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Chris Harris - One of the best experts on this subject based on the ideXlab platform.
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tectonic significance and redox state of paleoproterozoic eclogite and Pyroxenite components in the slave cratonic mantle lithosphere voyageur kimberlite arctic canada
Chemical Geology, 2017Co-Authors: Katie A Smart, Sebastian Tappe, Antonio Simonetti, Stefanie S Simonetti, Alan B Woodland, Chris HarrisAbstract:Abstract Mantle-derived eclogite and Pyroxenite xenoliths from the Jurassic Voyageur kimberlite on the northern Slave craton in Arctic Canada were studied for garnet and clinopyroxene major and trace element compositions, clinopyroxene Pb and garnet O isotopic compositions, and garnet Fe3 +/ΣFe contents. The Voyageur xenoliths record a wide range of pressures, but are cooler compared to mantle xenoliths derived from the nearby, coeval Jericho kimberlite. The CaO, TiO2 and Zr contents of Voyageur eclogites increase with depth, which is also observed in northern Slave peridotite xenoliths, demonstrating ‘bottom-up’ metasomatic processes within cratonic mantle lithosphere. The Voyageur eclogites have positive Eu anomalies, flat HREEN patterns, and major element compositions that are consistent with ultimate origins from basaltic and gabbroic protoliths within oceanic lithosphere. Clinopyroxene Pb isotope ratios intercept the Stacey-Kramers two-stage terrestrial Pb evolution curve at ca. 2.1 Ga, and form an array towards the host kimberlite, indicating isotopic mixing. The 2.1 Ga eclogite formation age broadly overlaps with known Paleoproterozoic subduction and collision events that occurred along the western margin of the Slave craton. Unlike the eclogites, the Voyageur Pyroxenites contain garnet with distinctive fractionated HREEN, sinusoidal REE patterns of calculated bulk rocks, and clinopyroxene with 206Pb/204Pb ratios that intercept the Stacey-Kramers curve at 1.8 Ga. This suggests a distinct origin as Paleoproterozoic high-pressure mantle cumulates. However, the Pyroxenite Pb isotope ratios fall within the eclogite array and could also be explained by protoliths formation at ca. 2.1 Ga followed by minor isotopic mixing during mantle metasomatism. Thus, an alternative scenario involves Pyroxenite formation within the mantle section of Paleoproterozoic oceanic lithosphere followed by variable metasomatism after incorporation into cratonic mantle lithosphere. This model allows for a linked petrogenesis of the Voyageur eclogites (crust) and Pyroxenites (mantle) as part of the same subducting oceanic slab. Oxygen fugacity determinations for one Pyroxenite and ten eclogite xenoliths show a range of 3 log units, from − 4.6 to − 1.6 ΔFMQ, similar to the range observed for nearby Jericho and Muskox eclogites (ΔFMQ − 4.2 to − 1.5). Importantly, the northern Slave eclogite and Pyroxenite mantle components are highly heterogeneous in terms of redox state provided that they range from reduced to oxidized relative to Slave peridotite xenoliths. Moreover, the Voyageur eclogites do not exhibit any trend between oxidation state and equilibration depth, which contrasts with the downward decrease in fO2 shown by Slave and worldwide cratonic peridotite xenoliths. Our investigation of mantle eclogite and Pyroxenite fO2 reinforces the important influence of recycled mafic components in upper mantle processes, because their high and variable redox buffering capacity strongly controls volatile speciation and melting relations under upper mantle conditions.
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relationship between footwall composition crustal contamination and fluid rock interaction in the platreef bushveld complex south africa
Mineralium Deposita, 2008Co-Authors: Julie Pronost, Chris HarrisAbstract:In the northern limb of the 2.06-Ga Bushveld Complex, the Platreef is a platinum group elements (PGE)-, Cu-, and Ni-mineralized zone of Pyroxenite that developed at the intrusion margin. From north to south, the footwall rocks of the Platreef change from Archaean granite to dolomite, hornfels, and quartzite. Where the footwall is granite, the Sr-isotope system is more strongly perturbed than where the footwall is Sr-poor dolomite, in which samples show an approximate isochron relationship. The Nd-isotope system for samples of Pyroxenite and hanging wall norite shows an approximate isochron relationship with an implied age of 2.17 ± 0.2 Ga and initial Nd-isotope ratio of 0.5095. Assuming an age of 2.06 Ga, the ɛNd values range from −6.2 to −9.6 (ave. −7.8, n = 17) and on average are slightly more negative than the Main Zone of the Bushveld. These data are consistent with local contamination of an already contaminated magma of Main Zone composition. The similarity in isotope composition between the Platreef Pyroxenites and the hanging wall norites suggests a common origin. Where the country rock is dolomite, the Platreef has generally higher plagioclase and pyroxene δ18O values, and this indicates assimilation of the immediate footwall. Throughout the Platreef, there is considerable petrographic evidence for sub-solidus interaction with fluids, and the Δplagioclase–pyroxene values range from −2 to +6, which indicates interaction at both high and low temperatures. Whole-rock and mineral δD values suggest that the Platreef interacted with both magmatic and meteoric water, and the lack of disturbance to the Sr-isotope system suggests that fluid–rock interaction took place soon after emplacement. Where the footwall is granite, less negative δD values suggest a greater involvement of meteoric water. Consistently higher values of Δplagioclase–pyroxene in the Platreef Pyroxenites and hanging wall norites in contact with dolomite suggest prolonged interaction with CO2-rich fluid derived from decarbonation of the footwall rocks. The overprint of post crystallization fluid–rock interaction is the probable cause of the previously documented lack of correlation between PGE and sulfide content on the small scale. The Platreef in contact with dolomite is the focus of the highest PGE grades, and this suggests that dolomite contamination played a role in PGE concentration and deposition, but the exact link remains obscure. It is a possibility that the CO2 produced by decarbonation of assimilated dolomite enhanced the process of PGE scavenging by sulfide precipitation.
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relationship between footwall composition crustal contamination and fluid rock interaction in the platreef bushveld complex south africa
Mineralium Deposita, 2008Co-Authors: Julie Pronost, Chris Harris, Christian PinAbstract:In the northern limb of the 2.06-Ga Bushveld Complex, the Platreef is a platinum group elements (PGE)-, Cu-, and Ni-mineralized zone of Pyroxenite that developed at the intrusion margin. From north to south, the footwall rocks of the Platreef change from Archaean granite to dolomite, hornfels, and quartzite. Where the footwall is granite, the Sr-isotope system is more strongly perturbed than where the footwall is Sr-poor dolomite, in which samples show an approximate isochron relationship. The Nd-isotope system for samples of Pyroxenite and hanging wall norite shows an approximate isochron relationship with an implied age of 2.17 ± 0.2 Ga and initial Nd-isotope ratio of 0.5095. Assuming an age of 2.06 Ga, the ɛNd values range from −6.2 to −9.6 (ave. −7.8, n = 17) and on average are slightly more negative than the Main Zone of the Bushveld. These data are consistent with local contamination of an already contaminated magma of Main Zone composition. The similarity in isotope composition between the Platreef Pyroxenites and the hanging wall norites suggests a common origin. Where the country rock is dolomite, the Platreef has generally higher plagioclase and pyroxene δ18O values, and this indicates assimilation of the immediate footwall. Throughout the Platreef, there is considerable petrographic evidence for sub-solidus interaction with fluids, and the Δplagioclase–pyroxene values range from −2 to +6, which indicates interaction at both high and low temperatures. Whole-rock and mineral δD values suggest that the Platreef interacted with both magmatic and meteoric water, and the lack of disturbance to the Sr-isotope system suggests that fluid–rock interaction took place soon after emplacement. Where the footwall is granite, less negative δD values suggest a greater involvement of meteoric water. Consistently higher values of Δplagioclase–pyroxene in the Platreef Pyroxenites and hanging wall norites in contact with dolomite suggest prolonged interaction with CO2-rich fluid derived from decarbonation of the footwall rocks. The overprint of post crystallization fluid–rock interaction is the probable cause of the previously documented lack of correlation between PGE and sulfide content on the small scale. The Platreef in contact with dolomite is the focus of the highest PGE grades, and this suggests that dolomite contamination played a role in PGE concentration and deposition, but the exact link remains obscure. It is a possibility that the CO2 produced by decarbonation of assimilated dolomite enhanced the process of PGE scavenging by sulfide precipitation.
Károly Hidas - One of the best experts on this subject based on the ideXlab platform.
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interplay between melt infiltration and deformation in the deep lithospheric mantle external liguride ophiolite north italy
Lithos, 2021Co-Authors: Andréa Tommasi, Károly Hidas, Giulio Borghini, Alberto Zanetti, E. RamponeAbstract:Abstract Mantle peridotites from the External Liguride Jurassic ophiolites (Northern Apennines, Italy) show diffuse occurrence of Pyroxenite bands, recording melt migration and crystallization at the lithosphere-asthenosphere boundary, during old, pre-Jurassic stages of their subcontinental lithospheric mantle evolution. We present coupled microstructural and geochemical study of profiles across various types of Pyroxenite-peridotite layering in these ophiolites, aiming to constrain the relative timing and potential interplay between melt infiltration and mantle deformation. The mantle sequence is composed of lherzolite and harzburgite, occasionally interleaved with dunite, crosscut by centimeter- to decimeter-wide Pyroxenite layers. The peridotites have a porphyroclastic texture and show a penetrative tectonic foliation subparallel to the Pyroxenite layering. Peridotite-Pyroxenite contacts are irregular at the grain scale. Olivine and pyroxenes in both peridotites and Pyroxenites record moderate to strong crystallographic preferred orientations (CPO) with alignment of olivine [100] and pyroxenes [001] axes subparallel to the stretching lineation marked by olivine and pyroxenes elongation. This is compatible with coeval deformation of olivine and pyroxenes during a high-temperature, spinel lherzolite-facies deformation event. The major and trace element compositions of peridotites record a metasomatic imprint that decreases with distance from the Pyroxenite layers, whereas the strength of the olivine CPO decreases from the country peridotites towards the Pyroxenite layers. The parallelism between Pyroxenite layers and the peridotite foliation, their irregular contacts, as well as the spatial correlation between CPO and geochemical changes, are consistent with syn- to late-kinematic emplacement of the Pyroxenites. These observations point to a strong interplay between deformation and melt transport processes in the mantle, characterized by melt focusing in conduits parallel to the foliation and changes in the mantle deformation processes due to the presence of melts. Exhumation of this mantle section in the Jurassic resulted in partial replacement of the spinel-facies assemblage by a plagioclase-bearing assemblage. Topotaxial relationship between plagioclase and precursor spinel-facies minerals suggests that this exhumation was not associated with pervasive deformation of the peridotite, but rather accommodated by deformation localized in discrete shear zones, not sampled in the present study.
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Strain Localization in Pyroxenite by Reaction-Enhanced Softening in the Shallow Subcontinental Lithospheric Mantle
Journal of Petrology, 2013Co-Authors: Károly Hidas, Carlos J Garrido, Andréa Tommasi, José Alberto Padrón-navarta, Marcel Thielmann, Zoltan Konc, Erwin Frets, Claudio MarchesiAbstract:We report structural evidence of ductile strain localization in mantle Pyroxenite from the spinel to plagioclase websterite transition in the Ronda Peridotite (southern Spain). Mapping shows that, in this domain, small-scale shear zones occurring at the base of the lithospheric section are systematically located within thin Pyroxenite layers, suggesting that the Pyroxenite was locally weaker than the host peridotite. Strain localization is associated with a sudden decrease of grain size and increasing volume fractions of plagioclase and amphibole as a result of a spinel to plagioclase phase transformation reaction during decompression. This reaction also fostered hydrogen extraction ('dehydroxylation') from clinopyroxene producing effective fluid saturation that catalyzed the synkinematic net-transfer reaction. This reaction produced fine-grained olivine and plagioclase, allowing the onset of grain-size sensitive creep and further strain localization in these Pyroxenite bands. The strain localization in the Pyroxenites is thus explained by their more fertile composition, which allowed earlier onset of the phase transition reactions. Geothermobarometry undertaken on compositionally zoned constituent minerals suggests that this positive feedback between reactions and deformation is associated with cooling from at least 1000°C to 700°C and decompression from 1*0 to 0*5 GPa.
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Mantle refertilization by melts of crustal-derived garnet Pyroxenite: Evidence from the Ronda peridotite massif, southern Spain
Earth and Planetary Science Letters, 2013Co-Authors: Claudio Marchesi, Jean-louis Bodinier, Carlos J Garrido, Delphine Bosch, Fernando Gervilla, Károly HidasAbstract:Geochemical studies of primitive basalts have documented the presence of crustal-derived garnet Pyroxenite in their mantle sources. The processes whereby melts with the signature of garnet Pyroxenite are produced in the mantle are, however, poorly understood and somewhat controversial. Here we investigate a natural example of the interaction between melts of garnet Pyroxenite derived from recycled plagioclase-rich crust and surrounding mantle in the Ronda peridotite massif. Melting of garnet Pyroxenite at ∼1.5 GPa generated spinel websterite residues with MREE/HREE fractionation and preserved the positive Eu anomaly of their garnet Pyroxenite precursor in whole-rock and clinopyroxene. Reaction of melts from garnet Pyroxenite with depleted surrounding peridotite generated secondary fertile spinel lherzolite. These secondary lherzolites differ from common spinel lherzolite from Ronda and elsewhere by their lower-Mg# in clinopyroxene, orthopyroxene and olivine, lower-Cr# in spinel and higher whole-rock Al2O3, CaO, Sm/Yb and FeO⁎ at a given SiO2. Remarkably, secondary spinel lherzolite shows the geochemical signature of ghost plagioclase in the form of positive Eu and Sr anomalies in whole-rock and clinopyroxene, reflecting the transfer of a low-pressure crustal imprint from recycled Pyroxenite to hybridized peridotite. Garnet Pyroxenite melting and melt-peridotite interaction, as shown in the Ronda massif, may explain how the signature of subducted or delaminated crust is transferred to the mantle and how a garnet Pyroxenite component is introduced into the source region of basalts. The efficiency of these processes in conveying the geochemical imprint of crustal-derived garnet Pyroxenite to extruded lavas depends on the reactivity of Pyroxenite melt with peridotite and the mantle permeability, which may be controlled by prior refertilization reactions similar to those documented in the Ronda massif. Highly fertile heterogeneities produced by Pyroxenite-peridotite interaction, such as secondary spinel lherzolite in Ronda, may nucleate magmatic channels that remain chemically isolated from the ambient mantle and act as preferential pathways for melts with the signature of recycled crust.
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a late oligocene suprasubduction setting in the westernmost mediterranean revealed by intrusive Pyroxenite dikes in the ronda peridotite southern spain
The Journal of Geology, 2012Co-Authors: Claudio Marchesi, Jean-louis Bodinier, Carlos J Garrido, Delphine Bosch, Károly Hidas, Jose Alberto Padronnavarta, Fernando GervillaAbstract:AbstractContrasting tectonic reconstructions of the westernmost Mediterranean have been proposed to explain the origin of the Alboran marine basin contemporaneously with Cenozoic convergence between the African and European plates. Cr-rich Pyroxenites in the Ronda massif record the geochemical processes occurring in the subcontinental mantle of the Alboran domain in the Late Oligocene, thus constraining the geodynamic scenario of Cenozoic extension in the western Mediterranean lithosphere. Clinopyroxene in intrusive Cr-rich websterite dikes crosscutting the Ronda peridotite is strongly depleted in Nb-Ta and enriched in light rare earth elements, as typically observed in arc magmas, and is in trace element equilibrium with Neogene subduction-related lavas from the western and central Mediterranean. Sr-Nd-Pb radiogenic isotopes indicate that the mantle source of the Ronda Pyroxenite dikes was contaminated by a subduction component released by detrital sediments likely deposited in passive continental margin...
W L Griffin - One of the best experts on this subject based on the ideXlab platform.
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lithospheric memory of subduction in mantle Pyroxenite xenoliths from rift related basalts
Earth and Planetary Science Letters, 2020Co-Authors: W L Griffin, J P Zheng, Romain Tilhac, Qing Xiong, Benat Oliveira, Suzanne Y OreillyAbstract:Abstract Petrological and geochemical studies have revealed the contribution of garnet Pyroxenites in basalt petrogenesis. However, whether primary mantle melts are produced with such signature or acquired it subsequently remains somewhat controversial. We here integrate new major-, trace-element and Sr-Nd-Hf isotopic compositions of garnet Pyroxenite xenoliths in Holocene alkali basalts from Lakes Bullenmerri and Gnotuk, Southeastern Australia, to relate their petrogenesis to mantle-wedge melt circulation and subsequent lithospheric evolution. Results show that the clinoPyroxenites have lower MgO and Cr2O3 contents than the associated websterites, and range in compositions from depleted LREE patterns and highly radiogenic Nd and Hf isotopic signatures in relatively low-MgO samples (Type 1), to enriched REE patterns with negative HFSE anomalies, unradiogenic Nd and Hf isotopes, and extremely radiogenic Sr-isotopic ratios in samples with higher MgO (Type 2). Such compositional variabilities suggest that these Pyroxenites represent segregates from melts derived from a recycled oceanic lithosphere with a potential contribution from pelagic sediments. Variable LREE contents and isotopic compositions between those of Type 1 and 2 clinoPyroxenites are observed in amphibole-bearing samples (Type 3), which are interpreted as Type 1-like protoliths metasomatized by the basaltic and carbonatitic melts, possibly parental to Type 2 clinoPyroxenites. The lithosphere beneath Southeastern Australia thus has received variable melt contributions from a heterogeneous mantle-wedge source, which notably includes a subducted oceanic slab package that has retained its integrity during subduction. On this basis, we suggest that the compositional heterogeneity and temporal evolution of the subsequent Southeastern Australian basaltic magmatism were probably affected by the presence of Pyroxenite fragments in the basalt source and formed by the tectonic reactivation of this lithosphere during Cenozoic rifting. This interpretation is notably consistent with a trend of Nd-Pb isotopes towards EMII in Older Volcanic Provinces (OVP basalts) and limited Sr-Nd-Pb isotopic variations towards HIMU in the Newer Volcanic Provinces (NVP basalts, including the host lavas), which also exhibit low SiO2, high FeO and high CaO/Al2O3 commonly interpreted as due to Pyroxenite contributions. Therefore, the identification of a subduction signature in these rift-related lavas attests to a “lithospheric memory” of earlier subduction episodes (as documented by the xenoliths), rather than a reflection of contemporaneous subduction tectonics.
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reworking of old continental lithosphere unradiogenic os and decoupled hf nd isotopes in sub arc mantle Pyroxenites
Lithos, 2020Co-Authors: Romain Tilhac, W L Griffin, Benat Oliveira, Suzanne Y Oreilly, Bruce F Schaefer, Olivier Alard, Georges Ceuleneer, Juan Carlos AfonsoAbstract:Abstract Mantle lithologies in orogenic massifs and xenoliths commonly display strikingly different Hf- and Nd-isotope compositions compared to oceanic basalts. While the presence of Pyroxenites has long been suggested in the source region of mantle-derived magmas, very few studies have reported their combined Hf Nd isotope compositions. We here report the first Lu Hf data along with Re Os data and S concentrations on the Cabo Ortegal Complex, where the Pyroxenite-rich Herbeira massif has been interpreted as remnants of a delaminated arc root. The Pyroxenites, chromitites and their host harzburgites show a wide range of whole-rock 187Re/188Os and 187Os/188Os (0.16–1.44), indicating that Re was strongly mobilized, partly during hydrous retrograde metamorphism but mostly during supergene alteration that preferentially affected low-Mg#, low Cu/S Pyroxenites. Samples that escaped this disturbance yield an isochron age of 838 ± 42 Ma, interpreted as the formation of Cabo Ortegal Pyroxenites. Corresponding values of initial 187Os/188Os (0.111–0.117) are relatively unradiogenic, suggesting limited contributions of slab-derived Os to primitive arc melts such as those parental to these Pyroxenites. This interpretation is consistent with radiogenic Os in arc lavas being mostly related to crustal assimilation. Paleoproterozoic to Archean Os model ages confirm that Cabo Ortegal Pyroxenites record incipient volcanic arc magmatism on the continental margin of the Western African Craton, as notably documented by zircon U Pb ages of 2.1 and 2.7 Ga. Lu Hf data collected on clinopyroxene and amphibole separates and whole-rock samples are characterized by uncorrelated 176Lu/177Hf and 176Hf/177Hf (0.2822–0.2855), decoupled from Nd-isotope compositions. This decoupling is ascribed to diffusional disequilibrium during melt-peridotite interaction, in good agreement with the results of percolation-diffusion models simulating the interaction of an arc melt with an ancient melt-depleted residue. These models notably show that Hf Nd isotopic decoupling such as recorded by Cabo Ortegal Pyroxenites and peridotites (ΔƐHf(i) up to +97) is enhanced during melt-peridotite interaction by slow diffusional re-equilibration and can be relatively insensitive to chromatographic fractionation. Finally, we discuss the hypothesis that arc-continent interaction may provide preferential conditions for such isotopic decoupling and propose that its ubiquitous recognition in peridotites reflects the recycling of sub-arc mantle domains derived from ancient, reworked SCLM.
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sources and timing of Pyroxenite formation in the sub arc mantle case study of the cabo ortegal complex spain
Earth and Planetary Science Letters, 2017Co-Authors: Michel Grégoire, W L Griffin, Romain Tilhac, Suzanne Y Oreilly, Hadrien Henry, Georges CeuleneerAbstract:Abstract Pyroxenites exposed in ophiolites and orogenic peridotite massifs may record petrogenetic processes occurring in mantle domains generated and/or transferred in supra-subduction environments. However, the timing of their formation and the geochemical characteristics of their source region commonly are obscured by metamorphic and metasomatic overprints. This is especially critical in arc-related environments, where Pyroxenites may be formed during the differentiation of primitive magmas. Our approach combines Sr- and Nd-isotope geochemistry and geochronology, and modelling of REE diffusion, to further constrain the origin of a well-characterized set of Pyroxenites from the arc-related Cabo Ortegal Complex, Spain. In the light of petrological constraints, Sr- and Nd-isotope systematics consistently indicate that cpx and amphibole have acquired disequilibrium during two main episodes: (1) a magmatic/metasomatic episode that led to the formation of the Pyroxenites, coeval with that of Cabo Ortegal granulites and corresponding to the incipient stage of a potential Cadomian arc (459–762 Ma; isochron and second-stage Nd model ages); (2) an episode of metamorphic amphibolitization upon the percolation of relatively unradiogenic and LREE-enriched hydrous fluids, subsequent to the delamination of the Pyroxenites from their arc-root settings during Devonian subduction. Calculations of diffusional timescale for the re-equilibration of REE are consistent with this scenario but provide only poor additional constraints due to the sensitivity of this method to grain size and sub-solidus temperature. We thus emphasize the necessity to combine isochron ages and Nd model ages corrected for radiogenic ingrowth to put time constraints on the formation of subduction- and/or collision-related Pyroxenites, along with petrological and geochemical constraints. Homogeneous age-corrected 143Nd/144Nd of 0.5121–0.5125 (eNd between 0 and +7.5) and 87Sr/86Sr of 0.7037–0.7048 provide information on the sources of the metasomatic agents involved (and potentially the parental melts) and notably indicate the contributions from enriched mantle components (EM I and/or II). This suggests the involvement of an old crustal component, which is consistent with the derivation of the Pyroxenites and granulites from an ensialic island arc, potentially built on the northern margin of either Gondwana or a pre-Gondwanan continental block. This case study thus documents the role of melt–rock reactions as major Pyroxenite-forming processes in the sub-arc mantle, providing further constraints on their sources and timing in the Cabo Ortegal Complex.
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lithological and age structure of the lower crust beneath the northern edge of the north china craton xenolith evidence
Lithos, 2015Co-Authors: Ying Wei, J P Zheng, W L GriffinAbstract:Abstract Deep-seated xenoliths in volcanic rocks offer direct glimpses into the nature and evolution of the lower continental crust. In this contribution, new data on the U–Pb ages and Hf isotopes of zircons in six felsic granulite xenoliths and one Pyroxenite xenolith from the Hannuoba Cenozoic basalts, combined with published data from mafic to felsic xenoliths, are used to constrain the lithological and age structure of the lower crust beneath the northern edge of the North China Craton. Two newly-reported felsic granulites contain Precambrian zircons with positive (+ 7.5–+ 10.6) and negative e Hf values (− 10.1 to − 3.7) corresponding to upper intercept ages of 2449 ± 62 Ma and 1880 ± 54 Ma, respectively, indicating crustal accretion in the late Archean and reworking in Paleoproterozoic time. Zircons in another four felsic xenoliths give Phanerozoic ages from 142 Ma to 73 Ma and zircons from one Pyroxenite xenolith give a concordant age of 158 Ma. The zircon e Hf values of these four felsic xenoliths range between − 23.3 and − 19.1, reflecting re-melting of the pre-existing lower crust. Integration of geothermobarometric, and geochronological data on the Hannuoba xenoliths with seismic refraction studies shows that the lower crust beneath the northern edge of the North China Craton is temporally and compositionally zoned: the upper lower crust (24–33 km) consists dominantly of Archean (~ 2.5 Ga with minor 2.7 Ga) felsic granulites with subordinate felsic granulites that reworked at 140–120 Ma; both Precambrian and late Mesozoic mafic granulites are important constituents of the middle lower crust (33–38 km); major late Mesozoic (140–120 Ma) and less Cenozoic (45–47 Ma) granulites and Pyroxenites are presented in the lowermost crust (38–42 km). The zoned architecture of the lower crust beneath Hannuoba suggests a complex evolution beneath the northern margin of the craton, including late Neoarchean (~ 2.5 Ga) accretion and subsequent episodic accretion and/or reworking during Paleoproterozoic (~ 1.8–1.9 Ga) and Phanerozoic (~ 220 Ma, 120–140 Ma, 45–47 Ma) times, with possible links to circum-craton subduction/collision events and the destruction of the North China Craton.
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magnetic mineralogy of Pyroxenite xenoliths from hannuoba basalts northern north china craton implications for magnetism in the continental lower crust
Journal of Geophysical Research, 2014Co-Authors: Jianping Zheng, Qingli Zeng, Qingsheng Liu, W L GriffinAbstract:Studies of the petrology, mineral chemistry, and rock magnetic properties of nine Pyroxenite xenoliths from Hannuoba basalts, northern North China Craton, have been made to determine the magnetization signature of the continental lower crust. These Pyroxenites are weakly magnetic with low average susceptibility (χ) and saturation isothermal remanent magnetization (Mrs) of 39.59 × 10−8 m3 kg−1 and 12.05 × 10−3 Am2 kg−1, respectively. The magnetic minerals are mainly magnetite, pyrrhotite, and Fe-rich spinel, which significantly contribute to χ and natural remanent magnetization. Magnetite occurs as interstitial microcrystals together with zeolite aggregates, indicating a secondary origin in a supergene environment. In contrast, pyrrhotite and Fe-rich spinel were formed prior to the xenoliths' ascent to the surface, as evidenced by their dominant occurrence as tiny inclusions and thin exsolution lamellae in pyroxene. The Fe-rich spinel has ~ 50% mole fraction of Fe3O4 and corresponds to the strongest magnetization, and its coexistence with Mg-rich spinel implies a reheating event due to the underplating of basaltic magma. Besides, armalcolite and ilmenite were found in the reaction rims between xenoliths and the basalt, but they contribute little to the whole rock magnetization. However, these Pyroxenite xenoliths would be nonmagnetic at in situ depths, as well as peridotite and mafic granulite xenoliths derived from the crust-mantle transition zone (~ 32–42 km). Therefore, we suggest the limiting depth of magnetization at the boundary between weakly magnetic deep-seated (lower crust and upper mantle) xenoliths and strongly magnetic Archean granulite facies rocks (~ 32 km) in Hannuoba, northern North China Craton.
Hongfu Zhang - One of the best experts on this subject based on the ideXlab platform.
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combined iron and magnesium isotope geochemistry of Pyroxenite xenoliths from hannuoba north china craton implications for mantle metasomatism
Contributions to Mineralogy and Petrology, 2017Co-Authors: Xinmiao Zhao, Hui Hui Cao, Noreen J Evans, Fang Huang, Hongfu ZhangAbstract:We present high-precision iron and magnesium isotopic data for diverse mantle Pyroxenite xenoliths collected from Hannuoba, North China Craton and provide the first combined iron and magnesium isotopic study of such rocks. Compositionally, these xenoliths range from Cr-diopside Pyroxenites and Al-augite Pyroxenites to garnet-bearing Pyroxenites and are taken as physical evidence for different episodes of melt injection. Our results show that both Cr-diopside Pyroxenites and Al-augite Pyroxenites of cumulate origin display narrow ranges in iron and magnesium isotopic compositions (δ57Fe = −0.01 to 0.09 with an average of 0.03 ± 0.08 (2SD, n = 6); δ26Mg = − 0.28 to −0.25 with an average of −0.26 ± 0.03 (2SD, n = 3), respectively). These values are identical to those in the normal upper mantle and show equilibrium inter-mineral iron and magnesium isotope fractionation between coexisting mantle minerals. In contrast, the garnet-bearing Pyroxenites, which are products of reactions between peridotites and silicate melts from an ancient subducted oceanic slab, exhibit larger iron isotopic variations, with δ57Fe ranging from 0.12 to 0.30. The δ57Fe values of minerals in these garnet-bearing Pyroxenites also vary widely (−0.25 to 0.08 in olivines, −0.04 to 0.25 in orthopyroxenes, −0.07 to 0.31 in clinopyroxenes, 0.07 to 0.48 in spinels and 0.31–0.42 in garnets). In addition, the garnet-bearing Pyroxenite shows light δ26Mg (−0.43) relative to the mantle. The δ26Mg of minerals in the garnet-bearing Pyroxenite range from −0.35 for olivine and orthopyroxene, to −0.34 for clinopyroxene, 0.04 for spinel and −0.68 for garnet. These measured values stand in marked contrast to calculated equilibrium iron and magnesium isotope fractionation between coexisting mantle minerals at mantle temperatures derived from theory, indicating disequilibrium isotope fractionation. Notably, one phlogopite clinoPyroxenite with an apparent later metasomatic overprint has the heaviest δ57Fe (as high as 1.00) but the lightest δ26Mg (as low as −1.50) values of all investigated samples. Overall, there appears to be a negative co-variation between δ57Fe and δ26Mg in the Hannuoba garnet-bearing Pyroxenite and in the phlogopite clinoPyroxenite xenoliths and minerals therein. These features may reflect kinetic isotopic fractionation due to iron and magnesium inter-diffusion during melt–rock interaction. Such processes play an important role in producing inter-mineral iron and magnesium isotopic disequilibrium and local iron and magnesium isotopic heterogeneity in the subcontinental mantle.
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diverse crustal components in Pyroxenite xenoliths from junan sulu orogenic belt implications for lithospheric modification invoked by continental subduction
Chemical Geology, 2013Co-Authors: Jifeng Ying, Hongfu Zhang, Yanjie Tang, Xinhua ZhouAbstract:Abstract Detailed mineralogical and geochemical studies have been carried out on a suite of Pyroxenite xenoliths captured in a late Mesozoic basaltic dike from Junan, eastern Shandong, which is tectonically situated in the Sulu ultrahigh pressure orogenic belt. Two types of Pyroxenites, namely websterite and garnet Pyroxenite were identified according to their mineral assemblages. The equilibrium temperatures (828–935 °C) of websterite and garnet Pyroxenite xenoliths, falling within the temperature range of lower crustal xenoliths, suggest that they were derived from the lower crust rather than the lithospheric mantle. The websterite xenoliths are characterized by higher MgO and lower Al2O3; their convex upward REE patterns, along with lower concentrations of highly incompatible elements indicate that they were high pressure cumulates. Their extremely unradiogenic Nd and radiogenic Sr isotopic compositions imply the contribution of crustal materials to their sources. It is suggested that the precursor melts of websterite xenoliths were derived from a mantle source which had been intensely modified by the subducted lower continental crust of the Yangtze craton following the collision with the North China craton in the Triassic. The compositional features of garnet Pyroxenites also suggest their cumulative origin, however, the sharp contrast in trace element concentrations and Sr–Nd isotopic compositions with those of websterite xenoliths indicate they have different precursor melts. Their Sr and Nd isotopic compositions are consistent with the suggestion that their precursor melts were derived from an enriched lithospheric mantle which had been metasomatized by melts released from the Proto-Tethyan oceanic crust which was subducted into the mantle prior to the subduction of continental crust of Yangtze craton. The occurrence of websterite and garnet Pyroxenite xenoliths provide evidence that the lithospheric mantle of the North China craton had been significantly modified by the recycled oceanic and continental crust resulting from the collision between the Yangtze craton and the North China craton.
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recycled crustal melt injection into lithospheric mantle implication from cumulative composite and Pyroxenite xenoliths
International Journal of Earth Sciences, 2010Co-Authors: Hongfu Zhang, Jifeng Ying, Eizo Nakamura, Katsura Kobayashi, Yanjie TangAbstract:A rare composite xenolith and abundant cumulative Pyroxenites obtained from the Mesozoic Fangcheng basalts on the eastern North China Craton record a complex history of melt percolation and circulation in the subcontinental lithospheric mantle. The composite xenolith has a dunite core and an olivine clinoPyroxenite rim. The dunite is of cumulative origin and has a granular recrystallized texture and extremely low Mg# [100 Mg/(Mg + Fe) = 81–82] contents in olivines. The olivine clinoPyroxenite contains larger clinopyroxene and/or orthopyroxene with a few fine-grained olivine and tiny phlogopite, feldspar, and/or carbonate minerals interstitial to clinopyroxene. The clinopyroxene has low Mg# (83–85). Compositional similarity between dunitic olivine and pyroxenitic one indicates a sequential crystallization of dunite and Pyroxenite from a precursor melt. Pyroxenite xenoliths include olivine websterites and clinoPyroxenites, both are of cumulative origin. Estimation of the melt from major oxides in olivines and REE concentrations in clinopyroxenes in these composite and Pyroxenite xenoliths suggests a derivation from subducted crustal materials, consistent with the highly enriched EMII-like Sr and Nd isotopic ratios observed in the Pyroxenites. Occurrence of phlogopite, feldspar and carbonate minerals in some xenoliths requires the melt rich in alkalis (K, Na), silica and volatiles (water and CO2) at the latest stage as well, similar to highly silicic and potassic melts. Thus, the occurrence of these composite and Pyroxenite xenoliths provides an evidence for voluminous injection of recycled crustal melts into the lithosphere beneath the southeastern North China Craton at the Late Mesozoic, a reason for the rapid lithospheric enrichment in both elemental and isotopic compositions.
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high pressure partial melting of mafic lithologies in the mantle
Journal of Petrology, 2004Co-Authors: Tetsu Kogiso, Marc M Hirschmann, M PertermannAbstract:We review experimental phase equilibria associated with partial melting of mafic lithologies ( Pyroxenites) at high pressures to reveal systematic relationships between bulk compositions of Pyroxenite and their melting relations. An important aspect of Pyroxenite phase equilibria is the existence of the garnet–pyroxene thermal divide, defined by the enstatite–Ca-Tschermaks pyroxene–diopside plane in CaO–MgO–Al2O3–SiO2 projections. This divide appears at pressures above 2 GPa in the natural system where garnet and pyroxenes are the principal residual phases in Pyroxenites. Bulk compositions that reside on either side of the divide have distinct phase assemblages from subsolidus to liquidus and produce distinct types of partial melt ranging from strongly nepheline-normative to quartz-normative compositions. Solidus and liquidus locations are little affected by the location of natural Pyroxenite compositions relative to the thermal divide and are instead controlled chiefly by bulk alkali contents and Mg-numbers. Changes in phase volumes of residual minerals also influence partial melt compositions. If olivine is absent during partial melting, expansion of the phase volume of garnet relative to clinopyroxene with increasing pressure produces liquids with high Ca/Al and low MgO compared with garnet peridotite-derived partial melts.
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high pressure partial melting of garnet Pyroxenite possible mafic lithologies in the source of ocean island basalts
Earth and Planetary Science Letters, 2003Co-Authors: Tetsu Kogiso, Marc M Hirschmann, Daniel J FrostAbstract:Abstract Many ocean island basalts (OIB) that have isotopic ratios indicative of recycled crustal components in their source are silica-undersaturated and unlike silicic liquids produced from partial melting of recycled mid-ocean ridge basalt (MORB). However, experiments on a silica-deficient garnet Pyroxenite, MIX1G, at 2.0–2.5 GPa show that some Pyroxenite partial melts are strongly silica-undersaturated [M.M. Hirschmann et al., Geology 31 (2003) 481–484]. These low-pressure liquids are plausible parents of alkalic OIB, except that they are too aluminous. We present new partial melting experiments on MIX1G between 3.0 and 7.5 GPa. Partial melts at 5.0 GPa have low SiO 2 ( 2 O 3 ( 12 wt%) at moderate MgO (12–16 wt%), and are more similar to primitive OIB compositions than lower-pressure liquids of MIX1G or experimental partial melts of anhydrous or carbonated peridotite. Solidus temperatures at 5.0 and 7.5 GPa are 1625 and 1825°C, respectively, which are less than 50°C cooler than the anhydrous peridotite solidus. The liquidus temperature at 5.0 GPa is 1725°C, indicating a narrow melting interval (∼100°C). These melting relations suggest that OIB magmas can be produced by partial melting of a silica-deficient Pyroxenite similar to MIX1G if its melting residue contains significant garnet and lacks olivine. Such silica-deficient Pyroxenites could be produced by interaction between recycled subducted oceanic crust and mantle peridotite or could be remnants of ancient oceanic lower crust or delaminated lower continental crust. If such compositions are present in plumes ascending with potential temperatures of 1550°C, they will begin to melt at about 5.0 GPa and produce appropriate partial melts. However, such hot plumes may also generate partial melts of peridotite, which could dilute the Pyroxenite-derived partial melts.
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alkalic magmas generated by partial melting of garnet Pyroxenite
Geology, 2003Co-Authors: Marc M Hirschmann, Tetsu Kogiso, Michael B Baker, Edward M StolperAbstract:Many oceanic-island basalts (OIBs) with isotopic signatures of recycled crustal components are silica poor and strongly nepheline (ne) normative and therefore unlike the silicic liquids generated from partial melting of recycled mid-oceanic-ridge basalt (MORB). High-pressure partial-melting experiments on a garnet Pyroxenite (MIX1G) at 2.0 and 2.5 GPa produce strongly ne-normative and silica-poor partial melts. The MIX1G solidus is located below 1350 and 1400 8C at 2 and 2.5 GPa, respectively, slightly cooler than the solidus of dry peridotite. Chemographic analysis suggests that natural garnet Pyroxenite compositions straddle a thermal divide. Whereas partial melts of compositions on the silica-excess side of the divide (such as recycled MORB) are silica saturated, those from silica-deficient garnet Pyroxenites can be alkalic and have similar- ities to low-silica OIB. Although the experimental partial melts are too rich in Al2O3 to be parental to highly undersaturated OIB suites, higher-pressure (4-5 GPa) partial melting of garnet pyrox- enite is expected to yield more appropriate parental liquids for OIB lavas. Silica-deficient garnet Pyroxenite, which may originate by mixing of MORB with peridotite, or by recycling of other mafic lithologies, represents a plausible source of OIB that may resolve the apparent contradiction of strongly alkalic composition with iso- topic ratios characteristic of a recycled component.
