Archaean to Proterozoic Crustal Evolution in the Central Zone of the Limpopo Belt (South Africa-Botswana): Constraints from Combined U-Pb and Lu-Hf Isotope Analyses of Zircon

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(T_DM^Hf) 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 initialT_DM^Hf between 3·46 and 3·01 Ga. These variableT_DM^Hf_initial 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 T_DM^Hf_initial 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     

A Metamorphosed, Early Archaean Chromitite from West Greenland: Implications for the Genesis of Archaean Anorthositic Chromitites

An early Archaean (>3·81 Ga) chromitite–ultramaficlayered body from the Ujaragssuit nunât area, west Greenland,may represent the Earth’s oldest chromitite. The layeredbody occurs as a large xenolith (800 m x 100 m) entrained withintonalitic gneisses and preserves primary igneous layering andtextures. New Re–Os and Pb–Pb isotope results supportthe view that it has been metamorphosed twice, in the earlyand late Archaean at 3·75 Ga and 2·8 Ga. Mineralchemistry and textures indicate that the chromite compositionspreserve two different evolutionary trends. There is a mainmagmatic trend in which Cr/(Cr + Al) ratios remain relativelyconstant but in which there is strong enrichment in Fe³⁺, Fe²⁺and Ti with progressive differentiation. This trend is a compositeof magmatic-liquidus, magmatic-cooling and subsolidus re-equilibrationprocesses. A second trend is defined by chromites from harzburgitesin the upper part of the layered body. These chromites showmagmatic replacement textures in which Fe-rich chromites arealtered to aluminous chromites. Chromites showing magmatic replacementtextures are thought to have formed by reaction with a late,interstitial melt during the solidification of the layered body.The close association between the Fe³⁺–Cr-chromites ofthe main trend and Al-rich chromites of the type found in otherArchaean megacrystic anorthosites suggest a magmatic-geneticrelationship between the two types of chromite. We propose thatanorthositic chromites form in an Fe-rich basaltic melt derivedfrom a komatiitic, boninitic or basaltic parent magma throughreaction between the melt and early-formed Fe-rich chromite. KEY WORDS: chromite; anorthosite; Archaean; Greenland; Re–Os     

Pan-African Tectonism in the Western Maud Belt: P-T-t Path for High-grade Gneisses in the H.U. Sverdrupfjella, East Antarctica

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     

Shear Zone-hosted Migmatites (Eastern India): the Role of Dynamic Melting in the Generation of REE-depleted Felsic Melts, and Implications for Disequilibrium Melting

In the Ranmal migmatite complex, non-anatectic foliated graniteprotoliths can be traced to polyphase migmatites. Structural–microtexturalrelations and thermobarometry indicate that syn-deformationalsegregation–crystallization of in situ stromatic and diatexiteleucosomes occurred at 800°C and 8 kbar. The protolith,the neosome, and the mesosome comprise quartz, K-feldspar, plagioclase,hornblende, biotite, sphene, apatite, zircon, and ilmenite,but the modal mineralogy differs widely. The protolith compositionis straddled by element abundances in the leucosome and themesosome. The leucosomes are characterized by lower CaO, FeO+MgO,mg-number, TiO₂ , P₂O₅ , Rb, Zr and total rare earth elements(REE), and higher SiO₂ , K₂O, Ba and Sr than the protolith andthe mesosome, whereas Na₂O and Al₂O₃ abundances are similar.The protolith and the mesosome have negative Eu anomalies, butprotolith-normalized abundances of REE-depleted leucosomes showpositive Eu anomalies. The congruent melting reaction for leucosomeproduction is inferred to be 0·325 quartz+0·288K-feldspar+0·32 plagioclase+0·05 biotite+0·014hornblende+0·001 apatite+0·001 zircon+0·002sphene=melt. Based on the reaction, large ion lithophile element,REE and Zr abundances in model melts computed using dynamicmelting approached the measured element abundances in leucosomesfor >0·5 mass fraction of unsegregated melts withinthe mesosome. Disequilibrium-accommodated dynamic melting andequilibrium crystallization of melts led to uniform plagioclasecomposition in migmatites and REE depletion in leucosome. KEY WORDS: migmatite; REE; trace element; partial melting; P–T conditions     

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

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     

The Shape and Volume of the Skaergaard Intrusion, Greenland: Implications for Mass Balance and Bulk Composition

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 3·4–4km 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     

High-Pressure Melting of Eclogite and the P-T-X History of Tonalitic to Trondhjemitic Zoisite-Pegmatites, Munchberg Massif, Germany

Zoisite-bearing high-pressure pegmatites from the MünchbergMassif, Germany, provide an excellent example of the characteristicsof the onset of metabasite melting at eclogite-facies conditions.The pegmatites were derived by partial melting of a mid-oceanridge basalt (MORB)-like eclogite at T 680°C/2·3GPa to 750°C/3·1 GPa, which produced small amountsof tonalitic to trondhjemitic melt. The melt concentrated locallyin isolated, small melt pockets and crystallized primary zoisiteas liquidus phase at P 2·3 GPa/680°C to 2·1GPa/750°C. Compositional zoning of pegmatite zoisite recordsan ensuing multi-stage uplift history with successive, discretecrystallization events at 1·4 ± 0·2 GPa/650–700°Cand 1·0 ± 0·1 GPa/620–650°C.Resorption textures indicate reheating and thermal perturbationof the whole system prior to each successive crystallizationevent. Final solidification of zoisite-pegmatites occurred at0·9 ± 0·1 GPa/620–650°C. Thedata suggest that isolated melt + zoisite crystal mush pocketsformed an integral part of the eclogite throughout uplift frommelt formation at T 680°C/2·3 GPa to 750°C/3·1GPa to final solidification at 0·9 GPa/620–630°C;that is, over a depth range of 45–60 km. The entire pegmatite-formingprocess was probably fluid conserving: fluid present duringmelt formation was trapped by fully or nearly water-saturatedsiliceous melts, whereas fluid liberated during pegmatite crystallizationinteracted with dehydrated eclogite-facies assemblages to formamphibolite-facies hydrous minerals. A set of empirical D^{melt/eclogite}values based on mean zoisite-pegmatite and eclogite compositionwere used to model the onset of partial high-pressure meltingof metabasites. KEY WORDS: adakite; high-pressure melting; pegmatite; trondhjemite; zoisite     

Kimberlite-like Metasomatism and 'Garnet Signature' in Spinel-peridotite Xenoliths from Sal, Cape Verde Archipelago: Relics of a Subcontinental Mantle Domain within the Atlantic Oceanic Lithosphere?

A mantle xenolith suite from two Late Tertiary necks on SalIsland (Cape Verde Archipelago) consists of nearly equivalentamounts of anhydrous spinel-bearing lherzolites and harzburgites,in which secondary metasomatic textural domains are superimposedon the original protogranular textures. Detailed petrographicstudies, coupled with in situ major and trace element analysesof the constituent minerals and interstitial glasses, revealthe complex evolutionary history of the Cape Verde lithosphericmantle, from depletion in the garnet facies to re-equilibrationand re-enrichment in the spinel stability field. Low CaO (16·4–18·0wt %) and heavy rare earth element (HREE; Yb_n = 2·4–4·8),and high Cr₂O₃ (1·06–1·84 wt %) contentsin the clinopyroxenes of the lherzolites can be quantitativelyaccounted for by (1) low-degree (4%) partial melting of a PrimitiveMantle-like garnet lherzolite followed by (2) partial re-equilibrationof the melting residuum from the garnet to the spinel stabilityfield. This model is further supported by thermobarometric estimates(T = 975–1210°C; P = 1·3–2·1 GPa),which cluster around the spinel–garnet boundary in theperidotite system. Secondary parageneses, regardless of theprimary lithologies, are characterized by (1) two clinopyroxenes,cpx2-O and cpx2-C, respectively related to orthopyroxene andclinopyroxene destabilization after reaction with metasomaticfluids, and (2) glasses with anomalously high, even for continentalsettings, K₂O contents (up to 8·78 wt %), together withK-feldspar. Major and trace element mass balance calculationsbetween the primary and secondary parageneses suggest infiltrationof a kimberlite-like metasomatizing agent (on volatile-freebasis, MgO 17–27 wt %; K₂O/Na₂O 1·6–3·2molar; (K₂O + Na₂O)/Al₂O₃ 1·1–3·0 molar;Rb 91–165 ppm; Zr 194–238 ppm). The kimberlite-likemetasomatism in the Cape Verde lithospheric mantle, togetherwith the presence of lherzolitic domains, partially re-equilibratedfrom the garnet to the spinel stability field, may suggest thepresence of subcontinental mantle lithosphere relicts left behindby drifting of the African Plate during the opening of the CentralAtlantic Ocean. KEY WORDS: Cape Verde; mantle metasomatism; garnet signatures; clinopyroxenes; kimberlites     

Calc-Alkaline Magmatism at the Archean Proterozoic Transition: the Caico Complex Basement (NE Brazil)

The Paleoproterozoic metaplutonic rocks of the CaicóComplex Basement (Seridó region, NE Brazil) provide importantand crucial insights into the petrogenetic processes governingcrustal growth and may potentially be a proxy for understandingthe Archean–Proterozoic transition. These rocks consistof high-K calc-alkaline diorite to granite, with Rb–Sr,U–Pb, Pb–Pb and Sm–Nd ages of c. 2·25–2·15Ga. They are metaluminous, with high Yb_N, K₂O/Na₂O and Rb/Sr,low I_Sr ratios, and are large ion lithophile elements (LILE)enriched. Petrographic and geochemical data demonstrate thatthey belong to differentiated series that evolved by low-pressurefractionation, thus resulting in granodioritic liquids. We proposea model in which the petrogenesis of the Caicó Complexorthogneisses begins with partial melting of a metasomaticallyenriched spinel- to garnet-bearing lherzolite (with high-silicaadakite melt as the metasomatic agent), generating a basic magmathat subsequently evolved at depth through fractional crystallizationof olivine, followed by low-pressure intracrustal fractionation.A subduction zone setting is proposed for this magmatism, toaccount for both negative anomalies in high field strength elements(HFSE) and LILE enrichment. Mantle-derived juvenile magmatismwith the same age is also known in the São Franciscoand West Africa cratons, as well as in French Guyana, and thusthe Archean–Proterozoic transition marks a very importantcontinental accretion event. It also represents a transitionfrom slab-dominated (in the Archean) to wedge-dominated post-Archeanmagmatism. KEY WORDS: calc-alkaline; magmatism; NE Brazil; Paleoproterozoic; petrogenesis     

Garnetite Xenoliths and Mantle-Water Interactions Below the Colorado Plateau, Southwestern United States

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 U–Pb agesand Hf isotopic compositions. Most U–Pb analyses ploton or near concordia in the range 60–85 Ma but a few arediscordant. The range in ¹⁷⁶Hf/¹⁷⁷Hf is about 0·2818–0·2828,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 85–60 Ma. The garnetites are interpretedas mantle analogues of rodingites, formed in metasomatic reactionzones caused by water–rock 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 5–10 Myr before tectonism related tolow-angle subduction. KEY WORDS: garnetite; Lu–Hf, mantle; rodingite; metasomatism     

The Concentration of the Platinum-Group Elements in South African Komatiites: Implications for Mantle Sources, Melting Regime and PGE Fractionation during Crystallization

We have analysed 18 samples of komatiite from five consecutivelava flows of the Komati Formation at Spinifex Creek, BarbertonMountain Land. Our samples include massive komatiite, varioustypes of spinifex-textured komatiite, and flow-top breccias.The rocks have low platinum-group element (PGE) contents andPd/Ir ratios relative to komatiites from elsewhere, at 0·45–2ppb Os, 1–1·4 ppb Ir, <1–5 ppb Ru, 0·33–0·79ppb Rh, 1·7–6 ppb Pt, 1·6–6·1ppb Pd, and Pd/Ir 3·3. Pt/Pd ratios are c. 1·1.Platinum-group elements are depleted relative to Cu (Cu/Pd =15 300). They display a tendency to increase in the less magnesiansamples, suggesting that the magmas were S-undersaturated uponeruption and that all PGE were incompatible with respect tocrystallizing olivine. Komatiites from the Westonaria Formationof the Ventersdorp Supergroup and the Roodekrans Complex nearJohannesburg have broadly similar PGE patterns and concentrationsto the Komati rocks, suggesting that the PGE contents of SouthAfrican ultrabasic magmas are controlled by similar processesduring partial mantle melting and low-P magmatic crystallization.Most workers believe that the Barberton komatiites formed byrelatively moderate-degree batch melting of the mantle at highpressure. Based on the concentration of Zr in the Komati samples,we estimate that the degree of partial melting was between 26and 33%. We suggest that the low PGE contents and Pd/Ir ratiosof all analysed South African komatiites are the result of sulphideshaving been retained in the mantle source during partial melting.The difference in Pd/Ir between our samples and Al-undepletedkomatiites from elsewhere further suggests that the PGE arefractionated during progressive partial melting of the mantle.Thus, our data are in agreement with other recent studies showingthat the PGE are hosted by different phases in the mantle, withPd being concentrated by interstitial Cu-rich sulphide, andthe IPGE (Os, Ir, Ru) and Rh resting in monosulphide solid solutionincluded within silicates. Pt is possibly controlled by a discreterefractory phase, as Pt/Pd ratios of most komatiites worldwideare sub-chondritic. KEY WORDS: platinum-group elements; komatiites; Barberton; mantle melting; South Africa     

Evolution from Oceanic Subduction to Continental Collision: a Case Study from the Northern Tibetan Plateau Based on Geochemical and Geochronological Data

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 (480–440Ma, with weighted mean ages of 464 ± 6 Ma for North Qilianand 457 ± 7 Ma for North Qaidam), determined by zirconU–Pb 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 Qilian–Qaidam 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     

Fractionation and Assimilation Processes in the Alkaline Augite Syenite Unit of the Ilimaussaq Intrusion, South Greenland, as Deduced from Phase Equilibria

The early augite syenite unit in the 1·13-Ga-old Ilímaussaqintrusive complex, South Greenland, consists of a magmatic assemblageof ternary alkali feldspar + fayalitic olivine + augite + titanomagnetite+ apatite + baddeleyite ± nepheline ± quartz ±ilmenite ± zircon. Feldspar, nepheline and QUILF thermometryyield T = 1000–700°C, at P = 1 kbar, which is derivedfrom fluid inclusion data from other parts of the complex. Ternaryfeldspar was the first major liquidus phase. It crystallizedat temperatures between 950 and 1000°C from a homogeneousmagma with a_SiO2 = 0·8 and f_O2 about 1·5–2log units below the fayalite–magnetite–quartz (FMQ)buffer. Later, closed system fractionation produced nepheline-bearingassemblages with a_SiO2 = 0·4 and log f_O2 = FMQ –3 to FMQ – 5. Assimilation of wall rocks produced localvariations of melt composition. Four traverses through the unitwere sampled parallel to the assumed direction of crystallization.They exhibit significant differences in their mineral assemblagesand compositions. The chemical zoning and calculated intensiveparameters of four sample suites reflect both closed systemfractional crystallization and local assimilation of wall rocks. KEY WORDS: alkaline magmatism; assimilation; fractionation; redox equilibria; QUILF     

Hybridization of a Shallow 'I-type' Granitoid Pluton and its Host Migmatite by Magma-Chamber Wall Collapse: the Tokuwa Pluton, Central Japan

The Miocene Tokuwa pluton of ‘I-type’ granitoidaffinity was emplaced discordantly into a Cretaceous to Paleogeneaccretionary complex and induced a contact aureole in whichvarious thermally metamorphosed rocks were developed, includinghornfels, metatexite, diatexite and cordierite-bearing tonalite(Crd-tonalite) of ‘S-type’ granite affinity. Thethermally metamorphosed rocks show low-pressure reaction texturesculminating in partial melting. Peak P–T conditions of3 kbar at 780°C are estimated on the basis of the TWQ thermobarometerfor the garnet-bearing rocks. The rocks in the contact aureoleexhibit a gradual transition from hornfels, through metatexiteand diatexite to Crd-tonalite. The Sr-isotopic composition atthe time of Tokuwa pluton emplacement at 12 Ma decreases systematicallyfrom metatexite (0·7100–0·7112) throughdiatexite (0·7078–0·7094) to Crd-tonalite(0·7067–0·7068); this trend is interpretedin terms of mixing between the Tokuwa magma and the aureolemigmatites. The field relationships, geochemical data, and isotopicdata collectively suggest that the emplacement of the Tokuwapluton triggered partial melting of the surrounding metasedimentaryrocks. Subsequent hybridization of the Tokuwa magma with themetatexite in variable proportions produced the Crd-tonaliteand diatexite. The hybridization was caused by invasion of theTokuwa magma into the migmatite zone, accompanied by gravitationalcollapse of the previously crystallized wall of the magma chamber.The data presented demonstrate that even a relatively low-temperature,shallow, ‘I-type’ granitoid pluton can induce contactanatexis and hybrid ‘S-type’ granitoid formationat the intrusive contact. KEY WORDS: contact metamorphism; hybridization; magma–host-rock interaction; migmatite; ‘S-type’ granitoid     

Petrology and Geochronology of Granulites from the McKaskle Hills, Eastern Amery Ice Shelf, Antarctica, and Implications for the Evolution of the Prydz Belt

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 P–T conditions of 9·0–9·5kbar and 880–950°C and 6·6–7·2kbar and 700–750°C, respectively. Similar but slightlyscattered peak P–T estimates of 7·9–10·1kbar and 820–980°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 microprobeU–Pb zircon dating reveals an evolutionary history forthe granulites, including a mafic and felsic igneous intrusionat 1174–1019 Ma, sedimentation after 932–916 Ma,and a high-grade metamorphism at 533–529 Ma. In contrast,Sm–Nd mineral–whole-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 P–T conditions and a decompression-dominated P–Tpath 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 P–T–t 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; P–T path; Prydz Belt; East Antarctica     

Lawsonite-Omphacite-Bearing Metabasites of the Pam Peninsula, NE New Caledonia: Evidence for Disrupted Blueschist- to Eclogite-Facies Conditions

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     

Water-deficient Calc-alkaline Plutonic Rocks of Northeastern Superior Province, Canada: Significance of Charnockitic Magmatism

Calc-alkaline batholiths of the Archaean Minto block, northeasternSuperior Province, Canada, have pyroxene- and hornblende-bearingmineral assemblages inferred to have crystallized from hot,water-undersaturated magmas at 2·729–2·724Ga. A regional amphibolite- to granulite-facies tectonothermalevent at 2·70 Ga resulted in mild to negligible metamorphiceffects on the dominantly granodioritic units. Geochemical,textural and thermobarometric studies define the crystallizationhistory in compositions ranging from cumulate pyroxenite throughquartz diorite, granodiorite, granite, and syn-magmatic gabbroicdykes. Early magmatic assemblages include orthopyroxene, clinopyroxene,plagioclase, biotite, Fe–Ti oxides and ternary feldspar,indicating crystallization from magmas containing <2 wt %H₂O at 1100–900°C. Water enrichment in the residualmelt induced hornblende crystallization at 5 ± 1 kbar,800–600°C. Characterized by a continuum of large ionlithophile element (LILE)-enriched, high field strength element(HFSE)-depleted compositions, the I-type suite resembles moderncontinental arc batholiths in composition and size but not primarymineralogy. Magmatic arcs produced between 2·75 and 1·85Ga commonly have charnockitic components, possibly because slab-derivedfluids interacted with mantle wedges at ambient temperatureshigher by 100°C than at present, producing large volumesof water-deficient magma. KEY WORDS: granitoid rocks; igneous pyroxenes; water-undersaturated magma; charnockite     

U-Pb Zircon Calendar for Namaquan (Grenville) Crustal Events in the Granulite-facies Terrane of the O'okiep Copper District of South Africa

The O'okiep Copper District is underlain by voluminous 1035–1210Ma 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 1700–2000 Ma zirconas xenocrysts in all of the intrusive rocks, and as detritalzircon in the metasediments of the Khurisberg Subgroup. Thesedata are consistent with published Sm–Nd model ages ofc. 1700 Ma (T_CHUR) and c. 2000 Ma (T_DM) of many of the intrusivesthat support a major crust-forming event in Eburnian (Hudsonian)times. In addition, U–Th–Pb analyses of zirconsfrom all major rock units define two tectono-magmatic episodesof the Namaquan Orogeny: (1) the O'okiepian Episode (1180–1210Ma), 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 (1020–1040Ma), 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 east–west-trending open folds [F₃(D_3a)], andby localized east–west-trending near-vertical ductilefolds [‘steep structures’; F₄(D_3b)] 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 800–850 Ma ago. KEY WORDS: U–Pb, zircon; O'okiep, Namaqualand; granite plutonism; granulite facies; Koperberg Suite; Namaquan (Grenville) Orogeny     

Fenitizing Processes Induced by Ferrocarbonatite Magmatism at Swartbooisdrif, NW Namibia

The southeastern margin of the anorthositic Kunene IntrusiveComplex, NW Namibia, has been subsequently invaded by Mesoproterozoicsyenite, nepheline syenite and ferrocarbonatite dykes alongNE- and SE-trending faults. The first generation of carbonatiteintrusions frequently contains fenitized anorthositic wall-rockfragments set in a ferrocarbonatite matrix; later, subordinateveins of massive ferrocarbonatite are almost xenolith-free andcut through the main carbonatite dykes. A mantle source forboth carbonatite generations is constrained by their respectiveoxygen and carbon isotope compositions of ankerite (¹⁸O_SMOW8·91–9·73; ¹³C_PDB –6·98 to–6·76). Na-rich fluids, released from the meltparental to the ferrocarbonatites, caused the fenitization ofboth the incorporated anorthosite xenoliths and the borderinganorthosite, syenite and nepheline syenite. This process ismainly characterized by the progressive transformation of Ca-richplagioclase, K-feldspar and nepheline into albite and/or sodalite.The changing mineral modes indicate that the fenitizing fluidswere sodium-rich and strongly Si-deficient solutions, whichalso contained significant amounts of Sr, Ba, Nb and the lightrare earth elements. On the basis of mineral equilibria studies,it is possible to reconstruct the temperature conditions forcarbonatite emplacement (c. 830 ± 200°C) and recrystallization(c. 480 ± 130°C), and for the metasomatic formationof sodalite (c. 700 ± 70°C). KEY WORDS: anorthosite; fenitization; ferrocarbonatite; sodalite; stable isotopes     

Petrogenesis of the Post-kinematic Magmatism of the Central Finland Granitoid Complex II; Sources and Magmatic Evolution

The 1·88–1·87 Ga post-kinematic granitoidsof the Central Finland Granitoid Complex (CFGC) provide a keygeochemical link to understanding granite formation in Paleoproterozoicorogenic and post-orogenic terrains. Thickness of the crustand intra-crustal differentiation processes played an importantrole in the formation of three granitoid types that shortlyfollowed the peak of the Svecofennian orogeny. In the easternCFGC, pyroxene-bearing plutons with C-type geochemical affinitiespredominate. These were formed from a mixture of low- to moderate-degreepartial melts (30%) of mafic mantle-derived (basaltic, 49% SiO₂)source rocks and partial melts of pre-existing mafic granulitelower crust at depth. In the western CFGC, high-silica, iron-rich,fluorite-bearing plutons with A-type granite characteristicspredominate. A higher thermal gradient, thinner upper and lowercrust, and significantly more shallow Moho depth resulted inhigher proportions of crustal melts (0·3–0·4vs 0·1–0·2 in the eastern CFGC) incorporatedinto the partial melts of a mafic mantle-derived source. A geochemicalmodel focusing on the Jämsä and Honkajoki plutonsof the post-kinematic suite is presented, constraining the natureof the source rock(s), the degree of partial melting, proportionsof partial melts and fractionation processes involved. KEY WORDS: magmatic evolution; geochemistry; Proterozoic; granite; Finland