Trace Element Compositions Of Clinopyroxenes Research Articles

Mafic to ultramafic xenoliths from Mauna Kea, Hawaii: clues to magma evolution from trace element compositions of clinopyroxene

Mafic to ultramafic xenoliths from Mauna Kea, Hawaii: clues to magma evolution from trace element compositions of clinopyroxene

Carboniferous mafic-ultramafic intrusions in the Eastern Pontides (Pulur Complex): Implications for the source of coeval voluminous granites

This study deals with the age and petrogenesis of mafic-ultramafic intrusions ranging in size from a few meters to 10 km within the Early Carboniferous high-grade gneisses of the Pulur Complex in the Eastern Pontides. The intrusions comprise dunite, wehrlite, gabbronorite, leucogabbro, anorthosite and ilmenite-bearing gabbronorite of cumulus origin, and are crosscut by dikes of ilmenite-bearing gabbronorite, leucogranite and microdiorite. U Pb dating on zircons from gabbronorite, anorthosite and leucogranite yielded igneous crystallization ages of 322–326 Ma, indicating that the intrusions were emplaced ca. 5–7 Ma after the peak of high-grade metamorphism, and form part of the Late Carboniferous high-volume magmatism in the region. In most cumulate rocks, Cr Al spinel, olivine and plagioclase were early crystallizing phases, followed by orthopyroxene, clinopyroxene and hornblende. Whole rock geochemical data suggest that wehrlite, gabbronorite, leucogabbro and anorthosite stem from a common magma, and ilmenite-bearing gabbronorite and dikes of leucogranite and microdiorite from different magmas. Application of mineral/melt partition coefficients to trace element compositions of clinopyroxene and hornblende in cumulate rocks suggests that the main cumulate body was derived from middle- to high-K calc-alkaline basic melts, and relatively late ilmenite-bearing gabbronorites from hypersthene-normative Ca-rich melts. All the rock types display radiogenic Sr and Pb isotopic signatures, and unradiogenic Nd isotopic ratios, which are indistinguishable from those of the coeval voluminous high-K calc-alkaline I-type granites in the region; the isotopic ratios are probably related to the metasomatism of the lithospheric mantle by sediment-derived melts. We suggest that the parental melts of the mafic-ultramafic intrusions and those of the high-K calc-alkaline granites were genetically related, and melts of the high-K calc-alkaline granites were probably derived from the melting of newly underplated calc-alkaline basic material at lower crustal depths, that were compositionally comparable to the parental magmas of the mafic-ultramafic intrusions. • The Pulur mafic-ultramafic intrusions (NE Turkey) formed at 326–322 Ma. • They are part of the Late Carboniferous high-volume magmatism. • Parental melts of the mafic-ultramafic intrusions were middle- to high-K calc-alkaline basic melts. • The high-volume magmatism probably resulted from the convective thinning of the lithosphere.

Generation of Continental Lithospheric Mantle by Tectonic Isolation of Oceanic Plate

AbstractContinental formation models invoke subduction or plume‐related processes to create the buoyant, refractory character of continental lithospheric mantle (CLM). From similarities in melt depletion, major element composition, modal clinopyroxene, and Os isotope systematics it has been proposed that oceanic mantle lithosphere is the likely protolith to non‐cratonic CLM, however, a direct link between the two has been difficult to ascertain. Using dredged mantle peridotite xenoliths from the Ferrel Seamount, off the west coast of Baja California, Mexico, we show that tectonic isolation of an oceanic plate may lead to formation of non‐cratonic CLM. Ferrel xenoliths are coarse‐grained spinel lherzolite, or rare harzburgite. Bulk‐rock and clinopyroxene trace element compositions reveal two‐stages of melt refertilization following melt depletion, with infiltration by mid‐ocean ridge basalt‐type melts, followed by melt addition from host alkali basalt. Melt depletion correlations with 187Os/188Os and highly siderophile element abundances indicate preserved melt depletion and refertilization processes are ancient. From these observations, the Ferrel xenoliths represent lithosphere from the abandoned Pacific‐Farallon ridge. The history of melt depletion, followed by MORB‐melt refertilization is consistent with the peridotites representing oceanic mantle lithosphere that was subsequently incorporated into the Baja‐Guadalupe microplate during “ridge jump.” These peridotites demonstrate that isolation of oceanic lithosphere that is rafted onto a continental margin provides a viable means for producing non‐cratonic CLM. We suggest that continuation of late‐stage, low degree melt refertilization may provide a link between oceanic lithosphere and non‐cratonic CLM and propose a tectonic model to preserve and facilitate this continued evolution.

Thermal history and parental magma composition of early olivine gabbros of the Troodos ophiolite, Cyprus

Samples of an early olivine gabbro in the lower plutonic sequence of the Troodos ophiolite were studied to constrain their thermal history and the compositions of their parental magma. These rocks exhibit typical orthocumulate textures and petrographic evidence of reaction between a migrating interstitial magma and a crystal mush, including embayed margins of olivine crystals and irregular ragged grain boundaries of clinopyroxenes, indicating dissolution of olivine and crystallization of clinopyroxene, followed by crystallization of plagioclase within the framework of olivine and clinopyroxene. All the minerals exhibit relatively homogeneous compositions, without any clear zoning in either their major or trace elements, indicating that the minerals and migrating magma reached equilibrium. The rare earth element (REE) distribution between clinopyroxene and plagioclase records an equilibrium temperature of about 1300 °C, but the Mg-exchange geothermometer yielded a lower temperature of about 1000 °C. These different closure temperatures indicate a rapid cooling history with a cooling rate of −2 °C/year log units. The low TiO 2 contents and additional trace element compositions of clinopyroxene show that the parental magma of the gabbro was related to boninitic magmas of the ophiolite. Consequently, we show that some boninitic lavas in the upper oceanic crust have been affected by melt-rock reaction process during their ascent through the lower crust, and therefore their composition cannot be directly used to constrain the composition of the primary boninites. • Gabbros of the Troodos ophiolite display petrographic evidence of reaction between a migrating magma and a crystal mush. • Trace elements of clinopyroxene indicate that the migrating magma is related to boninites in the UPL. • Some boninitic lavas affected by melt-rock reaction process cannot represent composition of the primary boninite.

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.

Clinopyroxene and Garnet Mantle Cargo in Kimberlites as Probes of Dharwar Craton Architecture and Geotherms, with Implications for Post-1·1 Ga Lithosphere Thinning Events Beneath Southern India

AbstractThe Wajrakarur Kimberlite Field (WKF) on the Eastern Dharwar Craton in southern India hosts several occurrences of Mesoproterozoic kimberlites, lamproites and ultramafic lamprophyres, for which mantle-derived xenoliths are rare and only poorly preserved. The general paucity of mantle cargo has hampered the investigation of the nature and evolution of the continental lithospheric mantle (CLM) beneath cratonic southern India. We present a comprehensive study of the major and trace element compositions of clinopyroxene and garnet xenocrysts recovered from heavy mineral concentrates for three c.1·1 Ga old WKF kimberlite pipes (P7, P9, P10), with the goal to improve our understanding of the cratonic mantle architecture and its evolution beneath southern India. The pressure-temperature conditions recorded by peridotitic clinopyroxene xenocrysts, estimated using single-pyroxene thermobarometry, suggest a relatively moderate cratonic mantle geotherm of 40 mW/m2 at 1·1 Ga. Reconstruction of the vertical distribution of clinopyroxene and garnet xenocrysts, combined with some rare mantle xenoliths data, reveals a compositionally layered CLM structure. Two main lithological horizons are identified and denoted as layer A (∼80–145 km depth) and layer B (∼160–190 km depth). Layer A is dominated by depleted lherzolite with subordinate amounts of pyroxenite, whereas layer B comprises mainly refertilised and Ti-metasomatized peridotite. Harzburgite occurs as a minor lithology in both layers. Eclogite stringers occur within the lower portion of layer A and at the bottom of layer B near the lithosphere–asthenosphere boundary at 1·1 Ga. Refertilisation of layer B is marked by garnet compositions with enrichment in Ca, Ti, Fe, Zr and LREE, although Y is depleted compared to garnet in layer A. Garnet trace element systematics such as Zr/Hf and Ti/Eu indicate that both kimberlitic and carbonatitic melts have interacted with and compositionally overprinted layer B. Progressive changes in the REE systematics of garnet grains with depth record an upward percolation of a continuously evolving metasomatic agent. The intervening zone between layers A and B at ∼145–160 km depth is characterized by a general paucity of garnet. This ‘garnet-paucity’ zone and an overlying type II clinopyroxene-bearing zone (∼115–145 km) appear to be rich in hydrous mineral assemblages of the MARID- or PIC kind. The composite horizon between ∼115–160 km depth may represent the product of intensive melt/rock interaction by which former garnet was largely reacted out and new metasomatic phases such as type II clinopyroxene and phlogopite plus amphibole were introduced. By analogy with better-studied cratons, this ‘metasomatic horizon’ may be a petrological manifestation of a former mid-lithospheric discontinuity at 1·1 Ga. Importantly, the depth interval of the present-day lithosphere–asthenosphere boundary beneath Peninsular India as detected in seismic surveys coincides with this heavily overprinted metasomatic horizon, which suggests that post-1·1 Ga delamination of cratonic mantle lithosphere progressed all the way to mid-lithospheric depth. This finding implies that strongly overprinted metasomatic layers, such as the ‘garnet-paucity’ zone beneath the Dharwar craton, present structural zones of weakness that aid lithosphere detachment and foundering in response to plate tectonic stresses.

Isotopic analyses of clinopyroxenes demonstrate the effects of kimberlite melt metasomatism upon the lithospheric mantle

The trace element and radiogenic isotope systematics of clinopyroxene have frequently been used to characterise mantle metasomatic processes, because it is the main host of most lithophile elements in the lithospheric mantle. To further our understanding of mantle metasomatism, both solution-mode Sr-Nd-Hf-Pb and in situ trace element and Sr isotopic data have been acquired for clinopyroxene grains from a suite of peridotite (lherzolites and wehrlites), MARID (Mica-Amphibole-Rutile-Ilmenite-Diopside), and PIC (Phlogopite-Ilmenite-Clinopyroxene) rocks from the Kimberley kimberlites (South Africa). The studied mantle samples can be divided into two groups on the basis of their clinopyroxene trace element compositions, and this subdivision is reinforced by their isotopic ratios. Type 1 clinopyroxene, which comprises PIC, wehrlite, and some sheared lherzolite samples, is characterised by low Sr (~100–200 ppm) and LREE concentrations, moderate HFSE contents (e.g., ~40–75 ppm Zr; La/Zr < 0.04), and restricted isotopic compositions (e.g., 87Sr/86Sri = 0.70369–0.70383; εNdi = +3.1 to +3.6) resembling those of their host kimberlite magmas. Available trace element partition coefficients can be used to show that Type 1 clinopyroxenes are close to being in equilibrium with kimberlite melt compositions, supporting a genetic link between kimberlites and these metasomatised lithologies. Thermobarometric estimates for Type 1 samples in this study indicate equilibration depths of 135–160 km within the lithosphere, thus showing that kimberlite melt metasomatism is prevalent in the deeper part of the lithosphere beneath Kimberley. In contrast, Type 2 clinopyroxenes occur in MARID rocks and coarse granular lherzolites in this study, which derive from shallower depths (<135 km), and have higher Sr (~350–1000 ppm) and LREE contents, corresponding to higher La/Zr of > ~ 0.05. The isotopic compositions of Type 2 clinopyroxenes are more variable and extend from compositions resembling the “enriched mantle” towards those of Type 1 rocks (e.g., εNdi = −12.7 to −4.4). To constrain the source of these variations, in situ Sr isotope analyses of clinopyroxene were undertaken, including zoned grains in Type 2 samples. MARID and lherzolite clinopyroxene cores display generally radiogenic but variable 87Sr/86Sri values (0.70526–0.71177), which are correlated with Sr contents and La/Zr ratios, and which might be explained by the interaction between peridotite and melts from different enriched sources within the lithospheric mantle. Most notably, the rims of these Type 2 clinopyroxenes trend towards compositions similar to those of the host kimberlite and Type 1 clinopyroxene from PIC and wehrlites. These results are interpreted to represent clinopyroxene overgrowth during late-stage (shortly before/during entrainment) metasomatism by kimberlite magmas. Our study shows that a pervasive, alkaline metasomatic event caused MARID to be generated and harzburgites to be converted to lherzolite in the lithospheric mantle beneath the Kimberley area, which was followed by kimberlite metasomatism during Cretaceous magmatism. This latter event is the time at which discrete PIC, wehrlite, and sheared lherzolite lithologies were formed, and MARID and granular lherzolites were partly modified.

Mixing of heterogeneous, high-MgO, plume-derived magmas at the base of the crust in the Central Iapetus Magmatic Province (Ma 610-550): Origin of parental magmas to a global LIP event

High-MgO (>12 wt%) magmas represent some of the most primary and high temperature melts from mantle plumes. The compositional diversity of high-MgO magmas gained by fractional melting within inhomogeneous mantle sources, is often overprinted by wall rock assimilation, magma mixing and fractional crystallisation within magma chambers at various depths within the crust. The deepest intrusions, at the base of the crust, are the first stop for magmas as they leave the mantle and such localities offer vital insights into the crustal processes that modify high-MgO melts. The Seiland Igneous Province (SIP) represents a rare exposure of a deep crustal magma conduit system that transported large volumes of mantle-derived melts through the lower continental crust. In this work, trace element compositions of clinopyroxenes in dunite, wehrlite and olivine clinopyroxenite samples from the Reinfjord intrusion were measured using laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The melts calculated to be in equilibrium with the clinopyroxenes represent derivatives of mantle-derived magmas and have steep rare earth element (REE) profiles with La/YbPM of 8.4–14., Sm/YbPM of 3.5–4.8, and negative anomalies in Nd, Zr and Hf. Assimilation, recharge and fractional crystallisation modelling shows that the ultramafic cumulates in the Reinfjord intrusion formed through concurrent fractional crystallisation and repetitive recharge (and mixing) of new primitive magmas from the mantle with very limited input from crustal sources. The recharge and mixing model is strongly supported by field and petrographic data. Two end-member ultramafic magmas are identified; the first end-member melt composition (MELTCPX0) is similar to the melt compositions that are in equilibrium with the most trace element depleted clinopyroxenes from Reinfjord. The second end-member melt composition (MELTPIC0) is similar to a number of picrite dykes found throughout the SIP, which have less steep LREE/HREE and lack strong negative anomalies in Zr and Hf. The REEBOX PRO melting model (Brown and Lesher, 2016) was used to forward model adiabatic decompression melting of lithologically homogeneous and heterogeneous sources containing anhydrous/hydrous peridotite ± pyroxenite. It was found that MELTPIC0 formed by melting of a peridotite source with up to 10% pyroxenite component and mantle potential temperatures of up to 1450 °C. MELTCPX0, however, formed by melting of a carbonate- metasomatised peridotite with potential temperatures of up to 1630 °C. These results indicate a compositionally and thermally inhomogeneous mantle plume beneath the SIP.

Integrated major and trace element study of clinopyroxene in basic, intermediate and acidic volcanic rocks from the middle Okinawa Trough: Insights into petrogenesis and the influence of subduction component

Magmatism is very common in the Okinawa Trough. However, the influence of magmatic processes, including fractional crystallization and magma mixing, on the petrogenesis of volcanic rocks is poorly understood. The in situ major and trace element compositions of clinopyroxene in co-genetic basalts, andesites, trachyandesites and rhyolites from the Iheya Ridge in the middle Okinawa Trough are examined to determine their petrogenesis and the influence of slab subduction. Regarding the types of mineral zoning, in basalt and andesite, clinopyroxene phenocrysts are unzoned. In trachyandesite, the clinopyroxene phenocrysts can be classified into three groups: normal zoned, oscillatory zoned and unzoned. Similarly, unzoned and reverse zoned clinopyroxene phenocrysts are the two main structures in rhyolites. According to the major element contents and distinct distribution patterns of rare earth elements (REEs), we identify various clinopyroxene domains in their host volcanic rocks. Type 1 clinopyroxene domains in basalts and trachyandesites are dark, have high Mg# values and are strongly depleted in light REEs (LREEs) with no significant Eu anomalies. The Type 2 clinopyroxene domains in basalts and trachyandesites and all clinopyroxene phenocrysts in andesites are bright, have low Mg# values and are strongly depleted in LREEs and have significant negative Eu anomalies. Clinopyroxene domains in rhyolites have significant negative Eu anomalies and can be divided into two types: Type 1 clinopyroxene domains are characterized by strong depletion in LREEs, Sr, P and high field strength elements (HFSEs), while Type 2 clinopyroxene domains are slightly depleted in LREEs and have small negative P anomalies. The continuously changing major element composition and parallel distribution of trace element spider diagrams suggest that all these clinopyroxene phenocrysts in the studied volcanic rocks were derived from co-genetic magmas and the compositional transition of clinopyroxene from basalts to rhyolites attributed to different extents of fractional crystallization of various minerals (e.g., clinopyroxene, orthopyroxene, plagioclase and apatite). The oscillatory zoned and reversed zoned clinopyroxene phenocrysts show that magma mixing and recharge events with co-genetic magmas are common in the middle Okinawa Trough magma system. The origin of clinopyroxene phenocrysts involves contributions from subducted sedimentary components of 2.5% sediment melts and 1.5% sediment fluids and is deduced based on the equilibrium melts of clinopyroxene phenocrysts in basalts. The geochemical study of clinopyroxene provides evidence for the contributions of magmatic processes and slab subduction to the petrogenesis of Okinawa Trough volcanic rocks.

Trace Element Constraints on the Differentiation and Crystal Mush Solidification in the Skaergaard Intrusion, Greenland

New major and trace element analyses of plagioclase and clinopyroxene in gabbros from the Skaergaard intrusion are used to understand the mechanisms of crystallization in the main magma body and in the crystal mush. Crystal cores show a continuous chemostratigraphic evolution in the Layered Series, with compatible elements (e.g. Cr and Ni in clinopyroxene) being progressively depleted from the bottom up, whereas incompatible elements (e.g. Sr, Ba, REE in plagioclase and clinopyroxene) become progressively enriched. We performed numerical models and showed that these trends can be explained by a simple process of fractional crystallization, except for the upper 20% of the intrusion where plagioclase and clinopyroxene trace element compositions depart from fractional crystallization trends. At this stage of magma chamber solidification, fractional crystallization becomes less efficient and is replaced by a major proportion of in situ, equilibrium crystallization. Trace elements also show significant variations in interstitial overgrowths on plagioclase and clinopyroxene cumulus crystals. They result from crystallization of the interstitial liquid in the liquid + crystal mush. However, incompatible elements, and especially REE, show a degree of enrichment in plagioclase rims (> 20 ppm Ce) that strongly exceeds the highest concentrations observed in plagioclase cores (up to 4 ppm Ce) at the top of the Layered Series. Such a strong enrichment is difficult to reconcile with a simple process of fractional crystallization of the interstitial liquid in the crystal mush, but may be related to the development of silicate liquid immiscibility in the crystal mush or to delayed nucleation of apatite. After the crystallization of plagioclase and clinopyroxene overgrowths, diffusive re-equilibration during a period of 0.1-0.25 Myr significantly changed the original zoning profiles. Ce flux from clinopyroxene into the plagioclase lattice of adjacent crystals could also have contributed to the strong Ce enrichment observed in plagioclase rims.

Immature intra-oceanic arc-type volcanism on the Izanagi Plate revealed by the geochemistry of the Daimaruyama greenstones in the Hiroo Complex, southern Hidaka Belt, central Hokkaido, Japan

The Izanagi Plate is assumed to have underlain the western Panthalassa Ocean to the east of Eurasia, and to have been subducting under the Eurasian continent. Although the Izanagi Plate has been lost to subduction, the subduction complexes of the circum-Panthalassa continental margins provide evidence that subduction-related volcanism occurred within the Panthalassa Ocean, and not just along its margins. The Daimaruyama mass is a kilometer-sized allochthonous greenstone body in the Hiroo Complex in the southeastern part of the Nakanogawa Group in the southern Hidaka Belt, northern Japan. The Hiroo Complex is a subduction complex that formed within the Paleo-Kuril arc–trench system at 57–48 Ma. The Daimaruyama greenstones consist mainly of coarse volcaniclastic rocks with lesser amount of lava. Red bedded chert, red shale, and micritic limestone are also observed as blocks associated with the greenstones. The presence of Early Cretaceous (Aptian–Albian) radiolaria in red bedded cherts within the greenstones indicates that the Daimaruyama greenstones formed after this time. An integrated major and trace element geochemical dataset for whole-rocks and clinopyroxenes of the greenstones indicates a calc-alkaline magmatic trend with low TiO2 contents and increases in SiO2 and decreases in FeO* with increasing differentiation. Negative anomalies of Nb, Ta, and Ti in normal mid-ocean-ridge basalt type normalized patterns are interpreted as “arc-signatures”. Using “rhyolite-MELTS”, we conducted a numerical simulation of magmatic differentiation under conditions of 1.5 kbar and H2O = 3 wt% to reproduce the liquid line of descent of the Daimaruyama greenstones. Back-calculations of the equilibrium melt compositions from the trace element chemistry of the clinopyroxenes generally agree with the whole-rock rare earth element compositions of the Daimaruyama greenstones, therefore providing support for the conditions used for the rhyolite-MELTS calculations as well as the actual results. Clinopyroxene trace element compositions indicate substantial enrichment of Ba in the magma, reflecting the participation of shallow subduction components such as aqueous fluids. Geochemical investigations reveal that the Daimaruyama greenstones were probably submarine volcanic rocks that formed as a result of the subduction of the Izanagi Plate within the Thalassa Ocean (the Thalassa Ocean was the eastern realm of Panthalassa, and represents the proto-Pacific Ocean) after the Early Cretaceous (Aptian–Albian; 125–101 Ma), and they were eventually accreted onto the Paleo-Kuril arc–trench system at 57–48 Ma to form an allochthonous block as part of the mélange facies of the Hiroo Complex on the landward slope of the trench.

Insights into the evolution of an alkaline magmatic system: An in situ trace element study of clinopyroxenes from the Ditrău Alkaline Massif, Romania

Clinopyroxene is a major constituent in most igneous rock types (hornblendite, diorite, syenite, nepheline syenite, camptonite, tinguaite and ijolite) of the Ditrău Alkaline Massif, Eastern Carpathians, Romania. Phenocryst and antecryst populations have been distinguished based on mineral zoning patterns and geochemical characteristics. Major and trace element compositions of clinopyroxenes reflect three dominant pyroxene types including primitive high-Cr Fe-diopside, intermediate Na-diopside-hedenbergite and evolved high-Zr aegirine-augite. Clinopyroxenes record two major magma sources as well as distinct magma evolution trends. The primitive diopside population is derived from an early camptonitic magma related to basanitic parental melts, whilst the intermediate diopside-hedenbergite crystals represent a Na-, Nb- and Zr-rich magma source recognised for the first time in the Ditrău magmatic system. This magma fractionated towards ijolitic and later phonolitic compositions. Field observations, petrography and clinopyroxene-melt equilibrium calculations reveal magma recharge and mingling, pyroxene recycling, fractional crystallisation and accumulation. Repeated recharge events of the two principal magmas resulted in multiple interactions between more primitive and more fractionated co-existing magma batches. Magma mingling occurred between mafic and felsic magmas by injection of ijolitic magma into fissures (dykes) containing phonolitic (tinguaite) magma. This study shows that antecryst recycling, also described for the first time in Ditrău, is a significant process during magma recharge and demonstrates that incorporated crystals can crucially affect the host magma composition and so whole-rock chemical data should be interpreted with great care.

Ultra-refractory mantle domains in the Luqu ophiolite (Tibet): Petrology and tectonic setting

Fresh mantle peridotites occur in the Luqu ophiolite, which is located at the central segment of the Yarlung-Tsangpo Suture Zone in the Tibetan Plateau. This study presents major and trace element compositions of clinopyroxene from harzburgites and dunites of the Luqu ophiolite, with the aim to study melt depletion and late-stage melt–rock interaction processes. The studied harzburgites have refractory to ultra-refractory compositions, as indicated by low whole-rock Al2O3 and high MgO contents, as well as high spinel Cr# values. Clinopyroxenes in the Luqu harzburgites are variably depleted in light rare earth elements (LREEs) relative to the middle and heavy rare earth elements (MREEs and HREEs), with Yb contents of 1.4–3.4 times of CI chondrites. Modeling results suggest that the Luqu harzburgites have experienced up to 8% garnet-facies melting followed by 10–18% fractional melting in the stability field of spinel. In both cases melting occurred under anhydrous conditions. This suggests that the Luqu ophiolite originated from a mid-ocean ridge setting rather than a supra-subduction zone environment. Nevertheless, the high degrees of partial melting are inconsistent with the thin oceanic crustal sequence preserved in the Luqu ophiolite, implying that the Luqu peridotites might have experienced melt extraction prior to entering into the spreading centers of the Neo-Tethyan Ocean during the Early Cretaceous. Clinopyroxenes in the dunites are interstitial among olivine grains. They display slightly LREE-enriched patterns and have higher REE contents than those in the harzburgites. They also show markedly positive anomalies in Sr, Zr and Hf. These features suggest that the dunites were generated through reaction between percolating melts and the harzburgites, during which the interstitial clinopyroxenes formed.

Metasomatic Processes Revealed by Trace Element and Redox Signatures of the Lithospheric Mantle Beneath the Massif Central, France

Trace element compositions of clinopyroxene and oxidation state measurements are both useful parameters for constraining metasomatic interactions within the subcontinental lithospheric mantle (SCLM). Laser ablation inductively coupled plasma mass spectrometry analyses of trace elements in clinopyroxene in combination with oxidation state data from the same samples provide a fuller picture of the metasomatic processes that occurred within the SCLM below the French Massif Central (FMC). A total of 89 spinel peridotite xenoliths from 39 localities across the FMC document variable degrees of partial melting and metasomatism at both regional and local scales. According to the degree of enrichment in light rare earth elements (LREE) and middle rare earth elements (MREE) in clinopyroxene, four main groups (A, B, C and D) are identified. Variations in other trace element concentrations are consistent with these groups. LREE enrichment correlates with a tendency to higher oxygen fugacity (fO2). These characteristics, along with their Ti/Eu–Zr/Hf systematics, are useful in assessing the different metasomatic agents that have interacted with the FMC SCLM. The behaviour of Lu and Hf additionally provides a means for distinguishing secondary clinopyroxene grains from those that have been overprinted. Most samples have been affected by cryptic metasomatism and different signatures indicate interaction with different types of metasomatic agent. Modal metasomatism is apparent by the presence of amphibole or secondary clinopyroxene. The lack of any notable trace element or redox signature related to the occurrence of amphibole suggests that it formed from small amounts of dilute H2O-rich fluids. Most samples from the northern sector of the FMC are fairly refractory and clinopyroxene exhibits a ‘tick-shaped’ REE signature (group D), consistent with metasomatism by subduction-related fluids during Variscan times. Such fluids were oxidizing (Δlog fO2 values > FMQ + 1, where FMQ is the fayalite–magnetite–quartz buffer) and responsible for LREE enrichment in clinopyroxene and low concentrations of the less mobile elements such as Ti, Eu, Zr and Hf. Clinopyroxene in several samples has very low Ti/Eu, elevated (La/Yb)N and suprachondritic Zr/Hf ratios, reflecting local carbonatite metasomatism. These samples also record oxidizing conditions, with Δlog fO2 values > FMQ + 1. Many other samples have clinopyroxenes with higher Ti/Eu and approximately chondritic Zr/Hf ratios, consistent with having undergone silicate melt metasomatism. Their chondrite-normalized REE element patterns vary from LREE-depleted to relatively flat (group A), indicating small but variable metasomatic enrichments in the more mobile elements, which can be attributed to chromatographic effects during reactive flow. The oxidation states of these samples record Δlog fO2 ∼ FMQ + 0·4 ± 0·5. Some samples have clinopyroxene with negative Zr and Hf anomalies and distinctly low Ti/Eu (<2000), but not as low as expected for carbonatite metasomatism. These may have interacted with carbonated-silicate melts, which also imposed somewhat elevated oxidation states (Δlog fO2 values up to ∼FMQ + 0·9). Some samples exhibit evidence for having been overprinted by more than one metasomatic event, in part indicated by two populations of geochemically distinct clinopyroxene. These samples, among others, also demonstrate that immobility of heavy REE (HREE) cannot be assumed when modeling the degree of melt extraction of strongly metasomatized peridotites. Addition of HREE by refertilization lowers the apparent degree of melting compared with calculations based upon Cr/(Cr + Al) in spinel. In other cases, a lowering of HREE concentrations via interaction with a low-HREE metasomatic agent produces higher apparent degrees of melting.

Refertilization Processes in the Subcontinental Lithospheric Mantle: the Record of the Beni Bousera Orogenic Peridotite (Rif Belt, Northern Morocco)

Correlations between major and minor transition elements in tectonically emplaced orogenic peridotites have been ascribed to variable degrees of melt extraction and melt–rock reaction processes, leading to depletion or refertilization. To elucidate how such processes are recorded in the subcontinental lithospheric mantle, we processed a large geochemical dataset for peridotites from the four tectono-metamorphic domains of the Beni Bousera orogenic massif (Rif Belt, northern Morocco). Our study reveals that variations in bulk-rock major and minor elements, Mg-number and modal mineralogy of lherzolites, as well as their clinopyroxene trace element compositions, are inconsistent with simple partial melting and mainly resulted from different reactions between melts and depleted peridotites. Up to 30% melting at < 3 GPa and cryptic metasomatism can account for the geochemical variations of most harzburgites. In Grt–Sp mylonites, melting and melt–rock reactions are masked by tectonic mixing with garnet pyroxenites and subsolidus re-equilibration. In the rest of the massif, lherzolites were mostly produced by refertilization of a refractory protolith (Mg-number ∼ 91, Ol ∼ 70%, Cpx/Opx = 0·4) via two distinct near-solidus, melt–rock reactions: (1) clinopyroxene and orthopyroxene precipitation and olivine consumption at melt/rock ratios <0·75 and variable mass ratio between crystallized minerals and infiltrated melt (R), which are recorded fairly homogeneously throughout the massif; (2) dissolution of orthopyroxene and precipitation of clinopyroxene and olivine at melt/rock ratios ≤1 and R = 0·2–0·3, which affected mainly the Ariegite–Seiland and Seiland domains. The distribution of secondary lherzolites in the massif suggests that the first refertilization reaction occurred prior to the differentiation of the Beni Bousera mantle section into petro-structural zones, whereas the second reaction was associated with the development of the tectono-metamorphic domains. Our data support a secondary, refertilization-related origin for most lherzolites in orogenic peridotite massifs.

The Ediacaran volcanic rocks and associated mafic dykes of the Ouarzazate Group (Anti-Atlas, Morocco): Clinopyroxene composition, whole-rock geochemistry and Sr-Nd isotopes constraints from the Ouzellarh-Siroua salient (Tifnoute valley)

Belonging to the huge Ouarzazate volcanic Group that covered the whole Anti-Atlas during the late Ediacaran (580-545 Ma), the Tifnoute valley volcanic formations are mainly pyroclastic and show a large composition, from trachybasalt to rhyolite and are crosscut by dolerite dykes. The Tifnoute valley volcanic rocks are located within a rigid salient of the Anti-Atlas that gives them special extreme characteristics. Due to the heavy greenschist alteration that affects this volcanic group, we focused the more immobile elements, but as REE can also be affected, we used the composition of unaltered clinopyroxene crystals to determine the nature of these volcanic rocks. The clinopyroxene is an augite diopside in the basalt, an augite in the andesite and an augite-salite in the dolerite. Petrography of the Tifnoute mafic volcanic rocks and clinopyroxene compositions indicate the presence of two magmatic series: (i) older high-K calc-alkaline (alkali-calcic) andesite and basalt characterized by the early crystallization of Fe-Ti oxides and of the late fractionation of plagioclase, the modal proportion of the latter increasing from the basalt to the andesite and (ii) younger alkalic dolerite dykes. With clinopyroxene trace element compositions obtained using laser ablation ICP-MS, we calculated the composition of the melts in equilibrium with the pyroxenes.The volcanic rocks of the Tifnoute Valley have positive εNd570 (+1.7 to +5.0), low Sri (<0.7063), and NdTDM model ages ranging from 0.80 to 1.14 Ga, indicating a mostly depleted Neoproterozoic source with limited involvement of the Eburnian lithosphere for the Tifnoute magmas. This depleted source is the young lithospheric mantle for the alkali-calcic series and the asthenosphere for the younger alkalic series. The Tifnoute Valley volcanic rocks emplaced in a Pan-African transtensive post-collisional environment that evolved towards the major rifting event that will give rise to the Rheic ocean, in a similar way to what occurred just after the Variscan orogeny during the Triassic period that evolved to the Tethys ocean opening.

Refertilization of lithospheric mantle beneath the Yangtze craton in south-east China: Evidence from noble gases geochemistry

The Yangtze craton (YC), in eastern China, is one of the oldest cratons in the world and is characterized by a complex tectonic and geodynamic evolution. This evolution regards most of the eastern China craton, which since Mesozoic time has undergone significant thinning (>200km) of Archean lithosphere. This thinning favored the refertilization of the old refractory subcontinental lithospheric mantle (SCLM) by the upwelling of younger fertile asthenosphere. Whether this feature is localized only beneath certain areas of eastern China or is a more widespread characteristic of the mantle, including the YC, is a matter of debate.In order to constrain the history of the YC SCLM, we have measured the He- and Ar-isotopic compositions of fluid inclusions hosted in mantle xenoliths in the Lianshan area, which is part of the poorly investigated YC in south-east China. We also report new mineral chemistry and trace element compositions of clinopyroxenes from the same suite of samples, for comparison with noble gases. Two distinct types of xenoliths can be identified: Type 1, characterized by mantle-like He-isotopic (3He/4He) ratios (up to 9.1Ra), represents fragments of a fertile lithospheric mantle; Type 2, showing 3He/4He values in the SCLM range (3He/4He<7Ra), represents shallow relicts of a refractory mantle. The patterns of rare-earth elements as well as the Y and Yb concentrations in the clinopyroxenes normalized to primitive mantle (YN and YbN, respectively) indicate that fractional partial melting might have affected the local mantle by <3% in Type 1 and up to 20% in Type 2 xenoliths from Lianshan, respectively. The range of 4He/40Ar* (40Ar* is corrected for atmospheric contamination) ranges from 4.9×10−4 to 3.6×10−1, which is below the typical production ratio of the mantle (4He/40Ar*=1–5); this range is however compatible with this fractional partial melting. The variable 3He/4He and 4He/40Ar* values in Lianshan xenoliths suggest that the local mantle source was also influenced by kinetic fractionation, possibly triggered by metasomatic melts. Metasomatism associated with carbonatitic melts, together with fluxing by CO2-rich fluids, have permeated the mantle beneath Lianshan, generating the observed decoupling between noble gases and trace elements. The interpretative framework is also applicable for other mantle xenoliths from eastern China, indicating that the refertilization of the SCLM by ascending mantle-like melts is common also to YC, which can be identified using noble gases.

Southward trench migration at ∼130–120 Ma caused accretion of the Neo-Tethyan forearc lithosphere in Tibetan ophiolites

The preservation of ultrahigh-pressure and super-reduced phases (diamond, moissanite, etc.) in the harzburgites and chromitites of the Yarlung Zangbo ophiolites (South Tibet, China) has major implications for mantle recycling and lithosphere evolution in the tectonic system related to the closing of the Neo-Tethyan Ocean. However, important aspects of the genesis of these enigmatic ophiolites and the related geodynamic evolution are still unclear. In the Zedang ophiolite of the eastern Yarlung Zangbo Suture, detailed mineral chemical data reveal that the harzburgite domain in the east [spinel Cr# (mole Cr3+/(Cr3+ + Al3+) = 0.62–0.33] is more depleted than the lherzolite domain in the west (spinel Cr# = 0.30–0.17) and shows much lower equilibration temperatures (by ∼250–150 °C) than the lherzolites. Clinopyroxene trace-element compositions indicate that the harzburgites underwent pervasive metasomatism after melt extraction, while the lherzolites did not. New zircon U–Pb ages show that the harzburgites were intruded by dolerite dykes with chilled margins at ∼130–128 Ma, consistent with the widespread mafic magmatism at ∼130–120 Ma in the Yarlung Zangbo ophiolites. Nd–Hf isotopic data indicate that the Zedang lherzolites subcreted the pre-emplaced harzburgites concurrently with the intrusion of the dolerite dykes into the harzburgites, and that the lherzolites and dolerites both were derived from upwelling asthenosphere with minor slab input. Available zircon geochronology and Hf-isotope data show that juvenile magmatism in the adjacent Gangdese Arc was almost completely interrupted from ∼130–120 Ma.We suggest that the extension of the overlying harzburgitic lithosphere, subcretion of lherzolites, intrusion of mafic dykes, and the waning of Gangdese-Arc magmatism all reflect a southward trench migration in the Neo-Tethyan subduction system from the Gangdese Arc to the oceanic forearc lithosphere. This magmatic relocation and tectonic linkage are inferred to be the far-field effects of plate reorganization related to the Lhasa-Qiangtang collision and the breakup of Gondwanaland in the early Cretaceous. This model provides a solution for the global “ophiolite conundrum”: supra-subduction-zone type ophiolites with mid-ocean-ridge features.

Trace-element abundances in the shallow lithospheric mantle of the North Atlantic Craton margin: Implications for melting and metasomatism beneath Northern Scotland

AbstractBulk rock geochemistry and major- and trace-element compositions of clinopyroxene have been determined for three suites of peridotitic mantle xenoliths from the North Atlantic Craton (NAC) in northern Scotland, to establish the magmatic and metasomatic history of subcontinental lithospheric mantle (SCLM) below this region. Spinel lherzolites from the southernmost locality (Streap Com'laidh) have non-NAC mantle compositions, while the two northern xenolith suites (Loch Roag and Rinibar) are derived from the thinned NAC marginal keel. Clinopyroxene compositions have characteristic trace-element signatures which show both 'primary' and 'metasomatic' origins. We use Zr and Hf abundances to identify ancient cryptic refertilization in 'primary' clinopyroxenes. We suggest that Loch Roag and Rinibar peridotite xenoliths represent an ancient Archaean-Palaeoproterozoic SCLM with original depleted cratonic signatures which were overprinted by metasomatism around the time of intrusion of the Scourie Dyke Swarm (∼2.4 Ga). This SCLM keel was preserved during Caledonian orogenesis, although some addition of material and/or metasomatism probably also occurred, as recorded by Rinibar xenoliths. Rinibar and Streap xenoliths were entrained in Permo-Carboniferous magmas and thus were isolated from the SCLM ∼200 Ma before Loch Roag xenoliths (in an Eocene dyke). Crucially, despite their geographical location, lithospheric mantle peridotite samples from Loch Roag show no evidence of recent melting or refertilization during the Palaeogene opening of the Atlantic.

Geochemical and petrological constraints on mantle composition of the Ohře(Eger) rift, Bohemian Massif: peridotite xenoliths from the České Středohoří Volcanic complex and northern Bohemia

The Ohře(Eger) rift in the Bohemian Massif, which represents part of the Cenozoic Central European Volcanic Province, contains abundant mantle xenoliths of spinel peridotite. Petrography, geothermometry, mineral compositions and whole-rock and clinopyroxene trace element compositions of xenolith suites from four localities in the central part Ceske Středohoři Volcanic complex (CSVC) and one locality in the eastern continuation (northern Bohemia) of the rift system display important differences that reveal small-scale compositional heterogeneity of the mantle. The xenoliths from the CSVC are mostly harzburgite that experienced high degrees of partial melting from ~17 to 21 %. However, xenoliths from one locality, Medvědický vrch, are predominantly fertile lherzolite. Subsequent pervasive metasomatism produced enrichments in light rare earth elements, large-ion lithophile elements, and U and Th, except for xenoliths from Medvědický vrch, which show marked depletions in all these elements. Such composition most likely reflects refertilization of the source rock by depleted melts. The trace element characteristics of the CSVC xenoliths indicate metasomatism of a depleted protolith by silicate melt at high melt/rock ratios. In contrast, harzburgite xenoliths from northern Bohemia experienced metasomatism at low melt/rock ratios by more evolved, alkaline and carbonate-rich melts.