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W L Griffin - One of the best experts on this subject based on the ideXlab platform.
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titanates of the lindsleyite mathiasite lima group reveal isotope disequilibrium associated with metasomatism in the mantle beneath kimberley south africa
Earth and Planetary Science Letters, 2018Co-Authors: Roland Maas, Andrea Giuliani, Jon D Woodhead, David Phillips, G R Davies, W L GriffinAbstract:Abstract Radiogenic isotope variations unrelated to radiogenic ingrowth are common between minerals found in metasomatised mantle Xenoliths entrained in kimberlite, basalts and related magmas. As the metasomatic minerals are assumed to have been in isotopic equilibrium originally, such variations are typically attributed to contamination by the magma host and/or interaction with mantle fluids during or before Xenolith transport to surface. However, the increasing evidence of metasomatism by multiple, compositionally distinct fluids permeating the lithospheric mantle, coeval with specific magmatic events, suggests that isotopic disequilibrium might be a consequence of discrete, though complex, metasomatic events. Here we provide clear evidence of elemental and Sr isotope heterogeneity between coeval Ti-rich LIMA (lindsleyite–mathiasite) minerals at the time of their formation in the mantle. LIMA minerals occur in close textural association with clinopyroxene and phlogopite in low-temperature (∼800–900 °C), strongly metasomatised mantle Xenoliths from the ∼84 Ma Bultfontein kimberlite (South Africa). Previous U/Pb dating of the LIMA phases was used to argue that each Xenolith recorded a single event of LIMA crystallisation at ∼180–190 Ma, coeval with the emplacement of Karoo magmas. SEM imaging reveals that up to four types of LIMA phases coexist in each Xenolith, and occasionally in a single LIMA grain. Major element and in situ Sr isotope analyses of the different LIMA types show that each phase has a distinct elemental composition and initial 87Sr/86Sr ratio (e.g., 0.7068–0.7086 and 0.7115–0.7129 for two LIMA types in a single Xenolith; 0.7053-0.7131 across the entire sample suite). These combined age and isotopic constraints require that multiple fluids metasomatised these rocks at broadly the same time (i.e. within a few thousands to millions of years), and produced similar mineralogical features. Elemental and isotopic variations between different LIMA types could be due to interaction between one (or more) Karoo-related Ti-rich silicate melts and previously metasomatised, phlogopite-rich lithospheric mantle. This study demonstrates that mantle metasomatic assemblages seemingly generated in a single event may instead result from the infiltration of broadly coeval fluids with variable compositions. This in turn implies that the isotopic variations recorded in mantle rocks may be an inherent feature of metasomatism, and that hot fluids infiltrating a rock do not necessarily cause equilibration at the cm scale, as has been assumed previously. Simple modelling of solid-state diffusion in mantle minerals shows that isotopic disequilibrium may be preserved for up to hundreds of Myr at mantle lithosphere temperatures (≤1100–1200 °C), unless subsequently affected by transient heating and/or fluid infiltration events. Radiogenic isotope disequilibrium associated with mantle metasomatism may therefore be a common feature of mantle Xenoliths.
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multiple metasomatism beneath the nograd gomor volcanic field northern pannonian basin revealed by upper mantle peridotite Xenoliths
Journal of Petrology, 2017Co-Authors: Suzanne Y Oreilly, Levente Patkó, Nóra Liptai, István János Kovács, Károly Hidas, Zsanett Pinter, Teresa Jeffries, Zoltan Zajacz, W L GriffinAbstract:Peridotite Xenoliths from the Nograd-Gomor Volcanic Field (NGVF) record the geochemical and structural evolution of the subcontinental lithospheric mantle beneath the northern margin of the Pannonian Basin. The Xenoliths are divided into a lherzolitic and a wehrlitic series based on their modal composition and textural features. This paper focuses on the lherzolite series and presents petrography and major- and trace-element geochemistry for Xenoliths 51 selected from all Xenolith-bearing localities of the NGVF. The Xenoliths consist of olivine, orthopyroxene, clinopyroxene, spinel ± amphibole. Protogranular textures are most frequently found in the northern part, while to porphyroclastic and equigranular textures are more dominant in the central and southern parts. The Xenoliths have undergone varying degrees (<5-30 %) of partial melting with overprinting by different metasomatic processes. Based on heir Mg#, the spinel lherzolite Xenoliths can be subdivided into two major groups. Group I has olivine Mg# between 89 and 92, whereas Group II has Mg# <89, and significant enrichment of Fe and Mn in olivine and pyroxenes, and of Ti in spinel. Trace-element contents of the Xenoliths vary widely, allowing a further division based on LREE enrichment or depletion in pyroxenes. REE patterns of amphiboles match those of clinopyroxenes in each Xenolith where they appear, and depict different origins based on their Nb (and other HFSE) content. It is inferred that Nb-poor amphiboles record the oldest metasomatic event, caused by volatile-bearing silicate melts, followed by at least two further metasomatic processes: one that resulted in U-Th-(Nb-Ta)- LREE-enrichment and crystallization of Nb-rich amphibole, affecting selective domains under the entire NGVF, and another evidenced by Fe-Mn-Ti-enrichment which overprinted previous geochemical signatures. We suggest that the metasomatic agents in both cases were basaltic silicate melts, compositionally similar to the host basalts. These melts were generated during the Miocene extension of the Pannonian Basin. The effects of heating and subsequent cooling are evident in significantly different equilibration temperatures.
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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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late mesozoic eocene mantle replacement beneath the eastern north china craton evidence from the paleozoic and cenozoic peridotite Xenoliths
International Geology Review, 2005Co-Authors: Jianping Zheng, Ru Y Zhang, Suzanne Y Oreilly, J G Liou, W L Griffin, Fengxiang LuAbstract:Xenolith-bearing Paleozoic kimberlites and Cenozoic basalts from the eastern part of the North China craton provide unusual insights into intraplate processes and Phanerozoic lithospheric evolution. Paleozoic peridotite Xenoliths represent samples of ancient cratonic mantle; P-T estimates show that a thick (˜230 km), cold (ca 40 mW/m2) lithosphere existed beneath the craton during mid- Ordovician time. However, Xenoliths from Tertiary basalts sample a thin (> 90 km), hot (mean geotherm ca 80 mW/m2), compositionally heterogeneous lithosphere beneath the same area in Cenozoic time. Fertile, spinel-facies mantle makes up much of the Cenozoic lithosphere beneath the eastern North China craton, especially in regions along the translithospheric Tanlu fault. However, refractory spinel-facies Xenoliths are found locally along the north-south gravity lineament in areas far away from the Tanlu fault. These refractory Xenoliths are interpreted as derived from shallow relics of the cratonic mantle embedded in more fe...
Nóra Liptai - One of the best experts on this subject based on the ideXlab platform.
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Metasomatism-induced wehrlite formation in the upper mantle beneath the Nógrád-Gömör Volcanic Field (Northern Pannonian Basin): Evidence from Xenoliths
Geoscience Frontiers, 2020Co-Authors: Levente Patkó, Nóra Liptai, László Előd Aradi, Rita Klébesz, Eszter Sendula, Robert J. Bodnar, István János Kovács, Károly Hidas, Bernardo Cesare, Attila NovákAbstract:Abstract Clinopyroxene-enriched upper mantle Xenoliths classified as wehrlites are common (~20% of all Xenoliths) in the central part of the Nograd-Gomor Volcanic Field (NGVF), situated in the northern margin of the Pannonian Basin in northern Hungary and southern Slovakia. In this study, we thoroughly investigated 12 wehrlite Xenoliths, two from each wehrlite-bearing occurrence, to determine the conditions of their formation. Specific textural features, including clinopyroxene-rich patches in an olivine-rich lithology, orthopyroxene remnants in the cores of newly-formed clinopyroxenes and vermicular spinel forms all suggest that wehrlites were formed as a result of intensive interaction between a metasomatic agent and the peridotite wall rock. Based on the major and trace element geochemistry of the rock-forming minerals, significant enrichment in basaltic (Fe, Mn, Ti) and high field strength elements (Nb, Ta, Hf, Zr) was observed, compared to compositions of common lherzolite Xenoliths. The presence of orthopyroxene remnants and geochemical trends in rock-forming minerals suggest that the metasomatic process ceased before complete wehrlitization was achieved. The composition of the metasomatic agent is interpreted to be a mafic silicate melt, which was further confirmed by numerical modelling of trace elements using the plate model. The model results also show that the melt/rock ratio played a key role in the degree of petrographic and geochemical transformation. The lack of equilibrium and the conclusions drawn by using variable lherzolitic precursors in the model both suggest that wehrlitization was the last event that occurred shortly before Xenolith entrainment in the host mafic melt. We suggest that the wehrlitization and the Plio–Pleistocene basaltic volcanism are related to the same magmatic event.
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multiple metasomatism beneath the nograd gomor volcanic field northern pannonian basin revealed by upper mantle peridotite Xenoliths
Journal of Petrology, 2017Co-Authors: Suzanne Y Oreilly, Levente Patkó, Nóra Liptai, István János Kovács, Károly Hidas, Zsanett Pinter, Teresa Jeffries, Zoltan Zajacz, W L GriffinAbstract:Peridotite Xenoliths from the Nograd-Gomor Volcanic Field (NGVF) record the geochemical and structural evolution of the subcontinental lithospheric mantle beneath the northern margin of the Pannonian Basin. The Xenoliths are divided into a lherzolitic and a wehrlitic series based on their modal composition and textural features. This paper focuses on the lherzolite series and presents petrography and major- and trace-element geochemistry for Xenoliths 51 selected from all Xenolith-bearing localities of the NGVF. The Xenoliths consist of olivine, orthopyroxene, clinopyroxene, spinel ± amphibole. Protogranular textures are most frequently found in the northern part, while to porphyroclastic and equigranular textures are more dominant in the central and southern parts. The Xenoliths have undergone varying degrees (<5-30 %) of partial melting with overprinting by different metasomatic processes. Based on heir Mg#, the spinel lherzolite Xenoliths can be subdivided into two major groups. Group I has olivine Mg# between 89 and 92, whereas Group II has Mg# <89, and significant enrichment of Fe and Mn in olivine and pyroxenes, and of Ti in spinel. Trace-element contents of the Xenoliths vary widely, allowing a further division based on LREE enrichment or depletion in pyroxenes. REE patterns of amphiboles match those of clinopyroxenes in each Xenolith where they appear, and depict different origins based on their Nb (and other HFSE) content. It is inferred that Nb-poor amphiboles record the oldest metasomatic event, caused by volatile-bearing silicate melts, followed by at least two further metasomatic processes: one that resulted in U-Th-(Nb-Ta)- LREE-enrichment and crystallization of Nb-rich amphibole, affecting selective domains under the entire NGVF, and another evidenced by Fe-Mn-Ti-enrichment which overprinted previous geochemical signatures. We suggest that the metasomatic agents in both cases were basaltic silicate melts, compositionally similar to the host basalts. These melts were generated during the Miocene extension of the Pannonian Basin. The effects of heating and subsequent cooling are evident in significantly different equilibration temperatures.
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the upper mantle beneath the nograd gomor volcanic field northern hungary southern slovakia an integratedstudy on spinel lherzolite Xenoliths
2015Co-Authors: Nóra Liptai, Levente Patkó, Károly Hidas, Zsanett Pinter, Teresa Jeffries, Zoltan Zajacz, Istvan Kovacs, Andrea Tommasi, Gyorgy Falus, Csaba SzaboAbstract:Peridotite Xenoliths of the Nograd-Gomor Volcanic Field (NGVF) record the geochemical and structural evolution of the lithospheric mantle beneath the northern margin of the Pannonian basin. We analyzed petrography, major and trace element geochemistry of mineral constituents, ‘water’ content of nominally anhydrous minerals (NAMs), and olivine crystal preferred orientation (CPO) in 17 selected spinel peridotite Xenoliths from the central and southern part of the NGVF. Xenolith textures from the central and southern localities differ in abundance: porphyroclastic texture predominates in the central part, whereas the southern part exhibits a wider variety, having a significant ratio of equigranular textures. Geochemical analyses indicate similar major element contents of minerals with a variability of Fe- and LREE-enrichment in Xenoliths from the central localities, indicating that cryptic metasomatism has affected the Xenoliths to different degrees. This is also supported by ‘water’ content of pyroxenes, which is generally lower in the southern part than in the central part. Strength of CPO also varies: porphyroclastic Xenoliths from the southern part generally display weak orthorhombic olivine CPOs, whereas equigranular textures are associated with axial–[010] olivine CPOs, respectively. On the other hand, all but one Xenoliths from the central part show strong orthorhombic patterns, independently from geochemical features. Based on these results, we presume that the NGVF Xenoliths sampled two geochemically and rheologically different domains of the upper mantle and have recorded imprints of major processes such as the Miocene extension and the accompanying melt production, which occurred during the Neogene evolution of the Pannonian Basin.
István János Kovács - One of the best experts on this subject based on the ideXlab platform.
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Metasomatism-induced wehrlite formation in the upper mantle beneath the Nógrád-Gömör Volcanic Field (Northern Pannonian Basin): Evidence from Xenoliths
Geoscience Frontiers, 2020Co-Authors: Levente Patkó, Nóra Liptai, László Előd Aradi, Rita Klébesz, Eszter Sendula, Robert J. Bodnar, István János Kovács, Károly Hidas, Bernardo Cesare, Attila NovákAbstract:Abstract Clinopyroxene-enriched upper mantle Xenoliths classified as wehrlites are common (~20% of all Xenoliths) in the central part of the Nograd-Gomor Volcanic Field (NGVF), situated in the northern margin of the Pannonian Basin in northern Hungary and southern Slovakia. In this study, we thoroughly investigated 12 wehrlite Xenoliths, two from each wehrlite-bearing occurrence, to determine the conditions of their formation. Specific textural features, including clinopyroxene-rich patches in an olivine-rich lithology, orthopyroxene remnants in the cores of newly-formed clinopyroxenes and vermicular spinel forms all suggest that wehrlites were formed as a result of intensive interaction between a metasomatic agent and the peridotite wall rock. Based on the major and trace element geochemistry of the rock-forming minerals, significant enrichment in basaltic (Fe, Mn, Ti) and high field strength elements (Nb, Ta, Hf, Zr) was observed, compared to compositions of common lherzolite Xenoliths. The presence of orthopyroxene remnants and geochemical trends in rock-forming minerals suggest that the metasomatic process ceased before complete wehrlitization was achieved. The composition of the metasomatic agent is interpreted to be a mafic silicate melt, which was further confirmed by numerical modelling of trace elements using the plate model. The model results also show that the melt/rock ratio played a key role in the degree of petrographic and geochemical transformation. The lack of equilibrium and the conclusions drawn by using variable lherzolitic precursors in the model both suggest that wehrlitization was the last event that occurred shortly before Xenolith entrainment in the host mafic melt. We suggest that the wehrlitization and the Plio–Pleistocene basaltic volcanism are related to the same magmatic event.
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multiple metasomatism beneath the nograd gomor volcanic field northern pannonian basin revealed by upper mantle peridotite Xenoliths
Journal of Petrology, 2017Co-Authors: Suzanne Y Oreilly, Levente Patkó, Nóra Liptai, István János Kovács, Károly Hidas, Zsanett Pinter, Teresa Jeffries, Zoltan Zajacz, W L GriffinAbstract:Peridotite Xenoliths from the Nograd-Gomor Volcanic Field (NGVF) record the geochemical and structural evolution of the subcontinental lithospheric mantle beneath the northern margin of the Pannonian Basin. The Xenoliths are divided into a lherzolitic and a wehrlitic series based on their modal composition and textural features. This paper focuses on the lherzolite series and presents petrography and major- and trace-element geochemistry for Xenoliths 51 selected from all Xenolith-bearing localities of the NGVF. The Xenoliths consist of olivine, orthopyroxene, clinopyroxene, spinel ± amphibole. Protogranular textures are most frequently found in the northern part, while to porphyroclastic and equigranular textures are more dominant in the central and southern parts. The Xenoliths have undergone varying degrees (<5-30 %) of partial melting with overprinting by different metasomatic processes. Based on heir Mg#, the spinel lherzolite Xenoliths can be subdivided into two major groups. Group I has olivine Mg# between 89 and 92, whereas Group II has Mg# <89, and significant enrichment of Fe and Mn in olivine and pyroxenes, and of Ti in spinel. Trace-element contents of the Xenoliths vary widely, allowing a further division based on LREE enrichment or depletion in pyroxenes. REE patterns of amphiboles match those of clinopyroxenes in each Xenolith where they appear, and depict different origins based on their Nb (and other HFSE) content. It is inferred that Nb-poor amphiboles record the oldest metasomatic event, caused by volatile-bearing silicate melts, followed by at least two further metasomatic processes: one that resulted in U-Th-(Nb-Ta)- LREE-enrichment and crystallization of Nb-rich amphibole, affecting selective domains under the entire NGVF, and another evidenced by Fe-Mn-Ti-enrichment which overprinted previous geochemical signatures. We suggest that the metasomatic agents in both cases were basaltic silicate melts, compositionally similar to the host basalts. These melts were generated during the Miocene extension of the Pannonian Basin. The effects of heating and subsequent cooling are evident in significantly different equilibration temperatures.
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a quartz bearing orthopyroxene rich websterite Xenolith from the pannonian basin western hungary evidence for release of quartz saturated melts from a subducted slab
Journal of Petrology, 2008Co-Authors: Eniko Bali, István János Kovács, Kalman Torok, Zoltan Zajacz, Cs. Szabó, Werner E Halter, Orlando Vaselli, Robert J. BodnarAbstract:An unusual quartz-bearing orthopyroxene-rich websterite Xenolith has been found in an alkali basaltic tuff at Szigliget, Bakony-Balaton Highland Volcanic Field (BBHVF), western Hungary. Ortho- and clinopyroxenes are enriched in light rare earth elements (LREE), middle REE and Ni, and depleted in Nb, Ta, Sr and Ti compared with ortho- and clinopyroxenes occurring in either peridotite or lower crustal granulite Xenoliths from the BBHVF. Both ortho- and clinopyroxenes in the Xenolith contain primary and secondary silicate melt inclusions, and needle-shaped or rounded quartz inclusions. The melt inclusions are rich in SiO 2 and alkalis and poor in MgO, FeO and CaO. They are strongly enriched in LREE and large ion lithophile elements, and display negative Nb, Ta and Sr anomalies, and slightly positive Pb anomalies. The Xenolith is interpreted to represent a fragment of an orthopyroxene-rich body that crystallized in the upper mantle from a hybrid melt that formed by interaction of mantle peridotite with a quartz-saturated silicate melt that was released from a subducted oceanic slab. Although the exact composition of the slab melt cannot be determined, model calculations on major and trace elements suggest involvement of a metasedimentary component.
Carlos Villaseca - One of the best experts on this subject based on the ideXlab platform.
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depleted lherzolite Xenoliths from the leucititic morron de villamayor volcano calatrava volcanic field spain
Lithos, 2021Co-Authors: Javier Serrano, Carlos Villaseca, Cecilia PerezsobaAbstract:Abstract This work reports petrographical, mineralogical and geochemical data of a suite of mantle Xenoliths from the Morron de Villamayor volcanic centre (MVM), which belongs to the Calatrava Volcanic Field (CVF), a part of the Circum-Mediterranean Anorogenic Cenozoic Igneous Province. The MVM volcano is the only centre belonging to the first stage of the CVF magmatism, an ultrapotassic stage that provides a sampling of the subcontinental lithospheric mantle beneath central Spain. The MVM peridotites record a variable host melt interaction (spongy texture and reaction zones), more intense toward Xenolith-host leucitite contacts that it was formed during transport within the volcanic melt. The studied mantle Xenoliths are spinel lherzolites equilibrated at lower temperatures (618–942 °C) and similar pressure ranges (8.8–13.6 kbar) than other studied CVF mantle Xenoliths, and record a lithospheric mantle before the main volcanic Calatrava event. The MVM lherzolites could be classified in two groups by their chondrite-normalized rare earth element (REE) clinopyroxene patterns: group-1 lherzolites show flat pattern, and group-2 lherzolites display a LREE-depleted (N-MORB like) pattern. Group-1 lherzolites represent a more residual lithospheric mantle (up to 9 partial melting %) than group-2 lherzolites (~ 5%), the latter with similar partial melting degrees than other mantle Xenoliths from the CVF. Moreover, group-1 lherzolites have undergone a post partial melting refertilization by different metasomatic agents: silicate and alkaline silicate melts different in composition to host leucitite. A primary clinopyroxene concentrate from these MVM lherzolites provides high Sr (87Sr/86Sr = 0.706104) and intermediate Nd (143Nd/144Nd = 0.512830) initial radiogenic ratios, not previously recorded in this volcanic field. This sampling proves the presence of an enriched isotope mantle pole in Xenolith suites of the Calatrava area, trending in composition to an intermediate DMM-EMII component.
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nature and composition of the lower continental crust in central spain and the granulite granite linkage inferences from granulitic Xenoliths
Journal of Petrology, 1999Co-Authors: Carlos Villaseca, Hilary Downes, L BarberoAbstract:Xenolith-bearing alkaline ultrabasic dykes were intruded into the of the Hercynian basement of the Spanish Central System in early Mesozoic times. The suite of lower-crustal Xenoliths in the dykes divided into three groups: felsic peraluminous granulites, metapelitic granulites and charnockitic granulites. The felsic granulites form ~95% of the total volume of the Xenoliths, whereas the charnockitic and metapelitic granulites are much less abundant (~0·01 5%, respectively). Thermobarometric calculations based on mineral paragenesis indicate equilibration conditions around 850–950°C, 7–11 kbar; thus the Xenoliths represent lower continental crustal material. Superimposed on this high-T high-P assemblage is a high-T low-P paragenesis represented mainly by kelyphitic coronas, reflecting re-equilibration during transport in the clearly restitic mineral assemblages, with up to 50% garnet and 37% sillimanite. Major and trace element modelling supports the idea that the late-Hercynian peraluminous granites of central Spain represent liquids in equilibrium with restitic material of similar composition to the studied lower-crustal Xenoliths. 87Sr/86Sr and eNd of the felsic Xenoliths, calculated at an average Hercynian age of 300 Ma, are in the range 0·706–0·712, and –1·4 to –8·2, respectively. These values match the isotopic composition of the outcropping late Hercynian granites. The Sr isotopic composition of the Xenoliths is lower than that of the outcropping mid-crustal lithologies (orthogneisses, pelites). A major contribution from the lower crust to the source of Hercynian granites greatly reduces the necessity of invoking a large mantle contribution in models of granite petrogenesis. The felsic nature of the lower continental crust in central Spain contrasts with the more mafic lower-crustal composition estimated in other European Hercynian areas, suggesting a non underplated crust in this region of the Hercynian orogenic belt.
Sobhi Nasir - One of the best experts on this subject based on the ideXlab platform.
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lithospheric petrology of the eastern arabian plate constraints from al ashkhara oman Xenoliths
Lithos, 2012Co-Authors: Sobhi Nasir, Robert J SternAbstract:article i nfo Mafic granulite and spinel lherzolite Xenoliths from Cenozoic alkaline basalts near Al-Ashkhara, eastern Oman, have been selected for a systematic mineralogical, geochemical and Sr-Nd-Pb isotopic study. This is the only place in E Arabia where samples of both lower crust and upper mantle can be examined. Lower crustal Xenoliths consist of two mineralogically and chemically distinct groups: gabbronorite (subequal abundances of ortho- and clino-pyroxene and plagioclase) and plagioclase pyroxenite (dominant pyroxene and subordinate plagioclase). Temperature estimates for lower crustal Xenoliths using the two pyroxene geothermometer (T-Wells) yield 810-865 °C. The mineral assemblage (spinel-pyroxene-plagioclase) and Al content in pyroxene indicate that plagioclase-bearing Xenoliths equilibrated at 5-8 kbar (13 and 30 km depth) in the lower crust. eNd and 87 Sr/ 86 Sr calculated at 700 Ma for Al-Ashkhara lower crustal Xenoliths (+6.4 to +6.6; 87 Sr/ 86 Sr=0.7028 to 0.7039) are consistent with the interpretation that juvenile, mafic melts were added to the lower crust during Neoproterozoic time and that there was no discernible contribution from pre-Neoproterozoic crust. Upper mantle Xenoliths consist of both dry and hydrous (phlog- opite-bearing) lherzolites. These peridotites are more Fe-rich than expected for primitive mantle or melt res- idues and probably formed by pervasive circulation of melts that have refertilized pre-existing mantle peridotites. Mineral equilibration temperatures range from 990 to 1070 °C. Isotopic compositions calculated at 700 Ma are eNd=+6.8 to +7.8 and 87 Sr/ 86 Sr=0.7016 to 0.7025, indicating depleted upper mantle. Pb isotopic compositions indicate that the metasomatism was relatively recent, perhaps related to Paleogene tectonics and basanite igneous activity. Nd model ages for the spinel peridotite Xenoliths range between 0.59 and 0.65 Ga. The Xenolith data suggest that eastern Arabian lower crust is of hotspot origin, in contrast to western Arabian lower crust, which mostly formed at a convergent plate margin. Geochemical and isotopic differences between lower crust and upper mantle indicate that these are unrelated, possibly because delam- ination replaced the E Arabian mantle root in Neoproterozoic time.
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composition of the lower crust of the arabian plate a Xenolith perspective
Lithos, 2004Co-Authors: Ali T Almishwat, Sobhi NasirAbstract:Abstract Petrological and geochemical data for a suite of mafic granulite Xenoliths in Cenozoic alkali basalts from Saudi Arabia, Jordan and Syria provide a unique opportunity to explore the composition and nature of the lower crust beneath the Arabian Plate. Two mineralogically and chemically distinct groups of Xenoliths occur. Group I is composed of two pyroxenes and plagioclase approximately in equal amounts. Group II is plagioclase rich and has variable proportions of orthopyroxene and clinopyroxene. The Xenolith mineral assemblages and geothermobarometry of coexisting minerals suggest that these Xenoliths represent basaltic cumulates that crystallized under high-pressure conditions in the lower crust. The Xenoliths possibly form a part of a lower crustal gabbroic intrusive complex that underlies the Arabian Plate and may represent mafic roots of an arc complex of Pan-African age beneath Arabia. The Xenolith data are compatible with available geophysical models on crust thickness and layering. The crust is between 20 and 40 km thick, and its lower part consists of mafic meta-igneous granulites. The chemical averages of Xenoliths from different parts of the Arabian Plate are more mafic than the estimated present-day average of model lower crust.
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lithospheric petrology beneath the northern part of the arabian plate in syria evidence from Xenoliths in alkali basalts
Journal of African Earth Sciences, 2000Co-Authors: Sobhi Nasir, Abdulrahman SafarjalaniAbstract:Abstract A petrological model for the upper mantle and lower crust under the northern part of the Arabian Plate (Syria) has been derived on the basis of petrology of upper mantle and lower crustal Xenoliths occurring in the Neogene to Quaternary alkali basalts of the Shamah volcanic fields. The Xenolith suite has been classified by texture mineralogy and chemistry into the following groups: (1) Type I metasomatised and dry Cr diopside Xenoliths with protogranular to porphyroclastic textures; (2) Type II Al augite spinal and garnet pyroxenite and websterite which have igneous and/or porphyroclastic textures and abundant phlogopite and/or amphibole; (3) Cr-poor megacrysts; and (4) mafic lower crustal Xenoliths. Estimates of Type I Xenolith temperatures are 990–1070°C with pressure between 13 and 19 kbar. Type II Xenoliths yield temperatures of 930–1150°C and pressures in the range 12—13 kbar. The lower crustal Xenolith mineral assemblages and geothermometry based on coexisting minerals suggest equilibration conditions between 6 and 8 kbar and 820–905°C. Mantle plumes, which may be the source of the volatile flux, have implications for melt generation in the Arabian basalt provinces. It is estimated that the lithosphere beneath the Arabian Plate is less than 80 km thick. Xenolith data and geophysical studies indicate that the Moho is located at a depth of 40–37 km and that the crust-mantle transition zone has a thickness of 8–5 km and occurs at a depth of 27–30 km. The boundary between an upper granitic crust and a lower mafic crust occurs at a depth of 19 km. Type I dry Xenoliths show a low overall concentration of REE (La/Yb =1–2 and Sm = 0.7–1.1 times chondrite), whereas Type I hydrous Xenoliths are LREE enriched (La/Yb=6–9 and Sm=1.1–1.3 times chondrite). Type II Xenoliths show high overall LREE enrichment. Petrological and geochemical data for the lower crustal Xenoliths indicate that these Xenoliths represent basaltic cumulates crystallised at lower crustal pressures.
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the lithosphere beneath the northwestern part of the arabian plate jordan evidence from Xenoliths and geophysics
Tectonophysics, 1992Co-Authors: Sobhi NasirAbstract:Abstract The Cenozoic alkali basaltic lavas of the northwestern part of the Arabian plate contain a wide variety of crustal and upper mantle Xenoliths. The compositional diversity of the Xenolith suite indicates a heterogeneous upper mantle beneath the northwestern part of the Arabian plate. It mainly consists of depleted and later metasomatized spinel Iherzolite injected with magmatic veins of pyroxenites. Thermobarometry of the lherzolites and pyroxenites yield temperatures of 900–1030°C at 12–18 kbar and 940–1020°C at 11–13 kbar respectively. The estimated δ log fO2 varies between −1.7 and −2.3. Calculated temperatures of the granulitic Xenoliths vary between 775° and 800°C at 5–11 kbar. The geotherm derived from the modelled Xenolith predicts a higher surface heat flow than the measured surface heat flow and suggest a widespread regional elevation of the geotherm below the Cenozoic basalt provinces in the Arabian plate. Correlation of Xenolith data and available geophysical studies suggests a cross section of the lithosphere below the northwestern part of the Arabian plate consisting of about 18–21 km thick granitic upper crust underlain by a mafic lower crust of gabbroic composition to a depth of about 37 km. The Moho appears to be a 5–8 km thick transition zone of spinel lherzolite intercalated with mafic rocks.
