High Field Strength Element Anomalies Research Articles

Origin and evolution of lithospheric mantle beneath the juvenile Arabian–Nubian Shield: Evidence from the Natash peridotite xenoliths, Southern Eastern Desert, Egypt

Compared with the Archean cratonic mantle, much less is known about the genesis and composition of the subcontinental lithospheric mantle (SCLM) beneath the juvenile terranes. In this study, we present petrology and mineral chemistry analyses of peridotite xenoliths carried by Cretaceous volcanics from the Natash area in the western Arabian–Nubian Shield (ANS). Combined with mantle xenoliths from the eastern ANS, we aim to constrain the nature, origin, and evolution of SCLM beneath the juvenile terrane. The Natash peridotite xenoliths, including well-preserved clinopyroxene (Cpx) and spinel (Sp), are categorized into two groups. Group A peridotites display exceptionally high Mg# (molar100 × Mg/(Mg + Fetotal)) in Cpx (95.6–96.7) and Cr# (molar100 × Cr/(Cr + Al)) in Sp (45.5–51.7), signifying a high degree of melting and refractory nature. Cpx in Group A demonstrates low Ti/Eu (532–1785) and high (La/Yb)N (5.34–26.29) ratios, indicative of carbonatitic metasomatism. Comparatively, Group B peridotites exhibit relatively high Mg# in Cpx (93.8–95.5) and Cr# in Sp (15.9–26.2), indicating a low degree of melting and fertile nature. Cpx trace element patterns subdivide Group B into three sub-groups. Cpx in Group B1 are characterized by high Ti/Eu (3934–5622) but low (La/Yb)N (0.91–2.98), implying silicate melt metasomatism. Both Groups B2 and B3 Cpx show steep rare earth element (REE) patterns, negative anomalies in high field-strength elements, and high (La/Yb)N (3.62–7.57). However, Cpx in B3 exhibit higher Ti/Eu ratios and water-soluble element contents than those in B2 peridotites. In addition, the spoon-shaped REE patterns are prominently unique to Cpx in Group B3. Hence, it is inferred that Group B2 were influenced by carbonatitic melts, whereas the evolved small-volume CO2-H2O-rich fluids infiltrated into Group B3 samples. Furthermore, the high Sr contents (71–404 ppm) but low 87Sr/86Sr (0.7003–0.7034) ratios of Cpx in Natash peridotites suggest that the various metasomatic agents all originated from the asthenosphere. Collectively, the Natash peridotite xenoliths exhibit similar nature and metasomatism processes with those from Cenozoic volcanics in the eastern ANS, and they all show affinity to continental peridotites. The pMELTS simulation suggests that the refractory peridotites have a high melting pressure (typically >3 GPa), while the fertile samples register a low melting pressure. Consequently, we infer that the SCLM beneath the juvenile ANS was formed through the cooling of upwelling asthenosphere, incorporating the ancient mantle fragments, and subsequently underwent asthenosphere-derived carbonatitic and silicate melt/fluid metasomatisms.

Exploring the orogenic nature of the Cariris Velhos event (ca. 1000-920 Ma) of the Borborema Province (NE Brazil)

The Cariris Velhos event (ca. 1000-920 Ma) is a significant marker in the early Neoproterozoic lithospheric record of the Borborema Province in NE Brazil. Despite its importance, the nature of this event remains unresolved having been linked to either continental rifting or accretionary-collisional orogeny. In this study, we support a compressive setting for the Cariris Velhos event based on compiled geochemical and isotopic data (Sm–Nd and Lu–Hf) from metagranitic rocks in the Alto Pajeú Terrane (central Borborema) and in the Pernambuco-Alagoas Super Terrane, as well as in the Riacho do Pontal and Sergipano fold belts (Southern Borborema). Geochemistry of these rocks share similarities with calcic to alkali-calcic and ferroan magmas, along with some Cordilleran magnesian members. They also correspond to meta-to peraluminous I- and S-type granitic rocks. Trace element distribution on spider diagrams reveals negative anomalies of HFSE (high field strength elements) such as Nb and Ta, which are typically associated with subduction-related settings. Additionally, chondrite normalized REE (rare-earth element) diagrams indicate mixing of both crustal and mantle material. Tectonic discriminant diagrams using elements with low mobility supports the hypothesis of a continental margin subduction-related setting. Calculated εNd and εHf values vary from negative to slightly positive, suggesting contributions from juvenile inputs and reworking of Meso-Paleoproterozoic crust, as indicated by the associated TDM model ages. Although the nature of the Cariris Velhos event requires further investigation, we suggest that, at least partially, it corresponded to an orogenic episode that might have occurred concurrently with continental break-up, similar to modern continental arc-retro arc foreland settings.

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.

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.

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.

Record of Early-Stage Rodingitization from the Purang Ophiolite Complex, Western Tibet

Rodingitization, commonly coupled with serpentinization of ultramafic rocks, bears significant information for fluid-rock interactions and element transfer from sea-floor to subduction zone environments. Numerous outcrops of rodingites are exposed along the Yarlung Zangbo suture zone (YZSZ) of southern Tibet, providing us an excellent opportunity to probe the petrogenetic processes, and unravel their implications for regional tectonic evolution. Several studies have been performed on rodingites from the eastern to central portions of the YZSZ, whereas limited work has ever been conducted on rodingitized rocks from the western segment of the YZSZ, precluding a comprehensive understanding of this lithological type. In this paper, we present detailed studies of petrology, mineral, whole-rock geochemistry and phase equilibrium modeling on a suite of newly recognized rodingites within the Purang ophiolite massif in the southwestern part of the YZSZ. The rodingites have a major metasomatic mineral association of chlorite, clinozoisite, amphibole and minor amounts of plagioclase, representing products of an early-stage rodingitization. They generally present compositions of low SiO2 (48.89 wt.%.53.57 wt.%), Fe2O3T (3.77 wt.%.5.56 wt.%), Na2O (1.31 wt.%.1.93 wt.%), Al2O3 (4.78 wt.%.8.84 wt.%), moderate CaO (9.69 wt.%.11.23 wt.%), and high MgO (24.11 wt.%.26.08 wt.%) concentrations with extremely high Mg# values [Mg#=100×Mg/(Mg+Fe2+) molar] of 89.92. Bulk-rock recalculation reveals that the rodingites have a protolith of mantle-derived olivine gabbro or gabbronorite. They have low rare earth element compositions (ΣREE=2.4 ppm.6.5 ppm) and are characterized by flat LREE and slightly enriched HREE patterns with positive Eu anomalies; they also exhibit positive anomalies in Sr, U and Pb and negative anomalies in high-field strength elements, including Nb, P and Ti, suggesting for a subduction-zone imprinting. Phase equilibrium modeling shows that the rodingitization did take place at P<2 kbar and T= ~350.400 °C, consistent with low greenschist facies conditions. Taking into account of all these petrological and geochemical features, we propose that the rodingites record evidence of early-stage fluid-rock interactions between olivine gabbroic rocks and Ca-rich fluids, which may have derived from weakly serpentinized ultramafic country rocks. Although this process may initially have occurred in a mid-ocean ridge setting, an obvious overprinting by supra-subduction zone fluids in a fore-arc environment is recognized.

Geochemistry, Geochronology, and Hf-S-Pb Isotopes of the Akechukesai IV Mafic-Ultramafic Complex, Western China

The Akechukesai IV mafic–ultramafic complex, located in the western segment of the eastern Kunlun orogenic belt (EKOB), represents a newly-discovered complex, containing Ni ores at grades of up to 0.98% Ni. It is dominated by olivine pyroxenite, pyroxenite, and gabbro units. The gabbros are enriched in lithophile elements (e.g., Rb, U, and K) and light rare-earth elements (LREE), with negative anomalies in high field-strength elements, except Zr, Ta. Nb/Ta(∼5) and Zr/Hf (∼10) ratios lower than the primitive mantle and chondrites, respectively, indicate the influence of the mantle metasomatic process or fractionation of accessory mineral phases. Zircon U–Pb dating of the gabbro yielded an age of 423.9 ± 2.6 Ma, indicating that the complex formed contemporaneously with the Xiarihamu Ni deposit (423 ± 1 Ma). The gabbro has negative εHf(t) values (−11.3 to −1.2) with corresponding TDM1 ages of 1535–1092 Ma. The vein-like and disseminated mineralization (i.e., pyrite and pyrrhotite) have δ34S values of 13.1‰–13.4‰ and 5.0‰–8.5‰, respectively, suggesting that the magmas that formed the complex assimilated crustal sulfur. They yield 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb values of 17.323–18.472, 15.422–15.626, and 37.610–38.327, respectively, indicating Pb derived from multiple sources (i.e., mantle crustal sources). Geochemical and Hf–S–Pb isotopic characteristics suggest that the complex formed from a primitive magma derived by partial melting of a spinel- and garnet-bearing lherzolite mantle at variable degree of 5%–10%. This source region was geochemically enriched by previous interaction with slab-related fluids. Tectonic reconstruction suggests that the Akechukesai IV complex was generated in a post-collisional extensional environment.

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.

Origin of the Eocene porphyries and mafic microgranular enclaves from the Beiya porphyry Au polymetallic deposit, western Yunnan, China: Implications for magma mixing/mingling and mineralization

The origin of magmas with high H2O, S and metals that are linked to porphyry economic mineralization in continental collisional belts is controversial. In this paper, we studied the host porphyries and associated microgranular enclaves (MMEs) including gabbroic enclaves and dioritic enclaves from the large Beiya porphyry Au deposit in Jinshajiang-Ailaoshan mineralization belt, which provide first hand evidence of crust–mantle hybridization in the formation of fertile magma for porphyry deposit. Zircon LA-ICPMS U–Pb ages indicate coeval formation for the MMEs and the felsic porphyries in Eocene time (~36Ma). The MMEs show spheroidal shapes, acicular apatites, and oscillatory zoning with repeated resorption surfaces in plagioclases, which suggest that the MMEs are globules of a more mafic magma that was injected into and mingled with the host felsic magma. One gabbroic enclave sample exhibits high MgO (5.6wt.%), high TiO2 (1.7wt.%), enriched in large-ion lithophile element (LILE) and depleted in high field strength element (HFSE), with negative Nb, Ta and Ti anomalies. This indicates that the gabbroic enclave is similar to those of the coeval potassic mafic rocks in the Western Yunnan, which originated from the enriched mantle metasomatized by the slab-derived fluids. The dioritic enclave exhibit both potassic and adakitic-like features, and are characterized by high contents of MgO (4.4–4.9wt.%), K2O (6.4–7.4wt.%), and compatible trace elements (e.g. Ni: 115–142ppm; Cr: 214–291ppm), as well as by high Sr/Y ratios. The dioritic enclave samples yield εHf (t) values (−1.9 to +3.6) similar to those of the host porphyries. Petrographic, elemental, and isotopic evidence suggests that the Beiya dioritic enclave were formed by mixing between potassic mafic and adakitc-like melts, derived from metasomatized lithospheric mantle and lower crust, respectively. In contrast, the Beiya host porphyries are characterized by high SiO2 (66.9–71.9wt.%) and Al2O3 (13.8–15.9wt.%) contents, high Sr/Y ratios (35–79), low compatible element contents (e.g. Ni=0.9–7.2ppm; Cr =1.0–8.2ppm), low Mg# values (0.19–0.35), positive LILE anomalies, marked negative high HFSE anomalies, negative εNd(t) values (−6 to −2), and high (87Sr/86Sr)i values (0.7071–0.7081). These features indicate that the host porphyries were mainly formed by partial melting of subduction-modified lower crust with a certain assimilation of mafic melts represented by MMEs. MMEs hosted in the Beiya porphyries, formed from a hydrous, mafic magma although they formed in a non-arc setting. High magmatic water content is probably related to an enriched mantle metasomatized by fluids from a Proterozoic subducted oceanic slab. High Cu (341–626ppm) and Au contents of the MMEs suggest they crystallized from a metal-rich mafic melt. Complex assemblages of pyrite–chalcopyrite–apatite–rutile with hornblende are observed in the least altered MME. We suggest that the mafic melt probably supplied part of water, and metal to the Beiya porphyry Au system.

The composition and genesis of the Mesoarchean Dagushan banded iron formation (BIF) in the Anshan area of the North China Craton

The Dagushan BIF-hosted iron deposit in the Anshan–Benxi area of the North China Craton (NCC) has two types of iron ore: quartz–magnetite BIF (Fe2O3T<57wt.%) and high-grade iron ore (Fe2O3T>90wt.%). Chlorite-quartz schist and amphogneiss border the iron orebodies and are locally present as interlayers with BIFs; chlorite-quartz schist and BIFs are enclosed by amphogneiss in some locations. The quartz–magnetite BIFs are enriched in HREEs (heavy rare earth elements) with positive La, Eu and Y anomalies, indicating their precipitation from marine seawater with a high-temperature hydrothermal component. Moreover, these BIFs have low concentrations of Al2O3, TiO2 and HFSEs (high field strength elements, e.g., Zr, Hf and Ta), suggesting that terrigenous detrital materials contributed insignificantly to the chemical precipitation. The high-grade iron ores exhibit similar geochemical signatures to the quartz–magnetite BIFs (e.g., REE patterns and Y/Ho ratios), implying that they have identical sources of iron. However, these ores have different REE (rare earth element) contents and Eu/Eu* values, and the magnetites contained within them exhibit diverse REE contents and trace element concentrations, indicating that the ores underwent differing formation conditions, and the high-grade ores are most likely the reformed product of the original BIFs.The chlorite-quartz schist and amphogneiss are characterized by high SiO2 and Al2O3 contents and exhibit variable abundances of REEs, enrichment in LREEs (light rare earth elements), negative anomalies in HFSEs (e.g., Nb, Ta, P and Ti) and positive anomalies in LILEs (large ion lithophile elements, e.g., Rb, Ba, U and K). A protolith reconstruction indicates that the protoliths of the chlorite-quartz schist are felsic volcanic rocks. SIMS and LA-ICP-MS zircon U–Pb dating indicate that this schist formed at approximately 3110 to 3101Ma, which could represent the maximum deposition age of the Dagushan BIF. However, two groups of zircons from the amphogneiss are identified: 3104 to 3089Ma zircons that are most likely derived from the chlorite-quartz schist and 2997 to 2995Ma zircons, which are interpreted to represent the time of protolith crystallization. Thus, the Dagushan BIF most likely formed before 2997 to 2995Ma. The ~3.1Ga zircons yield εHf(t) values of −8.07 to 5.46, whereas the ~3.0Ga zircons yield εHf(t) values of −3.96 to 2.09. These geochemical features suggest that the primitive magmas were derived from the depleted mantle with significant contributions of ancient crust.

Continental rift and oceanic protoliths of mafic–ultramafic rocks from the Kechros Complex, NE Rhodope (Greece): implications from petrography, major and trace-element systematics, and MELTS modeling

The whole-rock chemistry of eclogites, partially amphibolitized eclogites, and dyke amphibolites from the metamorphic Kechros complex in the eastern Rhodope Mountains preserves evidence of the geodynamic framework for the origin of their protoliths. Major and trace-element concentrations define two distinct protolith groups for the eclogites. The low-Fe–Ti (LFT) eclogites have low-TiO_2 content (<0.67 wt%), negative high field strength element anomalies, and variable enrichments in large ion lithophile elements (LILE). The rare earth element (REE) patterns are characterized by strong light-REE (LREE) enrichment and heavy-REE (HREE) depletion. The high-Fe–Ti (HFT) eclogites have small to moderate LILE enrichment and lack Nb anomalies. The REE patterns of the HFT eclogites are characterized by LREE depletion and relatively flat MREE–HREE patterns. The rock compositions and petrographic features of the LFT eclogites resemble gabbros formed in a continental rift environment with minor to moderate contamination of a mantle-derived mafic magma by continental crust, whereas the HFT eclogites resemble mafic rocks formed in extensional oceanic environments. We interpret the HFT suite to represent a later stage in an evolution from continental rift to open ocean, following the origin of the LFT suite. Dyke amphibolite compositions, except for probable SiO_2 loss associated with metamorphic dehydration reactions, appear to represent liquid compositions quenched in conduits through the lower crust. MELTS modeling shows that dyke amphibolite compositions can be related to each other by fractional crystallization under strongly oxidizing conditions at ~0.5 GPa pressure, and all can be derived from a low-degree melt of modified fertile peridotite from around 1.7 GPa. Cumulates crystallized from the parental liquids of the amphibolites under oxidizing conditions may have yielded the protoliths of the HFT suite.

Melt–peridotite interaction in the shallow lithospheric mantle of the North China Craton: evidence from melt inclusions in the quartz-bearing orthopyroxene-rich websterite from Hannuoba

To better understand the origin, migration, and evolution of melts in the lithospheric mantle and their roles on the destruction of the North China Craton (NCC), we conducted a petrological and geochemical study on a quartz-bearing orthopyroxene-rich websterite xenolith from Hannuoba, the NCC, and its hosted melt and fluid inclusions. Both clinopyroxene and orthopyroxene in the xenolith contain lots of primary and secondary inclusions. High-temperature microthermometry of melt inclusions combined with Raman spectroscopy analyses of coexisting fluid inclusions shows that the entrapment temperature of the densest inclusions was ~1215°C and the pressure ~11.47 kbar, corresponding to a depth of ~38 km, i.e. within the stability of the spinel lherzolite. Intermediate pressure inclusions probably reflect progressive fluid entrapment over a range of depths during ascent, whereas the low-pressure inclusions (P < 2 kbar) may represent decrepitated primary inclusions. In situ laser-ablation ICP-MS analyses of major and trace elements on individual melt inclusions show that the compositions of these silicate melt inclusions in clinopyroxene and orthopyroxene are rich in SiO2, Al2O3, and alkalis but poor in TiO2 and strongly enriched in light rare earth elements (LREEs) and large ion lithophile elements (LILEs), with negative anomalies of high-field strength elements (HFSEs). These characteristics suggest that the silica-rich melts could be derived from the partial melting of subducted oceanic slab. Therefore, this kind of quartz-bearing orthopyroxene-rich websterite may be produced by interaction between the slab-derived melts with the mantle peridotite. This study provides direct evidence for the origin, migration, and evolution of melts in the lithospheric mantle, which may play an important role in the destruction of the NCC.

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

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

Small-scale coexistence of island-arc- and enriched-MORB-type basalts in the central Vanuatu arc

We report here major, trace element and Sr–Nd–Pb isotopic data for a new set of basaltic lavas and melt inclusions hosted in Mg-rich olivines (Fo86–91) from Mota Lava, in the Banks islands of the Vanuatu island arc. The results reveal the small-scale coexistence of typical island-arc basalts (IAB) and a distinct type of Nb-enriched basalts (NEB) characterized by primitive mantle-normalized trace element patterns without high-field-strength element (HFSE) depletion. The IAB show trace element patterns with prominent negative HFSE anomalies acquired during melting of mantle sources enriched with slab-derived, H2O-rich components during subduction. In contrast, the NEB display trace element features that compare favourably with enriched-mid-ocean ridge basalt (MORB) and the most enriched basalts from the Vanuatu back-arc troughs. Both their trace element and Nd–Sr isotopic compositions require partial melting of an enriched-MORB-type mantle source, almost negligibly contaminated by slab-derived fluids (~0.2 wt%). The coexistence of these two distinct types of primitive magma, at the scale of one volcanic island and within a relatively short span of time, would reflect a heterogeneous mantle source and/or tapping of distinct mantle sources. Direct ascent of such distinct magmas could be favoured by the extensive tectonic setting of Mota Lava Island, allowing decompression melting and sampling of variable mantle sources. Significantly, this island is located at the junction of the N–S back-arc troughs and the E–W Hazel Home extensional zone, where the plate motion diverges in both direction and rate. More broadly, this study indicates that crustal faulting in arc contexts would permit basaltic magmas to reach Earth’s surface, while preserving the geochemical heterogeneity of their mantle sources.

Eoarchean within-plate basalts from southwest Greenland

Research Article| March 01, 2013 Eoarchean within-plate basalts from southwest Greenland F.E. Jenner; F.E. Jenner * 1Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road Northwest, Washington, DC 20015, USA2Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia *E-mail: fjenner@dtm.ciw.edu. Search for other works by this author on: GSW Google Scholar V.C. Bennett; V.C. Bennett 2Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia Search for other works by this author on: GSW Google Scholar G. Yaxley; G. Yaxley 2Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia Search for other works by this author on: GSW Google Scholar C.R.L. Friend; C.R.L. Friend 3Glendale, Tiddington, Oxon OX9 2LQ, UK Search for other works by this author on: GSW Google Scholar O. Nebel O. Nebel 2Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia Search for other works by this author on: GSW Google Scholar Geology (2013) 41 (3): 327–330. https://doi.org/10.1130/G33787.1 Article history received: 25 Jun 2012 rev-recd: 20 Sep 2012 accepted: 25 Sep 2012 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation F.E. Jenner, V.C. Bennett, G. Yaxley, C.R.L. Friend, O. Nebel; Eoarchean within-plate basalts from southwest Greenland. Geology 2013;; 41 (3): 327–330. doi: https://doi.org/10.1130/G33787.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The majority of >3 Ga metabasalts have chemical features, such as high field strength element (HFSE) depletions, that are characteristic of modern island-arc basalts. These compositions have been interpreted as evidence for subduction of oceanic crust early in Earth’s history. Alternatively, the apparent absence of Archean mafic rocks with mid-oceanic ridge basalt (MORB) and ocean island basalt (OIB) compositions and the ubiquitous occurrence of metabasalts with HFSE anomalies suggest that these chemical features may instead be a widespread characteristic of the Archean mantle related to early chemical differentiation and unrelated to modern-style recycling of crust. Here we present major- and trace-element data for a suite of metabasalts from Innersuartuut Island, southwest Greenland, which have a minimum age constraint of 3.75 Ga and are likely as old as ≥3.85 Ga. Samples from Innersuartuut show no evidence for crustal contamination or subduction-related magmatism, and have a petrogenesis comparable to modern OIB. The new data demonstrate that a compositional range for volcanic rocks comparable to that seen in the Phanerozoic existed in the Eoarchean. Therefore, rather than a global anomaly, subduction-related processes are the likely origin for the compositions of the most commonly preserved Archean mafic rocks with island-arc basalt characteristics. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Metasomatized Lithospheric Mantle beneath the Western Qinling, Central China: Insight into Carbonatite Melts in the Mantle

Mantle xenoliths from the Western Qinling, central China, are dominated by lherzolites, which can be divided into four subgroups—namely, garnet-facies, coexisting spinel-garnet, spinel-facies, and carbonate-bearing ones. All these rocks display light rare earth element enrichment, positive Sr and Ba anomalies, carbonatite-like trace element patterns, and Sr-Nd-Pb isotopic mixing between depleted mantle and enriched mantle type II end members, consistent with the geochemical features resulting from carbonatite metasomatism. The garnet-facies lherzolites show high trace element concentrations but low LaN/YbN ratios, and they show high Sr and Pb isotopic ratios that are similar to those of carbonatites, suggesting that they were highly metasomatized. The spinel-facies group has the lowest trace element concentrations but higher LaN/YbN ratios than the garnet-facies group; their lowest Sr and Pb isotopic ratios are closer to those of the depleted mantle end member, implying low-degree metasomatism. Geochemical variation of the coexisting spinel-garnet sample lies between that of the garnet and spinel groups. The elevated and highly variable trace element concentrations and Sr-Pb isotopic values of the carbonate-bearing lherzolite group are most likely related to the modal content of carbonate minerals. Collectively, these geochemical features indicate a rising front of carbonatite metasomatism in the lithospheric mantle beneath the Western Qinling. Combining experimental and empirical data, the positive Pb, Y, and high-field strength element anomalies in the peridotites might be ascribed to the involvement of a subduction component in the carbonatite melts. On the basis of the data presented in this article, we propose a general model for carbonatite metasomatism in the lithospheric mantle to interpret the different signatures recorded in the garnet-facies peridotites (chemical imprint) and spinel-facies peridotites (occurrence of carbonate minerals), which has potential application to other regions that have undergone carbonatite metasomatism.

Dunites from Isua, Greenland: A ca. 3720 Ma window into subcrustal metasomatism of depleted mantle

AbstractThe chemistry of surviving pieces of Eoarchaean mantle together with related crust helps us determine early crust-forming mechanisms. Two lenses of high-Mg, low-Al dunite within a ca. 3720 Ma part of the Isua supracrustal belt in Greenland are interpreted as relicts of Eoarchaean mantle with minimal crustal disturbance. The lenses are within altered, higher Al, Ca ultramafic schists and are intercalated with amphibolitized pillow basalts and gabbros with island arc chemical signatures, all intruded by 3715–3710 Ma tonalites. One variety of dunite is dominated by forsterite (Fo90–92) olivine with accessory chromite and rare clinopyroxene, which does not show high field strength element (HFSE) anomalies. Another variety contains olivine (Fo96–98), some intergrown with Ti-humite group minerals with strong positive HFSE anomalies that are complementary to the negative HFSE anomalies of the adjacent amphibolites. We propose that these dunites are tectonic slivers of ca. 3720 Ma subarc mantle that preserve evidence for varying interaction with mafic magmas in a ≥850 °C, 1.7–2.0 GPa subcrustal environment. These are by far the oldest direct geochemical link between coeval mantle and crustal rocks, and are new evidence for subduction zone–like environments on the early Earth.