Hydrothermal Metamorphism Research Articles

Rochas ultramáficas plutônicas do greenstone belt Rio das Velhas na porção central do Quadrilátero Ferrífero, Minas Gerais, Brasil

Em Amarantina, distrito de Ouro Preto, encontram-se rochas ultramáficas expostas em duas áreas com cerca de 500 m² cada. As rochas afloram no Complexo do Bação, que é o embasamento gnáissico do greenstone belt Rio das Velhas, na porção central do Quadrilátero Ferrífero (QF). O interesse no estudo petrogenético desses corpos deve-se à preservação parcial de minerais ígneos, ausentes na maior parte das rochas ultramáficas totalmente metamorfizadas do QF. Entre essas rochas, destacam-se os esteatitos e os serpentinitos, devido a sua importância econômica. As rochas ultramáficas de Amarantina possuem textura equigranular, fato que as caracteriza como tendo origem plutônica, isto é, trata-se de metaperidotitos. Possuem grãos maiores de olivina, piroxênio e espinélio da rocha ígnea original distribuídos em matriz metamórfica fina com talco, serpentinas, cloritas, anfibólios e minerais opacos. Escassas arita (NiSbAs) e breithauptita (NiSb) foram formadas a partir de pentlandita durante o metamorfismo associado a hidrotermalismo. A comparação da composição química com a de um metakomatiito com textura spinifex do QF, bem como com rochas komatitiiticas de outras partes do mundo, mostra que os metaperidotitos são, quimicamente, semelhantes aos komatiitos não-desfalcados em alumínio. Portanto é provável que as rochas ultramáficas estudadas correspondam à porção plutônica do magmatismo komatitiitico do Grupo Nova Lima, que é a unidade basal do greenstone belt Rio das Velhas.

Mineral Compositions and Textures of Jadeite Orebody and its Country Rock in Nammaw, Myanmar

This paper studies the mineral compositions and textures of seven rock specimens from jadeite orebody and its country rock in Nammaw, Myanmar through XRD and observation of hand specimens and thin sections. The jadeite orebody is mainly composed of jadeite and minor zeolite minerals. Phlogopite schist and chromite-bearing amphibolite occur between the orebody and its country rock. The country rock is antigorite serpentinite. Outside of serpentinite is schist consisting of chlorite, hastingsite and polylithionite. The specimens of jadeite orebody show mainly following texture types: radiation texture, inequigranular crystalloblastic texture, granular-prismatic crystalloblastic texture, metasomatic texture and mylonitic texture. These textures indicate that the formation of the orebody is related to the intrusion of some fused mass or hydrothermal solution and then the orebody underwent dynamical metamorphism and hydrothermal metamorphism.

Low-Temperature Hydrothermal Metamorphic Mineralization of Island-Arc Volcanics, South Apuseni Mountains, Romania

AbstractThe island-arc volcanics situated in the eastern part of the Căpîlnaş-Techereu nappe (South Apuseni Mountains, Romania) were studied to evaluate the temperature, fluid properties, and mineral chemistry during low-temperature metamorphism. Detailed observations of metamorphic mineral assemblages were conducted using powder X-ray diffraction and electron microprobe. The metamorphism involved albitization of plagioclase feldspar and the formation of mafic phyllosilicates, zeolites, and other hydrous Ca-Al-silicate minerals. Mafic phyllosilicates consisted of transitional dioctahedral-trioctahedral smectites, mixed-layer chlorite-smectite (C/S, 6–96% chlorite), and discrete chlorite. The zeolites were analcime, stilbite ± stellerite, heulandite, laumontite, epistilbite, and mordenite. Also present, as secondary minerals filling amygdales and veins, are prehnite, pumpellyite, and secondary amphibole. Two mineral assemblages were identified which provide important information about metamorphic conditions (temperature, reaction progress, and fluid properties): (1) heulandite + analcime + quartz; and (2) laumontite + albite + quartz + prehnite + pumpellyite ± amphibole. The types of and relations between minerals in the first assemblage suggest the occurrence of low-temperature hydrothermal metamorphism in the zeolite facies at ~125°C, whereas the second assemblage was metamorphosed at 200°C. The composition and variability of the mineral assemblages in the study area suggest that, due to slow reaction rates, the low-temperature transformations and mineral assemblages were influenced not only by temperature but also by local rock composition, fluid-rock ratio, and fluid chemistry.

Fluid inclusion microthermometry for P–T constraints on normal displacement along the Median Tectonic Line in Northern Besshi area, Southwest Japan

AbstractThe Median Tectonic Line (MTL) is a first‐order tectonic boundary that separates the Sanbagawa and Ryoke metamorphic belts. Documented large‐scale top‐to‐the‐north normal displacements along this fault zone have the potential to contribute to the exhumation of the Sanbagawa high‐pressure metamorphic belt. Fluid inclusion analyses of vein material formed associated with secondary faults within the Sanbagawa belt affected by movement on the MTL show normal movement was initially induced under temperatures greater than around 250°C. Microstructures of quartz and K‐feldspar comprising the vein material suggest a deformation temperature of around 300°C, supporting the results of fluid inclusion analyses and suggesting deformation at depths of around 10 km. The retrograde P–T path of the Sanbagawa metamorphic rocks and the estimated isochore of the fluid inclusions do not intersect. The semi‐ductile structures of surrounding rocks and lack of evidence for hydrothermal metamorphism around the veins imply the temperature of the rocks was similar to that of the fluid. These observations suggest fluid pressure of the veins was lower than lithostatic pressure close to the MTL.

Deformation and hydrothermal metamorphism of gabbroic rocks within the Godzilla Megamullion, Parece Vela Basin, Philippine Sea

Microstructural and petrologic analyses of 7 gabbroic rocks sampled from the medial area of the Godzilla Megamullion (site KH07-02-D18), located along the Parece Vela Basin spreading ridge (Parece Vela Rift), Philippine Sea, reveal the development of a high-temperature ductile shear zone associated with hydrothermal metamorphism in the lower crust. The deformed gabbroic rocks are petrographically classified into mylonites and an ultramylonite, and are characterized by porphyroclastic textures consisting mainly of coarse plagioclase and clinopyroxene/amphibole porphyroclasts in a fine-grained matrix. Plagioclase crystallographic-preferred orientations vary from (010)[100] and (001)[100] patterns in the mylonites to a weak (001)[100] pattern in the some mylonites and ultramylonite, suggesting a change in the deformation mechanism from dislocation creep to grain-size-sensitive creep with increasing intensity of deformation. The chemical composition of matrix plagioclase is generally more sodic than that of porphyroclasts. Secondary amphibole is ubiquitous, consisting mainly of pargasite and magnesiohornblende (brown hornblende) and actinolite (green hornblende). The mineral assemblage is consistent with the hydrothermal metamorphic reaction: clinopyroxene + calcic plagioclase + fluid → amphibole + sodic plagioclase. Compared with deformed gabbroic rocks from the breakaway and termination areas of the Godzilla Megamullion, the samples record ductile shearing under high temperature conditions, possibly related to the development of the Godzilla Megamullion, although hydrothermal activity in the medial area appears to have been less intense than in both the breakaway and termination areas.

Criteria for recognition of localization and timing of multiple events of hydrothermal alteration in sandstones illustrated by petrographic, fluid inclusion, and isotopic analysis of the Tera Group, Northern Spain

Stratigraphic relations, detailed petrography, microthermometry of fluid inclusions, and fine-scale isotopic analysis of diagenetic phases indicate a complex thermal history in Tithonian fluvial sandstones and lacustrine limestones of the Tera Group (North Spain). Two different thermal events have been recognized and characterized, which are likely associated with hydrothermal events that affected the Cameros Basin during the mid-Cretaceous and the Eocene. Multiple stages of quartz cementation were identified using scanning electron microscope cathodoluminescence on sandstones and fracture fills. Primary fluid inclusions reveal homogenization temperatures (Th) from 195 to 350°C in the quartz cements of extensional fracture fillings. The high variability of Th data in each particular fluid inclusion assemblage is related to natural reequilibration of the fluid inclusions, probably due to Cretaceous hydrothermal metamorphism. Some secondary fluid inclusion assemblages show very consistent data (Th = 281–305°C) and are considered not to have reequilibrated. They are likely related to an Eocene hydrothermal event or to a retrograde stage of the Cretaceous hydrothermalism. This approach shows how multiple thermal events can be discriminated. A very steep thermal gradient of 97–214°C/km can be deduced from δ18O values of ferroan calcites (δ18O −14.2/−11.8‰ V-PDB) that postdate quartz cements in fracture fillings. Furthermore, illite crystallinity data (anchizone–epizone boundary) are out of equilibrium with high fluid inclusion Th. These observations are consistent with heat-flux related to short-lived events of hydrothermal alteration focused by permeability contrasts, rather than to regional heat-flux associated with dynamo-thermal metamorphism. These results illustrate how thermal data from fracture systems can yield thermal histories markedly different from host-rock values, a finding indicative of hydrothermal fluid flow.

Deformation structures associated with the Tazo landslide (La Gomera, Canary Islands)

Deformation structures below the basal plane of gravitational slides can provide useful information about the state of stress undergone by rocks prior to the sliding process and about the triggering forces acting at each particular sliding event. In the present work we conducted a structural analysis of the rocks below the surface of the gravitational slide of Tazo (La Gomera, Canary Islands) and determined the epigenetic processes involved in the filling of the amphitheatre. We also inferred the possible triggering phenomena related to the Tazo landslide. The rocks located below the surface of the gravitational slide of Tazo -i.e., the basaltic lava flows, sills and dikes of the Lower Old Edifice and the submarine volcanic rocks, gabbros, pyroxenites and dikes of the Basal Complex of La Gomera- are strongly deformed close to this sliding surface. The lava flows and dikes of the Lower Old Edifice are folded, with fault breccias and gouges, and locally foliated, defining the sliding surface. The dikes of the Basal Complex are also folded, and the gabbros and pyroxenites are affected by a large number of small faults. In the Basal Complex, the sliding surface is defined by a foliated granular gouge. In the damage zone, the Basal Complex rocks show an incipient fracture cleavage. The sliding amphitheatre has been filled by the debris avalanche or cohesive debris flow generated within the slide, as well as by later debris flows, hyperconcentrated flows, sheet flows, and by interspersed lava flows from the Upper Old Edifice. We suggest here that the collapse of the north-western flank of the Lower Old Edifice at Tazo could in part have been triggered by continuous magma injection, associated with the emplacement of dikes in a rift zone with an ENE-WSW direction, enhanced by the mechanical weakness of the Basal Complex unit, which was affected by hydrothermal metamorphism under greenschist facies conditions and by the displacement along the Montana de Alcala and Guillama normal faults, which are deeply entrenched in the altered rocks of the Basal Complex.

Lithofacies and reservoir properties of Tertiary igneous rocks in Qikou Depression, East China

SUMMARY Tertiary igneous rocks in Qikou Depression, East China are primarily basic rocks. Two types of igneous reservoirs including the effusive basaltic reservoir and the near-surface intrusive doleritic reservoir have been identified so far and each has different reservoir spaces. In this paper, the lithofacies and reservoir properties of Tertiary igneous rocks are described through a systematic analysis of the characteristics of seismic reflection and well logs and selected cores observation. Both of the two types of igneous rocks present strong and low-frequency reflection events on seismic profiles, however, intrusive diabases exhibit the characteristic of strata penetration with a small intersection angle. Effusive basaltic lavas are divided into upper, middle and lower subfacies. This can be distinguished from the well log curves. Observation of sampled cores finds that primary vesicles and amygdales are abundant in the upper and lower subfacies of basaltic lavas. These vesicles can be favourable reservoir space if connected by cracks formed in the late-stage. Intrusive diabases can also be separated into inner subfacies as well as the upper and lower marginal subfacies, of which the former is well crystallized because of slowly cooling, whereas the latter is vitreous due to fast descending temperature at the contact zone between intrusive rocks and host rocks. Hydrothermal metamorphism left the surrounding mudstones metamorphosed to form shrunk fissures and structural fractures as reservoir space, making mudstones of poor porosity and permeability to be metamorphic and have certain reservoir potential.

Growth of bultfonteinite and hydrogarnet in metasomatized basalt xenoliths in the B/K9 kimberlite, Damtshaa, Botswana: insights into hydrothermal metamorphism in kimberlite pipes

Metamorphic assemblages within Karoo basalt xenoliths, found within volcaniclastic kimberlite of the B/K9 pipe, Damtshaa, Botswana, constrain conditions of kimberlite alteration. Bultfonteinite and chlorite partially replace the original augite-plagioclase assemblage, driven by the serpentinisation of the kimberlite creating strong chemical potential gradients for Si and Mg. Hydrogarnet and serpentine replace these earlier metamorphic assemblages as the deposits cool. The bultfonteinite (ideally Ca2SiO2[OH,F]4) and hydrogarnet assemblages require a water-rich fluid containing F−, and imply hydrothermal alteration dominated by external fluids rather than autometamorphism from deuteric fluids. Bultfonteinite and hydrogarnet are estimated to form at temperatures of ca. 350–250°C, which are similar to those for serpentinisation. Alteration within the B/K9 kimberlite predominantly occurs between 250 and 400°C. We attribute these conditions to increased efficiency of mass transfer and chemical reactions below the critical point of water and a consequence of volume-increasing serpentinisation and metasomatic reactions that take place over this temperature range. A comparison of the B/K9 kimberlite with kimberlites from Venetia, South Africa suggests that the composition and mineralogy of included xenoliths affects the alteration assemblages within kimberlite deposits.

Middle Archean ocean ridge hydrothermal metamorphism and alteration recorded in the Cleaverville area, Pilbara Craton, Western Australia

AbstractA hydrothermally metamorphosed greenstone complex, capped by bedded cherts and banded iron formations (BIFs), is exposed in the Cleaverville area, Pilbara Craton, Western Australia. It has been interpreted as an accretionary complex characterized by both a duplex structure and an oceanic plate stratigraphy, and is shown to represent a 3.2 Ga upper oceanic crust. Three metamorphic zones are identified in the basaltic greenstones. The metamorphic grade increases from sub‐greenschist facies (zones A and B) to greenschist facies (zone C) under low‐pressure conditions. The boundaries between three mineral zones are subparallel to the bedding plane of overlying chert/BIF, and metamorphic temperature increases stratigraphically downward. The zones correspond to the thermal structure of ocean‐floor metamorphism, at a mid‐ocean ridge.The uppermost greenstone in the study area is more pervasively altered and carbonatized than the modern upper oceanic crust. This indicates the enrichment of CO2 in the metamorphic fluid by which widespread formation of carbonate occurred, compared with a narrow stability region of Ca‐Al silicates. It is, therefore, suggested that the Archean hydrothermal alteration played a more important role in fixation of CO2 than present‐day ocean‐ridge hydrothermal alteration, as an interaction between sea water and oceanic crust.

Exothermic and endothermic soils of Iceland

This paper is based on the results of the study of soils and soil-forming rocks on five key sites in Iceland, including the coastal plain (the Hvammur key site), piedmont plain (the Hveragerði key site), the Holocene lava plateau (the Reykjanes key site), and the zone of modern hydrothermal activity in the north of the rift zone of Iceland (the Theistareykir and Namafjall key sites). The studied soils are subdivided by us into the groups of exothermic and endothermic soils. Exothermic soils are the soils that develop from the congealed volcanic deposits. These soils are specified by the homogeneous dark gray color and sandy texture. Their mineral components are weakly transformed. Exothermic soils developed under good drainage conditions have the low content of organic matter (about 1.5% Corg). The soil reaction is neutral within the entire profile. Under impeded drainage conditions, the organic carbon content in the exothermic soils reaches its maximum of about 7%, and the soil reaction varies from acid to neutral values. Endothermic soils are the soils that are subjected at present or were subjected in the past to the high-temperature hydrothermal metamorphism. They have mottled color patterns with sharp color contrast in the soil profile; their texture is loamy or clayey. These soils are rich in secondary minerals with a predominance of smectite; kaolinite, pyrite, anatase, gypsum, and other secondary minerals can also be found in them. The properties of endothermic soils are largely controlled by the provincial features of the hydrothermal activity, including the chemical composition of hydrothermal water, the soil temperature, the duration of hydrothermal activity, etc.

Trace element heterogeneity in hydrothermal diopside: evidence for Ti depletion and Sr-Eu-LREE enrichment during hydrothermal metamorphism of mantle harzburgite

We discovered a new kind of dyke in the mantle section of the Oman ophiolite composed mainly of diopside that we called diopsidite, although traces of forsterite, anorthite, titanite or andradite may be also observed. The texture and chemical composition, and the relationship between antigorite and diopside or rare forsterite, suggest that they formed by high-T metamorphism of serpentine (antigorite) and are the result of interaction between hydrothermal fluid and percolating magma in the shallow mantle. Chemical data, obtained by electron microprobe and in situ laser ablation coupled to ICP-MS, show large heterogeneities from one sample to another and between diopside crystals in each sample. The trace element patterns are characterised by strong positive anomalies in Sr and Eu and a negative anomaly in Ti. Small-scale heterogeneities reflect the disequilibrium conditions in the dyke formation and show that a relatively Ti-rich- and Al-Eu-Sr-LREE-poor phase may be included in the diopside. The exact nature of this phase is difficult to determine but the Eu negative anomaly, and the low Sr and LREE concentration suggest that it is probably some partially altered, relict protolith mineral.

PLAGIOGRANITIC ROCKS OF EVROS OPHIOLITE, NE GREECE

Dykes or small stocks of plagiogranitic rocL· occur below the extrusive sequence and in mutually interpenetrating association with the sheeted dyke complex of the Evros Ophiolite, NE Greece. They are classified as tonalités, low silica trondhjemites (LST) and high silica trondhjemites (HST). Pétrographie and geochemical data suggest they resemble oceanic plagiogranites ofSSZ origin. Their normalized rock/ORG diagrams reveal ORG compatible element values, slightly depleted relative to ORG incompatible elements and Ba, Ta and Nb negative anomalies. Plagiogranites also show subparallel, relatively flat REE patterns with variable Eu anomaly. As such geochemical features are also found in the dacitic to rhyodacitic lavas of Evros Ophiolite, it is assumed that plagiogranitic melts, especially of LST composition, presumably fed them. As a first approximation to plagiogranites origin, it is suggested tonalité and HST could have been generated by 5-15 % partial melting of oceanic gabbros, whereas LST may possibly derive by fractional crystallization of a MORB type source. In a later stage, intense hydrothermal metamorphism affected the plagiogranites causing formation of peculiar epidositic spherical clots and veinlets

Hydroschorlomite in altered basalts from Hole 1256D, ODP Leg 206: The transition from low‐temperature to hydrothermal alteration

Hydroschorlomite, a Ti‐, Ca‐, Fe‐rich andraditic garnet present in the deepest cores of basalts (661–749 mbsf) drilled in Hole 1256D during Ocean Drilling Program (ODP) Leg 206 (equatorial east Pacific), is reported here for the first time in oceanic crust. Detailed petrological and mineralogical studies by optical microscope, electron microprobe, scanning and transmission electron microscope, and micro‐Raman spectroscopy are used to characterize this hydrogarnet and its relationships with other minerals. Hydroschorlomite occurs in Hole 1256D as small (5–50 μm) anhedral or euhedral crystals associated either with celadonite in black halos adjacent to celadonite veins or with brown saponitic phyllosilicate in brown alteration halos adjacent to veins of saponite and iron oxyhydroxides. Both types of halos are formed at low temperature (less than about 100°C). Textural observations suggest that hydroschorlomite formation is contemporaneous with the phyllosilicates. Hydroschorlomite is rich in CaO (22.5–26.5 wt%), TiO2 (22.0–28.6 wt%), and FeOt (6.2–12.9 wt%) and contains significant F (up to 0.85 wt%) and Zr2O3 (up to 0.34 wt%). The presence of OH suggested by the low total percentages of oxides (95.2–97.3 wt%) is confirmed by the OH vibration at 3557 cm−1 in the micro‐Raman spectrum. Chemical mapping indicates that hydroschorlomite is not zoned and is always associated with either celadonitic or saponitic phyllosilicates. Some hydroschorlomite crystals partly include tiny (<10 μm) skeletal titanomagnetite. The occurrence of hydroschorlomite in Hole 1256D basalts coincides with a general downward increase in temperatures and overall intensity of alteration manifest by the alteration of plagioclase and the occurrence of small amounts of mixed‐layer chlorite‐smectite. The titanium necessary to form hydroschorlomite is provided by the breakdown of primary tiny (<10 μm) titanomagnetite, while calcium is provided by the replacement of plagioclase by albite. Hydroschorlomite is thus an indicator of alteration of titanomagnetite under conditions transitional from low‐temperature alteration to hydrothermal metamorphism with formation of titanite and may affect magnetic properties of the rocks.

Erratum

Journal of Metamorphic GeologyVolume 24, Issue 8 p. 779-779 Free Access Erratum First published: 13 October 2006 https://doi.org/10.1111/j.1525-1314.2006.00675.xAboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Stripp, G. R., Field, M., Schumacher, J. C., Sparks, R. S. J. & Cressey, G., 2006. Post-emplacement serpentinization and related hydrothermal metamorphism in a kimberlite from Venetia, South Africa. Journal of Metamorphic Geology, 24, 515–534 Two corrections were missed from the final published version of the article. They are given below. The publisher apologises for any confusion caused. On page 515 in the Abstract, 4th line from the bottom, should read “380 °C” rather than “370 °C”. On page 527, in the footnote to Table 2, the four sets of square brackets should have been removed, so that it now reads as follows: Weight % H2O is adjusted so that OH = 4. Totals include H2O and mineral formulas are calculated for seven ideal oxygen atoms and five cations. GI is best described as serpentine with a large interlayer smectite component. The XRD analysis of GI indicated this material is composed of serpentine and smectite. The proportions were estimated by assuming that the principal constituents are ideal end member lizardite ((Mg, Fe)3Si2O5(OH)4) and ideal trioctahedral smectite ((Mg,Fe)6-Si8O20(OH)4nH2O) with partial substitution along the following coupled-substitution vectors: (K,Na)AlIV = SiIV,CaAlIV AlIV = SiIV SiIV and AlVI AlIV = MgVI SiIV. Two estimates of the proportions of lizardite to smectite were made; one based on the amount of tetrahedral cations and one based on the sum of octahedral + tetrahedral cations. As the cation proportions are based on seven oxygens, the number of tetrahedral cations will vary between 2 (lizardite) and 2.545 (smectite based on seven oxygen) and the sum of octahedral + tetrahedral cations will vary between 5 (lizardite) and 4.455 (smectite based on 7 oxygen). The tetrahedral cations = Si + K + Na + 2 * Ca + (Al + Cr ) (K + Na + 2 * Ca)/2; the sum of octahedral + tetrahedral cations = Total cations based on seven oxygen–K–Na–Ca. The lever rule can be applied to obtain the proportions of lizardite and smectite. Averages were given and variations are less than ± 6%. These are only estimates. Reference StrippG. R., FieldM., SchumacherJ. C., SparksR. S. J., Cressey, G., 2006. Post-emplacement serpentinization and related hydrothermal metamorphism in a kimberlite from Venetia, South Africa. Journal of Metamorphic Geology, 24, 515– 534. Wiley Online LibraryCASADSWeb of Science®Google Scholar Volume24, Issue8October 2006Pages 779-779 ReferencesRelatedInformation

Post‐emplacement serpentinization and related hydrothermal metamorphism in a kimberlite from Venetia, South Africa

AbstractThe common serpentine–diopside matrix assemblage in volcaniclastic kimberlite (VK) at the Venetia Mine, South Africa is ascribed to a secondary origin, because of post‐emplacement serpentinization and associated hydrothermal metamorphism. Volcaniclastic deposits with 20–30% porosity infill kimberlite pipes in the waning stages of kimberlite eruptions. Olivine macrocrysts are typically rimmed by talc and are pseudomorphed by lizardite, with minor magnetite. The fine matrix consists of mixtures of lizardite, chlorite, smectite, brucite, calcite, titanite and andradite, an assemblage which either pseudomorphed microcrysts or in‐filled voids. Locally we recognize microcryst pseudomorphs rich in sub‐microscopic mixtures of lizardite with smectite, and other microcryst pseudomorphs and void‐filling matrix rich in chlorite and lizardite. Interstitial lizardite and associated phyllosilicates (brucite, smectite and chlorite) crystallized progressively from meteoric or hydrothermally derived pore waters, and Si4+ and Mg2+ released into the fluid phase during serpentinization of olivine macrocrysts. Radial‐fibrous fringes of diopside microlites around crystals display void‐filling textures because of unrestricted growth into pore spaces. Secondary diopside is attributed to Si4+, Mg2+ and Ca2+ cations released into the fluid phase by interaction with olivine, calcite and plagioclase in siliceous xenoliths. The paucity of primary, fine‐grained groundmass phases resistant to alteration, for example, perovskite and spinel, precludes an origin for the intergrain matrix as altered interstitial ash, glass or a late‐stage kimberlite melt. Isovolumetric replacement of olivine results in a volume increase of 60% so that pore spaces in the original deposit can be easily filled up with serpentine. The source of Al3+ to form chlorite and smectite is attributed to alteration of plagioclase in xenoliths which comprise 20–30 vol.% of the deposit. Titanite, hydro‐andradite and second‐generation diopside precipitate as hydrothermal minerals from calcium‐bearing serpentinizing fluids in replacement reactions and as void‐filling minerals. Consideration of mineral equilibria in the CaO‐MgO‐SiO2‐H2O‐CO2 system constrains the common matrix assemblage of lizardite and diopside in XCO2)–T space. At 300 bar, the assemblage is stable only at temperatures below 370 °C and XCO2 < 0.01. This upper limit on temperature is well below the plausible solidus of ultrabasic magmas. Furthermore, the requirement of trace CO2 in the fluid phase implies a post‐emplacement external source rather than ‘autometamorphism’ from kimberlite‐derived fluids, because of high PCO2 commonly inferred for kimberlite magmas.

Dynamical constraints on kimberlite volcanism

Kimberlite volcanism involves the ascent of low viscosity (0.1 to 1 Pa s) and volatile-rich (CO 2 and H 2O) ultrabasic magmas from depths of 150 km or greater. Theoretical models and empirical evidence suggest ascent along narrow (∼1 m) dykes at speeds in the range > 4 to 20 m/s. With typical dyke breadths of 1 to 10 km, magma supply rates are estimated in the range 10 2 to 10 5 m 3/s with eruption durations of many hours to months. Based on observations, theory and experiments we propose a four-stage model for kimberlite eruptions to explain the main geological relationships of kimberlites. In stage I magma reaches the Earth's surface along fissures and erupts explosively due to their high volatile content. The early flow exit conditions are overpressured with choked flow conditions; an exit velocity of ∼200 m/s is estimated as representative. Explosive expansion and near surface overpressures initiate crater and pipe formation from the top downwards. In stage II under-pressures (the difference between the lithostatic pressure and pressure of the erupting mixture) develop within the evolving pipe causing rock bursting at depth, undermining overlying rocks and causing down-faulting and crater rim slumping. Rocks falling into the pipe interior are ejected by the strong explosive flows. Stage II is the erosive stage of pipe formation. As the pipe widens and deepens larger under-pressures develop enhancing pipe wall instability. A critical threshold is reached when the exit pressure falls to one atmosphere. As the pipe widens and deepens further the gas exit velocity declines and ejecta becomes trapped within the pipe, initiating stage III. A fluidised bed of pyroclasts develops within the pipe as the eruption wanes to form typical massive volcaniclastic kimberlite. Marginal breccias represent the transition between stages II and III. After the eruption stage IV is a period of hydrothermal metamorphism (principally serpentinisation) and alteration as the pipe cools and meteoric waters infiltrate the hot pipe fill. Following an eruption an open crater can be filled by kimberlite- and country-rock derived sediments, forming the crater-facies.

The role of fluidisation in the formation of volcaniclastic kimberlite: Grain size observations and experimental investigation

Deep, steep-sided kimberlite pipes in southern Africa are normally partially filled with multiple units of a characteristically massive, structureless volcaniclastic rock, here termed massive volcaniclastic kimberlite (MVK). These units can be arranged in a pseudo-concentric, nested ‘pipes-within-pipes’ structure. Several key observations on typical MVK from the Venetia kimberlite pipes, South Africa, indicate that fluidisation may have been an important process during its emplacement. These include: (1) the thorough mixing together of juvenile components and of lithic components derived from all stratigraphic levels of the country rock and; (2) a matrix composed of void-filling minerals related to hydrothermal metamorphism (mainly diopside and serpentine), which indicates that the primary deposit had a high porosity (> 50%); and (3) particle size distributions poor in fine ash (< 0.0625 mm) and coarse lithic fragments (> 10 cm). Analogue experiments utilising a sand bed laced with marker horizons were conducted in order to investigate the role of fluidisation in the formation of MVK. High-pressure gas, fed through the bed from a point source, resulted in the formation of an upwardly diverging pipe structure of thoroughly mixed fluidised material bounded by undisturbed sand. Strong upward gas flow along the centre of the pipe is characteristically bubbly and results in internal circulation with downward moving regions at the pipe margins. Analysis of the experimental results suggests that the gas velocity decreases linearly with height and has a Gaussian distribution of vertical velocity in the cross-flow direction. A decrease in the gas flow rate resulted in the development of nested pipes reminiscent of the ‘pipes-within-pipes’ structures in kimberlite pipes. The results suggest that fluidisation could be a major control on the structure and sorting of deposits in kimberlite pipes. We propose that the fluidisation of particles in a kimberlite pipe occurs during the final stages of the eruption when erupting pyroclasts and incorporated fragmented lithic material can no longer be ejected out of the deep, wide conduit. This bed is subject to quasi-sustained fluidisation by the erupting gas-particle flow, which imparts on it the distinctive characteristics of a massive well-mixed, poorly sorted deposit.

A Variscan slow-spreading ridge (MOR-LHOT) in Limousin (French Massif Central): magmatic evolution and tectonic setting inferred from mineral chemistry

AbstractThe Limousin ophiolite (French Massif Central) occurs as elongate bodies forming a (nearly) continuous suture zone between two major lithotectonic units of the French Variscan belt. The mantle section of the ophiolite is made of diopside-bearing harzburgite, harzburgite and dunite characteristic of a lherzolite-harzburgite ophiolite type (LHOT). The plutonic section is essentially composed of troctolites, wehrlites and gabbros locally intruded by ilmenite-rich mafic dykes. All the rocks were strongly affected by an ocean-floor hydrothermal metamorphism. The composition and evolution of primary magmatic phases (olivine, clinopyroxene, plagioclase and spinel) throughout the lowermost magmatic sequence correspond to those described in oceanic cumulates (ODP data). The Limousin ophiolite is thus of MOR type instead of SSZ type. The whole lithological section, the mineral chemistry, the extensive hydrothermal oceanic alteration and the relatively thin crustal section are typical of a slow-spreading ridge ocean (i.e. Mid-Atlantic ridge). Comparison of the Limousin ophiolite with other ophiolites from European Variscides suggests that the oceanic domain was actively spreading during the Late Palaeozoic and extended from the Armorican massif to the Polish Sudetes.

ALTERAÇÃO METAMÓRFICO-HIDROTERMAL NOS DEPÓSITOS DE OURO TIPO-VEIO DE PORTO NACIONAL, TO, BRASIL: EVIDÊNCIAS DE QUÍMICA MINERAL E MICROESTRUTURAS NA ZONA DE CISALHAMENTO CACHIMBO

The Cachimbo shear zone (CSZ) is part of the Transbrasiliano lineament (LTB), Tocantins tectonic province, in West-Central Brazil. The CSZ has mylonites and phyllonites, which host gold deposits of quartzvein-type. The hostrocks are upper amphibolite facies metagranites of the Manducasuite (MS) composed of quartz, orthoclase, plagioclase, whitemica, and Fe chlorite. Based on mineral chemistry and microstructures, it is possible to characterize the physico-chemical conditions of shear deformation and metamorphic hydrothermal alteration in the CSZ. The first stage of shearing is marked by phyllic alteration and generation of early myllonites composed of muscovite-ferriphengite, chlorite, monazite and zircon. The metamorphic-hydrothermal events generate two distinct phyllonites: (a) phengite-gamet phyllonite, with an assemblage of quartz-gamet-chlorite-muscovite/phengite-tourmaline; and (b) paragonite-staurolite phyllonite, with an assemblage of quartz-staurolite-chlorite-paragonite-tourmaline ±calcite ±gamet. The paragonite-staurolite phyllonite occurs in more aluminous sites and the phengite-garnet phyllonite in more magnesian sites. Tourmaline inclusions in staurolite porphyroblasts sub-parallel to the main foliation characterize the porphyroblasts generated synkinematic to the phyllonite foliation. Gamet and staurolite indicate amphibolite facies conditions in the staurolite zone for the hydrothermal metamorphism. The paragonite-staurolite phyllonite and the phengite-garnet phyllonite evolve to the phengitephyllonite, with a simpler assemblage of chlorite-muscovite/phengite-pyrite and chalcopyrite. Calcite±pyrite±chalcopyrite veinlets and monomyneralie pyrite veinlets cross-cut the phyllonites. Graphite phyllonite marks the potential mineralized zone, with an assemblage of quartz-graphite-pyrite±gold. Graphite phyllonites host the mineralized quartz vein system with an assemblage of quartz±graphite±pyrite±chalcopyrite±gold. The high crystallinity of graphiteis similar to the spectrum of amphibolite facies, staurolite zone conditions. It seems to demonstrate the persistence of hydrothermal-metamorphic conditions during the CSZ evolution. The low oxygen fugacity for the graphite formation, together with pyrite mineral, indicates a reduced environment for gold precipitation.