Intraplutonic Quench Zones in the Kap Edvard Holm Layered Gabbro Complex, East Greenland

The Kap Edvard Holm Layered Gabbro Complex is a large layeredgabbro intrusion (>300 km²) situated on the opposite sideof the Kangerdlugssuaq fjord from the Skaergaard Intrusion.It was emplaced in a continental margin ophiolite setting duringearly Tertiary rifting of the North Atlantic. Gabbroic cumulates, covering a total stratigraphic thicknessof >5 km, have a typical four-phase tholeiitic cumulus mineralogy:plagioclase, clinopyroxene, olivine, and Fe–Ti oxides.The cryptic variation is restricted (plagioclase An_81–51,olivine Fo_85–66, clinopyroxene Wo_43–41 En_46–37Fs_20–11) and there are several reversals in mineral chemistry.Crystallization took place in a low-pressure, continuously fractionatingmagma chamber system which was periodically replenished andtapped. Fine-grained (0•2–0•4 mm) equigranular, thin(0•5–3 m), laterally continuous basaltic zones occurwithin an {small tilde}1000 m thick layered sequence in theTaco Point area. Twelve such zones define the bases of individualmacrorhythmic units with an average thickness of {small tilde}80m. The fine-grained basaltic zones grade upwards, over a fewmetres, into medium-grained (>1 mm) poikilitic, olivine gabbrowith smallscale modal layering. Each fine-grained basaltic zoneis interpreted as an intraplutonic quench zone in which magmachilled against the underlying layered gabbros during influxalong the chamber floor. Supercooling by {small tilde}50C isbelieved to have caused nucleation of plagioclase, olivine,and clinopyroxene in the quench zone. The nucleation rate isbelieved to have been enhanced as the result of in situ crystallizationin a continuously flowing magma. The transition to the overlyingpoikilitic olivine gabbro reflects a decreasing degree of supercooling. Compositional variation in the Taco Point sequence is typicalfor an open magma chamber system: olivine (Fo_{77–68 5})and plagioclase cores (An_80–72) show a zig-zag crypticvariation pattern with no overall systematic trend. Olivinehas the most primitive compositions in the quench zones andmore evolved compositions in the olivine gabbro; plagioclasecores show the opposite trend. Although plagioclase cores arebelieved to retain their original compositions, olivines re-equilibratedby reaction with trapped liquid. Some plagioclase cores containrelatively sodic patches which retain quench compositions. Whole-rock compositions of nine different quench zones varyover a range from 10 to 18% MgO although the mg-number remainsconstant at {small tilde}0•78. The average composition(47•7% SiO₂, 13•3%MgO, 1•57% Na₂O+K₂O) is takenas a best estimate of the parental magma composition, and isequivalent to a high-magnesian olivine tholeiite. The compositionalvariation of the quench zones is believed to reflect burstsof nucleation and growth of olivine and plagioclase during quenching. Magma emplacement is believed to have taken place by separatetranquil influxes which flowed along the interface between alargely consolidated cumulus pile and the residual magma. Theresident magma was elevated with little or no mixing. At certainlevels in the layered sequence the magma drained back into thefeeder system; such a mechanism is referred to as a surge-typemagma chamber system.     

Petrology of the Younger Andesites and Dacites of Iztacc?huatl Volcano, Mexico: I. Disequilibrium Phenocryst Assemblages as Indicators of Magma Chamber Processes

Disequilibrium phenocryst assemblages in the Younger Andesitesand Dacites of Iztacc?huatl, a major Quaternary volcano in theTrans-Mexican Volcanic Belt, provide an excellent record ofepisodic replenishment, magma mixing, and crystallization processesin calc-alkaline magma chambers. Phenocryst compositions andtextures in ‘mixed’ lavas, produced by binary mixingof primitive olivine-phyric basalt and evolved hornblende dacitemagmas, are used to evaluate the mineralogical and thermal characteristicsof end-members and the physical and chemical interactions thatattend mixing. Basaltic end-members crystallized olivine (FO_90–88) andminor chrome spinel during ascent into crustal magma chambers.Resident dacite magma contained phenocrysts of andesine (An_45–35),hypersthene (En67–61), edenitic-pargasitic hornblende,biotite, quartz, .titanomagnetite, and ilmenite. On reachinghigh-level reservoirs, basaltic magmas were near their liquidiat temperatures of about 1250–1200?C according to theolivine-liquid geothermometer. Application of the Fe-Ti-oxidegeothermometer-oxygen barometer indicates that hornblende dacitemagma, comprising phenocrysts (<30 vol. per cent) and coexistingrhyolitic liquid, had an ambient temperature between 940 and820?C at f_O2s approximately 0?3 log units above the nickel-nickeloxide buffer assemblage. Mixing induced undercooling of hybridliquids and rapid crystallization of skeletal olivine (Fo_88–73),strongly-zoned clinopyroxene (endiopside-augite), calcic plagioclase(An_65–60); and orthopyroxene (bronzite), whereas low-temperaturephenocrysts derived from hornblende dacite were resorbed ordecomposed by hybrid melts. Quartz reacted to form coronas ofacicular augite and hydroxylated silicates were heated to temperaturesabove their thermal stability limit ({small tilde}940?C foramphibole, according to clinopyroxene-orthopyroxene geothermometry,and {small tilde}880?C for biotite). Calculations of phenocrystresidence times in hybrid liquids based on reaction rates suggestthat the time lapse between magma chamber recharge and eruptionwas extremely short (hours to days). It is inferred that mixing of magmas of diverse compositionis driven by convective turbulence generated by large differencesin temperature between end-members. The mixing mechanism involves:(1)rapid homogenization of contrasting residual liquid compositionsby thermal erosion and diffusive transfer (liquid blending);(2) assimilation of phenocrysts derived from the low-temperatureend-member; and (3) dynamic fractional crystallization of rapidlyevolving hybrid liquids in a turbulent boundary layer separatingbasaltic and dacitic magmas. The mixed lavas of lztacc?huatlrepresent samples of this boundary layer quenched by eruption.     

High-Pressure Metamorphism in the SW Tauern Window, Austria: P-T Paths from Hornblende-Kyanite-Staurolite Schists

The hornblende garbenschist horizon of the Lower Schieferhulleseries (LSH) in the SW Tauern Window, Austria, contains theassemblage hornblende + kyanite + staurolite + garnet + biotite+ epidote + plagioclase + ankerite + quartz + rutile + ilmenite,with either chlorite or paragonite present in all samples. Theseassemblages are divariant in the system SiO₂-Al₂O₃-TiO₂-Fe₂O₃-MgO-FeO-MnO-CaO-Na₂O-K₂O-H₂O-CO₂.Garnet-biotite geothermometry yields temperatures of final equilibrationof {small tilde}550 °C, and garnet-plagioclase-kyanite-quartzgeobarometry indicates pressures of 6–8 kb for the matrixassemblage and 9–10 kb for plagioclase inclusions in garnet.Quantitative modelling of zoned garnet, hornblende, and plagioclaseindicates growth and equilibration along a decompression pathfrom {small tilde}530 °C, 10 kb to {small tilde}550 °C,7 kb. Fluid inclusion data constrain the uplift path to havepassed through a point at {small tilde} 375 °C, 1.5 kb. These data permit the construction of a relatively completeP-T loop for metamorphism associated with the Alpine orogeniccycle in the LSH of the SW Tauern Window. The maximum pressureconditions ({small tilde}10 kb at 530 °C) recorded alongthis loop are considerably higher than previous estimates of5–7 kb for the region. Simple overthrust models developedfor the Tauern Window cannot account for pressures of this magnitude;a more likely scenario involves partial subduction of the rocksto a depth of {small tilde}35 km, followed by prolonged heatingin response to decay of the subduction isotherms. Initial upliftappears to have been rapid and occurred along a nearly isothermalpath. Significant cooling did not occur until the rocks werewithin {small tilde}5 km of the surface. Detailed tectonic modelsfor the evolution of the Tauern Window must be able to accountfor the quantitative features of the P-T loop.     

Mineral Chemistry and Crystallization Conditions of the Mboutou Layered Gabbro-Syenite-Granite Complex, North Cameroon

The Mboutou complex is one of a line of early Tertiary ringcomplexes which runs from Lake Chad to the Gulf of Guinea, noneof which has hitherto been described in detail. The main rocktypes are layered gabbros and gabbronorites, with minor bodiesof quartz-syenodiorite, quartz-syenite and hypersolvus granite.Feldspars form a continuum with exceptional compositional range,from An₈₅Ab₁₃Or₂ to around An₁Ab₄₆Or₅₅, and form an entirelyhypersolvus sequence with very strong zoning in the syenodiorites.Ca-rich clinopyroxenes (salite and calcic augite) and olivines(Fo_78–62) have restricted range. Orthopyroxene-bearingleucogabbros and syenodiorites contain minor orthopyroxene (En₆₂Fs₃₅Wo₃)and quartz; olivine and orthopyroxene never coexist. In moreevolved rocks amphibole (magnesio-hornblende to ferroedenite)and minor biotite, showing progressive Fe-enrichment, are theonly mafic silicates.Major-element rock chemistry, minor elementsin clinopyroxenes and biotite chemistry show that, notwithstandingits thoroughly anorogenic setting, Mboutou was, at the outset,only very mildly alkaline. Its more evolved members embarkedon a line of evolution with some calc-alkaline characteristics,probably because of ingress of water into residual batches ofmagma, a possibility supported by stable isotope data. Thischange in behaviour corresponded with the sudden appearanceof quartz and orthopyroxene, which was not in equilibrium withclinopyroxene on the two-pyroxene surface. Amphibole then becamethe main mafic silicate with further increase in . The more evolved rocks are relatively highly altered,but Fe-Ti oxide pairs suggest that was maintained near to and above the QFM buffer and the rangeof biotite compositions further suggests crystallization undera regime of decreasing . Biotites maintain alkaline characteristics throughout the sequence. Zoningpatterns in the ternary feldspars in the syenodiorites, andthe hypersolvus character of the final granite, limit maximumvalues of to < 1 kb, and suggest minimum temperatures for the end of crystallizationin the syenodiorites of{small tilde} 850 ?C.     

Crystallization and Breakdown of Metasomatic Phases in Graphite-bearing Peridotite Xenoliths from Marsabit (Kenya)

Mantle-derived xenoliths from the Marsabit shield volcano (easternflank of the Kenya rift) include porphyroclastic spinel peridotitescharacterized by variable styles of metasomatism. The petrographyof the xenoliths indicates a transition from primary clinopyroxene-bearingcryptically metasomatized harzburgite (light rare earth element,U, and Th enrichment in clinopyroxene) to modally metasomatizedclinopyroxene-free harzburgite and dunite. The metasomatic phasesinclude amphibole (low-Ti Mg-katophorite), Na-rich phlogopite,apatite, graphite and metasomatic low-Al orthopyroxene. Transitionalsamples show that metasomatism led to replacement of clinopyroxeneby amphibole. In all modally metasomatized xenoliths melt pockets(silicate glass containing silicate and oxide micro-phenocrysts,carbonates and empty vugs) occur in close textural relationshipwith the earlier metasomatic phases. The petrography, majorand trace element data, together with constraints from thermobarometryand fO₂ calculations, indicate that the cryptic and modal metasomatismare the result of a single event of interaction between peridotiteand an orthopyroxene-saturated volatile-rich silicate melt.The unusual style of metasomatism (composition of amphibole,presence of graphite, formation of orthopyroxene) reflects lowP –T conditions (850–1000°C at < 1·5GPa) in the wall-rocks during impregnation and locally low oxygenfugacities. The latter allowed the precipitation of graphitefrom CO₂. The inferred melt was possibly derived from alkalinebasic melts by melt–rock reaction during the developmentof the Tertiary–Quaternary Kenya rift. Glass-bearing meltpockets formed at the expense of the early phases, mainly throughincongruent melting of amphibole and orthopyroxene, triggeredby infiltration of a CO₂-rich fluid and heating related to themagmatic activity that ultimately sampled and transported thexenoliths to the surface. KEY WORDS: graphite; peridotite xenoliths; Kenya Rift; modal metasomatism; silicate glass     

Melting Relations of Basalt with Equilibrium Water Pressure Less Than Total Pressure

Equilibrium H₂O pressure (P_eH2O) was fixed at values less thantotal pressure (P_T) in melting experiments on mixtures of 1921Kilauea tholeiite, H₂O, and CO₂ (58.5 mole per cent H₂O, 41.5mole per cent CO₂), buffered by Ni+NiO. New determinations ofthe beginning of melting of mixtures of 1921 Kilauea tholeiiteand H₂O buffered by quartz+fayalite+magnetite were made at 2and 3 kb. Microprobe analyses of coexisting glass, clinopyroxene,?olivine, ?amphibole were determined for several runs. Decreasing H₂O fugacity (fH₂O) to about six-tenths the fugacityof pure H₂O (f?II₂O) raises the solidus and the upper stabilitylimit of plagioclase. Plagioclase and clinopyroxene coexistin equilibrium with liquid-a feature not observed in the pureH₂O system. Amphibole is stable to about 970 ?C at 2 kb, 1025?C at 5 kb and 1060 ?C at 8 kb. The Al (VI)+Ti contents of theamphibole increase with P, yielding kaersutite at 1050 ?C and8 kb. Calculated modes for the condensed phases reveal large differencesin the amount of glass (liquid) present and large differencesin liquid composition below and above the breakdown temperatureof amphibole at 5 and 8 kb. Liquids coexisting with amphibole,clinopyroxene, olivine, and magnetite are dacitic near the solidusand silica-rich andesites around 1000 ?C at 5 and 8 kb. Theresults of this study substantiate the model for the generationof the calc-alkaline suite by partial melting of H₂O-rich basalts.     

Clinoenstatite in a Volcanic Rock from the Cape Vogel Area, Papua

A porphyritic volcanic rock from Cape Vogel, Papua, containsabundant phenocrysts of multiply twinned clinoenstatite, andless common phenocrysts of orthopyroxene, set in a groundmassof pyroxene microlites, glass, and zeolites. The rock contains54% SiO₂, 13–16% MgO, and 6–7% FeO, but only 7–8%Al₂O₃, 45–5% CaO, and 06–08% Na₂O. Microprobeanalyses show that the clinoenstatite phenocrysts range fromEn₉₂ to En₈₂, and have very low Al₂O₃ and extremely low CaOcontents. Their composition differs consistently from that ofthe orthopyroxene phenocrysts, which range from En₈₇ to at leastas Fe-rich as En₇₈. The clinoenstatite phenocrysts are a metastableinversion-product from primary protoenstatite. The crystallizationof protoenstatite as the liquidus phase is attributed directlyto the unique magma composition.     

Melting of Biotite + Plagioclase + Quartz Gneisses: the Role of H2O in the Stability of Amphibole

Biotite + plagioclase + quartz (BPQ) is a common assemblagein gneisses, metasediments and metamorphosed granitic to granodioriticintrusions. Melting experiments on an assemblage consistingof 24 vol. % quartz, 25 vol. % biotite (XMg = 0·38–0·40),42 vol. % plagioclase (An_26–29), 9 vol. % alkali feldsparand minor apatite, titanite and epidote were conducted at 10,15 and 20 kbar between 800 and 900°C under fluid-absentconditions and with small amounts (2 and 4 wt %) of water addedto the system. At 10 kbar when 4 wt % of water was added tothe system the biotite melting reaction occurred below 800°Cand produced garnet + amphibole + melt. At 15 kbar the meltingreaction produced garnet + amphibole + melt with 2 wt % addedwater. At 20 kbar the amphibole occurred only at high temperature(900°C) and with 4 wt % added water. In this last case themelting reaction produced amphibole + clinopyroxene ±garnet + melt. Under fluid-absent conditions the melting reactionproduced garnet + plagioclase II + melt and left behind a plagioclaseI ± quartz residuum, with an increase in the modal amountof garnet with increasing pressure. The results show that itis not possible to generate hornblende in such compositionswithout the addition of at least 2–4 wt % H₂O. This reflectsthe fact that conditions of low aH₂O may prevent hornblendefrom being produced with peraluminous granitic liquids fromthe melting of biotite gneiss. Thus growth of hornblende inanatectic BPQ gneisses is an indication of addition of externalH₂O-rich fluids during the partial melting event. KEY WORDS: biotite; dehydration; gneisses; hornblende; melt     

Regional Metamorphism of the Waits River Formation, Eastern Vermont: Delineation of a New Type of Giant Metamorphic Hydrothermal System

Metamorphic isograds and time-integrated fluid fluxes were mappedover the 1500 km2 exposure of the Waits River Formation, easternVermont, south of latitude 4430'N. Isograds based on the appearanceof oligoclase, biotite, and amphibole in metacarbonate rocksdefine elongated metamorphic highs centered on the axes of twolarge antiforms. The highest-grade isograd based on the appearanceof diopside is closely associated spatially with synmetamorphicgranitic plutons. Pressure, calculated from mineral equilibria,was fairly uniform in the area, 7 1.5 kb; calculated temperatureincreases from {small tilde} 480C at the lowest grades in thearea to {small tilde} 575C in the diopside zone. CalculatedX_co2f equilibrium metamorphic fluid increases from <0-03at the lowest grades to 0.2 in the amphibole zone and decreasesto 0.07 in the diopside zone. Time-integrated fluid fluxesincrease with increasing metamorphic grade, with the followingmean values for each metamorphic zone (in cm³/cm²): ankerite-oligoclasezone, 1 x 10⁴; biotite zone, 7 x 10⁴; amphibole zone, 2 x 10⁵;diopside zone, 7 x 10⁵. The mapped pattern of time-integrated fluxes delineates twolarge deep-seated ({small tilde} 25-km depth) regional metamorphichydrothermal systems, each centered on one of the major antiforms.Fluid flowed subhorizontally perpendicular to the axis of theantiforms from their low-temperature flanks to their hot axialregions and drove prograde decarbonation reactions as they went.Along the axes of the antiforms fluid flow was further focusedaround synmetamorphic granitic intrusions. In the hot axialregion fluid changed direction and flowed subvertically outof the metamorphic terrane, precipitating quartz veins. Estimatesof the total recharge, based on progress of prograde decarbonationreactions, nearly match estimates of the total discharge, basedon measured quartz vein abundance, (2-10) x 10¹² cm3 fluid percm system measured parallel to the axes of the antiforms. Withinthe axial regions fluids had lower X_CO2 and rocks record greatertime-integrated fluxes close to the intrusions than at positionsmore than {small tilde} 5 km from them. The differences in bothfluid composition and time-integrated flux can be explainedby mixing close to the intrusions of regional metamorphic fluidsof X_CO2/ with fluids from another source with X_CO2{small tilde}0 in the approximate volume ratio of 1:2.     

High-Pressure Granulites (Retrograded Eclogites) from the Hengshan Complex, North China Craton: Petrology and Tectonic Implications

Both high- and medium-pressure granulites have been found asenclaves and boudins in tonalitic–trondhjemitic–granodioriticgneisses in the Hengshan Complex. Petrological evidence fromthese rocks indicates four distinct metamorphic assemblages.The early prograde assemblage (M₁) is preserved only in thehigh-pressure granulites and represented by quartz and rutileinclusions within the cores of garnet porphyroblasts, and omphacitepseudomorphs that are indicated by clinopyroxene + sodic plagioclasesymplectic intergrowths. The peak assemblage (M₂) consists ofclinopyroxene + garnet + sodic plagioclase + quartz ±hornblende in the high-pressure granulites and orthopyroxene+ clinopyroxene + garnet + plagioclase + quartz in the medium-pressuregranulites. Peak metamorphism was followed by near-isothermaldecompression (M₃), which resulted in the development of orthopyroxene+ clinopyroxene + plagioclase symplectites and coronas surroundingembayed garnet grains, and decompression-cooling (M₄), representedby hornblende + plagioclase symplectites on garnet. The THERMOCALCprogram yielded peak (M₂) P–T conditions of 13·4–15·5kbar and 770–840°C for the high-pressure granulitesand 9–11 kbar and 820–870°C for the medium-pressuregranulites, based on the core compositions of garnet, matrixpyroxene and plagioclase. The P–T conditions of pyroxene+ plagioclase symplectite and corona (M₃) were estimated at     

Evolution of Low-Al Orthopyroxene in the Horoman Peridotite, Japan: an Unusual Indicator of Metasomatizing Fluids

Unusually alumina-poor orthopyroxene is found in a spinel peridotitefrom the Horoman Peridotite Complex, Japan. Al₂O₃, Cr₂O₃ andCaO contents in the low-Al orthopyroxene (named Low-Al OPX hereafter)are <0·25 wt %, <0·04 wt % and <0·3wt %, respectively, and are distinctively lower than those inorthopyroxene porphyroclasts. The Low-Al OPX occurs in two modes,both at the margin of olivine. The first mode of occurrenceis as the rim of a large orthopyroxene porphyroclast in contactwith olivine. This type of Low-Al OPX occurs only locally (15µm x 45 µm), and the orthopyroxene rim in contactwith olivine more commonly has normal Al₂O₃ contents (>2wt %). In the second mode of occurrence, the Low-Al OPX occursas a thin film, 5 µm x 50 µm in dimension, at agrain boundary between olivine and clinopyroxene. Trace elementcompositions of porphyroclast clinopyroxene in the sample indicatethat the sample having the Low-Al OPX underwent metasomatismalthough there are no hydrous minerals around the Low-Al OPX.Petrographic observations and trace element compositions ofclinopyroxene combined with an inferred P–T history ofthe Horoman peridotite suggest that the Low-Al OPX was formedthrough a very local reaction between peridotite and invasivefluids, probably formed by dehydration of a subducted slab,in a late stage of the history of the Horoman peridotite. Crystallizationof orthopyroxene, representing addition of silica to mantlelherzolite via a CO₂ + H₂O-bearing fluid phase, is a mechanismfor metasomatic alteration of mantle wedge peridotite. KEY WORDS: Horoman Peridotite Complex; low-Al orthopyroxene; metasomatism; mantle wedge     

The Petrology of the Lower Old Red Sandstone Lavas of the Eastern Sidlaw Hills, Perthshire, Scotland

The calc-alkaline lava sequence of the eastern Sidlaw Hillsforms a small part of an extensive volcanic province of LowerOld Red Sandstone (Devonian) age in Scotland and N. England.The Sidlaw lavas ranging from olivine basalt to dacite are allporphyritic with combinations of olivine, plagioclase, clinopyroxene,orthopyroxene, and opaque oxide pheno-crysts. Chemically, thelavas are slightly more alkalic than modern calc-alkaline lavas.There is considerable variation in the ‘incompatible elements’.The differentiation of the lavas can be accounted for by fractionationof olivine+plagioclase+minor ore from a chemically variable,immediately parental magma at low pressure (c. 1 kb P_H2O). Itis suggested that fractionation of variable amounts of olivineand clinopyroxene from an olivine tholeiite at moderate P_H2Ocould give rise to this chemically variable, high alumina, immediatelyparental magma.     

Melting Relations in Part of the System CaO-MgO-Al2O3-SiO2-Na2O-H2O under 5kb Pressure

In the system CaO-MgO-Al₂O₃-SiO₂-Na₂O-H₂O under 5 kb pressurethe invariant equilibrium forsterite-orthopyroxene-Ca-rich clinopyroxene-amphibole-plagioclase-liquid-vapourhas been identified at 960?12 ?C. A similar invariant assemblagewith spinel replacing Ca-rich clinopyroxene exists at 950?8?C. The liquid in the former equilibrium contains 16.5 per cent(wt.) normative quartz and 3 per cent Na₂O; the plagioclaseis more calcic than An₈₇; the pyroxenes contain about 3 percent Al₂O₃ and the amphibole is hypersthene-normative. Two anhydrousthermal maxima, the olivine-Ca-rich clinopyroxene-plagioclaseand the orthopyroxene-Ca-rich clinopyroxene-plagioclase dividezones are not encountered in this system, and nepheline-normativeliquids may crystallize amphibole?olivine?Ca-rich clinopyroxeneto produce quartz-normative residual liquids of andesite-typecomposition. A thermal maximum involving amphibole-olivine-Ca-richclinopyroxene-liquid-vapour exists for liquids containing approximately11 per cent normative nepheline and liquids more undersaturatedthan this will crystallize these phases to produce extremelynephelinitic liquids. Phase diagrams are presented which facilitate the predictionof crystallization sequences and liquid evolution paths forany basic or intermediate composition under the conditions employedhere.     

Ca-rich Garnet-Clinopyroxene Rocks at Hujialin in the Su-Lu Terrane (Eastern China): Deeply Subducted Arc Cumulates?

Layers of Ca-rich garnet–clinopyroxene rocks enclosedin a serpentinite body at Hujialin, in the Su–Lu terraneof eastern China, preserve igneous textures, relict spinel ingarnet, and exsolution lamellae of Ca-rich garnet, ilmenite/magnetite,Fe-rich spinel, and also amphibole in clinopyroxene. In termsof their major and trace element compositions, the studied samplesform a trend from arc cumulates towards Fe–Ti gabbros.Reconstructed augite compositions plot on the trend for clinopyroxenein arc cumulates. These data suggest that the rocks crystallizedfrom mantle-derived magmas differentiated to various extentsbeneath an arc. The Ca-rich garnet + diopside assemblage isinferred to have formed by compressing Ca-rich augite, whereasthe relatively Mg-rich cores of garnet porphyroblasts may haveformed at the expense of spinel. The protolith cumulates weresubducted from near the crust–mantle boundary (c. 1 GPa)deep into the upper mantle (4·8 ± 0·6 GPaand 750 ± 50°C). Negatively sloped P–T pathsfor the garnet–clinopyroxene rocks and the corollary ofcorner flow induced subduction of mantle wedge peridotite arenot supported by the available data. Cooling with, or without,decompression of the cumulates after the igneous stage probablyoccurred prior to deep subduction. KEY WORDS: arc cumulates; Ca-rich garnet; garnet–clinopyroxene rocks; Su–Lu terrane; UHP metamorphism     

Metamorphic Conditions and Fluid Compositions of Scapolite-Bearing Rocks from the Lapis Lazuli Deposit at Sare Sang, Afghanistan

Scapolite and other halogen-rich minerals (phlogopite, amphibole,apatite, titanite and clinohumite) occur in some high-pressureamphibolite facies calc-silicates and orthopyroxene-bearingrocks at Sare Sang (Sar e Sang or Sar-e-Sang), NE Afghanistan.The calc-silicates are subdivided into two groups: garnet-bearingand garnet-free, phlogopite-bearing. Besides garnet and/or phlogopite,the amphibolite facies mineral assemblages in the calc-silicatesinclude clinopyroxene, calcite, quartz and one or more of theminerals scapolite, plagioclase, K-feldspar, titanite, apatiteand rarely olivine. Orthopyroxene-bearing rocks consist of clinopyroxene,garnet, plagioclase, scapolite, amphibole, quartz, calcite andaccessory dolomite and alumosilicate (kyanite?). Retrogradephases in the rocks are plagioclase, scapolite, calcite, amphibole,sodalite, haüyne, lazurite, biotite, apatite and dolomite.The clinopyroxene is mostly diopside and rarely also hedenbergite.Aegirine and omphacite with a maximum jadeite content of 29mol % were also found. Garnet from the calc-silicates is Grs_45–95Py_0–2and from the orthopyroxene-bearing rocks is Grs_10–15Py_36–43.Peak P–T metamorphic conditions, calculated using availableexchange thermobarometers and the TWQ program, are 750°Cand 1·3–1·4 GPa. Depending on the rock type,the scapolite exhibits a wide range of composition (from Eq_An= 0·07, X_Cl =0·99 to Eq_An = 0·61, X_Cl =0·07).Equilibria calculated for scapolite and coexisting phases atpeak metamorphic conditions yield X_CO2 = 0·03–0·15.X_NaCl (fluid), obtained for scapolite, ranges between 0·04and 0·99. Partitioning of F and Cl between coexistingphases was calculated for apatite–biotite and amphibole–biotite.Fluorapatite is present in calc-silicates, but orthopyroxene-bearingrocks contain chlorapatite. Cl preferentially partitions intoamphibole with respect to biotite. All these rocks have sufferedvarious degrees of retrogression, which resulted in removalof halogens, CO₂ and S. Halogen- and S-bearing minerals formedduring retrogression and metasomatism are fluorapatite, sodalite,amphibole, scapolite, clinohumite, haüyne, pyrite, andlazurite, which either form veins or replace earlier formedphases. KEY WORDS: scapolite; fluid composition; high-pressure; amphibolite facies; Western Hindukush; Afghanistan     

Experimental Investigation of the Upper Thermal Stability of Mg-rich Actinolite; Implications for Kiruna-Type Iron Deposits

The occurrence of actinolite in magnetite deposits of possiblemagmatic origin has prompted an experimental investigation ofthe upper thermal stability of Mg-rich actinolite to determinehow the stability of actinolite changes with increasing Fe content.Experiments were carried out primarily on the compositionalre-equilibration of natural tremolite [molar Fe/(Fe + Mg) =Fe-number = 0·014] in the presence of synthetic clinopyroxene(Ca_0·80Fe_0·67Mg_0·54Si_2·00O₆), syntheticpigeonite/orthopyroxene (Ca_0·08Fe_1·19Mg_0·70Si_2·02O₆),quartz, and water to a more Fe-rich actinolite over the rangeof 600–880°C, 1 and 4 kbar, at the Ni–NiO oxygenbuffer, for durations of 1–2 weeks. The bulk compositionof the mineral mixture is close to actinolite with Fe-number= 0·5. These experiments constitute a half-reversal ofthe amphibole composition, which, when approached from a Mg-richstarting composition, provides information on the minimum Fecontent of actinolite at a given temperature. Compositionalchanges were monitored by electron microprobe analysis of amphibolerim compositions and/or overgrowths on the original tremolite.At 4 kbar and 880–800°C, tremolite shows strong re-equilibrationwith overgrowths of an Fe-rich but low-Ca (1·7 > Ca> 1·4) actinolite; Fe-rich cummingtonite (Ca <0·7)begins to nucleate at 860°C. At 800–700°C, tremoliteshows weak compositional re-equilibration but strong nucleationof Fe-rich cummingtonite. Similar results were observed at 1kbar, with tremolite showing strong re-equilibration to low-Caactinolite at 790–600°C with cummingtonite nucleationat 800°C and below. The wide variation in Ca contents ofthe re-equilibrated amphiboles was unexpected. Additional univariantreversal experiments were carried out on the thermal decompositionof a natural actinolite (Fe-number = 0·22) from PleitoMelón, Chile, indicating the breakdown of actinoliteto clinopyroxene, orthopyroxene, quartz, and water at 780°Cand 1 kbar, and 850°C and 4 kbar. Considering only amphiboleswith Ca >1·7 a.p.f.u., the thermal stability of actinoliteis observed to decrease in a linear manner over the P–Trange investigated with a dT/dFe-number slope of –372°C/Fe-numberat 1 kbar and –546°C/Fe-number at 4 kbar. The highthermal stabilities (750–900°C) of actinolites withFe-numbers in the range of 0–0·4 overlap with therange of water-saturated melting for a typical andesite or tonalite.These conditions also overlap the field of experimental Fe–P-richmelt formation, suggesting that actinolite may have an igneousorigin in Kiruna-type ore deposits. KEY WORDS: actinolite; mineral stability; Kiruna deposits, thermodynamic values; cummingonite     

Granulite Metamorphism, Fluid Buffering, and Dehydration Melting in the Madras Charnockites and Metapelites

Interlayered and cofolded charnockites and metapelites of thetype charnockite area near Madras were metamorphosed under granulitefades conditions. Fe-Mg partitioning between orthopyroxene,garnet, and biotite indicates that chemical equilibrium wasapproached under similar P-T conditions in the two rock suites.Several geothennometers and geobarometers give P-T values whichconverge at 750–800?C and 6.5–7.5 kb. Computations utilizing data from high pressure phase equilibriumexperiments of Bohlen et al. (1983a) and Wones & Dodge (1977)point to several significant relations regarding the behaviourof H₂O during the granulite metamorphism. a_H2O values, computedfrom Bohlen et al.'s (1983a) reversal data and the a-X modelfor phlogopite after Bohlen et al. (1980), show distinctly lowermagnitudes in metapelites (0.10–0.16) than in charnockites(0.23–0.34). No systematic spatial gradients exist withinthe charnockites or metapelites, and a_H2O has similar valuesin metapelite exposures widely separated in the field. Theseimply an internal, rather than an external (e.g., by CO2 influx),control of the fluids. Applying the algebraic method developed by Rumble (1976), Gibbsanalysis in the system K₂O-MgO-FeO-Fe₂O₃-Al₂O₃-SiO₂-TiO₂-H₂Oshows that the chemical potentials of H₂O and to O₂, as monitoredagainst biotite composition and , exhibit gradients with respect to X_Mg in the two rock suites under isothermal-isobaricconditions. µ_H2O was found to decrease with X_Mg^bt in both,while µ_O2 increases with decreasing X_Mg^bt in metapelitesbut increases sympathetically with X_Mg^bt in charnockites. Thesefindings point out again that µ_H2O and µ_O2 wereinternally buffered. The absence of graphite in the metapelites,at an estimated f_O2 = 10^–14.7 b, also argues against anexternal influx of CO₂ and, inter alia, supports internal buffering.A complementary enquiry into variations of a_TIO2 reveals aninverse relation between a_TIO2 and a_H2O, suggesting a similarcontrol for a_TIO2. The inferences from biotite dehydration equilibria, when combinedwith the P-T data and with several field and chemical featuresof these rocks noted earlier (Sen, 1974), make dehydration meltinga distinct possibility for the Madras rocks. It is argued thatthe low a_H2O and high a_TIO2 ({small tilde} 0.9) observed inthe metapelites have been caused by a greater extent of meltingin the precursors of metapelites, which were more hydrous thanthose of charnockites, coupled with preferential partitioningof Ti into the residual rocks—thus strengthening the casefor dehydration melting.     

Metasomatism and Melting in Carbonated Peridotite Xenoliths from the Mantle Wedge: The Gobernador Gregores Case (Southern Patagonia)

Spinel-facies mantle xenoliths occur in a diatreme cutting throughthe Neogene Southern Patagonia Plateau at Gobernador Gregores(Santa Cruz Province, Argentina). This plateau is in a back-arcposition with respect to the Chile trench. Xenoliths differin their whole-rock composition from other South America occurrences,having higher CaO/Al₂O₃ ratios and, in some samples, TiO₂ enrichment,whereas the Na₂O/Al₂O₃ variation range is similar. Three assemblagescan be distinguished. Assemblage 1, in anhydrous protogranularlherzolites and harzburgites, contains clinopyroxene with adepleted major and trace element composition, indicating pre-metasomaticdepletion processes. This assemblage fully recrystallized toAssemblage 2 (amphibole ± phlogopite ± Cl-apatite-bearing)during a metasomatic episode. This causes clinopyroxene to acquiregeochemical characteristics often attributed to carbonate-meltmetasomatism. Noticeably, amphibole is markedly enriched inNb (up to 298 ppm), especially when depleted in Ti. A furtherevent, related to decompression during xenolith uplift to thesurface, induces closed-system (perhaps with the exception ofCO₂ addition) disequilibrium melting of Assemblage 2, dominantlyof amphibole. It is found in pockets (where amphibole is a residualphase) consisting of Na–Si-rich glass and carbonate (Mg-richcalcite) drops, and in veins originating from the pockets (Assemblage3). Euhedral olivine, clinopyroxene and spinel crystallize onlyin the silicate glass. So do new, euhedral apatite crystalswhen glass is in contact with previous Assemblage 2 apatite.Textural evidence and comparison with experimental work suggestthat silicate glass and carbonates are the result of unmixingof a former homogeneous melt. Because of the different flowrates of carbonate and silicate melt, the xenoliths become enrichedin carbonate, which is found in the veins during their migration.Thus, the high CaO/Al₂O₃ ratio of whole rocks provides inconclusiveevidence of carbonatite metasomatism. This factor, and otherminor deviations from the expected results of carbonatite metasomatism,lead us to hypothesize an aqueous, Cl-rich fluid, possibly slabderived, as an alternative agent. Amphibole, resulting fromreactive porous flow of this agent in the mantle, could fullyexplain the observed geochemical features, as indicated by estimatesof its partition coefficients. KEY WORDS: carbonated xenoliths; Gobernador Gregores; LAM–ICP-MS; mantle metasomatism; silicate glass     

High-Grade Fluid Metasomatism on both a Local and a Regional Scale: the Seward Peninsula, Alaska, and the Val Strona di Omegna, Ivrea-Verbano Zone, Northern Italy. Part I: Petrography and Silicate Mineral Chemistry

K-feldspar–plagioclase–quartz mineral textures aswell as biotite and hornblende compositions are compared forsuites of metamorphosed mafic rocks from two widely separatedtraverses. A portion of either traverse has experienced a high-gradedehydration event transforming it from an H₂O-rich, hornblende-bearingzone to an H₂O-poor, hornblende-free, orthopyroxene-bearing,‘granulite facies’ zone at 700–800°C and7–8 kbar. In the Kigluaik Mountains, Seward Peninsula,Alaska, dehydration took place over an 85 cm thick layer ofmetatonalite in contact with a marble during regional metamorphismand involved a CO₂-rich fluid, whereas for the Val Strona diOmegna traverse, Ivrea–Verbano Zone, northern Italy, dehydrationtook place over a 3–4 km thick sequence of metabasitesinterlayered with metapelites in a contact metamorphic eventinvolving basaltic magmas intruded at the base of the sequence.Orthopyroxene-bearing samples from both dehydration zones showmicro-veins of K-feldspar along quartz and plagioclase grainboundaries as well as replacement antiperthite in plagioclase.K came primarily from the breakdown of hornblende + quartz toorthopyroxene ± clinopyroxene, feldspar and fluid. Biotiteeither was stabilized or formed in the dehydration zones andis enriched in Ti, Mg, F and Cl relative to biotite in the amphibolitefacies zone. KEY WORDS: KCl–NaCl brines; metasomatism; granulite facies metamorphism; charnockite–enderbite; orthopyroxene; K-feldspar; biotite; hornblende     

Ultramafic Xenoliths from Bullenmerri and Gnotuk Maars, Victoria, Australia: Petrology of a Sub-Continental Crust-Mantle Transition

The basanite tuffs of Bullenmerri and Gnotuk maars, Victoria,enclose abundant xenoliths of spinel lherzolites, many of whichcontain amphibole ± apatite ± phlogopite. Thexenolith suite also includes cumulate wehrlites, spinel metapyroxenitesand garnet metapyroxenites. All xenolith types contain abundantlarge CO₂-rich fluid inclusions. Microstructural evidence forthe exsolution of spinel, orthopyroxene, garnet and rare plagioclasefrom complex clinopyroxenes suggests that all of the metapyroxeniteshave formed from clinopyroxene (± spinel ± orthopyroxene)cumulates by exsolution and recrystallization during coolingto the ambient geotherm. Pyroxene chemistry implies that a rangeof parental magma types was involved. Garnet pyroxenites showa series of reactions to successively finer-grained, lower-Pmineral assemblages, which imply a relatively slow initial upwardtransport of the xenoliths in the magma, prior to explosiveeruption. The same process has allowed crystallization of phenocrystsfrom small patches of interstitial melt within xenoliths oflherzolite, wehrlite and metapyroxenite. Critically selected P-T estimates for 16 garnet websteritesare consistent with published experimental studies of the spinel/garnetpyroxenite transition, and define a geotherm from 900 °C,11 kb to 1100 °C, 16 kb. Other published data extend thecurve down to c. 7 kb and up to 25 kb. This elevated geothermsuggests that the high regional heat flow is related to convectiveheat transfer by dike injection accompanying the vulcanism.T estimates for the lherzolites range from 850–1050 °C;comparison with the derived geotherm implies that the spinellherzolites are derived from depths of 30–55 km. Thiszone has low seismic velocities (V_p = 6.8–7.8 km/sec)and has thus previously been regarded as a thick, largely maficlower crust. The xenolith data show that this Mower crust' isdominantly ultramafic, with layers, dikes and some large bodiesof pyroxenites and mafic granulites. The anomalously low V_pmay be due to the high T, the high proportion of fluid-filledpore volume, and the magnesian composition of the lherzolites.The seismically defined Moho (V_p >8.0 km/sec) coincides withthe experimentally determined position of the spinel lherzolite-garnetlherzolite transition.