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1.
Extensive high-grade polydeformed metamorphic provinces surroundingArchaean cratonic nuclei in the East Antarctic Shield recordtwo tectono-thermal episodes in late Mesoproterozoic and lateNeoproterozoicCambrian 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 newUPb sensitive high-resolution ion microprobe and conventionalsingle zircon and monazite age data, and ArAr data onhornblende and biotite, with thermobarometric calculations onrocks from the H.U. Sverdrupfjella, northern Maud Belt, resultedin a more complex PTt 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 garnetomphaciteassemblage within strain-protected mafic boudins indicate thatthe peak metamorphic conditions recorded by most rocks in thearea (T = 687758°C, P = 9·411·3kbar) were attained subsequent to decompression from P >12·9 kbar. By analogy with limited UPb 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; PTt path, East Antarctica 相似文献
2.
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
3.
Resetting of the U-Pb Zircon System in Cambro-Ordovician Intrusives of the Deep Freeze Range, Northern Victoria Land, Antarctica 总被引:5,自引:0,他引:5
Bomparola R. M.; Ghezzo C.; Belousova E.; Griffin W. L.; O'Reilly Suzanne Y. 《Journal of Petrology》2007,48(2):327-364
A detailed in situ isotopic (UPb, LuHf) and geochemicalstudy of zircon populations in a composite sequence of foliatedto massive Cambro-Ordovician intrusions in the Deep Freeze Range(North Victoria Land, Antarctica), has highlighted great complexityin zircon systematics. Zircons in deformed granitoids and tonalitesdisplay complex internal textures, a wide spread of concordantUPb ages (between 522 and 435 Ma) and unusual trace-elementcompositions (anomalous enrichment of light rare earth elements,U, Th and Y) within single zircon grains. In contrast, zirconsfrom undeformed samples display a limited range of UPbages and trace-element compositions. Zircons from all age andtextural populations in most of the deformed and undeformedsamples show a relatively narrow range of Hf values, suggestingthat the LuHf system remained undisturbed. Inferred emplacementages cover a time interval of about 30 Myr: from 508 to 493Ma for the oldest strongly foliated synkinematic Howard Peaksmegacrystic monzogranites and high-K calc-alkaline mafic tointermediate rocks of the Corner Tonalite unit;from about 489 to 481 Ma for the younger massive shoshoniticmafic dyke suite and the high-K calc-alkaline Keinath granite.The observed isotopic and chemical variations in zircon areattributed to a sub-solidus recrystallization under hydrousconditions and varying temperature, in a setting characterizedby a transpressional to extensional stress regime. KEY WORDS: Antarctica; Cambro-Ordovician intrusives; Ross Orogen; zircon UPb geochronology 相似文献
4.
U-Pb Age, Setting and Tectonic Significance of the Anorthosite-Mangerite-Charnockite-Granite Suite, Lofoten-Vesteralen, Norway 总被引:1,自引:0,他引:1
Mangerites, charnockites, anorthosites, gabbros and granitesoccur within a high-grade metamorphic complex in the LofotenVesterålenislands of northern Norway. UPb dating of zircon, titaniteand monazite indicates a three-stage magmatic history beginningat 18701860 Ma with the emplacement of the Lødingenand Hopen plutons, followed by a dominant stage at 18001790Ma that formed the bulk of the suite, and concluded by the emplacementof pegmatites, local rehydration and retrogression between 1790and 1770 Ma. On the scale of the Baltic Shield the 18701860Ma episode corresponds to contraction, amalgamation of arcs,and regional deformation. By contrast, the episode at 18001790Ma was characterized by major shifts in plate convergence, byintraplate deformation, and by a diversity of magmatic associationsincluding suites derived from the subcontinental mantle andwidespread granitoid rocks extracted from the continental crust.The diversity of concurrent magmatic events across the Svecofennianorogen, and the temporal coincidence with collisional eventsin coeval orogenic belts, suggests that the genesis of the suiteof magmatic rocks may have been related to tectonically drivenmechanisms of magma generation. KEY WORDS: anorthositemangeritecharnockitegranite; lithospheric processes; LofotenVesterålen; Svecofennian orogen; UPb geochronology 相似文献
5.
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
6.
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
7.
Evolution from Oceanic Subduction to Continental Collision: a Case Study from the Northern Tibetan Plateau Based on Geochemical and Geochronological Data 总被引:41,自引:0,他引:41
SONG SHUGUANG; ZHANG LIFEI; NIU YAOLING; SU LI; SONG BIAO; LIU DUNYI 《Journal of Petrology》2006,47(3):435-455
Two apparently distinct, sub-parallel, paleo-subduction zonescan be recognized along the northern margin of the Tibetan Plateau:the North Qilian Suture Zone (oceanic-type) with ophioliticmélanges and high-pressure eclogites and blueschistsin the north, and the North Qaidam Belt (continental-type) inthe south, an ultrahigh-pressure (UHP) metamorphic terrane comprisingpelitic and granitic gneisses, eclogites and garnet peridotites.Eclogites from both belts have protoliths broadly similar tomid-ocean ridge basalts (MORB) or oceanic island basalts (OIB)in composition with overlapping metamorphic ages (480440Ma, with weighted mean ages of 464 ± 6 Ma for North Qilianand 457 ± 7 Ma for North Qaidam), determined by zirconUPb sensitive high-resolution ion microprobe dating.Coesite-bearing zircon grains in pelitic gneisses from the NorthQaidam UHP Belt yield a peak metamorphic age of 423 ±6 Ma, 40 Myr younger than the age of eclogite formation, anda retrograde age of 403 ± 9 Ma. These data, combinedwith regional relationships, allow us to infer that these twoparallel belts may represent an evolutionary sequence from oceanicsubduction to continental collision, and continental underthrusting,to final exhumation. The QilianQaidam Craton was probablya fragment of the Rodinia supercontinent with a passive marginand extended oceanic lithosphere in the north, which was subductedbeneath the North China Craton to depths >100 km at c. 423Ma and exhumed at c. 403 Ma (zircon rim ages in pelitic gneiss). KEY WORDS: HP and UHP rocks; subduction belts; zircon SHRIMP ages; Northern Tibetan Plateau 相似文献
8.
Garnetite Xenoliths and Mantle-Water Interactions Below the Colorado Plateau, Southwestern United States 总被引:1,自引:0,他引:1
Garnetite xenoliths from ultramafic diatremes in northeasternArizona provide insights into hydration and metasomatism inthe mantle. The garnetites contain more than 95% garnet, someof which has complex compositional zonation related to growthin fractures within grains. Accessory minerals include rutile,ilmenite, chlorite, clinopyroxene, and zircon. Zircon grainsin one rock were analyzed in situ to determine UPb agesand Hf isotopic compositions. Most UPb analyses ploton or near concordia in the range 6085 Ma but a few arediscordant. The range in 176Hf/177Hf is about 0·28180·2828,with grains zoned to more radiogenic Hf from interiors to rims.The garnetite protolith contained zircons at least 1·8Ga in age, and garnet and additional zircon crystallized episodicallyduring the interval 8560 Ma. The garnetites are interpretedas mantle analogues of rodingites, formed in metasomatic reactionzones caused by waterrock interactions in Proterozoicmantle during late Cretaceous and Cenozoic subduction of theFarallon plate. Associated eclogite xenoliths may have beenparts of these same reaction zones. The rodingite hypothesisrequires serpentinization in the mantle wedge 700 km from thetrench, beginning 510 Myr before tectonism related tolow-angle subduction. KEY WORDS: garnetite; LuHf, mantle; rodingite; metasomatism 相似文献
9.
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
10.
Episodic Silicic Volcanism in Patagonia and the Antarctic Peninsula: Chronology of Magmatism Associated with the Break-up of Gondwana 总被引:10,自引:2,他引:8
New SHRIMP UPb zircon, RbSr whole-rock, and 40Ar39Ardata are presented for the Jurassic silicic volcanic rocks andrelated granitoids of Patagonia and the Antarctic Peninsula.UPb is the only reliable method for dating crystallizationin these rocks; RbSr is prone to hydrothermal resettingand ArAr is additionally affected by initial excess 40Ar.Volcanism spanned more than 30 My, but three episodes are definedon the basis of peak activity: V1 (188178 Ma), V2 (172162Ma) and V3 (157153 Ma). The first essentially coincideswith the KarooFerrar 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; UPb; zircon 相似文献
11.
《地学前缘(英文版)》2019,10(6):2045-2061
The Southern Irumide Belt(SIB) is an orogenic belt consisting of a number of lithologically varied Mesoproterozoic and Neoproterozoic terranes that were thrust upon each other.The belt lies along the southwest margin of the Archaean to Proterozoic Congo Craton,and bears a Neoproterozoic tectonothermal overprint relating to the Neoproterozoic-Cambrian collision between the Congo and Kalahari cratons.It preserves a record of about 500 million years of plate interaction along this part of the Congo margin.Detrital zircon samples from the SIB were analysed for U-Pb and Lu-Hf isotopes,as well as trace element compositions.These data are used to constrain sediment-source relationships between SIB terranes and other Gondwanan terranes such as the local Congo Craton and Irumide belt and wider afield to Madagascar(Azania) and India.These correlations are then used to interpret the Mesoproterozoic to Neoproterozoic affinity of the rocks and evolution of the region.Detrital zircon samples from the Chewore-Rufunsa and Kacholola(previously referred to as Luangwa-Nyimba) terranes of the SIB yield zircon U-Pb age populations and evolved ε_(Hf)(t) values that are similar to the Muva Supergroup found throughout eastern Zambia,primarily correlating with Ubendian-Usagaran(ca.2.05-1.80 Ga) phase magmatism and a cryptic basement terrane that has been suggested to underlie the Bangweulu Block and Irumide Belt.These data suggest that the SIB was depositionally connected to the Congo Craton throughout the Mesoproterozoic.The more eastern Nyimba-Sinda terrane of the SIB(previously referred to as Petauke-Sinda terrane) records detrital zircon ages and ε_(Hf)(t) values that correlate with ca.1.1-1.0 Ga magmatism exposed elsewhere in the SIB and Irumide Belt.We ascribe this difference in age populations to the polyphase development of the province,where the sedimentary and volcanic rocks of the Nyimba-Sinda terrane accumulated in extensional basins that developed in the Neoproterozoic.Such deposition would have occurred following late-Mesoproterozoic magmatism that is widespread throughout both the Irumide and Southern Irumide Belts,presently considered to have occurred in response to collision between a possible microcontinental mass and the Irumide Belt.This interpretation implies a multi-staged evolution of the ocean south of the Congo Craton during the mid-Mesoproterozoic to late-Neoproterozoic,which ultimately closed during collision between the Congo and Kalahari cratons. 相似文献
12.
Zeh Armin; Gerdes Axel; Klemd Reiner; Barton Jackson M. Jr 《Journal of Petrology》2007,48(8):1605-1639
A combined set of UPb and LuHf 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 textureage relationships,as obtained by cathodoluminescence images and UPb 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 UPb and LuHfdata 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 2·022·06 Ga. TheMokgware granite (2019 ± 9 Ma) contains zircon xenocrystswith PbPb ages of 2·522·65 Ga and2·93 Ga and hafnium model ages of 3·03·4Ga, indicating that this granite is derived from remelting ofArchaean crust. In contrast, uniform TDMHfinitial ages of 2·612·67Ga 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, UPb dating; LuHf isotopes; LA-ICP-MS 相似文献
13.
In north-east Brazil, Archean and Paleoproterozoic cratonicblocks are enclosed within a network of Brasiliano-age (0·70·55Ga) metasedimentary foldbelts. The unfoliated Coronel JoãoSá granodiorite pluton, which contains magmatic epidoteand strongly resorbed clinopyroxene, intrudes the SergipanoFoldbelt. Zircons yield a concordant UPb crystallizationage of 625 ± 2 Ma; titanite ages are approximately 621Ma. Discordant zircons suggest inheritance from at least twomagma sources of ages <1·8 and >2·2 Ga.Model calculations based on diffusion parameters and RbSrisotope data from separated minerals indicate that the plutoncooled at a rate of 36°C/Myr. Whole-rock element compositionsand initial SrNd isotopic compositions that are heterogeneouson all length scales suggest magma mixing. Trace-element concentrationsand Nd isotope data argue against a contribution from a contemporaneousmantle-derived magma. Values of magmatic Nd (at 625 Ma) resemblecontemporary Nd for local supracrustal rocks and basement, compatiblewith anatexis of a crustal source. In north-east Brazil, cratonicblocks could have amalgamated with foldbelts that originatedas: (1) a mosaic of island arcs and arc basins (traditionalallochthonous model), or as (2) extensional continental sedimentarybasins (but not oceanic crust) later involved in collision (autochthonousmodel). The Coronel João Sá isotopic and chemicaldata support an autochthonous origin. KEY WORDS: Brasiliano Orogeny; granodiorite pluton; RbSr isotopes, SmNd isotopes; UPb isotopes, magma cooling rate 相似文献
14.
Temperature and Bulk Composition Control on the Growth of Monazite and Zircon During Low-pressure Anatexis (Mount Stafford, Central Australia) 总被引:5,自引:0,他引:5
The formation, age and trace element composition of zircon andmonazite were investigated across the prograde, low-pressuremetamorphic sequence at Mount Stafford (central Australia).Three pairs of inter-layered metapelites and metapsammites weresampled in migmatites from amphibolite-facies (T 600°C)to granulite-facies conditions (T 800°C). Sensitive high-resolutionion microprobe UPb dating on metamorphic zircon rimsand on monazite indicates that granulite-facies metamorphismoccurred between 1795 and 1805 Ma. The intrusion of an associatedgranite was coeval with metamorphism at 1802 ± 3 Ma andis unlikely to be the heat source for the prograde metamorphism.Metamorphic growth of zircon started at T 750°C, well abovethe pelite solidus. Zircon is more abundant in the metapelites,which experienced higher degrees of partial melting comparedwith the associated metapsammites. In contrast, monazite growthinitiated under sub-solidus prograde conditions. At granulite-faciesconditions two distinct metamorphic domains were observed inmonazite. Textural observations, petrology and the trace elementcomposition of monazite and garnet provide evidence that thefirst metamorphic monazite domain grew prior to garnet duringprograde conditions and the second in equilibrium with garnetand zircon close to the metamorphic peak. Ages from sub-solidus,prograde and peak metamorphic monazite and zircon are not distinguishablewithin error, indicating that heating took place in less than20 Myr. KEY WORDS: accessory phases; anatexis; trace element partitioning; UPb dating 相似文献
15.
Zircon Crystal Morphology, Trace Element Signatures and Hf Isotope Composition as a Tool for Petrogenetic Modelling: Examples From Eastern Australian Granitoids 总被引:24,自引:0,他引:24
In situ laser ablation inductively coupled plasma mass spectrometryanalysis of trace elements, UPb ages and Hf isotopiccompositions of magmatic zircon from I- and S-type granitoidsfrom the Lachlan Fold Belt (Berridale adamellite and Kosciuskotonalite) and New England Fold Belt (Dundee rhyodacite ignimbrite),Eastern Australia, is combined with detailed studies of crystalmorphology to model petrogenetic processes. The presented examplesdemonstrate that changes in zircon morphology, within singlegrains and between populations, generally correlate with changesin trace element and Hf-isotope signatures, reflecting the mixingof magmas and changes in the composition of the magma throughmingling processes and progressive crystallization. The zircondata show that the I-type Kosciusko tonalite was derived froma single source of crustal origin, whereas the S-type Berridaleadamellite had two distinct sources including a significantI-type magma contribution. Complex morphology and Hf isotopevariations in zircon grains indicate a moderate contributionfrom a crustal component in the genesis of the I-type Dundeerhyodacite. The integration of data on morphology, trace elementsand Hf isotope variations in zircon populations provides a toolfor the detailed analysis of the evolution of individual igneousrocks; it offers new insights into the contributions of differentsource rocks and the importance of magma mixing in granite petrogenesis.Such information is rarely obtainable from the analysis of bulkrocks. KEY WORDS: granite source origins; zircon Hf isotopes; zircon petrogenesis; zircon morphology; zircon UPb ages 相似文献
16.
JANOUSEK VOJTECH; GERDES AXEL; VRANA STANISLAV; FINGER FRITZ; ERBAN VOJTECH; FRIEDL GERTRUDE; BRAITHWAITE COLIN J. R. 《Journal of Petrology》2006,47(4):705-744
The Liov Granulite Massif differs from neighbouring granulitebodies in the Moldanubian Zone of southern Bohemia (Czech Republic)in including a higher proportion of intermediatemaficand orthopyroxene-bearing rocks, associated with spinel peridotitesbut lacking eclogites. In addition to dominantly felsic garnetgranulites, other major rock types include quartz dioritic two-pyroxenegranulites, tonalitic granulites and charnockites. Minor bodiesof high-pressure layered gabbroic garnet granulites and spinelperidotites represent tectonically incorporated foreign elements.The protoliths of the maficintermediate granulites (quartz-dioriticand tonalitic) crystallized 360370 Ma ago, as indicatedby laser ablation inductively coupled plasma mass spectrometryUPb ages of abundant zircons with well-preserved magmaticzoning. Strongly metamorphically recrystallized zircons giveages of 330340 Ma, similar to those of other Moldanubiangranulites. For the overwhelming majority of the Liov granulitespeak metamorphic conditions probably did not exceed 800900°Cat 45 kbar; the equilibration temperature of the pyroxenegranulites was 670770°C. This is in sharp contrastto conditions of adjacent contemporaneous Moldanubian granulites,which are characterized by a distinct HPHT signature.The maficintermediate Liov granulites are thought tohave originated during Viséan metamorphic overprintingof metaluminous, medium-K calc-alkaline plutonic rocks thatformed the mid-crustal root of a Late Devonian magmatic arc.The protolith resembled contemporaneous calc-alkaline intrusionsin the European Variscan Belt. KEY WORDS: low-pressure granulites; geothermobarometry; laser-ablation ICP-MS zircon dating; whole-rock geochemistry; SrNd isotopes; Moldanubian Zone 相似文献
17.
Sensitive high-resolution ion microprobe UPb dating showsthat a biotite orthogneiss from the Hercynian belt of westerncentral Iberia contains 1000300 Ma zircon. Older, 1000570Ma ages within this range represent inherited, detrital materialamong which four age components may be recognized: 相似文献
18.
BRUECKNER HANNES K.; VAN ROERMUND HERMAN L. M.; PEARSON NORMAN J. 《Journal of Petrology》2004,45(2):415-437
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 ablationmicroprobeinductively coupled plasma mass spectrometryReOs 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 ReOs 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 SrNd isotope ratios,and by parallel large ion lithophile-enriched trace elementpatterns in clinopyroxene from pyroxenite and the immediatelyadjacent peridotite. Clinopyroxene and whole-rock model SmNdages (TDM = 1·12·2 Ga) indicate that fertilizationtook place in Proterozoic times. SmNd garnet2clinopyroxene2wholerock ± 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
19.
DI VINCENZO GIANFRANCO; TONARINI SONIA; LOMBARDO BRUNO; CASTELLI DANIELE; OTTOLINI LUISA 《Journal of Petrology》2006,47(7):1439-1465
Different lithologies (impure marble, eclogite and graniticorthogneiss) sampled from a restricted area of the coesite-bearingBrossascoIsasca Unit (Dora Maira Massif) have been investigatedto examine the behaviour of 40Ar39Ar and RbSrsystems in phengites developed under ultrahigh-pressure (UHP)metamorphism. Mineralogical and petrological data indicate thatzoned phengites record distinct segments of the PT path:prograde, peak to early retrograde in the marble, peak to earlyretrograde in the eclogite, and late retrograde in the orthogneiss.Besides major element zoning, ion microprobe analysis of phengitein the marble also reveals a pronounced zoning of trace elements(including Rb and Sr). 40Ar39Ar apparent ages (3562Ma, marble; 89170 Ma, eclogite; 3552 Ma, orthogneiss),determined through Ar laserprobe data on phengites (step-heatingand in situ techniques), show wide intra-sample and inter-samplevariations closely linked to within-sample microchemical variations:apparent ages decrease with decreasing celadonite contents.These data confirm previous reports on excess Ar and, more significantly,highlight that phengite acted as a closed system in the differentlithologies and that chemical exchange, not volume diffusion,was the main factor controlling the rate of Ar transport. Conversely,a RbSr internal isochron from the same eclogite yieldsan age of 36 Ma, overlapping with the time of the UHP metamorphicpeak determined through UPb data and thereby corroboratingthe previous conclusion that UHP metamorphism and early retrogressionoccurred in close succession. Different phengite fractions ofthe marble yield calcitephengite isochron ages of 36to 60 Ma. Although this time interval matches Ar ages from thesame sample, RbSr data from phengite are not entirelyconsistent with the whole dataset. According to trace elementvariations in phengite, only RbSr data from two wet-groundphengite separates, yielding ages of 36 and 41 Ma, are internallyconsistent. The oldest age obtained from a millimetre-sizedgrain fraction enriched in progradepeak phengites mayrepresent a minimum age estimate for the prograde phengite relics.Results highlight the potential of the in situ 40Ar39Arlaser technique in resolving discrete PT stages experiencedby eclogite-facies rocks (provided that excess Ar is demonstrablya negligible factor), and confirm the potential of RbSrinternal mineral isochrons in providing precise crystallizationages for eclogite-facies mineral assemblages. KEY WORDS: 40Ar39Ar dating; RbSr dating; phengite; SIMS; UHP metamorphism 相似文献
20.
Assigning Dates to Thin Gneissic Veins in High-Grade Metamorphic Terranes: A Cautionary Tale from Akilia, Southwest Greenland 总被引:10,自引:0,他引:10
A granodiorite from Akilia, southwest Greenland, previouslysuggested to date putative life-bearing rocks to 3·84Ga, is re-investigated using whole-rock major and trace-elementgeochemistry, and detailed cathodoluminescence image-guidedsecondary ion mass spectrometer analyses of zircon UThPband rare earth elements. Complex zircon internal structure revealsthree episodes of zircon growth and/or recrystallization datedto c. 3·84 Ga, 3·62 Ga and 2·71 Ga. Rareearth element abundances imply a significant role for garnetin zircon generation at 3·62 Ga and 2·71 Ga. The3·62 Ga event is interpreted as partial melting of ac. 3·84 Ga grey gneiss precursor at granulite facieswith residual garnet. Migration of this 3·62 Ga magma(or meltcrystal mush) away from the melt source placesa maximum age limit on any intrusive relationship. These earlyArchaean relationships have been complicated further by isotopicreworking in the 2·71 Ga event, which could have includeda further episode of partial melting. This study highlightsa general problem associated with dating thin gneissic veinsin polyphase metamorphic terranes, where field relationshipsmay be ambiguous and zircon inheritance can be expected. KEY WORDS: Archaean; geochronology; Greenland; secondary ion mass spectrometry; zircon 相似文献