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1.
Mg-Al Sapphirine- and Ca-Al Hibonite-bearing Granulite Xenoliths from the Chyulu Hills Volcanic Field, Kenya 总被引:2,自引:0,他引:2
Basanites of the Chyulu Hills (Kenya Rift) contain mafic MgAland CaAl granulite xenoliths. Their protoliths are interpretedas troctolitic cumulates; however, the original mineral assemblageswere almost completely transformed by subsolidus reactions.MgAl granulites contain the minerals spinel, sapphirine,sillimanite, plagioclase, corundum, clinopyroxene, orthopyroxeneand garnet, whereas CaAl granulites are characterizedby hibonite, spinel, sapphirine, mullite, sillimanite, plagioclase,quartz, clinopyroxene, corundum, and garnet. In the MgAlgranulites, the first generation of orthopyroxene and some spinelmay be of igneous origin. In the CaAl granulites, hibonite(and possibly some spinel) are the earliest, possibly igneous,minerals in the crystallization sequence. Most pyroxene, spineland corundum in MgAl and CaAl granulites formedby subsolidus reactions. The qualitative PT path derivedfrom metamorphic reactions corresponds to subsolidus cooling,probably accompanied, or followed by, compression. Final equilibrationwas achieved at T 600740°C and P <8 kbar, inthe stability field of sillimanite. The early coexistence ofcorundum and pyroxenes (± spinel), as well as the associationof sillimanite and sapphirine with clinopyroxene and the presenceof hibonite, makes both types of granulite rare. The CaAlhibonite-bearing granulites are unique. Both types enlarge thespectrum of known CaAlMg-rich granulites worldwide. KEY WORDS: granulite xenoliths; corundum; sapphirine; hibonite; Kenya Rift 相似文献
2.
The Southern Yenisey Range (Eastern Siberia) consists of thegranulite-facies Kanskiy complex bordered by the lower-gradeYeniseyskiy and Yukseevskiy complexes. Samples of metapeliteof the Kanskiy complex typically show characteristic garnet-formingreaction textures and near-isobaric cooling PT paths.An important new result of this study concerns the differencein shape of the PT paths from different parts of theKanskiy granulite complex: metapelites collected 8 km from theboundary with the Yeniseyskiy complex followed a linear pathwith dP/dT 0·006 kbar/°C; metapelites collected3 km from this boundary reveal a kinked PT path withan interval of burial cooling (dP/dT 0·006 kbar/°C).The difference in the shape of the PT paths is supportedby the chemical zoning of garnet studied in the second groupof samples. A mechanism of buoyant exhumation of granulite issuggested by comparison with the results of numerical modelling,which indicate that such a diversity of PT paths mayresult from a transient disturbance of the thermal structureby rapid differential movement of material from different crustallevels. To arrive at a correct tectonic interpretation, thewhole assemblage of interrelated PT paths of metamorphicrocks collected from different localities within the same complexmust be studied. KEY WORDS: crustal diapirism; exhumation; granulites; numerical modelling; PT path 相似文献
3.
The Archaean rocks of West Greenland are predominantly bandedquartzofeldspathic gneisses enclosing thin migmatized sheetsof older metabasic and metasedimentary lithologies. Isotopicdating (Moorbath & Pankhurst, 1976) indicates that muchof the Archaean crust presently exposed in southern West Greenlandwas generated from predominantly upper mantle sources 3000-2800m.y. ago. Parts of this crust crystallized under prograde granulitefacies conditions 2950-2750 m.y. ago. The granulite gneisseswere severely depleted in some of the lithophile and heat producingelements, Si, Na, Sr, Rb, U and Th, during the granulite metamorphism.These elements appear to have been transferred to higher crustallevels by the migration of a dispersed vapour phase. The pressureand temperature recorded by amphibolite and granulite faciesassemblages have been estimated using thermometers and barometerscalibrated against the results of phase equilibrium experiments.These estimates (800 °C, 10.5 kb and 630 °C, 7.3 kbfor granulite and amphibolite facies respectively) indicatethat the Archaean continental crust in southern West Greenlandwas at least 3040 km thick 2800 m.y. ago. Water vapourpressures in the granulites were extremely low, 0.3 to 0.1 Ptotal.The thermal evolution of the Archaean crust during the period3000-2700 m.y. was controlled by the emplacement of large volumesof acid-intermediate melts into a relatively thin metabasiccrust. The thermal perturbations generated by this convectivetransfer of heat from the upper mantle to the crust relaxedduring the period immediately following crustal accretion. Progradegranulite facies assemblages could have developed under stronglydehydrating conditions and progressively falling temperatures,or during a phase of rising temperature in the lower crust. 相似文献
4.
Three types of garnet have been distinguished in pelitic schistsfrom an epidoteblueschist-facies unit of the Ambin andSouth Vanoise Briançonnais massifs on the basis of texture,chemical zoning and mineral inclusion characterization. Type-1garnet cores with high Mn/Ca ratios are interpreted as pre-Alpinerelicts, whereas Type-1 garnet rims, Type-2 inclusion-rich porphyroblastsand smaller Type-3 garnets are Alpine. The latter are all characterizedby low Mn/Ca ratios and a coexisting mineral assemblage of blueamphibole, high-Si phengite, epidote and quartz. Prograde growthconditions during Alpine D1 high-pressure (HP) metamorphismare recorded by a decrease in Mn and increase in Fe (±Ca)in the Type-2 garnets, culminating in peak PT conditionsof 1416 kbar and 500°C in the deepest parts of theAmbin dome. The multistage growth history of Type-1 garnetsindicates a polymetamorphic history for the Ambin and SouthVanoise massifs; unfortunately, no age constraints are available.The new metamorphic constraints on the Alpine event in the massifsdefine a metamorphic T gap between them and theirsurrounding cover (Briançonnais and upper Schistes Lustrésunits), which experienced metamorphism only in the stabilityfield of carpholitelawsonite (T < 400°C). Thesedata and supporting structural studies confirm that the Ambinand South Vanoise massifs are slices of eclogitizedcontinental crust tectonically extruded within the SchistesLustrés units and Briançonnais covers. The correspondingtectonic contacts with top-to-east movement are responsiblefor the juxtaposition of lower-grade metamorphic units on theAmbin and South Vanoise massifs. KEY WORDS: Alpine HP metamorphism; Ambin and South Vanoise Briançonnais basements; metamorphic gaps; multistage garnets; Western Alps 相似文献
5.
Mesoproterozoic Reworking of Palaeoproterozoic Ultrahigh-temperature Granulites in the Central Indian Tectonic Zone and its Implications 总被引:3,自引:0,他引:3
BHOWMIK S. K.; SARBADHIKARI A. BASU; SPIERING B.; RAITH M. M. 《Journal of Petrology》2005,46(6):1085-1119
In the southern periphery of the Sausar Mobile Belt (SMB), thesouthern component of the Central Indian Tectonic Zone (CITZ),a suite of felsic and aluminous granulites, intruded by gabbro,noritic gabbro, norite and orthopyroxenite, records the polymetamorphicevolution of the CITZ. Using sequences of prograde, peak andretrograde reaction textures, mineral chemistry, geothermobarometricresults and petrogenetic grid considerations from the felsicand the aluminous granulites and applying metamorphosed maficdyke markers and geochronological constraints, two temporallyunrelated granulite-facies tectonothermal events of Pre-Grenvillianage have been established. The first event caused ultrahigh-temperature(UHT) metamorphism (M1) (T 950°C) at relatively deepercrustal levels (P 9 kbar) and a subsequent post-peak near-isobariccooling PT history (M2). M1 caused pervasive biotite-dehydrationmelting, producing garnetorthopyroxene and garnetrutileand sapphirinespinel-bearing incongruent solid assemblagesin felsic and aluminous granulites, respectively. During M2,garnetcorundum and later spinelsillimanitebiotiteassemblages were produced by reacting sapphirinespinelsillimaniteand rehydration of garnetcorundum assemblages, respectively.Applying electron microprobe (EMP) dating techniques to monazitesincluded in M1 garnet or occurring in low-strain domains inthe felsic granulites, the UHT metamorphism is dated at 20402090Ma. Based on the deep crustal heatingcooling PTtrajectory, the authors infer an overall counterclockwise PTpath for this UHT event. During the second granulite event,the Palaeoproterozoic granulites experienced crustal attenuationto 6·4 kbar at T 675°C during M3 and subsequentnear-isothermal loading to 8 kbar during M4. In the felsic granulites,the former is marked by decomposition of M1 garnet to orthopyroxeneplagioclasesymplectites. During M4, there was renewed growth of garnetquartzsymplectites in the felsic granulites, replacing the M3 mineralassemblage and also the appearance of coronal garnetquartzclinopyroxeneassemblages in metamorphosed mafic dykes. Using monazites frommetamorphic overgrowths and metamorphic recrystallization domainsfrom the felsic granulite, the M4 metamorphism is dated at 15251450Ma. Using geochronological and metamorphic constraints, theauthors interpret the M3M4 stages to be part of the sameMesoproterozoic tectonothermal event. The result provides thefirst documentation of UHT metamorphism and Palaeo- and Mesoproterozoicmetamorphic processes in the CITZ. On a broader scale, the findingsare also consistent with the current prediction that isobaricallycooled granulites require a separate orogeny for their exhumation. KEY WORDS: Central Indian Tectonic Zone; UHT metamorphism; counterclockwise PT path; monazite chemical dating 相似文献
6.
Mg- and Al-rich granulites of the central Highland Complex,Sri Lanka preserve a range of reaction textures indicative ofa multistage PT history following an ultrahigh-temperaturemetamorphic peak. The granulites contain a near-peak assemblageof sapphirinegarnetorthopyroxenesillimanitequartzK-feldspar,which was later overprinted by intergrowth, symplectite andcorona textures involving orthopyroxene, sapphirine, cordieriteand spinel. Biotite-rims, kornerupine and orthopyroxene-rimson biotite are considered to be late assemblages. Thermobarometriccalculations yield an estimated PT of at least 1100°Cand 12 kbar for the near-peak metamorphism. Isopleths of Al2O3in orthopyroxene are consistent with a peak temperature above1150°C. The PT path consists of four segments. Initialisobaric cooling after peak metamorphism (Segment A), whichproduced the garnetsapphirinequartz assemblage,was followed by near-isothermal decompression at ultrahigh temperature(Segment B), which produced the multiphase symplectites. Furtherisobaric cooling (Segment C) resulted in the formation of biotiteand kornerupine, and late isothermal decompression (SegmentD) formed orthopyroxene rims on biotite. This evolution canbe correlated with similar PT paths elsewhere, but thereare not yet sufficient geochronological and structural dataavailable from the Highland Complex to allow the tectonic implicationsto be fully assessed. KEY WORDS: central Highland Complex; granulites; multistage evolution; Sri Lanka; UHT metamorphism 相似文献
7.
A high-grade metamorphic terrane in the southern part of theCalabrian massif (South Italy) has been petrographically mappedand the dominant rock types petrologically investigated. Bothmethods of investigation have led to the recognition of a continuoussection through a former lower crust which is 7 km thick. Itslower part consists predominantly of metabasic rocks togetherwith minor felsic granulites, its upper part of metapeliteswith minor metabasic and metacarbonate rocks. The rocks experienced a common two-stage prograde metamorphicevolution in which the second stage occurred after the lastpenetrative deformation. The prograde metamorphism which, accordingto radiometric dates, ended in late Hercynian time, was of themedium-pressure type of Miyashiro (1961), and equilibrationoccurred in the medium-pressure granulite field(characterized by the instability of olivine-plagioclase aswell as garnet-clinopyroxene-quartz). Estimates of the highestPT conditions of prograde metamorphism give 78kb and approximately 800°C at the base, but 56 kband 650700°C at the top of the section, at whichthe paragenesis staurolite-quartz indicates the transition tothe amphibolite facies. The existence of a metamorphic gradientin the lower crust section is demonstrated by the systematicchange in the compositions of ferro-magnesian minerals in divariantmetapelitic assemblages. The metamorphic evolution during the excavation history of theformer lower crust has been reconstructed using the numerousdisequilibrium reaction textures preserved in most rock types.The highest metamorphic conditions ended with a pressure decreaseof approximately 1.5 to 2 kb, which was followed by a periodof quasi-isobaric cooling in the middle crust. During this cooling,the stability field of the high-pressure granulites(garnet-clinopyroxene-quartz) was reached. The pressure decrease, which induced the end of the high-temperaturehistory of the lower crust, is interpreted as reflecting theerosion of the uppermost crustal levels as a response to overlappingof large crustal segments during the Hercynian orogeny. Consequently,the deduced PT path of the upper, i.e. overthrust crustalsegment is thought to have been tectonically controlled. 相似文献
8.
Thermobarometry and Geotectonic Significance of High-Pressure Granulites: Examples from the Moldanubian Zone of the Bohemian Massif in Lower Austria 总被引:14,自引:1,他引:14
Petrographic details together with mineral and whole-rock compositiondata are provided for acid-intermediate garnet granulites fromexposed granulite complexes in Lower Austria. Thermobarometricevaluation integrated with available isotopic age data indicatesthe initial equilibration of early Variscan ({small tilde}370Ma) high-pressure granulite assemblages at {small tilde}16 kbarand 1000C and their partial overprinting by retrograde assemblageswhich reflect the blocking of mineral exchange reaction equilibriaat {small tilde}65 kbar and 725C during subsequent Variscanuplift and cooling. These calculated P-T estimates, togetherwith general phase equilibria constraints and evidence frompreserved prograde coronitic reaction textures and garnet compositionalzoning profiles, indicate a clockwise P-T-t evolutionary pathof the type expected during crustal thickening in a major platecollision orogen and characterized by near-isothermal decompressionduring initial uplift. Geochemical characterization of the rockprotoliths as calc-alkaline igneous rocks and the high metamorphictemperatures suggest that garnet granulite formation involvedthe subduction of a magmatic arc at a continental plate margin. Reviewed evidence from granulites in the Central European Variscidesruns counter to suggestions by Bohlen (1987, 1991) that high-pressuregranulites are of little regional geotectonic significance incomparison with low- to medium-pressure granulites. The differentevolutionary P-T paths for these two important groups of granulitespoint to formation in contrasting plate settings. However, questionsare raised regarding petrogenetic models for low- to medium-pressuregranulites which have emphasized the importance of magmatic,rather than tectonic, crustal thickening and the recognitionof stabilization along deduced anti-clockwise P-T-t paths characterizedby post-peak near-isobaric cooling. It is suggested here that the reality of stabilization of atleast some low- to medium-pressure granulites in a collisionaltectonic regime may have been concealed either because lower-pressureassemblages have overprinted mineralogical evidence for an earlierhigh-pressure history at deeper crustal levels or through invaliddeduction of near-isobaric cooling trajectories as a resultof the different closure temperatures for the mineral reactionsused to monitor the equilibration temperatures and pressuresin granulites. However, the sequential underthrusting modelfavoured for the tectonometamorphic evolution of the Variscannappe pile in the Bohemian Massif renders it unlikely that alllate Variscan low- to medium-pressure granulites have experiencedthe early Variscan high-pressure metamorphism. 相似文献
9.
Processes of crystal separation in a magma heavily laden withcrystals without phase change are investigated from observationson frozen magma systems: Nosappumisaki and other shoshoniteintrusions in the Nemuro peninsula, Japan, for which the originof the crystals and the initial conditions are well constrained.The Nosappumisaki intrusion is 120 m in thickness and extendsfor more than 1·5 km. It exhibits a wide range of lithologicalvariation, principally as a result of crystal redistributionafter intrusion. Crystals in each lithology can be clearly dividedinto two kinds according to their composition and texture: thosepresent before the intrusion of the magma (phenocrysts)and those that crystallized in situ after intrusion. From thevertical change in mode and size of phenocrysts,it is shown that (1) augite phenocrysts were rapidlydeposited, with little overgrowth after intrusion, by significantcoagulation or clustering on a time-scale of more than a fewyears, and (2) plagioclase phenocrysts, definitelydenser than the melt but concentrated in the upper level, floatedby counter flow of massive deposition of augite phenocrysts.These results indicate that in a magma heavily laden with crystalsof a few millimeters in size (>20 vol. %), crystalcrystaland crystalmelt interaction play an important role inthe separation of crystals from the host melt. KEY WORDS: magma chamber; sill; crystal settling; plagioclase flotation; Nosappumisaki 相似文献
10.
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 PT conditions of 9·09·5kbar and 880950°C and 6·67·2kbar and 700750°C, respectively. Similar but slightlyscattered peak PT estimates of 7·910·1kbar and 820980°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 microprobeUPb zircon dating reveals an evolutionary history forthe granulites, including a mafic and felsic igneous intrusionat 11741019 Ma, sedimentation after 932916 Ma,and a high-grade metamorphism at 533529 Ma. In contrast,SmNd mineralwhole-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 PT conditions and a decompression-dominated PTpath 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 PTt 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; PT path; Prydz Belt; East Antarctica 相似文献
11.
Temperatures of Granulite-facies Metamorphism: Constraints from Experimental Phase Equilibria and Thermobarometry Corrected for Retrograde Exchange 总被引:17,自引:7,他引:17
PATTISON DAVID R. M.; CHACKO THOMAS; FARQUHAR JAMES; MCFARLANE CHRISTOPHER R. M. 《Journal of Petrology》2003,44(5):867-900
This study assesses temperatures of formation of common granulitesby combining experimental constraints on the PT stabilityof granulite-facies mineral associations with a garnetorthopyroxene(GrtOpx) thermobarometry scheme based on Al-solubilityin Opx, corrected for late FeMg exchange. We appliedthis scheme to 414 granulites of mafic, intermediate and aluminousbulk compositions. Our findings suggest that granulites aremuch hotter than traditionally assumed and that the PTconditions of the amphibolitegranulite transition portrayedin current petrology textbooks are significant underestimatesby over 100°C. For aluminous and intermediate granulites,mean corrected temperatures based on our method are 890 ±17 and 841 ± 11°C, respectively (uncertainties reportedas 95% confidence limits on the mean), consistent with minimumtemperatures for orthopyroxene production by fluid-absent partialmelting in these bulk compositions. In contrast, mean temperaturesbased on GrtOpx FeMg exchange equilibria, usingthe same thermodynamic data, are 732 ± 22 and 723 ±11°C, respectively, well below the minimum temperaturesfor Opx stability. For mafic granulites, the mean correctedtemperature using our method is 816 ± 12°C, similarto the mean temperature of 793 ± 13°C from FeMgexchange. Reasons for the differences between the mafic granulitesand aluminousintermediate granulites are unclear butmay be due to the lower Al concentrations in Opx in the maficrocks and possible deficiencies in the thermodynamic modellingof these low concentrations. We discuss a number of well-knowngranulite terrains in the context of our findings, includingthe Adirondacks, the Acadian granulites of New England, theincipient charnockites of southern India and Sri Lanka, andthe Kerala Khondalite Belt. Our findings carry implicationsfor thermotectonic models of granulite formation. A computerprogram to perform our thermobarometry calculations, RCLC, isavailable from the Journal of Petrology website at http://www.petrology.oupjournals.orgor from the authors at http://www.geo.ucalgary.ca/~pattison/drm_pattison-rclc.htm. KEY WORDS: granulite-facies metamorphism; thermobarometry; garnet; orthopyroxene 相似文献
12.
The Shape and Volume of the Skaergaard Intrusion, Greenland: Implications for Mass Balance and Bulk Composition 总被引:6,自引:0,他引:6
Re-examination of the Skaergaard intrusion in the context ofits regional setting, combined with new data from explorationdrilling, has resulted in a revised structural model for theintrusion. It is modelled as an irregular box, c. 11 km fromnorth to south, up to 8 km from east to west, and 3·44km from the lower to the upper contact. The walls of the intrusionare inferred to follow pre-existing and penecontemporaneoussteep faults, and the floor and roof seem largely controlledby bedding planes in the host sediments and lavas, similar toregional sills. The suggested shape and volume are in agreementwith published gravimetric modelling. Crystallization alongall margins of the intrusion concentrated the evolving meltin the upper, central part of the intrusion, best visualizedas an onion-skin structure inside the box. Thetotal volume is estimated to c. 280 ± 23 km3, of which13·7% are referred to the Upper Border Series (UBS),16·4% to the Marginal Border Series (MBS) and 69·9%to the Layered Series (LS). In the LS, the Lower Zone (LZ) isestimated to constitute 66·8%, the Middle Zone (MZ) 13·5%and the Upper Zone (UZ) 19·7%. The new volume relationshipsprovide a mass balance estimate of the major and trace elementbulk composition of the intrusion. The parental magma to theSkaergaard intrusion is similar to high-Ti East Greenland tholeiiticplateau basalts with Mg number c. 0.45. The intrusion representsthe solidification of contemporary plateau basalt magma trappedand crystallized under closed-system conditions in a crustalreservoir at the developing East Greenland continental margin. KEY WORDS: bulk composition; emplacement; mass proportions; Skaergaard intrusion; structure 相似文献
13.
Nehring Franziska; Foley Stephen F.; Holtta Pentti; Van Den Kerkhof Alfons M. 《Journal of Petrology》2009,50(1):3-35
Fault bound blocks of granulite and enderbite occur within upperamphibolite-facies migmatitic tonalitic–trondhjemitic–granodioritic(TTG) gneisses of the Iisalmi block of Central Finland. Theseunits record reworking and partial melting of different levelsof the Archean crust during a major tectonothermal event at2·6–2·7 Ga. Anhydrous mineral assemblagesand tonalitic melts in the granulites formed as a result ofhydrous phase breakdown melting reactions involving amphiboleat peak metamorphic conditions of 8–11 kbar and 750–900°C.A nominally fluid-absent melting regime in the granulites issupported by the presence of carbonic fluid inclusions. Thegeochemical signature of light rare earth element (LREE)-depletedmafic granulites can be modelled by 10–30 wt % partialmelting of an amphibolite source rock leaving a garnet-bearingresidue. The degree of melting in intermediate granulites isinferred to be less than 10 wt % and was restricted by the availabilityof quartz. Pressure–temperature estimates for the TTGgneisses are significantly lower than for the granulites at660–770°C and 5–6 kbar. Based on the P–Tconditions, melting of the TTG gneisses is inferred to haveoccurred at the wet solidus in the presence of an H2O-rich fluid.A hydrous mineralogy, abundant aqueous fluid inclusions andthe absence of carbonic inclusions in the gneisses are in accordancewith a water-fluxed melting regime. Low REE contents and strongpositive Eu anomalies in most leucosomes irrespective of thehost rock composition suggest that the leucosomes are not meltcompositions, but represent plagioclase–quartz assemblagesthat crystallized early from felsic melts. Furthermore, similarplagioclase compositions in leucosomes and adjacent mesosomesare not a migmatite paradox, as both record equilibrationwith the same melt phase percolating along grain boundaries. KEY WORDS: Archean continental crust; fluid inclusion; granulite; migmatite; partial melting 相似文献
14.
Mantle Preconditioning by Melt Extraction during Flow: Theory and Petrogenetic Implications 总被引:2,自引:0,他引:2
Mantle preconditioning may be defined as the extraction of smallmelt fractions from mantle asthenosphere during its flow tothe site of magma generation. Equations may be written for mantlepreconditioning, assuming that the mantle comprises enrichedplums in a depleted matrix. The equations takeinto account variations in mass fraction of plums, the relativerate of melting of plums and matrix, the temperature and pressureof melt extraction, the mass fraction of melt extracted, theextent of chemical exchange between plums and matrix, and theefficiency of melt extraction. Monitoring mineralogical changesand variations in partition coefficients along the inferredPTt path of the mantle asthenosphere allows theequations to be correctly applied to the conditions under whichmelt extraction takes place. Numerical experiments demonstratethe influence of petrogenetic variables on the shape of meltextraction trajectories and provide new criteria for distinguishingbetween melt extraction and mixing as the cause of regionalgeochemical gradients. Representative examples of arcback-arcsystems (Scotia), continental break-up (Afar) and plumeridgeinteraction (Azores) indicate that the compositions of the mantlesources of mid-ocean ridge basalts and island arc basalts maybe determined, at least in part, by the melt extraction historiesof their asthenospheric sources. KEY WORDS: geochemical modelling; mantle flow; isotope ratios; trace elements 相似文献
15.
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-precisionUPb 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 PT conditionsof 690750°C and 57 kbar, corresponding toan intrusion depth of 1925 km. At 337 ± 1 Ma themagmaticmetamorphic suite was intruded by granites, syenitesand diorites at a shallow crustal level of some 711 km.This is inferred from a diorite, and conforms to PT pathsobtained from the metasediments, indicating a nearly isothermaldecompression from 57 to 24 kbar at 690750°C.Subsequently, the metamorphicmagmatic sequence underwentaccelerated cooling to below 400°C, as constrained by garnetgeospeedometry and a previously published KAr muscoviteage of 333 ± 7 Ma. With respect to PTDtdata from surrounding units, rapid exhumation of the KCC canbe interpreted to result from NW-directed crustal shorteningduring the Viséan. KEY WORDS: contact metamorphism; UPb dating; hornblende; garnet; Mid-German Crystalline Rise; PT pseudosection 相似文献
16.
Metamorphic Evolution of the Ribeira Belt: Evidence from Outcrops in the Rio de Janeiro Area, Brazil
Migmatitic granulites and arc-related felsic intrusives of Pan-Africanage form the bedrock in the Rio de Janeiro area, SE Brazil.These rocks preserve a partial record of three parageneses.The earliest assemblage (M1) grew during fabric formation inthe rocks (D1) and is characterized by the mineral assemblagePl + Bt + Sil + Kfs + Qtz. Peak metamorphic conditions (M2)are characterized by the assemblage Bt + Crd + Kfs + Pl + Grt+ liq + Qtz and are inferred to have developed during D2 foldingof the rocks at T = 750800°C and P = 7 kbar. M3 reactiontextures overprint the M2 assemblage and comprise symplectiticintergrowth of cordierite(II) and quartz that formed after garnet,whereas secondary biotite formed as a result of reactions betweengarnet and K-feldspar. By comparing the observed modal abundanceswith modal contours of garnet, cordierite and quartz on therelevant pseudosection a post M2 PT vector indicatingcontemporaneous cooling and decompression can be deduced. Theinferred equilibrium assemblage and reaction textures are interpretedto reflect a clockwise PT path involving heating followedby post-peak decompression and associated cooling. We inferthat metamorphism occurred in response to advective heatingby the abundant syn-collisional (arc-related) I-type granitoidsin the region, consistent with the unusually high peak T/P ratio. KEY WORDS: advective heating; Ribeira belt; granulite; partial melting; PT pseudosection 相似文献
17.
GARRIDO CARLOS J.; BODINIER JEAN-LOUIS; BURG JEAN-PIERRE; ZEILINGER GEROLD; HUSSAIN S. SHAHID; DAWOOD HAMID; CHAUDHRY M. NAWAZ; GERVILLA FERNANDO 《Journal of Petrology》2006,47(10):1873-1914
We report the results of a geochemical study of the Jijal andSarangar complexes, which constitute the lower crust of theMesozoic Kohistan paleo-island arc (Northern Pakistan). TheJijal complex is composed of basal peridotites topped by a gabbroicsection made up of mafic garnet granulite with minor lensesof garnet hornblendite and granite, grading up-section to hornblendegabbronorite. The Sarangar complex is composed of metagabbro.The Sarangar gabbro and Jijal hornblende gabbronorite have melt-like,light rare earth element (LREE)-enriched REE patterns similarto those of island arc basalts. Together with the Jijal garnetgranulite, they define negative covariations of LaN, YbN and(La/Sm)N with Eu* [Eu* = 2 x EuN/(SmN + GdN), where N indicateschondrite normalized], and positive covariations of (Yb/Gd)Nwith Eu*. REE modeling indicates that these covariations cannotbe accounted for by high-pressure crystal fractionation of hydrousprimitive or derivative andesites. They are consistent withformation of the garnet granulites as plagioclasegarnetassemblages with variable trapped melt fractions via eitherhigh-pressure crystallization of primitive island arc basaltsor dehydration-melting of hornblende gabbronorite, providedthat the amount of segregated or restitic garnet was low (<5wt %). Field, petrographic, geochemical and experimental evidenceis more consistent with formation of the Jijal garnet granuliteby dehydration-melting of Jijal hornblende gabbronorite. Similarly,the Jijal garnet-bearing hornblendite lenses were probably generatedby coeval dehydration-melting of hornblendites. Melting modelsand geochronological data point to intrusive leucogranites inthe overlying metaplutonic complex as the melts generated bydehydration-melting of the plutonic protoliths of the Jijalgarnet-bearing restites. Consistent with the metamorphic evolutionof the Kohistan lower arc crust, dehydration-melting occurredat the mature stage of this island arc when shallower hornblende-bearingplutonic rocks were buried to depths exceeding 2530 kmand heated to temperatures above c. 900°C. Available experimentaldata on dehydration-melting of amphibolitic sources imply thatthickening of oceanic arcs to depths >30 km (equivalent toc. 1·0 GPa), together with the hot geotherms now postulatedfor lower island arc crust, should cause dehydration-meltingof amphibole-bearing plutonic rocks generating dense garnetgranulitic roots in island arcs. Dehydration-melting of hornblende-bearingplutonic rocks may, hence, be a common intracrustal chemicaland physical differentiation process in island arcs and a naturalconsequence of their maturation, leading to the addition ofgranitic partial melts to the middleupper arc crust andformation of dense, unstable garnet granulite roots in the lowerarc crust. Addition of LREE-enriched granitic melts producedby this process to the middleupper island arc crust maydrive its basaltic composition toward that of andesite, affordinga plausible solution to the arc paradox of formationof andesitic continental-like crust in island arc settings. KEY WORDS: island arc crust; Kohistan complex; Jijal complex; amphibole dehydration-melting; garnet granulite; continental crustal growth 相似文献
18.
An Experimentally Constrained Petrogenetic Grid in the Silica-Saturated Portion of the System KFMASH at High Temperatures and Pressures 总被引:4,自引:0,他引:4
Experiments in the quartz-saturated part of the system KFMASHunder fO2 conditions of the haematitemagnetite bufferand using bulk compositions with XMg of 0·81, 0·72,0·53 define the stability limits of several mineral assemblageswithin the PT field 912 kbar, 8501100°C.The stability limits of the mineral assemblages orthopyroxene+ spinel + cordierite ± sapphirine, orthopyroxene + garnet+ sapphirine, sapphirine + cordierite + orthopyroxene and garnet+ orthopyroxene + spinel have been delineated on the basis ofPT and TX pseudosections. Sapphirine did not appearin the bulk composition of XMg = 0·53. A partial petrogeneticgrid applicable to high MgAl granulites metamorphosedat high fO2, developed in our earlier work, was extended tohigher pressures. The experimental results were successfullyapplied to several high-grade terranes to estimate PTconditions and retrograde PT trajectories. KEY WORDS: KFMASH equilibria; experimental petrogenetic grid at high fO2 相似文献
19.
CLIFFORD TOM N.; BARTON ERIKA S.; STERN RICHARD A.; DUCHESNE JEAN-CLAIR 《Journal of Petrology》2004,45(4):669-691
The O'okiep Copper District is underlain by voluminous 10351210Ma granite gneiss and granite with remnants of metamorphosedsupracrustal rocks. This assemblage was intruded by the 1030Ma copper-bearing Koperberg Suite that includes jotunite, anorthosite,biotite diorite and hypersthene-bearing rocks ranging from leuconoriteto hypersthenite. New sensitive high-resolution ion microprobeage data demonstrate the presence of 17002000 Ma zirconas xenocrysts in all of the intrusive rocks, and as detritalzircon in the metasediments of the Khurisberg Subgroup. Thesedata are consistent with published SmNd model ages ofc. 1700 Ma (TCHUR) and c. 2000 Ma (TDM) of many of the intrusivesthat support a major crust-forming event in Eburnian (Hudsonian)times. In addition, UThPb analyses of zirconsfrom all major rock units define two tectono-magmatic episodesof the Namaquan Orogeny: (1) the O'okiepian Episode (11801210Ma), represented by regional granite plutonism, notably theNababeep and Modderfontein Granite Gneisses and the Concordiaand Kweekfontein Granites that accompanied and outlasted (e.g.Kweekfontein Granite) regional tectonism [F2(D2)] and granulite-faciesmetamorphism (M2); (2) the Klondikean Episode (10201040Ma), which includes the intrusion of the porphyritic RietbergGranite and of the Koperberg Suite that are devoid of regionalplanar or linear fabrics. Klondikean tectonism (D3) is reflectedby major eastwest-trending open folds [F3(D3a)], andby localized eastwest-trending near-vertical ductilefolds [steep structures; F4(D3b)] whose formationwas broadly coeval with the intrusion of the Koperberg Suite.A regional, largely thermal, amphibolite- to granulite-faciesmetamorphism (M3) accompanied D3. This study demonstrates, interalia, that the complete spectrum of rock-types of the KoperbergSuite, together with the Rietberg Granite, was intruded in ashort time-interval (<10 Myr) at c. 1030 Ma, and that therewere lengthy periods of about 150 Myr of tectonic quiescencewithin the Namaquan Orogeny: (1) between the O'okiepian andKlondikean Episodes; (2) from the end of the latter to the formalend of Namaquan Orogenesis 800850 Ma ago. KEY WORDS: UPb, zircon; O'okiep, Namaqualand; granite plutonism; granulite facies; Koperberg Suite; Namaquan (Grenville) Orogeny 相似文献
20.
Na-rich Partial Melts from Newly Underplated Basaltic Crust: the Cordillera Blanca Batholith, Peru 总被引:102,自引:0,他引:102
The late Miocene Cordillera Blanca Batholith lies directly overthick (50 km) crust, inboard of the older Cretaceous CoastalBatholith. Its peraluminous S type mineralogyand its position suggest recycling of continental crust, whichis commonly thought to be an increasingly important componentin magmas inboard of continental margins. However, the peraluminous,apparent S type character of the batholith isan artefact of deformation and uplift along a major crustallineament. The batholith is a metaluminous I typeand the dominant high-silica rocks (>70%) are Na rich withmany of the characteristics of subducted oceanic slab melts.However, the position of the batholith and age of the oceaniccrust at the trench during the Miocene preclude slab melting.Instead, partial melting of newly underplated Miocene crustis proposed. In this dynamic model newly underplated basalticmaterial is melted to produce high-Na, low HREE, high-Al trondhjemitictype melts with residues of garnet, clinopyroxene and amphibole.Such Na-rich magmas are characteristic of thick Andean crust;they are significantly different from typical cole-alkaline,tonalite-grano-diorite magmas, and their presence along thespine of the Andes provokes questions about models of trondhjemitegenesis by melting of subducted oceanic crust, as well as anygeneralized, circum-Pacific model involving consistent isotopicor chemical changes inboard from the trench. KEY WORDS: batholith; modified I type granite; Na-rich magma; thick crust
* Corresponding author. 相似文献