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1.
The spinel–garnet transition in Cr/Al-enriched peridotiticbulk compositions is known from experimental investigationsto occur at 20–70 kbar, within the pressure range sampledby kimberlites. We show that the Cr2O3–CaO compositionsof concentrate garnets from kimberlite have maximum Cr/Ca arrayscharacterized by Cr2O3/CaO 0·96–0·81, andinterpret the arrays as primary evidence of chromite–garnetcoexistence in Cr-rich harzburgitic or lherzolitic bulk compositionsderived from depth within the lithosphere. Under Cr-saturatedconditions on a known geotherm, each Cr/Ca array implicitlydelineates an isobar inside a garnet Cr2O3–CaO diagram.This simplification invites a graphical approach to calibratean empirical Cr/Ca-in-pyrope barometer. Carbonaceous chromite–garnetharzburgite xenoliths from the Roberts Victor kimberlite tightlybracket a graphite–diamond constraint (GDC) located atCr2O3 = 0·94CaO + 5·0 (wt %), representing a pivotalcalibration corresponding to 43 kbar on a 38 mW/m2 conductivegeotherm. Additional calibration points are established at 14,17·4 and 59·1 kbar by judiciously projecting garnetcompositions from simple-system experiments onto the same geotherm.The garnet Cr/Ca barometer is then simply formulated as follows(in wt %):
if Cr2O3 0·94CaO + 5, then P38 (kbar) = 26·9+ 3·22Cr2O3 – 3·03CaO, or
if Cr2O3 <0·94CaO + 5, then P38 (kbar) = 9·2+ 36[(Cr2O3+ 1·6)/(CaO + 7·02)].
A small correction to P38 values, applicable for 35–48mW/m2 conductive geotherms, is derived empirically by requiringconventional thermobarometry results and garnet concentratecompositions to be consistent with the presence of diamondsin the Kyle Lake kimberlite and their absence in the Zero kimberlite.We discuss application of the P38 barometer to estimate (1)real pressures in the special case where chromite–garnetcoexistence is known, (2) minimum pressures in the general casewhere Cr saturation is unknown, and (3) the maximum depth ofdepleted lithospheres, particularly those underlying Archaeancratons. A comparison with the PCr barometer of Ryan et al.(1996, Journal of Geophysical Research 101, 5611–5625)shows agreement with P38 at 55 ± 2 kbar, and 6–12%higher PCr values at lower P38. Because the PCr formulationsystematically overestimates the 43 kbar value of the GDC by2–6 kbar, we conclude that the empirical Cr/Ca-in-garnetbarometer is preferred for all situations where conductive geothermsintersect the graphite–diamond equilibrium. KEY WORDS: Cr-pyrope; chromite; P38 barometer; mantle petrology; lithosphere thickness  相似文献   

2.
Clay minerals in 32 samples collected from the upper 6 meters of drill core LF 82/1-3 in Lake Frome, Australia have been examined using X-ray powder diffraction techniques. The results show that the clay mineral species are predominated by illite, kaolinite and a small amount of montmorillonite. Minor chlorite and vermiculite can also be identified in some of the samples. In terms of clay mineral species, crystallinity and other characteristics, the upper 6-m core can be divided into 3 sections, the boundaries of which are marked at depths of about 125–150 cm and 415–455 cm with ages of 14 and 19 thousand years B.P., respectively. The middle section at a burial depth of 150–415 cm may have been deposited under moist conditions while the other two under relatively dry conditions. Additionally, some slight climatic fluctuations seem possible in different stages.  相似文献   

3.
Fluid-saturated subsolidus experiments from 2·0 to 6·5GPa, and from 680 to 800°C have been performed on threemodel peridotites in the system Na2O–CaO–FeO–MgO–Al2O3–SiO2–H2O(NCFMASH). Amphibole and chlorite coexist up to 2·4 GPa,700°C. Chlorite persists to 4·2 GPa at 680°C.Starting from 4·8 GPa, 680°C a 10 Å phase structurereplaces chlorite in all compositions. The 10 Å phasestructure contains significant Al2O3 (up to 10·53 wt%) deviating from the MgO–SiO2–H2O 10 Å phase(MSH 10 Å phase). A mixed layered structure (chlorite–MSH10 Å phase) is proposed to account for aluminium observed.In the Tinaquillo lherzolite amphibole breakdown occurs viathe reaction Thermal stabilityof chlorite (chlorite + orthopyroxene = forsterite + garnet+ H2O) is shifted towards lower temperatures, compared withthe system MASH. Furthermore, the chlorite thermal breakdownis also related to the degenerate reaction Chlorite and the Al-10 Å phase structurecontribute significantly to the water budget in subduction zonesin the depth range relevant for arc magmatism, whereas amphibole-relatedfluid release is restricted to the forearc region. Chloriteand Al-10 Å phase breakdowns might explain the occurrenceof a double seismic zone by dehydration embrittlement. KEY WORDS: amphibole; chlorite; high pressure; peridotites; subduction zones  相似文献   

4.
Manganiferous chemical sediments of Neoproterozoic age in Namibiawere subjected to high-T–low-P metamorphism during theDamara Orogeny and display unique phase assemblages. The manganeseformations are embedded in iron formations and siliciclasticcountry rocks. This sequence is petrographically subdividedinto restricted lithotypes which bear specific mineral assemblagesand compositions depending on their protolith type. In puremanganese ores the critical assemblage braunite + haematite+ jacobsite + rhodonite is frequently developed, whereas interlayeredimpure silicate ores bear various proportions of spessartine,Mn3+-bearing andradite–calderite and andradite garnets,rhodonite, manganoan aegirine–augite, aegirine, Ba–K–Na-feldspars,barite and rare kinoshitalite. Petrological constraints derivedfrom country rock lithologies indicate peak metamorphic conditionsof 660–700C at estimated pressures of 35–45 kbar.Numerous Ba-rich pegmatitic veins restricted to the ore horizonstestify to the production of partial melts from siliciclasticstrata within the manganese formations. They are correlatedwith peak pressure conditions between 5 and 6 kbar, accompanyingthe main deformation event and pre-dating the thermal peak.An early H2O-rich generation of fluid inclusions is interpretedas a manifestation of prograde dehydration reactions in theore horizons. This caused hydraulic fracturing of the ores and,subsequently, triggered the formation of partial melts whichintruded the fracture planes in situ. Peak metamorphism thenoccurred under strainfree conditions allowing equilibrium recrystallizationof all minerals to develop. Phase relationships of manganeseoxides and silicates modelled in the system Mn–Fe–Si–Oreveal variable chemical compositions of braunites, jacobsitesand haematites depending on their paragenesis. They indicatevery restricted oxygen reservoirs within specific strata ofthe manganese ores and eliminate a prominent mass exchange evenon a small scale. This is supported by 18O analyses of silicateassemblages which further exclude mass transfer between manganeseores and country rocks, and indicate preservation of the exchangeequilibria during cooling. The uplift path of the sequence canbe constrained using different decrepitation patterns of H2Ofluid inclusions and a syn-to late-metamorphic CO2-rich fluidinclusion population, which indicate high geothermal gradientsof 70C/km and more. The P–T–D evolution of thishigh-T–low-P metamorphic belt conforms with the palaeotectonicsetting of the study area at the southernmost part of the CongoCraton, representing the continental buttress colliding withthe Kalahari Craton during the Pan-African orogeny. KEY WORDS: manganiferous sediments; Damara Orogeny; Namibia; metamorphism; oxygen isotopes; fluid inclusions *Corresponding author. Present address Institut fr Geowissenschaften und Lithosphrenforschung, Senckenbergstrasse 3, D-35390 Giessen, Germany  相似文献   

5.
Suprasolidus phase relations at pressures from 4 to 7 GPa andtemperatures from 1000 to 1700C have been determined experimentallyfor a sanidine phlogopite lamproite from North Table Mountain,Leucite Hills, Wyoming. The lamproite is silica rich and hasbeen postulated to be representative of the magmas which wereparental to the Leucite Hills volcanic field. Near-liquidusphases above 5 GPa are pyrope-rich garnet and jadeite-rich pyroxene.Below 5 GPa, jadeite-poor pyroxene is the only near-liquidusphase. Near-solidus assemblages consist of clinopyroxene, titanianpotassium richterite and titanian phlogopite with either potassiumtitanian silicate above 5 GPa or potassium feldspar below 5GPa. The potassium titanian silicate is a newly recognized high-pressurephase ranging in composition from K4Ti2Si7O20 to K4TiSi8O20.It coexists with coesite at pressures above 6 GPa at 1100–1400C.A previously unrecognized K-Ba-phosphate is a common near-solidusphase. The phase relationships are interpreted to suggest thatlamproites cannot be derived by the partial melting of simplelherzolitic sources. However, it is proposed that sanidine phlogopitelamproites an derived by high degrees of partial melting ofancient metasomatic veins within a harzburgitic–lherzoliticlithospheric substrate mantle. The veins are considered to consistof phlogopite, K–Ti-richterite, K–Ba-phosphate andK–Ti-silicates. KEY WORDS: lamproilte; experimental petrology; upper mantle *Corresponding author  相似文献   

6.
We have observed apparent decoupling of the Re–Os andSm–Nd isotopic systems in sulphide-saturated magmas thatsuggests that bulk two-component or assimilation–fractionalcrystallization (AFC) mixing modelling, based on Re–Osisotopic data, is inappropriate for chalcophile isotopic systemsin turbulent sulphide-saturated magmas. This behaviour is observedin three Palaeoproterozic layered mafic–ultramafic intrusionsin the Halls Creek Orogen of Western Australia. All intrusionsclearly have a basaltic parental magma based on primitive olivineand spinel compositions. The intrusions are light rare earthelement enriched and define a narrow range of initial Nd-isotopicsignatures (  相似文献   

7.
New SHRIMP U–Pb zircon, Rb–Sr whole-rock, and 40Ar–39Ardata are presented for the Jurassic silicic volcanic rocks andrelated granitoids of Patagonia and the Antarctic Peninsula.U–Pb is the only reliable method for dating crystallizationin these rocks; Rb–Sr is prone to hydrothermal resettingand Ar–Ar is additionally affected by initial excess 40Ar.Volcanism spanned more than 30 My, but three episodes are definedon the basis of peak activity: V1 (188–178 Ma), V2 (172–162Ma) and V3 (157–153 Ma). The first essentially coincideswith the Karoo–Ferrar mafic magmatism of South Africa,Antarctica and Tasmania. The silicic products of V1 are lower-crustalmelts that have incorporated upper-crustal material. The geochemistryof V2 and V3 ignimbrites is more characteristic of destructiveplate margins, but the presence of inherited zircon still pointsto a crustal source. The pattern of volcanism corresponds inspace and in time to migration away from the Karoo mantle plumetowards the proto-Pacific margin of Gondwana during riftingand break-up. The heat required to initiate bulk crustal fusionmay have been supplied by the spreading plume-head, but thinningof the crust during continental dispersion would also have facilitatedanatexis. KEY WORDS: Antarctic Peninsula; ignimbrites; Jurassic; Patagonia; U–Pb; zircon  相似文献   

8.
The kinetics of the quartz–coesite phase transition hasbeen studied in situ by X-ray diffraction in the 2·1–3·2GPa, 500–1010°C pressure–temperature range.Analysis of the data within Cahn's model of nucleation and growthat grain boundaries reveals that the prograde and retrogradereactions have different kinetics. The quartz  相似文献   

9.
Pressure–temperature–time (P–T–t) pathsof orogenic granulites provide important information on thethermal and chemical structure of the lower continental crustthrough time, and constraints on tectonic processes. We presentthe first detailed petrological investigation of granulitesfrom the Variscan Schwarzwald. Pelitic granulites from the CentralSchwarzwald Gneiss Complex (CSGC) are characterized by the peakassemblage garnet + rutile + kyanite + antiperthite ±quartz. Felsic to intermediate granulites from the SouthernSchwarzwald Gneiss Complex (SSGC) exhibit different peak assemblageswith clinopyroxene, orthopyroxene, ternary feldspar, garnet,quartz and sillimanite, and manifold retrograde reaction textures.Peak P–T conditions were calculated by two-feldspar thermometry,garnet–orthopyroxene thermometry and various geobarometers.Minimum estimates for peak conditions are 950–1010°Cand 1·4–1·8 GPa for the granulites of theCSGC, which followed a clockwiseP–T path. The retrogradepath is characterized by initial isothermal decompression, associatedwith partial melting, followed by isobaric cooling. Peak conditionsfor the SSGC are 1015°C and 1·5 GPa (minimum temperature,maximum pressure). No prograde relics are preserved, and isothermaldecompression was less pronounced than in the CSGC. Other VariscanHP–HT granulites from Central Europe show similar lithologies,equilibration temperatures and ages (340–335 Ma). Theheat for widespread high-temperature metamorphism in the Variscanlower crust could have been supplied by repeated intrusion ofsubduction-related basic magmas. Rapid, near-isothermal decompressionof the granulites may have been facilitated by considerablevolumes of partial melt and by orogenic extension. KEY WORDS: granulites; near-isothermal decompression; two-feldspar thermometry; HT metamorphism; Variscan Schwarzwald  相似文献   

10.
Talc–kyanite schists (whiteschists), magnesiohornblende–kyanite–talc–quartzschists and enstatite–sapphirine–chlorite schistsoccur at Mautia Hill in the East African Orogen of Tanzania.They are associated with metapelites and garnet–clinopyroxene–quartzmetabasites. Geobarometry (GASP/GADS equilibria) applied tothe latter two rock types indicates a peak pressure of P = 10–11kbar. These results are confirmed by the high fO2 assemblagehollandite–kyanite–quartz and late-stage manganianandalusite that contains up to 19·5 mol. % Mn2SiO5. Maximumtemperatures of T = 720°C are inferred from late-stage yoderite+ quartz. A clockwise P–T evolution is constrained byprograde kyanite inclusions in metapelitic garnet and late-stagereaction rims of cordierite between green yoderite and talcthat reflect conditions at least 3–4 kbar below the peakpressure. Oxidizing conditions are recorded throughout the metamorphichistory of the whiteschists and chlorite schists, as indicatedby the presence of haematite coexisting with pseudobrookiteand/or rutile. Increasing water activity near peak pressuresis thought to have led to the breakdown of the high-pressureassemblages (Tlc–Ky–Hem and Mg-Hbl–Ky–Hem)and the subsequent formation of certain uncommon minerals, e.g.yellow sapphirine, Mn–andalusite, green and purple yoderite,piemontite and boron-free kornerupine. The proposed increasein water activity is attributed to fluid infiltration resultingfrom the devolatilization of underlying sediments during metamorphism. KEY WORDS: fluid infiltration; high-pressure amphibolite facies; East African Orogen; Pan-African; whiteschist  相似文献   

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