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1.
A suite of garnetiferous amphibolites and mafic granulites occuras small boudins within layered felsic migmatite gneiss in thenorthern part of the Sausar Mobile Belt (SMB), the latter constitutingthe southern component of the Proterozoic Central Indian TectonicZone (CITZ). Although the two types of metabasites are in variousstages of retrogression, textural, compositional and phase equilibriastudies attest to four distinct metamorphic episodes. The earlyprograde stage (Mo) is represented by an inclusion assemblageof hornblende1 + ilmenite1 + plagioclase1 ± quartz andgrowth zoning preserved in garnet. The peak assemblage (M1)consists of porphyroblastic garnet + clinopyroxene ±quartz ± rutile ± hornblende in mafic granulitesand garnet + quartz + hornblende in amphibolites and stabilizedat pressuretemperature conditions of 910 kbarand 750800°C and 8 kbar and 675°C, respectively.This was followed by near-isothermal decompression (M2), andpost-decompression cooling (M3) events. In mafic granulites,the former resulted in the development of early clinopyroxene2Ahornblende2Aplagioclase2Asymplectites at 8 kbar and 775°C (M2A stage), synchronouswith D2 and later anhydrous clinopyroxene2Bplagioclase2Bilmenite2Bsymplectites and coronal assemblages at 7 kbar, 750°C (M2Bstage) and post-dating D2. In amphibolites, ilmenite + plagioclase+ quartz ± hornblende symplectites appeared during M2at 6·4 kbar and 700°C. During M3, coronal garnet+ clinopyroxene + quartz ± hornblende-bearing symplectitesin metabasic dykes and hornblende3plagioclase3 symplectitesembaying garnet in mafic granulites were formed. PT estimatesshow near-isobaric cooling from 7 kbar and 750°C to 6 kbarand 650°C during M3. It is argued that the decompressionin the mafic granulites is not continuous, being punctuatedby a distinct heating (prograde?) event. The latter is alsocoincident with a period of extension, marked by mafic dykeemplacement. The combined PT path of evolution has aclockwise sense and provides evidence for a major phase of earlycontinental subduction in parts of the CITZ. This was followedby a later continentcontinent collision event duringwhich granulites of the first phase became tectonically interleavedwith younger lithological units. This tectonothermal event,of possibly Grenvillian age, marks the final amalgamation ofthe North and the South Indian Blocks along the CITZ to producethe Indian subcontinent. KEY WORDS: Central Indian Tectonic Zone; clockwise PT path; continental collision; metabasite 相似文献
2.
Lawsonite-Omphacite-Bearing Metabasites of the Pam Peninsula, NE New Caledonia: Evidence for Disrupted Blueschist- to Eclogite-Facies Conditions 总被引:2,自引:0,他引:2
The Diahot terrane of NE New Caledonia contains an interbeddedsequence of Cretaceous to Eocene metasediments, felsic and maficmetavolcanics that experienced c. 40 Ma high-P/T metamorphism.Metabasaltic assemblages define two prograde events (M1 andM2) and a tectonically disrupted crustal profile that extendsfrom lawsoniteblueschist conditions in the SW to paragoniteeclogiteconditions in the NE. Weakly deformed metabasalts from lowest-gradeparts of the Diahot terrane contain M1 omphacite, chlorite,lawsonite and glaucophane-bearing assemblages that partiallypseudomorph igneous plagioclase and augite, and reflect P =0·71·0 GPa and T = 350400°C.M1 assemblages are enveloped by a steeply SW-dipping S2 foliationthat becomes progressively more intense towards the NE overa distance of c. 15 km. S2 assemblages are divided into fourzones: (1) lawsoniteomphacite; (2) lawsoniteclinozoisitespessartine;(3) clinozoisitehornblendealmandine; (4) almandineomphacite.S2 assemblages reflect a PT gradient that spans the exposed15 km of the Diahot terrane from P = 0·81·0GPa and T = 350400°C (Zone 1) to P = 1·61·7GPa and T = 550600°C (Zone 4). The systematic mineralogicalchanges reflect parts of a PT array between 1·0and 1·7 GPa that was extensively disrupted by tectonicthinning during exhumation. KEY WORDS: blueschist; eclogite; New Caledonia; CNFMASH; pseudosection 相似文献
3.
High-Pressure Granulites (Retrograded Eclogites) from the Hengshan Complex, North China Craton: Petrology and Tectonic Implications 总被引:55,自引:1,他引:55
ZHAO GUOCHUN; CAWOOD PETER A.; WILDE SIMON A.; LU LIANGZHAO 《Journal of Petrology》2001,42(6):1141-1170
Both high- and medium-pressure granulites have been found asenclaves and boudins in tonalitictrondhjemiticgranodioriticgneisses 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) PT conditions of 13·415·5kbar and 770840°C for the high-pressure granulitesand 911 kbar and 820870°C for the medium-pressuregranulites, based on the core compositions of garnet, matrixpyroxene and plagioclase. The PT conditions of pyroxene+ plagioclase symplectite and corona (M3) were estimated at 相似文献
4.
Polymetamorphism in the NE Shackleton Range, Antarctica: Constraints from Petrology and U-Pb, Sm-Nd, Rb-Sr TIMS and in situ U-Pb LA-PIMMS Dating 总被引:1,自引:0,他引:1
Metapelitic rock samples from the NE Shackleton Range, Antarctica,include garnet with contrasting zonation patterns and two agespectra. Garnet porphyroblasts in K-rich kyanitesillimanitestaurolitegarnetmuscovitebiotite schistsfrom Lord Nunatak show prograde growth zonation, and give SmNdgarnet, UPb monazite and RbSr muscovite ages of518 ± 5, 514 ± 1 and 499 ± 12 Ma, respectively.Geothermobarometry and PT pseudo-section calculationsin the model system CaONa2OK2O TiO2MnOFeOMgOAl2O3SiO2H2Oare consistent with garnet growth during prograde heating from540°C/7 kbar to 650°C/7·5 kbar, and partial resorptionduring a subsequent PT 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 kyanitesillimanite staurolitegarnetcordieritebiotite-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 UPb 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. SmNd garnet datingyields an age of 1571 ± 40 Ma and RbSr 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 PT conditions of about 650°C/7·0kbar and a retrograde stage at c. 575°C/4·0 kbar. KEY WORDS: garnet microtexture; PT pseudosection; geochronology; polymetamorphism; Shackleton Range; Antarctica 相似文献
5.
Experimental studies were carried out to evaluate phase relationsinvolving titaniteFAl-titanite solid solutionin the system CaSiO3Al2SiO5TiO2CaF2. Theexperiments were conducted at 9001000°C and 1·14·0GPa. The average F/Al ratio in titanite solid solution in theexperimental run products is 1·01 ± 0·06,and XAl ranges from 0·33 ± 0·02 to 0·91± 0·05, consistent with the substitution [TiO2+]1[AlF2+]1.Analysis of the phase relations indicates that titanite solidsolutions coexisting with rutile are always low in XAl, whereasthe maximum XAl of titanite solid solution occurs with fluoriteand either anorthite or Al2SiO5. Reaction displacement experimentswere performed by adding fluorite to the assemblage anorthite+ rutile = titanite + kyanite. The reaction shifts from 1·60GPa to 1·15 ± 0·05 GPa at 900°C, from1·79 GPa to 1·375 ± 0·025 GPa at1000°C, and from 1·98 GPa to 1·575 ±0·025 GPa at 1100°C. The data show that the activityof CaTiSiO4O is very close to the ideal molecular activity model(XTi) at 1100°C, but shows a negative deviation at 1000°Cand 900°C. The results constrain 相似文献
6.
Al2SiO5 reaction textures in aluminous schist and quartziteof the northern Picuris range, north-central New Mexico, recorda paragenetic sequence of kyanite to sillimanite to andalusite,consistent with a clockwise PT loop, with minor decompressionnear the Al2SiO5 triple-point. Peak metamorphic temperaturesare estimated at 510525°C, at 4·04·2kbar. Kyanite and fibrolite are strongly deformed; some prismaticsillimanite, and all andalusite are relatively undeformed. Monaziteoccurs as inclusions within kyanite, mats of sillimanite andcentimetre-scale porphyroblasts of andalusite, and is typicallyaligned subparallel to the dominant regional foliation (S0/S1or S2) and extension lineation (L1). Back-scatter electron imagesand X-ray maps of monazite reveal distinct core, intermediateand rim compositional domains. Monazitexenotime thermometryfrom the intermediate and rim domains yields temperatures of405470°C (±50°C) and 500520°C(±50°C), respectively, consistent with the progradeto peak metamorphic growth of monazite. In situ, ion microprobeanalyses from five monazites yield an upper intercept age of1417 ± 9 Ma. Near-concordant to concordant analyses yield207Pb206Pb ages from 1434 ± 12 Ma (core) to 1390± 20 Ma (rim). We find no evidence of older regionalmetamorphism related to the 1650 Ma Mazatzal Orogeny. KEY WORDS: Al2SiO5; metamorphism; monazite; thermochronometry; triple-point 相似文献
7.
Variation in Metamorphic Style along the Northern Margin of the Damara Orogen, Namibia 总被引:2,自引:0,他引:2
The northern margin of the Inland Branch of the Pan-AfricanDamara Orogen in Namibia shows dramatic along-strike variationin metamorphic character during convergence between the Congoand Kalahari Cratons (M3 metamorphic cycle). Low-P contact metamorphismwith anticlockwise PT paths dominates in the westerndomains (Ugab Zone and western Northern Zone), and high-P Barrovianmetamorphism with a clockwise PT path is documented fromthe easternmost domain (eastern Northern Zone). The sequenceof M3 mineral growth in contact aureoles shows early growthof cordierite porphyroblasts that were pseudomorphed to biotitechloritemuscoviteat the same time as an andalusitebiotitemuscovitetransposed foliation was developed in the matrix. The peak-Tmetamorphic assemblages and fabrics were overprinted by crenulationsand retrograde chloritemuscovite. The KFMASH PTpseudosection for metapelites in the Ugab Zone and western NorthernZone contact aureoles indicates tight anticlockwise PTloops through peak metamorphic conditions of 540570°Cand 2·53·2 kbar. These semi-quantitativePT loops are consistent with average PT calculationsusing THERMOCALC, which give a pooled mean of 556 ± 26°Cand 3·2 ± 0·6 kbar, indicating a high averagethermal gradient of 50°C/km. In contrast, the eastern NorthernZone experienced deep burial, high-P/moderate-T Barrovian M3metamorphism with an average thermal gradient of 21°C/kmand peak metamorphic conditions of c. 635°C and 8·7kbar. The calculated PT pseudosection and garnet compositionalisopleths in KFMASH, appropriate for the metapelite sample fromthis region, document a clockwise PT path. Early plagioclasekyanitebiotiteparageneses evolved by plagioclase consumption and the growthof garnet to increasing XFe, XMg and XCa and decreasing XMncompositions, indicating steep burial with heating. The developedkyanitegarnetbiotite peak metamorphic parageneseswere followed by the resorption of garnet and formation of plagioclasemoats, indicating decompression, which was followed by retrogressivecooling and chloritemuscovite growth. The clockwise PTloop is consistent with the foreland vergent foldthrustbelt geometry in this part of the northern margin. Earlier formed(580570 Ma) pervasive matrix foliations (M2) were overprintedby contact metamorphic parageneses (M3) in the aureoles of 530± 3 Ma granites in the Ugab Zone and 553514 Magranites in the western Northern Zone. Available geochronologicaldata suggest that convergence between the Congo and KalahariCratons was essentially coeval in all parts of the northernmargin, with similar ages of 535530 Ma for the main phaseof deformation in the eastern Northern Zone and Northern Platformand 538505 Ma high-grade metamorphism of the CentralZone immediately to the south. Consequently, NNESSW-directedconvergent deformation and associated M3 metamorphism of contrastingstyles are interpreted to be broadly contemporaneous along thelength of the northern margin of the Inland Branch. In the westheat transfer was dominated by conduction and externally drivenby granites, whereas in the east heat transfer was dominatedby advection and internally driven radiogenic heat production.The ultimate cause was along-orogen variation in crustal architecture,including thickness of the passive margin lithosphere and thicknessof the overlying sedimentary succession. KEY WORDS: Pan-African Orogeny; PT paths; pseudosections; low-P metamorphism; contact metamorphism; Barrovian metamorphism 相似文献
8.
The Sulfide Capacity and the Sulfur Content at Sulfide Saturation of Silicate Melts at 1400{degrees}C and 1 bar 总被引:2,自引:0,他引:2
The solubility of sulfur as S2 has been experimentallydetermined for 19 silicate melt compositions in the system CaOMgOAl2O3SiO2(CMAS)± TiO2 ± FeO, at 1400°C and 1 bar, using COCO2SO2gas mixtures to vary oxygen fugacity (fO2) and sulfur fugacity(fS2). For all compositions, the S solubility is confirmed tobe proportional to (fS2/fO2)1/2, allowing the definition ofthe sulfide capacity (CS) of a silicate melt as CS = [S](fO2/fS2)1/2.Additional experiments covering over 150 melt compositions,including some with Na and K, were then used to determine CSas a function of melt composition at 1400°C. The resultswere fitted to the equation 相似文献
9.
Complex Growth Textures in a Polymetamorphic Metabasite from the Kraubath Massif (Eastern Alps) 总被引:1,自引:0,他引:1
Zoned garnet and amphibole occur in metabasites of the KraubathMassif, Eastern Alps, that contain relic magmatic clinopyroxene.The amphibole composition gradually changes from core (XMg =0·83) to rim (XMg = 0·60·7). A numberof compositional varieties of garnet occur in the metabasite.An older porphyroblastic garnet (Py2327, Alm4143,Grs2933) has two different compositional domains, onerelatively rich in Mg (Py2730) and the other rich inCa (Grs3538) with a low Mg (Py2025) content. Theyoungest variety, which forms rims on, or microveins in, theporphyroblastic garnet, has high Ca and low Mg (Grs4057,Py27, Alm4651). The amphibole cores and garnetporphyroblasts are interpreted to represent minerals formedduring Variscan regional metamorphism under amphibolite-faciesconditions. Alpine metamorphism is represented by the most recentCa-rich and Mg-poor variety of garnet that coexists with theamphibole rims, epidote and chlorite. Fracturing in the porphyroblasticgarnet probably originated during retrogression of the Variscanamphibolite-facies assemblages. Textural relations suggest thatthe garnet in the microveins formed by dehydration of hydrousphases during an Alpine metamorphic overprint that reached PTconditions of 550583°C at 1·0 GPa. KEY WORDS: microveins; garnet; metabasites; Kraubath Massif; Eastern Alps 相似文献
10.
Hydrothermal synthesis and investigations of stability relationsof MgAl pumpellyite were conducted using high-pressurecold-seal apparatus over the temperature range 250600°C and 28 kb Pfluid. MgAl pumpellyite Ca4Al5MgSi6O21(OH)7was synthesized from partially crystalline gel mixtures of stoichiometriccomposition at 275410 °C, 69 kb Pfluid, andruns of 790 days. Pure monomineralic synthetic MgAlpumpellyite has refractive index nß = 1.624 (2) andcell dimensions = 8.825 (8) Á, b = 5.875 (5) Á,c = 19.10 (1) Á, and ß = 97.39 (7)°. The high temperature assemblage of the equivalent bulk compositionconsists of clinozoisite, hydrogrossular/grossular, aluminousseptechlorite/chlorite, quartz, and H2O. Hydrogrossular wassynthesized in the presence of quartz at 8 kb from 400500°C, and hydrogrossular + quartz are unstable with respectto grossular + H2O at 400 °C and 8 kb Pfluid. At 8 kb Pfluid,aluminous septechlorite forms at temperatures below 500 °Cwhereas aluminous 14 Á chlorite crystallizes at 500600°C. The equilibrium relations of MgAl pumpellyite were determinedusing subequal mixtures of synthetic MgAl pumpellyiteand its high temperature assemblage. The reaction 9 MgAlpumpellyite = 9 clinozoisite + 6 grossular + 2 chlorite + 4quartz + 19 H2O occurs at temperatures of 390 °C at 8 kb,368 °C at 5 kb, and near 325 °C at 2 kb Pfluid. Thereversal data yield an approximate value of 3141 joules/mole°K for the standard entropy of formation for the syntheticMgAl pumpellyite. The Schreinemakers' relations for pumpellyite, prehnite, clinozoisite,tremolite, grossular, and amesite in the presence of excessquartz and fluid were constructed in the pseudo-ternary systemCaOAl2O3MgO(SiO2H2O). The results, togetherwith reconnaissance experiments on the reaction 4 MgAlpumpellyite + 2 quartz = 8 prehnite + aluminous septechlorite+ 2 H2O, locate the invariant point [TR] at approximately 5.7kb Pfluid and 375 °C. The results of the present study arenot compatible with previous experimental data on the invariantpoint [GR]. The PT oriented phase relations are used to interpretsome natural parageneses developed in low-grade metabasalticrocks recrystallized under conditions of low co2. The high-temperaturestability relations of MgAl pumpellyite are useful todenote the onset of greenschist facies metamorphism in rocksof basaltic composition. 相似文献
11.
MARKS MICHAEL; VENNEMANN TORSTEN; SIEBEL WOLFGANG; MARKL GREGOR 《Journal of Petrology》2003,44(7):1247-1280
The Puklen complex of the Mid-Proterozoic Gardar Province, SouthGreenland, consists of various silica-saturated to quartz-bearingsyenites, which are intruded by a peralkaline granite. The primarymafic minerals in the syenites are augite ± olivine +FeTi oxide + amphibole. Ternary feldspar thermometryand phase equilibria among mafic silicates yield T = 950750°C,aSiO2 = 0·71 and an fO2 of 13 log unitsbelow the fayalitemagnetitequartz (FMQ) bufferat 1 kbar. In the granites, the primary mafic minerals are ilmeniteand Li-bearing arfvedsonite, which crystallized at temperaturesbelow 750°C and at fO2 values around the FMQ buffer. Inboth rock types, a secondary post-magmatic assemblage overprintsthe primary magmatic phases. In syenites, primary Ca-bearingminerals are replaced by Na-rich minerals such as aegirineaugiteand albite, resulting in the release of Ca. Accordingly, secondaryminerals include ferro-actinolite, (calcitesiderite)ss,titanite and andradite in equilibrium with the Na-rich minerals.Phase equilibria indicate that formation of these minerals tookplace over a long temperature interval from near-magmatic temperaturesdown to 相似文献
12.
Solubility of Anhydrite, CaSO4, in NaCl-H2O Solutions at High Pressures and Temperatures: Applications to Fluid-Rock Interaction 总被引:1,自引:0,他引:1
Anhydrite solubility in H2ONaCl solutions was measuredat 614 kbar, 600800°C and NaCl mole fractions(XNaCl) of 00·3 in pistoncylinder apparatus.Solubilities were determined by weight changes of natural anhydritein perforated Pt envelopes confined with fluid in larger Ptcapsules. In initially pure H2O at 10 kbar and 800°C, CaSO4concentration is low (0·03 molal), though much largerthan at the same temperature and 1 kbar. Hematite-buffered experimentsshowed slightly lower solubilities than unbuffered runs. CaSO4solubility increases enormously with NaCl activity: at 800°Cand 10 kbar and XNaCl of 0·3, CaSO4 molality is 200 timeshigher than with pure H2O. Whereas CaSO4 solubility in pureH2O decreases with rising T at low T and P, the high-P resultsshow that anhydrite solubility increases with T at constantP at all XNaCl investigated. The effects of salinity and temperatureare so great at 10 kbar that critical mixing between sulfate-richhydrosaline melts and aqueous salt solutions is probable at900°C at XNaCl 0·3. Recent experimental evidencethat volatile-laden magmas crystallizing in the deep crust mayevolve concentrated salt solutions could, in light of the presentwork, have important implications regarding such diverse processesas Mount Pinatubo-type S-rich volcanism, high-f O2 regionalmetamorphism, and emplacement of porphyry CuMo ore bodies,where anhydritehematite alteration and fluid inclusionsreveal the action of very oxidized saline solutions rich insulfur. KEY WORDS: anhydrite; sulfur; solubility; metamorphic brines; granulites 相似文献
13.
We present an experimental and petrological study aimed at estimatingthe pre-eruptive conditions of a Holocene dacitic lava fromVolcán San Pedro (36°S, Chilean Andes). Phase-equilibriumexperiments were performed at temperatures (T) from 800 to 950°C,and mainly at 200 MPa, but also at 55, 150, and 406 MPa. Oxygenfugacity (fO2) ranged from the NiNiO buffer (NNO) to3·5 log units above (NNO + 3·5), and water contentsfrom 相似文献
14.
Multianvil melting experiments in the system CaOMgOAl2O3SiO2CO2(CMASCO2) at 38 GPa, 13401800°C, involvingthe garnet lherzolite phase assemblage in equilibrium with CO2-bearingmelts, yield continuous gradations in melt composition betweencarbonatite, kimberlite, melilitite, komatiite, picrite, andbasalt melts. The phase relations encompass a divariant surfacein PT space. Comparison of the carbonatitic melts producedat the low-temperature side of this surface with naturally occurringcarbonatites indicates that natural magnesiocarbonatites couldbe generated over a wide range of pressures >2·5 GPa.Melts analogous to kimberlites form at higher temperatures alongthe divariant surface, which suggests that kimberlite genesisrequires more elevated geotherms. However, the amount of waterfound in some kimberlites has the potential to lower temperaturesfor the generation of kimberlitic melts by up to 150°C,provided no hydrous phases are present. Compositions resemblinggroup IB and IA kimberlites are produced at pressures around56 GPa and 10 GPa, respectively, whereas the compositionsof some other kimberlites suggest generation at higher pressuresstill. At pressures <4 GPa, an elevated geotherm producesmelilitite-like melt in the CMASCO2 system rather thankimberlite. Even when a relatively CO2-rich mantle compositioncontaining 0·15 wt % CO2 is assumed, kimberlites andmelilitites are produced by <1% melting and carbonatitesare generated by even smaller degrees of melting of <0·5%. KEY WORDS: carbonatite; CO2; kimberlite; melilitite; melt generation 相似文献
15.
The unmetamorphosed equivalents of the regionally metamorphosedclays and marls that make up the Alpine Liassic black shaleformation consist of illite, irregular mixed-layer illite/montmorillonite,chlorite, kaolinite, quartz, calcite, and dolomite, with accessoryfeldspars and organic material. At higher grade, in the anchizonalslates, pyrophyllite is present and is thought to have formedat the expense of kaolinite; paragonite and a mixed-layer paragonite/muscovitepresumably formed from the mixed-layer illite/montmorillonite.Anchimetamorphic illite is poorer in Fe and Mg than at the diageneticstage, having lost these elements during the formation of chlorite.Detrital feldspar has disappeared. In epimetamorphic phyllites, chloritoid and margarite appearby the reactions pyrophyllite + chlorite = chloritoid + quartz+ H2O and pyrophyllite + calcite ± paragonite = margarite+ quartz + H2O + CO2, respectively. At the epi-mesozone transition,paragonite and chloritoid seem to become incompatible in thepresence of carbonates and yield the following breakdown products:plagioclase, margarite, clinozoisite (and minor zoisite), andbiotite. The maximum distribution of margarite is at the epizone-mesozoneboundary; at higher metamorphic grade margarite is consumedby a continuous reaction producing plagioclase. Most of the observed assemblages in the anchi-and epizone canbe treated in the two subsystems MgO (or FeO)-Na2OCaOAl2O3(KAl3O5SiO2H2OCO2).Chemographic analyses show that the variance of assemblagesdecreases with increasing metamorphic grade. Physical conditions are estimated from calibrated mineral reactionsand other petrographic data. The composition of the fluid phasewas low in XCO2 throughout the metamorphic profile, whereasXCH4 was very high, particularly in the anchizone where aH2Owas probably as low as 0.2. P-T conditions along the metamorphicprofile are 12 kb/200300 °C in the anchizone(Glarus Alps), and 5 kb/500550 °C at the epi-mesozonetransition (Lukmanier area). Calculated geothermal gradientsdecrease from 50 °C/km in the anchimetamorphic Glarus Alpsto 30 °C/km at the epi-mesozone transition of the Lukmanierarea. 相似文献
16.
Kistufell: Primitive Melt from the Iceland Mantle Plume 总被引:5,自引:2,他引:5
This paper presents new geochemical data from Kistufell (64°48'N,17°13'W), a monogenetic table mountain situated directlyabove the inferred locus of the Iceland mantle plume. Kistufellis composed of the most primitive olivine tholeiitic glassesfound in central Iceland (MgO 10·56 wt %, olivine Fo89·7).The glasses are interpreted as near-primary, high-degree plumemelts derived from a heterogeneous mantle source. Mineral, glassand bulk-rock (glass + minerals) chemistry indicates a low averagemelting pressure (15 kbar), high initial crystallization pressuresand temperatures (1015 kbar and 1270°C), and eruptiontemperatures (1240°C) that are among the highest observedin Iceland. The glasses have trace element signatures (Lan/Ybn<1, Ban/Zrn 0·550·58) indicative ofa trace element depleted source, and the SrNdPbisotopic ratios (87Sr/86Sr 0·703040·70308,143Nd/144Nd 0·5130580·513099, 206Pb/204Pb18·34318·361) further suggest a long-termtrace element depletion relative to primordial mantle. HighHe isotopic ratios (15·316·8 R/Ra) combinedwith low 207Pb/204Pb (15·4215·43) suggestthat the mantle source of the magma is different from that ofNorth Atlantic mid-ocean ridge basalt. Negative Pb anomalies,and positive Nb and Ta anomalies indicate that the source includesa recycled, subducted oceanic crustal or mantle component. PositiveSr anomalies (Srn/Ndn = 1·391·50) furthersuggest that this recycled source component involves lower oceaniccrustal gabbros. The 相似文献
17.
Magmatic Evolution of the La Pacana Caldera System, Central Andes, Chile: Compositional Variation of Two Cogenetic, Large-Volume Felsic Ignimbrites 总被引:10,自引:3,他引:10
LINDSAY J. M.; SCHMITT A. K.; TRUMBULL R. B.; DE SILVA S. L.; SIEBEL W.; EMMERMANN R. 《Journal of Petrology》2001,42(3):459-486
La Pacana is one of the largest known calderas on Earth, andis the source of at least two major ignimbrite eruptions witha combined volume of some 2700 km3. These ignimbrites have stronglycontrasting compositions, raising the question of whether theyare genetically related. The Toconao ignimbrite is crystal poor,and contains rhyolitic (7677 wt % SiO2) tube pumices.The overlying Atana ignimbrite is a homogeneous tuff whose pumiceis dacitic (6670 wt % SiO2), dense (4060% vesicularity)and crystal rich (3040 % crystals). Phase equilibriaindicate that the Atana magma equilibrated at temperatures of770790°C with melt water contents of 3·14·4wt %. The pre-eruptive Toconao magma was cooler (730750°C)and its melt more water rich (6·36·8 wt% H2O). A pressure of 200 MPa is inferred from mineral barometryfor the Atana magma chamber. Isotope compositions are variablebut overlapping for both units (87Sr/86Sri 0·70940·7131;143Nd/144Nd 0·512220·51230) and are consistentwith a dominantly crustal origin. Glass analyses from Atanapumices are similar in composition to those in Toconao tubepumices, demonstrating that the Toconao magma could representa differentiated melt of the Atana magma. Fractional crystallizationmodelling suggests that the Toconao magma can be produced by30% crystallization of the observed Atana mineral phases. Toconaomelt characteristics and intensive parameters are consistentwith a volatile oversaturation-driven eruption. However, thelow H2O content, high viscosity and high crystal content ofthe Atana magma imply an external eruption trigger. KEY WORDS: Central Andes; crystal-rich dacite; eruption trigger; high-silica rhyolite; zoned magma chamber 相似文献
18.
GARCIA-CASCO ANTONIO; HAISSEN FAOUZIYIA; CASTRO ANTONIO; EL-HMIDI HASSAN; TORRES-ROLDAN RAFAEL LUIS; MILLAN GUILLERMO 《Journal of Petrology》2003,44(10):1727-1757
We document experiments on a natural metapelite in the range650775°C, 614 kbar, 10 wt % of added water,and 700850°C, 410 kbar, no added water. Staurolitesystematically formed in the fluid-present melting experimentsabove 675°C, but formed only sporadically in the fluid-absentmelting experiments. The analysis of textures, phase assemblages,and variation of phase composition and FeMg partitioningwith P and T suggests that supersolidus staurolite formed at(near-) equilibrium during fluid-present melting reactions.The experimental results are used to work out the phase relationsin the system K2ONa2OFeOMgOAl2O3SiO2H2Oappropriate for initial melting of metapelites at the upperamphibolite facies. The PT grid developed predicts theexistence of a stable PT field for supersolidus staurolitethat should be encountered by aluminous Fe-rich metapelitesduring fluid-present melting at relatively low temperature andintermediate pressures (675700°C, 610 kbarfor XH2O = 1, in the KNFMASH system), but not during fluid-absentmelting. The implications of these findings for the scarcityof staurolite in migmatites are discussed. KEY WORDS: metapelites; migmatites; partial melting; PT grid; staurolite 相似文献
19.
The redox state of sillimanite zone (650700°C, 56kbar) metasediments of the Barrovian type area, Scotland, wasinvestigated using estimates of metamorphic oxygen fugacity(fO2), sulfur fugacity (fS2), and fluid chemistry based on newdeterminations of mineral and rock compositions from 33 samples.A total of 94% of the samples lack graphite, contain both ilmenitehematitesolid solutions (RHOMOX) and magnetite, and had metamorphicfO2 about 2 log10 units above the quartzfayalitemagnetite(QFM) buffer. The regional variation in metamorphic fO2 forthese rocks was minimal, about ±0·3 log10 units,reflecting either a protolith that was homogeneous with respectto redox state, or an initially variable protolith whose redoxstate was homogenized by metamorphic fluidrock interaction.RHOMOX inclusions in garnet porphyroblasts that become richerin ilmenite from the interior to the edge of the host porphyroblastsuggest that at least some syn-metamorphic reduction of rockoccurred. Significant variations in bulk-rock oxidation ratio(OR) that are probably inherited from sedimentary protolithsare found from one layer to the next; OR ranges mostly between 相似文献
20.
Boninites are an important end-member supra-subductionzone magmatic suite as they have the highest H2O contents andrequire the most refractory of mantle wedge sources. The pressuretemperatureconditions of boninite origins in the mantle wedge are importantto understanding subduction zone initiation and subsequent evolution.Reaction experiments at 1·5 GPa (13501530°C),2 GPa (14001600°C) and 2·5 GPa (14501530°C)between a model primary high-Ca boninite magma composition anda refractory harzburgite under anhydrous and H2O-undersaturatedconditions (23 wt % H2O in the melt) have been completed.The boninite composition was modelled on melt inclusions occurringin the most magnesian olivine phenocrysts in high-Ca boninitesfrom the Northern Tongan forearc and the Upper Pillow Lavasof the Troodos ophiolite. Direct melting experiments on a modelrefractory lherzolite and a harzburgite composition at 1·5GPa under anhydrous conditions (14001600°C) havealso been completed. Experiments establish a P, T meltinggrid for refractory harzburgite at 1·5, 2 and2·5 GPa and in the presence of 23 wt % H2O. Theeffect of 23 wt % dissolved H2O produces a liquidus depressionin primary boninite of 相似文献