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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 pressure–temperature conditions of 9–10 kbarand 750–800°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 clinopyroxene2A–hornblende2A–plagioclase2Asymplectites at 8 kbar and 775°C (M2A stage), synchronouswith D2 and later anhydrous clinopyroxene2B–plagioclase2B–ilmenite2Bsymplectites 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 hornblende3–plagioclase3 symplectitesembaying garnet in mafic granulites were formed. P–T 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 P–T 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 continent–continent 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 P–T path; continental collision; metabasite  相似文献   

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 lawsonite–blueschist conditions in the SW to paragonite–eclogiteconditions 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 =0·7–1·0 GPa and T = 350–400°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) lawsonite–omphacite; (2) lawsonite–clinozoisite–spessartine;(3) clinozoisite–hornblende–almandine; (4) almandine–omphacite.S2 assemblages reflect a P–T gradient that spans the exposed15 km of the Diahot terrane from P = 0·8–1·0GPa and T = 350–400°C (Zone 1) to P = 1·6–1·7GPa and T = 550–600°C (Zone 4). The systematic mineralogicalchanges reflect parts of a P–T 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.
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  相似文献   

4.
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   

5.
Experimental studies were carried out to evaluate phase relationsinvolving titanite–F–Al-titanite solid solutionin the system CaSiO3–Al2SiO5–TiO2–CaF2. Theexperiments were conducted at 900–1000°C and 1·1–4·0GPa. 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.
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 P–T paths dominates in the westerndomains (Ugab Zone and western Northern Zone), and high-P Barrovianmetamorphism with a clockwise P–T 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 biotite–chlorite–muscoviteat the same time as an andalusite–biotite–muscovitetransposed foliation was developed in the matrix. The peak-Tmetamorphic assemblages and fabrics were overprinted by crenulationsand retrograde chlorite–muscovite. The KFMASH P–Tpseudosection for metapelites in the Ugab Zone and western NorthernZone contact aureoles indicates tight anticlockwise P–Tloops through peak metamorphic conditions of 540–570°Cand 2·5–3·2 kbar. These semi-quantitativeP–T loops are consistent with average P–T 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 P–T pseudosection and garnet compositionalisopleths in KFMASH, appropriate for the metapelite sample fromthis region, document a clockwise P–T path. Early plagioclase–kyanite–biotiteparageneses evolved by plagioclase consumption and the growthof garnet to increasing XFe, XMg and XCa and decreasing XMncompositions, indicating steep burial with heating. The developedkyanite–garnet–biotite peak metamorphic parageneseswere followed by the resorption of garnet and formation of plagioclasemoats, indicating decompression, which was followed by retrogressivecooling and chlorite–muscovite growth. The clockwise P–Tloop is consistent with the foreland vergent fold–thrustbelt geometry in this part of the northern margin. Earlier formed(580–570 Ma) pervasive matrix foliations (M2) were overprintedby contact metamorphic parageneses (M3) in the aureoles of 530± 3 Ma granites in the Ugab Zone and 553–514 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 535–530 Ma for the main phaseof deformation in the eastern Northern Zone and Northern Platformand 538–505 Ma high-grade metamorphism of the CentralZone immediately to the south. Consequently, NNE–SSW-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; P–T paths; pseudosections; low-P metamorphism; contact metamorphism; Barrovian metamorphism  相似文献   

7.
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 P–T loop, with minor decompressionnear the Al2SiO5 triple-point. Peak metamorphic temperaturesare estimated at 510–525°C, at 4·0–4·2kbar. 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. Monazite–xenotime thermometryfrom the intermediate and rim domains yields temperatures of405–470°C (±50°C) and 500–520°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 yield207Pb–206Pb 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   

8.
The solubility of sulfur as S2– has been experimentallydetermined for 19 silicate melt compositions in the system CaO–MgO–Al2O3–SiO2(CMAS)± TiO2 ± FeO, at 1400°C and 1 bar, using CO–CO2–SO2gas 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.
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 = 0·6–0·7). A numberof compositional varieties of garnet occur in the metabasite.An older porphyroblastic garnet (Py23–27, Alm41–43,Grs29–33) has two different compositional domains, onerelatively rich in Mg (Py27–30) and the other rich inCa (Grs35–38) with a low Mg (Py20–25) content. Theyoungest variety, which forms rims on, or microveins in, theporphyroblastic garnet, has high Ca and low Mg (Grs40–57,Py2–7, Alm46–51). 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 P–Tconditions of 550–583°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 Mg—Al pumpellyite were conducted using high-pressurecold-seal apparatus over the temperature range 250–600°C and 2–8 kb Pfluid. Mg—Al pumpellyite Ca4Al5MgSi6O21(OH)7was synthesized from partially crystalline gel mixtures of stoichiometriccomposition at 275–410 °C, 6–9 kb Pfluid, andruns of 7–90 days. Pure monomineralic synthetic Mg—Alpumpellyite 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 400–500°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 500–600°C. The equilibrium relations of Mg—Al pumpellyite were determinedusing subequal mixtures of synthetic Mg—Al pumpellyiteand its high temperature assemblage. The reaction 9 Mg—Alpumpellyite = 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 syntheticMg—Al 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 systemCaO–Al2O3–MgO(SiO2–H2O). The results, togetherwith reconnaissance experiments on the reaction 4 Mg—Alpumpellyite + 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 P–T 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 Mg—Al pumpellyite are useful todenote the onset of greenschist facies metamorphism in rocksof basaltic composition.  相似文献   

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