Internally consistent gahnitic spinel-cordierite-garnet equilibria in the FMASHZn system: geothermobarometry and applications

The equilibrium (Mg, Fe, Zn)₃Al₂Si₃O₁₂+2Al₂SiO₅=3(Mg, Fe, Zn)Al₂O₄+5SiO₂ garnet + sillimanite/kyanitc = spinel + quartz was calibrated in the piston-cylinder apparatus between 11 and 30 kbar, and over the temperature range of 950 to 1200°C. Three experimental mixes of Mg no. [100^*MgO/(MgO+FeO)] 40, 47 and 60, in the FeO –MgO–Al₂O₃–SiO₂–ZnO (FMASZn) system were used under low oxygen fugacities and anhydrous conditions. We derive a ternary Fe–Mg–Zn symmetric mixing model for aluminous spinels in equilibrium with garnet, to quantify the increase in gahnitic end-member of spinel with increasing pressure and descreasing temperature. Further experiments in the spinel-cordieritequartz-sillimanite field were combined with garnet-cordierite data from the literature to produce a consistent set of equations describing the exchange reactions in FMASHZn relevant to quartz-sillimanite bearing rocks at granulite facies conditions. As spinel is an important mineral participating in many rocks of aluminous composition at granulite-facies conditions, and as zinc contributes to an enlargement of spinel's stability field towards higher pressures and lower temperatures, the thermobarometric calibrations presented here will be most significant in delineating the prograde and retrograde trajectory of P-T paths.     

Ultrahigh-pressure metamorphism and exhumation of garnet peridotite in Pohorje, Eastern Alps

New evidence for ultrahigh‐pressure metamorphism (UHPM) in the Eastern Alps is reported from garnet‐bearing ultramafic rocks from the Pohorje Mountains in Slovenia. The garnet peridotites are closely associated with UHP kyanite eclogites. These rocks belong to the Lower Central Austroalpine basement unit of the Eastern Alps, exposed in the proximity of the Periadriatic fault. Ultramafic rocks have experienced a complex metamorphic history. On the basis of petrochemical data, garnet peridotites could have been derived from depleted mantle rocks that were subsequently metasomatized by melts and/or fluids either in the plagioclase‐peridotite or the spinel‐peridotite field. At least four stages of recrystallization have been identified in the garnet peridotites based on an analysis of reaction textures and mineral compositions. Stage I was most probably a spinel peridotite stage, as inferred from the presence of chromian spinel and aluminous pyroxenes. Stage II is a UHPM stage defined by the assemblage garnet + olivine + low‐Al orthopyroxene + clinopyroxene + Cr‐spinel. Garnet formed as exsolutions from clinopyroxene, coronas around Cr‐spinel, and porphyroblasts. Stage III is a decompression stage, manifested by the formation of kelyphitic rims of high‐Al orthopyroxene, aluminous spinel, diopside and pargasitic hornblende replacing garnet. Stage IV is represented by the formation of tremolitic amphibole, chlorite, serpentine and talc. Geothermobarometric calculations using (i) garnet‐olivine and garnet‐orthopyroxene Fe‐Mg exchange thermometers and (ii) the Al‐in‐orthopyroxene barometer indicate that the peak of metamorphism (stage II) occurred at conditions of around 900 °C and 4 GPa. These results suggest that garnet peridotites in the Pohorje Mountains experienced UHPM during the Cretaceous orogeny. We propose that UHPM resulted from deep subduction of continental crust, which incorporated mantle peridotites from the upper plate, in an intracontinental subduction zone. Sinking of the overlying mantle and lower crustal wedge into the asthenosphere (slab extraction) caused the main stage of unroofing of the UHP rocks during the Upper Cretaceous. Final exhumation was achieved by Miocene extensional core complex formation.     

Dehydration melting and the granulite transition in metapelites from southern Namaqualand,S. Africa

In a prograde amphibolite-granulite transition zone in the Namaqualand Metamorphic Complex, metapelites show an interbanding of the amphibolite facies association biotite+sillimanite+quartz with the granulite facies association garnet+cordierite+K-feldspar. Relict graded bedding shows that compositional banding is of sedimentary origin. The garnet-cordierite-K-feldspar gneisses contain quartzofeldspathic segregations surrounding garnets, and have more Fe-rich bulk compositions than the biotite-sillimanite schists.The contrasting asemblages could have formed at the same pressure and temperature provided that a(H₂O) was systematically lower in the garnet-cordierite-K-feldspar layers. The a(H₂O) reduction resulted from the production of silicate melt by a vapour-absent continuous Fe-Mg reaction such as biotite+sillimanite+quartz=garnet+K-feldspar+liquid which affects Fe-rich compositions before vapour-absent melting occurs in more Mg-rich rocks. The segregations represent the solid and liquid products of the reaction.Such processes imply local control of a(H₂O), and indicate that this granulite transition did not result from a regional influx of metasomatising fluids.     

Multiple Foliations Defined by Different Sillimanite Crystal Habits,Partial Melting and the Late Metamorphic Development of the Cannington Ag-Pb-Zn Deposit,Northeast Australia

A succession of foliations defined by different sillimanite-bearing structural fabrics suggests that the macroscopic, isoclinal synform that dominates the geometry of the Cannington Ag-Pb-Zn deposit, northwest Queensland, Australia formed during D₂. The five foliations in this succession, S₁ to S₅, are defined by aligned sillimanite with habits ranging from individual crystals in S₁ through S₄ to clusters of fibrolite in S₅ in both the matrix and as inclusion trails within garnet and gahnite (Zn-rich spinel) porphyroblasts. S₁, S₃ and S₅ formed as sub-horizontal foliations, whereas S_1a, S₂ and S₄ formed sub-vertically. Foliation intersection/inflexion axes (FIAs) within garnet and gahnite porphyroblasts preserve a succession of trends that shifts from W-E to N-S. This succession indicates that this region experienced N-S followed by W-E bulk crustal shortening. N-S shortening occurred during D₁and D_1a, and W-E shortening occurred from D₂ to D₅. Prismatic and rhombic sillimanite produced during D₁-D₄ accompanied prograde metamorphism to ca. 634 62°C and 4.8 1.3 kbar. The coexistence of fibrous, prismatic and rhombic sillimanite resulted from post peak metamorphic reactivation of the early foliations during D₅. The synformal D₂ fold was intensified during D₄ by W-E bulk shortening. Local partial melting occurred after D₁ in the appropriate bulk rock compositions, based on calculation of P-T pseudosections in the chemical systems KFMASH, KFMASHTO, NCKFMASH and MnNCKFMASH. Zn mineralization related to gahnite growth occurred during D₃ through D₄, and was redistributed by partial melting into structural and rheological sites during D₄ and D₅shearing.     

Spinel-anthophyllite rocks of the Kokchetav massif (northern Kazakhstan)

Spinel-anthophyllite rocks that may be classified as ultrabasic low-Ca spinel amphibolites have been first discovered in the Kokchetav collision zone (northern Kazakhstan). They outcrop 2 km west of Enbek-Berlyk Village among schists and quartzites and are closely associated with spinel harzburgites and garnet pyroxenites. The main hosted minerals are spinel (hercynite) and anthophyllite. The rocks bear magnetite-hornblende-spinel-anthophyllite pseudomorphs with rounded and polygonal sections, which might have been resulted from the replacement of garnet grains. The prismatic anthophyllite crystals and scarce olivine relics contain elongate parallel spinel inclusions resembling spinel-olivine syntactic intergrowths in the Enbek-Berlyk spinel harzburgites. The spinel-anthophyllite rocks are similar to the associated spinel harzburgites in CaO, MnO, SiO₂, and Al₂O₃ contents but are richer in FeO and poorer in MgO (F = FeO/(FeO + MgO) = 57% against 35% in the harzburgites). Geological, mineralogical, and geochemical data suggest that the spinel-anthophyllite rocks formed during the isochemical contact metamorphism of garnet-bearing spinel harzburgites, which contained more FeO and less MgO than garnet-free harzburgites of the same area. Variations in FeO and MgO contents in both types of harzburgites seem to be due to different chemical compositions of the chlorite protoliths of these rocks.     

Gahnite in the metamorphosed stratiform massive sulfide deposits of the Mineral District,Virginia, U.S.A.

Summary Ferroan-gahnite, (Zn,Fe)Al₂O₄, is a common accessory mineral found in association with the metamorphosed volcanogenic massive sulfide ore bodies of the Mineral District, Virginia. Gahnite is present throughout the mineralized zones that contain pyrite, sphalerite, chalcopyrite, and sulfosalts. Although constituting only 1–2 modal percent of most samples and displaying not well-defined distribution pattern, gahnite is especially abundant at the margins of the mineralized zones and locally constitutes as much as 35 modal percent. The compositions vary slightly from one deposit to another but all lie within the range of 68 to 88 mol.% gahnite (ZnAl₂O₄), 8 to 25 mol.% hercynite (FeAl₂O₄), and 1–17 mol.% spinel (MgAl₂O₄). Gahnite occurs within a wide variety of mineral assemblages, with or without coexisting sphalerite.The reflectance and unit cell dimension are sensitive to composition and decrease with increasing gahnite component. The approximate 8% reflectance at 540 nm is intermediate between most silicates and ore minerals such as sphalerite and magnetite and thus provides a ready method for recognition in reflected light. The gahnite is believed to have formed during amphibolite grade regional metamorphism as a result of either sphalerite desulfidation or the reaction of original zinc oxide phases.

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Gahnit in metamorphen schichtgebundenen Sulfid-Lagerstätten des Mineral-Distriktes, Virginia, U.S.A.

Zusammenfassung Eisenhaltiger Gahnite (Zn,Fe)Al₂O₄ ist in den metamorphen vulkanogenen massiven Sulfiderzkörpern Virginias als akzessorisches Mineral weit verbreitet. Er kommt durchwegs in den Pyrit, Zinkblende, Kupferkies und Sulfosalze enthaltenden mineralisierten Zonen vor. Obwohl die meisten Proben nur 1–2 Modal % Gahnit enthalten, und keine klar definierte Verteilung zu erkennen ist, tritt er besonders häufig an den Rändern der mineralisierten Zonen auf und erreicht lokal 35 Modal %. Die Zusammensetzung schwankt leicht von Vorkommen zu Vorkommen, liegt aber stets zwischen 68 und 88 Mol.% Gahnit (ZnAl₂O₄) 8 und 25 Mol.% Hercynit (FeAl₂O₄) und 1 und 17 Mol.% Spinell (MgAl₂O₄) Gahnit ist Bestandteil sehr verschiedener Mineralparagenesen mit und ohne Zinkblende.

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With 7 Figures     

Zn-rich högbomite formed from gahnite in the metabauxites of the Menderes Massif,SW Turkey

Gahnite, ZnAl₂O₄, present as an accessory mineral in regionally metamorphosed low-grade diasporites, has reacted in adjacent higher-grade, corundum-bearing metabauxite equivalents (emeries) to form Zn-rich högbomite, (Zn,Fe²⁺,Mg,Ni)_t-2x (Ti,Sn)_xAl₂O₄, of the 4H polytype. Commonly, the initial högbomite crystals grew epitactically along the octahedral faces of gahnite, which was subsequently dissolved, so that högbomite now forms spectacularly intergrown sets of eight crystals in perfect crystallographic orientation to each other. This indicates a metamorphic reaction, probably involving a fluid, transporting mainly the elements Zn and Al. Reactant Ti minerals in the diasporites were rutile and titanian hematite (10–15 mol% FeTiO₃). In the emeries högbomite coexists with still more Ti-rich hematites containing between 26 and 37 mol% FeTiO₃. The overall reaction relations involving partial reduction may be subdivided into the intial univariant reaction, gahnite+diaspore+Ti-hematite+rutile=högbomite+H₂O+O₂. This was followed, in the absence of gahnite, by compositional readjustments of högbomite and Ti-hematite and the appearance of magnetite. Core to rim zoning profiles indicate that, with continued growth, the högbomite crystals became poorer in Zn and Ti, but richer in Fe²⁺, while the Ti-contents of coexisting hematite increased. Högbomite formation at the expense of gahnite started at temperatures as low as about 400° C for an estimated pressure of 5–6 kbar.     

Indicator minerals as guides to base metal sulphide mineralisation in Betul Belt, central India

Zn-bearing minerals that act as indicator minerals for base metal sulphide mineralization from the Proterozoic Betul Belt, central India with special emphasis on their genetic significance have been discussed. Sulphide mineralisation is hosted by the felsic volcanic rocks and has similarities with volcanic-hosted massive sulphide deposits in other parts of the world. Synvolcanic hydrothermal alteration is crudely zoned with an inner high Mg-Ca core and an outer wider envelop of Al-Fe rich mineral assemblage. Most of the prospects have strata bound, moderately to steeply dipping, multiple, sub-parallel sheet like ore bodies composed of disseminated and semi-massive to massive ores. Zn-bearing spinel, staurolite, biotite and ilmenite typically occur within the foot-wall alteration zones in close proximity to the sulphide mineralization. Zincian spinel is ubiquitous irrespective of the nature of alteration zone. Zincian staurolite is nearly absent in Mg-Ca alteration zones but commonly present in Al-Fe alteration zone along with zincian ilmenite. Zn-bearing biotite in intimate association with zincian spinel is generally found in Mg-Ca alteration zone and in the transition to Al-Fe alteration zone. Most of these indicator minerals can be considered as products of desulphidation of sphalerite during metamorphism. Mechanisms other than desulphidation like formation of gahnite by overstepping of the zinc saturation limit of biotite during retrogression to chlorite and formation of zincian staurolite at the expense of gahnite is also recorded. Field presence of these minerals has immense significance in exploration in Betul Belt as they occur in close spatial relationship with the sulphide rich zones and therefore act as direct vectors to ore.     

Metasedimentary rocks of the paleoarchean Dniester-Bug Group,Ukrainian Shield: Composition,age, and sources

Paleoarchean granulite-facies metasedimentary rocks (quartzites, garnet quartzites, garnet-pyroxene gneisses, pyroxene-magnetite and magnetite quartzites) attributed to the Dniester-Bug Group of the Ukrainian Shield were studied. On the basis of geochemical data, including REE, the primary composition of these rocks was reconstructed as association of Fe-rich sandstones and sublitharenites, Fe-shales, and BIFs. This sedimentary association is similar to the rocks of other ancient greenstone belts and ascribed to the Algama-type iron formation. The sum of Al₂O₃, CaO, Na₂O, and TiO₂, high Zr contents (>100 ppm in quartzites), and the presence of detrital zircon grains of different ages are consistent with the terrigenous nature of sedimentary rocks. The Sm/Nd, Ti/Zr, Sc/Zr, and Ni/Zr ratios indicate the predominance of granitoid rocks in the source areas. The elevated Cr contents suggest that, in addition to granitoids, the source area contained ultramafic rocks. Geochemical characteristics, such as Fe/Mn ratio, low REE contents, and variations of REE versus the sum of Ni, Co, and Cu testify that sedimentation occurred under shallow-water conditions on the continent or its slope, similarly as the formation of ancient (3.5–3.2 Ga) basalt-komatiitic series intercalated with sedimentary rocks in the Pilbara Craton. The age of supracrustal rocks of the Dniester-Bug Group was constrained within the time interval of 3.4–3.2 Ga on the basis of U-Pb zircon dating and determination of Nd isotope composition. The DM model age of quartzites varies from 3.37 to 3.5 Ga. Sedimentary rocks together with volcanic rocks represent the oldest supracrustal association of the East European Platform.     

Gahnite in Rare-Metal Metasomatic Rocks of the Verkhneurmiysky Ore Cluster,Amur River Region

A new finding of Zn-spinel (gahnite) in biotite metasomatic rock of the Verkhneurmiysky copper–tungsten–tin ore cluster is described in this paper, including the chemical composition of gahnite, its paragenesis, and place in the regional evolution history of hydrothermal processes. Gahnite is commonly localized in biotite–microcline metasomatic rocks at the lower subore stage of the greisen metasomatic column in proximity to the exocontact of the rare-metal granite massif. Gahnite from rare-metal ore occurrences belongs to the hercynite–gahnite–franklinite isomorphic series and contains significant isomorphic admixtures of iron (24.3%) and manganese (0.65%). Based on our results and earlier published data, it is suggested that gahnite can be a mineral-indicator of rare-metal metasomatism.     

Zincian staurolite in the Dry River South volcanic-hosted massive sulfide deposit,northern Queensland,Australia: an assessment of its usefulness in exploration

Staurolite from the Dry River South volcanic-hosted massive sulfide deposit in northern Queensland, Australia is enriched in ZnO (2.5–6.8%) only within the massive sulfide lens or in highly pyritic biotite-chlorite schist just below the massive sulfide lens. In the footwall alteration zone, staurolite ZnO levels vary between 0.4 and 1.3%, whereas in hanging wall metagraywackes and metapelites, the ZnO content is mainly below 1.0%.As staurolite from metapelitec rocks contains up to 7.9% ZnO, high ZnO levels in staurolite do not necessarily indicate a relationship to Zn-rich massive sulfide. Staurolite grains from the Dry River South amd other massive sulfide lenses have low TiO₂ concentrations (mainly <0.4%) relative to staurolite grains from metasediments and alteration zones (mainly >0.4%). The low concentration of TiO₂ in staurolite from the massive sulfide lens results from the low initial Ti concentration in exhalative ores. High ZnO and low TiO₂ values are indicative of staurolite associated with Zn-rich massive sulfide.Zincian staurolite is a potential exploration indicator at both prospect and reconnaissance scales. At the prospect scale, zinc levels have potential in distinguishing true from false gossans, distinguishing low grade portions of massive sulfide lenses from barren massive pyrite bodies, and characterizing alteration zones. At the reconnaissance scale, staurolite can be collected in the heavy mineral fraction of stream sediments, and multiple grains can be analyzed rapidly using modern electron microprobes.     

Petrological investigation of the Ganyu peridotite in the Sulu ultrahigh-pressure terrane, eastern China

The ultramafic body sampled in the Chinese Continental Scientific Drilling (CCSD) Hole PP3 is located in the eastern part of the Dabie-Sulu UHP metamorphic belt near Donghai County. It is about 480 m thick, and consists chiefly of garnet peridotite, dunite and serpentinite. The principal minerals include olivine, chromium spinel, diopside, enstatite, garnet with minor secondary augite, phlogopite and amphibole. Both the olivine and orthopyroxene are highly magnesian, and the garnet is pyropic with 5.4-6.4% CaO and 0.3-3.3% Cr₂O₃. Two generations of clinopyroxene are present; an early diopside followed by augite. Chromium spinels are highly variable with Cr#s (100Cr / (Cr + Al)) between 51 and 89, and their compositions reflect different processes of formation, namely partial melting and eclogite, amphibole and greenschist facies metamorphism. The Mg#s (100 Mg / (Mg + Fe²⁺))of the spinels correlate positively with the Cr#s but negatively with oxygen fugacity. Based on the spinel compositions the ultramafic rocks originated in the shallow mantle, then subducted to depths of more than 100 km and finally exhumed to the surface. They underwent partial melting at shallow depths, mostly in the spinel facies, and were later transformed into garnet peridotites during deep subduction. All of the rocks were weakly metasomatized during exhumation and were subjected to retrograde metamorphism.     

The Cu-Zn Obrazek ore deposit,Czechoslovakia: A volcanogenic deposit included in the Ransko intrusive complex

The Cu-Zn Obrazek ore deposit is reinterpreted as metamorphosed volcanogenic-type rather than epigenetic vein-type. Enclosed by undeformed, non-metamorphosed mafic-ultramafic rocks of the Ransko complex, the ore is a highly folded, intensely metamorphosed (Zn-rich) association of banded, massive sphalerite-barite-pyrite-pyrrhotite-chalcopyrite and (Cu-rich)pyrite-chalcopyrite-pyrrhotite. Cu-rich ores are disseminated in deformed metamorphic assemblages of quartz, sillimanite, cordierite, anthophyllite, orthopyroxene and gahnite. Textural and compositional features in the metamorphic rocks suggest that the cordierite-anthophyllite assemblage was produced by regional metamorphism of rocks associated with the ore deposit. Inclusion of the Cu-Zn deposit and associated rocks in the Ransko intrusive complex produced contact metamorphic hornfelses of quartz, cordierite, orthopyroxene and Al-spinel. The occurrence and compositions of Zn-rich chromian spinel and minor intercumulus sulfides in anorthosite, troctolite and norite of the Ransko complex near the Obrazek ore body are interpreted to result from contamination of the Ransko parent magma by the Cu-Zn deposit and associated rocks.     

Petrology of spinel granulites from Araku, Eastern Ghats, India, and a petrogenetic grid for sapphirine-free rocks in the system FMAS

Abstract Spinel-quartz-cordierite and spinel-quartz are found as relic prograde assemblages in Fe-rich granulites from the Araku area, Eastern Ghats belt, India. Subsequent reactions produced orthopyroxene + sillimanite in the former association and garnet + sillimanite in the latter. The first reaction is univariant in the FMAS system, but is trivariant in the present case because of the presence of Zn and Fe³⁺ in spinel. The second reaction also has high variance because of Zn and Fe³⁺, but also because of the presence of Ca in garnet. Thermobarometry shows that the metamorphic conditions were approximately 950° C and 8.5 kbar and the fo ₂ was near the NNO buffer. In Fe-rich bulk compositions and low- P -high- T conditions of metamorphism, two of the univariant reactions around the invariant point [Sa], namely (Sa, Hy) and (Sa, Cd), change topology due to reverse partitioning of Fe-Mg between coexisting garnet and spinel. An alternative partial petrogenetic grid in the system FMAS is constructed for such conditions and is applied satisfactorily to several sapphirine-free spinel granulites. It is shown that bulk composition ( X _Fe and Zn) exerts greater control on the stability of spinel + quartz than fo ₂. The effect of the presence of Zn and Fe³⁺ in spinel on the proposed grid is evaluated. Reaction textures in the Araku spinel granulites can be explained from the petrogenetic grid as due to near-isobaric cooling.     

Multiple Foliations Defined by Different Sillimanite Crystal Habits, Partial Melting and the Late Metamorphic Development of the Cannington Ag-Pb-Zn Deposit, Northeast Australia

A succession of foliations defined by different sillimanite-bearing structural fabrics suggests that the macroscopic, isoclinal synform that dominates the geometry of the Cannington Ag-Pb-Zn deposit, northwest Queensland, Australia formed during D₂. The five foliations in this succession, S₁ to S₅, are defined by aligned sillimanite with habits ranging from individual crystals in S₁ through S₄ to clusters of fibrolite in S₅ in both the matrix and as inclusion trails within garnet and gahnite (Zn-rich spinel) porphyroblasts. S₁, S₃ and S₅ formed as sub-horizontal foliations, whereas S_1a, S₂ and S₄ formed sub-vertically. Foliation intersection/inflexion axes (FIAs) within garnet and gahnite porphyroblasts preserve a succession of trends that shifts from W-E to N-S. This succession indicates that this region experienced N-S followed by W-E bulk crustal shortening. N-S shortening occurred during D₁and D_1a, and W-E shortening occurred from D₂ to D₅. Prismatic and rhombic sillimanite produced during D₁-D₄ accompanied prograde metamorphism to ca. 634 62°C and 4.8 1.3 kbar. The coexistence of fibrous, prismatic and rhombic sillimanite resulted from post peak metamorphic reactivation of the early foliations during D₅. The synformal D₂ fold was intensified during D₄ by W-E bulk shortening. Local partial melting occurred after D₁ in the appropriate bulk rock compositions, based on calculation of P-T pseudosections in the chemical systems KFMASH, KFMASHTO, NCKFMASH and MnNCKFMASH. Zn mineralization related to gahnite growth occurred during D₃ through D₄, and was redistributed by partial melting into structural and rheological sites during D₄ and D₅shearing.     

Ultramafic xenoliths from the Mt. Carmel area (Karem Maharal Volcano), Israel

A suite of xenoliths from the Mt. Carmel area, Israel, is predominantly composed of high-Al clinopyroxene + pyrope-almandine garnet + spinel ± plagioclase. Some garnet clinopyroxenites show exsolution features of the aluminous phases garnet, spinel and amphibole while the garnet granulites have recrystallized completely to an equigranular texture. The garnet granulites were last reequilibrated under maximum pressures of 8 ± 1.5 kbar and temperatures of 600–700°C. The garnet clinopyroxenites equilibrated at temperatures of 880 ± 87°C and were probably extracted from greater depths. The textural and chemical data suggest that the xenoliths are pieces of a gabbroic lower crust formed by accretion of aluminous clinopyroxene basaltic cumulates that have recrystallized extensively before incorporation in the transporting magma.     

The stratigraphic interpretation of a highly deformed proterozoic region in central bushmanland,South Africa: First correlation of structurally separated metasediments of the aggeneys subgroup

It is demonstrated that highly deformed and structurally separated Proterozoic metasediments, which have previously defied stratigraphic and sedimentological classification, may be correlated and characterised. This is done by using a multidisciplinary approach, including stratigraphic and structural mapping and sedimentological interpretation, developed during the Geodynamics Programme in Western Namaqualand (1975–1980). The metasediments under consideration belong to the economically important Aggeneys Subgroup, which hosts stratabound massive sulphide deposits at Aggeneys and Gamsberg (Gams). Four isolated areas are considered and it is shown that the Dabenoris, Kabas and Pella Formations, and the lower part of the Achab Formation are equivalent to the lower part of the Wortel Formation, and that the upper part of the stratigraphic column is composed of the upper units of the Achab Formation, the Gams Formation and the Blomhoek Formation. The Zuurwater Formation is, for the most part, equivalent to the lower portion of the Wortel Formation, but the uppermost strata are correlated with the Blomhoek Formation. These results illustrate that it is possible to obtain the information necessary for a palaeobasin analysis from even the most complex Proterozoic regions.     

Biotite breakdown and the formation of gahnite in metapelitic rocks from Kemiö, southwest Finland

The formation of ferroan gahnite by breakdown of zinc-bearing biotite has been studied in metapelitic gneisses from the Kemiö region in southwestern Finland. Spinel compositions range from herc₅₀ gahn₄₀spin₁₀ to herc₁₅gahn₈₂spin₃; biotite contains up to 0.24 wt.% ZnO. Individual spinel grains show a gradual compositional zoning owing to increasing zinc contents towards the grain margins.Chemical and textural evidence indicate that spinel has been formed by breakdown of zinc-bearing biotite. During prograde metamorphism ferroan gahnite formed according to the reaction: biotite+sillimanite +quartz=cordierite+spinel+fluid, and during retrogressive metamorphism ferroan gahnite was formed by chloritization of biotite. It is proposed that the zinc-saturation limit of biotite depends on the metamorphic conditions. Decrease of this limit during retrogression to values below the zinc content of biotite initiated release of zinc and formation of ferroan gahnite. The activity of the Fe and Mg end members of retrogressively formed spinel are fixed by the coexisting biotite.     

Mineral chemistry and zircon geochronology of xenocrysts and altered mantle and crustal xenoliths from the Aries micaceous kimberlite: Constraints on the composition and age of the central Kimberley Craton,Western Australia

The Neoproterozoic (∼ 820 Ma) Aries micaceous kimberlite intrudes the central Kimberley Basin, northern Western Australia, and has yielded a suite of 27 serpentinised ultramafic xenoliths, including spinel-bearing and rare, metasomatised, phlogopite–biotite and rutile-bearing types, along with minor granite xenoliths. Proton-microprobe trace-element analysis of pyrope and chromian spinel grains derived from heavy mineral concentrates from the kimberlite has been used to define a ∼ 35–40 mW/m² Proterozoic geotherm for the central Kimberley Craton. Lherzolitic chromian pyrope highly depleted in Zr and Y, and Cr-rich magnesiochromite xenocrysts (class 1), probably were derived from depleted garnet peridotite mantle at ∼ 150 km depth. Sampling of shallower levels of the lithospheric mantle by kimberlite magmas in the north and north-extension lobes entrained high-Fe chromite xenocrysts (class 2), and aluminous spinel-bearing xenoliths, where both spinel compositions are anomalously Fe-rich for spinels from mantle xenoliths. This Fe-enrichment may have resulted from Fe–Mg exchange with olivine during slow cooling of the peridotite host rocks. Fine exsolution rods of aluminous spinel in diopside and zircon in rutile grains in spinel- and rutile-bearing serpentinised ultramafic xenoliths, respectively, suggest nearly isobaric cooling of host rocks in the lithospheric mantle, and indicate that at least some aluminous spinel in spinel-facies peridotites formed through exsolution from chromian diopside. Fe–Ti-rich metasomatism in the spinel-facies Kimberley mantle probably produced high-Ti phlogopite–biotite + rutile and Ti, V, Zn, Ni-enriched aluminous spinel ± ilmenite associations in several ultramafic xenoliths. U–Pb SHRIMP ²⁰⁷Pb/²⁰⁶Pb zircon ages for one granite (1851 ± 10 Ma) and two serpentinised ultramafic xenoliths (1845 ± 30 Ma; 1861 ± 31 Ma) indicate that the granitic basement and lower crust beneath the central Kimberley Basin are at least Palaeoproterozoic in age. However, Hf-isotope analyses of the zircons in the ultramafic xenoliths suggest that the underlying lithospheric mantle is at least late Archean in age.     

Reaction relationships of chrome-spinels in igneous rocks — further evidence from the layered intrusions of Rhum and Mull,Inner Hebrides,Scotland

Compositions of chrome-spinels in peridotites from the layered igneous rocks of Rhum and of the Ben Buie intrusion, Mull, show evidence of post-depositional reaction relationships with the cumulus olivine and/or the intercumulus liquid. Some seam-forming spinels in the Ben Buie intrusion are highly aluminous, more so than those in the Rhum intrusion. Occasional zoned spinels, in both intrusions, show enrichment in Al towards their rims. The evidence suggests that the spinel reaction trend involving Cr-Al exchange proceeds in the direction of Al-enrichment. Equilibrium between chrome-spinel and cumulus olivine appears to have been attained rarely, thus limiting the application of the olivine-spinel geothermometer.