Palaeo-Tethys in Southeast Asia

The time of final closure of the Palaeo-Tethys and the Sibumasu-Indochina collision in Southeast Asia represents a major unresolved geologic problem. Here, we present zircon chronology, whole-rock elemental, Sr–Nd, and zircon Hf isotopic geochemistry for newly discovered mafic dikes from the northern segment of the Sibumasu terrane, to provide constraints on this issue. Zircon U–Pb data indicate that the dikes were emplaced at 240 ± 3 Ma. These are the earliest Mesozoic magmatic rocks reported so far in the Sibumasu terrane, the late Palaeozoic passive margin of the Palaeo-Tethys. They are subalkaline tholeiites, showing geochemical characteristics similar to those of enriched mid-ocean ridge basalts (E-MORBs). They have 87Sr/86Sr(t) ratios of 0.703161–0.703826, ϵNd(t) of +4.8 to +7.5, and zircon ϵHf(t) of +9.2 to +13.3, implying strong mantle depletion. They were derived by partial melting of asthenospheric mantle and underwent subsequent fractional crystallization and lithospheric assimilation. The geologic–petrologic evidence suggests that the mafic dikes were generated in a collisional setting, when suturing of the Baoshan and Simao subterranes (the two subterranes are part of the Sibumasu and Indochina terranes, respectively) occurred. These early Middle Triassic mafic dikes provide an upper limit for Sibumasu–Indochina collision. In conjunction with previous work, we conclude that the final closure of the Palaeo-Tethys and collision of the Sibumasu and Indochina terranes took place during the late Permian to Early Triassic.

The Kultuk Volcano: spatial-temporal change of magmatic sources at the western terminus of the South Baikal basin between 18 and 12 Ma

The Kultuk volcano erupted at the axial South Baikal basin of the Baikal rift zone (BRZ). Now it exhibits facies of subvolcanic bodies, land-lava eruptions and subaqueous pillow lavas and hyaloclastites. The volcano was controlled by the Obruchev fault that is currently a border of the basin which amplitude of vertical movements is rapidly decreasing in the westward direction. It is found that the Kultuk volcano was active at the beginning and end of the volcanic activity period of the Kamar, Stanovaya, and Bystrinskaya volcanic zones, which took place 18–12 Ma ago. In previous papers, it was assumed that dominant structures in the area under study were major Cenozoic shear displacements along the Main Sayan fault and/or along the Tunka rift valley; however, at the current stage of our study, linear configurations of the volcanic zones do not reveal any of such displacements. Based on analyses of distribution of volcanic rocks in the relief at the western coast of Lake Baikal, distinct vertical crustal movements are revealed; such movements started in the Early Miocene and continue to the present time. It is concluded that volcanism was controlled by the trans-tensional system of volcanic zones. Sources are identified for the shallow lithospheric mantle melt with the substantial admixture of the low-crust component and deeper asthenospheric mantle melts in the Kamar and Stanovaya volcanic zones; for the Bystrinskaya volcanic zone, only components from the deeper source are revealed. The local shallow mantle magmatism occurred only within the lithosphere extension zone beneath the South Baikal basin. The lithosphere thinning is reflected in the change of activity from the sub-lithospheric to lithospheric sources under the Kamar zone. Rifting of the axial structure is recorded at the root of the Slyudyanka lithospheric block that was subjected to the collision-related Early Paleozoic metamorphism. Geochemical characteristics of the collision-type components were inherited by the Miocene basaltic melts.

Post-orogenic extension in the eastern part of the Jiangnan orogen: Evidence from ca 800–760 Ma volcanic rocks

We present a systematic geochronological and geochemical study on ca 800–760Ma volcanic rocks in the eastern part of the Jiangnan orogen. The Xucun composite dykes are dated at ca 805Ma; the mafic components have OIB-like trace-element patterns and positive anomalies in Zr and Hf. The least-contaminated sample has relatively depleted Nd isotopic features, suggesting the Xucun mafic dykes may have been generated from the partial melting of OIB-like asthenosphere with later crustal contamination. The Xucun felsic dykes have decoupled Nd–Hf isotopes, and the Hf-isotope compositions of zircons indicate that the dykes may be derived from the partial melting of the early Neoproterozoic juvenile crustal materials, with minor incorporation of Paleoproterozoic crustal components. The ca 800–790Ma Shangshu volcanics include two compositional series: calc-alkaline and tholeiitic. The Shangshu calc-alkaline volcanics in the Minjiawu area have low abundances of LILE, HFSE and high Na2O contents and Sr/Y ratios, similar to adakitic rocks. The evident arc-like geochemical features and radiogenic Nd isotopes (ɛNd(t) values of +3.7 to +4.8) suggest that these rocks may have been generated from the partial melting of juvenile lithospheric mantle metasomatized by Na-rich melts released from the subducted slab. The tholeiitic mafic rocks from the Shangshu bimodal volcanics represent two different magma sources. The partial melting of metasomatized lithospheric mantle led to the formation of arc-like basalts with low TiO2 contents, negative anomalies in Zr and Hf, and high values of Mg# and ɛNd(t) (+6.2), whereas the partial melting of asthenospheric mantle generated volcanic rocks with high TiO2 contents and low positive ɛNd(t) (+1.4 to +2.7), without negative anomalies of Nb, Ta, Zr and Hf. The Shangshu felsic rocks were formed by the reworking of early Neoproterozoic juvenile arc crustal materials. The ca 760Ma mafic rocks from the Puling bimodal volcanics generally have low TiO2 contents (<0.9wt%), nearly flat REE distributions and arc-like trace-element patterns. They may have been generated from the high-degree partial melting of metasomatized lithospheric mantle. One sample has a high TiO2 content (2.41wt%) and high ɛNd(t) (+6.2), with overall OIB-like trace-element patterns, implying the local partial melting of asthenospheric mantle. The occurrence of significant volumes of bimodal volcanics in the eastern part of the Jiangnan orogen suggests an extensional setting in the period ca 800–760Ma. The evident partial melting of newly-metasomatized lithospheric mantle and subordinate partial melting of asthenosphere suggest that post-orogenic extension shortly after the Neoproterozoic orogenesis may be a better explanation for the genesis of the mid-Neoproterozoic magmatic rocks in the eastern part of the Jiangnan orogen. Post-orogenic extension may be diachronous along the whole orogenic belt, and probably has no direct relationship with the Rodinia rifting event. A more detailed model is presented to illustrate the evolution of the eastern part of the Jiangnan orogen.

The Nebo-Babel Ni-Cu-PGE Sulfide Deposit (West Musgrave Block, Australia): Pt. 2. Constraints on Parental Magma and Processes, with Implications for Mineral Exploration

The Nebo-Babel Ni-Cu-platinum-group element (PGE) sulfide deposit (West Musgrave, Australia) is hosted in a gabbronorite chonolith emplaced into sulfur-free country-rock orthogneiss at ca. 1068 Ma. Five different types of mafic dikes are found in the area or are spatially associated with the Nebo-Babel intrusion. The mafic dikes are divided into three groups based on petrography, whole-rock major, trace, and PGE chemistry, Sm-Nd isotope ratios, and silicate mineral chemistry: (1) low Ti basalts (NB-1, NB-2, and NB-3); (2) high Ti basalts (NB-4); and (3) alkali basalts (NB-5). Recent age constraints on similar magma suites intruding in the west Musgrave indicate the low and high Ti basalts are coeval with the Nebo-Babel intrusion and hence can potentially be equivalent to its parental magma composition(s). Based on this, we use whole-rock major, trace, and PGE chemistry, Sm-Nd isotope data, silicate mineral chemistry, and geochemical modeling to constrain the petrogenesis of each mafic dike type and its potential relationships with the Nebo-Babel intrusion. Our results indicate that crustal contamination did not play a major role in the generation or evolution of the different magma series. The geochemical variations observed are rather interpreted to reflect different mantle source compositions and different degrees of partial melting. The low Ti basalts exhibit subduction-related geochemical signatures and are interpreted to have been generated by 5 to 10 percent partial melting of a hydrous spinel-bearing mantle. The melting is inferred to have been triggered as a result of mantle plume impingement in an area of the mantle that has previously been metasomatized. The high Ti basalts are interpreted to have formed by mixing of the sublithospheric mantle with deeper asthenospheric mantle melts from the mantle plume. The decompressional melting of the plume head led to the formation of alkali basalts (characterized by typical ocean island basalt (OIB)-like compositions) generated by 4 to 5 percent of partial melting of a garnet-bearing lherzolite. Our results show that the Nebo-Babel intrusion and its associated Ni-Cu-PGE sulfide mineralization formed between the intrusion of the low- and high Ti basalts and may have originated from either mixing between these two magma types, or as a result of continuous change in the melting conditions between these two magmas types. The spatial analysis of the repartition of the different magma suites allowed highlighting zones where the two magma types are spatially associated and where Ni-Cu-PGE sulfide anomalies were discovered, suggesting that magma mixing may be a key factor in the ore genesis at Nebo-Babel. This observation is strengthened by the modeling of the sulfur concentrations at sulfide saturation, which indicates that the assimilation of country-rock orthogneiss by the mixed magma caused a drop in sulfur solubility which in turn led to sulfide saturation in the early stages of the mixing and/or assimilation and potentially to the formation of the Nebo-Babel Ni-Cu-PGE sulfide deposit. Calculated metal concentrations of the parental magma from which the Ni-Cu-PGE mineralization formed are: 165 ppm Ni, 0.15 ppb Ir, 0.30 ppb Ru, 0.22 ppb Rh, 3.40 ppb Pt, 3.30 ppb Pd, and 150 ppm Cu. These metal concentrations are similar to those obtained from the low Ti basalt compositions after they experienced a small amount (<0.001%) of sulfide segregation prior to their final emplacement.

Post-collisional plutons in the Balikun area, East Chinese Tianshan: Evolving magmatism in response to extension and slab break-off

The Late Carboniferous to Permian is a critical period for final amalgamation of the Central Asian Orogenic Belt (CAOB). Gabbroic and granitic intrusions formed in this period at Balikun, eastern Tianshan, and therefore provide important clues for understanding the tectonic events that took place at this time. The Shiquanzi gabbro was formed at 301 ± 6 Ma and consists of low-Si (LS) (SiO 2 < 47 wt.%) and high-Si (HS) (SiO 2 > 48 wt.%) members, and both are characterized by high Nb (> 9 ppm), high εNd (t) (+ 7.12–+ 8.71) and low initial 87Sr/ 86Sr ratios (0.7030–0.7041). The HS gabbros have HFSE (Nb = 17.5–24.2 ppm) contents, and Nb/Y (0.56–0.72) and Ta/Yb (0.52–0.60) ratios, higher than those of the LS samples and exhibit E-MORB-like characteristics, while the LS samples have higher Ba/Th ratios (150–236) and more prominent subduction-related signatures than those of the HS samples (Ba/Th = 10–91). The HS rocks probably resulted from decompressional melting of asthenospheric mantle, whereas the LS rocks originated from a depleted mantle source metasomatised by slab-derived fluid. The Shiquanzi gabbro therefore reflects an extensional environment, in which asthenospheric mantle upwelled and triggered partial melting of metasomatised depleted mantle at a shallower level. Granitic intrusions at Balikun (Daliugou Pluton, Dajiashan Pluton and Barkol Tagh Batholith) formed in the Early Permian (284–288 Ma), and generally have alkali-calcic compositions and exhibit characteristics of A2-type granite. Their variable Nd (εNd (t) = + 1.92–+ 6.03) and Sr ( 87Sr/ 86Sr (i) = 0.7040–0.7152) isotope compositions and young T DM model ages (0.56–0.75 Ga) suggest that they were derived from juvenile crustal sources, probably earlier arc intrusions. From the Late Carboniferous to Early Permian, magmatism at Balikun evolved from gabbroic bodies to large A2-type granitic intrusions, implying a gradually increasing impact of mantle-derived magma on the crust. A slab break-off regime, following accretion of the Harlik Arc onto the Angara continent, may explain the post-collisional magmatism and geodynamic transition of the eastern Tianshan.

Volcanism of the Nanpu Sag in the Bohai Bay Basin, Eastern China: Geochemistry, petrogenesis, and implications for tectonic setting

The recently discovered large Nanpu Oilfield is located in the NW part of the Bohai Bay Basin, eastern China. Extensive drilling and geophysical research have revealed widespread occurrence of Eocene to Miocene mafic volcanic rocks. On the basis of stratigraphic constraints, three cycles of volcanism have been recognised. Petrological and geochemical studies of drill core samples indicate that most of these mafic rocks are alkaline basalts. Some basalts experienced fractional crystallization, dominated by clinopyroxene and FeTi oxides. All samples have trace elements patterns similar to oceanic island basalts (OIB). Sr and Nd isotopic analyses show that 87Sr/ 86Sr i values range from 0.703577 to 0.708160 and εNd( t) varies from 0.7 to 6.8. Pb isotope analyses yield Pb ratios ranging from 17.43 to 19.19 for 206Pb/ 204Pb, from 15.43 to 15.67 for 207Pb/ 204Pb and from 37.35 to 38.64 for 208Pb/ 204Pb. The trace element and isotopic characteristics indicate that the Nanpu Sag basalts are similar to oceanic island basalts possibly with some contributions of subcontinental lithospheric mantle. Considering that there is widespread and coeval basaltic volcanism in the Bohai Bay Basin, which has comparable geochemistry, we adopt a model of episodic upwelling and melting of asthenospheric mantle for the origin of these basalts. The old subcontinental lithospheric mantle had interacted with new asthenospheric mantle-derived melts. The replacement of old North China Craton mantle to new and fertile asthenospheric mantle is still ongoing. Early-stage magmas were generated from deeper in the mantle with a higher degree of partial melting than late stage magma. The upward migration of the melting column supported active asthenosphere mantle upwelling or mantle plume induced magmatism under an extensional regime.

Causes and effects of geochemical variations in late Cenozoic volcanism of the Foça volcanic centre, NW Anatolia, Turkey

The Foça volcanic centre (FVC) occupies a NNE–SSW-oriented highland between two EW-trending structural grabens in western Anatolia, and includes early–middle Miocene mafic and felsic extrusive suites. Its evolutionary history consists of an older volcano stage (16.6–16.1 Ma) and a younger volcano stage (15.2–14.1 Ma), which are characterized by different eruption styles and compositional and geochemical features. The older units include high-K calc-alkaline basalt, andesite, trachyandesite, rhyolite, and associated pyroclastic rocks, which formed during ignimbrite eruptions and plinian–subplinian air-fall episodes. The younger sequences are composed of shoshonitic–alkaline basalt lavas and dikes, trachytes, phonolites, and phonolitic ignimbrites that formed strombolian cones. The Foça volcanic rocks display high initial 87Sr/86Sr ratios (0.7075–0.7082 for the calc-alkaline mafic lavas, 0.7073–0.7064 for calc-alkaline felsic lavas, and 0.7063–0.7075 for the alkaline series) and low 143Nd/144Nd (0.5123–0.5125 in both series with ϵNd values varying from − 1.3 to − 6.0). These FVC geochemical features are consistent with those of other volcanic centres in western Anatolia (i.e. Bodrum, Urla-Cumaovası) and on the Aegean islands (i.e. Samos, Patmos, Chios). The geochemical and Sr–Nd isotopic compositions of the Foça volcanic units suggest that both lithospheric and asthenospheric mantle melts were involved in their evolution; however, the mantle lithosphere fingerprint was diminished by the middle Miocene, as the asthenospheric mantle melt input became dominant. These findings, combined with the bimodal character of post-collisional volcanism in the study area, suggest that geochemical variations in the nature of volcanism from calc-alkaline to alkaline and the changes in tectonic regimes through time may have been caused by successive thermal relaxations associated with possible ‘piecemeal’ removal of the base of subcontinental lithospheric mantle beneath western Anatolia. This interpretation is more plausible than a catastrophic collapse or wholesale delamination of the entire lithospheric mantle. Asthenospheric upwelling caused by this inferred convective thinning provided underplating of mantle-derived magmas, which interacted with the previously metasomatized lithospheric mantle and the overlying crust, resulting in their partial melting and in production of high-K calc-alkaline to mildly alkaline, incompatible element enriched magmas in separate magma chambers in which fractional crystallization occurred.

Petrology, geochemistry and U–Pb zircon geochronology of lower crust pyroxenites from northern Apennine (Italy): insights into the post-collisional Variscan evolution

Spinel pyroxenites occur locally as clasts in polygenic breccias from the Late Cretaceous sedimentary melanges of the Northern Apennine (Italy). They are of cumulus origin and formed in the deep crust by early precipitation of clinopyroxene and minor olivine and late crystallisation of orthopyroxene, spinel, Ti-pargasite and sulphides. Pyroxenites underwent high-temperature (~850°C) subsolidus re-equilibration and ductile deformation with development of mylonitic bands made of clinopyroxene, orthopyroxene, Ti-pargasite and spinel. U–Pb geochronology on zircons revealed the occurrence of inherited grains of Early Proterozoic to Late Devonian age. The inherited zircons are locally rimmed by recrystallised zircon domains. The oldest rims yield a mean concordia U–Pb age at 306 ± 8 Ma, which is considered to date the emplacement of the pyroxenites, in the framework of the post-Variscan lithospheric extension. The incompatible element compositions of calculated melts in equilibrium with clinopyroxenes from the pyroxenites are characterised by Ba, Nb, LREE and Sr enrichment relative to N-MORB. The depleted Nd isotopic signature of the pyroxenites (initial eNd values of +5.3 to +6.1) may be thus linked to primary magmas produced by low degrees of melting of asthenospheric mantle. In addition, the pyroxenites locally record the infiltration of plagioclase-saturated hydrous melts, most likely evolved through fractional crystallisation and enriched in highly incompatible elements, within the clinopyroxene-dominated crystal mush. A thermal event in Late Permian–Middle Triassic caused the partial resetting of zircon U–Pb system.

Middle to late Jurassic felsic and mafic magmatism in southern Hunan province, southeast China: Implications for a continental arc to rifting

Extensive middle to late Jurassic felsic and mafic magmatism occurred in the southern Hunan province, southeast China. SHRIMP zircon U–Pb dating, mineral chemical, element geochemical and Sr–Nd–Hf isotopic data have been determined for these rocks. The results indicate that the middle Jurassic (178–170 Ma) Changchengling and Ningyuan basaltic rocks belong to tholeiitic series and alkaline series, respectively, and were formed by fractional crystallization of the parent magmas that were derived from partial melting of asthenospheric mantle triggered by the slab-released fluids of the subducted Paleo-Pacific plate. The late Jurassic (152–146 Ma) Daoxian basalts and Guiyang lamprophyre dikes are low-Ti, high-Mg potassic rocks. They were derived directly by partial melting of shallower (60–100 km) lithospheric mantle composed of amphibole- and phlogopite-bearing lherzolite in response to thermal perturbations associated with the rifting of continental arc. The middle Jurassic (164 Ma) Tongshanling granitic rocks belong to calc-alkaline series and are strongly peraluminous. They were formed by partial melting of Palaeoproterozoic metasedimentary basement in the lower-crust plus additional input from coeval basaltic magma. The late Jurassic (156 Ma) Jinjiling and Xishan granitic rocks show A 2 subtype affinity. They were generated by partial melting of granulitized Palaeoproterozoic metamorphic basement in the lower crust in response to injection of coeval low-Ti, high-Mg potassic magmas. A moderate degree (56–58%) of fractionation of these pure crustal melts could account for more felsic end-member of granitic rocks. Detailed petrologic and geochemical data for the middle to late Jurassic felsic and mafic rocks from the southern Hunan imply that during the middle Jurassic time SE China on the southeast of the Shi-Hang zone was a continental arc coupled with the subduction of the Palaeo-Pacific plate and since the beginning of late Jurassic time an intra-arc rift has been formed along the Shi-Hang zone as a consequence of slab roll-back.

Geochronology and Geochemistry of the Kuwei Mafic Intrusion, Southern Margin of the Altai Mountains, Northern Xinjiang, Northwest China: Evidence for Distant Effects of the Indo‐Eurasia Collision

The Kuwei mafic intrusion, consisting of hornblende gabbro, gabbro, gabbro norite, and olivine norite, lies in the southern Altai Mountains, northern Xinjiang. A combined field, geochronological, and geochemical study of the Kuwei intrusion is reported here. This study provides the first reliable SHRIMP U‐Pb zircon dating results for the intrusion, and these yielded an age of $$47\pm 1$$ Ma, which is the first documented report of Eocene magmatism in the region. The chondrite‐normalized rare earth element patterns for the Eocene intrusions are flat, and most of the incompatible elements are comparably depleted. Thus, geochemical data suggest that the Kuwei mafic intrusion was produced by partial melting of asthenospheric mantle that was slightly contaminated by lithospheric material. We interpret the 47‐Ma magmatism to result from asthenospheric mantle upwelling following the progressive India‐Eurasian collision. Although the Kuwei intrusion is laterally beyond the limit of Eocene deformation normally attributed to the India‐Asia collision, the timing of magmatism in the intrusion suggests that lateral extension may have initially affected a wider region than the area later thickened by convergence in the Tibetan Plateau. The Kuwei intrusion and other plutons likely related to it may have been emplaced into dilational jogs in fault systems activated by the India‐Asia collision. The emplacement depth is estimated to be ∼6 km, based on geobarometric determinations. Erosion was imperceptible before 25 Ma but has worn away an average of 0.024 cm of uplift every year since 25 Ma. The 6 km of exhumation since the late Oligocene is also attributed to far‐field effects of the India‐Asia collision.

Cenozoic Crustal Evolution and Mantle Dynamics of Post-Collisional Magmatism in Western Anatolia

Post-collisional magmatism in western Anatolia followed a continental collision event in the Early Eocene, and occurred in discrete pulses that appear to have propagated from north to south over time. The first episode occurred during the Eocene and Oligo-Miocene and was subalkaline in nature, producing medium-to high-K calc-alkaline granitoids and mafic to felsic volcanic rocks. Partial melting and assimilation-fractional crystallization of enriched subcontinental lithospheric mantle-derived magma(s) were important processes in the genesis and evolution of the parental magmas, which experienced decreasing subduction influence and increasing crustal contamination through the Early Eocene-Early Miocene. This magmatic episode coincided with continued regional compression and development of a thick orogenic crust, and was influenced by an influx of asthenospheric heat and melts provided by lithospheric slab break-off. Extensional tectonics replaced the regional compression by the Middle Miocene, following the initial collapse of the western Anatolian orogenic welt, and resulted in the development of metamorphic core complexes and horst-graben structures. The second main episode of magmatism occurred during the Middle Miocene (16-14 Ma) and produced mildly alkaline rocks that show a decreasing amount of crustal contamination and subduction influence through time. Although melting of a subduction-modified lithospheric mantle continued, an asthenospheric mantle-derived melt contribution played a major role in the generation of these mildly alkaline magmas. The inferred asthenospheric melt contribution was a result of delamination of the lowermost part of the lithospheric mantle and/or partial convective removal of the sub-continental lithospheric mantle (SCLM). The third episode of post-collisional magmatism started around ~12 Ma and continued through the Late Quaternary. The main melt source for this phase carried no subduction component and was generated by the decompressional melting of asthenospheric mantle, which flowed in beneath the attenuated continental lithosphere in the Aegean extensional province. Lithospheric-scale extensional fault systems acted as natural conduits for the transport of uncontaminated alkaline magmas to the surface. Post-collisional magmatism in western Anatolia thus displays compositionally distinct episodes controlled by slab break-off, lithospheric delamination, and asthenospheric upwelling and decompressional melting, reflecting the geodynamic evolution of the eastern Mediterranean region throughout the Cenozoic. These events and the associated processes in the mantle took place primarily in response to the plate tectonic evolution of the region and collectively constitute a time-progressive template for the mode and nature of the post-collisional magmatism common to most alpine-style orogenic belts.