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1.
Claudia Cannatelli 《Mineralogy and Petrology》2012,104(3-4):177-195
Mantle peridotites from the Veneto Volcanic Province (VPP) have been investigated in order to constrain P-T conditions of mantle events, determine the style of the metasomatic reactions, and the compositions of the metasomatic agents. Studied rocks show dominant protogranular and transitional textures; only one sample shows effect of pyrometamorphism. Clinopyroxenes in protogranular lherzolites show depleted LREE patterns, while those of transitional rocks are characterised by spoon-shaped REE patterns (La up to 60 times chondrite), and variable enrichments in LILE. Two generations of fluid inclusions are recognised: 1) Type I (CO2 ± CO ± C fluid) found only in orthopyroxene of transitional xenoliths which may contain very small amphibole; 2) Type II (CO2-rich fluid) found in all minerals of all xenoliths. Most of inclusions homogenize to liquid, with ThL ranging between ?44 and 31°C. The densest CO2 fluid inclusions (d?=?1.13?g/cm3), indicates a trapping pressure of ~10?kbar at 800°C. We propose that the mantle beneath the VVP equilibrated at pressures of 10?kbar, at about 800°C. Traces of an aqueous fluid preserved as fluid inclusions in orthopyroxene suggest the existence of an older subduction related metasomatic event and the occurrence of two stages metasomatism in the lithosphere beneath the SE Alps. 相似文献
2.
He Sun Yilin Xiao Yongjun Gao Jianqing Lai Zhenhui Hou Yangyang Wang 《Contributions to Mineralogy and Petrology》2013,165(5):885-901
Melt inclusions and fluid inclusions in the Fangcheng basalt were investigated to understand the magma evolution and fluid/melt-peridotite interaction. Primary silicate melt inclusions were trapped in clinopyroxene and orthopyroxene phenocrysts in the Fangcheng basalt. Three types of melt inclusions (silicate, carbonate, and sulfide) coexisting with fluid inclusions occur in clinopyroxene xenocrysts and clinopyroxene in clinopyroxenite xenoliths. In situ laser-ablation ICP-MS analyses of major and trace element compositions on individual melt inclusions suggest that the silicate melt inclusions in clinopyroxene and orthopyroxene phenocrysts were trapped from the same basaltic magma. The decoupling of major and trace elements in the melt inclusions indicates that the magma evolution was controlled by melt crystallization and contamination from entrapped ultramafic xenoliths. Trace element patterns of melt inclusions are similar to those of the average crust of North China Craton and Yangtze Craton, suggesting a considerable crustal contribution to the magma source. Calculated parental melt of the Fangcheng basalt has features of low MgO (5.96 wt%), high Al2O3 (16.81 wt%), Sr (1,670 ppm), Y (>35 ppm), and high Sr/Y (>40), implying that subducted crustal material was involved in the genesis of the Fangcheng basalt. The coexisting fluid and melt inclusions in clinopyroxene xenocrysts and in clinopyroxene of xenoliths record a rare melt-peridotite reaction, that is olivine + carbonatitic melt1 (rich in Ca) = clinopyroxene + melt2 ± CO2. The produced melt2 is enriched in LREE and CO2 and may fertilize the mantle significantly, which we consider to be the cause for the rapid replacement of lithospheric mantle during the Mesozoic in the region. 相似文献
3.
Lherzolite xenoliths containing fluid inclusions from the Ichinomegata volcano, located on the rear-arc side of the Northeast Japan arc, may be considered as samples of the uppermost mantle above the melting region in the mantle wedge. Thus, these fluid inclusions provide valuable information on the nature of fluids present in the sub-arc mantle. The inclusions in the Ichinomegata amphibole-bearing spinel–plagioclase lherzolite xenoliths were found to be composed mainly of CO2–H2O–Cl–S fluids. At equilibrium temperature of 920 °C, the fluid inclusions preserve pressures of 0.66–0.78 GPa, which correspond to depths of 23–28 km. The molar fraction of H2O and the salinity of fluid inclusions are 0.18–0.35 and 3.71 ± 0.78 wt% NaCl equivalent, respectively. These fluid inclusions are not believed to be fluids derived directly from the subducting slab, but rather fluids exsolved from sub-arc basaltic magmas that are formed through partial melting of mantle wedge triggered by slab-derived fluids. 相似文献
4.
Summary ?Fluid inclusions from two Mesoproterozoic, metamorphosed layered intrusive complexes, Niquelandia and Barro Alto, Goiás State,
Brazil record multiple fluid influx events from the magmatic to granulitic and retrograde metamorphic stages.
1. The oldest inclusions contain high density CO2 ± N2 ± CH4 and are found as primaries in plagioclase and orthopyroxene in mafic granulite with homogenization temperatures between − 48
and − 28 °C. These inclusions may correspond to the early, magmatic stage. This type was found in samples from both the Niquelandia
and the Barro Alto complexes.
2. Intragranular, relatively high density CO2 + N2 inclusions (Th between − 33 and − 26 °C) together with decrepitated and reequilibrated N2 inclusions (Th between − 160 and − 151 °C) in the rock-forming minerals can be associated with the granulite facies metamorphism. Such inclusions
were found only in the Barro Alto complex.
3. Transgranular, high density, CO2–N2 inclusions (93% CO2 and 7% N2, according to Raman analysis, with Th between − 66.6 and − 50.4 °C) as well as the low density, secondary CO2 ± N2 ± CH4 inclusions (Th between − 13.0 and + 18.7 °C) and the H2O–NaCl–CaCl2 hypersaline inclusions (with halite dissolution temperature between 132 and 354 °C, and Th between 212 and 490 °C) are attributed to different fluid influx events during the retrograde metamorphism. This inclusion
type can be found both in the Niquelandia and in the Barro Alto complexes.
The fluid inclusion textures and compositions show several stages of fluid evolution. The fluid inclusion measurements and
the geothermobarometric data indicate an anticlockwise P-T path for both the Barro Alto and the Niquelandia complexes.
Received October 16, 2000; revised version accepted November 20, 2001 相似文献
5.
High‐ to ultrahigh‐temperature metamorphism in the lower crust: An example resulting from Hikurangi Plateau collision and slab rollback in New Zealand
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Jean‐Baptiste Jacob James M. Scott Rose E. Turnbull Matthew S. Tarling Matthew W. Sagar 《Journal of Metamorphic Geology》2017,35(8):831-853
Lower crustal xenoliths erupted from an intraplate diatreme reveal that a portion of the New Zealand Gondwana margin experienced high‐temperature (HT) to ultrahigh‐temperature (UHT) granulite facies metamorphism just after flat slab subduction ceased at c. 110–105 Ma. P–T calculations for garnet–orthopyroxene‐bearing felsic granulite xenoliths indicate equilibration at ~815 to 910°C and 0.7 to 0.8 GPa, with garnet‐bearing mafic granulite xenoliths yielding at least 900°C. Supporting evidence for the attainment of HT and UHT conditions in felsic granulite comes from re‐integration of exsolution in feldspar (~900–950°C at 0.8 GPa), Ti‐in‐zircon thermometry on Y‐depleted overgrowths on detrital zircon grains (932°C ± 24°C at aTiO2 = 0.8 ± 0.2), and correlation of observed assemblages and mineral compositions with thermodynamic modelling results (≥850°C at 0.7 to 0.8 GPa). The thin zircon overgrowths, which were mainly targeted by drilling through the cores of grains, yield a U–Pb pooled age of 91.7 ± 2.0 Ma. The cause of Late Cretaceous HT‐UHT metamorphism on the Zealandia Gondwana margin is attributed to collision and partial subduction of the buoyant oceanic Hikurangi Plateau in the Early Cretaceous. The halt of subduction caused the fore‐running shallowly dipping slab to rollback towards the trench position and permitted the upper mantle to rapidly increase the geothermal gradient through the base of the extending (former) accretionary prism. This sequence of events provides a mechanism for achieving regional HT–UHT conditions in the lower crust with little or no sign of this event at the surface. 相似文献
6.
Toshiaki Tsunogae M. Santosh Yasuhito Osanai Masaaki Owada Tsuyoshi Toyoshima Tomokazu Hokada 《Contributions to Mineralogy and Petrology》2002,143(3):279-299
The ultrahigh-temperature (UHT) metamorphism of the Napier Complex is characterized by the presence of dry mineral assemblages, the stability of which requires anhydrous conditions. Typically, the presence of the index mineral orthopyroxene in more than one lithology indicates that H2O activities were substantially low. In this study, we investigate a suite of UHT rocks comprising quartzo-feldspathic garnet gneiss, sapphirine granulite, garnet-orthopyroxene gneiss, and magnetite-quartz gneiss from Tonagh Island. High Al contents in orthopyroxene from sapphirine granulite, the presence of an equilibrium sapphirine-quartz assemblage, mesoperthite in quartzo-feldspathic garnet gneiss, and an inverted pigeonite-augite assemblage in magnetite-quartz gneiss indicate that the peak temperature conditions were higher than 1,000 °C. Petrology, mineral phase equilibria, and pressure-temperature computations presented in this study indicate that the Tonagh Island granulites experienced maximum P-T conditions of up to 9 kbar and 1,100 °C, which are comparable with previous P-T estimates for Tonagh and East Tonagh Islands. The textures and mineral reactions preserved by these UHT rocks are consistent with an isobaric cooling (IBC) history probably following an counterclockwise P-T path. We document the occurrence of very high-density CO2-rich fluid inclusions in the UHT rocks from Tonagh Island and characterize their nature, composition, and density from systematic petrographic and microthermometric studies. Our study shows the common presence of carbonic fluid inclusions entrapped within sapphirine, quartz, garnet and orthopyroxene. Analysed fluid inclusions in sapphirine, and some in garnet and quartz, were trapped during mineral growth at UHT conditions as 'primary' inclusions. The melting temperatures of fluids in most cases lie in the range of -56.3 to -57.2 °C, close to the triple point for pure CO2 (-56.6 °C). The only exceptions are fluid inclusions in magnetite-quartz gneiss, which show slight depression in their melting temperatures (-56.7 to -57.8 °C) suggesting traces of additional fluid species such as N2 in the dominantly CO2-rich fluid. Homogenization of pure CO2 inclusions in the quartzo-feldspathic garnet gneiss, sapphirine granulite, and garnet-orthopyroxene gneiss occurs into the liquid phase at temperatures in the range of -34.9 to +4.2 °C. This translates into very high CO2 densities in the range of 0.95-1.07 g/cm3. In the garnet-orthopyroxene gneiss, the composition and density of inclusions in the different minerals show systematic variation, with highest homogenization temperatures (lowest density) yielded by inclusions in garnet, as against inclusions with lowest homogenization (high density) in quartz. This could be a reflection of continued recrystallization of quartz with entrapment of late fluids along the IBC path. Very high-density CO2 inclusions in sapphirine associated with quartz in the Tonagh Island rocks provide potential evidence for the involvement of CO2-rich fluids during extreme crustal temperatures associated with UHT metamorphism. The estimated CO2 isochores for sapphirine granulite intersect the counterclockwise P-T trajectory of Tonagh Island rocks at around 6-9 kbar at 1,100 °C, which corresponds to the peak metamorphic conditions of this terrane derived from mineral phase equilibria, and the stability field of sapphirine + quartz. Therefore, we infer that CO2 was the dominant fluid species present during the peak metamorphism in Tonagh Island, and interpret that the fluid inclusions preserve traces of the synmetamorphic fluid from the UHT event. The stability of anhydrous minerals, such as orthopyroxene, in the study area might have been achieved by the lowering of H2O activity through the influx of CO2 at peak metamorphic conditions (>1,100 °C). Our microthermometric data support a counterclockwise P-T path for the Napier Complex. 相似文献
7.
Jane Selverstone 《Contributions to Mineralogy and Petrology》1982,79(1):28-36
Granulite xenoliths within alkali olivine basalts of the Pali-Aike volcanic field, southern Chile, contain the mineral assemblage orthopyroxene + clinopyroxene + plagioclase + olivine + green spinel. These granulites are thought to be accidental inclusions of the lower crust incorporated in the mantle-derived basalt during its rise to the surface. Symplectic intergrowths of pyroxene and spinel developed between olivine and plagioclase imply that the reaction olivine+plagioclase = Al-orthopyroxene + Al-clinopyroxene + spinel (1) occurred during subsolidus cooling and recrystallization of a gabbroic protolith of the granulites.Examination of fluid inclusions in the granulites indicates the ubiquitous presence of an essentially pure CO2 fluid phase. Inclusions of three different parageneses have been recognized: Type I inclusions occur along exsolution lamellae in clinopyroxene and are thought to represent precipitation of structurally-bound C or CO2 during cooling of the gabbro. These are considered the most primary inclusions present. Type II inclusions occur as evenly distributed clusters not associated with any fractures. These inclusions probably represent entrapment of a free fluid phase during recrystallization of the host grains. IIa inclusions are found in granoblastic grains and have densities of 0.68–0.88 g/cm3. Higher density (=0.90–1.02 g/cm3) IIb inclusions occur only in symplectite phases. Secondary Type III CO2+glass inclusions with =0.47–0.78 g/cm3 occur along healed fractures where basalt has penetrated the xenoliths. Type III inclusions appear related to exsolution of CO2 from the host basalt during its ascent to the surface. These data suggest that CO2 is an important constituent of the lower crust under conditions of granulite facies metamorphism, indicated by Type I and II fluid inclusions, and of the mantle, as indicated by Type III inclusions.Correlation of fluid inclusion densities with P-T conditions calculated from both two-pyroxene geothermometry and reation (1) indicate emplacement of a gabbroic pluton at 1,200–1,300° C, 4–6 kb; cooling was accompanied by a slight increase in pressure due to crustal thickening, and symplectite formation occurred at 850±35° C, 5–7 kb. Capture of the xenoliths by the basalt resulted in heating of the granulites, and CO2 from the basalt was continuously entrapped by the xenoliths over the range 1,000–1,200° C, 4–6 kb. Examination of fluid inclusions of different generations can thus be used in conjunction with other petrologic data to place tight constraints on the specific P-T path followed by the granulite suite, in addition to indicating the nature of the fluid phase present at depth. 相似文献
8.
Following ultrahigh temperature granulite metamorphism at ∼1 Ga, the Eastern Ghats Province of India was intruded by the Koraput Alkaline Complex, and was subsequently re-metamorphosed in the granulite facies in the mid-Neoproterozoic time. Fluid inclusion studies were conducted on silica undersaturated alkali gabbro and syenites in the complex, and a pre-metamorphic pegmatitic granite dyke that intrudes it. High density (1.02–1.05 g/cc), pseudo-secondary pure CO2 inclusions are restricted to metamorphic garnets within the gabbro and quartz within the granite, whereas moderate (∼0.92–0.95 g/cc) and low density (∼0.75 g/cc) secondary inclusions occur in garnet, magmatic clinopyroxene, plagioclase, hornblende and quartz. The isochores calculated for high density pseudo-secondary inclusions pass very close to the peak metamorphic window (∼8 kbar, 750 °C), and are interpreted to represent the fluid present during peak metamorphism that was entrapped by the growing garnet. Microscopic round inclusions of undigested, relict calcite in garnet suggest that the CO2 present during metamorphism of the complex was internally derived through carbonate breakdown. Pure to low salinity (0.00–10.1 wt% NaCl equivalent) aqueous intra-/intergranular inclusions showing unimodal normal distribution of final ice-melting temperature (Tm) and temperature of homogenization (Th) are present only in quartz within the granite. These represent re-equilibrated inclusions within the quartz host that were entrapped at the metamorphic peak. Rare, chemically precipitated graphite along the walls of carbonic inclusions is interpreted as a post-entrapment reaction product formed during decompression. The fluid inclusion evidence is consistent with rapid exhumation of a thickened lower crust following the mid-Neoproterozoic granulite facies metamorphic event. The study suggests that mantle CO2, transported by alkaline magma into the crust, was locked up within carbonates and released during granulite metamorphism. 相似文献
9.
A. M. VAN DEN. KERKHOF J. L. R. TOURET R. KREULEN 《Journal of Metamorphic Geology》1994,12(3):301-310
The enderbites from Tromøy in the central, granulite facies part of the Proterozoic Bamble sector of southern Norway contain dominantly CO2 and N2 fluid inclusions. CO2 from fluid inclusions in quartz segregations in enderbites was extracted by mechanical (crushing) and thermal decrepitation and the δ13C measured. Measurement was also made on samples washed in 10% HCl, oxidized with CuO at high temperatures, and step-wise extracted with progressive heating. Results between the different techniques are systematic. The main results show δ13C of -4.5±1.5% for crushing and -7±2% for thermal decrepitation. δ13C is about constant for CO2 extracted at different temperatures and points to a homogeneous isotopic composition. Due to the presence of carbonate particles and/or induced contaminations for the extraction by thermal decrepitation, the results for the crushing experiments are assumed the most reliable for fluid-inclusion CO2. Very low values of δ13C have not been found in enderbite samples and δ13C combined with δ18O of the host quartzes (8-11%) indicates juvenile values. In addition, the fluid inclusions were examined by microthermometry and Raman analysis and host quartz by acoustic emission and cathodoluminescence. CO2 fluid inclusions have varying densities with a frequency maximum of 0.92 g cm-3 and generally do not concur with trapping densities at granulite conditions. Textures show that CO2 must have been trapped in fluid inclusions in one early event, but transformed to different extents during late isothermal uplift without important fractionation of isotope compositions. The present data support a model of intrusion and crystallization of a CO2-rich enderbitic magma at granuiite conditions. 相似文献
10.
《International Geology Review》2012,54(9):1122-1137
We have studied melt and fluid inclusions in minerals from alkali basalts, mantle xenoliths, and dawsonite-bearing sandstones from the Shuangliao volcanic field in southern Songliao Basin, Northeast China. The inclusions have been investigated using petrographic, geochemical, and laser Raman spectroscopic techniques. Volcanic rocks of the Shuangliao field are predominantly alkali olivine basalts that contain rare mantle xenoliths. Silicate melt and fluid inclusions are common in both olivine phenocrysts and the mantle xenoliths. The fluid inclusions are mainly composed of CO2 with small amounts of CO, CH4, N2, and H2O, which is consistent with an upper mantle origin. CO2 gas reservoirs in the southern Songliao Basin are mostly derived from a mantle–magmatic source. Coeval fluid-inclusion homogenization temperatures, coupled with the thermal burial history, show that the CO2 gas reservoirs in the southern Songliao Basin are Cenozoic (40–63 Ma) and coeval with the magmatism in the Shuangliao volcanic field. Despite the relatively small scale of this volcanic activity, it released large amounts of CO2. Much of the magma was not erupted, and CO2- and H2O-rich magma was probably intruded into the basin along deep faults, acting as a major source of inorganic CO2 gas in the southern Songliao Basin. 相似文献
11.
Nitrogen-rich fluid in the upper mantle: fluid inclusions in spinel dunite from Lanzarote,Canary Islands 总被引:2,自引:0,他引:2
T. Andersen E. A. J. Burke E. -R. Neumann 《Contributions to Mineralogy and Petrology》1995,120(1):20-28
Fluid inclusions, ranging from pure N2 to pure CO2, occur in olivine porphyroclasts in spinel dunite xenoliths (chrome-diopside suite) from two localities within the Quaternary to Historic alkaline lavas of Lanzarote, Canary Islands. This is the first report of fluid inclusions containing major amounts of N2 in mantle xenoliths. The nitrogen-rich fluid inclusions predate at least one generation of nitrogen-free carbon dioxide inclusions; textural evidence indicates that the inclusions were trapped within the upper mantle. Some of the nitrogen-rich fluid inclusions are intimately associated with solid inclusions of spinel. The nitrogen-rich fluid was most likely produced in-situ, by oxidation-dehydration reactions destabilizing ammonium-bearing silicate minerals (phlogopite, amphibole), increasing oxygen fugacity or, possibly, increasing temperature of the mantle. This process could be related to an event of CO2 and silicate melt injection at 6–8 kbar (Neumann et al., in press), or to some earlier event in the evolution of the mantle beneath Lanzarote. The existence of a N2-rich fluid phase in at least some mantle lithology(ies) at certain conditions is demonstrated by these data. This discovery has consequences for the understanding of the evolution of the mantle below the Canary Islands, as well as for the global nitrogen budget. 相似文献
12.
Chanchal Sarbajna Sankar Bose V. Rajagopalan Kaushik Das Anjan Som A. K. Paul K. Shivkumar K. Umamaheswar Anjan Chaki 《Mineralogy and Petrology》2013,107(4):553-571
High Mg-Al granulite occurs as enclave within granite gneisses at Karimnagar, southern India, and it contains coarse granoblastic aggregates of orthopyroxene and sapphirine with minor amount of cordierite, spinel and phlogopite. An important chemical characteristic of these minerals is their extremely high MgO content and the high Cr2O3 in sapphirine and spinel. Textural analysis shows sapphirine + orthopyroxene + cordierite as the peak-metamorphic assemblage that possibly evolved though the breakdown of a spinel-bearing assemblage. Cation exchange geothermometers involving orthopyroxene, sapphirine and spinel yield temperatures of 600–800 °C with a maximum of 860 °C implying an event of high temperature (HT) metamorphism. Pseudosection analysis in the FeO–MgO–Al2O3–SiO2 chemical system shows the stability of the peak- assemblage below 6.2 kbar. Subsequently, the rock underwent hydration and cooling with the appearance of phlogopite in the assemblage. Chromium enrichment is possibly inherited from the protolith and its presence presumably stabilized sapphirine and spinel below their high-temperature stability field. The recorded Rb–Sr age of ca. 2,500 Ma in host granite gneiss marks the upper age limit of HT metamorphism. Presence of patchy, lobate grains as well as veinlets of uraninite and brannerite is also a characteristic feature of the rock. Uranium mineralization took place during the post peak metamorphic stage, sulfide mineralization represented by tiny grains and veinlets of pyrite, millerite and pentlandite coincided with, and outlasted the uranium mineralization. The U–Th–Pb chemical ages of uraninite grains suggest ca. 2,200?±?12 Ma for the age of uranium mineralization in the granulite. Based on the field relations, it is surmised that the granulite metamorphism in the study area is older than ca. 2,500 Ma and is comparable with an event in the other parts of Eastern Dharwar Craton. It can be conceived as a widespread event in southern India. 相似文献
13.
Rare dunite and 2-pyroxene gabbro xenoliths occur in banded trachyte at Puu Waawaa on Hualalai Volcano, Hawaii. Mineral compositions suggest that these xenoliths formed as cumulates of tholeiitic basalt at shallow depth in a subcaldera magma reservoir. Subsequently, the minerals in the xenoliths underwent subsolidus reequilibration that particularly affected chromite compositions by decreasing their Mg numbers. In addition, olivine lost CaO and plagioclase lost MgO and Fe2O3 during subsolidus reequilibration. The xenoliths also reacted with the host trachyte to form secondary mica, amphibole, and orthopyroxene, and to further modify the compositions of some olivine, clinopyroxene, and spinel grains. The reaction products indicate that the host trachyte melt was hydrous. Clinopyroxene in one dunite sample and olivine in most dunite samples have undergone partial melting, apparently in response to addition of water to the xenolith. These xenoliths do not contain CO2 fluid inclusions, so common in xenoliths from other localities on Hualalai, which suggests that CO2 was introduced from alkalic basalt magma between the time CO2-inclusion-free xenoliths erupted at 106±6 ka and the time CO2-inclusion-rich xenoliths erupted within the last 15 ka. 相似文献
14.
The high-pressure granulites of the Uluguru Mountains are part of the Pan-African belt of Tanzania, the metamorphic evolution
of which is characterized by an anticlockwise P-T path. Mineral assemblages that represent distinct metamorphic stages are selected for fluid inclusion studies in order to
deduce the fluid evolution in metapelites and pyroxene granulites from the prograde to the retrograde stage. Fluid inclusion
data improve the petrologically derived P-T path and confirm the anticlockwise evolution. Fluid inclusions in quartz enclosed in garnet porphyroblasts in metapelites
preserve prograde fluids of CO2–N2 composition and later-trapped pure CO2. During isochoric heating at temperatures near the peak of metamorphism, deformation and recrystallization led to fluid homogenization
yielding N2-poor CO2 composition in the metapelites. Near-peak CO2–N2 fluid inclusions in quartz of metapelites and CO2 inclusions in garnet-pyroxene granulites are characterized by perfect negative crystal shape. Garnet formed in veins and
as coronas around orthopyroxene represent the near-isochoric/isobaric cooling stage which is characterized by high-density
CO2-rich fluid inclusions. Up to 15 mol% N2 in some primary CO2 inclusions in corona garnet indicate small-scale fluid heterogeneity during the static garnet growth. The fact that high-density
fluid inclusions are preserved, suggests a shallow dP/dT slope of the uplift path. Nevertheless, some fluid inclusions decrepitated or re-equilibrated and low-density CO2 inclusions were trapped in the garnet-pyroxene granulite while N2–CH4 inclusions formed in the metapelites. Different fluid compositions in metapelite and metabasite argue for an internal control
of the fluid composition by phase equilibria. In shear zones where the pyroxene granulite was transformed into scapolite-biotite
schist, CO2–N2 and low-density N2–CH4 fluid inclusions indicate several stages of tectonic activity and suggest fluid influx from the nearby metapelites. High-
and low-salinity aqueous inclusions observed beside CO2 inclusions in garnet-pyroxene granulites, in vein quartz and shear zones could be of high-grade origin but are mainly re-equilibrated
or re-trapped along healed microfractures during lower-grade stages.
Received: 21 May 1997 / Accepted: 6 October 1997 相似文献
15.
Cliff S. J. Shaw 《Contributions to Mineralogy and Petrology》1999,135(2-3):114-132
A large body of recent work has linked the origin of Si-Al-rich alkaline glass inclusions to metasomatic processes in the
upper mantle. This study examines one possible origin for these glass inclusions, i.e., the dissolution of orthopyroxene in
Si-poor alkaline (basanitic) melt. Equilibrium dissolution experiments between 0.4 and 2 GPa show that secondary glass compositions
are only slightly Si enriched and are alkali poor relative to natural glass inclusions. However, disequilibrium experiments
designed to examine dissolution of orthopyroxene by a basanitic melt under anhydrous, hydrous and CO2-bearing conditions show complex reaction zones consisting of olivine, ± clinopyroxene and Si-rich alkaline glass similar
in composition to that seen in mantle xenoliths. Dissolution rates are rapid and dependent on volatile content. Experiments
using an anhydrous solvent show time dependent dissolution rates that are related to variable diffusion rates caused by the
saturation of clinopyroxene in experiments longer than 10 minutes. The reaction zone glass shows a close compositional correspondence
with natural Si-rich alkaline glass in mantle-derived xenoliths. The most Si-and alkali-rich melts are restricted to pressures
of 1 GPa and below under anhydrous and CO2-bearing conditions. At 2 GPa glass in hydrous experiments is still Si-␣and alkali-rich whereas glass in the anhydrous and
CO2-bearing experiments is only slightly enriched in SiO2 and alkalis compared with the original solvent. In the low pressure region, anhydrous and hydrous solvent melts yield glass
of similar composition whereas the glass from CO2-bearing experiments is less SiO2 rich. The mechanism of dissolution of orthopyroxene is complex involving rapid incongruent breakdown of the orthopyroxene,
combined with olivine saturation in the reaction zone forming up to 60% olivine. Inward diffusion of CaO causes clinopyroxene
saturation and uphill diffusion of Na and K give the glasses their strongly alkaline characteristics. Addition of Na and K
also causes minor SiO2 enrichment of the reaction glass by increasing the phase volume of olivine. Olivine and clinopyroxene are transiently stable
phases within the reaction zone. Clinopyroxene is precipitated from the reaction zone melt near the orthopyroxene crystal
and redissolved in the outer part of the reaction zone. Olivine defines the thickness of the reaction zone and is progressively
dissolved in the solvent as the orthopyroxene continues to dissolve. Although there are compelling reasons for supporting
the hypothesis that Si-rich alkaline melts are produced in the mantle by orthopyroxene – melt reaction in the mantle, there
are several complications particularly regarding quenching in of disequilibrium reaction zone compositions and the mobility
of highly polymerized melts in the upper mantle. It is considered likely that formation of veins and pools of Si-rich alkaline
glass by orthopyroxene – melt reaction is a common process during the ascent of xenoliths. However, reaction in situ within
the mantle will lead to equilibration and therefore secondary melts will be only moderately siliceous and alkali poor.
Received: 24 August 1998 / Accepted: 2 December 1998 相似文献
16.
Pin Wang Yan-Jing Chen Bin Fu Yong-Fei Yang Mei Mi Zhong-Lie Li 《International Journal of Earth Sciences》2014,103(3):777-797
The Yaochong porphyry Mo deposit in Xinxian County, Henan Province, China, is located in the Hong’an terrane, that is, the western part of the Dabie orogen. The Dabie orogen is part of a >1,500 km long, Triassic continental collision belt between the North China Block and the South China Block. Four types of vein are present. Paragenetically, from early to late, they are as follows: stage 1 quartz + K-feldspar ± pyrite ± magnetite vein; stage 2 quartz + K-feldspar + molybdenite ± pyrite vein; stage 3 quartz + polymetallic sulfides ± K-feldspar vein; and stage 4 quartz ± carbonate ± fluorite vein. Four compositional types of fluid inclusion, pure CO2, CO2 bearing, aqueous, and solid bearing, are present in quartz from the first three stages; only low-salinity aqueous fluid inclusions occur in quartz from the last stage. All the estimated salinities are ≤13.1 wt% NaCl eq., and no halite crystals were identified. Homogenization temperatures for the fluid inclusions from stages 1 to 4 are in the ranges of 262–501, 202–380, 168–345, and 128–286 °C, respectively, and estimated depths decrease from 6.9 to 8.9 km, through 6.2–7.2, to ~4.7 km. Quartz separates from the veins yielded a δ18O value of 7.7–11.2 ‰, corresponding to δ18OH2O values of ?1.3 to 6.9 ‰ using temperature estimates from fluid inclusion data; δDH2O values of fluid inclusion vary from ?80 to ?55 ‰, and δ13CCO2 from ?2.3 to 2.7 ‰, suggesting that the ore-fluids evolved from magmatic to meteoric sources. We conclude that the ore-forming fluid system at Yaochong was initially high temperature, high salinity, and CO2-rich and then progressively evolved to CO2-poor, lower salinity, and lower temperature, by mixing with meteoric water, which results in ore precipitation. 相似文献
17.
This paper reviews the origin and evolution of fluid inclusions in ultramafic xenoliths,providing a framework for interpreting the chemistry of mantle fluids in the different geodynamic settings.Fluid inclusion data show that in the shallow mantle,at depths below about 100 km,the dominant fluid phase is CO_2±brines,changing to alkali-,carbonate-rich(silicate) melts at higher pressures.Major solutes in aqueous fluids are chlorides,silica and alkalis(saline brines;5-50 wt.%NaCl eq.).Fluid inclusions in peridotites record CO_2 fluxing from reacting metasomatic carbonate-rich melts at high pressures,and suggest significant upper-mantle carbon outgassing over time.Mantle-derived CO_2(±brines) may eventually reach upper-crustal levels,including the atmosphere,independently from,and additionally to magma degassing in active volcanoes. 相似文献
18.
《Russian Geology and Geophysics》2007,48(10):811-824
Results of study of different types of inclusions in minerals from mantle xenoliths from the Bele pipe basanites are presented. Two groups of inclusions were recognized in the host minerals according to their genesis. The first group includes single, apparently primary, fluid inclusions. They were discovered only in orthopyroxenes and consist of CO2 (95 mol.%) and N2 (5 mol.%). These inclusions had partly leaked. The densities of two least leaked inclusions from different xenoliths are 1.05 and 1.14 g/cm3, and their trapping pressures are estimated at >8.5 and 12 kbar, respectively. The second group includes syngenetic secondary fluid, melt, and crystalline inclusions. In composition the secondary fluid inclusions differ from the primary ones in higher concentrations of N2 (up to 7 mol.%). Their maximum density is 0.57 g/cm3, which corresponds to 2.4–2.6 kbar and 1100–1200 °C (homogenization temperature of secondary melt inclusions). Comparison of data on melt inclusions in xenolith minerals and host-basanite phenocrysts shows that the secondary inclusions in the xenoliths are, most likely, the result of infiltration and partial reaction of basanitic melt with the xenoliths. On the ascent, the basanitic melt vigorously reacted with mantle xenoliths, which led to the appearance of secondary inclusions in nodule minerals at shallow depths and interstitial mineral assemblages in the xenoliths. 相似文献
19.
《Lithos》1986,19(1):1-10
Forming the southwestern segment of the Precambrian granulite facies terrain of the Indian shield, the Kerala region largely comprises charnockites, khondalites and migmatitic gneisses. Fluid inclusions in quartz from the charnockites show distinct distribution patterns consistent with three generations of inclusions. The early monophase type records entrapment of high-density CO2-rich fluid (0.95–1.0 g cm−3). A subsequent monophase type with lower-density CO2-rich fluid (0.65–0.75 g cm−3) coexists with CO2H2O inclusions having an average degree of filling of 0.2 (H2O = 20%; CO2 = 80%). Late aqueous biphase inclusions show coexistence with a second category of CO2H2O inclusions showing a degree of filling of 0.6 (H2O = 60%; CO2 = 40%). The CO2-isochores for early carbonic inclusions yield a pressure range of 4.6–6.1 kbar at granulite facies temperatures of 650–800°C, depicting the entrapment of fluids present during or close to the peak metamorphic stage. A definite sequence of fluid evolution is traceable for the subsequent stages. Thus, the coexisting CO2 and CO2H2O inclusions were entrapped at 510°C and 2.2 kbar, marking the waning of carbonic regime and the beginning of aqueous regime. At 330°C and 0.4 kbar, fluid unmixing occurred, leading to the simultaneous entrapment of mixed CO2H2O and H2O inclusions along rehealed microfractures. The data presented indicate that the metamorphic fluids evolved from early high-density carbonic through mixed carbonic-aqueous to late aqueous types. The dry granulite mineral assemblage of charnockites is a result of metamorphic equilibration under water-deficient and high-PCO2 conditions. 相似文献
20.
《International Geology Review》2012,54(2):112-133
Understanding Neoproterozoic crustal evolution is fundamental to reconstructing the Gondwana supercontinent, which was assembled at this time. Here we report evidence of Cryogenian crustal reworking in the Madurai Block of the Southern Granulite Terrane of India. The study focuses on a garnet-bearing granite–charnockite suite, where the granite shows in situ dehydration into patches and veins of incipient charnockite along the contact with charnockite. The granite also carries dismembered layers of Mg–Al-rich granulite. Micro-textural evidence for dehydration of granite in the presence of CO2-rich fluids includes the formation of orthopyroxene by the breakdown of biotite, neoblastic zircon growth in the dehydration zone, at around 870°C and 8 kbar. The zircon U–Pb ages suggest formation of the granite, charnockite, and incipient charnockite at 836 ± 73, 831 ± 31, and 772 ± 49 Ma, respectively. Negative zircon εHf (t) (?5 to ?20) values suggest that these rocks were derived from a reworked Palaeoproterozoic crustal source. Zircon grains in the Mg–Al-rich granulite record a spectrum of ages from ca. 2300 to ca. 500 Ma, suggesting multiple provenances ranging from Palaeoproterozoic to mid-Neoproterozoic, with neoblastic zircon growth during high-temperature metamorphism in the Cambrian. We propose that the garnet-bearing granite and charnockite reflect the crustal reworking of aluminous crustal material indicated by the presence of biotite + quartz + aluminosilicate inclusions in the garnet within the granite. This crustal source can be the Mg–Al-rich layers carried by the granite itself, which later experienced high-temperature regional metamorphism at ca. 550 Ma. Our model also envisages that the CO2 which dehydrated the garnet-bearing granite generating incipient charnockite was sourced from the proximal massive charnockite through advection. These Cryogenian crustal reworking events are related to prolonged tectonic activities prior to the final assembly of the Gondwana supercontinent. 相似文献