Formation, Crystallization, and Migration of Melt in the Mid-orogenic Crust: Muskoka Domain Migmatites, Grenville Province, Ontario

Migmatitic orthogneisses in the Muskoka domain, southwesternGrenville Province, Ontario, formed during the Ottawan stage(c. 1080–1050 Ma) of the Grenvillian orogeny. Stromaticmigmatites are volumetrically dominant, comprising granodioriticgneisses with 2–5 cm thick granitic leucosomes, locallyrimmed by thin melanosomes, that constitute 20–30 vol.%, and locally 40–50 vol. %, of the outcrops. Patch migmatitesin dioritic gneisses form large (>10 m) pinch-and-swell structureswithin the stromatic migmatites, and consist of decimetre-scale,irregular patches of granitic leucosome, surrounded by medium-grainedhornblende–plagioclase melanosomes interpreted as restite.The patches connect to larger networks of zoned pegmatite dykes.Petrographic and geochemical evidence suggests that the patchleucosomes formed by 20–40% fluid-present, equilibriummelting of the dioritic gneiss, followed by feldspar-dominatedcrystallization. The dyke networks may have resulted from hydraulicfracturing, probably when the melts reached water saturationduring crystallization. Field and geochemical data from thestromatic migmatites suggest a similar petrogenesis to the patchmigmatites, but with significant additions of externally derivedmelts, indicating that they acted as conduits for melts derivedfrom deeper structural levels within the orogen. We hypothesizethat the Muskoka domain represents a transfer zone for meltsmigrating to higher structural levels during Grenvillian deformation. KEY WORDS: migmatite geochemistry; partial melting; melt crystallization; melt transport; Grenville orogen     

Metamorphic Evolution of the Ribeira Belt: Evidence from Outcrops in the Rio de Janeiro Area, Brazil

Migmatitic granulites and arc-related felsic intrusives of Pan-Africanage form the bedrock in the Rio de Janeiro area, SE Brazil.These rocks preserve a partial record of three parageneses.The earliest assemblage (M₁) grew during fabric formation inthe rocks (D₁) and is characterized by the mineral assemblagePl + Bt + Sil + Kfs + Qtz. Peak metamorphic conditions (M₂)are characterized by the assemblage Bt + Crd + Kfs + Pl + Grt+ liq + Qtz and are inferred to have developed during D₂ foldingof the rocks at T = 750–800°C and P = 7 kbar. M₃ reactiontextures overprint the M₂ assemblage and comprise symplectiticintergrowth of cordierite(II) and quartz that formed after garnet,whereas secondary biotite formed as a result of reactions betweengarnet and K-feldspar. By comparing the observed modal abundanceswith modal contours of garnet, cordierite and quartz on therelevant pseudosection a post M₂ P–T vector indicatingcontemporaneous cooling and decompression can be deduced. Theinferred equilibrium assemblage and reaction textures are interpretedto reflect a clockwise P–T path involving heating followedby post-peak decompression and associated cooling. We inferthat metamorphism occurred in response to advective heatingby the abundant syn-collisional (arc-related) I-type granitoidsin the region, consistent with the unusually high peak T/P ratio. KEY WORDS: advective heating; Ribeira belt; granulite; partial melting; P–T pseudosection     

Metapelites of the Kanskiy Granulite Complex (Eastern Siberia): Kinked P-T Paths and Geodynamic Model

The Southern Yenisey Range (Eastern Siberia) consists of thegranulite-facies Kanskiy complex bordered by the lower-gradeYeniseyskiy and Yukseevskiy complexes. Samples of metapeliteof the Kanskiy complex typically show characteristic garnet-formingreaction textures and near-isobaric cooling P–T paths.An important new result of this study concerns the differencein shape of the P–T paths from different parts of theKanskiy granulite complex: metapelites collected 8 km from theboundary with the Yeniseyskiy complex followed a linear pathwith dP/dT 0·006 kbar/°C; metapelites collected3 km from this boundary reveal a kinked P–T path withan interval of burial cooling (dP/dT –0·006 kbar/°C).The difference in the shape of the P–T paths is supportedby the chemical zoning of garnet studied in the second groupof samples. A mechanism of buoyant exhumation of granulite issuggested by comparison with the results of numerical modelling,which indicate that such a diversity of P–T paths mayresult from a transient disturbance of the thermal structureby rapid differential movement of material from different crustallevels. To arrive at a correct tectonic interpretation, thewhole assemblage of interrelated P–T paths of metamorphicrocks collected from different localities within the same complexmust be studied. KEY WORDS: crustal diapirism; exhumation; granulites; numerical modelling; P–T path     

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     

Quantitative Constraints on Metamorphism in the Variscides of Southern Brittany--a Complementary Pseudosection Approach

Previous studies of metapelitic rocks from the core of the southernBrittany metamorphic belt suggest a complex clockwise P–Tevolution. We use pseudosections calculated for an average subaluminousmetapelite composition in the MnNCKFMASH system and averageP–T calculations to investigate in more detail the metamorphicevolution of these rocks. For migmatites, sequential occurrenceof kyanite, kyanite + staurolite and sillimanite suggests thata prograde evolution to P > 8 kbar at T     

Exhumation Paths of High-Pressure-Low-Temperature Metamorphic Rocks from the Lycian Nappes and the Menderes Massif (SW Turkey): a Multi-Equilibrium Approach

The Menderes Massif and the overlying Lycian Nappes occupy anextensive area of SW Turkey where high-pressure–low-temperaturemetamorphic rocks occur. Precise retrograde P–T pathsreflecting the tectonic mechanisms responsible for the exhumationof these high-pressure–low-temperature rocks can be constrainedwith multi-equilibrium P–T estimates relying on localequilibria. Whereas a simple isothermal decompression is documentedfor the exhumation of high-pressure parageneses from the southernMenderes Massif, various P–T paths are observed in theoverlying Karaova Formation of the Lycian Nappes. In the uppermostlevels of this unit, far from the contact with the MenderesMassif, all P–T estimates depict cooling decompressionpaths. These high-pressure cooling paths are associated withtop-to-the-NNE movements related to the Akçakaya shearzone, located at the top of the Karaova Formation. This zoneof strain localization is a local intra-nappe contact that wasactive in the early stages of exhumation of the high-pressurerocks. In contrast, at the base of the Karaova Formation, alongthe contact with the Menderes Massif, P–T calculationsshow decompressional heating exhumation paths. These paths areassociated with severe deformation characterized by top-to-the-eastshearing related to a major shear zone (the Gerit shear zone)that reflects late exhumation of high-pressure parageneses underwarmer conditions. KEY WORDS: exhumation; high-pressure–low-temperature metamorphism; multi-equilibrium P–T estimates; Lycian Nappes; Menderes Massif     

Melting of Amphibole-bearing Wehrlites: an Experimental Study on the Origin of Ultra-calcic Nepheline-normative Melts

Olivine + clinopyroxene ± amphibole cumulates have beenwidely documented in island arc settings and may constitutea significant portion of the lowermost arc crust. Because ofthe low melting temperature of amphibole (1100°C), suchcumulates could melt during intrusion of primary mantle magmas.We have experimentally (piston-cylinder, 0·5–1·0GPa, 1200–1350°C, Pt–graphite capsules) investigatedthe melting behaviour of a model amphibole–olivine–clinopyroxenerock, to assess the possible role of such cumulates in islandarc magma genesis. Initial melts are controlled by pargasiticamphibole breakdown, are strongly nepheline-normative and areAl₂O₃-rich. With increasing melt fraction (T > 1190°Cat 1·0 GPa), the melts become ultra-calcic while remainingstrongly nepheline-normative, and are saturated with olivineand clinopyroxene. The experimental melts have strong compositionalsimilarities to natural nepheline-normative ultra-calcic meltinclusions and lavas exclusively found in arc settings. Theexperimentally derived phase relations show that such naturalmelt compositions originate by melting according to the reactionamphibole + clinopyroxene = melt + olivine in the arc crust.Pargasitic amphibole is the key phase in this process, as itlowers melting temperatures and imposes the nepheline-normativesignature. Ultra-calcic nepheline-normative melt inclusionsare tracers of magma–rock interaction (assimilative recycling)in the arc crust. KEY WORDS: experimental melting; subduction zone; ultra-calcic melts; wehrlite     

Mesoproterozoic Reworking of Palaeoproterozoic Ultrahigh-temperature Granulites in the Central Indian Tectonic Zone and its Implications

In the southern periphery of the Sausar Mobile Belt (SMB), thesouthern component of the Central Indian Tectonic Zone (CITZ),a suite of felsic and aluminous granulites, intruded by gabbro,noritic gabbro, norite and orthopyroxenite, records the polymetamorphicevolution of the CITZ. Using sequences of prograde, peak andretrograde reaction textures, mineral chemistry, geothermobarometricresults and petrogenetic grid considerations from the felsicand the aluminous granulites and applying metamorphosed maficdyke markers and geochronological constraints, two temporallyunrelated granulite-facies tectonothermal events of Pre-Grenvillianage have been established. The first event caused ultrahigh-temperature(UHT) metamorphism (M₁) (T 950°C) at relatively deepercrustal levels (P 9 kbar) and a subsequent post-peak near-isobariccooling P–T history (M₂). M₁ caused pervasive biotite-dehydrationmelting, producing garnet–orthopyroxene and garnet–rutileand sapphirine–spinel-bearing incongruent solid assemblagesin felsic and aluminous granulites, respectively. During M₂,garnet–corundum and later spinel–sillimanite–biotiteassemblages were produced by reacting sapphirine–spinel–sillimaniteand rehydration of garnet–corundum assemblages, respectively.Applying electron microprobe (EMP) dating techniques to monazitesincluded in M₁ garnet or occurring in low-strain domains inthe felsic granulites, the UHT metamorphism is dated at 2040–2090Ma. Based on the deep crustal heating–cooling P–Ttrajectory, the authors infer an overall counterclockwise P–Tpath for this UHT event. During the second granulite event,the Palaeoproterozoic granulites experienced crustal attenuationto 6·4 kbar at T 675°C during M₃ and subsequentnear-isothermal loading to 8 kbar during M₄. In the felsic granulites,the former is marked by decomposition of M₁ garnet to orthopyroxene–plagioclasesymplectites. During M₄, there was renewed growth of garnet–quartzsymplectites in the felsic granulites, replacing the M₃ mineralassemblage and also the appearance of coronal garnet–quartz–clinopyroxeneassemblages in metamorphosed mafic dykes. Using monazites frommetamorphic overgrowths and metamorphic recrystallization domainsfrom the felsic granulite, the M₄ metamorphism is dated at 1525–1450Ma. Using geochronological and metamorphic constraints, theauthors interpret the M₃–M₄ stages to be part of the sameMesoproterozoic tectonothermal event. The result provides thefirst documentation of UHT metamorphism and Palaeo- and Mesoproterozoicmetamorphic processes in the CITZ. On a broader scale, the findingsare also consistent with the current prediction that isobaricallycooled granulites require a separate orogeny for their exhumation. KEY WORDS: Central Indian Tectonic Zone; UHT metamorphism; counterclockwise P–T path; monazite chemical dating     

Temperature and Bulk Composition Control on the Growth of Monazite and Zircon During Low-pressure Anatexis (Mount Stafford, Central Australia)

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 U–Pb 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; U–Pb dating     

A Low-Variance Mineral Assemblage with Talc and Phengite in an Eclogite from the Saxonian Erzgebirge, Central Europe, and its P-T Evolution

A light-coloured, fine-grained eclogite sample from near thevillage of Hammerunterwiesenthal in the Erzgebirge (NW BohemianMassif) preserves the low-variance mineral assemblage of garnet,omphacite, phengite, talc, amphibole, clinozoisite, quartz,rutile, and accessory phases. Porphyroblasts of amphibole, clinozoisite,and phengite formed during a late stage (III) of metamorphism.Paragonite joined the assemblage late in this stage (IIIb).The chemical zonation of the minerals was carefully studied.Various geothermobarometric methods were applied, especiallyinvolving phengite and talc. The constrained P–T pathfor the eclogite starts at about 480°C and 25 kbar (stageIb), followed by a significant temperature rise (stage II) atslightly increasing pressure. At the peak P–T conditionsof 720°C and 27 kbar, blastesis of amphibole, clinozoisite,and phengite was caused by infiltrating hydrous fluids. Theresulting density reduction may have allowed buoyant upliftof the eclogite. Subsequently, significant cooling occurredat high pressures. Stage IIIb is characterized by P–Tconditions around 520°C and 18 kbar at reduced water activities.This unusual late P–T evolution might explain the freshnessof the eclogite, including the preservation of chemical zonationon the micrometre scale. KEY WORDS: eclogite; Saxonian Erzgebirge; P–T evolution; talc; phengite     

Origin of Exceptionally Abundant Phonolites on Ua Pou Island (Marquesas, French Polynesia): Partial Melting of Basanites Followed by Crustal Contamination

On the basis of the first systematic mapping of Ua Pou, longknown for its exceptionally abundant phonolites, we estimatethat these rocks cover 65% of the surface of the island whereasmafic lavas cover 27% and intermediate ones 8%. The silica-undersaturatedsuite was erupted in a restricted time span (2·9–2·35Myr), following the emplacement of tholeiites derived from ayoung HIMU-type source at c. 4 Ma. Primitive basanites, derivedfrom a heterogeneous mantle source with a dominant EM II + HIMUsignature, represent likely parental magmas. The series is characterizedby a Daly gap defined by a lack of phonotephrites. We considerthat the most likely model for the origin of evolved lavas ispartial melting at depth of primitive basanites, leaving anamphibole-rich residuum and producing tephriphonolitic magmas.These tephriphonolitic magmas may have evolved by closed-systemfractional crystallization towards Group A phonolites. Threeother groups of phonolites could have been derived from tephriphonoliticmagmas by open-system fractional crystallization processes,characterized respectively by seawater contamination (GroupB), assimilation of nepheline syenite-type materials (GroupC) and extreme fractionation coupled with assimilation of theunderlying oceanic crust (Group D). The prominence of evolvedlavas is a consequence of their origin from partial meltingof mafic precursors followed by crustal contamination. KEY WORDS: Marquesas; French Polynesia; phonolite; partial melting; contamination     

Partial Melting and Counterclockwise P T Path of Subducted Oceanic Crust (Sierra del Convento Melange, Cuba)

Partial melting of subducted oceanic crust has been identifiedin the Sierra del Convento mélange (Cuba). This serpentinite-matrixmélange contains blocks of mid-ocean ridge basalt (MORB)-derivedplagioclase-lacking epidote ± garnet amphibolite intimatelyassociated with peraluminous trondhjemitic–tonalitic rocks.Field relations, major element bulk-rock compositions, mineralassemblages, peak metamorphic conditions (c. 750°C, 14–16kbar), experimental evidence, and theoretical phase relationssupport formation of the trondhjemitic–tonalitic rocksby wet melting of subducted amphibolites. Phase relations andmass-balance calculations indicate eutectic- and peritectic-likemelting reactions characterized by large stoichiometric coefficientsof reactant plagioclase and suggest that this phase was completelyconsumed upon melting. The magmatic assemblages of the trondhjemitic–tonaliticmelts, consisting of plagioclase, quartz, epidote, ±paragonite, ± pargasite, and ± kyanite, crystallizedat depth (14–15 kbar). The peraluminous composition ofthe melts is consistent with experimental evidence, explainsthe presence of magmatic paragonite and (relict) kyanite, andplaces important constraints on the interpretation of slab-derivedmagmatic rocks. Calculated P–T conditions indicate counterclockwiseP–T paths during exhumation, when retrograde blueschist-faciesoverprints, composed of combinations of omphacite, glaucophane,actinolite, tremolite, paragonite, lawsonite, albite, (clino)zoisite,chlorite, pumpellyite and phengite, were formed in the amphibolitesand trondhjemites. Partial melting of subducted oceanic crustin eastern Cuba is unique in the Caribbean realm and has importantconsequences for the plate-tectonic interpretation of the region,as it supports a scenario of onset of subduction of a youngoceanic lithosphere during the early Cretaceous (c. 120 Ma).The counterclockwise P–T paths were caused by ensuingexhumation during continued subduction. KEY WORDS: amphibolite; Cuba; exhumation; partial melting; trondhjemite; subduction     

Discontinuous Melt Extraction and Weak Refertilization of Mantle Peridotites at the Vema Lithospheric Section (Mid-Atlantic Ridge)

Melting processes beneath the Mid-Atlantic Ridge were studiedin residual mantle peridotites sampled from a lithospheric sectionexposed near the Vema Fracture Zone at 11°N along the Mid-AtlanticRidge. Fractional and dynamic melting models were tested basedon clinopyroxene rare earth element and high field strengthelement data. Pure fractional melting (non-modal) cannot accountfor the observed trends, whereas dynamic melting with criticalmass porosity <0·01 fits better the measured values.Observed microtextures suggest weak refertilization with 0·1–1%quasi-instantaneous or partially aggregated melts trapped duringpercolation. The composition of the melts is evaluated, togetherwith their provenance, with respect to the garnet–spineltransition. Partial melts appear to be aggregated over shortbut variable intervals of the melting column. Deep melts (generatedwithin the garnet stability field at the base of the meltingcolumn) escape detection, being separated from the residuesby transport inside conduits or fractures. The temporal evolutionof the melting process along the exposed section shows a steadyincrease of mantle temperature from 20 Ma to present. KEY WORDS: mantle partial melting; abyssal peridotite; trace element; refertilization; Vema Fracture Zone     

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     

Petrogenesis of Ultramafic Rocks from the Ultrahigh-pressure Metamorphic Kimi Complex in Eastern Rhodope (NE Greece)

Widespread bodies of garnet–spinel metaperidotites withpyroxenitic layers occur in the ultrahigh-pressure metamorphicKimi Complex. In this study we address the origin of such peridotite–pyroxeniteassociations in the context of polybaric melting regimes. Weconduct a detailed geochemical investigation of major and traceelement relations and compare them with a range of major elementmodelling scenarios. With increasing bulk-rock MgO content,the garnet–spinel metaperidotites exhibit decreasing CaO,Al₂O₃, TiO₂, and Na₂O along with increasing Ni and a graduallyincreasing Zr/Zr* anomaly, consistent with an origin as residuesafter variable degrees of melt extraction. The major elementmodelling further suggests a polybaric adiabatic decompressionmelting regime beginning at high to ultrahigh pressure, withan intermediate character between pure batch and fractionalmelting and a mean extent of melting of 9–11%. The pyroxenitesexhibit major element compositions that cannot be reproducedby experimental or calculated melts of peridotite. Moreover,the Kimi pyroxenites have highly variable Ni and Sc contentsand a wide range of Mg-number (0· 76–0·89), inconsistent with an origin as frozen melts or the productsof melt–peridotite interaction. However, both the majorelement systematics and the observed rare earth element patterns,with both convex and concave shapes, can be explained by anorigin as clinopyroxene-rich, high-pressure cumulates involvinggarnet and/or Cr-spinel. KEY WORDS: peridotite; pyroxenite; partial melting; UHP metamorphism; cumulate     

Possible Non-melted Remnants of Subducted Lithosphere: Experimental and Geochemical Evidence from Corundum-Bearing Mafic Rocks in the Horoman Peridotite Complex, Japan

We report experimental results and whole-rock trace-elementcharacteristics of a corundum-bearing mafic rock from the Horomanperidotite complex, Japan. Coronitic textures around corundumin the sample suggest that corundum was not stable in maficrock compositions during the late-stage P–T conditionsrecorded in the complex (P < 1 GPa, T < 800°C). Basedon the experimental results, corundum is stable in aluminousmafic compositions at pressures of 2–3 GPa under dry conditions,suggesting that the corundum-bearing mineral assemblages developedunder upper-mantle conditions, probably within the surroundingperidotite. Variations in the trace-element compositions ofthe corundum-bearing mafic rock and related rocks can be controlledby modal variations of plagioclase, clinopyroxene and olivine,suggesting that they formed as gabbroic rocks at low-pressureconditions, and that the corundum-bearing mafic rock was derivedfrom a plagioclase-rich protolith. A complex P–T trajectory,involving metamorphism of the plagioclase-rich protolith ata pressure higher than that at which it was first formed, isneeded to explain the origin of the corundum-bearing mafic rocks.They show no evidence for partial melting after their formationas low-pressure cumulates. The Horoman complex is an exampleof a large peridotite body containing possible remnants of subductedoceanic lithosphere still retaining their original geochemicalsignatures without chemical modification during subduction andexhumation. KEY WORDS: Horoman; mafic rock; corundum; experiment; P–T history; recycling     

Provenance and Magmatic-Metamorphic Evolution of a Variscan Island-Arc Complex: Constraints from U-Pb Dating, Petrology, and Geospeedometry of the Kyffhauser Crystalline Complex, Central Germany

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-precisionU–Pb 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 P–T conditionsof 690–750°C and 5–7 kbar, corresponding toan intrusion depth of 19–25 km. At 337 ± 1 Ma themagmatic–metamorphic suite was intruded by granites, syenitesand diorites at a shallow crustal level of some 7–11 km.This is inferred from a diorite, and conforms to P–T pathsobtained from the metasediments, indicating a nearly isothermaldecompression from 5–7 to 2–4 kbar at 690–750°C.Subsequently, the metamorphic–magmatic sequence underwentaccelerated cooling to below 400°C, as constrained by garnetgeospeedometry and a previously published K–Ar muscoviteage of 333 ± 7 Ma. With respect to P–T–D–tdata from surrounding units, rapid exhumation of the KCC canbe interpreted to result from NW-directed crustal shorteningduring the Viséan. KEY WORDS: contact metamorphism; U–Pb dating; hornblende; garnet; Mid-German Crystalline Rise; P–T pseudosection     

Using In Situ Trace-Element Determinations to Monitor Partial-Melting Processes in Metabasites

Peak metamorphism (800–850°C, 8–10 kbar) inthe Harts Range Meta-Igneous Complex (Harts Range, central Australia)was associated with localized partial melting by the reactionhornblende + plagioclase + quartz + H₂O = garnet + clinopyroxene+ titanite + melt. In situ trace-element determinations of prograde,peak and retrograde minerals in migmatitic metabasites and associatedtonalitic melts using laser-ablation ICP–MS has allowedmonitoring of a range of partial-melting processes (melting,melt segregation and back-reaction between crystallizing meltand restitic minerals). Mass balance calculations indicate thattitanite is a major carrier of trace elements such as Ti, Nb,Ta, Sm, U and Th, and therefore may be an important accessoryphase to control the redistribution of these elements duringthe partial melting of amphibolites. Titanite preferentiallyincorporates Ta over Nb and, hence, residual titanite mightassist in the formation of melts with high Nb/Ta. The fact thatsingle minerals record different rare earth element (REE) patterns,from prograde to peak to retrograde conditions, demonstratesthat REE diffusion is not significant up to 800°C. Therefore,trace-element analysis in minerals can be a powerful tool toinvestigate high-grade metamorphic processes beyond the limitsgiven by major elements. KEY WORDS: Harts Range; laser-ablation ICP–MS; metabasites; partial melting; trace elements     

Role of Syn-eruptive Cooling and Degassing on Textures of Lavas from the AD 1783-1784 Laki Eruption, South Iceland

The Laki eruption involved 10 fissure-opening episodes thatproduced 15·1 km³ of homogeneous quartz-tholeiite magma.This study focuses on the texture and chemistry of samples fromthe first five episodes, the most productive period of the eruption.The samples comprise pumiceous tephra clasts from early falloutdeposits and lava surface samples from fire-fountaining andcone-building activity. The fluid lava core was periodicallyexposed at the surface upon lobe breakout, and its characteristicsare preserved in glassy selvages from the lava surface. In allsamples, plagioclase is the dominant mineral phase, followedby clinopyroxene and then olivine. Samples contain <7 vol.% of euhedral phenocrysts (>100 µm) with primitivecores [An* = 100 x Ca/(Ca + Na) >70; Fo > 75; En* = 100x Mg/(Mg + Fe) >78] and more evolved rims, and >10 vol.% of skeletal, densely distributed groundmass crystals (<100µm), which are similar in composition to phenocryst rims(tephra: An*_58–67, Fo_72–78, En*_72–81; lava:An*_49–70, Fo_63–78, En_57–78). Tephra and lavahave distinct vesicularity (tephra: >40 vol. %; lava: <40vol. %), groundmass crystal content (tephra: <10 vol. %;lava: 20–30 vol. %), and matrix glass composition (tephra:5·4–5·6 wt % MgO; lava: 4·3–5·0wt % MgO). Whole-rock and matrix glass compositions define atrend consistent with liquid evolution during in situ crystallizationof groundmass phases. Plagioclase–glass and olivine–glassthermometers place the formation of phenocryst cores at 10 kmdepth in a melt with 1 wt % H₂O, at near-liquidus temperatures(1150°C). Phenocryst rims and groundmass crystals formedclose to the surface, at 10–40°C melt undercoolingand in an 10–20°C cooler drier magma (0–0·1wt % H₂O), causing an 10 mol % drop in An content in plagioclase.The shape, internal zoning and number density of groundmasscrystals indicate that they formed under supersaturated conditions.Based on this information, we propose that degassing duringascent had a major role in rapidly undercooling the melt, promptingintensive shallow groundmass crystallization that affected themagma and lava rheology. Petrological and textural differencesbetween tephra and lava reflect variations in the rates of magmaascent and the timing of surface quenching during each eruptiveepisode. That in turn affected the time available for crystallizationand subsequent re-equilibration of the melt to surface (degassed)conditions. During the explosive phases, the rates of magmaascent were high enough to inhibit crystallization, yieldingcrystal-poor tephra. In contrast, pervasive groundmass crystallizationoccurred in the lava, increasing its yield strength and causinga thick rubbly layer to form during flow emplacement. Lava selvagescollected across the flow-field have strikingly homogeneousglass compositions, demonstrating the high thermal efficiencyof fluid lava transport. Cooling is estimated as 0·3°C/km,showing that rubbly surfaced flows can be as thermally efficientas tube-fed phoehoe lavas. KEY WORDS: lava; crystallization; basalt; cooling rate; pressure; geobarometry; P–T conditions; plagioclase; degassing; Laki, Iceland     

Immiscible Transition from Carbonate-rich to Silicate-rich Melts in the 3 GPa Melting Interval of Eclogite + CO2 and Genesis of Silica-undersaturated Ocean Island Lavas

We explore the partial melting behavior of a carbonated silica-deficienteclogite (SLEC1; 5 wt % CO₂) from experiments at 3 GPa and comparethe compositions of partial melts with those of alkalic andhighly alkalic oceanic island basalts (OIBs). The solidus islocated at 1050–1075 °C and the liquidus at 1415 °C.The sub-solidus assemblage consists of clinopyroxene, garnet,ilmenite, and calcio-dolomitic solid solution and the near solidusmelt is carbonatitic (<2 wt % SiO₂, <1 wt % Al₂O₃, and<0·1 wt % TiO₂). Beginning at 1225 °C, a stronglysilica-undersaturated silicate melt (34–43 wt % SiO₂)with high TiO₂ (up to 19 wt %) coexists with carbonate-richmelt (<5 wt % SiO₂). The first appearance of carbonated silicatemelt is 100 °C cooler than the expected solidus of CO₂-freeeclogite. In contrast to the continuous transition from carbonateto silicate melts observed experimentally in peridotite + CO₂systems, carbonate and silicate melt coexist over a wide temperatureinterval for partial melting of SLEC1 carbonated eclogite at3 GPa. Silicate melts generated from SLEC1, especially at highmelt fraction (>20 wt %), may be plausible sources or contributingcomponents to melilitites and melilititic nephelinites fromoceanic provinces, as they have strong compositional similaritiesincluding their SiO₂, FeO^*, MgO, CaO, TiO₂ and Na₂O contents,and CaO/Al₂O₃ ratios. Carbonated silicate partial melts fromeclogite may also contribute to less extreme alkalic OIB, asthese lavas have a number of compositional attributes, suchas high TiO₂ and FeO^* and low Al₂O₃, that have not been observedfrom partial melting of peridotite ± CO₂. In upwellingmantle, formation of carbonatite and silicate melts from eclogiteand peridotite source lithologies occurs over a wide range ofdepths, producing significant opportunities for metasomatictransfer and implantation of melts. KEY WORDS: carbonated eclogite; experimental phase equilibria; partial melting; liquid immiscibility; ocean island basalts