Depletion In High Field Strength Elements Research Articles

Recycling of sediment into the mantle source of K-rich mafic rocks: Sr–Nd–Hf–O isotopic evidence from the Fushui complex in the Qinling orogen

Potassium (K)-rich mafic rocks are viewed as being derived from partial melting of an enriched mantle source, but it is controversial about which processes cause the mantle enrichment. The Fushui intrusive complex is the largest early Paleozoic K-rich intrusive complex in the eastern Qinling orogen. Therefore, detailed studies on the Fushui complex can contribute not only to understanding of the petrogenesis of K-rich mafic rocks, but also to unraveling the Paleozoic evolution of the Qinling orogen. In this study, we provide an integrated investigation of in situ zircon U–Pb dating and Hf–O isotopes, in combination with whole-rock major and trace elements, as well as Sr–Nd–Hf isotopes, for the Fushui intrusive complex. In situ zircon secondary ion mass spectrometry (SIMS) / laser ablation induction coupled plasma mass spectrometry (LA-ICPMS) U–Pb dating reveals that different rock types of the Fushui complex have identical formation ages of 488–484 Ma. The Fushui complex belongs to the shoshonitic series, and is characterized by extreme large ion lithophile element (LILE, e.g., Ba, U, Th and Sr) and Pb enrichment and depletion of high field-strength elements (HFSEs, e.g. Nb, Ta, Zr, Hf, P and Ti). It shows high initial Sr isotopic ratios (0.7100–0.7151), negative whole-rock e Nd(t) (−3.97 to −5.68) and negative to slight positive whole-rock (−2.24 to 2.38) and zircon (−2.85 to 0.34) e Hf(t) values, as well as high zircon δ18O values (6.86 ± 0.13 ‰). The Hf–Nd isotopic systems are decoupled with positive Δe Hf values (3.85–5.37). These geochemical features indicate that the mantle source has incorporated subducted zircon–barren oceanic sediments. A simple two-end-members mixing model constrains the amount of subducted sediments in the Fushui mantle source to 5–8 %. The Fushui complex originated from 1 to 6 % equilibrium melting of a phlogopite-bearing garnet lherzolite by non-modal melting. As shoshonitic magmas have been discovered in modern nascent arcs, we suggest that the generation of the Fushui complex was induced by the subduction of the Paleotethyan Ocean, when it jumped from the northern to the southern boundary of the North Qinling microcontinent.

Constraints on the mobilization of Zr in magmatic-hydrothermal processes in subduction zones from in situ fluid-melt partitioning experiments

The partitioning of Zr between high P-T aqueous fluids and melts has been investigated in situ in the haplogranite-H2O and haplogranite-(F)-H2O systems to assess the mobilization of high field strength elements (HFSE) in magmatic-hydrothermal processes in subduction zones. The partition coefficients Df/Zmr were determined from Zr concentrations measured in situ by synchrotron X-ray fluorescence (SXRF) in both aqueous fluids and F-free or F-bearing hydrous haplogranite melts equilibrated in diamond-anvil cells at 575 to 800 °C and 0.3 to 2.4 GPa. This experimental approach eliminates the need for internal or external calibrations of the SXRF signal and/or post-mortem analysis of the melt phase, hence decreasing the total uncertainties on Df/Zmr below 16%. Above 0.6 GPa, Zr partitions favorably into the hydrous silicate melt in both F-free and F-bearing systems, with Df/Zmr that range between 0.19 ± 0.02 and 0.38 ± 0.03. However, the relatively high Df/Zmr values indicate that alkali-silica rich aqueous fluids generated by metamorphic devolatilization may contribute significantly to the recycling of HFSE in subduction zones. The efficient uptake of Zr (and likely other HFSE) by subduction zone fluids, regardless of their nature (aqueous fluid, hydrous melt, or supercritical fluid), supports the idea that the typical HFSE depletion recorded in arc magmas does not result from their incompatibility in water-rich slab-derived fluids but most probably originates from complex fluid-melt-rock interactions occurring at the slab interface and within the mantle wedge. At shallow crustal pressure conditions (800 °C and 0.3 GPa), Zr partitions reversely into the aqueous fluid in the presence of fluorine (Df/Zmr = 1.40 ± 0.10) as observed for Nb at similar conditions by Webster et al. (1989). The enrichment of the aqueous phase in HFSE (Zr, Nb) at shallow crustal conditions is likely related to the enhanced peralkalinity of low pressure, F-bearing aqueous fluid with temperature, that provides the favorable conditions for their mobilization via the formation of HFSE-O-Si/Na clusters. This mechanism may control the enrichment in HFSE (and plausibly other rare metals such as REE) in early magmatic fluids exsolved from granitic melts, leading to the formation of HFSE-enriched aggregates in shallow magmatic-hydrothermal environments (e.g., Strange Lake and Thor Lake Nechalacho deposit, Canada; Galineiro complex, Spain).

Composition and structure of the lithospheric mantle beneath NE Iran: Constraints from mantle xenoliths

A detailed study on petrology and mineral chemistry of 32 mantle xenoliths has been conducted to decipher the physical and chemical characteristics of the lithosphere beneath NE Iran. Spinel lherzolite, the most abundant xenolith type, is made up of olivine, orthopyroxene, clinopyroxene, and spinel. Clinopyroxenes in the spinel lherzolites display a primitive mantle-like composition, typical of non-cratonic peridotites. Pyroxenite, another major xenolith type, shows equilibrated textures and highly variable compositions including olivine websterite, websterite and clinopyroxenite. These pyroxenites, together with an equigranular dunite, delineate a clear metasomatic trend, characterized by systematic Mg#, Cr#, Al 2 O 3 , and TiO 2 variations in the constituent minerals, coupled with light rare earth element enrichment and high field strength element depletion in clinopyroxene. The pyroxenites are therefore suggested to have formed by the interaction between garnet-bearing peridotites within the lithospheric mantle and melts from a stagnant slab within the asthenosphere. The lithospheric mantle may have undergone multiple stages of partial melting. The earliest stage, evidenced by the equigranular dunite, resulted in significant NiO depletion in olivine, low Al 2 O 3 and TiO 2 coupled with high Mg# and Cr# in clinopyroxene, and high Cr# in spinel. The second stage occurred more widely and gave rise to the large ion lithophile element depletion in clinopyroxenes of all rock types. The extent of melting is lower in the spinel lherzolites than that in the pyroxenites, implying that the partial melting was not caused by decompression and thus most likely related to Tethyan subduction. A third and more recent melting stage, responsible for the spongy texture in some clinopyroxenes, is attributed to the extensional tectonic regime that started in the middle Miocene in the region. Temperature estimates show that both the spinel lherzolites and pyroxenites equilibrated at ~ 900–1000 °C. Based on our new data and published data we propose the lithospheric structure beneath NE Iran. The xenolith-defined geotherm appears to be hotter than that typical in cratonic and oceanic settings, but colder than that suggested by seismic tomographic modeling for the entire Iranian Plateau. This difference is probably due to the widespread occurrence of pyroxenites within the lithospheric mantle, which we argue could have played a significant role in causing not only the seismic and gravitational anomalies but also late Cenozoic volcanism on the Iranian Plateau. • Primitive mantle-like composition of the upper lithospheric mantle beneath NE Iran; • Subduction-related mantle metasomatism contributing to the genesis of the pyroxenites; • NE Iranian lithospheric mantle reconstructed using data from unique xenolith suite.

Hafnium isotope evidence for slab melt contributions in the Central Mexican Volcanic Belt and implications for slab melting in hot and cold slab arcs

This study presents evidence that Quaternary frontal arc calc-alkaline lavas from Central Mexican Volcanic Belt (CMVB) contain contributions from partial melts of the subducting garnet-bearing eclogitic oceanic crust and sediment, based on chemical and Hf–Nd isotope data. The CMVB includes both calc-alkaline lavas with arc-type trace element patterns such as aqueous fluid mobile element enrichments and high field strength element depletions; and “high-Nb” alkaline lavas with trace element patterns similar to ocean island basalts. The two types of lavas are closely related geographically and temporally. Distinct from the high-Nb lavas, the calc-alkaline lavas show trends toward higher 176Hf/177Hf and 143Nd/144Nd ratios coupled with lower Lu/Hf. The high Hf–Nd isotope ratios fingerprint contributions of subducted basaltic ocean crust, while the correlation with low Lu/Hf indicates melting in the presence of residual garnet, which reflects conversion of the subducted oceanic crust to eclogite. Isotopic and chemical mass balance considerations indicate that the slab melts are ~80% basaltic oceanic crust and ~20% subducted sediment. The calc-alkaline lavas have higher SiO2 at a given Mg# compared to the high-Nb alkaline lavas, also reflecting melt contributions from the subducted slab.A survey of global arc lavas shows that calc-alkaline lavas with low Lu/Hf ratios, reflecting melting in the presence of residual garnet and preferential mobilization of Hf over Lu from the subducted slab, are generally associated with hot slab conditions. These include arcs where young (<30Ma old) ocean crust is subducted (e.g. Mexican Volcanic Belt, Cascades, Austral Andes, Luzon, Setouchi), where slab tearing occurred and hot asthenospheric mantle could upwell through the slab window (e.g., western Aleutians, Sunda, southern Scotia), and where oblique or slow subduction leads to higher slab temperatures (e.g. Lesser Antilles, western Aleutians). In some of these hot slab arcs, where low Lu/Hf ratios are coupled with high Nd–Hf isotope ratios, slab melt contributions are dominated by partial melts from the subducted oceanic basalt (e.g., Mexican Volcanic Belt, Aleutians and Cascades). In other hot slab arcs, low Lu/Hf ratios are coupled with low Nd–Hf isotope ratios, reflecting slab contributions dominated by sediment melts (e.g. Setouchi, Lesser Antilles, Luzon, Sunda, and southern Scotia). Arcs associated with colder subducted oceanic crust (e.g. Izu–Bonin–Marianas, Tonga–Kermadec, central and northern Scotia) erupt lavas with high Lu/Hf along with high Hf–Nd isotope ratios, similar to mid-ocean ridge basalts, thus they lack the signature of residual garnet as well as significant slab melt input.

Petrogenesis and tectonic significance of Paleoproterozoic meta-mafic rocks from central Liaodong Peninsula, northeast China: Evidence from zircon U–Pb dating and in situ Lu–Hf isotopes, and whole-rock geochemistry

The central Liaodong Peninsula is located in the northwestern part of the Jiao-Liao-Ji Belt in the Eastern Block of the North China Craton. Surface rocks in this region are voluminous meta-mafic rocks, which mainly comprise amphibolite and metamorphic gabbro or diabase. We present zircon cathodoluminescence images, U–Pb dates, and Lu–Hf isotope data, as well as whole-rock geochemical data for these rocks, in order to constrain the tectonic nature and evolution of the Jiao-Liao-Ji Belt. Zircons from these meta-mafic rocks can be divided into two groups, with one having a typical magmatic origin and the other a metamorphic origin. Zircon U–Pb dating and Lu–Hf isotopic results show that the magmatic zircons have two peak ages at 2503 and 2154Ma, with the former have ɛHf=+7.1 to +8.1 and single-stage model ages (THf)=2.50–2.55Ga, indicating that these captured or inherited zircons formed at ca. 2.50Ga by partial melting of coeval juvenile crust. The younger magmatic zircons have ɛHf=+3.2 to +9.6, mainly varying from +7.4 to +9.6 (average=+8.7), and THf=2.15–2.34Ga (peak at 2.19Ga), suggesting that the meta-mafic rocks were emplaced at ca. 2.15Ga and derived by partial melting of depleted mantle. The metamorphic zircons yield a peak age of 1897Ma and have ɛHf=−4.5 to +6.8. These zircons can be further divided into three groups on the basis of Hf isotope data (i.e., ɛHf=−4.5 to −2.3, −0.6 to +1.0, and +3.1 to +6.8; THf=2.39–2.54, 2.26–2.39, and 2.03–2.11Ga, respectively), indicating they were derived by reworking of Neoarchean–Paleoproterozoic ancient crust and some Paleoproterozoic juvenile crust. Thus there existed two crustal growth events at 2.50 and 2.15Ga and one reworking event at ca. 1.90Ga in the Eastern Block. Geochemical data show that these meta-mafic rocks are characterized by: SiO2=47.3–49.8wt.%; TiO2=0.97–1.59wt.%;K2O=0.39–1.95wt.%; Na2O/K2O=1.03–4.85; Mg#=39–52; Cr=29–247ppm; and Ni=24.4–73.4ppm. These features, combined with low total rare earth element concentrations (ΣREE), relative enrichment in light REE, significant positive Rb, Ba, K, and P anomalies, and depletions in high-field strength elements (e.g., Nb, Ta, and Ti), indicate derivation by partial melting of depleted lithospheric mantle that had been metasomatized by subduction-derived fluids, with little or no crustal contamination during magma ascent. Given our dating results and the regional geological framework, we propose that the subduction in the eastern Longgang Block took place at least in age of 2.15Ga, and that the meta-mafic rocks in the central Liaodong Peninsula may have formed in a back-arc basin. However, the tectonic location of this magmatic arc and its relationship with the Nangrim Block need to be further studied to confirm this hypothesis.

Melt– and Fluid–Rock Interaction in Supra-Subduction Lithospheric Mantle: Evidence from Andesite-hosted Veined Peridotite Xenoliths

We report petrographic, major and trace element data for xenoliths from the andesitic Avacha volcano (Kamchatka), which host orthopyroxene (opx)-rich veins of mantle origin formed either by rapid crystallization of intruded melts or by their interaction with the host harzburgite. Studies of such veins may give better insights into sub-arc mantle processes (in particular on a millimetre to centimetre scale) than those of (1) arc xenoliths that do not preserve solidified initial metasomatizing agents, (2) massif peridotites, probably modified during their emplacement, or (3) arc magmatic rocks, which provide indirect information. We seek to trace the evolution of these agents as they react with the host peridotite and to assess their impact on the wall-rocks. The veins cross-cut spinel harzburgite and consist mainly of opx with minor olivine, clinopyroxene (cpx) and/or amphibole. We identify 'rapidly crystallized' veins that cut wall-rock olivine without petrographic evidence of reaction, and 'reactive' veins subdivided into 'thick' (0*5-1 mm) and 'thin' (<€0*5 mm). Minerals in the rapidly crystallized veins are depleted in rare earth elements (REE) and high field strength elements (HFSE) and enriched in fluid-mobile elements relative to REE. Minerals in the reactive veins have higher Ti, Al, Cr and alkalis than minerals in the rapidly crystallized veins, as well as highly variable trace element abundances, especially in reaction zones, thin veins and related metasomatic pockets in the host peridotite. They commonly show U-shaped REE patterns and positive Zr and Hf spikes in normalized trace element patterns. Our data, supported by recent reports, show that the rapidly crystallized veins formed between 1200°C and 900°C from a liquid derived by fluid-fluxed melting of a refractory (harzburgitic) mantle source depleted in heavy REE. The reactive veins formed via 'fractionation-reactive percolation' from fractionated hydrous derivatives of the melts that precipitated the rapidly crystallized veins. These liquids re-equilibrated with the host through diffusion and fluid-assisted dissolution-precipitation reactions, whose end-products are thin reactive veins and metasomatic pockets with distinctive U-shaped REE patterns and Zr-Hf spikes. Some Avacha xenoliths contain veins of Fe-rich amphibole deposited from the host magma that penetrated fractures in the peridotite fragments during their transport to the surface. These products of contamination were mistakenly attributed to mantle metasomatism in previous studies of other suites of Avacha mantle xenoliths. Trace element abundances in such veins are higher than for reactive veins of mantle origin, but both have similar trace element patterns (U-shaped REE patterns and Zr-Hf spikes) suggesting that 'fractionation-reactive percolation' also took place during their formation and is common during interaction of refractory peridotites with percolating melts and fluids. Metasomatic pockets of cpx and amphibole replacing coarse opx and spinel in the host peridotites commonly occur in the vicinity of fractures that lead to reactive opx-rich veins. The cpx and amphiboles in the pockets show progressive depletion in middle REE and HFSE at constant light REE and/or large ion lithophile elements away from the veins towards the host. This indicates that residual hydrous fluids expelled from the source veins enriched the wall-rock peridotites in incompatible elements but were progressively modified by reaction with the host with increasing percolation distance. This process produces disseminated pockets of metasomatic minerals with a broad range of compositions from a single initial liquid and strongly affects the trace element budgets of the harzburgite xenoliths from Avacha. We show that melts and fluids are likely to undergo profound transformation as they travel through and react with the refractory host mantle, even on a millimetre to centimetre scale. The composition of the initial metasomatizing agents can only be inferred from the composition of the metasomatic phases in mantle rocks combined with appropriate partition coefficients if these phases come from well-equilibrated mineral assemblages located close to melt and/or fluid sources.

Neoarchaean crustal growth by combined arc–plume action: evidence from the Kadiri Greenstone Belt, eastern Dharwar craton, India

Abstract Field and geochemical studies combined with laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U–Pb dating set important constraints on the timing and petrogenesis of volcanic rocks of the Neoarchaean Kadiri greenstone belt and the mechanism of crust formation in the eastern Dharwar craton (EDC). The volcanic rocks are divided into three suites: tholeiitic basalts, calc-alkaline high-Mg# andesites and dominant dacites–rhyolites. The basalts (pillowed in places) show flat rare earth element (REE) and primordial mantle-normalized trace element patterns, but have minor negative Nb and Ta anomalies. They are interpreted as mantle plume-related oceanic plateau basalts whose source contained minor continental crustal input. The andesites are characterized by high Mg# (0.66–0.52), Cr and Ni, with depletion of high-field strength elements (HFSE) and enrichment of light REE (LREE) and large-ion lithophile elements (LILE). They were probably derived from a metasomatized mantle wedge overlying a subducted slab in a continental margin subduction zone. The dacites–rhyolites are silicic rocks (SiO 2 =61–72 wt%) with low Cr and Ni, K 2 O/Na 2 O mostly 0.5–1.1, highly fractionated REE patterns, enrichments of LILE and distinctly negative HFSE anomalies. One rhyolite sample yielded a zircon U–Pb age of 2353±32 Ma. This suite is similar to potassic adakites and is explained as the product of deep melting of thickened crust in the arc with a significant older crustal component. Collision between a continental margin arc with an oceanic plateau followed by slab break-off, upwelling of hot asthenosphere and extensive crustal reworking in a sustained compressional regime is proposed for the geodynamic evolution of the area. This is in corroboration with the scenario of EDC as a Neoarchaean hot orogen as suggested recently by some workers. Supplementary material: Details of whole-rock major and trace element determination, Nd isotope analysis and zircon U–Pb dating and trace element analysis, the geographical coordinates of the samples and the values of the international rock standards analysed are available at http://www.geolsoc.org.uk/SUP18660

Petrochemical Characteristics and Timing of Middle Eocene Granitic Magmatism in Kooh‐Shah, Lut Block, Eastern Iran

Abstract:The Kooh‐Shah region located in a Tertiary volcanic–plutonic belt of the Lut Block in eastern Iran comprises several subvolcanic intermediate to acidic intrusive rocks, diorite to syenite in composition, which have intruded into volcanic rocks. The Kooh‐Shah granitoid rocks are characterized by enrichment in large ion‐lithophile elements (LILE: e.g. Sr, Ba, Rb) and depletion in high field‐strength elements (HFSE: e.g. Nb, Ta, Ti). The chondrite‐normalized REE patterns are characterized by moderate LREE enrichment (La/Yb)N=6.01–10.01, medium‐heavy REE enrichment, and absence of Eu anomalies. The Kooh‐Shah intrusive rocks are metaluminous, shoshonitic with calc‐alkaline affinity and high values of magnetic susceptibility, and classified as the magnetite‐series of oxidant I‐type granitoids. The age of Kooh‐Shah granitoid rocks based on zircon U–Pb age dating is 39.7±0.7 Ma (=Middle Eocene) and the ranges of their initial 87Sr/86Sr and 143Nd/144Nd ratios are from 0.704812 to 0.704920 and 0.512579 to 0.512644, respectively, when recalculated to an age of 39 Ma. The initial cNd isotope values for the Kooh‐Shah intrusive rocks range from −0.18 to 1.09. This geochemical data indicates that the Kooh‐Shah granitoid rocks formed from depleted mantle in an island arc setting. The geochemical signature of the studied granitoid rocks represents a characteristic guide for future exploration of copper‐gold porphyry‐type deposits in the Lut block.

Back-arc Paleo-Tethys related blueschist from Central Iran, south of Chupanan, Isfahan Province

Late Paleozoic blueschists present good exposures in the Pateyar metamorphic complex (south of Chupanan, Isfahan Province). They are formed by metamorphism of primitive basaltic lavas. Petrography and microprobe analyses show that the studied rocks consist of glaucophane, actinolite, actinolitic hornblende, plagioclase (albite), sphene, magnetite, quartz and apatite. Secondary minerals are epidote, chlorite, pyrite, hematite and calcite. Mineralogical assemblages are consistent with blueschist facies metamorphism, which is followed by a retrograde metamorphism in greenschist facies. Estimation of the metamorphic conditions suggests 300–400°C and 7–11 kbar. Chemical signatures of the studied metamorphic rocks conclude that they retain main geochemical characteristics of the protoliths, which allow the petrochemical interpretations. Geochemical analyses of these blueschists show that they were originally tholeiitic basalts. Evident negative anomalies of Nb, Ta and Ti relative to Th, La and Ce, in the primitive mantle normalized spider-gram, reveal subduction role in their petrogenesis. The studied metavolcanics exhibit an intermediate chemistry between the N-MORB (normal mid-ocean ridge basalt) and IATB (island arc tholeitic basalt). Enrichment in LREE (light rare earth elements) and LILE (large ion lithophile elements) and relative depletion in HFSE (high field strength elements) suggest a back-arc basin paleotectonic setting for the Chupanan samples. The primitive magma of the analyzed samples possibly have been produced by 8–13% melting of a spinel lherzolite. The field and petrological data propose that the studied Paleozoic metavolcanics were formed in a back-arc basin above the northward subduction of Paleo-Tethys oceanic lithosphere in Central Iran. The chemical criteria of the LILE/HFSE ratio suggests that the subduction zone was young and immature during the volcanism.

Geochronology and geochemistry of submarine volcanic rocks in the Yamansu iron deposit, Eastern Tianshan Mountains, NW China: Constraints on the metallogenesis

The Yamansu skarn iron deposit is hosted in Early Carboniferous submarine lava flow and volcaniclastic rocks of the Yamansu Formation in Eastern Tianshan Mountains, NW China. The lava flows are predominantly basaltic, with minor andesites. Laser ablation inductively coupled plasma mass spectrometry (LAICP-MS) U–Pb zircon dating of the basalts and skarns yields almost coeval ages of 324.4±0.94 and 323.47±0.95Ma, respectively. The basalts contain clinopyroxene and plagioclase phenocrysts with a considerable amount of Fe–Ti oxide minerals in the groundmass as interstitial phases, probably suggesting that olivine–, clinopyroxene- and plagioclase fractionated within the magma chamber. Geochemically, the basalts are characterized by slight variations in SiO2 (42.90–46.61wt.%), P2O5 (0.08–0.12wt.%), MnO (0.35–0.97wt.%) and TiO2 (0.74–0.82wt.%), and relatively large variations in CaO (6.93–15.13wt.%), Al2O3 (14.71–19.93wt.%), total Fe2O3 (8.14–12.66wt.%) and MgO (4.96–8.52wt.%). They possess flat to light rare earth element (REE)-depleted patterns and display variable degrees of depletions in high field-strength elements (HFSE), suggesting a transitional feature between MORB and arc volcanic rocks, and indicating a back-arc tectonic setting. Furthermore, the geochemical signature also suggests that the volcanic rocks of Yamansu Formation were produced by partial melting of the spinel-facies, asthenospheric mantle peridotite which had been metasomatized by slab-derived fluids. The broadly overlapping ages of the basalts and skarn mineralization suggests that the skarn formation in the Yamansu deposit is related to subaqueous volcanism. In combination with the available information including fluid inclusions and stable isotope data, we infer that the hydrothermal fluids that generated the skarns could be a mixture of evolved magma-derived fluids and convecting sea water driven by the heat from the shallow active magma chamber. The Yamansu basalts provided the source of iron for the skarn mineralization. We envisage the submarine volcanism, skarn alteration and iron mineralization in the Yamansu iron deposit as a continuous process, different from either conventional intrusion-related skarn type or submarine volcanic exhalation sedimentation type.

A 100 Ma bimodal composite dyke complex in the Jiamusi Block, NE China: An indication for lithospheric extension driven by Paleo-Pacific roll-back

A 125m-wide bimodal composite dyke complex, consisting of rhyolite and dolerite dykes, was emplaced into Cretaceous volcanic strata of the Songmuhe Formation in the Jiamusi Block of NE China. The dolerite dykes are sub-vertical, strike north-south, and intruded into both the country rocks and rhyolite dykes soon after the latter solidified. SHRIMP zircon U–Pb dating shows that the rhyolite dykes were emplaced at 100±2Ma and the dolerite dykes were also most likely emplaced at 100±2Ma. The rhyolite is characterized by enrichment in large-ion lithophile elements (LILE) and light rare earth elements (LREE), and depletion in high-field strength elements (HFSE). It shows a significant negative Eu anomaly, and has εNd(t) values ranging from 0.49 to 1.66 and two groups of initial 87Sr/86Sr ratios at 0.7045 and 0.7061. The rhyolite displays the compositional signature of Peraluminous Ferroan Granitoid, indicating it was derived by either differentiation of basalt and/or low pressure partial melting of crust. The dolerite is also characterized by enrichment in LILE and LREE, and depletion in HFSE. It has a weak negative Eu anomaly and has εNd(t)=−1.22 to +3.26, and (87Sr/86Sr)i=0.7057–0.7074. The dolerite originated from partial melting of lithospheric mantle which was affected by subducted oceanic crust, and experienced different amounts of crustal contamination. Such bimodal dyke complexes are an important indicator of crustal extension under the influence of mantle processes. Thus the dyke complex in the Jiamusi Block indicates mid-Cretaceous intra-plate extension in NE China related to the subduction of the paleo-Pacific plate along the eastern Eurasian continental margin. When compared with Mesozoic bimodal magmatism further to the west, our new data support a temporal eastward migration of magmatism over a distance >1000km from ~160Ma to ~100Ma. This was most likely associated with roll-back of the paleo-Pacific Plate and consequent upwelling of asthenospheric mantle.

Slab–mantle interaction in continental subduction channel: Geochemical evidence from Mesozoic gabbroic intrusives in southeastern North China

A combined study of zircon U–Pb ages and Lu–Hf isotopes, whole-rock major-trace elements and Sr–Nd isotopes as well as whole-rock and mineral O isotopes was carried out for Mesozoic gabbroic intrusives in the southeastern edge of the North China Block (NCB). The zircon U–Pb dating yields consistent ages of 131±2 to 136±2Ma for magma crystallization. The gabbroic rocks are characterized by arc-like trace element distribution patterns, enrichment of LILE (large ion lithophile elements) and LREE (light rare earth elements) but depletion of HFSE (high field strength elements). They exhibit variable enrichment of whole-rock Sr–Nd and zircon Hf–O isotopic compositions relative to the depleted MORB (mid-ocean ridge basalts) mantle. These geochemical features indicate their derivation from enriched mantle sources with incorporation of continental crustal-derived materials. The arc-like trace element distribution patterns and enriched Sr–Nd–Hf isotopic compositions for these Early Cretaceous mafic igneous rocks are comparable to those of Triassic ultrahigh-pressure metaigneous rocks in the Dabie–Sulu orogenic belt. Furthermore, they have two-stage whole-rock Nd and zircon Hf model ages of Mesoproterozoic to Paleoproterozoic, which are characteristic time of crustal growth for the subducted continental crust of the South China Block (SCB). These results suggest contributions of the subducted SCB continental crust to the mantle sources for the gabbroic intrusives. It is thus inferred that the mantle sources were generated by melt-peridotite reaction in continental subduction channel, in which the overlying NCB subcontinental lithospheric mantle (SCLM) wedge peridotite was metasomatized by felsic melts derived from the subducted SCB continental crust during the Triassic continental collision. The gabbroic intrusives in the southeastern NCB edge exhibit geochemical gradients from southeast to northwest, with decreases of their 87Sr/86Sr, K2O/Na2O, Ce/Pb and La/Nb ratios but increase of their εNd(t) and εHf(t) values. This suggests the spatial effect of crustal metasomatism on the mantle sources with regard to distance from the Dabie–Sulu orogenic belt. Therefore, there are variable contributions from the subducted SCB continental crust to the mantle sources, with a possible range of about 200–300km from the continental collision zone for the slab–mantle interaction in the subduction channel.

Age, geochemistry and tectonic setting of the Neoproterozoic ( ca 830 Ma) gabbros on the southern margin of the North China Craton

Many studies have addressed the Paleozoic to Triassic collisional processes along the southern margins of the North China Craton (NCC), but little is known about the evolution of this margin during Precambrian time. This paper examines the geochronology and geochemistry of gabbros from the Luanchuan Group, which is located on the southern margin of the NCC. These gabbros are dated at ca 830 Ma by SHRIMP and LA–ICP-MS zircon U–Pb methods. They are characterized by high contents of TiO 2 (2.21–4.45 wt%) and light-REEs (rare earth elements) ((La/Yb) N = 9.54–7.71). The gabbros have overall OIB (ocean island basalts)-like trace element patterns, without the positive Pb and Sr anomalies and negative Nb–Ta anomalies. In addition, they have low La/Nb (<1.5) and La/Ta (<30) ratios, indicating an origin in the asthenospheric mantle. The depletions of HFSEs (high field strength elements) (e.g. Zr–Hf and Ti) probably suggest that their source has been metasomatized by carbonates. The gabbros show negative ɛ Nd( t) (−1.5 to −3.0), suggesting crustal contamination or mixing with metasomatized lithospheric mantle. However, the low Th and U contents, constant incompatible-element ratios (e.g. Zr/Nb, La/Nb, La/Sm) and the absences of correlations of ɛ Nd( t) with MgO, Nb/Nb*, SiO 2 or 1/Nd preclude significant crustal contamination. Alternatively, the wide range of Hf–isotope ratios in zircons ( ɛ Hf( t) from −3.6 to +6.3) may imply the interaction between asthenosphere-derived melts and the metasomatized lithospheric mantle. Petrological modeling suggests that the gabbros may have been generated from the low-degree (2–3%) partial melting of lherzolite with 2% garnet at depths greater than 85 km. The gabbros may have been generated in a within-plate rift setting. The gabbros may best be correlated with the Neoproterozoic magmatic rocks in the NQB (North Qinling Belt), representing the relicts onto the southern margin of the NCC following the Neoproterozoic rifting between the NCC and the NQB. Therefore, the NCC and NQB may have been connected at ca 830 Ma. Moreover, the occurrence of the gabbros implies that the present southern margin of the NCC may have been mixed with late Mesoproterozoic to Neoproterozoic crustal materials from the NQB. The new findings imply that the southern margin of the NCC has been the locus of at least three extension-convergence cycles, and the reactivation of such tectonic margins may be more common in the geological record than previously recognized.

Geochemical and Sm–Nd isotopic study of titanite from granitoid rocks of the eastern Dharwar craton, southern India

Titanite occurs as an accessory phase in a variety of igneous rocks, and is known to concentrate geologically important elements such as U, Th, rare earth element (REE), Y and Nb. The differences in the abundances of the REEs contained in titanite from granitoid rocks could reflect its response to changes in petrogenetic variables such as temperature of crystallization, pressure, composition, etc. Widespread migmatization in the granodiorite gneisses occurring to the east of Kolar and Ramagiri schist belts of the eastern Dharwar craton resulted in the enrichment of the REEs in titanite relative to their respective host rocks. A compositional influence on the partitioning of REEs between titanite and the host rock/magma is also noticed. The relative enrichment of REEs in titanite from quartz monzodiorite is lower than that found in the granodioritic gneiss. Depletion of REE and HFSE (high field-strength elements) abundances in granitic magmas that have equilibrated with titanite during fractional crystallization or partial melting has been modelled. As little as 1% of titanite present in residual phases during partial melting or in residual melts during fractional crystallization can significantly lower the abundances of trace elements such as Nb, Y, Zr and REE which implies the significance of this accessory mineral as a controlling factor in trace element distribution in granitoid rocks. Sm–Nd isotope studies on titanite, hornblende and whole rock yield isochron ages comparable to the precise U–Pb titanite ages, invoking the usefulness of Sm–Nd isochron ages involving minerals like titanite.

Halaqin volcano-sedimentary succession in the central-northern margin of the North China Craton: Products of Late Paleoproterozoic ridge subduction

The Halaqin volcano-sedimentary succession (HVSS) is tectonically situated on top of the largely Late Archean to Early Paleoproterozoic Yinshan terrane in the central-northern margin of the North China Craton. It comprises greenschist- to amphibolite-facies schists, quartzites, marbles and amphibolites, which were originally pelites, pebbly sandstones, sandstones, limestones and volcanic rocks. The presence of tight to isoclinal folds and thrusts of the HVSS indicates tectonic transport from southeast to northwest (present coordinates). U–Pb ages of detrital zircon grains in the clastic rocks suggest provenance from both the Fengzhen belt in the south and the Yinshan terrane in the north. Zircon U–Pb ages of the HVSS volcanic rocks, e.g., 1909 ± 10 Ma, 1894 ± 6 Ma, 1887 ± 6 Ma, and 1884 ± 8 Ma indicate volcanism in the period ∼1910–1880 Ma. Detrital zircon age study of the neighbouring Majiadian Group reveals significant ∼2700 Ma crust growth and ∼2500 Ma metamorphism events in the Yinshan terrane. The HVSS volcanic rocks have a trimodal geochemical distribution demonstrated by SiO 2-contents of 45–50 wt.% (basalts), 53–60 wt.% (andesites), and 70–76 wt.% (dacites–rhyolites). The basalts and andesites are tholeiitic in chemistry, and show distinct depletion in high-field strength elements (HFSE, e.g., Nb, Zr, Ti), compared to neighbouring elements in spider-diagrams. Specifically, low-Mg basalts and andesites (MgO < 5 wt.%) have enriched light REE patterns (La/Yb N = 2.79–7.89), and enriched large ion lithophile elements (LILE). However, high-Mg rocks (MgO > 5 wt.%) have more or less flat REE patterns (La/Yb N = 0.45–1.76) and LILE. ɛNd t ( t = 1890 Ma) values of the basalts and andesites vary from −4.8 to + 0.4. The rhyolites and dacites have strongly enriched light trace element patterns (La/Yb N = 8.49–47.9) and LILE, but depleted HFSE (e.g., Nb, Zr, Ti). The basalts and basaltic andesites most likely originated from the asthenosphere contaminated by melts from the lithosphere, whereas the dacites and rhyolites likely originated from the crust. We propose that the basalts and andesites are extrusive counterparts of the coeval and geochemically similar Xuwujia gabbronorites, and the dacites and rhyolites are upper crustal equivalents of the contemporaneous, chemically comparable Liangcheng granitoids, which are in the originally underlying Fengzhen belt. We suggest that the HVSS was most likely generated by ridge subduction below active continental margin just prior to the final amalgamation of the North China Craton. This ridge subduction contributed to the ultra-high temperature metamorphism in this area. Terminal collision tectonics was responsible for the regional metamorphism and high deformation of the Halaqin succession, when it was transported northwestwards onto the Yinshan terrane.

Early Permian high-K calc-alkaline volcanic rocks from NW Inner Mongolia, North China: geochemistry, origin and tectonic implications

Abstract: Knowledge of the characteristics of the extensive late Palaeozoic volcanic rocks across the northern China–Mongolia tract is essential for understanding the tectonic evolution and continental crustal growth in the Central Asian Orogenic Belt. This geochronological and geochemical study documents the Early Permian mafic and felsic volcanic rocks from northwestern Inner Mongolia. The mafic rocks form two magma series with distinctive geochemical characteristics; one showing large ion lithophile element (LILE) enrichment relative to high field strength elements (HFSE) and an asthenosphere-like Sr–Nd–Pb isotopic signature, and the other featuring an elevated Nb and lithospheric isotopic signature. This result indicates that two mantle source components are involved in the magma generation: the subduction-related metasomatized asthenosphere and lithospheric mantle. The felsic rocks show strong enrichment of LILE and light REE, depletion in HFSE, and indistinguishable isotopic compositions from mafic ones. Such features are consistent with partial melts of mixed sources composed of predominant juvenile basaltic underplates and minor ancient crustal materials. These mafic and felsic rocks constitute a post-subduction high-potassium calc-alkaline magmatic suite possibly under a geodynamic regime of Palaeo-Asian Ocean slab breakoff. This regime not only provides a feasible trigger for the flipping of subduction polarity in the Solonker suture zone, but also presents a favourable venue for vertical continental crustal growth.

Permian bimodal volcanism in the Zhangguangcai Range of eastern Heilongjiang Province, NE China: Zircon U–Pb–Hf isotopes and geochemical evidence

In this paper, we report on zircon U–Pb dating, Hf isotopes, major and trace elements, and Sr–Nd isotope data, with the aim of constraining the petrogenesis and regional tectonic evolution of late Paleozoic volcanic rocks in the Zhangguangcai Range of eastern Heilongjiang Province, NE China. Located in the eastern segment of the Central Asian Orogenic Belt (CAOB), between the Siberian and North China cratons, the late Paleozoic volcanic rocks in the Zhangguangcai Range are composed mainly of basalt, basaltic andesite, rhyolite, and minor dacite. The results of LA–ICP–MS zircon U–Pb dating for two basaltic rocks, three rhyolites, and one dacite indicate that they formed in the early Permian (ca. 292 Ma). The mafic rocks have SiO 2 = 50.13–53.80 wt.%, K 2O = 0.98–2.28 wt.%, Mg# = 0.51–0.71, Cr = 144–541 ppm, Ni = 74–260 ppm, ( 87Sr/ 86Sr) i = 0.7044, and ε Nd ( t) = +4.28, whereas the felsic rocks have SiO 2 = 69.12–77.98 wt.%, K 2O = 3.09–5.33 wt.%, Mg# = 0.17–0.36, ( 87Sr/ 86Sr) i = 0.7032, and ε Nd ( t) = +4.32. These data are typical of bimodal volcanism. The mafic volcanic rocks are characterized by a strong enrichment in large ion lithophile elements (LILEs) such as Rb, Ba, Sr, and Pb, depletion in high field-strength elements (HFSEs) such as Nb, Ta, and Ti, depletion in heavy rare-earth elements (HREEs), and weak negative Eu anomalies (Eu/Eu * = 0.88–0.94). On the other hand, the felsic rocks show a strong depletion in Nb, Ta, Sr, P, and Ti, enrichment in Th, U, and K, and relatively large negative Eu anomalies (Eu/Eu * = 0.28–0.95). Taken together, these data suggest that the mafic magma was derived from the partial melting of a depleted lithospheric mantle, modified by subducted slab-derived fluids, and that the felsic magma originated by partial melting of newly accreted crust. The early Permian bimodal volcanic rocks, together with the coeval A-type granites, indicate an extensional environment, similar to a back-arc basin. Such a setting was possibly related to subduction of the Paleo-Asian oceanic plate beneath the Jiamusi and Songnen–Zhangguangcai Range massifs.

中央北海道南部の後期漸新世火山岩の岩石学的検討―背弧海盆拡大の初期火山活動―

The Oligocene volcanism in central Hokkaido may have taken place as the volcanic arc of the Eurasian continental margin prior to the formation of the Japan Sea back-arc basin. The Oligocene volcanic rocks in the Minami-Naganuma Formation recovered from the Nanporo drill cores consist of alkali olivine basalt, basaltic andesite, and andesite, and is classified into Lower (3742.2-3744.6 m depth), Middle (3488.0-3489.4 m), and Upper (3243.2-3245.1 m) members. The Oligocene volcanic rocks are characterized by greater large ion lithophile element and rare earth element enrichments, and also show a depletion in high-field strength elements. The Oligocene volcanic rocks have enriched isotopic compositions (higher 87Sr/86Sr and lower 143Nd/144Nd), similar to the Early Miocene lavas in the precursory stage of the Japan Sea. The geochemical characteristics of the Oligocene volcanic rocks are consistent with magma generation beneath central Hokkaido area from subduction-modified lithospheric mantle. The felsic andesites from the Middle member have adakite-like characteristics; high Sr/Y ratios, low Y contents, which can be produced in garnet stability field. In contrast to the felsic andesites, the basalts and basaltic andesites in the Lower and Upper members have similar isotopic compositions with systematically different REE abundances. Alkali basalts from the Upper member was inferred to have generated through small degrees of partial melting at pressure >2 GPa (>70 km depth), which is similar to that for the Quaternary NE Honshu back-arc basalts. In comparison with the NE Honshu arc magma, petrological evidence of the Oligocene basalts is consistent with the existence of a thick lithospheric lid beneath central Hokkaido.

Plio-Pleistocene intra-plate magmatism from the southern Sulu Arc, Semporna peninsula, Sabah, Borneo: Implications for high-Nb basalt in subduction zones

New analyses of major and trace element concentrations and Sr, Nd and Pb isotopic ratios are presented for Plio-Pleistocene basalts and basaltic andesites from the Semporna peninsula in Sabah, Borneo, at the southern end of the Sulu Arc. Depletion of high field strength elements (HFSE), which is characteristic of many subduction-related magmatic suites, is present in more evolved Semporna rocks but is associated with radiogenic Sr and Pb, and less radiogenic Nd isotopic ratios and results from contamination of mafic melt by, possibly ancient, crustal basement. The most mafic lavas from Semporna, and elsewhere in the Sulu Arc, display no HFSE depletion relative to other elements with similar compatibility. High-Nb basalt from Semporna formed when mantle resembling the source of Ocean Island Basalt (OIB) upwelled into lithospheric thin spots created during earlier subduction. This mantle did not experience enrichment by fluids or melt derived from subducted crust. The presence of similar lavas throughout the Sulu Arc and around the South China Sea suggests that the OIB-like component resides in the convecting upper mantle. Depletion of light rare earth elements, with respect to other incompatible elements, throughout the Sulu Arc could result from melt–mantle interaction during magma transport through the lithosphere. Such depletion is absent in suites from the South China Sea, where magma probably migrated along large, lithosphere-penetrating structures. Semporna high-Nb basalts are not associated with adakitic magmatism which is a frequent, but not ubiquitous, association in some active subduction zones. Both geochemical signatures are developed early in the history of a melt pulse, either in the source (high-Nb basalt) or during deep differentiation (adakite). Preservation of these distinctive geochemical signatures is favoured in settings that minimise (1) interaction with other, more copious melt types, and/or (2) subsequent differentiation in the shallow crust. Where found, the high-Nb basalt–adakite association is a result of transport through favourable lithospheric conditions and not due to any link between their mantle sources.

Tracing the metasomatic and magmatic evolution of continental mantle roots with Sr, Nd, Hf and and Pb isotopes: A case study of Middle Atlas (Morocco) peridotite xenoliths

Abstract We studied clinopyroxenes from spinel-facies peridotite xenoliths sampled by the Quaternary intra-plate volcanism of the Middle Atlas (Morocco) and present new trace element and Sr–Nd–Hf isotope data. However, we focus in particular on Pb isotope data and 238 U/ 204 Pb and 232 Th/ 204 Pb ratios of these clinopyroxenes. This data allows us to investigate: (a) the timing of metasomatic events, (b) the prevalence and persistence of elevated 238 U/ 204 Pb, 232 Th/ 238 U and 232 Th/ 204 Pb in continental mantle roots and (c) the 238 U/ 204 Pb and 232 Th/ 204 Pb composition of putative basaltic melts generated from such metasomatised sub-continental lithospheric mantle (SCLM). Incompatible trace element concentrations in these clinopyroxenes are elevated, marked by high-field strength element depletion and fractionated elemental ratios (e.g., U/Nb, Zr/Hf) most consistent with enrichment due to carbonatitic liquids. Sr, Nd and Hf isotopes have an affinity to HIMU. U, Th and Pb abundances in the clinopyroxenes generally exceed estimates of primitive mantle clinopyroxene. Pb isotope compositions of these clinopyroxenes are radiogenic and vary between 206 Pb/ 204 Pb = 19.93–20.25, 207 Pb/ 204 Pb = 15.63–15.66 and 208 Pb/ 204 Pb = 39.72–40.23. These Pb isotope systematics result in generally negative Δ7/4 but positive Δ8/4; setting these samples distinctly apart from typical HIMU. These Pb isotope compositions are also distinct from the associated host volcanic rocks. 238 U/ 204 Pb and 232 Th/ 204 Pb of these clinopyroxenes, which range from 26 to 81 and 136 to 399, respectively, are elevated and more extreme than estimates of MORB- and HIMU-source mantle. The Pb isotope evolution of the clinopyroxenes suggests that the metasomatic enrichment is younger than 200 Ma, which discounts the volcanic activity due to the opening of the Atlantic and the onset of the collision of the African and Eurasian plates as processes generating the lithophile element and isotope composition of this continental mantle root. Instead, the enrichment is thought to be associated with the Quaternary intra-plate volcanism in the Middle Atlas. However, the erupted mafic melts have unradiogenic Pb isotopes and lower 238 U/ 204 Pb, 232 Th/ 204 Pb and 232 Th/ 238 U relative to the clinopyroxene and do not seem to have equilibrated with the clinopyroxenes. The high Th abundances and the high 232 Th/ 238 U also suggest that the metasomatism was due to carbonatitic liquids. When literature data for Pb isotopes in mantle minerals are considered, the Pb isotope range of Archean, Proterozoic and Phanerozoic continental mantle roots is remarkable in that they are similar to the convecting mantle. This observation does not support the existence of sub-continental lithospheric mantle with high 238 U/ 204 Pb and 232 Th/ 204 Pb for long periods of time. Consequently, the narrow range of Pb isotopes in SCLM worldwide suggests that only the youngest metasomatic events are recorded by incompatible elements such as U, Th and Pb. Numerical modelling of putative magmas generated from Middle Atlas SCLM by fractional, non-modal melting calculations yield extremely high 238 U/ 204 Pb and 232 Th/ 204 Pb ratios. For example, pure SCLM magmas generated from 0.5% to 10% melting are anticipated to have 232 Th/ 204 Pb ratios exceeding those known from terrestrial basalts.