Distinct Magma Sources Research Articles

Magnetite geochemistry of giant alkalic-type epithermal gold deposits: Insights into the magmatic evolution of mineralized alkaline systems

Magnetite, a common mineral in alkaline magmatic complexes hosting giant epithermal Au deposits, is examined to obtain insights into the magmatic evolution and ore fertility of alkaline systems. Textural and compositional analyses of magmatic magnetite from Cripple Creek, USA, and Lihir Island, Papua New Guinea, reveal distinct magma sources and evolution paths. High HFSE concentrations and high La/Yb ratios in magnetite from Cripple Creek suggest relatively low-degree partial melting of lithospheric and asthenospheric mantle sources, whereas magnetite from Lihir Island indicates higher degree partial melting from subduction-modified mantle. Chromium concentrations track magma evolution, while V systematics record decreasing oxygen fugacity with magma differentiation. Sulfide saturation is observed at Cripple Creek, potentially affecting ore fertility. Magnetite itself bears the capability to scavenge Au from melts as indicated by Au concentrations of up to 40 ng/g. This might point towards elevated Au concentrations in the parental magmas, although experimental data on Au partitioning is lacking. Magnetite compositions overlap across different mineralization styles, challenging its reliability as an exploration tool but analysis of noble metals in magnetite and its inclusion cargo could offer promising alternative approaches. Overall, magnetite serves as a valuable tool for investigating ore deposit petrogenesis and holds potential for exploration.

Cleaning up the record – revised U-Pb zircon ages and new Hf isotope data from southern Sweden

ABSTRACT Ten samples of felsic plutonic rocks from the Eastern Segment of the Sveconorwegian Orogen in southern Sweden, previously dated by ID-TIMS on zircon, have been dated anew using SIMS spot analysis of individual zircon grains, leading to more reliable and in most cases also more precise revised magmatic crystallization ages. A gneissic monzonite within the Protogine Zone in Småland yields a revised U-Pb age of ca. 1725 Ma, four samples of orthogneiss from Skåne all yield revised ages between 1700 and 1690 Ma, while two samples of coarse-grained granitic gneiss in the same region yield ages between 1690 and 1680 Ma. These revised ages are between 15 and 250 m.y. older than previously obtained TIMS ages. Two samples of the Gumlösa-Glimåkra granite along the Protogine Zone in northern Skåne and one sample of related syenite yield ages around 1220 Ma, similar but more precise compared to the previous ages. The U-Pb zircon data have been complemented by Hf isotope analysis by LA-ICP-MS on the same grains, and previously obtained initial Sr and Nd whole-rock isotope data have been recalculated to the revised crystallization ages. The Sr isotope data scatter, while the revised initial εNd values fall between +1 and +2 for the older rocks, and close to 0 for the younger intrusives along the Protogine Zone. Initial εHf in magmatic undisturbed zircons shows relatively little spread within each sample, between 2 and 4 Epsilon units, disregarding a few outliers, with average values for the 1725 to 1680 Ma rocks between +3 and +5.5 and for the 1220 Ma rocks at ca. +1.5. Covariation between initial εNd and initial εHf in the older rocks suggests either mixing between two isotopically distinct magma sources or one magma source which was isotopically heterogeneous. The isotopic signatures of the 1220 Ma intrusive rocks along the Protogine Zone are indicative of juvenile mantle input to their magmas, rather than pure crustal melting.

Paleozoic post-collisional magmatism and high-temperature granulite-facies metamorphism coupling with lithospheric delamination of the East Kunlun Orogenic Belt, NW China

Lithosphere extension and upwelling of asthenosphere at post-collisional stage of an orogenic cycle generally induce diverse magmatism and/or associated high-temperature metamorphism. Nevertheless, the intimate coexistence of post-collisional magmatic activity and high-temperature metamorphism is rare. In this contribution, a lithological assemblage composing of diverse magmatic rocks deriving from distinct magma sources and coeval high-temperature metamorphism was identified in eastern Kunlun. Petrography, ages, mineral chemistry and whole-rock geochemistry demonstrated that those intimately coexistent diverse rocks were genetically related to post-collisional extension. The garnet-bearing mafic granulites in Jinshuikou area interior of the East Kunlun Orogenic Belt are mainly composed of garnet, orthopyroxene, and plagioclase, with peak metamorphic P–T conditions of ~ 701–756 °C and 5.6–7.0 kbar, representing a granulite-facies metamorphism at 409.7 ± 1.7 Ma. The diverse contemporaneous magmatic rocks including hornblendites, gabbros and granites yield zircon U–Pb ages of 408.6 ± 2.5 Ma, 413.4 ± 4.6 Ma, and 387–407 Ma, respectively. The hornblendites show N-MORB-like REE patterns with (La/Sm)N values of 0.85–0.94. They have positive zircon εHf(t) values of 0.1–4.9 and whole-rock εNd(t) values of 3.9–4.7 but relatively high (87Sr/86Sr)i values of 0.7081 to 0.7088. These features demonstrate that the hornblendites derived from a depleted asthenospheric mantle source with minor continental crustal materials in source. As for the gabbros, they exhibit arc-like elemental signatures, low zircon εHf(t) values (−4.3 to 2.5) and variable whole-rock εNd(t) values (−4.9 to 1.2) as well as high (87Sr/86Sr)i values (0.7068 to 0.7126), arguing for that they were originated from partial melting of heterogeneous lithospheric mantle anteriorly metasomatized by subducted-sediment released melts. Geochemistry of the granites defines their strongly peraluminous S-type signatures. Zircons from the granites yield a large range of εHf(t) values ranging from −30.8 to −5.1, while the whole-rock samples yield consistent (87Sr/86Sr)i values (0.7301 to 0.7342) and negative εNd(t) values (−10.1 to −12.4). These features indicate that the S-type granites could be generated by reworking of an ancient crust. Taken together, the penecontemporaneous magmatism and metamorphic event, demonstrated the early-middle Devonian transition from crustal thickening to extensional collapse. The post-collisional mantle-derived magmas serve as an essential driving force for the high-temperature granulite-facies metamorphism and anataxis of the crust associated with formation of S-type granite. This study not only constructs a more detail Proto-Tethys evolution process of the eastern Kunlun, but also sheds new light on better understanding the intimate relationship between magmatism and metamorphism during post-collisional extensional collapse.

Petrogenesis of ca. 113 Ma volcanic rocks in the central Lhasa subterrane, southern Tibet: Implications for the tectonic setting and continental crustal reworking

Voluminous Early Cretaceous volcanic rocks in the central Lhasa subterrane provide an ideal opportunity for understanding the mantle–crust interaction and tectonic‐magmatic evolution of the Lhasa Terrane. Here, we report zircon U–Pb ages, geochemical and Sr–Nd–Pb–Hf isotopic data for the Early Cretaceous volcanic rocks, including high‐silica rhyolites (117.2 ± 1.1 Ma) and contemporaneous andesites (114.1 ± 0.8 Ma) in the Sailipu area, central Lhasa subterrane. All these rocks show enriched light rare earth elements (LREE), and radioactive heat‐generating elements (e.g., Th, U, K, and Pb) but depleted high‐field‐strength elements (HFSEs, e.g., Nb, Ta, P, and Ti). The different bulk‐rock Sr–Nd–Pb and zircon Hf isotopic compositions of the rhyolites and andesites suggest that these rocks have distinct magma sources and petrogenetic history rather than an identical source involving assimilation‐fractional crystallization process. The Sailipu high‐silica rhyolites exhibit the characteristics of fractional crystallization and have varying zircon εHf(t) values (−12.2 to +5.2), negative εNd(t) values (−9.5 to −1.6), high and variable initial Sr isotopic compositions ([87Sr/86Sr]i = 0.7047–0.7116) and radiogenic Pb isotopic signatures (206Pb/204Pb = 18.784–18.802, 207Pb/204Pb = 15.722–15.737, and 208Pb/204Pb = 39.382–39.492). A combined process of magma mixing (involving crustal‐derived felsic melts and mantle‐derived mafic melts) and subsequent fractional crystallization were mainly responsible for the formation of these high‐silica rhyolites. The Sailipu andesites show more enriched Sr–Nd isotopic compositions [(87Sr/86Sr)i = 0.7088–0.7154, εNd(t) = −9.9 to −6.7] relative to rhyolites, and less radiogenic Pb isotopic compositions (206Pb/204Pb = 18.632–18.669, 207Pb/204Pb = 15.686–15.689, 208Pb/204Pb = 39.140–39.201). They were likely derived from partial melting of an enriched mantle wedge previously metasomatized by melts derived from subducted sediments. We interprete these Early Cretaceous volcanic rocks as the product of slab break‐off during the southward subduction of the Bangong‐Nujiang Tethyan oceanic seafloor. The dominantly positive εHf(t) and ancient TDMC ages (0.85–1.90 Ga) of Sailipu high‐silica rhyolites further imply that, under the stress relaxation regime, the mantle‐derived melts locally reworked the ancient basement of the central Lhasa subterrane.

Oxygen-Hafnium-Neodymium Isotope Constraints on the Origin of the Talnakh Ultramafic-Mafic Intrusion (Norilsk Province, Russia)

Abstract The ultramafic-mafic Talnakh intrusion in the Norilsk province (Russia) hosts one of the world’s major platinum group element (PGE)-Cu-Ni sulfide deposits. This study employed a multitechnique approach, including in situ Hf-O isotope analyses of zircon combined with whole-rock Nd isotope data, in order to gain new insights into genesis of the Talnakh economic intrusion. Zircons from gabbrodiorite, gabbroic rocks of the layered series, and ultramafic rocks have similar mantle-like mean δ18O values (5.39 ± 0.49‰, n = 27; 5.64 ± 0.48‰, n = 34; and 5.28 ± 0.34‰, n = 7, respectively), consistent with a mantle-derived origin for the primary magma(s) parental to the Talnakh intrusion. In contrast, a sulfide-bearing taxitic-textured troctolite from the basal part of intrusion has high δ18O (mean of 6.50‰, n = 3), indicating the possible involvement of a crustal component during the formation of sulfide-bearing taxitic-textured rocks. The Hf isotope compositions of zircon from different rocks of the Talnakh intrusion show significant variations, with ɛHf(t) values ranging from –3.2 to 9.8 for gabbrodiorite, from –4.3 to 11.6 for unmineralized layered-sequence gabbroic rocks, from 2.3 to 12 for mineralized ultramafic rocks, and from –3.5 to 8.8 for mineralized taxitic-textured rocks at the base of the intrusion. The significant range in the initial 176Hf/177Hf values is ascribed to interaction of distinct magma sources during formation of the Talnakh intrusion. These include (1) a juvenile source equivalent to the depleted mantle, (2) a subcontinental lithospheric source, and (3) a minor crustal component. Initial whole-rock Nd isotope compositions of the mineralized taxitic-textured rocks from the base of the intrusion (mean ɛNd(t) = –1.5 ± 1.8) differ from the other rocks, which have relatively restricted ranges in initial ɛNd (mean ɛNd = 0.9 ± 0.2). The major set of ɛNd values around 1.0 at Talnakh is attributed to limited crustal contamination, presumably in deep magma chambers, whereas the smaller set of negative ɛNd values in taxitic-textured rocks is consistent with greater involvement of a crustal component and reflects an interaction with the wall rocks during emplacement.

Hafnium isotope composition of zircon from the Kondyor clinopyroxenite-dunite massif (Khabarovsk territory, Russia)

This study evaluates for the first time Hf-isotope characteristics of zircon from dunite of the Kondyor massif, which is closely related to an economic platinum placer deposit. The significant range in εHf(t) values (from -8.4 ± 0.8 to 10.5 ± 1.3) in Mesozoic zircons indicates the interaction of a ‘juvenile’ mantle source with distinct magma sources, equivalent to a subcontinental lithospheric mantle and/or a continental crust. The peculiarities of the Hf-isotopic composition of Precambrian zircons (εHf(t) from -2.3 to +0.3) is consistent with their xenogenic origin and inheritance from basement rocks of the Siberian Craton.

Hafnium Isotope Composition of Zircon from the Kondyor Clinopyroxenite-Dunite Massif (Khabarovsk Territory, Russia)

This study evaluates for the first time Hf-isotope characteristics of zircon from dunite of the Kondyor massif, which is closely related to an economic platinum placer deposit. The significant range in eHf(t) values (from –8.4 ± 0.8 to 10.5 ± 1.3) in Mesozoic zircons indicates the interaction of a “juvenile” mantle source (eHf(t) = +15) with distinct magma sources, equivalent to a subcontinental lithospheric mantle and/or a continental crust. The peculiarities of the Hf-isotopic composition of Precambrian zircons (eHf(t) from –2.3 to +0.3) is consistent with their xenogenic origin and inheritance from basement rocks of the Siberian Craton.

The Petrology of the Kaiserstuhl Volcanic Complex, SW Germany: The Importance of Metasomatized and Oxidized Lithospheric Mantle for Carbonatite Generation

Abstract The Miocene Kaiserstuhl Volcanic Complex (Southwest Germany) consists largely of tephritic to phonolitic rocks, accompanied by minor nephelinitic to limburgitic and melilititic to haüynitic lithologies associated with carbonatites. Based on whole-rock geochemistry, petrography, mineralogy and mineral chemistry, combined with mineral equilibrium calculations and fractional crystallization models using the Least Square Fitting Method, we suggest that the Kaiserstuhl was fed by at least two distinct magma sources. The most primitive rock type of the tephritic to phonolitic group is rare monchiquite (basanitic lamprophyre) evolving towards tephrite, phonolitic tephrite, phonolitic noseanite, nosean phonolite and tephritic phonolite by fractional crystallization of variable amounts of clinopyroxene, amphibole, olivine, spinel/magnetite, garnet, titanite, plagioclase and nosean. During this evolution, temperature and silica activity (aSiO2) decrease from about 1100°C and aSiO2 = 0·6–0·8 to 880°C and aSiO2 = ∼0·2. At the same time, oxygen fugacity (fO2) increases from ΔFMQ* = +2–3 to ΔFMQ* = +3–5, with ΔFMQ* being defined as the log fO2 deviation from the silica activity-corrected FMQ buffer curve. Nephelinitic rocks probably derive by fractionation of mostly olivine, spinel/magnetite, melilite, perovskite and nepheline from an olivine melilititic magma. The nephelinitic rocks were formed at similarly high crystallization temperatures (>1000°C) and evolve towards limburgite (hyalo-nepheline basanite) by an increase of silica activity from about aSiO2 = 0·4–0·5 to aSiO2 = 0·5–0·9, whilst redox conditions are buffered to ΔFMQ* values of around +3. Haüyne melilitite and the more evolved (melilite) haüynite may equally be derived from an olivine melilitite by more intense olivine and less melilite fractionation combined with the accumulation of haüyne, clinopyroxene and spinel. These rocks were crystallized at very low silica activities (aSiO2 ≤0·2) and highly oxidized conditions (ΔFMQ* = +4–6). Even higher oxygen fugacities (ΔFMQ* = +6–7) determined for the carbonatite suggests a close genetic relation between these two groups. The assemblage of carbonatites with highly oxidized silicate rocks is typical of many carbonatite occurrences worldwide, at least for those associated with melilititic to nephelinitic silicate rocks. Therefore, we suggest that the existence of highly oxidized carbonate-bearing sublithospheric mantle domains is an important prerequisite to form such complexes.

Oxidation state inherited from the magma source and implications for mineralization: Late Jurassic to Early Cretaceous granitoids, Central Lhasa subterrane, Tibet

Arc magmas are more oxidized than mid-ocean ridge basalts; however, there is continuing debate as to whether this higher oxidation state is inherited from the source magma or developed during late-stage magmatic differentiation processes. Well-constrained Late Jurassic to Early Cretaceous arc-related intermediate to felsic rocks derived from distinct magma sources provide us with a good opportunity to resolve this enigma. A series of granitoids from the western Central Lhasa subterrane were analyzed for whole-rock magnetic susceptibility, Fe2O3/FeO ratios, and trace elements in zircon. Compared to Late Jurassic samples (1.8 ± 2.0 × 10−4 emu g−1 oe−1, Fe3+/Fetotal = 0.32 ± 0.07, zircon Ce4+/Ce3+* = 15.0 ± 13.4), Early Cretaceous rocks show higher whole-rock magnetic susceptibility (5.8 ± 2.5 × 10−4 emu g−1 oe−1), Fe3+/Fetotal ratios (0.43 ± 0.04), and zircon Ce4+/Ce3+* values (23.9 ± 22.3). In addition, positive correlations among whole-rock magnetic susceptibility, Fe3+/Fetotal ratios, and zircon Ce4+/Ce3+* reveal a slight increase in oxidation state from fO2 = QFM to NNO in the Late Jurassic to fO2 = ∼NNO in the Early Cretaceous. Obvious linear correlation between oxidation indices (whole-rock magnetic susceptibility, zircon Ce4+/Ce3+*) and source signatures (zircon eHf(t), TDM C ages) indicates that the oxidation state was predominantly inherited from the source with only a minor contribution from magmatic differentiation. Thus, the sources for both the Late Jurassic and Early Cretaceous rocks were probably influenced by mantle wedge-derived magma, contributing to the increased fO2. Compared to ore-forming rocks at giant porphyry Cu deposits, the relatively low oxidation state (QFM to NNO) and negative eHf(t) (−16 to 0) of the studied granitoids implies relative infertility. However, this study demonstrates two potential fast and effective indices (fO2 and eHf(t)) to evaluate the fertility of granitoids for porphyry-style mineralization. In an exploration context for the west Central Lhasa subterrane, features indicative of potential fertility might include more oxidized, positive eHf(t), young rocks (<130 Ma).

Geochemistry and geochronology of the Late Permian mafic intrusions along the boundary area of Jiamusi and Songnen-Zhangguangcai Range massifs and adjacent regions, northeastern China: Petrogenesis and implications for the tectonic evolution of the Mudanjiang Ocean

This paper presents zircon U-Pb ages, whole-rock major and trace element data, and Hf isotope data for the metagabbros from the Zhushan pluton and gabbros from the Taiping pluton along the boundary area of Jiamusi and Songnen-Zhangguangcai Range massifs and adjacent regions, which will not only place important constraints on the rock-forming ages, source characteristics and tectonic setting of these gabbros, but will also provide insights into understanding the Permian tectonic evolution between the Jiamusi Massif and the Songnen-Zhangguangcai Range Massif. Zircon U-Pb dating, determined using laser ablation-inductively coupled plasma-mass spectrometry and secondary-ion mass spectrometry, indicates that the magmatic zircons from the Zhushan and Taiping plutons yield 206Pb/238U ages of 256±2Ma and 259±3Ma, respectively, interpreted as the emplacement ages of the intrusions. The metagabbros from the Zhushan pluton display the geochemical characteristics of calc-alkaline series rocks, and are enriched in light rare earth and large ion lithophile elements, and depleted in Nb, Ta, P, Zr and Hf. The εHf(t) values of magmatic zircons in these metagabbros vary from −5.47 to +0.74. All these geochemical features indicate that the primary magma of the Zhushan pluton was derived from an enriched lithospheric mantle source that was metasomatized by subducted slab-derived fluids. The gabbros from the Taiping pluton are also enriched in large ion lithophile elements (e.g., Rb, Ba and U) relative to high field strength elements, and have negative Nb-Ta-P anomalies, with εHf(t) values of −4.02 to −1.70. It is inferred that they also formed from a primary magma generated by the partial melting of enriched lithospheric mantle that was metasomatized by subducted slab-derived fluids. The rocks from the Zhushan and Taiping plutons have similar petrogenetic processes, but their primary magmas are likely to be derived from two distinct magma sources based on geochemical and isotopic compositions. According to zircon U-Pb ages, geochemical data, Hf isotope data and regional geological investigations, we conclude that the generation of the metagabbros in the Zhushan pluton was most likely related to westward subduction of the Mudanjiang oceanic plate beneath the Songnen-Zhangguangcai Range Massif, and the gabbros from the Taiping pluton were the result of eastward subduction of the Mudanjiang oceanic plate beneath the Jiamusi Massif. Hence, we suggest that the Jiamusi Massif and Songnen-Zhangguangcai Range Massif were not amalgamated before the Late Permian.

Multiple Mixing and Hybridization from Magma Source to Final Emplacement in the Permian Yamatu Pluton, the Northern Alxa Block, China

A combined study of zircon U–Pb ages, whole-rock chemistry, Sr–Nd isotopes and in situ Lu–Hf isotopic ratios in zircons was carried out on Permian monzogranites and mafic microgranular enclaves (MME) and coeval massive gabbros in the northern Alxa block, North China. The data obtained were used to constrain magma sources and petrogenetic processes involved in the generation of this igneous suite. Zircon U–Pb dating yields ages of 271 ± 1 Ma, 270 ± 1 Ma and 276–270 Ma for the monzogranites, MME and massive gabbros, respectively. Two populations of MME, gabbroic (SiO 2 <49 wt %) and dioritic (SiO 2 >53 wt %) enclaves, are identified. They represent mafic to intermediate magmas quenched in a partially crystallized granitic (sensu lato) host; evidence to support this conclusion includes their fine-grained textures, sinuous margins and diffuse contacts with the host monzogranites. Back-veins and xenocrysts of quartz and plagioclase, as well as various disequilibrium textures and mineral assemblages, indicate mingling or mixing processes. The two magma systems, mafic and felsic, have broadly similar isotopic characteristics with whole-rock initial 87 Sr/ 86 Sr(i) ratios ranging from 0·7075 to 0·7077 in the monzogranite host and from 0·7067 to 0·7069 in the MME, with eN d (t) values ranging from –10·2 to –12·4 in monzogranites and from –8·2 to –9·9 in the MME. Zircon eHf(t) values of the monzogranites, gabbroic enclaves and massive gabbros show a wide range and significant overlap from –1·2 to –15·2, –4 to –13·3 and –5·4 to –19·5, respectively; the maximum frequency value of the Hf model age is almost coincident with the whole-rock Nd model age. Mafic, gabbroic rocks similar in composition to some enclaves form layered synplutonic intrusions several metres in thickness and more than 100 m in lateral extent. A mixing test based on mass balance for whole-rock major element compositions reveals that mixing was an efficient process between two coeval magmas. The fraction of felsic magma involved in the hybrid rocks ranged from 0·19–0·29 in the dioritic enclaves to 0·84 in granodiorite and good linear fits (r 2 > 0·9) are obtained using the average composition of gabbroic enclaves and the most felsic monzogranite as end-members. Although the monzogranites and enclaves may be derived from distinct magma sources, they share similar isotopic signatures, pointing to interaction processes in the source region. The combination of information from geochronology, petrology, whole-rock geochemistry and isotopic compositions leads us to conclude that the processes of hybridization and magma mixing were effective at both the level of emplacement in the shallow crust and at depth in the magma source region. Hybridization in a source region within the lithospheric mantle, involving mantle and crustal source rocks, produced magma bimodality with strong geochemical affinities between end-members and clear calc-alkaline arc signatures, compatible with a subduction setting. A plausible subduction erosion plus relamination model, which differs from classical models based on mafic magma underplating, is proposed. Accordingly, the Yamatu monzogranites are argued to have been generated from granitic melt segregated and ponded in buoyant silicic diapirs, which formed by melting of subducted melanges in the lithospheric mantle and eventually relaminated to the lower crust. They represent the melts that metasomatized the mantle region during a pre-Permian subduction event. The massive gabbros were formed by decompression melting of the previously metasomatized mantle. The gabbroic enclaves and gabbro layers of the Yamatu pluton are interpreted as magmas formed by decompression melting of this modified hydrated mantle. The dioritic enclaves represent hybrid liquids generated from reaction between granitic melts, which were derived from subducted melanges, and the hydrated mantle or melts derived from it.

Spatial distribution of helium isotopes in volcanic gases and thermal waters along the Vanuatu (New Hebrides) volcanic arc

We report the first helium isotope survey of volcanic gases, hot springs and some olivine phenocrysts along the Vanuatu island arc, from Tanna in the south to Vanua Lava in the north. Low CO2 content and low 3He/4He ratios in thermal fluids of Epi (4.0±0.1Ra), Efate (4.5±0.1Ra) and Pentecost (5.3±0.5Ra) islands coherently indicate reduced mantle gas leakage and crustal contamination by radiogenic helium on these extinct volcanic systems of the former (Pliocene) arc. Instead, presently active Vanuatu volcanoes display 3He/4He and C/3He ratios typical of subduction-related volcanic arcs: 3He/4He ratios range from 6.4±0.5Ra in southernmost Tanna and 7.23±0.09Ra in northernmost Vanua Lava to typical MORB values in the central islands of Gaua (7.68±0.06Ra), Ambrym (7.6±0.8Ra) and Ambae (7±2Ra in groundwaters, 7.9±1.4Ra in olivine phenocrysts, and 8.0±0.1Ra in summit fumaroles of Aoba volcano). On Ambrym, however, we discover that hydrothermal manifestations separated by only 10–15km on both sides of a major E–W transverse fault zone crossing the island are fed by two distinct helium sources, with different 3He/4He signatures: while fluids in southwest Ambrym (Baiap and Sesivi areas) have typical arc ratios (7.6±0.8Ra), fluids on the northwest coast (Buama Bay area) display both higher 3He/4He ratios (9.8±0.2Ra in waters to 10.21±0.08Ra in bubbling gases) and lower C/3He ratios that evidence a hotspot influence. We thus infer that the influx of Indian MORB mantle beneath the central Vanuatu arc, from which Ambrym magmas originate, also involves a 3He-rich hotspot component, possibly linked to a westward influx of Samoan hotspot material or another yet unknown local source. This duality in magmatic He source at Ambrym fits with the bimodal composition and geochemistry of the erupted basalts, implying two distinct magma sources and feeding systems. More broadly, the wide He isotopic variations detected along the Vanuatu arc further verify the complex tectonic and magmatic framework of this intra-oceanic island arc.

The Middle Triassic Meiwu Batholith, West Qinling, Central China: Implications for the Evolution of Compositional Diversity in a Composite Batholith

An integrated study involving whole-rock and Sr–Nd–Hf isotope geochemistry and zircon geochronology and trace element combined with detailed field investigation was carried out for the composite Meiwu batholith in the West Qinling orogenic belt of central China to probe the origins of its compositional diversity and its emplacement history. The batholith is composed of quartz diorite, granodiorite and biotite granite, with abundant mafic magmatic enclaves and minor tonalitic enclaves in the granodiorite. The crystallization age of the batholith is ∼240–245 Ma. Geochemical and Sr–Nd–Hf isotopic data indicate that the magmas that formed the quartz diorite and the mafic enclaves were derived by partial melting of enriched lithosphere mantle, followed by variable degrees of hybridization with crustal magmas in deep crustal hot zones. These initially heterogeneous, hybrid magmas successively intruded into the upper crust and coalesced into a large magma chamber. Zircon trace element and Hf isotopic compositions suggest that the outer fine-grained part of the quartz diorite pluton crystallized from a less differentiated magma as a result of rapid cooling and thus preserved its initial heterogeneities, whereas the inner medium-grained part of the quartz diorite pluton crystallized from a convecting, isotopically homogeneous magma that had undergone advanced magmatic differentiation. The Sr–Nd–Hf isotope compositions of the mafic magmatic enclaves are strikingly similar to those of the host granodiorite, implying their isotopic equilibration. The tonalitic enclaves have high Sr/Y ratios and most probably represent magmas derived from partial melting of thickened mafic lower crust. The Sr–Nd–Hf isotope data suggest that the granodiorite and biotite granite were dominantly derived from isotopically heterogeneous crustal sources. However, the granodiorite also has relatively high Mg#, Cr, and Ni, indicating a minor contribution from a mantle source. The granodiorite was constructed incrementally from a number of discrete melt batches that were generated by partial melting of mafic lower crust under variable water fugacity. These melt batches did not assemble into a large magma chamber and thus preserved their source chemical features. The granodiorite magma was also replenished by mafic and high Sr/Y magmas, resulting in abundant and compositionally diverse magmatic enclaves. The biotite granite formed by the successive accumulation of discrete magma pulses generated by dehydration melting of mafic lower crust under water-absent conditions. These magma pulses coalesced into a small single magma chamber where they underwent fractional crystallization. The various rock types exhibit distinct geochemical variations, indicating that the Meiwu batholith was constructed from multiple injections of magma over a protracted period. Fractional crystallization, assimilation, magma mixing and/or mingling occurred during magma ascent and at the emplacement level. Distinct magma sources played a primary role in controlling the chemical diversity of the igneous bodies at pluton and batholith scale.

Locating magma reservoirs using InSAR and petrology before and during the 2011–2012 Cordón Caulle silicic eruption

We combine petrological and InSAR observations to probe the origin of deformation of Cordón Caulle volcano in Southern Chile before and during the 2011–2012 eruption. The erupted rhyodacite is crystal-poor and was stored at about 900 °C with ca. 4 wt% H2O and 220 ppm CO2 in the melt, corresponding to a storage depth of about 6–7 km. In addition to a previously documented 2007–2008 deformation episode, we find three pre-eruptive deformation episodes: the first between 2008 and 2009 that covers ∼400 km2 with a maximum uplift of 18.5 cm, the second between February and March 2010 that covers ∼20 km2 with a maximum uplift of 10 cm, and the third between March and May 2011 that covers ∼40 km2 with a maximum uplift of 6 cm. Modeling of the 2008–2009 deformation episode suggests source depths of about 5–9 km, consistent with the petrologically determined magma storage depth. The sum of modeled source volume changes from 2007 to 2009 is about 12 times smaller than the observed erupted volume. Our calculation of magma compressibility shows that there must have been a significant volume of pre-existing magma in the reservoir to account for this difference. Co-eruptive subsidence shows evidence for three distinct magma sources, and pre-eruptive inflation is seen at two of the three sources. Modeled deformation sources are located 2–10 km from the 2011–2012 eruptive vent and coincide with the vents from previous historical eruptions (1921–22, 1960) and with the neighboring Puyehue volcano. This implies the existence of an interconnected magma plumbing system that allows for pressure transfer and which covers an area of at least 20 km2. Thus, the deformation and geochemical characteristics of Cordón Caulle resemble those of a large silicic caldera system. The presence of a mafic crystal clot demonstrates the existence of basaltic melts in the silicic reservoir, but we find that the mafic magmas reached the reservoir at least several hundred years before eruption. Therefore, if the pre-eruptive deformation was caused by replenishment to a shallow reservoir, the new magma was silicic and similar to the pre-existing magma, and/or there was no interaction between the two. We find that pre-eruptive deformation can occur in pulses and reach its peak years before eruption, and thus lack of deformation does not necessarily imply that eruption is not imminent.

Hafnium-neodymium constraints on source heterogeneity of the economic ultramafic-mafic Noril'sk-1 intrusion (Russia)

The ultramafic–mafic Noril'sk-1 intrusion in Polar Siberia (Russia) hosts one of the world's major platinum-group-element (PGE)-CuNi sulphide deposits. In situ Hf-isotope analyses of zircon and baddeleyite, combined with whole-rock Nd-isotope results, identify three distinct clusters of HfNd isotope values restricted to different lithological units (e.g., gabbro-diorite, unmineralised layered sequence gabbros, and mineralised portions, represented by ultramafic and taxitic-textured rocks). Hf-isotope signatures of zircon and baddeleyite from unmineralized mafic rocks (εHf from 7.3±1.1 to 11.4±0.3) reflect the dominant role of mantle-derived magmas and suggest that a juvenile mantle was one of the main sources for the ultramafic–mafic Noril'sk-1 intrusion. The less radiogenic Hf-isotope values for zircons from mineralised rocks (εHf from 4.9±1.4 to 6.4±1.2) and gabbro-diorite (εHf−1.2±1.9) are indicative of involvement of distinct source components, possibly equivalent to a subcontinental lithospheric mantle and a continental crust, respectively. The significant range in the initial 176Hf/177Hf values of unmineralised rocks and mineralised lithologies indicates interaction of distinct magma sources during formation of the Noril'sk-1 intrusion. Our new findings, in conjunction with whole-rock Nd-isotope results, imply that economic intrusions hosting PGE–CuNi deposits of the Noril'sk area have a far more complex magmatic history than is commonly assumed.

Characteristics and geodynamic evolution of Indosinian magmatism in South China: A case study of the Guikeng pluton

We examined the geochronology, petrology, and geochemistry of the two-stage Guikeng pluton, Wuyi Mountain Range, South China, with the aim of assessing the influence on Indosinian magmatism in South China of collision/amalgamation between the Sibumasu Block and the Indo-China and South China blocks, and between the North China and Yangtze blocks. An additional aim was to summarise the characteristics and geodynamic evolution of Indosinian magmatism in South China. The Guikeng pluton was then compared with the Indosinian Fucheng–Hongshan and Shiwandashan–Darongshan plutons in the Wuyi–Yunkai Mountain Range Belt, two groups of Indosinian granites in Hunan Province (South China Inland Belt), and Indosinian granitoids in the Sulu–South Korea Belt.The Guikeng pluton consists of medium- to fine-grained biotite monzogranites and biotite granites in its marginal belt, medium- to coarse-grained K-feldspar phenocryst-rich biotite granites in its interior part, and late-stage fine-grained muscovite granites. The granites have high Rb, Cs values, (87Sr/86Sr)i, and Rb/Sr ratios, low Sr, Ba, and εNd(t) values, and slightly to moderately negative Eu anomalies. These geochemical characteristics suggest that the granites were derived from late Paleo-proterozoic meta-sedimentary basement (1.7–2.0Ga).The rocks of the marginal belt and the main part of the pluton originated from two distinct magma sources. The fine-grained muscovite granites were produced by intensive differentiation of K-feldspar phenocryst-rich biotite granites. The granites from the marginal belt and the main part of the pluton yield SHRIMP U–Pb zircon ages of 234±3 and 220.4±3Ma, respectively, consistent with early and late Indosinian magmatism in South China. The early Indosinian metaluminous to weakly peraluminous foliated biotite granites were formed in a collision/compressional setting, whereas late Indosinian strongly peraluminous massive granites were formed in a post-collision setting.The rock assemblage, source region of Indosinian granites, and tectonic setting differ between South China and Sulu–South Korea, indicating the metamorphic basement of the Ogcheon Metamorphic Belt in North China is different to that of the Nanhua Rift in South China.

The Heerenveen Batholith, Barberton Mountain Land, South Africa: Mesoarchaean, Potassic, Felsic Magmas Formed by Melting of an Ancient Subduction Complex

The Heerenveen batholith exemplifies an aerially extensive group of � 3·1 Ga felsic potassic batholiths that mark the final magmatic event associated with the cratonization of the central^eastern Kaapvaal Craton, and the transition from the earlier sodic tonalite^ trondhjemite^granodiorite (TTG) to the more recent style of monzogranitic and granitic magmatism. As is typical of these bodies, it is geochemically and mineralogically heterogeneous, sheeted in structure, and was assembled incrementally, with eight main units in four assembly stages. Apart from a metaluminous quartz monzonite, all the Heerenveen rocks are peraluminous.There are few identifiable geochemical lineages among the rocks of the batholith. The geochemical heterogeneity is largely due to protolith internal heterogeneity, tapping of multiple sources, pulsed magma extraction related to sequential occurrence of discrete melting reactions, and varying degrees of entrainment of peritectic minerals formed during partial melting of the crustal protoliths, together with varying structural controls on magma extraction and ascent, which allowed distinct magma sources to be tapped, at different periods during batholith assembly. Fractional crystallization, magma mixing and wall-rock assimilation were not significant processes. TTG rocks are not suitable source materials for any of the Heerenveen magmas; nor are highly aluminous metasediments. For magmatic rocks with SiO2570 wt %, the likely source rocks were intermediate to mafic igneous rocks, with variable degrees of K-enrichment. For those rocks with SiO2472 wt %, the sources were K-enriched intermediate to felsic rocks. It seems likely that the partial melting reactions that produced the Heerenveen magmas occurred at moderate temperatures and that hornblende was present in the residues. This suggests that initial melting may have occurred in the presence of an H2O-rich fluid. There is no evidence that the genesis of the potassic granitic magmas involved either recycling of TTG crust or the presence of aluminous metasediments in the deep crust. The K-enrichment in the protoliths was most probably due to their ultimate derivation from mantle rocks metasomatized by fluids evolved in subduction-like processes. The monzonite in the batholith may have been derived directly by partial melting of this metasomatized mantle.

Geochemistry, geochronology, and cathodoluminescence imagery of the Salihli and Turgutlu granites (central Menderes Massif, western Turkey): Implications for Aegean tectonics

The Menderes Massif (western Turkey) is an important metamorphic core complex located in the Aegean region; geochemical and geochronological data from this extensional domain facilitates our understanding of large-scale extension of the Earth's lithosphere. S-type, peraluminous granites (Salihli and Turgutlu) that intrude the Alasehir detachment which bounds the northern edge of the central Menderes Massif may have been generated due to subduction of the Eastern Mediterranean floor along the Hellenic trench. In situ Th–Pb ion microprobe monazite ages from the granites range from 21.7 ± 4.5 Ma to 9.6 ± 1.6 Ma (± 1 σ ). Higher uncertainty in the ages is attributed to monazite common Pb, but the range is consistent with cathodoluminescence (CL) imagery that document complex textures within the samples. Salihli and Turgutlu granites share many similar characteristics, including multiple generations of plagioclase (some with shocked cores), plagioclase replacing K-feldspar and the development of myrmekite, evidence for fluid interaction, and multiple generations of microcracks and microfaults. The granites may have evolved from compositionally distinct magma sources, as Salihli samples in general contain allanite as the major accessory mineral, whereas Turgutlu granites contain monazite. However, the CL imagery document similar alteration textures. The youngest Turgutlu monazite grain (11.5 ± 0.8 Ma) is located in the rim of a K-feldspar grain close to myrmekite, and the youngest Salihli monazite grain (9.6 ± 1.6 Ma) is located in the outer edge of an altered plagioclase crystal. These ages likely time episodes of deformation as the granites were subjected to exhumation along the Alasehir detachment. Ages reported here are similar to dates constraining extension reported elsewhere in the Aegean, but indicate a level of complexity when linking movement within the Menderes Massif to the large-scale geodynamic processes that created other metamorphic core complexes in the region. Difficulties exist in linking the ages obtained from the granites to specific tectonic events due to the presence of secondary alteration textures, generations of mineral growth, and multiple episodes of deformation.

40Ar/ 39Ar age and geochemistry of the post-collisional Miocene Yamadağ volcanics in the Arapkir area (Malatya Province), eastern Anatolia, Turkey

The Neogene Yamadağ volcanics occupy a vast area between Sivas and Malatya in eastern Anatolia, Turkey. These volcanic rocks are characterized by pyroclastics comprising agglomerates, tuffs and some small outcrops of basaltic–andesitic–dacitic rocks, overlain upward by basaltic and dacitic rocks, and finally by basaltic lava flows in the Arapkir area, northern Malatya Province. The basaltic lava flows in the Arapkir area yield a 40Ar/ 39Ar age of 15.8 ± 0.2 Ma, whereas the dacitic lava flows give 40Ar/ 39Ar ages ranging from 17.6 through 14.7 ± 0.1 to 12.2 ± 0.2 Ma, corresponding to the Middle Miocene. These volcanic rocks have subalkaline basaltic, basaltic andesitic; alkaline basaltic trachyandesitic and dacitic chemical compositions. Some special textures, such as spongy-cellular, sieve and embayed textures; oscillatory zoning and glass inclusions in plagioclase phenocrysts; ghost amphiboles and fresh biotite flakes are attributable to disequilibrium crystallization related to magma mixing between coeval magmas. The main solidification processes consist of fractional crystallization and magma mixing which were operative during the soldification of these volcanic rocks. The dacitic rocks are enriched in LILE, LREE and Th, U type HFSE relative to the basaltic rocks. The basaltic rocks also show some marked differences in terms of trace-element and REE geochemistry; namely, the alkaline basaltic trachyandesites have pronounced higher HFSE, MREE and HREE contents relative to the subalkaline basalts. Trace and REE geochemical data reveal the existence of three distinct magma sources – one subalkaline basaltic trachyandesitic, one alkaline basaltic and one dacitic – in the genesis of the Yamadağ volcanics in the Arapkir region. The subalkaline basaltic and alkaline basaltic trachyandesitic magmas were derived from an E-MORB type enriched mantle source with a relatively high- and low-degree partial melting, respectively. The magmatic melt of dacitic rocks seem to be derived from an OIB-type enriched lithospheric mantle with a low proportion of partial melting. The enriched lithospheric mantle source reflect the metasomatism induced by earlier subduction-derived fluids. All these coeval magmas were generated in a post-collisional extensional geodynamic setting in Eastern Anatolia, Turkey.

Trace element constraints on mantle sources during mid-Proterozoic magmatism: evidence for a link between the Gardar (South Greenland) and Abitibi (Canadian Shield) mafic rocks

Trace and major element compositions of mid-Proterozoic (1.20–1.16 Ga) basaltic lava flows and dikes from the Gardar Province (South Greenland) provide evidence for two geochemically distinct magma sources. Based on distinct features of incompatible trace element ratios, such as Th/Ta, Th/Tb, or Th/Hf, they differ by the composition of their mantle source and by their partial melting trends. One mantle source is compositionally transitional between mid-ocean ridge basalt (MORB)-type and ocean-island basalt (OIB)-type sources with relatively low Ta/Hf ratios (~0.2), moderate enrichment in light rare-earth elements (LREE), and slightly positive initial εNdvalues (+2). It can be attributed to either a lithospheric mantle source or a depleted astenospheric mantle plume component that has been enriched shortly prior to eruption. The other mantle source is characterized by high Ta/Hf ratios (~0.6), a more pronounced LREE enrichment, and initial εNdvalues around 0. Elevated CeN/YbN(7.0–9.8) and TbN/YbNratios (1.6–1.8) of the rocks derived from this source indicate the presence of garnet during melting, suggesting melt generation at depths &gt; 70 km. This mantle source has the geochemical characteristics of an OIB-type source and is interpreted as originating from a mantle plume. Samples from the slightly younger (1.14 Ga) Abitibi dike swarm (Superior Province, Canada), spatially connected to the Gardar Province, show very similar trace element characteristics and the same two distinct magma sources can be identified. The geochemical similarities between the magma sources in South Greenland and Canada support the idea of a genetic link between the two magmatic provinces. This link strengthens the idea that the system was a long-lived major intracontinental rift zone.