Negative High Field Strength Element Research Articles

Multistage melt/fluid modification of lithospheric mantle beneath the circum-cratonic orogenic belt: Evidence from the Tuoyun peridotite xenoliths

The juvenile and moderate refractory mantle beneath the circum-cratonic orogenic belt is traditionally believed to be hotter and thinner than the ancient refractory cratonic mantle; it is thus more unstable and subject to modification by melts/fluids. Understanding these modification processes would help to elucidate the evolution of Earth’s continents. Peridotite xenoliths carried by the Tuoyun Cenozoic lamprophyre from the southwestern Tianshan belt show evidence of widespread multistage melt/fluid modification of the unstable circum-cratonic orogenic belt mantle. Tuoyun peridotites mainly consist of moderately refractory to fertile lherzolites (Mg# in Ol: 85.5−90.7; Cr# in Sp: 12.7−26.5) and show strong mechanical modification. They can be divided into four groups (A, B, C1, and C2) based on petrography and mineral chemistry. Group A lherzolites show relatively high basaltic components (Al2O3, CaO, TiO2, and FeO) and are enriched in large ion lithophile elements (LILEs) and rare earth elements (REEs), which indicates melt-peridotite reaction processes at high melt/rock ratios. The high modal pyroxene content in Group A suggests that the addition of high-Si melts caused the transition from olivine to pyroxene. Group B lherzolites show high modal pyroxene but relatively depleted incompatible elements, which should be superimposed by later melt extraction. Comparatively, Group C lherzolites exhibit higher modal olivine but lower basaltic components. The clinopyroxene cores of Group C1 are characterized by high (La/Yb)N and low Ti/Eu content, negative high field strength element (HFSE) anomalies, and relatively high 87Sr/86Sr ratios (cores: 0.70331−0.70457), which suggest metasomatism by carbonatite melts originating from recycled sedimentary carbonate. The Group C1 clinopyroxene (spongy rims) and Group C2 clinopyroxene (cores and spongy rims) have positive Sr anomalies, depleted HFSEs, and spoon-shaped REE patterns, which suggest modification by evolved small-volume and volatile-rich silicate melts. In addition, the melt pockets around spinels and the reactive zones of pyroxenes near the lamprophyre reveal the recent incongruent dissolution induced by the host rock. Based on our research and previously reported geological data, we propose that the high-Si melts and carbonatite melts are the products of dehydration and partial melting of the Paleo-Asian oceanic crust, and lithospheric delamination and fracturing (e.g., the Talas-Fergana strike-slip fault) provided the opportunity for small-volume and volatile-rich silicate melts and basaltic melts to modify the peridotites. Multistage melts/fluids and the deformation process are the protagonists in the evolutionary process of the circum-cratonic lithospheric mantle, with important implications for mantle destabilization and multilayered interaction.

Sr-Nd-Ca isotopic variations of Cenozoic calc-alkaline and alkaline volcanic rocks above a slab tear in Western Anatolia, Turkey

The tectonic mechanisms that may trigger a transition from calc-alkaline to alkaline volcanic activity above a subduction zone are enigmatic. We report major/trace elemental and Sr-Nd-Ca isotopic compositions of a suite of magmatic samples from the Miocene Gördes calc-alkaline dacites and the Quaternary Kula alkaline basalts from Western Anatolia, Turkey. Zircon sensitive high-resolution ion microprobe dating shows that the Gördes dacites formed at ca. 18 Ma. They are characterized by high-K calc-alkaline affinities, enrichment of light rare earth elements and large-ion lithophile elements (Rb, Th, U), positive Pb anomalies and negative Eu and high field strength element (HFSE) (Nb, Ta, and Ti) anomalies with high 87Sr/86Sri (0.70977−0.71010) and low εNd(t) (−7.3 to −7.0). They have small δ44/40Ca (0.59‰−0.69‰) values lower than that of mid-ocean ridge basalt (0.83% ± 0.11%). The Kula basalts exhibit alkaline affinities and are more enriched in incompatible elements (e.g., Rb, Ba, Th, Nb, Ta, Pb, and Sr) than those of typical oceanic island basalts. Their low 87Sr/86Sri (0.70307−0.70343) and high εNd(t) (+4.2 to +6.5) are consistent with an asthenospheric mantle source. Their δ44/40Ca values, ranging from 0.67‰ to 0.81‰, are higher than those of the dacites and lower than that of the bulk silicate earth (0.94‰ ± 0.10‰). Geochemical modeling suggests that variable degrees of partial melting of an asthenospheric mantle involving recycled oceanic crust may have resulted in low δ44/40Ca values of the Kula basalts. Combined with reported mantle tomographic images, we interpret that sub-slab asthenospheric mantle upwelling through a slab tear during subduction roll-back may have played a key role in the Sr-Nd-Ca isotopic variability from Miocene calc-alkaline dacites to Quaternary alkaline basalts in Western Anatolia.

Zircon U–Pb age, whole-rock geochemistry and Nd–Sr–Pb isotope constraints on petrogenesis of the Eocene Zajkan gabbro–monzogranite intrusion, Tarom-Hashtjin magmatic belt, NW Iran

The late Eocene high-K to shoshonitic Zajkan intrusion, situated in the Tarom-Hashtjin magmatic belt (THMB), NW Iran, ranges in composition from gabbro to pyroxene quartz monzodiorite, pyroxene quartz monzonite and monzogranite, and is classified as an I-type metaluminous granitoid. On the primitive mantle-normalised spider diagram, these rocks demonstrate negative high field strength element (HFSE) anomalies, large-ion lithophile element (LILE) enrichment, and positive Pb anomaly, indicating a subduction-related signature. Furthermore, the granitoids are characterised by coherent chondrite-normalised rare earth element patterns with high light rare earth elements/heavy rare earth elements (LREE/HREE) and moderate to weakly negative Eu anomalies (Eu/Eu* = 0.6–0.86). LA-ICP-MS zircon U–Pb dating demonstrates that the Zajkan intrusion crystallised at ca 36.5 Ma. Relatively low to moderate 87Sr/86Sr ratios (0.7046–0.7061) and positive εNd(t) values (1.52–1.88) of the Zajkan samples are consistent with an enriched lithospheric mantle source that was metasomatised by an earlier subducted slab. Geochemical features show that the parental magma of the Zajkan intrusion likely resulted from a partial melting of metasomatised amphibole-bearing mantle source. Sr–Nd isotope modelling is consistent with the Zajkan intrusion formed from a mix of ∼80% of a mantle-derived magma and ∼20% of a juvenile lower crustal-derived magma. Partial melting of the metasomatised mantle may have resulted from late Eocene lithospheric thinning and hot asthenospheric upwelling through slab roll-back or lithospheric delamination. KEY POINTS The late Eocene high-K to shoshonitic Zajkan intrusion is located within the Tarom-Hashtjin magmatic belt, NW Iran. U–Pb zircon dating revealed an age of ca 36.5 Ma for emplacement of the Zajkan intrusion. Sr–Nd isotope modelling is consistent with the Zajkan intrusion formed from a mix of ∼80% of mantle-derived magma and ∼20% of juvenile lower crustal-derived magma.

Genesis and magmatic-hydrothermal evolution of the Shapoling Mo deposit, East Qinling, China: Insights from geochronology, petrogenesis and fluid evolution

The Shapoling deposit, located in Xiong'ershan area, East Qinling, China, is characterized by a magmatic-hydrothermal Mo vein system. We present whole-rock geochemical, zircon U-Pb-Hf-O isotopic, quartz H-O isotopic and fluid inclusion data for the Shapoling deposit. We use these data to investigate the petrogenesis of granitoids associated with Mo mineralization and the processes of magmatic-hydrothermal evolution that formed the Mo deposit. The granitoids from the Shapoling deposit are metaluminous, alkalic to calc-alkalic, shoshonitic to high-K series, and belong to highly fractionated I-type granitoids. These granitoids are strongly enriched in light rare earths (LREEs) and large ion lithophile elements (LILE), but depleted in heavy rare earths (HREEs) with negative europium anomalies and high field-strength elements (HFSE), mostly plotting in the range of the Taihua Group rocks. Most of the Shapoling granitoids show relatively lower Sr/Y and (La/Yb)N than the typical adakitic rocks, showing that the parental magmas were generated under lower pressure. Therefore, the Shapoling granitoids were generated at thin crustal levels by lithospheric thinning process linked to the tectonic transition from compression to extension. Zircon U-Pb ages of the Shapoling granitoids range from 132.9 ± 1.3 Ma to 126.5 ± 0.7 Ma. The εHf(t) values and δ18Ozircon values show a variation trend from −19.1 to −8.0 and 5.33‰ to 6.30‰, which are consistent with Hf-O isotopic features of mixing of the partially melted Taihua Group with mantle-derived components. FIluid inclusion microthermometry results suggest that the hydrothermal fluids forming the Shapoling Mo deposit changed from early moderate- to low-salinity CO2-rich fluids in the H2O-NaCl-CO2 system, to late H2O-NaCl low-salinity fluids. The corresponding quartzs have δD values varying from −93.3‰ to −78.2‰ and δ18O values from −5.14‰ to 0.92‰, indicating a dominant magmatic source but were diluted by meteoric water in the late stage. Fluid boiling is the key factor that controlling the precipitation of molybdenite. Our study provides insights into the formation of Mo system associated with a prolonged and active magmatic system, upwelling of hot asthenosphere and intense mantle-crust interaction. The prolonged magmatic system is steadily replenished by mantle-derived materials in the magma chamber, which feed the hydrothermal systems and govern the formation of the Shapoling Mo deposit.

Origin of ultramafic–mafic bodies on the Isles of Lewis and Harris (Scotland, UK): Constraints on the Archean–Paleoproterozoic evolution of the Lewisian Gneiss Complex, North Atlantic Craton

The Lewisian Gneiss Complex (LGC) is a tonalite-trondhjemite-granodiorite (TTG)-dominated fragment of the North Atlantic Craton (NAC) in northwest Scotland. End-member models describe the LGC as representing either a continuous piece of Archean crust or up to 12 geologically distinct Archean terranes, with interpretations sitting on a spectrum between these end-members. There is particular uncertainty over the correlations between the Archean–Paleoproterozoic magmatic and metamorphic events recorded by mainland part of the LGC and the part exposed on the Outer Hebridean islands of Lewis and Harris. In this paper, we present the results of field mapping, petrography, and major, trace and platinum-group element (PGE) bulk-rock geochemistry for four ultramafic–mafic bodies in Lewis and Harris, namely: Maaruig, Loch Mhorsgail, Coltraiseal Mor and Beinn a’ Chuailean. We consider the effects of metamorphism and element mobility, their petrogenesis, and potential correlations with ultramafic–mafic rocks elsewhere in the LGC. Our data indicate that the studied ultramafic–mafic rocks can be subdivided into two petrologically distinct groups. Metaperidotites and metapyroxenites from Maaruig and Loch Mhorsgail are interpreted as Archean (>2.8 Ga) cumulates distinct from anything currently identified in the mainland LGC, with this interpretation based on distinctive modal layering, a discordance with surrounding TTG gneiss, fractionated PGE patterns ([Pd/Ir]N = 1.3–6.6) and negative HFSE anomalies ([Nb/La]N = 0.2–0.8). Metagabbronorites from Coltraiseal Mor and Beinn a’ Chuailean, which also exhibit negative high field strength-element (HFSE) anomalies ([Nb/La]N = 0.2–0.7) and show mildly fractionated ([Pd/Ir]N = 1.2–2.8) PGE patterns, most likely represent deformed Paleoproterozoic dykes. These occurrences could be correlatives of a suite of ca. 2.4 Ga mafic dykes exposed throughout the mainland LGC (the Scourie Dykes), with the Outer Hebridean occurrences having experienced more intense Paleoproterozoic (Laxfordian) deformation/reworking. These interpretations suggest that the LGC lithologies of Lewis and Harris were proximal to the mainland LGC’s Central Region by the early Paleoproterozoic but raises the possibility that they were distinct crustal blocks in the Mesoarchean.

Paleoproterozoic ophiolitic mélanges and orogenesis in the northern Yangtze Craton: Evidence for the operation of modern-style plate tectonics

Ophiolitic mélange that forms along convergent plate margins plays an important role in reconstructing the tectonic evolution of ancient orogenic belts and associated geodynamic settings. However, ophiolitic mélanges have rarely been documented in the Archean and Paleoproterozoic geological records, which led to the controversy surrounding the onset or not of plate tectonics at that time. Here we present an integrated study of petrography, geochemistry, zircon U-Pb-Lu-Hf isotope, and metamorphic P–T estimations on the newly-recognized 2.12–2.03 Ga ophiolitic mélange that contains amphibolite and serpentinite blocks in the North Kongling Complex (NKC), northern Yangtze Craton to better constrain the tectonic evolution of the northern Yangtze Craton and prevailing geodynamic backgrounds. The amphibolites and serpentinites in the mélange exhibit slightly-enriched light rare earth elements (LREEs), large-ion lithophile elements (LILEs, e.g., Rb, Ba) and negative high field strength elements (HFSEs, e.g., Nb, Ta, Zr, Hf, and Ti) anomalies, suggesting their derivation from a subarc mantle wedge metasomatized by slab-derived fluids/melts. The mafic–ultramafic components likely represent the dismembered fragments of a ca. 2.12 Ga suprasubduction zone (SSZ) arc/forearc complex. The garnet amphibolite underwent intermediate T/P metamorphism at ca. 2.03 Ga, with a clockwise P–T path. The estimated (near) peak P–T conditions of ∼10.1–10.6 kbar/660–670 °C correspond to thermobaric ratios of ∼625–635 °C/GPa, consistent with subduction-collision-related geothermal gradients. Collectively, the mélange recorded an intra-oceanic subduction-accretion event at ca. 2.12 Ga and subsequent collision at ca. 2.03 Ga. In combination with regional geological data, the accretionary-to-collisional orogenic events in the NKC of the northern Yangtze Craton could be a response to global-scale convergence and assembly of the Paleoproterozoic supercontinent Nuna/Columbia. The Paleoproterozoic (2.12–2.03 Ga) ophiolitic mélanges and orogenesis in the northern Yangtze Craton thus support the operation of the modern-style plate tectonic regime at that time.

Petrology of the Machangqing Complex in Southeastern Tibet: Implications for the Genesis of Potassium-rich Adakite-like Intrusions in Collisional Zones

Abstract Many intrusions with adakite-like affinities in collisional zones have obviously higher K2O contents and K2O/Na2O ratios compared with counterparts in subduction zones. A suite of Eocene post-collisional high-K2O adakite-like intrusions, mafic microgranular enclaves, and potassic–ultrapotassic lamprophyres in the Machangqing complex are associated with the Indian–Asian collision within the western Yangtze Craton, southeastern Tibet. The potassic–ultrapotassic lamprophyres, with a zircon U–Pb age of 34·1 ± 0·2 Ma, have high K2O and MgO contents, are enriched in light rare earth elements and large ion lithophile elements, and display high Rb/Sr, and low Ba/Rb and Nb/U ratios. They show enriched isotopic compositions [i.e. (87Sr/86Sr)i = 0·7070–0·7082, εNd(t) = −3·2 to −2·8], and zircon εHf(t) values (−1·6 to +2·6). Their parental magmas are inferred to have been derived from partial melting of an enriched lithospheric mantle, metasomatized by subduction-related fluids. The adakite-like intrusions, with zircon U–Pb ages of 35·4 ± 0·4 and 35·2 ± 0·3 Ma, are characterized by high SiO2 (68·8–71·1 wt%) and Al2O3 (14·0–15·3 wt%) contents, high Sr/Y (41–118) ratios, and low Y (5·3–14·7 ppm) contents. They show low contents of compatible elements (e.g. Ni = 9·5–36·2 ppm) and total REE, and lower Mg# values than the lamprophyres and mafic microgranular enclaves. The adakite-like intrusions have positive large ion lithophile element anomalies, especially potassium, negative high field strength element anomalies, negative εNd(t) (−5·5 to −3·3), and high (87Sr/86Sr)i (0·7064–0·7070) and zircon εHf(t) values (0·0 to +2·7), indicating that they were formed by partial melting of the juvenile lower crust. Mafic microgranular enclaves hosted in the adakite-like intrusions, with U–Pb ages similar to the lamprophyre of c. 34 Ma, exhibit disequilibrium textures, and some of them contain phlogopite. They exhibit potassic–ultrapotassic affinity, and relatively high compatible element contents. They are also characterized by enriched isotopic compositions with (87Sr/86Sr)i = 0·7063–0·7074, εNd(t) = −6·6 to −4·1, and variable zircon εHf(t) values (−0·6 to +3·2). Petrological and geochemical evidence suggests that the mafic microgranular enclaves were formed by magma mixing between potassic–ultrapotassic and pristine adakite-like melts. We propose a magma mixing model for the origin of the high-K2O adakite-like intrusions from the Machangqing complex. In this model, the formation of high-K2O adakite-like intrusions occurred in three stages: (1) partial melting of metasomatized lithospheric mantle generated potassic–ultrapotassic mafic melts; (2) underplating of these mafic melts beneath thickened juvenile lower crust resulted in partial melting of juvenile mafic lower crust and the generation of adakite-like melts; (3) magma mixing involved 80 % pristine adakite-like melts and 20 % potassic–ultrapotassic melts. This leads to the enrichment of K2O in these adakite-like intrusions, and magma differentiation further promotes K2O enrichment. These results are applicable to compositionally similar adakite-like rocks produced in other collisional zones, such as the Tibet, Sulu–Dabie and Zagros orogenic belts. From which we conclude that in continental collision zones, the post-collisional mantle-derived magmas characterized by potassic–ultrapotassic affinities are spatially associated with coeval collision-related adakite-like intrusions that originated from lower crustal melting. The emplacement of adakite-like and potassic–ultrapotassic rocks is controlled by the same fault systems, which increases the possibility of interaction between these two magma suites.

Petrogenesis and tectonic implications of the neoproterozoic mafic-ultramafic rocks in the western Jiangnan Orogen: Insights from in situ analysis of clinopyroxenes

The Huaqiao clinopyroxenite and Maodu gabbro are Neoproterozoic mafic-ultramafic rocks located in the western Jiangnan Orogen. Both rocks contain fresh clinopyroxenes. The in situ analysis of fresh clinopyroxenes from these rocks provides essential information about their parental magmas and mantle sources. Here, we present in situ major and trace element compositions of clinopyroxenes from these rocks to determine their petrogenesis and tectonic setting. Clinopyroxene grains from the Huaqiao clinopyroxenite exhibit relatively high values of Mg# (80.6–84.1), Cr2O3 (0.38–1.00 wt%), TiO2 (0.44–1.84 wt%), Ni (324–460 ppm), V (314–611 ppm), and Co (37–46 ppm); the calculated Mg# values of the parental liquid(s) are 52–58 (average 55). Clinopyroxene grains from the Maodu gabbro display lower Mg# (71.5–79.1), Cr2O3 (0–0.88 wt%), Al2O3 (1.72–4.75 wt%), and Na2O (0.24–0.88 wt%) contents but higher TiO2 (0.88–1.89 wt%) contents than those from the Huaqiao clinopyroxenite; they show relatively high Cr (15–4145 ppm, with an average of 1774 ppm), V (34–663 ppm, with an average of 535 ppm), and Co (32–60 ppm) contents; the Mg# values of the parental liquid(s) are estimated to be 39–49 (average 45). Therefore, we infer that clinopyroxene grains from the Huaqiao clinopyroxenite and Maodu gabbro crystallized from evolved basaltic melts. Mineral-melt partition coefficients reveal that the estimated parental magma composition in equilibrium with clinopyroxene for the Huaqiao clinopyroxenite exhibits slight enrichments in light rare earth elements (LREEs, LaN/YbN = 1.12–2.71; subscript N: chondrite normalized) with flat heavy REEs (HREEs, GdN/YbN = 1.15–1.85). It displays negative high field strength element (HFSE, such as Nb, Zr, and Hf) anomalies, resembling those of typical arc basalt. The calculated parental melt for the Maodu gabbro is characterized by pronounced enrichments in LREEs with LaN/YbN ratios of 11.36–19.02, and in large ion lithophile elements (LILEs) relative to HFSEs and lacks significant Nb, Zr, and Hf troughs, comparable to those of ocean island basalts (OIBs). Hence, we infer that the magma source of the Huaqiao clinopyroxenite was an arc mantle metasomatized by subduction melts and/or fluids from the subducted slab formed in a subduction setting; the nature of the source for the Maodu gabbro was the asthenospheric mantle that experienced upwelling and partial melting in an extensional setting.

Transformation from oxidized to reduced alkaline magmas in the northern North China Craton

The oxygen fugacity of a magma controlling element distribution and mobility is influenced by multiple factors including source composition and magmatic processes (e.g., assimilation and degassing). Here, whole-rock and in situ mineral analyses of the alkaline complex from the northern North China Craton are reported to investigate the influence of source composition and magmatic processes on the evolution of the melt oxidation state. The alkaline complex comprises orthoclase pyroxenites, pyroxene syenites, and syenites with similar mineral assemblages and trace element patterns. Consistent zircon and titanite U-Pb age of ~234 Ma is presented for the alkaline rocks. Their high large ion lithophile elements content, negative high field strength element anomalies, and high 87Sr/86Sr ratio suggest a metasomatized mantle origin by recycled crustal material. Magnetite and hematite inclusions were found in the early-crystallized minerals including clinopyroxene and apatite. Fluorapatites are characterized by high S (SO3 up to 0.64 wt%) and low Mn contents, with estimated ΔFMQ higher than +3. These features together indicate a primary oxidized magma inherited from the metasomatized mantle.However, inclusion-rich fluorapatites with decreasing S and increasing Mn content from the core to the rim were found in some pyroxene syenites. Moreover, the S content of inclusion-rich fluorapatite from these pyroxene syenites is lower than that of other rock types in the complex, which may correspond to a decrease in oxygen fugacity of ~2.5 log units. These features suggest that the oxidized magma was later transformed into reduced magma. Oriented Cu-rich sulfide in inclusion-rich fluorapatite formed during the reducing event. δ65Cu of the pyroxene syenite with inclusion-rich fluorapatite (−0.06 to −0.24‰) and sulfides in fluorapatite (−0.26‰) are lighter than those of other rock types (+0.30 to +0.43‰), suggesting that the sulfides were derived from extraneous materials via magma assimilation. Variable and high 87Sr/86Sr ratios (0.7071–0.7085) in titanites and high-207Pb/206Pb rims of zoned K-feldspar in the pyroxene syenite imply that the reduced extraneous materials were derived from ancient crust. Overall, in situ mineral analyses and Cu isotopes indicate a direct link between the transformation of alkaline magma from an oxidized to a reduced state with source composition and magmatic processes, and improve our understanding of element mobility and distribution.

Petrogenesis and Geological Implications of the Oligocene Mingze monzodiorites, Southern Lhasa

The Mingze Cu-Mo deposit is located in the southern margin of the Lhasa block of the Himalayan Tibetan Plateau. Here, we report the geochronological and geochemical data from Mingze monzodiorites, which hosts the Mingze deposit. Zircon dating indicates that the Mingze monzodiorites were emplaced at ca. 31 Ma (i.e., the Oligocene). The monzodiorites have variable SiO2 and MgO contents, strongly negative high field-strength element (HFSE, such as Ta, Nb, Zr and Hf) anomalies on the normalized trace element diagram and show uniform (87Sr/86Sr)i (0.7066–0.7076), εNd(t) (−2.50 to −4.04) and εHf(t) (+1.50 to +7.50). Their geochemical compositions are different from coeval (40–30 Ma) adakite-like rocks but comparable to coeval mafic enclaves and gabbros. We propose that Mingze monzodiorites were derived from partial melting of the lithospheric mantle, which previously metasomatized by the subducted Indian continental plate that probably subducted into the overlying mantle. The concurrency of the genetically related mafic enclaves and associated intermediate to mafic rocks implies the heterogeneity of the Lhasa lower crust.

Identification of the Neoarchean Jianping pyroxenite-mélange in the Central Orogenic Belt, North China Craton: A fore-arc accretional assemblage

Mélanges are widespread in accretionary and collisional orogens at convergent plate margins. We document a Neoarchean mélange belt exposed in the Jianping Complex, North China Craton (NCC). The mélange belt contains a structurally chaotic tectonic mixture of paragneiss, metapelites mixed with exotic tectonic blocks of mafic and ultramafic rocks including pyroxenites, metagabbros, metadiabases, metabasalts, and TTG gneisses. The blocks have formation ages of ca. 2.53–2.51 Ga and all units in the mélange have been intruded by undeformed pegmatite dykes (ca. 2.49 Ga). Geochemical studies show that the pyroxenites in the mélange have high contents of MgO (15.84–20.87%), Mg# (68–80), Ni (211–446 ppm) and Cr (963–2491 ppm). They are characterized by slightly negative Eu anomalies, enrichment in light rare earth elements (LREE) and pronounced negative high field strength elements (HFSE) anomalies on chondrite- and primitive mantle- normalized diagrams. We suggest that the pyroxenites are cumulates derived from an arc-related lithospheric mantle source. The pyroxenites in the mélange are suggested to have formed in a subduction-related fore-arc environment whose mantle source was metasomatized by fluids derived from the subducted slab. The recognition of the Neoarchean Jianping pyroxenite-mélange and regional tectonic analysis of the NCC indicate that the mélange belt in the Jianping Complex is one segment of a 2.5 Ga suture zone that crosses the NCC, stitching a forearc segment of an oceanic arc terrane in the Central Orogenic Belt (COB) with the Eastern Block. The Jianping pyroxenite-mélange is therefore correlative with the Zunhua, Zanhuang, and Dengfeng mélanges in more southern segments of the Central Orogenic Belt, defining a Neoarchean suture that extends for approximately 1600 km across the craton. The geochemistry of the arc-derived magmas, and the tectonic association with a subduction-related mélange is similar in petrology, chemistry, structural style, and size to modern-day forearc settings, negating any calls for non-plate tectonic settings for arc-like magmas in the Archean.

Petrology and geochemistry of a boninite dyke from the western Bastar craton of central India

The Dongargarh Supergroup along with the basal Amgaon Gneissic Complex constitutes the northwestern part of the central Indian Bastar craton. In the present study, we report a new finding of a boninite dyke intruded in the Amgaon gneisses of this area. The dyke composed of mainly pyroxenes, amphiboles and subordinate amount of plagioclase. The higher contents of \(\hbox {SiO}_{2}\) (51–54 wt.%), MgO (12–14 wt.%), Ni (375–473 ppm), Cr (1416–1580 ppm) and very low \(\hbox {TiO}_{2}\) (0.2–0.4 wt.%) are consistent with the boninite nature of the dyke as well as the unevolved primary nature of the source magma. The extraordinarily high CaO content (15.97–17.7 wt.%) with higher \(\hbox {CaO/Al}_{2}\hbox {O}_{3}\) (3.13–3.96) ratios classifies it as high-Ca boninite. The trace element ratios including Zr/Ti, Ti/V, Ti/Sc and Ti/Yb further show its geochemical similarity with the Archaean boninite. The dyke also shows negative high-field strength element (Nb, Ta and Ti) anomalies which are the characteristics of the boninite rocks reported elsewhere and along with the enriched light rare earth element pattern, it shows more affinity particularly with the northern Bastar boninite dyke. The mineralogical and geochemical similarities of the boninite dykes from the Bastar craton indicate a widespread boninitic event during the Palaeoproterozoic having a similar origin. These boninite dykes indicate the preservation of subduction-related signatures in the lithospheric mantle beneath the Bastar craton at the time of its evolution or may be during the convergence of the Bastar and Bundelkhand cratons.

40Ar-39Ar ages and petrogenesis of middle Eocene post-collisional volcanic rocks along the Izmir-Ankara-Erzincan suture zone, NE Turkey

The central and northeastern of Turkey were shaped by the transition from collisional to post-collisional tectonics and coeval magmatism during Early Cenozoic. However, the temporal and spatial evolution of the magmatism during the middle Eocene remains elusive. In this paper, we provide petrological, isotopic and radiometric data from two middle Eocene volcano-sedimentary successions, from the northern (Almus) and southern (Yıldızeli) parts of the Izmir–Ankara–Erzincan Suture Zone, and discuss their generation in the post-collisional setting. Our results indicate that the volcanic units from these regions display similar stratigraphic and petrological evolution during the narrow window of activity (44.4–44.8 Ma), which is constrained by Ar–Ar dating. The lower parts of the volcano-stratigraphic sections contain hawaiite, and trachybasalts that have a mildly alkaline, nepheline-normative character (V1a subseries), together with clinopyroxene-hornblendite xenoliths. The middle portions of the sections contain basaltic andesitic to dacitic lavas that become tholeiitic/calc-alkaline in character (V1b subseries). In the middle to upper parts of the successions, tholeiitic/calc-alkaline basaltic andesite (V2a subseries) and nepheline-normative mildly alkaline basalts (V2b subseries) become dominant, marking the end of the volcanic episode.All of these units display negative high field strength element values, and enriched large-ion lithophile and light rare earth element values, which mimic subduction-related signatures. The Na-rich character, high Ba/Rb and low Rb/Sr ratios, together with FC3MS values (wt.% FeO/CaO-3 * MgO/SiO2), of these series indicate the presence of amphibole and pyroxene minerals in the source area. Also, Dy/Yb values from the most basic suites, V1a and V2b, are supportive of a spinel-bearing source area. Low-degree partial melting (∼1–5%) modelling of cumulate clinopyroxene-hornblendite xenoliths in the lava series provided the observed geochemical patterns in our most basic lava samples. Petrological observations indicate that the V1a and V1b subseries were shaped by fractional crystallization and assimilation-related modifications. In addition, the V2a subseries displays basic magma replenishment, with inversed clinopyroxene cores and mantle/rim transects. The V2b subseries were most likely shaped by fractional crystallization of homogenized basic magma mush.Post-collisional magmatism across the region was therefore generated by thinning of the lithospheric mantle during the middle Eocene.

Petrogenesis of Cenozoic high–Sr/Y shoshonites and associated mafic microgranular enclaves in an intracontinental setting: Implications for porphyry Cu-Au mineralization in western Yunnan, China

Cenozoic high–Sr/Y shoshonites in western Yunnan, China, are spatially and temporally associated with porphyry deposits and mafic volcanic rocks that formed at a post–collisional setting. However, the petrogenesis of these rocks and the origins of the associated mineralization remain unclear. Here we present new geochemical data for the high–Sr/Y shoshonites and associated mafic microgranular enclaves (MMEs) in the Jinshajiang–Ailaoshan mineralization belt, which formed as a result of India–Asia collision during the Cenozoic. Both fertile and barren shoshonitic porphyries were emplaced during the Eocene–Oligocene (32–37 Ma) and are characterized by enrichment in large–ion lithophile elements (LILE), depletion in high field strength elements (HFSE), high K2O (4–6 wt%), Sr contents (674–1370 ppm), and low Y contents (7–18 ppm). The barren Zhanhe, Yongsheng, Songgui, and Jianchuan, and fertile Machangqing (Cu–Au–Mo mineralization) porphyry intrusions have variable zircon εHf(t) values of −6 to +9 and old TDM2 ages of 1433 to 900 Ma. However, the barren Shigu porphyry intrusion yields relatively positive zircon εHf(t) values (−4 to +8) and younger TDM2 ages (903–631 Ma). These features indicate that the Shigu intrusion was derived from late Permian juvenile lower crust, and the other high Sr/Y shoshonites were derived mainly from Neoproterozoic juvenile lower crust. Fertile and barren porphyries have variable whole–rock εNd(t) values (−5 to −0.7) and (87Sr/86Sr) i ratios (0.70595–0.70788) that are within the range of coeval mafic magmas in western Yunnan. The MMEs comprise mafic enclaves and andesitic enclaves that formed during the Eocene (34–35 Ma) and have spheroidal shapes, igneous textures and contain acicular apatites. This indicates that the MMEs are globules of coeval mafic magma that was injected into and mixed with the host shoshonitic magma. The enclaves show low TiO2 contents (0.7–0.8 wt%), high MgO (6–9 wt%), Cr (184–450 ppm) and Ni contents (98–157 ppm), positive LILE anomalies, marked negative HFSE anomalies, a narrow range of εNd(t) values (−1.2 to −0.6), and (87Sr/86Sr) i values of 0.70556–0.70635. The andesitic enclaves show adakitic geochemical affinities and have similar zircon εHf(t) values (−2.7 to +2.5) to the host porphyries, whereas, the mafic enclaves have relatively negative zircon εHf(t) values (−8.7 to +1.5), similar to enriched mantle. These evidences suggest the high−Sr/Y shoshonites were produced by partial melting of juvenile lower crust mixed with ultrapotassic or potassic mafic magmas as represented by the andesitic enclaves. The mafic enclave melts were derived from enriched mantle metasomatized by slab–derived fluids, which elevated the Mg#, Cr, Ni contents of the host porphyries at Machangqing, Zhanhe and Yongsheng.Zircon saturation temperatures (TZr) of the high–Sr/Y shoshonitic rocks range from 735 to 777 °C and indicate the shoshonites were derived mainly from water–fluxed melting of lower crust. The Manchangqing fertile intrusion had a high oxidation state (zircon Ce4+/Ce3+ = 38–180, average = 91) and was derived by mixing between juvenile lower–crustal melts and voluminous coeval lamprophyres. In contrast, the barren Jianchuan, Songgui, Shigu, Yongsheng and Zhanhe high–Sr/Y shoshonites formed by partial melting of juvenile lower crust and some mixing with mafic lavas, and are characterized by low oxygen fugacity (zircon Ce4+/Ce3+ = 0.5–77, average = 20). As such, we suggest the mafic magmas triggered water–fluxed, moderate–degree partial melting of the lower crust and caused amphibole breakdown during melting. This process could not provide enough water for the formation of porphyry Cu-Au deposits in western Yunnan, and therefore formed low–fO2, relatively low–H2O, and barren high–Sr/Y magmas. However, ultrapotassic magmas (e.g., lamprophyres) with high H2O are able to trigger intense water–fluxed, partial melting of metal–fertilized lower crust, which generated high–fO2, relatively high–H2O, fertile, high–Sr/Y shoshonitic magmas in this intracontinental setting.

Assessing the Validity of Negative High Field Strength-Element Anomalies as a Proxy for Archaean Subduction: Evidence from the Ben Strome Complex, NW Scotland

The relative depletion of high field strength elements (HFSE), such as Nb, Ta and Ti, on normalised trace-element plots is a geochemical proxy routinely used to fingerprint magmatic processes linked to Phanerozoic subduction. This proxy has increasingly been applied to ultramafic-mafic units in Archaean cratons, but as these assemblages have commonly been affected by high-grade metamorphism and hydrothermal alteration/metasomatism, the likelihood of element mobility is high relative to Phanerozoic examples. To assess the validity of HFSE anomalies as a reliable proxy for Archaean subduction, we here investigate their origin in ultramafic rocks from the Ben Strome Complex, which is a 7 km2 ultramafic-mafic complex in the Lewisian Gneiss Complex of NW Scotland. Recently interpreted as a deformed layered intrusion, the Ben Strome Complex has been subject to multiple phases of high-grade metamorphism, including separate granulite- and amphibolite-facies deformation events. Additional to bulk-rock geochemistry, we present detailed petrography, and major- and trace-element mineral chemistry for 35 ultramafic samples, of which 15 display negative HFSE anomalies. Our data indicate that the magnitude of HFSE anomalies in the Ben Strome Complex are correlated with light rare earth-element (LREE) enrichment likely generated during interaction with H2O and CO2-rich hydrothermal fluids associated with amphibolitisation, rather than primary magmatic (subduction-related) processes. Consequently, we consider bulk-rock HFSE anomalies alone to be an unreliable proxy for Archaean subduction in Archaean terranes that have experienced multiple phases of high-grade metamorphism, with a comprehensive assessment of element mobility and petrography a minimum requirement prior to assigning geodynamic interpretations to bulk-rock geochemical data.

Sub-lithospheric origin of Na-alkaline and calc-alkaline magmas in a post-collisional tectonic regime: Sr-Nd-Pb isotopes in recent monogenetic volcanism of Cappadocia, Central Turkey

Intense Cenozoic magmatism accompanied the convergence and collision of the Eurasian, African and Arabian plates. In the Cappadocia Region of Central Anatolia, widespread and abundant volcanism developed from the late Miocene to the Quaternary. Calc-alkaline pyroclastic products and lava erupted diffusely during the late Miocene and Pliocene. Both calc-alkaline and Na-alkaline volcanic rocks were emplaced in close spatial and temporal proximity during the Plio-Pleistocene. Here we focus on the Plio-Pleistocene monogenetic activity, which produced two types of rocks: calc-alkaline volcanic rocks ranging in composition from basalts to rhyolites and occurring around the Acıgöl caldera, the Göllü Dağ dome and the Hasan Dağ and Erciyes Dağ composite volcanoes; Na-alkaline volcanic rocks ranging from alkali basalts to mugearites and cropping out in monogenetic centres in Karapınar, along the WSW flank of Hasan Dağ, in Acıgöl and near the Kızılırmak River, northwest of Nevşehir.The coeval occurrence of calc-alkaline and Na-alkaline volcanism observed here is in striking contrast with activity in the surrounding Western and Eastern Anatolia regions, where the calc-alkaline volcanic rocks are distinctly older than the Na-alkaline ones. The Sr and Nd isotopic compositions of the mantle sources of both rock types show a narrow, overlapping range of values (87Sr/86Sr~0.70395–0.70474 and 143Nd/144Nd~0.51268–0.51281 for the calc-alkaline products, 0.70334–0.70524 and 0.51268–0.51293 for the Na-alkaline ones). In addition, most of the Na-alkaline products, like the calc-alkaline rocks, show an arc-type distribution of incompatible trace elements, with marked enrichments in LILE (Large-Ion Lithophile Elements) and negative HFSE (High Field Strength Elements) anomalies. True intraplate magmas are absent in Cappadocia, where the genesis of Na-alkaline basalts is ascribed to mixing between different percentages of within-plate (OIB) and calc-alkaline magmas. Modelling reveals that the addition of 13% and 30% OIB-type melt to a calc-alkaline magma is enough to change a SiO2-oversaturated magma into an ol-hy-normative and ne-normative one, respectively, whereas the addition of 15% calc-alkaline magma to an OIB-type magma results in a negative NbTa anomaly in the primitive mantle-normalized spider diagrams.The various types of products of recent volcanism in Cappadocia reflect the complex tectonic setting of the Central Anatolian Block: characterised by strike-slip tectonics and local extensional pull-apart basins, different magma types formed contemporaneously through decompression melting at different depths.In this scenario, the calc-alkaline magmas derived from partial melting of a mantle wedge modified by a subduction component, whereas the within-plate (OIB) magmas derived from a deeper, unmodified sub-slab mantle source. Mixing between calc-alkaline and within-plate magmas during their rise to the surface produced the Na-alkaline Cappadocia basalts. The very low rate of extension allowed within-plate magmas (OIB) to reach the surface after having interacted with the overlying calc-alkaline magmas in the late stages of Cappadocia volcanism.

Geochemistry and tectonic setting of the Chah-Bazargan sub-volcanic mafic dykes, south Sanandaj–Sirjan Zone (SSZ), Iran

The Chah-Bazargan sub-volcanic mafic dykes (trachybasalt and basaltic trachyandesite) are located in the south of the Sanandaj–Sirjan Zone (SSZ), Iran. The dyke mineralogy mostly comprises amphibole, clinopyroxene, olivine, orthopyroxene, and plagioclase as phenocrysts and fine-grained plagioclase and some ferromagnesian minerals in the matrix. The rocks are alkaline and shoshonitic in composition. The mafic melts relate to Neotethys subduction activity beneath the southern SSZ in the ~Eocene–Miocene interval. Markedly positive Ba, U, K, Pb, and Sr and negative HFSE (high field strength elements: Nb, Ta, Zr, Hf, P, and Ti) anomalies demonstrate this subduction. The sub-volcanic mafic dykes were produced from a metasomatized upper lithospheric mantle wedge at a depth consistent with the stability field of phlogopite-spinel (or -spinel/garnet) lherzolite. Geochemical studies on the basis of the rare earth elements (REE) and HFSE, and large ion lithophile elements (LILE) display that the mantle wedge underwent degrees of partial melting averaging between 5 and 15% to form the Chah-Bazargan sub-volcanic mafic dykes. It is possible that the chemical composition of the rocks was changed due to fractional crystallization and crustal contamination during emplacement.

Eclogite-facies metamorphism in impure marble from north Qaidam orogenic belt: Geodynamic implications for early Paleozoic continental-arc collision

In the North Qaidam ultrahigh-pressure (UHP) metamorphic belt, impure marble and interbedded eclogite represent a particular sedimentary provenance and tectonic setting, which have important implications for a controversial problem - the dynamic evolution of early Paleozoic subduction-collision complexes. In this contribution, detailed field work, mineral chemistry, and whole-rock geochemistry are presented for impure marble to provide the first direct evidence for the recycling of carbonate sediments under ultrahigh-pressures during subduction and collision in the Yuka terrane, in the North Qaidam UHP metamorphic belt. According to conventional geothermobarometry, pre-peak subduction to 0.8–1.3GPa/485–569°C was followed by peak UHP metamorphism at 2.5–3.3GPa/567–754°C and cooling to amphibolite facies conditions at 0.6–0.7GPa/571–589°C. U-Pb dating of zircons from impure marble reveals a large group with ages ranging from 441 to 458Ma (peak at ~450Ma), a smaller group ranging from 770 to 1000Ma (peak at 780Ma), and minor >1.8Ga zircon aged ca. 430Ma UHP metamorphism. The youngest detrital zircons suggest a maximum depositional age of ca. 442Ma and a burial rate of ca. 1.0–1.1cm/yr when combined with P–T conditions and UHP metamorphic age. The REE and trace element patterns of impure marble with positive Sr and U anomalies, negative high field strength elements (Nb, Ta, Zr, Hf, and Ti), and Ce anomalies imply that the marble had a marine limestone precursor. Impure marble intercalated with micaschist and eclogite was similar to limestone and siltstone protoliths deposited in continental fore-arc or arc setting with basic volcanic activity. Therefore, the Yuka terrane most likely evolved in a continental island arc setting during the Paleozoic. These data suggest that metasediments were derived from a mixture of Proterozoic continental crust and juvenile early Paleozoic oceanic and/or island arc crust. In addition, their protoliths were likely deposited in a terrigenous-dominated forearc marine basin rather than an intracontinental basin environment, further evidence that some continental arc volcanic rock may have been the source of eclogite in the North Qaidam. These sediments, formed in a forearc basin close to the Qaidam Block to the north, were transported in the subduction zone to 100–110km depth with UHP metamorphism prior to exhumation. Meanwhile, the new results suggest that subduction erosion occurred along the active continental margin during the Qaidam Block with north-dipping subduction, indicating that the North Qaidam UHP metamorphic belt may have formed during continental-arc collision.

In-situ trace element and Sr isotopic compositions of mantle xenoliths constrain two-stage metasomatism beneath the northern North China Craton

Subduction and collision are the key processes triggering geochemical refertilization of the lithospheric mantle beneath cratons. However, the way that the subducted plate influences the cratonic lithospheric mantle remains unclear. Here, in-situ major and trace-element and Sr isotopic compositions of peridotite and pyroxenite xenoliths carried by the Dongbahao Cenozoic basalts, located close to the northern margin of North China Craton (NCC), were examined to investigate the effects of the subducted Paleo-Asian oceanic plate on the lithospheric mantle of the NCC. Based on petrographic and geochemical features, peridotites were subdivided into two types recording two-stage metasomatism. Clinopyroxene (Cpx) in both types of peridotites show chemical zoning. In those peridotites we refer to as Type 1 peridotites, Cpx exhibit uniform convex-upward rare earth element (REE) patterns but core-rim variations in 87Sr/86Sr ratios (0.7065–0.7082 in the cores and 0.7043–0.7059 in the spongy rims), and have high (La/Yb)N ratios (>1.12) (N means normalized to chondrite), relatively low Ti/Eu ratios (<3756) and negative high field strength element (HFSE) (Nb, Ta, Zr, Hf and Ti) anomalies in the cores, indicating early-stage metasomatism by carbonatitic melts derived from the subducted sedimentary carbonate rocks. Cpx in the Type 2 peridotites have highly variable REE patterns (from light rare earth element (LREE)-depleted to LREE-enriched) and feature zoned Sr isotopic compositions contrasting to those in Type 1, i.e., increasing 87Sr/86Sr ratios from the cores (0.7020–0.7031) to the spongy rims (0.7035–0.7041). Accompanying variations of 87Sr/86Sr ratios, Cpx in both types of peridotites display increasing Nb/La ratios from the cores to the spongy rims. In addition, Cpx in the Type 2 peridotites show remarkably increased (La/Yb)N, Ca/Al, Sm/Hf and Zr/Hf ratios but decreased Ti/Eu and Ti/Nb ratios from the cores to the spongy rims. These features imply a later-stage metasomatism by CO2-rich silicate melts derived from carbonated eclogites. Pyroxenites were also classified into two types. Both types of pyroxenites show higher Ni content in Cpx and orthopyroxene than peridotites at the same Mg# (=100∗Mg/(Mg+Fe), atomic number) level. Their Cpx show high Ti/Eu, Ti/Sr ratios and similar 87Sr/86Sr ratios (0.7039–0.7055) to the Cpx spongy rims in peridotites, suggesting that pyroxenites originated from silicate melt-peridotite reactions in the later-stage metasomatism.These observations collectively indicate that the lithospheric mantle beneath the northern NCC presents evidence for two distinct mantle metasomatic events. We propose that both were caused by the subduction of the Paleo-Asian oceanic plate, which could have contributed significantly to the transformation of the lithospheric mantle beneath the northern NCC.

Paleo-Asian oceanic slab under the North China craton revealed by carbonatites derived from subducted limestones

Abstract It is widely accepted that the lithospheric mantle under the North China craton (NCC) has undergone comprehensive refertilization due to input from surrounding subducted slabs. However, the possible contribution from the Paleo-Asian oceanic slab to the north is poorly constrained, largely because of the lack of convincing evidence for the existence of this slab under the NCC. We report here carbonatite intruding Neogene alkali basalts in the Hannuoba region, close to the northern margin of the NCC. Trace element patterns with positive Sr and U anomalies, negative high field strength elements (Nb, Ta, Zr, Hf, and Ti) and Ce anomalies, high 87Sr/86Sr ratios (0.70522-0.70796), and high δ18OSMOW (standard mean ocean water) values (22.2‰–23‰) indicate that this carbonatite had a limestone precursor. However, the presence of coarse-grained mantle-derived clinopyroxene, orthopyroxene, and olivine, and chemical features of the carbonates suggest that the carbonate melts were derived from the mantle. The carbonates have high 143Nd/144Nd ratios (0.51282-0.51298) and show negative correlation between CaO and Ni contents, resulting from reaction between carbonate melt and peridotite. Considering the regional tectonic setting, the carbonatite probably formed by melting of subducted sedimentary carbonate rocks that formed part of the Paleo-Asian oceanic slab, and thus could provide the first direct evidence for the presence of the Paleo-Asian oceanic slab beneath the NCC.