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1.
The Palaeoproterozoic Ni–Cu sulphide deposits of the PechengaComplex, Kola Peninsula, occur in the lower parts of ferropicriticintrusions emplaced into the phyllitic and tuffaceous sedimentaryunit of the Pilgujärvi Zone. The intrusive rocks are comagmaticwith extrusive ferropicrites of the overlying volcanic formation.Massive lavas and chilled margins from layered flows and intrusionscontain <3–7 ng/g Pd and Pt and <0·02–2·0ng/g Ir, Os and Ru with low Pd/Ir ratios of 5–11. Theabundances of platinum group elements (PGE) correlate with eachother and with chalcophile elements such as Cu and Ni, and indicatea compatible behaviour during crystallization of the parentalmagma. Compared with the PGE-depleted central zones of differentiatedflows (spinifex and clinopyroxene cumulate zones) the olivinecumulate zones at the base contain elevated PGE abundances upto 10 ng/g Pd and Pt. A similar pattern is displayed in intrusivebodies, such as the Kammikivi sill and the Pilgujärvi intrusion.The olivine cumulates at the base of these bodies contain massiveand disseminated Ni–Cu-sulphides with up to 2 µg/gPd and Pt, but the PGE concentrations in the overlying clinopyroxenitesand gabbroic rocks are in many cases below the detection limits.The metal distribution observed in samples closely representingliquid compositions suggests that the parental magma becamesulphide saturated during the emplacement and depleted in chalcophileand siderophile metals as a result of fractional segregationof sulphide liquids. Relative sulphide liquid–silicatemelt partition coefficients decrease in the order of Ir >Rh > Os > Ru > Pt = Pd > Cu. R-factors (silicate-sulphidemass ratio) are high and of the order of 104–105, andthey indicate the segregation of only small amounts of sulphideliquid in the parental ferropicritic magma. In differentiatedflows and intrusions the sulphide liquids segregated and accumulatedat the base of these bodies, but because of a low silicate–sulphidemass ratio the sulphide liquids had a low PGE tenor and Pt/Irand Cu/Ir ratios similar to the parental silicate melts. Duringcooling the sulphide liquid crystallized 40–50% of monosulphidesolid solution (mss) and the residual sulphide liquid becameenriched in Cu, Pt and Pd and depleted in Ir, Os and Ru. TheCu-rich sulphide liquid locally assimilated components of thesurrounding S-rich sediments as suggested by the radiogenicOs isotopic composition of some sulphide ores (  相似文献   

2.
The Northern Apennine ophiolites are remnants of the MiddleJurassic–Early Cretaceous lithosphere from the LigurianTethys. New trace element and Nd–Sr isotope investigationswere performed on: (1) the rare gabbros associated with thesubcontinental mantle rocks from the External Liguride ophiolites;(2) the gabbro–peridotite association from the poorlyknown ophiolitic bodies from Cecina valley (Southern Tuscany).Clinopyroxenes from the External Liguride and Cecina valleygabbros have similar trace element compositions, which are consistentwith formation from normal mid-ocean ridge basalt (N-MORB) magmas.Sm–Nd mineral isochron ages are 179 ± 9 Ma foran External Liguride gabbro and 170 ± 13 Ma and 173·5± 4·8 Ma for two different gabbroic bodies fromthe Cecina valley ophiolites. These ages are interpreted todate the igneous crystallization of the gabbros and are slightlyolder than the oldest pelagic sediments of the Ligurian Tethys.Initial  相似文献   

3.
The high-grade Archean Ashuanipi complex contains an older sequenceof granulite-facies migmatitic paragneiss and tonalite cut byabundant orthopyroxene-bearing, enclave-laden granitoid bodies(diatexite) of strongly peraluminous (garnet-bearing) and mildlyperaluminous (garnet—absent) granodioritic composition,inferred to be magmatic in origin. Temperature estimates forgarnet–orthopyroxene–biotite–plagioclase–quartzassemblages in both metamorphic and igneous rock types are mainlyin the range 700– 835 ?C, but apparent pressures are higher(0?6–0?65 GPa) in a wide belt of paragneiss and associatedtonalite than in the enclosing diatexites (0?35–0?55 GPa),possibly owing to fluid-enhanced retrograde re-equilibrationwithin the crystallizing igneous assemblages. Paragneiss has bulk compositions typical of Archean greywacke(58–68 wt. % SiO2), including high Cr (110–250 ppm),Ni (20–100 ppm), and LREE [(70–100) ?chondrites].Garnet-bearing diatexites have compositions virtually identicalto paragneiss whereas garnet-absent diatexites are characterizedby marked HREE depletion. High degrees of fusion of a sourcesuch as paragneiss, with entrainment of crystalline phases suchas garnet and orthopyroxene, are required to explain the compositionof garnet-bearing diatexites, whereas lower amounts of melting,leaving residual garnet, may account for the origin of the garnet-absentvarieties. CO2 may have been a melt component in diatexite, based on severalobservations: (l)the high degrees of fusion implied in the genesisof diatexite require either extreme temperatures (> 1000?C)for which there is no mineralogical evidence, or some fluxingagent other than H2O (cf. Peterson & Newton, 1990); (2)some xenoliths have orthopyroxene-rich (dehydration) margins,implying relatively anhydrous melt conditions; and (3) orthopyroxeneis unaltered, suggesting that low aH2O conditions persistedduring crystallization. U–Pb zircon geochronology constrains the time for heatingand magma production to <18 Ma (2700 Ma for detrital zirconin paragneiss; 2682 Ma for crystallization of igneous zirconin diatexite). Combined with the evidence for high crustal temperaturesand possible CO2 involvement, the rapid heating implies thatunderplated basaltic magmas played a key role as heat and fluidsources driving high-grade metamorphism and granitoid melt production.  相似文献   

4.
The Panzhihua gabbroic layered intrusion is associated withthe 260 Ma Emeishan Large Igneous Province in SW China. Thissill-like body hosts a giant Fe–Ti–V oxide depositwith 1333 million ton ore reserves, which makes China a majorproducer of these metals. The intrusion has a Marginal zoneof fine-grained hornblende-bearing gabbro and olivine gabbro,followed upward by Lower, Middle, and Upper zones. The Lowerand Middle zones consist of layered melanogabbro and gabbrocomposed of cumulate clinopyroxene, plagioclase, and olivine.These zones also contain magnetite layers. The Upper zone consistschiefly of leucogabbro composed of plagioclase and clinopyroxenewith minor olivine. Most rocks in the body show variable-scalerhythmic modal layering in which dark minerals, primarily clinopyroxene,dominate in the lower parts of each layer, and lighter minerals,primarily plagioclase, dominate in the upper parts. The oxideores occur as layers and lenses within the gabbros and are concentratedin the lower parts of the intrusion. Ore textures and associatedmineral assemblages indicate that the ore bodies formed by verylate-stage crystallization of V-rich titanomagnetite from animmiscible oxide liquid in a fluid-rich environment. The rocksof the Panzhihua intrusion become more evolved in chemistryupward and follow a tholeiitic differentiation trend with enrichmentin Fe, Ti, and V. They are enriched in light rare earth elementsrelative to heavy rare earth elements, and exhibit positiveNb, Ta, and Ti anomalies and negative Zr and Hf anomalies. Thesilicate rocks and oxide ores of the Panzhihua intrusion formedfrom highly evolved Fe–Ti–V-rich ferrobasaltic orferropicritic magmas. The textures of the ores and the abundanceof minor hydrous phases indicate that addition of fluids fromupper crustal wall-rocks induced the separation of the immiscibleoxide melts from which the Fe–Ti–V oxide ore bodiesin the lower part of the intrusion crystallized. KEY WORDS: magnetite; Fe–Ti-rich gabbro; layered intrusion; Panzhihua; SW China  相似文献   

5.
Peridotites in the Ulten Zone (Upper Austroalpine, Eastern Alps),occur as small bodies within lower-crustal rocks (gneisses andmigmatites) subducted at eclogite-facies conditions during theVariscan orogeny. They record a complex metamorphic and deformationevolution as indicated by the transition from coarse-grainedspinel-bearing peridotites to fine-grained garnet + amphibole-bearingperidotites, and are interpreted as portions of mantle wedgethat were incorporated in a downgoing slab of cold continentalcrust. The transition from spinel- to garnet-bearing assemblagewas accompanied by significant input of metasomatic agents,as shown by the crystallization of abundant amphibole. Herewe present trace-element mineral chemistry data for selectedUlten peridotites, with the aim of unravelling the nature ofthe metasomatic processes. Amphiboles display significant lightrare earth element (LREE) enrichment [CeN/YbN = 3·90–11·50;LREE up to (20–50) x C1], high Sr (150–250 ppm),K (1910–7280 ppm) and Ba (280–800 ppm) contents,and low concentrations of high field strength elements (HFSE)(Zr = 14–25 ppm, Y = 6·7–16 ppm, Ti = 1150–2500ppm, Nb = 2–7 ppm). On the basis of (1) the evidence formodal orthopyroxene decrease as a result of the garnet-formingreaction rather than abundant orthopyroxene crystallization,(2) the high modal amounts of amphibole (up to 23%) in the mostmetasomatized peridotites and (3) the strong large ion lithophileelement (LILE)/HFSE fractionation in amphiboles, we infer thatthe metasomatic agent was an H2O–CO2 fluid with a lowCO2/H2O ratio. Petrological investigations and geochronologicaldata indicate that the host metapelites experienced in situpartial melting and migmatization concomitantly with the garnet+ amphibole-facies recrystallization in the enclosed peridotites.We infer that the metasomatizing hydrous fluids could representthe residual fluids left after the crystallization of leucosomes,starting from water-undersaturated melts produced during migmatizationof the host gneisses. KEY WORDS: garnet peridotite; crustal metasomatism; amphibole; hydrous fluids  相似文献   

6.
The metabasites within the Tokoro belt of eastern Hokkaido,Japan, suffered pervasive high–P/ Tetamorphism. Mineralassemblages and compositions of more than 400 metabasites fromthe Saroma–Tokoro district were investigated. The metabasites are divided into six metamorphic zones basedon mineral assemblages. The laumontite (Lm) zone is definedby the presence of laumontite. The prehnite–pumpellyite(Pr–Pp) zone is characterized by the association of prehnite+ pumpellyite. The lawsonite–sodic. pyroxene (Lw–Napx)zone is defined by the assemblage lawsonite + pumpellyite +sodic pyroxene + chlorite. The epidote–sodic pyroxene(Ep–Napx)(1) and (2) zones are charecterized by the assemblage epidote+ pumpellyite + sodic pyroxene + chlorite. The former is characterizedby the absence of aragonite, sodic amphibole, and winchite,as well as the presence of jadeite–poor sodic pyroxene(maxJd mol% = 13), whereas these minerals occur in the Ep–Napx(2)zone, together with jadeite–rich sodic pyroxene (max.Jd mol % = 34). In the epidote–actinolite (Ep–Act)zone, the most common assemblages contain epidote+ actionolite+ pumpellyite + chlorite. The Lm zone corresponds to the zeolite facies (150–200?Cand 1–2 kb) and the Pr–Pp zone is equivalent tothe prehnite–pumpellyite facies (200–250?C and 2–2–5kb). The Ep–Napx(I) zone appears to be stable at 200–250?C and 2? 5?3?5 kb. The pressure conditions in the Lw–Napx,Ep-Napx(2), and Ep–Act zones appear to range from 5 to6 kb, and the temperatures are estimated to be 200–230,230–270, and 270–300? C, respectively. The sequenceof the metamorphic zones is charaterized by the curved P–Tpath. The stability field of pumpellyite+ sodic+ pyroxene+ chloritein Fe3+ bearing metabasites is located in the lower–temperatureand higher–pressure part of the pumpellyite–actionolitefacies. On the basis of Schreinmaker's method, the stabilityfield of the assemblage is bounded by a high–pressurereaction Pp+ Napx+ Chl+ Ab+ Qz+ H2O= Lw+ Gl, and by a high-temperaturereaction Pp Napx+ Chl+ Ab+ Qz = Ep + Gl + H2O.  相似文献   

7.
Orogenic peridotites occur enclosed in Proterozoic gneissesat several localities in the Western Gneiss Region (WGR) ofwestern Norway; garnet peridotites typically occur as discretezones within larger bodies of garnet-free, chromite-bearingdunite and are commonly closely associated with pyroxenitesand eclogites. The dunites of the large Almklovdalen peridotitebody have extremely depleted compositions (Mg-number 92–93·6);the garnet peridotites have lower Mg-number (90·6–91·7)and higher whole-rock Ca and Al contents. Post-depletion metasomatismof both rock types is indicated by variable enrichment in thelight rare earth elements, Th, Ba and Sr. The dunites can bemodelled as residues after very high degrees (>60%) of meltextraction at high pressure (5–7 GPa), inconsistent withthe preservation of lower degrees of melting in the garnet peridotites.The garnet peridotites are, therefore, interpreted as zonesof melt percolation, which resulted in refertilization of thedunites by a silicate melt rich in Fe, Ca, Al and Na, but notTi. Previous Re–Os dating gives Archaean model ages forthe dunites, but mixed Archaean and Proterozoic ages for thegarnet peridotites, suggesting that refertilization occurredin Proterozoic time. At least some Proterozoic lithosphere mayrepresent reworked and transformed Archaean lithospheric mantle. KEY WORDS: Archaean mantle; Proterozoic mantle; Western Gneiss Region, Norway; mantle metasomatism; garnet peridotite  相似文献   

8.
A combined petrological and geochronological study was carriedout on mafic granulites and associated felsic gneisses fromthe McKaskle Hills, eastern Amery Ice Shelf, East Antarctica.Garnet-bearing mafic granulites exhibit reaction textures andexsolution textures that indicate two-stage metamorphic evolution.Thermobarometric estimates from matrix and symplectite assemblagesyield peak and retrograde P–T conditions of 9·0–9·5kbar and 880–950°C and 6·6–7·2kbar and 700–750°C, respectively. Similar but slightlyscattered peak P–T estimates of 7·9–10·1kbar and 820–980°C are obtained from the core compositionsof minerals from felsic para- and orthogneisses. Evidence forthe prograde history is provided by muscovite inclusions ingarnet from a paragneiss. Sensitive high-resolution ion microprobeU–Pb zircon dating reveals an evolutionary history forthe granulites, including a mafic and felsic igneous intrusionat 1174–1019 Ma, sedimentation after 932–916 Ma,and a high-grade metamorphism at 533–529 Ma. In contrast,Sm–Nd mineral–whole-rock dating mainly yields asingle age population at 500 Ma. This suggests that the McKaskleHills form part of the Prydz Belt, and that the relatively highpeak P–T conditions and a decompression-dominated P–Tpath for the rocks resulted from a single Cambrian metamorphiccycle, rather than two distinct metamorphic events as formerlyinferred for the granulites from Prydz Bay. The age data alsoindicate that the Precambrian history of the McKaskle Hillsis not only distinct from that of the early Neoproterozoic terranein the northern Prince Charles Mountains, but also differentfrom that of other parts of the Prydz Belt. The existence ofmultiple basement terranes, together with considerable crustalthickening followed by tectonic uplift and unroofing indicatedby the clockwise P–T–t evolution, suggests thatthe Prydz Belt may represent a collisional orogen that resultedin the assembly of Gondwana during the Cambrian period. KEY WORDS: Mesoproterozoic basement; Cambrian metamorphism; P–T path; Prydz Belt; East Antarctica  相似文献   

9.
CORFU  F. 《Journal of Petrology》2004,45(9):1799-1819
Mangerites, charnockites, anorthosites, gabbros and granitesoccur within a high-grade metamorphic complex in the Lofoten–Vesterålenislands of northern Norway. U–Pb dating of zircon, titaniteand monazite indicates a three-stage magmatic history beginningat 1870–1860 Ma with the emplacement of the Lødingenand Hopen plutons, followed by a dominant stage at 1800–1790Ma that formed the bulk of the suite, and concluded by the emplacementof pegmatites, local rehydration and retrogression between 1790and 1770 Ma. On the scale of the Baltic Shield the 1870–1860Ma episode corresponds to contraction, amalgamation of arcs,and regional deformation. By contrast, the episode at 1800–1790Ma was characterized by major shifts in plate convergence, byintraplate deformation, and by a diversity of magmatic associationsincluding suites derived from the subcontinental mantle andwidespread granitoid rocks extracted from the continental crust.The diversity of concurrent magmatic events across the Svecofennianorogen, and the temporal coincidence with collisional eventsin coeval orogenic belts, suggests that the genesis of the suiteof magmatic rocks may have been related to tectonically drivenmechanisms of magma generation. KEY WORDS: anorthosite–mangerite–charnockite–granite; lithospheric processes; Lofoten–Vesterålen; Svecofennian orogen; U–Pb geochronology  相似文献   

10.
ROACH  IAN C. 《Journal of Petrology》2004,45(4):739-758
Intraplate basalts of the Eocene–Oligocene Monaro VolcanicProvince (MVP), in southeastern New South Wales, include lower-crustaland refractory to weakly metasomatized upper-mantle xenoliths.Lower-crustal-derived xenoliths appear to be all two-pyroxeneplagioclase granulites (CpxFe:Mg:Ca 0·17–0·56:0·63–0·77:0·28–0·89OpxFe:Mg:Ca 0·39–0·52:1·37–1·47:0·02An72–86 and An48–50) but may also include garnetpyroxenites at depth. Mantle-derived xenoliths are principallyspinel-bearing lherzolites (Fo89·8–90·6CpxFe:Mg:Ca 0·07–0·45:0·70–1·70:0·01–0·94OpxFe:Mg:Ca 0·16–0·19:1·62–1·75:0·01–0·10)but also include amphibole ± spinel-bearing lherzolite(Fo88·7–89·1 CpxFe:Mg:Ca 0·09–0·21:0·61–0·91:0·73–0·93OpxFe:Mg:Ca 0·09–0·31:0·70–1·54:0·03–0·91),spinel-bearing harzburgite (Fo90·5–90·7CpxFe:Mg:Ca 0·08:0·91–0·93:0·74–0·84OpxFe:Mg:Ca 0·16–0·18:1·73–1·79:0·00–0·02),wehrlite, pyroxenite (CpxFe:Mg:Ca 0·08–0·10:0·84–0·90:0·80–0·85OpxFe:Mg:Ca 0·16–0·33:1·51–1·73:0·02–0·03)and rare garnet pyroxenite (GtFe:Mg:Ca 0·83–0·95:1·60–1·70:0·45–0·48CpxFe:Mg:Ca 0·14–0·21:0·69–0·77:0·78–0·86Opx Fe:Mg:Ca 0·31–0·42:1·43–1·56:0·02–0·03)and amphibole–apatite composites. Xenolith textures aregenerally weakly to moderately foliated, a few are mosaic-porphyroblasticand rare samples are veined or highly strained. MVP xenolithsappear to have equilibrated under similar pressure–temperature(P–T) conditions to other southeastern Australian xenolithsequivalent to the South Eastern Australia (SEA) palaeogeotherm.P–T estimates for the MVP suite of xenoliths reveal aheterogeneous lower crust and upper mantle that is thickly underplatedto c. 1·8 GPa or c. 50 km depth. MVP xenolith P–Tdata are compared with those used to derive the SEA palaeogeotherm,which is shown to be in need of revision using more modern geothermometersand geobarometers and new xenolith coexisting mineral data. KEY WORDS: xenolith; petrography; texture; geotherm; Monaro; eastern Australia  相似文献   

11.
Metapelitic rock samples from the NE Shackleton Range, Antarctica,include garnet with contrasting zonation patterns and two agespectra. Garnet porphyroblasts in K-rich kyanite–sillimanite–staurolite–garnet–muscovite–biotite schistsfrom Lord Nunatak show prograde growth zonation, and give Sm–Ndgarnet, U–Pb monazite and Rb–Sr muscovite ages of518 ± 5, 514 ± 1 and 499 ± 12 Ma, respectively.Geothermobarometry and P–T pseudo-section calculationsin the model system CaO–Na2O–K2O– TiO2–MnO–FeO–MgO–Al2O3–SiO2–H2Oare consistent with garnet growth during prograde heating from540°C/7 kbar to 650°C/7·5 kbar, and partial resorptionduring a subsequent P–T decrease to <650°C at <6kbar. All data indicate that rocks from Lord Nunatak were affectedby a single orogenic cycle. In contrast, garnet porphyroblastsin K-poor kyanite–sillimanite– staurolite–garnet–cordierite–biotite-schistsfrom Meade Nunatak show two growth stages and diffusion-controlledzonation. Two distinct age groups were obtained. Laser ablationplasma ionization multicollector mass spectrometry in situ analysesof monazite, completely enclosed by a first garnet generation,yield ages of c. 1700 Ma, whereas monazite grains in open garnetfractures and in most matrix domains give c. 500 Ma. Both agegroups are also obtained by U–Pb thermal ionization massspectrometry analyses of matrix monazite and zircon, which fallon a discordia with lower and upper intercepts at 502 ±1 and 1686 ± 2 Ma, respectively. Sm–Nd garnet datingyields an age of 1571 ± 40 Ma and Rb–Sr biotiteanalyses give an age of 504 ± 1 Ma. Integrated geochronologicaland petrological data provide evidence that rocks from MeadeNunatak underwent a polymetamorphic Barrovian-type metamorphism:(1) garnet 1 growth and subsequent diffusive garnet annealingbetween 1700 and 1570 Ma; (2) garnet 2 growth during the RossOrogeny at c. 500 Ma. During the final orogenic event the rocksexperienced peak P–T conditions of about 650°C/7·0kbar and a retrograde stage at c. 575°C/4·0 kbar. KEY WORDS: garnet microtexture; P–T pseudosection; geochronology; polymetamorphism; Shackleton Range; Antarctica  相似文献   

12.
In the Speik Complex (Eastern Alps, Austria), highly melt-depleted,metamorphosed harzburgites with abundant pods and layers ofchromitite are interlayered with a suite of metamorphosed orthopyroxenites,clinopyroxenites and gabbros. Coarse-grained orthopyroxenitesoccur as centimetre- to metre-wide veinlets and pods, but alsoas intrusive plugs several tens of metres wide. Intimately associatedmetaclinopyroxenite and metagabbro are present as bodies upto several metres thick at a distinct stratigraphic level withinthe complex. In the ultramafic rocks, relict magmatic olivine,orthopyroxene, clinopyroxene and spinel have been overprintedby a metamorphic assemblage of forsterite, diopside, tremolite,anthophyllite, chlorite, serpentine, talc and Cr–Fe-richspinel. Hornblende, epidote, zoisite and chlorite dominate themetamorphic paragenesis in metagabbros, in addition to rarerelicts of clinopyroxene and two phases of Ca-rich garnet. Thepolymetamorphic evolution of the Speik Complex includes rarelypreserved pre-Variscan (400 Ma) eclogite-facies conditions,Variscan (330 Ma) amphibolite-facies conditions (600–700°C,>5 kbar) and Eoalpine (100 Ma) greenschist- to amphibolite-faciesconditions reaching 550°C and 7–10 kbar. Orthopyroxenitesare characterized by high concentrations of SiO2, MgO and Cr,and by U-shaped chondrite-normalized rare earth element (REE)patterns similar to those of their harzburgite hosts. The REEpatterns of the clinopyroxenites are flat to slightly enrichedin light REE. Metagabbro compositions are variable, but generallycharacterized by low SiO2 and high mg-numbers (61–78).Their REE patterns all have GdN/YbN > 1; some samples havelarge positive Eu anomalies implying the original presence ofcumulus plagioclase. In the orthopyroxenites, clinopyroxenitesand some peridotites, Pt, Pd and Re are distinctly enrichedcompared with Os, Ir and Ru, whereas most harzburgites haveunfractionated to slightly fractionated platinum-group element(PGE) patterns with respect to average upper mantle. The Re–Osisotope compositions of the pyroxenites define an errorchronat 550 ± 17 Ma and a supra-chondritic 187Os/188Os of0·179 ± 0·003. An isochron age of 554 ±37 Ma with Nd(i) +0·7 is indicated by the Sm–Ndisotope compositions of whole-rock pyroxenite and gabbro samples,whereas the harzburgites plot on an errorchron of 745 ±45 Ma and Nd(i) +6. The pyroxenites and gabbros probably representa cogenetic suite of magmatic dykes intruded into uppermost,highly depleted, suboceanic mantle below the crust–mantletransition zone in an oceanic basin close to the northwesternmargin of Gondwana. KEY WORDS: pyroxenite; metagabbro; geochemistry; Re–Os isotopes; Sm–Nd isotopes  相似文献   

13.
The largest accumulations of rhyolitic melt in the upper crustoccur in voluminous silicic crystal mushes, which sometimeserupt as unzoned, crystal-rich ignimbrites, but are most frequentlypreserved as granodioritic batholiths. After approximately 40–50%crystallization, magmas of intermediate composition (andesite–dacite)typically contain high-SiO2 interstitial melt, similar to crystal-poorrhyolites commonly erupted in mature arc and continental settings.This paper analyzes the feasibility of system-wide extractionof this melt from the mush, a mechanism that can rationalizea number of observations in both the plutonic and volcanic record,such as: (1) abrupt compositional gaps in ignimbrites; (2) thepresence of chemically highly evolved bodies at the roof ofsubvolcanic batholiths; (3) the observed range of ages (up to200–300 ka) recorded by zircons in silicic magmas; (4)extensive zones of low P-wave velocity in the shallow crustunder active silicic calderas. We argue that crystal–meltsegregation occurs by a combination of several processes (hinderedsettling, micro-settling, compaction) once convection is hamperedas the rheological locking point of the crystal–melt mixture(  相似文献   

14.
Chemical variations along with changes in microstructure ofthe principal constituent minerals make it possible to identifyat least four equilibrium stages in the evolution of the Yangkougarnet peridotite in the Su-Lu ultrahigh-pressure metamorphicbelt, eastern China: Stage I—a primary garnet lherzolitestage represented by coarse-grained (a few millimeters size)porphyroclastic aluminous pyroxenes + chromian spinel ±garnet; Stage II—an ultrahigh-pressure (UHP) stage definedby fine-grained matrix phases (0·1–0·3 mmsize) of garnet + extremely low-Al orthopyroxene + high-Na clinopyroxene+ chromite; Stage III—a medium-pressure stage definedby fine-grained mineral aggregates (<0·1–0·2mm size) mainly composed of aluminous spinel + high-Al orthopyroxenein the matrix; Stage IV—an amphibolite- to greenschist-faciesstage defined by poikiloblastic amphibole. Orthopyroxene–clinopyroxenethermometry and an empirical spinel barometer give temperaturesof around 800–830°C and pressures of 1·2–2·9GPa for porphyroclasts of Stage I. Garnet–orthopyroxene,garnet–clinopyroxene and empirical spinel geothermobarometersgive relatively uniform P–T conditions for the matrixgarnet–orthopyroxene–clinopyroxene–chromiteassemblage of Stage II (  相似文献   

15.
A clinopyroxene suite from leucite–bearing lavas locatedin the Sabatini district of the Roman Volcanic Region has beeninvestigated by single–crystal X–ray diffractionmethods combined with electron probe microanalysis. The main aim of the study was to obtain crystallographic datanecessary to evaluate intracrystaUine order–disorder relationshipsin natural clinopyroxenes and related equilibria–disequilibriaphenomena in different host–rock types. Generally, the polyhedral site configurations of the clinopyroxenesinvestigated are interdependent and follow specific variationtrends related to specific host–rock types. The majorvariations are found in the tetrahedron and the octahedron;while the polyhedron M2 is essentially unchanged and occupiedby(Ca+Na). The degree of Mg–Fe2+; ordering in M1–M2, respectively,depends on the configuration of these sites, particularly M2,and is generally lower in late–crystallized (rim) clinopyroxenesthan in the corresponding higher–temperature (core) fromthe same crystals. This is' attributed to configuration relationshipsunfavourable to the Mg–Fe2+ ordering and largely dependingon the occupancy of M2. Polygenetic, early–crystallized clinopyroxenes, occasionallywith resorbed margins, indicate crystal-liquid disequilibriarelated to crystal and–or magma mixing processes.  相似文献   

16.
The Kyffhäuser Crystalline Complex, Central Germany, formspart of the Mid-German Crystalline Rise, which is assumed torepresent the Variscan collision zone between the East Avalonianterrane and the Armorican terrane assemblage. High-precisionU–Pb zircon and monazite dating indicates that sedimentaryrocks of the Kyffhäuser Crystalline Complex are youngerthan c. 470 Ma and were intruded by gabbros and diorites between345 ± 4 and 340 ± 1 Ma. These intrusions had magmatictemperatures between 850 and 900°C, and caused a contactmetamorphic overprint of the sediments at P–T conditionsof 690–750°C and 5–7 kbar, corresponding toan intrusion depth of 19–25 km. At 337 ± 1 Ma themagmatic–metamorphic suite was intruded by granites, syenitesand diorites at a shallow crustal level of some 7–11 km.This is inferred from a diorite, and conforms to P–T pathsobtained from the metasediments, indicating a nearly isothermaldecompression from 5–7 to 2–4 kbar at 690–750°C.Subsequently, the metamorphic–magmatic sequence underwentaccelerated cooling to below 400°C, as constrained by garnetgeospeedometry and a previously published K–Ar muscoviteage of 333 ± 7 Ma. With respect to P–T–D–tdata from surrounding units, rapid exhumation of the KCC canbe interpreted to result from NW-directed crustal shorteningduring the Viséan. KEY WORDS: contact metamorphism; U–Pb dating; hornblende; garnet; Mid-German Crystalline Rise; P–T pseudosection  相似文献   

17.
Garnet-bearing assemblages of K-rich and K-poor metapelitesfrom the Ilesha Schist belt, SW Nigeria, are investigated. K-richsamples contain the assemblages (A) garnet–staurolite–muscovite–chlorite–magnetite,(B) andalusite–garnet–staurolite–muscovite–chlorite–magnetiteand (C) sillimanite–andalusite–garnet–muscovite–chlorite–magnetite.K-poor samples contain the assemblages (D) garnet–staurolite–cordierite–chloriteand (E) garnet–cordierite–chlorite ± staurolite.All assemblages contain quartz, plagioclase, biotite and ilmenite.P–T pseudosections calculated in the system CaO–Na2O–K2O–TiO2–MnO–FeO–MgO–Al2O3–SiO2 –H2O ± O2 suggest peak metamorphismat 590 ± 20°C at 5 ± 0·5 kbar, followedby retrogression to 550°C at 3·0 kbar, in agreementwith field evidence, domain assemblages, mineral compositions,modes and geothermobarometry. The absence of compositional zonationshows that garnet in all investigated rocks nucleated and grewat constant P–T–X in equilibrium with associatedminerals on the thin-section scale. However, the garnet-in reactiondid not begin until the establishment of a significant temperatureoverstep of  相似文献   

18.
The 2·63 Ga Louis Lake batholith, a calc-alkalic plutonexposed in Wind River Range of western Wyoming, consists ofminor diorite, quartz diorite, granodiorite, and granite. Atshallow structural levels the batholith is pyroxene free, butat deeper levels, all units of the batholith contain pyroxenes.On its northern margin the batholith was emplaced at P = 5–6kbar, T = 775–800°C, fO2 at FMQ (fayalite–magnetite–quartz)+ 1·5 to FMQ + 1·8, and aH2O  相似文献   

19.
Kilauea East Rift Zone Magmatism: an Episode 54 Perspective   总被引：1，自引：0，他引：1  
On January 29–30, 1997, prolonged steady-state effusionof lava from Pu'u'O'o was briefly disrupted by shallow extensionbeneath Napau Crater, 1–4 km uprift of the active Kilaueavent. A 23-h-long eruption (episode 54) ensued from fissuresthat were overlapping or en echelon with eruptive fissures formedduring episode 1 in 1983 and those of earlier rift zone eruptionsin 1963 and 1968. Combined geophysical and petrologic data forthe 1994–1999 eruptive interval, including episode 54,reveal a variety of shallow magmatic conditions that persistin association with prolonged rift zone eruption. Near-ventlava samples document a significant range in composition, temperatureand crystallinity of pre-eruptive magma. As supported by phenocryst–liquidrelations and Kilauea mineral thermometers established herein,the rift zone extension that led to episode 54 resulted in mixtureof near-cotectic magma with discrete magma bodies cooled to  相似文献   

20.
Numerous lenticular bodies of ultramafic rocks occur withinthe upper amphibolite- to granulitefacies metamorphic terraneof the Austrides between the Non and Ultimo valleys (Nonsbergregion), northern Italy. The ultramafic rocks are divided intotwo textural types: (a) coarse-type; and (b) finetype. The coarse-typerocks have the protogranular texture and are predominantly spinellherzolite. Some coarse-type spinel lherzolites have partlytransformed to garnet lherzolite. The fine-types are consideredto be metamorphic derivatives of the former, and the observedmineral assemblages are: (1) olivine + orthopyroxene + clinopyroxene+ garnet + amphibole ? spinel, (2) olivine + orthopyroxene +garnet + amphibole + spinel; (3) olivine + orthopyroxene + amphibole+ spinel; and (4) olivine+ orthopyroxene + amphibole + chlorite.Based on the microprobe analyses of constituent minerals fromten representative peridotite samples, physical conditions ofthe metamorphism, particularly that of the spinel to garnetlherzolite transformation, are estimated. Applications of pyroxenegeothermometry yield temperature estimates of 1100–1300?Cfor the formation of the primary spinel lherzolite, and 700–800?Cfor that of the fine-type peridotites. A pressure range of 16–28kb is obtained for the garnet lherzolite crystallization dependingon the choice of geobarometers. Two alternative P-T paths, i.e.(1) isobaric cooling or (2) pressure-increase and temperaturedecrease are considered and their geodynamic implications discussed.  相似文献   

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