Pb Hf Research Articles

Geochemical constraints on the nature of mantle source for Cenozoic continental basalts in east-central China

Cenozoic continental basalts from east-central China were analyzed for their whole-rock major and trace elements as well as Sr–Nd–Hf–Pb and O isotopes. An integrated interpretation of all the results is used to place tight constraints on the nature of mantle source. The basalts are alkaline in lithochemistry and show the OIB-like patterns of trace element distribution. Their radiogenic isotopic compositions are relatively depleted, similar to those of Cenozoic MORB. But their ΔNb values, and Nb/U and Ce/Pb ratios are inconsistent with the MORB-type mantle source. They have high Fe/Mn ratios, almost constant Nb/Ta ratios, and relatively high Ni contents of olivine phenocrysts. Some clinopyroxene phenocrysts have lower δ 18O values than normal mantle-derived clinopyroxenes. Taken all these observations together, it appears that the dehydrated low δ 18O oceanic basalt was involved in the mantle source of continental basalts, and the silica-deficient pyroxenite is the best candidate for source lithology in addition to peridotite. Thus, the dehydrated oceanic metabasalt is assumed to produce low δ 18O adakitic melts during subduction, which metasomatized the overlying mantle-wedge peridotite to generate the juvenile subcontinental lithospheric mantle (SCLM) that is isotopically depleted but fertile for OIB-like basaltic magmatism. The melt–peridotite reaction is assumed to occur during low-angle subduction of the oceanic crust beneath the continental lithosphere, converting the mantle-wedge peridotite to the pyroxenite. Thus the mantle source of continental basalts is composed of pyroxenite–peridotite mixtures, whose partial melting at the lithosphere–asthenosphere thermal boundary due to continental rifting in the Cenozoic gave rise to the alkalic basaltic magmas. Therefore, the interaction between the oceanic crustal-derived melt and the mantle-wedge peridotite is a key to formation of the juvenile SCLM source, and the composition of continental basalts provides a snapshot of different SCLM sources with respect to the regime of plate tectonics.

Magmatism at the Eurasian–North American modern plate boundary: Constraints from alkaline volcanism in the Chersky Belt (Yakutia)

The Chersky seismic belt (NE-Russia) forms the modern plate boundary of the Eurasian−North American continental plate. The geodynamic evolution of this continent−continent setting is highly complex and remains a matter of debate, as the extent and influence of the Mid-Arctic Ocean spreading center on the North Asian continent since the Eocene remains unclear. The progression from a tensional stress regime to a modern day transpressional one in the Chersky seismic belt, makes the understanding even more complicated. The alkaline volcanism that has erupted along the Chersky range from Eocene through to the Recent can provide constraints on the geodynamic evolution of this continental boundary, however, the source and petrogenetic evolution of these volcanic rocks and their initiating mechanisms are poorly understood.We studied basanites of the central Chersky belt, which are thought to represent the first alkaline volcanic activity in the area, after initial opening of the Arctic Ocean basin. We present mineral and bulk rock geochemical data as well as Sr–Nd–Pb–Hf isotopes of the alkaline suite of rocks combined with new precise K–Ar and 40Ar/39Ar dating, and discuss an integrated tectono-magmatic model for the Chersky belt. Our findings show that the basanites were generated from a homogeneous asthenospheric mantle reservoir with an EM-1 isotopic flavor, under relatively ‘dry’ conditions at segregation depths around 110km and temperatures of ~1500°C. Trace element and isotope systematics combined with mantle potential temperature estimates offer no confirmation of magmatism related to subduction or plume activity. Mineral geochemical and petrographical observations together with bulk geochemical evidence indicate a rapid ascent of melts and high cooling rates after emplacement in the continental crust. Our preferred model is that volcanism was triggered by extension and thinning of the lithosphere combined with adiabatic upwelling of the underlying mantle at 37Ma. This suggests that at that time, rift tectonics in the Mid-Arctic Ocean most likely had also affected the North-Asian continent, causing volcanic activity in the Chersky belt, before the regional geodynamic regime changed from a tensional to compressional. Our conclusions contribute not only to the understanding of volcanism in the Chersky seismic belt (NE-Russia) but also to general aspects of plate dynamics between the Eurasian and North American continent.

Geochemistry and Sr–Nd–Pb–Hf isotopes of the Mesozoic Dadian alkaline intrusive complex in the Sulu orogenic belt, eastern China: Implications for crust–mantle interaction

The Dadian alkaline intrusive complex is located within the Sulu orogenic belt, and includes hornblende syenite, syenite, quartz syenite and syenite porphyry. All the rocks from the complex show high SiO 2, K 2O + Na 2O and LREE and low CaO, FeO T, MgO and HFSE (Nb, Ta, P, Ti) concentrations. The Sr, Nd, and Hf isotopic compositions (( 87Sr/ 86Sr) i = 0.708505–0.70876, ε Nd( t) = − 16.5 to − 17.9, ε Hf( t) = − 21.5 to − 16.4) of the rocks fall within the compositional field of mafic dykes occurring nearby, suggesting that the Dadian alkaline complex was mainly derived from an enriched lithospheric mantle. The initial Pb compositions (( 206Pb/ 204Pb) i = 16.706–16.779, ( 207Pb/ 204Pb) i = 15.427–15.450, ( 208Pb/ 204Pb) i = 37.266–37.352) of the Dadian rocks are lower than those of the Yangtze lithospheric mantle, albeit similar to that reported from beneath the North China Craton, implying that the mantle source of the Dadian alkaline complex most likely belong to the North China Craton. The relatively high SiO 2 and HREE contents and low Nb/Ta ratios as well as strongly negative Eu and Sr anomalies of some of the rocks from the Dadian complex suggest that crustal material was also involved in the formation of this complex and that the crustal component was probably derived through partial melting at low pressure. The high crystallization temperatures revealed by zircon saturation thermometry and low pressures suggested by Al-in-hornblende barometry indicate that the Dadian alkaline complex crystallized at high temperature and intruded into shallow crustal level. This intrusive complex does not show any relationship with post-orogenic process in the region, and its petrogenesis is inferred to be associated with the Paleo-Pacific subduction tectonics.

Archaean to Palaeoproterozoic crustal evolution of the Aravalli mountain range, NW India, and its hinterland: The U–Pb and Hf isotope record of detrital zircon

In situ U–Pb–Hf isotope analyses of detrital zircons from the Alwar quartzite, the oldest member of the North Delhi fold belt in the Aravalli mountain range, indicate that the basement of NW India shares a common Archaean-Palaeoproterozoic evolutionary history, and is a consequence of several crust-forming and crust-reworking events between 3.70 and 1.71 Ga. This is suggested by a zircon age spectrum with most ages at 1.87-1.71 Ga (75%), minor age peaks at 2.5-2.2 and 2.9-2.7 Ga, and Palaeoarchaean ages of 3.27-3.23 Ga. Lu–Hf isotope data reveal that the oldest crust, either in the Aravalli mountain range or in its hinterland, was formed from a depleted mantle source at around 3.7 Ga. Furthermore, the data also indicate that the hinterland of the Alwar sediments was affected by several periods of new “juvenile” crust formation at 3.25, 2.89, 2.67, 2.51, and 1.87-1.80 Ga, while crustal reworking was predominant at 2.62-2.45, 2.2 and 1.87-1.77 Ga. Zircons with xenocrystic cores and metamorphic overgrowths, additionally suggest that the source region was affected by high-grade (?) metamorphic events at 2.5 and 2.2 Ga. Based on our new data, in conjunction with field relationships and existing zircon ages in the Archaean basement of NW India, it can be concluded that the Palaeo- to Neoarchaean detritus (3.3-2.5 Ga) in the Alwar quartzite stems from proximal sources. The predominant late Palaeoproterozoic zircon detritus (1.87-1.71 Ga) was supplied from a nearby basement, which was affected by subduction-accretion tectonics at around 1.85 Ga, and by a first, perhaps post-collisional rift-related phase at 1.77 Ga. The latter event is unrecognized so far, and predates a second phase of rift-related A-type magmatism at 1.72-1.70 Ga. The youngest detrital zircon age of ca. 1.71 Ga suggests that the sedimentation and metamorphism in the North Delhi fold belt postdate the second phase of rift-related A-type magmatic activity.

Geochemistry and zircon U–Pb–Hf isotopic systematics of the Neoarchean Yixian–Fuxin greenstone belt, northern margin of the North China Craton: Implications for petrogenesis and tectonic setting

The Yixian–Fuxin greenstone belt in the Western Liaoning metamorphic terrane is situated in the middle segment of the northern margin of the North China Craton (NCC) and is composed mainly of plagioclase amphibolites, pyroxene amphibolites and hornblende plagioclase gneisses. LA-ICPMS zircon U–Pb isotopic dating of three representative samples of hornblende plagioclase gneisses reveals that the magmatic precursors of the Yixian–Fuxin greenstone belt were formed during 2589 Ma–2534 Ma and subjected to subsequent ~ 2490 Ma metamorphism. Lu–Hf isotopic analyses of the zircons dated from the three samples yield positive εHf(t) values of + 8.3 to + 2.7 and T DM from 2.76 Ga to 2.57 Ga. The youngest T DM value is close to their crystallization ages, suggesting that they were mainly derived from partial melting of the depleted mantle materials. These zircon chronological data and Lu–Hf isotopic features indicate an important ~ 2.5 Ga episode of crustal growth in the Western Liaoning metamorphic terrane. Based on whole rock geochemical characteristics, these meta-volcanic rocks in the Yixian–Fuxin greenstone belt may be subdivided into four groups. Samples in Group #1 show high MgO contents from 7.19% to 7.93% and Cr and Ni contents of 208–395 ppm and 113–210 ppm with Mg numbers (Mg#) of 53.4 to 55.1, and are characterized by slight LREE-depleted to flat REE patterns (La/Sm) N = 0.77–1.05, flat patterns on the primitive mantle normalized multiple trace elements diagram, and Nb/Ta ratios between 12.3 and 15.9, implying that they were derived from partial melting of depleted mantle source. Samples in Group #2 also display higher MgO (7.52%–14.51%), Cr (372–570 ppm) and Ni (240–320 ppm) contents with high Mg# (53.5–70.2), low TiO 2 contents (0.52%–0.64%), high Nb/Ta ratios (15.8–19.9), and have right-declined REE patterns with (La/Sm) N ratios from 1.73 to 3.6, and negative Nb, Ta and Ti anomalies, indicative of their derivation from partial melting of a metasomatized refractory mantle source. Samples in Group #3 show MgO content from 1.81% to 3.56% with Mg# of 35.9 to 43.5 and high Cr (1179–1280 ppm) and Ni (712–721 ppm) contents, lower Y and Yb contents (6.7–9.42 ppm and 0.64–0.88 ppm, respectively) and higher Nb/Ta ratios (18.4–21.8), and are characterized by the highest (La/Yb) N ratios (14.91–22.67) and negative Nb, Ta and Ti anomalies, revealing that they formed from partial melting of garnet-bearing oceanic slab, and the slab melts were contaminated by mantle peridotite and subjected to crustal contamination during their ascent. Samples in Group #4 exhibit high MgO content (5.98%–10.52%) with high Mg numbers (52.4–70.8), Cr (198–376 ppm) and Ni (112–177 ppm) contents and lower Nb/Ta ratios between 12.7 and 14.6, and have (La/Yb) N ratios ranging from 4.12 to 6.78 (lower than those of the Group #3) and strongly negative Nb, Ta and Ti anomalies, suggesting that they were derived from partial melting of mantle peridotite metasomatized by slab-derived fluids. Thus, the Yixian–Fuxin greenstones display geochemical features of the normal-mid-ocean ridge basalts (N-MORBs) (Group #1), boninite-like rocks (Group #2), adakite-like rocks (Group #3) and high magnesium andesites (HMA) (Group #4), suggesting that the protoliths of the greenstone belt might have formed in an active continental margin similar to the Andean-type orogen.

Origin of the Dexing Cu-bearing porphyries, SE China: elemental and Sr–Nd–Pb–Hf isotopic constraints

The Dexing porphyry copper deposit, part of the circum-Pacific porphyry copper ore belt, is the largest porphyry copper deposit in China. We present new LA–ICP–MS zircon U–Pb and molybdenite Re–Os dating, bulk-rock elemental and Sr–Nd–Pb isotopic as well as in situ zircon Hf isotopic geochemistry for these ore-bearing porphyries, in an attempt to better constrain their petrogenesis. LA–ICP–MS zircon U–Pb dating shows that the Dexing porphyries were emplaced in the early Middle Jurassic (∼171 Ma); molybdenite Re–Os dating indicates that the associated Cu–Mo mineralization was contemporaneous (∼171 Ma) with the igneous intrusion. The rocks are mainly high-K calc-alkaline and show adakitic affinities, including high Sr and low Y and Yb contents, high Sr/Y and La/Yb ratios, and high Mg# (higher than pure crustal melts). These porphyries have initial 87Sr/86Sr ratios of 0.7044−0.7047, ϵNd(T) values of –1.5 to +0.6, and ϵHf(T) (in situ zircon) values of +2.6 to +4.6. They show unusually radiogenic Pb isotopic compositions with initial 206Pb/204Pb ratios up to 18.41 and 207Pb/204Pb up to 15.61. These isotopic compositions are distinctly different from either Pacific MORB or Yangtze lower crust but are similar to the subducting sediments in the western Pacific trenches. Detailed elemental and isotopic data suggest that the Dexing porphyries were emplaced in a continental arc setting coupled with westward subduction of the palaeo-Pacific plate. Partial melting involved the subducted slab (mainly the overlying sediments), with generated melts interacting with the lithospheric mantle wedge, thereby forming the investigated high-K calc-alkaline porphyry magmas.

Age and geochemistry of the oceanic Manihiki Plateau, SW Pacific: New evidence for a plume origin

We present 40Ar/ 39Ar age and geochemical (major and trace element and Sr–Nd–Hf–Pb isotope) data from submarine samples recovered from the basement of the Manihiki Plateau during the R/V Sonne research expedition SO193. The samples, predominately tholeiites, with minor occurrences of basaltic andesites and hawaiites, give a mean age of 124.6 ± 1.6 Ma from four different localities on the plateau. Based on TiO 2 content, we define two groups of volcanic rocks that differ in trace element and isotopic compositions. Partial melting modeling suggests that the low-Ti group lavas were derived through large degrees of melting (c. 30%) of a peridotitic source at mantle potential melting temperatures of c. T p = 1510 °C, more than 100 °C above the ambient mantle potential melting temperature. Since the primary water contents of both groups of lavas are low (0.1–0.3g wt.%) and the source is peridotitic, excess temperature is most likely the reason for the large degrees of melting producing the large volume of plateau basalts, consistent with the involvement of a mantle plume. The incompatible element contents of the low-Ti group lavas show a multistage history with enrichment in the most incompatible elements of a previously highly depleted source. They have isotopic compositions similar to enriched mid-ocean-ridge basalt (EMORB) and similar to the common focal zone (FOZO) component. The high-Ti group lavas have more enriched incompatible element compositions overall. Their isotopic compositions tend towards an enriched mantle (EMI)-type endmember, similar, although less extreme, than lavas from the Pitcairn Islands. The geochemistry of the Manihiki Plateau can best be explained by a plume containing three components: 1) a dominant peridotitic FOZO-type component, 2) delaminated EMI-type subcontinental lithospheric mantle (SCLM), and 3) a HIMU (recycled oceanic crustal)-type component possibly in the form of eclogite/pyroxenite. The similarity in age and geochemical composition of Manihiki, Hikurangi and Ontong Java basement lavas, including volcanism in some adjacent basins, suggests that the Greater Ontong Java Volcanic Event covered c. 1% of the Earth's surface with volcanism.

Crustal contamination and mantle source characteristics in continental intra-plate volcanic rocks: Pb, Hf and Os isotopes from central European volcanic province basalts

We report new Os–Pb–Hf isotope data for a suite of alkaline to basaltic (nephelinites, basanites, olivine tholeiites to quartz-tholeiites) lavas from the Miocene Vogelsberg (Germany), the largest of the rift-related continental volcanic complexes of the Central European Volcanic Province (CEVP). 187Os/ 188Os in primitive (high-MgO) alkaline lavas show a much wider range than has been observed in alkaline basalts and peridotite xenoliths from elsewhere in the CEVP, from ratios similar to those in modern MORB and OIB (0.1260–0.1451; 58.9–168 ppt Os) to more radiogenic ratios (0.1908 and 0.2197; 27.6–15.1 ppt Os). Radiogenic Os is associated with high ε Hf and ε Nd, low 87Sr/ 86Sr and does not correlate with Mg ∗ or incompatible trace elements (e.g. Ce/Pb), suggesting the presence of a radiogenic endmember in the mantle rather than crustal contamination as the source of radiogenic Os. This contrasts with another high-Mg alkaline lava characterized by highly radiogenic 187Os/ 188Os (0.4344, 10.3 ppt Os), lower ε Hf and ε Nd, higher 87Sr/ 86Sr, and Pb isotope signatures than the other alkaline lavas with similar trace element composition suggestive of contamination with crustal material. Hafnium (ε Hf: +8.9 to +5.0) and Pb isotope compositions ( 206Pb/ 204Pb: 19.10–19.61; 207Pb/ 204Pb: 15.56–15.60) of the alkaline rocks fall within the range of enriched MORB and some OIB. The Vogelsberg tholeiites show even more diverse 187Os/ 188Os, ranging from 0.1487 in Os-rich olivine tholeiite (31.7 ppt) to ratios as high as 0.7526 in other olivine-tholeiites and in quartz-tholeiites with lower Os concentrations (10.3–2.0 ppt). Low- 187Os/ 188Os tholeiites show Pb–Hf isotope ratios ( 206Pb/ 204Pb:18.81; 207Pb/ 204Pb: 15.61; ε Hf: +2.7) that are distinct from those in alkaline lavas with similar 187Os/ 188Os and originate from a different mantle source. By contrast, the combination of radiogenic Os and low 206Pb/ 204Pb and ε Hf in the other tholeiites probably reflects crustal contamination. The association at Vogelsberg of primitive alkaline and tholeiitic lavas with a range of MORB- to OIB-like Os–Pb–Hf–Nd–Sr isotopic characteristics requires at least two asthenospheric magma sources. This is consistent with trace element modelling which suggests that the alkaline and tholeiitic parent magmas represent mixtures of melts from garnet and spinel peridotite sources (both with amphibole), implying an origin of the magmas in the garnet peridotite-spinel peridotite transition zone, probably at the asthenosphere–lithosphere interface. We propose that uncontaminated Vogelsberg lavas originated in ‘metasomatized’ mantle, involving a 3-stage model: (1) early carbonatite metasomatism several 10–100 Ma before the melting event (2) deposition of low-degree asthenospheric melts from carbonated peridotite at the lithosphere–asthenosphere thermal boundary produces hydrous amphibole-bearing veins or patches, and (3) remobilization of this modified lithospheric mantle into other asthenospheric melts passing through the same area later. In keeping with ‘metasomatized’ mantle models for other continental basalt provinces, we envisage that stage (2) is short-lived (few Ma), thus producing a prominent lithospheric trace element signature without changing the asthenospheric isotopic signatures. Models of this type can explain the peculiar mix of lithospheric (prominent depletions of Rb and K) and asthenospheric (OIB-like high 187Os/ 188Os, 143Nd/ 144Nd and 176Hf/ 177Hf) signatures observed in the Vogelsberg and many other continental basalt suites.

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.

Geochemistry of 24 Ma basalts from NE Egypt: source components and fractionation history

Abstract Subalkaline basalts from NE Egypt represent an episode of magmatism at c . 24 Ma, coincident with widespread eruptive activity in northern Africa. New geochemical data provide insight into the mineralogical and isotopic characteristics of the underlying mantle. The basalts show little geochemical variation, with incompatible trace element abundances similar to those of ocean island basalts. They display fairly smooth primitive mantle-normalized incompatible trace element patterns. Trace element abundances and Sr–Nd–Pb–Hf isotopic signatures are consistent with contributions from two distinct source regions, one similar to the Afar plume and the other located within the metasomatized spinel-facies subcontinental lithosphere. Mixing of melts from these two domains was followed by minor crustal contamination during prolonged ascent or emplacement. Integrating the geochemical data with available tomographic information allows us to develop a framework for understanding mid-Tertiary magmatic activity throughout northern Africa. A model for this widespread volcanism involves ascent of upwelling mantle derived from the margins of the South African Superplume rooted at the core–mantle boundary and/or through small-scale convection at the 660 km discontinuity. Ascent of magmas to the surface was facilitated by pre-existing structures within the lithosphere, including those associated with incipient rifting of the Red Sea. Supplementary material: Mineral chemistry data are available at http://www.geolsoc.org.uk/SUP18483 .

Multiple crust–mantle interactions for the destruction of the North China Craton: Geochemical and Sr–Nd–Pb–Hf isotopic evidence from the Longbaoshan alkaline complex

In situ zircon U–Pb ages and Hf isotopic data, major and trace elements, and Sr–Nd–Pb isotopic compositions are reported for the Longbaoshan alkaline intrusive complex in the western Shandong Province (Luxi Block), southeastern North China Craton. The Longbaoshan complex, which consists of quartz syenite, aegirine–augite syenite, hornblende syenite, monzonite, and syenodiorite, was emplaced at 129.4–131.7Ma. The complex is characterized by high concentrations of SiO2, K2O+Na2O, Al2O3, LILEs (e.g., Sr and Ba), and LREEs, and low concentrations of CaO, Fe2O3, MgO, and HFSEs (e.g., Nb, Ta, P and Ti). The Sr, Nd, Pb, and Hf isotopic values of this complex are similar to the nearby mafic rocks, which were derived from EM2-type lithospheric mantle, indicating that the Longbaoshan complex was mainly derived from partial melting of the EM2 source. Combined with geochemical and isotopic features, the high Nb/Ta ratios (average 19.2) of the Longbaoshan complex suggest that the EM2 source was induced by modification of crust-derived melts, which were in equilibrium with rutile-bearing eclogite. This was attributed to the subduction of the Yangtze continental crust. The presence of inherited zircons (2.51–2.64Ga) with positive εHf(t) values (0.2–6.2) suggest that the ancient crust of the North China Craton was also involved in the formation of the Longbaoshan complex. Magma mixing modelling shows that those 10–35% crustal materials were assimilated in the complex. This complex may have experienced hornblende, apatite/monazite, and titanite crystal fractionation before emplacement in the shallow crust level. Two stages of crust–mantle interaction can be identified through the formation of the Longbaoshan complex, including (1) the lithospheric mantle was metasomatized by the melts/fluids derived from the subducted crust and then transformed into an EM2 counterpart, and (2) magma derived from the EM2 source underplated the ancient crust and assimilated the crust-derived materials. We thus propose that multiple crust–mantle interactions are the essential mechanism for the destruction of the NCC. The first intensive crust–mantle interaction was aroused by crustal subduction and was responsible for the lithospheric mantle destruction, and the second major crust–mantle interaction was induced by mantle-derived magma underplating and was responsible for the crustal activation.

Timescales of magma differentiation from basalt to andesite beneath Hekla Volcano, Iceland: Constraints from U-series disequilibria in lavas from the last quarter-millennium flows

Measurements of 238U– 230Th– 226Ra disequilibria, Sr–Nd–Pb–Hf isotopes and major-trace elements have been conducted for lavas erupted in the last quarter-millennium at Hekla volcano, Iceland. The volcanic rocks range from basalt to dacite. Most of the lavas (excluding dacitic samples) display limited compositional variations in radiogenic Sr–Nd–Pb–Hf isotopes ( 87Sr/ 86Sr = 0.70319–0.70322; 143Nd/ 144Nd = 0.51302–0.51305; 206Pb/ 204Pb = 19.04–19.06; 207Pb/ 204Pb = 15.53–15.54; 208Pb/ 204Pb = 38.61–38.65; 176Hf/ 177Hf = 0.28311–0.28312). All the samples possess ( 230Th/ 238U) disequilibrium with 230Th excesses, and they show systematic variations in ( 230Th/ 232Th) and ( 238U/ 232Th) ratios. The highest 226Ra excesses occur in the basalt and most differentiated andesite lavas, while some basaltic-andesite lavas have ( 226Ra/ 230Th) ratio that are close to equilibrium. The 238U– 230Th– 226Ra disequilibria variations cannot be produced by simple closed-system fractional crystallization with radioactive decay of 230Th and 226Ra in a magma chamber. A closed-system fractional crystallization model and assimilation and fractional crystallization (AFC) model indicate that the least differentiated basaltic andesites were derived from basalt by fractional crystallization with a differentiation age of ∼24 ± 11 kyr, whereas the andesites were formed by assimilation of crustal material and fractionation of the basaltic-andesites within 2 kyr. Apatite is inferred to play a key role in fractionating the parent–daughter nuclides in 230Th– 238U and 226Ra– 230Th to make the observed variations. Our proposed model is that several batches of basaltic-andesite magmas that formed by fractional crystallization of a basaltic melt from a deeper reservoir, were periodically injected into the shallow crust to form individual magma pockets, and subsequently modifying the original magma compositions via simultaneous assimilation and fractional crystallization. The assimilant is the dacitic melt, which formed by partial melting of the crust.

Age and geochemistry of volcanic rocks from the Hikurangi and Manihiki oceanic Plateaus

Here we present the first radiometric age data and a comprehensive geochemical data set (including major and trace element and Sr–Nd–Pb–Hf isotope ratios) for samples from the Hikurangi Plateau basement and seamounts on and adjacent to the plateau obtained during the R/V Sonne 168 cruise, in addition to age and geochemical data from DSDP Site 317 on the Manihiki Plateau. The 40Ar/ 39Ar age and geochemical data show that the Hikurangi basement lavas (118–96 Ma) have surprisingly similar major and trace element and isotopic characteristics to the Ontong Java Plateau lavas (ca. 120 and 90 Ma), primarily the Kwaimbaita-type composition, whereas the Manihiki DSDP Site 317 lavas (117 Ma) have similar compositions to the Singgalo lavas on the Ontong Java Plateau. Alkalic, incompatible-element-enriched seamount lavas (99–87 Ma and 67 Ma) on the Hikurangi Plateau and adjacent to it (Kiore Seamount), however, were derived from a distinct high time-integrated U/Pb (HIMU)-type mantle source. The seamount lavas are similar in composition to similar-aged alkalic volcanism on New Zealand, indicating a second wide-spread event from a distinct source beginning ca. 20 Ma after the plateau-forming event. Tholeiitic lavas from two Osbourn seamounts on the abyssal plain adjacent to the northeast Hikurangi Plateau margin have extremely depleted incompatible element compositions, but incompatible element characteristics similar to the Hikurangi and Ontong Java Plateau lavas and enriched isotopic compositions intermediate between normal mid-ocean-ridge basalt (N-MORB) and the plateau basement. These younger (∼52 Ma) seamounts may have formed through remelting of mafic cumulate rocks associated with the plateau formation. The similarity in age and geochemistry of the Hikurangi, Ontong Java and Manihiki Plateaus suggest derivation from a common mantle source. We propose that the Greater Ontong Java Event, during which ∼1% of the Earth’s surface was covered with volcanism, resulted from a thermo-chemical superplume/dome that stalled at the transition zone, similar to but larger than the structure imaged presently beneath the South Pacific superswell. The later alkalic volcanism on the Hikurangi Plateau and the Zealandia micro-continent may have been part of a second large-scale volcanic event that may have also triggered the final breakup stage of Gondwana, which resulted in the separation of Zealandia fragments from West Antarctica.

Petrogenesis and Origins of Mid-Cretaceous Continental Intraplate Volcanism in Marlborough, New Zealand: Implications for the Long-lived HIMU Magmatic Mega-province of the SW Pacific

Continental intraplate basalts with a HIMU-like mantle signature in the SW Pacific (e.g. New Zealand, Australia and Antarctica) have been erupted over an area of ∼25 million km2 sporadically over the last c. 100 Myr. This long-lived HIMU signature has recently been ascribed to both melting in the subcontinental lithospheric mantle and the asthenospheric mantle. The Lookout Volcanics represent the oldest samples of this HIMU magmatic mega-province and are remnants of extensive mid-Cretaceous (98 Ma) intraplate volcanism preserved in South Island, New Zealand, which occurred prior to the rifting of Zealandia from Gondwana. We have carried out a detailed petrographic, chemical and isotopic study of the Lookout Volcanics based on sampling of a 700 m composite stratigraphic section that comprises predominantly mafic alkaline rocks. Initial Sr–Nd–Hf–Pb isotopic variations (87Sr/86Sr = 0·7030–0·7038; 143Nd/144Nd = 0·51272–0·51264; 176Hf/177Hf = 0·28283–0·28278; 206Pb/204Pb = 20·3–18·8) are explained by mixing between melts of a HIMU-like mantle source and up to 22% of Early Cretaceous upper crust. Oxygen isotope data for six lava flows yielded δ18Oolivine = 4·7–5·0‰, δ18Ocpx cores = 4·8–5·4‰ and δ18Ocpx rims = 3·9–5·5‰. Average olivine and clinopyroxene core values are in oxygen isotopic equilibrium and comparable with data for phenocrysts from HIMU ocean island basalts. Oxygen isotopic disequilibrium between clinopyroxene cores and rims records phenocryst growth in a shallow magma chamber interacting with an active meteoric water system. The scavenging of such phenocrysts suggests that volcanic phenocrysts with low δ18O cannot always be used as a mantle fingerprint of recycled crustal components with low δ18O. Clinopyroxene and plagioclase phenocrysts from the most evolved sample have elevated δ18O and 87Sr/86Sr consistent with crustal contamination. Variations in incompatible trace element ratios (e.g. La/YbN = 12·5–20·2; Dy/YbN = 1·90–2·20) are consistent with small degrees (c. 2–4%) of partial melting of an amphibole-bearing garnet pyroxenite or peridotite mantle source. Moreover, the elevated NiO contents of olivine phenocrysts from the Lookout Volcanics may also be consistent with melt generation from a pyroxenitic mantle source. However, given the documented effect of increasing pressure causing a decrease in the melt–olivine partition coefficient for Ni, the elevated Ni in the olivine phenocrysts may simply reflect high Ni contents in the primary magmas of the Lookout Volcanics as a result of deep, garnet-facies, mantle melting. The pervasive HIMU mantle source throughout Zealandia was most probably produced along the East Gondwana subduction margin by cooling of subduction-modified upper mantle and its incorporation into the subcontinental lithospheric mantle that accompanied the crustal formation of Zealandia.

Ridge subduction and crustal growth in the Central Asian Orogenic Belt: Evidence from Late Carboniferous adakites and high-Mg diorites in the western Junggar region, northern Xinjiang (west China)

The Central Asian Orogenic Belt (CAOB) is a natural laboratory for the study of accretionary tectonics and crustal growth owing to its massive generation of juvenile crust in the Paleozoic. There is a debate, however, on the mechanism of this growth. In the Baogutu area of the western Junggar region, northern Xinjiang (west China), diorite–granodiorite porphyry plutons and dikes are widely associated with Cu–Au mineralization. In this study, we present new results of zircon U–Pb geochronology, major and trace elements, and Sr–Nd–Pb–Hf isotope analyses for two diorite–granodiorite porphyry plutons and two dikes from this area. LA-ICP-MS zircon U–Pb analyses of four plutonic and dike samples yield Late Carboniferous ages of 315–310 Ma. The Baogutu diorite–granodiorite porphyries exhibit low-Fe and calc-alkaline compositions. They are also characterized by high Sr (346–841 ppm) contents, low Y (9.18–16.5 ppm) and Yb (0.95–1.60 ppm) contents, and relatively high Sr/Y (31–67) ratios, which are similar to those of typical adakites. In addition, some samples have relatively high MgO (2.35–8.32 wt.%) and Mg # (48–75), and Cr (22.7–291 ppm) and Ni (32.0–132 ppm) values, which are similar to those of high-Mg andesites. All rock samples exhibit mid-oceanic ridge basalt (MORB)-like Nd–Sr–Pb–Hf isotope features: high ε Nd(t) (+ 5.8–+8.3) and ε Hf(t) (+ 13.1–+15.7) values, and relatively low ( 87Sr/ 86Sr) i (0.7033 to 0.7054) and ( 206Pb/ 204Pb) i (17.842–18.055). The Baogutu adakitic rocks also contain reversely zoned clinopyroxene phenocrysts, which have low MgO cores and relatively high MgO rims. Geochemical modeling indicates that the Baogutu adakitic rocks could have been derived by mixing ~ 95% altered oceanic crust-derived melts with ~ 5% sediment-derived melts. Taking into account the regional geology, I- and A-type granitoids and Cu–Au mineralization, and the presence of Carboniferous ophiolite mélanges in northern Xinjiang, we suggest that the Baogutu adakitic rocks were most probably generated by partial melting of a slab edge close to a subducting spreading ridge in the Late Carboniferous. Ridge subduction and the resultant slab window probably caused strong extension in the overlying lithosphere, extensive melting of subducting oceanic crust, mantle and juvenile lower crust, and interaction between slab-derived melts and the mantle. Thus, events associated with ridge subduction are likely to have played an important role in crustal growth in the CAOB in addition to previously recognized accretion of subduction and arc complexes and post-collisional crustal melting.

The influence of small-scale mantle heterogeneities on Mid-Ocean Ridge volcanism: Evidence from the southern Mid-Atlantic Ridge (7°30′S to 11°30′S) and Ascension Island

Volcanism along Mid-Ocean Ridges is known to exhibit significant isotopic and elemental variations, traditionally regarded as reflecting global scale variations in mantle depletion history and refertilization processes. The ∼ 400 km long Mid-Atlantic Ridge (MAR) between the Ascension and Bode Verde Fracture Zones (7°30′S to 11°30′S) has been sampled at high spatial resolution (c. 10 km along-axis scale) and large elemental and isotopic variations have been identified covering almost the entire compositional spectrum of MAR basalts. In this paper we employ a multi-isotope (Sr–Nd–Hf–Pb) and trace element approach in order to explore the geodynamic implications of along-ridge compositional variations and their relationship to off-axis volcanism on nearby Ascension Island, commonly regarded as plume-related. The studied portion of the MAR consists of four major segments. Isotopic data from the deeply incised northern and southern segments A1 and A4 define a trend involving a high-εHf depleted mantle endmember. This isotopic signature is inferred to result from an ancient melting event in the garnet stability field, causing high time-integrated Lu/Hf and elevated 176Hf/ 177Hf. In contrast, a low-εHf depleted mantle endmember is indicated by the compositional trend for samples from the topographically elevated central portion (segments A2 and A3). Both trends converge at a common enriched endmember with least diluted compositions represented by samples from the subaerial eruptive stage of nearby Ascension Island. Because linear co-variations between isotope compositions and the respective parent–daughter ratios cannot be explained as representing “mantle isochrons” we infer that our data reflect arrays related to physical mixing of depleted and enriched mantle domains. This implies that distinctive mantle domains at kilometer scale may survive convection processes over time spans in the order of 1–2 Ga. This finding corroborates the results of recent Os-isotope studies of abyssal peridotite which returned similar conclusions. The systematics of along-axis isotopic variations show that mantle upwelling is highly variable. A systematic decrease of the enriched isotopic signature from the central segment A3 toward the northern termination of the segment A2 suggests a northward flow of enriched mantle material. In marked contrast, the compositional variations along the marginal segments A1 and A4 are random indicating heterogeneous mixing of mantle domains at a spatial resolution of < 50 km to 10 km (along-axis scale). With regard to Ascension Island we show that the submarine volcanic stage, sampled by a 3216 m long drill hole, was fed by a distinctive enriched mantle source currently inaccessible to partial melting. We interpret this observation within the framework of recent local-scale geophysical investigations and infer an on-axis origin of the bulk of the Ascension volcanic edifice at around 5 to 6 Ma, synchronous with the surrounding oceanic crust. Off-axis partial melting of the current common enriched mantle endmember accounts for rejuvenation of volcanism on Ascension generating the volumetrically subordinate subaerial portion of the island.

Melt–Peridotite Reactions and Fluid Metasomatism in the Upper Mantle, Revealed from the Geochemistry of Peridotite and Gabbro from the Horoman Peridotite Massif, Japan

An investigation of the petrology and geochemistry of peridotites and gabbros in the Horoman massif, Hokkaido, Japan was undertaken to constrain geochemical processes in the upper mantle. Two types of sample were studied: one type comprises peridotites and gabbros forming thin layers varying from a few millimeters to centimeters in scale (thin-layer peridotites and gabbros); the other comprises thick layers (>1 m scale; massive peridotites and gabbros). There is no clear trace element evidence for metasomatism in the thin-layer peridotites. Instead, they have melt–rock reaction textures interpreted in terms of the formation of secondary pyroxene at the expense of primary porphyroclastic olivine and dissolution of primary porphyroclastic pyroxene to form secondary olivine. The thin-layer gabbros also exhibit no metasomatic effects; they have incompatible element depleted trace element characteristics and mid-ocean ridge basalt (MORB)-like isotopic signatures consistent with the presence of a new type of gabbro that previously has not been described from the Horoman Massif. The whole-rock chemistry of the thin-layer peridotites and thin-layer gabbros can be explained by melt–peridotite reactions between isotopically highly depleted MORB mantle (represented by the thin-layer peridotites) and melt with geochemical affinity to Pacific MORB (represented by the thin-layer gabbros). Sm–Nd and Lu–Hf isotope systematics suggest that these reactions might have occurred at ∼300 Ma. Some of the plagioclase lherzolites and all of the spinel lherzolites and harzburgites within the massive peridotites show enrichment in incompatible trace elements and more radiogenic Hf–Nd–Pb isotopic compositions than the incompatible-element depleted thin-layer peridotites. The analyzed massive gabbros are interpreted as subduction-related magmas formed in a MORB-source mantle wedge, which have subsequently interacted with a fluid or melt in the Hidaka subduction zone. Hf–Nd–Pb isotope systematics reveal that this interaction may have occurred at an age younger than ∼50 Ma. Melt- and fluid-related processes occurring in the upper mantle are systematically identified from the samples of the Horoman Massif based on petrography, major and trace element, and Sr–Nd–Pb–Hf isotope geochemistry. These processes occurred in different tectonic settings such as the convecting oceanic mantle and supra-subduction zone mantle wedge and have variably modified the original chemistry of residual mantle protolith, formed by partial melting of a depleted MORB source mantle at ∼1 Ga.

Compositional Characteristics and Spatial Distribution of Enriched Icelandic Mantle Components

We present compositional data on a suite of 18 primitive neovolcanic alkali basalts from three flank-zone regions in Iceland (Vestmannaeyjar in the south, Snaefell in the east, and Snaefellsnes in the west) that are peripheral to the main rift zones that are dominated by tholeiitic basalts. This study integrates He isotope data with radiogenic isotope data (Sr–Nd–Pb–Hf), stable isotope data (δ18O), and trace element data to characterize the compositional features of the trace element enriched components of the Icelandic mantle. We also present high-precision Pb isotope data on an additional 57 lava samples from the flank zones (including Oraefajokull in the SE) and the Northern and Eastern rift zones. Most Icelandic lavas have negative Δ207Pb (–4 to –1), with higher values (–1 to +4) found only in samples from Oraefajokull, Snaefell, and parts of the Reykjanes Peninsula. At Snaefell, this EM1-type component is characterized by a low δ18Oolivine signature (+4·1‰ to +4·6‰), moderate 206Pb/204Pb values (18·4–18·6) and mid-ocean ridge basalt (MORB)-like 3He/4He (6·9–7·5 R/RA). Samples from Vestmannaeyjar and Snaefellsnes have mantle-like δ18Oolivine (+4·9‰ to +5·0‰), and radiogenic 206Pb/204Pb values (18·9–19·3) that fall on the Northern Hemisphere Reference Line for 208Pb/204Pb (Δ208Pb –5 to +5). Compared with the Vestmannaeyjar lavas, Snaefellsnes lavas have higher La/YbN (5–11 vs 3–5), lower eNd (5·5–6·5 vs 6·8–7·6) and lower 3He/4He (6·3–8·6 R/RA vs 11·4–13·5 R/RA). Therefore, the most trace element enriched components in the Icelandic mantle are not the carriers of the high 3He/4He values (>15 R/RA) found in some lavas on Iceland and the adjacent ridges, and instead are consistent with degassed, recycled components. Even after excluding the EM1-type high Δ207Pb samples, high-precision Pb isotope data produce a kinked array on a 206Pb/204Pb vs 208Pb/204Pb plot, which is not consistent with simple binary mixing between two end-members. This requires significant lateral heterogeneity within the Icelandic mantle and the presence of more than just two compositionally distinct local mixing end-member components. Samples from each of the main axial rift zones define different trends. Despite the tectonic continuity between the Northern Volcanic Zone and the Eastern Volcanic Zone, lavas from these two rift zones define separate sub-parallel linear arrays. Lavas from the adjacent Western Volcanic Zone and the Eastern Volcanic Zone define oblique linear arrays that converge on a common local end-member that is not involved in the magmatism of the Northern Volcanic Zone. Therefore, there is a distinct NE–SW compositional heterogeneity within the Icelandic mantle.

Slab and Mantle Controls on the Sr–Nd–Pb–Hf Isotope Evolution of the Post 42 Ma Izu–Bonin Volcanic Arc

The arc outflux is a hybrid of material fluxes from slab, mantle and crustal sources, whose proportions and composition may vary through time. Here we investigate the causes of temporal variability in the post 42Ma eruptive products of the Izu^Bonin arc in the NW Pacific Ocean basin using Sr^Nd^Pb^Hf isotopes in combination with major and trace element systematics in mafic to evolved arc magmas. Following arc initiation at 50Ma, Izu^Bonin arc formation was continuous except for a short period of quiescence ( 20^23Ma) during back-arc basin formation in the early Miocene.The Sr^Nd^Pb^Hf isotope trends do not co-vary through time. An increasing trend of Pb isotope ratios with time reflects a change in the trench sediment input as it becomes richer in radiogenic Pb. Sr isotopes show no unidirectional trend with time owing to the dominant contribution of unradiogenic Sr from the subducting igneous crust ( 66% of arc Sr) and mantle ( 31%) that buffer the minor amount of radiogenic Sr contributed from subducting sediment or altered oceanic crust. Nd and Hf isotope compositions are both controlled by the depleted sub-arc mantle. Nd isotope ratios increase significantly with time, whereas any temporal increase in Hf isotope ratios remains negligible relative to the inherent data variability. This temporal divergence is due to the much higher parent/daughter ratios of the sub-arc mantle source (Sm/Nd1⁄4 0·24; Lu/Hf1⁄4 0·04), which result in significant ingrowth in Nd but a negligible ingrowth of Hf. Overall, the isotopic evolution is unaffected by along-strike arc rifting and back-arc basin formation, or by the evolution of the siliceous Izu^Bonin arc crust. Compared with the greater isotopic variability of the broader Izu^Bonin^Mariana arc system, the post 42Ma Izu^Bonin arc exemplifies the long-term evolution of a simple arc system with comparatively uniform input from slab and mantle sources. Its isotopic evolution is hence not representative of that of other arcs, and this may reflect the different subduction geometry and components involved.

On the difficulty of assigning crustal residence, magmatic protolith and metamorphic ages to Lewisian granulites: constraints from combined in situ U–Pb and Lu–Hf isotopes

Abstract Zircons from two granulite facies gneisses from the central region of the Lewisian Complex have been investigated by high spatial resolution ion-microprobe U–Pb dating and laser ablation combined Pb–Hf isotope methods. The ion-microprobe data reveal a complex pattern of zircon ages distributed along the concordia curve between the time of granulite facies metamorphism at c . 2.5 Ga and the oldest zircon in each sample (respectively 2.89 Ga and 3.04 Ga). This Pb-loss pattern complicates assignment of an unambiguous magmatic protolith age to the zircon although cathodoluminescence (CL) imaging is used to suggest a preferred age of c . 2.85 Ga for both samples, with older grains being inherited. In situ Hf isotopes show a larger spread in the sample containing older grains which is also consistent with inheritance and further suggests that several crust extraction events are represented in the inherited population. Comparison of Hf isotope compositions with plausible model evolution curves suggests that crustal precursors to the Lewisian granulites were derived from their mantle source at c . 3.05–3.2 Ga. Supplementary material: Analytical methods and data tables are available at: http://www.geolsoc.org.uk/SUP18395