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J W Harris - One of the best experts on this subject based on the ideXlab platform.
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Trace-element geochemistry of diamond-hosted olivine inclusions from the Akwatia Mine, West African Craton: implications for diamond Paragenesis and geothermobarometry
Contributions to Mineralogy and Petrology, 2019Co-Authors: J. C. M. De Hoog, T. Stachel, J W HarrisAbstract:Trace-element concentrations in olivine and coexisting garnets included in diamonds from the Akwatia Mine (Ghana, West African Craton) were measured to show that olivine can provide similar information about equilibration temperature, diamond Paragenesis and mantle processes as garnet. Trace-element systematics can be used to distinguish harzburgitic olivines from lherzolite ones: if Ca/Al ratios of olivine are below the mantle lherzolite trend (Ca/Al
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diamond precipitation and mantle metasomatism evidence from the trace element chemistry of silicate inclusions in diamonds from akwatia ghana
Contributions to Mineralogy and Petrology, 1997Co-Authors: Thomas Stachel, J W HarrisAbstract:Trace element concentrations in the four principal peridotitic silicate phases (garnet, olivine, orthopyroxene, clinopyroxene) included in diamonds from Akwatia (Birim Field, Ghana) were determined using SIMS. Incompatible trace elements are hosted in garnet and clinopyroxene except for Sr which is equally distributed between orthopyroxene and garnet in harzburgitic Paragenesis diamonds. The separation between lherzolitic and harzburgitic inclusion parageneses, which is commonly made using compositional fields for garnets in a CaO versus Cr2O3 diagram, is also apparent from the Ti and Sr contents in both olivine and garnet. Titanium is much higher in the lherzolitic and Sr in the harzburgitic inclusions. Chondrite normalised REE patterns of lherzolitic garnets are enriched (10–20 times chondrite) in HREE (LaN/YbN = 0.02–0.06) while harzburgitic garnets have sinusoidal REEN patterns, with the highest concentrations for Ce and Nd (2–8 times chondritic) and a minimum at Ho (0.2–0.7 times chondritic). Clinopyroxene inclusions show negative slopes with La enrichment 10–100 times chondritic and low Lu (0.1–1 times chondritic). Both a lherzolitic and a harzburgitic garnet with very high knorringite contents (14 and 21 wt% Cr2O3 respectively) could be readily distinguished from other garnets of their parageneses by much higher levels of LREE enrichment. The REE patterns for calculated melt compositions from lherzolitic garnet inclusions fall into the compositional field for kimberlitic-lamproitic and carbonatitic melts. Much more strongly fractionated REE patterns calculated from harzburgitic garnets, and low concentrations in Ti, Y, Zr, and Hf, differ significantly from known alkaline and carbonatitic melts and require a different agent. Equilibration temperatures for harzburgitic inclusions are generally below the C-H-O solidus of their Paragenesis, those of lherzolitic inclusions are above. Crystallisation of harzburgitic diamonds from CO2-bearing melts or fluids may thus be excluded. Diamond inclusion chemistry and mineralogy also is inconsistent with known examples of metasomatism by H2O-rich melts. We therefore favour diamond precipitation by oxidation of CH4-rich fluids with highly fractionated trace element patterns which are possibly due to “chromatographic” fractionation processes.
Christian Nicollet - One of the best experts on this subject based on the ideXlab platform.
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Eocene ultra-high temperature (UHT) metamorphism in the Gruf complex (Central Alps): constraints by LA-ICPMS zircon and monazite dating in petrographic context
Journal of the Geological Society, 2018Co-Authors: Christian Nicollet, Valérie Bosse, Maria Spalla, Federica SchiaviAbstract:The Gruf complex in the Lepontine Alps is one of the rare occurrences of Phanerozoic ultra-high temperature (UHT) metamorphism in the world, but its age is still a matter of debate. Herewe present LA-ICPMS dating in a petrographic context of zircon and monazite from a UHT restitic granulite. Zircons and monazites are both included in large crystals and in retrograde symplectites. In such restitic rocks, partial melting or fluid interactions are unlikely, precluding resetting of the monazite chronometers. Zircon cores yield Permian ages, which are interpreted as the age of charnockitization. They are sometimes surrounded by a narrow rim at 32 Ma. Monazites are strongly zoned, but all yield a 31.8 ± 0.3 Ma age interpreted as the time of complete (re-)crystallization during the UHT Paragenesis. We propose that the zircons dated a post-Hercynian metamorphism which is responsible for the widespread formation of granulites in the Southern Alps and the crust differentiation. This fluidabsent melting event produced refractory lithologies, such as restites in charnockites. We suggest that Gruf UHT Paragenesis is alpine in age and crystallized from these refractory lithologies.We conclude that the lower restitic crust produced in the Permian had the ability to achieve UHT conditions during the fast exhumation and heating related to lithospheric thinning in Alpine time.
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Experimental study of the Ultra High T metamorphism of a restitic metapelitic granulite: role of a previous partial melting event on the UHT metamorphism and influence of the redox state
2018Co-Authors: Christian Nicollet, D. Vielzeuf, R Savoye, V. BosseAbstract:The origin of UHT metamorphism prevailing in the continental crust is still matter of debate. Partial melting probably plays a key role. UHT metamorphism occurs at temperatures above the fluid-absent melting of most crustal rocks. Partial melting is an endothermic process that consumes heat and buffers the temperature at around 750-850°C. Thus, partial melting prevents a fertile crust to attain UHT. Conversely, UHT conditions can be more easily reached upon refractory / restitic rocks and occur preferentially in terranes that underwent previous partial melting event and melt loss. The Gruf complex (Lepontine Alps) is a field area that confirms this scenario; it is one of the rare occurrences of Eocene UHT metamorphism in the world. This complex previously suffered the post Hercynian high-thermal regime responsible for the widespread formation of granulites in the Austro-alpine domain and Southern Alps. We propose that the typical UHT parageneses of the Gruf complex crystallized from refractory/restitic lithologies. The refractory character was acquired through fluid-absent melting reactions during the post Hercynian metamorphic event, while UHT conditions were reached during the Alpine cycle. The typical mineral assemblage diagnostic of UHT metamorphism Spr + Qtz is sometimes replaced by the Spl + Qz assemblage under similar P-T conditions. The reason for this dichotomy is not yet understood. In order to bring new elements to this discussion, we conducted experiments in an internally-heated pressure vessel on a metapelitic granulite from the Ivrea zone composed of Q-Grt-Sil-Kf±Pl±Bt±Rt. This rock is representative of a component of the lower crust after melt extraction. It would represent a restite associated with the process of differentiation of the crust. The P and T intervals considered during these experiments were 0.3-0.8GPa and 950-1050°C; their duration lasted 7 or 13 days. Between 0.8-0.6 GPa, the Paragenesis is Spr-Opx-Sil-Qz-TiMag-Melt (and Grt at T < 950°C) for the long-time experiments. For the short-duration experiments (same, P, T, and X), the Paragenesis is Spl-Qz-Sil-TiMag-Melt. We interpret the change of Paragenesis to a change from reducing (short-duration) to more oxydizing conditions (long-duration) due to progressive loss of hydrogen during the experiments at very high temperatures. At 0.3GPa, the stable assemblage is Spl-Qz-Sil-TiMag-Ilm-Melt-Crd and/or Osm and Grt at T>900°C, whatever the duration of the experiment. In conclusion, our experiments show that UHT conditions applied to a restite from the lower crust produce typical parageneses of UHT metamorphism. They also suggest that Spl-Qz assemblages are indicative of reducing conditions while Spr-Qz assemblages prevail under more oxidised conditions.
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Eocene ultra high temperature (UHT) metamorphism in the Gruf complex (Central Alps): constraints by LA-ICPMS zircon and monazite dating in petrographic context.
2018Co-Authors: Christian Nicollet, Valérie Bosse, Maria SpallaAbstract:The Gruf complex in the Lepontine Alps is one of the rare occurrences of Phanerozoic UHT metamorphism in the world but its age is still a matter of debate. Here we present LA-ICPMS dating in petrographic context of zircon and monazite from an UHT restitic granulite. Zircons and monazites are both included in large crystals and in retrograde symplectites. In such restitic rocks, partial melting or fluid interactions are unlikely precluding resetting of the monazite chronometers. Zircon cores yield Permian ages interpreted as age of charnockitisation. They are sometimes surrounded by a narrow rim at 32 Ma. Monazites are strongly zoned, but all yield a 31.8 ± 0.3 Ma 15 age interpreted as the time of complete (re-)crystallisation during the UHT Paragenesis. We propose that the zircons dated a post Hercynian metamorphism which is responsible of the widespread formation of granulites in the Southern Alps and the crust differentiation. This fluid-absent melting event produced refractory lithologies such as restites in charnockites. We suggest that Gruf UHT Paragenesis is alpine in age and cristallised from these refractory lithologies. We conclude that the lower restitic crust produced at the Permian time had the ability to achieve UHT conditions during the fast exhumation and heating related to lithospheric thinning in Alpine time.
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Two ultra high temperature (UHT) metamorphic events in the Gruf complex (Central Alps) ? Constraints by in situ dating of zircon and monazite
2017Co-Authors: Christian Nicollet, Valérie Bosse, Iole SpallaAbstract:The Gruf complex in the Lepontine Alps is one of the two occurrences of Phanerozoic UHT metamophism in the world. This area is thus of major interest to understand the geodynamic signification of such extreme metamor-phic conditions. However, the age of the UHT metamorphism is currently a matter of debate. Based on zircon U/Pb dating, Galli et al. (2013, Swiss J. Geosc. p33 and ref herein) have proposed a Permian age. Minerals of the charnockitic Paragenesis are included within the zircon cores. Rims of these zircon grains yield 34-29 Ma ages interpreted as dating the Alpine amphibolite facies migmatisation. A different interpretation is proposed by Liati and Gebauer (2003, Schweiz. Miner. Petrog., p159) who consider that the zircon Alpine rims grew during the UHT metamorphic event. Based on monazite dating, Schmitz et al. (2009, Eur. J. Mineral., p927) follow this interpretation , whereas Galli et al. (2013) suggest that the Alpine age is the result of monazite resetting processes during Alpine migmatisation. In order to try to solve this controversy, we have realized LA-ICPMS in situ dating on zircon and monazite from a restitic granulite within charnockite showing the typical well preserved UHT mineral assemblage (Spr-Al-rich Opx-Sil-Crd-Grt-Bt ± mesoperthite-Qtz-Spl). Only complex symplectitic Crd-Spl-Spr-Opx assemblages correspond to the beginning of the retrograde evolution. Both zircons and monazites are included in the large crystals from the UHT assemblage as well as in the late symplectites. In such restitic rocks, a significant fluid interaction is unlikely, precluding a fluid mediated resetting of the monazite. U/Pb ages in zircon and Th/U/Pb ages in monazite measured in the same sample confirm the ages previously measured. Zircon cores yield Permian ages (from around 250 to 304 Ma), sometimes surrounded by a narrow rim at 33.2 ± 1.2 Ma. Intermediate ages may reflect mixing between core and very thin rim. Monazites are present in the core of large Spr, Opx, Crd crystals or form clusters of small grains in the late symplectites. All the grains are strongly zoned in Th, U and Y, but all yield a 31.8 ± 0.3 Ma age interpreted as the time of complete (re-)crystallisation of the monazite in equilibrium with the UHT Paragenesis. In agreement with Galli et al. (2013), these results show that the Permian age preserved in the zircon cores is related to the charnockitisation. But the monazite age also demonstrates that the Spr-Opx-Sil UHT Paragenesis in the restites in the charnockites equilibrated at 32 Ma, in agreement with Liati and Gebauer (2003) and Schmitz et al. (2009). We propose that this typical UHT Paragenesis cristallised from refractory lithologies such as restites or schlieren in the charnockites. The refractory character was acquired during the previous metamorphic event, although it is difficult to precise what were the mineral assemblages in these rocks during the Permian (U?)HT metamorphism.
E. Jagoutz - One of the best experts on this subject based on the ideXlab platform.
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Neodymium and strontium isotope systematics of eclogite and websterite Paragenesis inclusions from single diamonds, Finsch and Kimberley Pool, RSA
Geochimica et Cosmochimica Acta, 1991Co-Authors: C.b. Smith, John J. Gurney, Jeff W. Harris, M. L. Otter, Melissa B. Kirkley, E. JagoutzAbstract:Abstract Garnet inclusions of the eclogite Paragenesis from large single diamonds (1.8 to 4.2 carats) from the 120 Ma Finsch kimberlite have Nd model ages ranging from 1443 to 2408 Ma. Ages of 1443 ± 166 and 1657 ± 77 Ma in two samples with 87 Sr 86 Sr = 0.7042 may represent the age of diamond formation and are within error of a 1580 ± 50 Ma Sm-Nd isochron age on clinopyroxene and garnet reported by Richardson et al. (1990). Older model ages in three other samples (1824, 2183, and 2408 Ma) correspond with unsupported radiogenic Sr ( 87 Sr 86 Sr = 0.7095 to 0.7158 ) and anomalously high MnO (0.7 to 1.2 wt%.) suggestive of multistage evolution. Carbon isotopic compositions of the diamonds range from −3.6 to −7.7%., and do not correlate with isotopic or chemical features of the inclusions. Compared to world-wide compositional ranges of most eclogite Paragenesis garnet inclusions in diamonds, four of five analyzed here are anomalously enriched in Fe and Mn, possibly a characteristic feature of the Finsch eclogitic diamond Paragenesis suite. A large diamond with a websterite inclusion assemblage (clinopyroxene, orthopyroxene, garnet) from the Kimberley Pool (85 Ma emplacement age) has clinopyroxene with a Nd model age of 2111 ± 120 Ma. In contrast to the high Sm/Nd and radiogenic Nd of the eclogite garnets, the websterite cpx has low Sm/Nd and unradiogenic Nd more akin to that in garnet of harzburgite association diamonds from the Finsch kimberlite. However, neither the eclogite nor the websterite group diamonds can be related to the harzburgite suite. Analysis of inclusions from single diamonds eliminates the problems inherent in compositing possibly unrelated inclusions from large numbers of stones, but the large diamond size required to yield large inclusions may represent only a specific and possibly rare component of the general diamond population. The samples analyzed here may have been derived ultimately from subducted oceanic crust.
Thomas Stachel - One of the best experts on this subject based on the ideXlab platform.
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Diamondiferous peridotitic microxenoliths from the Diavik Diamond Mine, NT
Contributions to Mineralogy and Petrology, 2008Co-Authors: Steven Creighton, Hayley Mclean, Dave Eichenberg, Thomas Stachel, Karlis Muehlenbachs, Antonio Simonetti, R LuthAbstract:Twenty-five diamonds recovered from 21 diamondiferous peridotitic micro-xenoliths from the A154 South and North kimberlite pipes at Diavik (Slave Craton) match the general peridotitic diamond production at this mine with respect to colour, carbon isotopic composition, and nitrogen concentrations and aggregation states. Based on garnet compositions, the majority of the diamondiferous microxenoliths is lherzolitic (G9) in Paragenesis, in stark contrast to a predominantly harzburgitic (G10) inclusion Paragenesis for the general diamond production. For garnet inclusions in diamonds from A154 South, the lherzolitic Paragenesis, compared to the harzburgitic Paragenesis, is distinctly lower in Cr content. For microxenolith garnets, however, Cr contents for garnets of both the parageneses are similar and match those of the harzburgitic inclusion garnets. Assuming that the microxenolith diamonds reflect a sample of the general diamond population, the abundant Cr-rich lherzolitic garnets formed via metasomatic overprinting of original harzburgitic diamond sources subsequent to diamond formation, conversion of original harzburgitic diamond sources occurred in the course of metasomatic overprint re-fertilization. Metasomatic overprinting after diamond formation is supported by the finding of a highly magnesian olivine inclusion (Fo_95) in a microxenolith diamond that clearly formed in a much more depleted environment than indicated by the composition of its microxenolith host. Chondrite normalized REE patterns of microxenolith garnets are predominantly sinusoidal, similar to observations for inclusion garnets. Sinusoidal REE_N patterns are interpreted to indicate a relatively mild metasomatic overprint through a highly fractionated (very high LREE/HREE) fluid. The predominance of such patterns may explain why the proposed metasomatic conversion of harzburgite to lherzolite appears to have had no destructive effect on diamond content.
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GARNET XENOCRYSTS FROM THE DIAVIK MINE, NWT, CANADA: COMPOSITION, COLOR, AND Paragenesis
The Canadian Mineralogist, 2007Co-Authors: Hayley Mclean, Steven Creighton, Anetta Banas, Sean Whiteford, Robert W. Luth, Thomas StachelAbstract:Recently sampled mantle-derived xenoliths from the A154–S kimberlite pipe, at the Diavik diamond mine, Northwest Territories, Canada, are dominantly eclogitic in Paragenesis, suggesting that either the abundance of eclogite at depth exceeds that of peridotite, or that eclogite is sampled or preserved preferentially. To determine the relative abundance of eclogite and peridotite sampled by the ascending kimberlite more accurately and to evaluate the utility of color alone as a guide to composition, we studied xenocrysts of garnet recovered from coarse heavy-mineral concentrates. Over 10,000 garnet crystals were divided into 15 groups according to their color. From these groups, 174 crystals were analyzed for major and minor elements using an electron microprobe (WDS). Concentrations of trace elements of 55 garnet xenocrysts were measured by LA–ICP–MS. Orange xenocrysts have 2 O 3 , and are classified as having an eclogitic derivation according to two sets of criteria. Pink, red, and purple xenocrysts contain >2 wt.% Cr 2 O 3 , and are inferred to have a peridotitic derivation. The range of Cr content of purple xenocrysts extends to higher values (~14 wt.% Cr 2 O 3 ) than do those of pink (~10 wt.%) or red crystals (~9 wt.%). Orange-red and red-orange xenocrysts have Cr concentrations overlapping those of the orange and red groups, and may be either eclogitic or peridotitic. Chondrite-normalized REE (REE N ) patterns are either “normal”, with low LREE N , increasing concentrations into the MREE N , and relatively flat HREE at ~10–30× chondrite, or sinusoidal, with normalized concentrations increasing from La to Nd, decreasing from Nd to Ho, and then increasing again to Lu. “Normal” patterns are typical of orange and orange-red xenocrysts, although some show a LREE enrichment. The sinusoidal patterns are characteristic of purple and some pink and red xenocrysts. Garnet xenocrysts of pyroxenitic (including eclogitic and websteritic) and peridotitic affinity are sufficiently distinct in color to permit assignment of the >10,000 garnet crystals in our sample to one of the two suites based solely on color. Color alone, however, could not differentiate unambiguously between different peridotitic parageneses. In contrast to the xenolith data, we infer from the relative abundances of the xenocrystic garnet that the A154–S mantle sample is ~96% peridotite.
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diamond precipitation and mantle metasomatism evidence from the trace element chemistry of silicate inclusions in diamonds from akwatia ghana
Contributions to Mineralogy and Petrology, 1997Co-Authors: Thomas Stachel, J W HarrisAbstract:Trace element concentrations in the four principal peridotitic silicate phases (garnet, olivine, orthopyroxene, clinopyroxene) included in diamonds from Akwatia (Birim Field, Ghana) were determined using SIMS. Incompatible trace elements are hosted in garnet and clinopyroxene except for Sr which is equally distributed between orthopyroxene and garnet in harzburgitic Paragenesis diamonds. The separation between lherzolitic and harzburgitic inclusion parageneses, which is commonly made using compositional fields for garnets in a CaO versus Cr2O3 diagram, is also apparent from the Ti and Sr contents in both olivine and garnet. Titanium is much higher in the lherzolitic and Sr in the harzburgitic inclusions. Chondrite normalised REE patterns of lherzolitic garnets are enriched (10–20 times chondrite) in HREE (LaN/YbN = 0.02–0.06) while harzburgitic garnets have sinusoidal REEN patterns, with the highest concentrations for Ce and Nd (2–8 times chondritic) and a minimum at Ho (0.2–0.7 times chondritic). Clinopyroxene inclusions show negative slopes with La enrichment 10–100 times chondritic and low Lu (0.1–1 times chondritic). Both a lherzolitic and a harzburgitic garnet with very high knorringite contents (14 and 21 wt% Cr2O3 respectively) could be readily distinguished from other garnets of their parageneses by much higher levels of LREE enrichment. The REE patterns for calculated melt compositions from lherzolitic garnet inclusions fall into the compositional field for kimberlitic-lamproitic and carbonatitic melts. Much more strongly fractionated REE patterns calculated from harzburgitic garnets, and low concentrations in Ti, Y, Zr, and Hf, differ significantly from known alkaline and carbonatitic melts and require a different agent. Equilibration temperatures for harzburgitic inclusions are generally below the C-H-O solidus of their Paragenesis, those of lherzolitic inclusions are above. Crystallisation of harzburgitic diamonds from CO2-bearing melts or fluids may thus be excluded. Diamond inclusion chemistry and mineralogy also is inconsistent with known examples of metasomatism by H2O-rich melts. We therefore favour diamond precipitation by oxidation of CH4-rich fluids with highly fractionated trace element patterns which are possibly due to “chromatographic” fractionation processes.
Carlos Fernandez - One of the best experts on this subject based on the ideXlab platform.
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metacarbonatites in the basal complex of fuerteventura canary islands the role of fluid rock interactions during contact metamorphism and anatexis
Lithos, 2011Co-Authors: R Casillas, Attila Demeny, Geza Nagy, A Ahijado, Carlos FernandezAbstract:Abstract The Basal Complex of Fuerteventura, in the Canary Islands, contains an ultra-alkaline complex formed by alkaline pyroxenites, melteigites, ijolites, alkaline gabbros, syenites and carbonatite dykes cut by a basaltic dyke swarm. A later basaltic magma, which formed a pyroxenite-gabbro pluton, intruded the entire series and caused a strong contact metamorphism and partial melting of the rocks near the contact. The anatexis appears in the form of “zebra rocks” in the contact-metamorphosed silicate rocks. Carbonatite dykes have been found in the metamorphic aureole and in the anatexite. Typical skarn mineralogy, including diopside, grossular-andradite garnet and vesuvianite, developed between the carbonatites and silicate rocks (e.g. ijolites, pyroxenites). In the metacarbonatites and skarns, the metamorphic Paragenesis varies with the distance to the contact zone. The original igneous mineralogy disappeared completely in the anatectic zone, and a metamorphic association composed of wollastonite, monticellite, diopside, vesuvianite, garnet, calcite, perovskite, alabandite, pyrrhotite and Nb–Zr–Ca silicates (cuspidine–niocalite–baghdadite series) was formed. This Paragenesis indicates that the carbonatites within the anatectic zone have undergone a thermal metamorphism under hornblende–hornfels facies (550–600 °C). Metamorphic reactions were also associated with the infiltration of F-rich aqueous fluids, which produced an intense metasomatism and caused Sr, Ba and S enrichment in the carbonatite. As a result, alkali elements such as K, Na, Rb or Th were leached and the remaining chemical elements were redistributed into the neoformed metacarbonatite Paragenesis. Most of the metacarbonatites show significant C and O isotope deviations as compared with the primary isotopic compositions, due to fluid/rock interactions. The metacarbonatites with wollastonite have δ 13 C and δ 18 O values indicating CO 2 release, in agreement with devolatilization reactions that took place in the metacarbonatite and in the skarn. In addition, in the metacarbonatites with diopside, vesuvianite and garnet, a δ 18 O decrease relative to the primary isotopic compositions due to late fluid/rock interactions can be observed. The circulation of hot meteoric water fluids heated by the pyroxenite intrusion was responsible for the metamorphic-metasomatic reactions that caused the mineralogical, chemical and isotopic changes in the carbonatitic rocks and brought about skarn formation.
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Metacarbonatites in the Basal Complex of Fuerteventura (Canary Islands). The role of fluid/rock interactions during contact metamorphism and anatexis
Lithos, 2011Co-Authors: R Casillas, Attila Demeny, Geza Nagy, A Ahijado, Carlos FernandezAbstract:Abstract The Basal Complex of Fuerteventura, in the Canary Islands, contains an ultra-alkaline complex formed by alkaline pyroxenites, melteigites, ijolites, alkaline gabbros, syenites and carbonatite dykes cut by a basaltic dyke swarm. A later basaltic magma, which formed a pyroxenite-gabbro pluton, intruded the entire series and caused a strong contact metamorphism and partial melting of the rocks near the contact. The anatexis appears in the form of “zebra rocks” in the contact-metamorphosed silicate rocks. Carbonatite dykes have been found in the metamorphic aureole and in the anatexite. Typical skarn mineralogy, including diopside, grossular-andradite garnet and vesuvianite, developed between the carbonatites and silicate rocks (e.g. ijolites, pyroxenites). In the metacarbonatites and skarns, the metamorphic Paragenesis varies with the distance to the contact zone. The original igneous mineralogy disappeared completely in the anatectic zone, and a metamorphic association composed of wollastonite, monticellite, diopside, vesuvianite, garnet, calcite, perovskite, alabandite, pyrrhotite and Nb–Zr–Ca silicates (cuspidine–niocalite–baghdadite series) was formed. This Paragenesis indicates that the carbonatites within the anatectic zone have undergone a thermal metamorphism under hornblende–hornfels facies (550–600 °C). Metamorphic reactions were also associated with the infiltration of F-rich aqueous fluids, which produced an intense metasomatism and caused Sr, Ba and S enrichment in the carbonatite. As a result, alkali elements such as K, Na, Rb or Th were leached and the remaining chemical elements were redistributed into the neoformed metacarbonatite Paragenesis. Most of the metacarbonatites show significant C and O isotope deviations as compared with the primary isotopic compositions, due to fluid/rock interactions. The metacarbonatites with wollastonite have δ13C and δ18O values indicating CO2 release, in agreement with devolatilization reactions that took place in the metacarbonatite and in the skarn. In addition, in the metacarbonatites with diopside, vesuvianite and garnet, a δ18O decrease relative to the primary isotopic compositions due to late fluid/rock interactions can be observed. The circulation of hot meteoric water fluids heated by the pyroxenite intrusion was responsible for the metamorphic-metasomatic reactions that caused the mineralogical, chemical and isotopic changes in the carbonatitic rocks and brought about skarn formation.
