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1.
The Koperberg Suite comprises some 1700 small bodies of intrusiverocks largely composed of andesine anorthosite, biotite diorite,and leuconorite, norite and melanorite-hypersthenite; 30 mineshave been established in the O'okiep District in the cupriferousrocks of this anorthosite-charnockite kindred. The suite isintrusive into a sequence of granite gneiss and metavolcanicand metasedimentary rocks, and intrusive granite, that wereelevated to the granulite fades of regional metamorphism.TheSm-Nd model ages for the country rocks and the Koperberg Suiteare all 1700 Ma (TCHUR) and 2000 Ma (TDM) supporting a majorcrustforming event in this portion of Namaqualand at the endof Lower Proterozoic times. The granulite fades metamorphismin the O'okiep District is recorded by a Rb-Sr isochron ageof 1223 48 Ma on the Nababeep Granite Gneiss, and by (1197 15)-Ma-old inherited cores of zircons in the Koperberg Suite.The time of intrusion of the Concordia and Rietberg Granitesis believed to be reflected by their Rb-Sr whole-rock age of1105 24 Ma. The mean U-Pb age of 1029 10 Ma on individualzircon grains and zircon rims from the Koperberg Suite recordsthe time of its intrusion, and this is supported by the Sm-Ndwhole-rock age of 1022 42 Ma for the suite. Subsequent coolingand reheating events are recorded by the Ar-Ar ages of 800–850Ma for the Koperberg Suite, and of 500–550 Ma for thesuite and certain country rocks, respectively.An Nd value of-7,and its volume and composition, suggest a crustal-melt sourcefor the intrusive Concordia Granite. Moreover, the age-correctedhigh lSr (07061-07272) and low Nd (-9), and the high µ2(101), that characterize the Koperberg Suite also imply a crustalsource, and a model is presented for the generation of the majorpart of the suite by partial melting of granulites of overallintermediate (diorite) composition in the lower crust. Corresponding author  相似文献   

2.
The O’okiep Copper District is the oldest formal mining area in South Africa. Between 1852 and 2002, the 2,500 km2 area yielded two million tons of copper from 32 mines ranging in ore tonnages from 140,000 to 37 million tons. This paper summarizes the calendar of events from the formation of the first primitive crust 1,700–2,000?Ma ago to early Cambrian times ~500?Ma ago, with particular emphasis on the Namaquan (Grenville) Orogeny, notably: the O’okiepian Episode (1,180–1,210?Ma ago) of alpine-type folding, regional granite plutonism, and granulite facies metamorphism and the Klondikean Episode (1,020–1,040?Ma ago) of open and tight folding and the intrusion of the Rietberg Granite and the Koperberg Suite. Almost all of the copper in the O’okiep District occurs in the Koperberg Suite, of which there are 1,700 small bodies that constitute 0.7% of the outcrop area. The suite comprises jotunite, anorthosite, biotite diorite, and hypersthenic rocks ranging from leuconorite to hypersthenite, and it is one of only two world examples of economic copper mineralization in rocks of the anorthosite–charnockite kindred; the second example is Caraiba, Brazil. High I Sr and low ε Nd (for a 1,030 Ma intrusion age), and high μ 2 of 10.1, for Koperberg rock-types indicate a crustal progenitor for the suite, and the presence of jotunite suggests a (subducted) crustal source at ca. 40–50 km depth. The magmatic sulphide paragenesis in the Koperberg Suite is chalcopyrite?+?pyrrhotite (Narrap-type ore) that, in a number of ore-bodies, has been inverted under upper amphibolite facies conditions to bornite?+?Ti-free magnetite (Carolusberg-type ore). Meteoric fluids resulted in supergene Cu enrichment in Koperberg bodies to ~500?m below the pre-Nama peneplane, and lower greenschist facies metamorphism 500–570?Ma ago is reflected by inter alia Hoits-type ore bearing second-generation bornite?+?chalcopyrite(±?covellite?±?chalcocite).  相似文献   

3.
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   

4.
We have performed time series experiments for periods rangingfrom 3 min to 44 h on the interaction of granite melt and partiallymolten basalt at 920C and 10 kbar, in the presence of 5 wt.%water. With time, the assemblage of the basalt domain changesfrom predominantly amphibole+plagioclase to clinopyroxene+garnet;the melt fraction increases from {small tilde}2•5 to 40%;and between the two domains, the melt compositions progressivelyequilibrate. Initially in each run, melts of the basalt domainhave uniform plateau concentrations for SiO2, Al2O3, CaO, MgO,and FeO because the activities of these components are regulatedby the mineral assemblage, but at advanced stages of reaction,no such control is evident. We have derived analytical expressionsto describe and simulate the diffusion profiles. The concentrationprofiles for SiO2, Al2O3, CaO, and Na2O in the granite, emanatingfrom the basalt–granite interface, have been used to estimateeffective diffusivities. The values from the shorter runs arecompared with those of the experiment of longest duration forwhich we assumed finite couples in our calculations. In thediffusion calculations for K2O the difference in melt fractionbetween the two domains is accounted for. The resulting values(in cm2/s) are: DNa2O=6 10–7, DK2O=3 10–7, DMgO=9 10–8, DCaO=(4–6) 10–8, and DSiO2 and DAl2O3=(3–0•6) 10–8. They are in reasonable agreement with values fromother studies. On the basis of our experiments we calculatethat mafic enclaves of magmatic origin should equilibrate toa large degree with their host magma in slowly cooling non-convectinggranitic plutons. Enclaves approaching complete re-equilibrationretain distinctly higher modal amounts of mafic minerals. Theydo not compositionally resemble binary magma mixtures, but aremore like host magma with accumulated crystals. We show thatthe modal differences between enclave and host are indicativeof the temperature of homogenization and that, in principle,this temperature can be deduced from equilibrium phase diagrams. * Present address: Mineralogisch-Petrologisches Institut, Universitt Gttingen, Goldschmidtstrasse 1, 3400 Gttingen, Germany  相似文献   

5.
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   

6.
A combined set of U–Pb and Lu–Hf in situ laser ablationICP-(MC)-MS zircon analyses were obtained from orthogneissesand granitoids in the Central Zone of the Limpopo Belt, whichcomprises the Beit Bridge and Mahalapye complexes. The resultsindicate that by combining the two isotope systems primary magmaticzircon domains can be distinguished from those formed duringlater metamorphic events, even if the distinct zircon domainsunderwent multiple Pb loss and the texture–age relationships,as obtained by cathodoluminescence images and U–Pb analyses,are ambiguous. Furthermore, the applied technique allows distinctionof zircon grains formed in juvenile magmas from those generatedby melting of older continental crust or affected by substantialcrustal contamination. The combined U–Pb and Lu–Hfdata reveal that the Sand River gneiss suite of the Beit BridgeComplex was emplaced at 3283 ± 8 Ma and formed from meltingof an older Archaean crust, which was derived from a depletedmantle source at around 3·65 Ga. The hafnium model age(TDMHf) is significantly older than those obtained from zirconsfrom numerous Neoarchaean granitoids of the Beit Bridge Complex,comprising the Singelele gneiss (2647 ± 12 Ma), the Bulaigranite (2612 ± 7 Ma), the Regina gneiss (2649 ±9 Ma) and two samples of the Zanzibar gneiss (2613 ±6 Ma). These granitoids show initial Hf(t) values between +0·5 and –7·1, which correspond to initialTDMHf between 3·46 and 3·01 Ga. These variableTDMHfinitial and Hf(t)initial values are interpreted to be theresult of different mixtures of reworked 3·65 Ga Palaeoarchaeancrust with juvenile magmas extracted from the depleted mantleduring the Neoarchaean at 2·65 Ga. This conclusion issupported by results obtained from the Mahalapye Complex, whichwas affected by migmatization and granite intrusions duringthe Palaeoproterozoic at 2·02–2·06 Ga. TheMokgware granite (2019 ± 9 Ma) contains zircon xenocrystswith Pb–Pb ages of 2·52–2·65 Ga and2·93 Ga and hafnium model ages of 3·0–3·4Ga, indicating that this granite is derived from remelting ofArchaean crust. In contrast, uniform TDMHfinitial ages of 2·61–2·67Ga obtained from a diorite gneiss (2061 ± 6 Ma) of theMahalapye Complex indicate that its protolith may have beenformed from remelting of a Neoarchaean juvenile crust. VariableHf(t)initial values from –3·7 to +6·3 ofzircon cores (2711 ± 11 Ma) in an adjacent leucosomealso support a model of mixing of juvenile mantle derived matterwith older crust in the Neoarchaean. KEY WORDS: Archaean; Palaeoproterozoic; Limpopo Belt; zircon, U–Pb dating; Lu–Hf isotopes; LA-ICP-MS  相似文献   

7.
Anorogenic granites of the Brandberg igneous complex in NW Namibiaformed during early Cretaceous rifting and continental break-upof Gondwana. A metaluminous series [SiO2 = 62–77 wt %,molar (Na + K)/Al = 0·8–0·95] includes anearly monzonite body, major biotite–hornblende granite,late biotite granite segregations and peripheral dykes of trachydacite.Volumetrically minor peralkaline granites of the Amis complex[SiO2 = 72–77 wt %, (Na + K)/Al = 1·0–1·5]intrude the main granite and adjacent country rocks. Comparedwith the metaluminous main granite, these are in part highlyenriched in Zr, Nb, Y, U and Th. Initial Nd and Sr isotope ratiosof the metaluminous suite are  相似文献   

8.
Mineralogical, isotopic, geochemical and geochronological evidencedemonstrates that the Friningen body, a garnet peridotite bodycontaining garnet pyroxenite layers in the Seve Nappe Complex(SNC) of Northern Jämtland, Sweden, represents old, certainlyProterozoic and possibly Archean, lithosphere that became incorporatedinto the Caledonian tectonic edifice during crustal subductioninto the mantle at c. 450 Ma. Both garnet peridotite and pyroxenitecontain two (M1 and M2) generations of garnet-bearing assemblagesseparated by the formation of two-pyroxene, spinel symplectitearound the M1 garnet and the crystallization of low-Cr spinel1Cin the matrix. These textures suggest initial high-pressure(HP) crystallization of garnet peridotite and pyroxenite succeededby decompression into the spinel stability field, followed byrecompression into the garnet peridotite facies. Some pyroxenitelayers appear to be characterized solely by M2 assemblages withstretched garnet as large as several centimeters. Laser ablationmicroprobe–inductively coupled plasma mass spectrometryRe–Os analyses of single sulfide grains generally definemeaningless model ages suggesting more than one episode of Reand/or Os addition and/or loss to the body. Pentlandite grainsfrom a single polished slab of one garnet peridotite, however,define a linear array on an Re–Os isochron diagram that,if interpreted as an errorchron, suggests an Archean melt extractionevent that left behind the depleted dunite and harzburgite bodiesthat characterize the SNC. Refertilization of this mantle bymelts associated with the development of the pyroxenite layersis indicated by enriched clinopyroxene Sr–Nd isotope ratios,and by parallel large ion lithophile-enriched trace elementpatterns in clinopyroxene from pyroxenite and the immediatelyadjacent peridotite. Clinopyroxene and whole-rock model Sm–Ndages (TDM = 1·1–2·2 Ga) indicate that fertilizationtook place in Proterozoic times. Sm–Nd garnet2–clinopyroxene2–wholerock ± orthopyroxene2 mineral isochrons from three pyroxenitelayers define overlapping ages of 452·1 ± 7·5and 448 ± 13 Ma and 451 ± 43 Ma (2  相似文献   

9.
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   

10.
New SHRIMP U–Pb zircon, Rb–Sr whole-rock, and 40Ar–39Ardata are presented for the Jurassic silicic volcanic rocks andrelated granitoids of Patagonia and the Antarctic Peninsula.U–Pb is the only reliable method for dating crystallizationin these rocks; Rb–Sr is prone to hydrothermal resettingand Ar–Ar is additionally affected by initial excess 40Ar.Volcanism spanned more than 30 My, but three episodes are definedon the basis of peak activity: V1 (188–178 Ma), V2 (172–162Ma) and V3 (157–153 Ma). The first essentially coincideswith the Karoo–Ferrar mafic magmatism of South Africa,Antarctica and Tasmania. The silicic products of V1 are lower-crustalmelts that have incorporated upper-crustal material. The geochemistryof V2 and V3 ignimbrites is more characteristic of destructiveplate margins, but the presence of inherited zircon still pointsto a crustal source. The pattern of volcanism corresponds inspace and in time to migration away from the Karoo mantle plumetowards the proto-Pacific margin of Gondwana during riftingand break-up. The heat required to initiate bulk crustal fusionmay have been supplied by the spreading plume-head, but thinningof the crust during continental dispersion would also have facilitatedanatexis. KEY WORDS: Antarctic Peninsula; ignimbrites; Jurassic; Patagonia; U–Pb; zircon  相似文献   

11.
Petrological and geochemical variations are used to investigatethe formation of granite magma from diatexite migmatites derivedfrom metasedimentary rocks of pelitic to greywacke compositionat St. Malo, France. Anatexis occurred at relatively low temperaturesand pressures (<800°C, 4–7 kbar), principally throughmuscovite dehydration melting. Biotite remained stable and servesas a tracer for the solid fraction during melt segregation.The degree of partial melting, calculated from modal mineralogyand reaction stoichiometry, was <40 vol. %. There is a continuousvariation in texture, mineralogy and chemical composition inthe diatexite migmatites. Mesocratic diatexite formed when metasedimentaryrocks melted sufficiently to undergo bulk flow or magma flow,but did not experience significant melt–residuum separation.Mesocratic diatexite that underwent melt segregation duringflow generated (1) melanocratic diatexites at the places wherethe melt fraction was removed, leaving behind a biotite andplagioclase residuum (enriched in TiO2, FeOT, MgO, CaO, Sc,Ni, Cr, V, Zr, Hf, Th, U and REE), and (2) a complementary leucocraticdiatexite (enriched in SiO2, K2O and Rb) where the melt fractionaccumulated. Leucocratic diatexite still contained 5–15vol. % residual biotite (mg-number 40–44) and 10–20vol. % residual plagioclase (An22). Anatectic granite magmadeveloped from the leucodiatexite, first by further melt–residuumseparation, then through fractional crystallization. Most biotitein the anatectic granite is magmatic (mg-number 18–22). KEY WORDS: anatexis; diatexite; granite magma; melt segregation; migmatite  相似文献   

12.
Experiments defining the distribution of H2O [Dw = wt % H2O(melt)/wt% H2O(crd)]) between granitic melt and coexisting cordieriteover a range of melt H2O contents from saturated (i.e. coexistingcordierite + melt + vapour) to highly undersaturated (cordierite+ melt) have been conducted at 3–7 kbar and 800–1000°C.H2O contents in cordierites and granitic melts were determinedusing secondary ion mass spectrometry (SIMS). For H2O vapour-saturatedconditions Dw ranges from 4·3 to 7 and increases withrising temperature. When the system is volatile undersaturatedDw decreases to minimum values of 2·6–5·0at moderate to low cordierite H2O contents (0·6–1·1wt %). At very low aH2O, cordierite contains less than 0·2–0·3wt % H2O and Dw increases sharply. The Dw results are consistentwith melt H2O solubility models in which aH2O is proportionalto Xw2 (where Xw is the mole fraction of H2O in eight-oxygenunit melt) at Xw  相似文献   

13.
The 2·63 Ga Louis Lake batholith, a calc-alkalic plutonexposed in Wind River Range of western Wyoming, consists ofminor diorite, quartz diorite, granodiorite, and granite. Atshallow structural levels the batholith is pyroxene free, butat deeper levels, all units of the batholith contain pyroxenes.On its northern margin the batholith was emplaced at P = 5–6kbar, T = 775–800°C, fO2 at FMQ (fayalite–magnetite–quartz)+ 1·5 to FMQ + 1·8, and aH2O  相似文献   

14.
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   

15.
Extensive high-grade polydeformed metamorphic provinces surroundingArchaean cratonic nuclei in the East Antarctic Shield recordtwo tectono-thermal episodes in late Mesoproterozoic and lateNeoproterozoic–Cambrian times. In Western Dronning MaudLand, the high-grade Mesoproterozoic Maud Belt is juxtaposedagainst the Archaean Grunehogna Province and has traditionallybeen interpreted as a Grenvillian mobile belt that was thermallyoverprinted during the Early Palaeozoic. Integration of newU–Pb sensitive high-resolution ion microprobe and conventionalsingle zircon and monazite age data, and Ar–Ar data onhornblende and biotite, with thermobarometric calculations onrocks from the H.U. Sverdrupfjella, northern Maud Belt, resultedin a more complex P–T–t evolution than previouslyassumed. A c. 540 Ma monazite, hosted by an upper ampibolite-faciesmineral assemblage defining a regionally dominant top-to-NWshear fabric, provides strong evidence for the penetrative deformationin the area being of Pan-African age and not of Grenvillianage as previously reported. Relics of an eclogite-facies garnet–omphaciteassemblage within strain-protected mafic boudins indicate thatthe peak metamorphic conditions recorded by most rocks in thearea (T = 687–758°C, P = 9·4–11·3kbar) were attained subsequent to decompression from P >12·9 kbar. By analogy with limited U–Pb singlezircon age data and on circumstantial textural grounds, thisearlier eclogite-facies metamorphism is ascribed to subductionand accretion around 565 Ma. Post-peak metamorphic K-metasomatismunder amphibolite-facies conditions is ascribed to the intrusionof post-orogenic granite at c. 480 Ma. The recognition of extensivePan-African tectonism in the Maud Belt casts doubts on previousRodinia reconstructions, in which this belt takes a pivotalposition between East Antarctica, the Kalahari Craton and Laurentia.Evidence of late Mesoproterozoic high-grade metamorphism duringthe formation of the Maud Belt exists in the form of c. 1035Ma zircon overgrowths that are probably related to relics ofgranulite-facies metamorphism recorded from other parts of theMaud Belt. The polymetamorphic rocks are largely derived froma c. 1140 Ma volcanic arc and 1072 ± 10 Ma granite. KEY WORDS: Maud Belt; Pan-African orogeny; geochronology; P–T–t path, East Antarctica  相似文献   

16.
New compositional data and petrogenetic models are presentedfor pre-Upper Miocene volcanism in the northern Puna of Argentina(22°S–24°S). Two phases of volcanism producedsmall dome complexes of mainly silicic andesite to low-SiO2rhyolite. The Upper Oligocene–Lower Miocene phase (UOLM,20–17 Ma), produced two distinct groups of rocks. TheUOLM-1 group is metaluminous and mainly andesitic, with isotopiccompositions like those of the recent arc (87Sr/86SrT  相似文献   

17.
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   

18.
The igneous complex of Ballachulish is a composite calc-alkalinepluton of Caledonian age (412 ? 28 Ma), emplaced in Dalradianmetasediments at a pressure of 3 ? 0–5 kb (c. 10 km depth).The 4 by 7 km intrusion is composed of a zoned monzodiorite-quartzdiorite envelope with a distinct flowand deformation-foliation,surrounding a younger core of porphyritic granite. Two-pyroxene thermometry, Fe-Ti oxide thermobarometry, and stabilityrelationships of ternary feldspars, biotite, and amphibolesare used to calibrate the 3 kb isobaric crystallization sequencewith respect to the following parameters: the fractionationstage of the host rocks, the water content of the magmas, phasecompositions, and oxygen fugacity. Plagioclase, augite, andoxides generally yielded submagmatic temperatures due to theextensive recrystallization and re-equilibration of these phasesin the 900–l550?C subsolidus range. The ‘dry’monzodiorites apparently contained less than 1 wt. % initialmagmatic water, and remained H2O-deficient and vapor-absentthroughout their entire crystallization range. In contrast,2.5–3 wt.% initial H2O is estimated for the more fractionatedquartz diorites and the younger granites. The main crystallizationinterval for Opx–Cpx–Plg primocrysts in the dioritescovers c. 1100–950?C. Late-magmatic biotite and alkalifeldspar join the paragenetic sequence below 980?860?C, at fO2near NNO. A solidus temperature of c. 900?C is inferred forthis ‘dry’ system, in which amphiboles are entirelysubsolidus. At the present level of emplacement, crystallizationintervals of {small tilde} 1050–690?C and{small tilde}900–680?C are suggested for the quartz diorites and thegranites, which probably terminated crystallization in the presenceof a hydrous fluid.  相似文献   

19.
The inferred crystallization history of the troctolitic LowerZone of the Kiglapait Intrusion in Labrador is tested by meltingmineral mixtures from the intrusion, made to yield the observedcrystal compositions on the cotectic trace of liquid, plagioclase,and olivine. Melting experiments were made in a piston-cylinderapparatus, using graphite capsules at 5 kbar. Lower Zone assemblagescrystallized from 1245°C, 5% normative augite in the liquid,to 1203°C, 24% normative augite in the liquid at saturationwith augite crystals. This transit is consistent with modaldata and the large volume of the Lower Zone. The 1245°Ccotectic composition matches the average Inner Border Zone composition.Quenched troctolitic liquid from the Upper Border Zone, andothers from nearby Newark Island, plot on or near our experimentalcotectic, supporting a common fractionation history. Olivine–plagioclaseintergrowths from cotectic troctolitic melt show mosaic texturesreflecting the differing barriers to nucleation of these twophases. The linear partitioning of XAb in plagioclase–meltyields an intercept constant KD = 0·524 for these maficmelts. Observed subsolidus exchange of Ca between plagioclaseand olivine elucidates the loss of Ca from plutonic olivines.The bulk composition of the intrusion is revised downward inFo and An. KEY WORDS: experimental; olivine; plagioclase; Kiglapait; partitioningAbbreviations: AP, MT, IL, OR, AB, AN, DI, HY, OL, FO, NE, Q, FSP, AUG: (Oxygen) Normative components; Ap, Aug, Ilm, Ol, Pl: Phases; Ab, An, Di, Fa, Fo, Or, Wo: Phase components; also ternary endmembers; BSE: Back-scattered electron; CaTs: Calcium Tschermak's component, CaAlAlSiO6; D: Partition coefficient; f: Fugacity; FL: Fraction of the system present as liquid = 1 – (PCS/100); FMQ: Fayalite = magnetite + quartz buffer; IBZ: Inner Border Zone; IW: Iron = wüstite buffer; kbar: kilobar, 108 pascal; KD: Exchange coefficient; KI: Kiglapait Intrusion; L: Liquid phase; LLD: Liquid line of descent; Ma: Mega-annum, age; Myr: Mega-year, time; OLHY: Normative OL + HY; OLRAT: The ratio OLHY/(OLHY + AUG); P: Pressure; P: Phosphorus; PCS: Percent solidified (volume); SMAR: South Margin average composition; T: Temperature, °C; UBZ: Upper Border Zone; WM: Wüstite = magnetite buffer; Wo: Wollastonite component of pyroxene; X: Mole fraction; XMg: Molar ratio Mg/(Mg + Fe2+); , XMg(0): Initial XMg before MT is formed in the norm calculation; X: Coordinate, horizontal axis; Y: Coordinate, vertical axis  相似文献   

20.
The Cretaceous lava sequence and associated mafic dyke swarmin central–western Madagascar (Mailaka and Bemaraha areas)range in composition from picrite basalts to cordierite–orthopyroxene-bearingrhyodacites (MgO from 14 to 0·6 wt %). Petrographic andchemical data indicate the presence of both tholeiitic and transitionalmagma series, with variable degree of rare earth element enrichment[(La/Nd)n = 1–1·4 for tholeiites vs (La/Nd)n =0·65–1 for transitional rocks]. Initial (at 88Ma) 87Sr/86Sr and  相似文献   

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