Accessory phase controls on the geochemistry of crustal melts and restites produced during water-undersaturated partial melting

The profound geochemical conseqences of accessory phase behaviour during partial melting of highgrade metapelites are demonstrated with reference to two geochemically distinct crustal melts produced by biotite dehydration melting reactions under granulite facies (kbar, 860°CC) conditions. These two leucogneiss suites, from the Brattstrand Bluffs coastline, eastern Antarctica, have similar field relations, transport distances (10–100 s of metres) and major element chemistry. Type 1 leucogneisses have low Zr, Th and LREE, positive Eu anomalies and Zr/Zr^* and LREE_t/LREE_t ^* values less than 1.0 (i.e. less than required to saturate the melt). Mass balance constraints suggest that these melts have been extracted before equilibration with host melanosomes. The dry, peraluminous nature of vapour-undersaturated melts inhibits monazite and zircon solubility and results in concentration of these phases in the residue. Melts are consequently depleted in LREE and HREE. Melanosomes show complementary enrichment in LREE, while HREE patterns are dominated by residual garnet. Type 2 leucogneisses, in contrast, have strongly enriched Zr, Th and LREE abundances, negative Eu anomalies and Zr/Zr^* and LREE_t/LREE_t ^*>1 resulting from accessory phase entrainment. Vapour-absent partial melting under moderate (6–8 kbar) pressure granulite-facies conditions of a pelitic source containing monazite is likely to give disequilibrium melts depleted in LREE and HREE as monazite and garnet are concentrated in the residue. If temperatures are high enough (850–870° C) to permit relatively large degrees of partial melting then the feldspar component of the source will be removed almost completely, giving melts with large positive Eu anomalies. Melts formed under vapour-present conditions are unlikely to show such extreme LREE and HREE depletion or positive Eu anomalies, even at high degrees of partial melting. Disequilibrium melting coupled with source entrainment could fortuitously produce REE and trace element signatures similar to those typical of S-type granites and usually ascribed to equilibrium melting conditions.     

Geology and rare-earth element geochemistry of highly evolved, molybdenite-bearing granitic plutons, Southeastern Desert, Egypt

The field relations, mineralogy, and major and trace elements (including REE analyses of whole-rock samples and minerals) of granites and their associated molybdenite uranium mineralized aplites in Southeastern Desert, Egypt, have been studied. The granites are leucocratic and mostly peraluminous in nature with muscovite increasing at the expense of biotite. The chemical and mineralogical characteristics of the granitic rocks indicate that their melts originated from the LILE-enriched mantle wedge by partial melting and are contaminated by crustal melts, followed by thermogravitational processes. Leucogranites with higher Na2O/K2O ratios from Um Dargag and Um Maiat crystallized under H2O-saturated equilibrium conditions in which the exsolved vapor continuously migrated away. The REE patterns of the granites studied are characterized by LREE enrichments and negative Eu anomalies. In comparison, the potassic aplites and the more sodic leucogranites are depleted in LREE, enriched in HREE and show more remarkable negative Eu anomalies. Allanite and monazite are the most important REE carriers in the granites. These minerals are strongly enriched in LREE, whereas fluorite and xenotime, which are more abundant in the aplites, are enriched in HREE. The average Lu/Ce ratio represents the fractionation trend with respect to HREE. It is 0.71 for radioactive fluorite, and it increases to 1.22 for non-radioactive fluorite. The high REE contents of molybdenite represent re-deposition of the mobilized Mo and REE. Due to the strong control of accessory minerals, the REEs are of limited use in petrogenetic modelling of highly evolved granitic systems.     

Behaviour of rare-earth elements in highly evolved granitic systems: Evidence from Proterozoic molybdenite mineralized aplites and associated granites in northern Sweden

Field relations, mineralogy, major- and trace-element contents (including REE analyses of whole-rock samples and minerals) of three Proterozoic granites and their associated molybdenite mineralized aplites have been studied at Allebuoda, Munka and Kåtaberget in northern Sweden. The granites crystallized from melts that were not saturated with water. The mineralized potassic aplites formed by quenching of residual melts caused by sudden pressure drop, H₂O saturation and vapour escape during tectonic rupturing. Leucogranites with higher Na₂O/K₂O ratios from Allebuoda and Munka crystallized during H₂O-saturated equilibrium conditions in which the exsolved vapour could continuously migrate away. The pressure was probably 3 kbar at Munka, and somewhat lower at Allebuoda.

The granites have REE patterns characterized by LREE enrichments and negative Eu anomalies. In comparison, the potassic aplites and the more sodic leucogranites are depleted in LREE, enriched in HREE and have larger negative Eu anomalies. Allanite and monazite are the most important REE carriers in the granites. These minerals are strongly enriched in LREE, whereas fluorite and xenotime, which are more abundant in the aplites, are most enriched in HREE. Due to the strong control of accessory minerals on the REE balance, REE are of limited use in petrogenetic modelling of highly evolved granitic systems.     

北京云蒙山浅色花岗岩脉及韧性剪切变形的地球化学特征

对云蒙山地区的花岗岩、糜棱岩化花岗岩及周围太古宙花岗片麻岩中的浅色花岗岩脉的主要及稀土元素地球化学研究结果表明：与未糜棱岩化花岗岩相比，浅色花岗岩脉具有较低的LREE和P2O5含量及（La／Gd）N比值，Sm／Nd比值较高，而剪切应变岩石的LREE相对富集；糜棱岩化花岗岩具有近平行于未糜棱岩化花岗岩的稀土元素配分模式；浅色花岗岩脉BH-2-5和BH-2-6具有和未糜棱岩化花岗岩相似的重稀土元素配分模式；浅色花岗岩脉BH-2-3的稀土元素配分模式和所有分析的样品都不一样，推断BH-2-3有可能是古老变质岩部分熔融的产物。在部分熔融过程中，诸如独居石这样的富含轻稀土元素的副矿物以残留体的形式出现，不参与部分熔融，导致BH-2-3具有很低的LREE和P2O5含量及高达0．4122的Sm／Nd比值。而其他两条浅色花岗岩脉有可能是云蒙山花岗岩后期岩浆分异的产物，随分异程度的增强和富含轻稀土元素的副矿物的分离结晶作用，导致最后岩浆的SiO2增高、LREE含量减少及Sm／Nd比值发生变化。     

Residence of REE, Y, Th and U in Granites and Crustal Protoliths; Implications for the Chemistry of Crustal Melts

A systematic study with laser ablation—ICP-MS, scanningelectron microscopy and electron microprobe revealed that 70–95wt% of REE (except Eu), Y, Th and U in granite rocks and crustalprotoliths reside within REEYThU-rich accessories whose nature,composition and associations change with the rock aluminosity.The accessory assemblage of peraluminous granites, migmatitesand high-grade rocks is composed of monazite, xenotime (in low-Cavarieties), apatite, zircon, Thorthosilicate, uraninite andbetafite-pyrochlore. Metaluminous granites have allanite, sphene,apatite, zircon, monazite and Thorthosilicaie. Peralkaline graniteshave aeschinite, fergusonite, samarskite, bastnaesite, fluocerite,allanite, sphene, zircon, monazite, xenotime and Th-orthosilicate.Granulite-grade garnets are enriched in Nd and Sm by no lessthan one order of magnitude with respect to amphibolite-gradegarnets. Granulitegrade feldspars are also enriched in LREEwith respect to amphibolite-grade feldspars. Accessories causenon-Henrian behaviour of REE, Y, Th and U during melt—solidpartitioning. Because elevated fractions of monazite, xenotimeand zircon in common migmatites are included within major minerals,their behaviour during anatexis is controlled by that of theirhost. Settling curves calculated for a convecting magma showthat accessories are too small to settle appreciably, beingseparated from the melt as inclusions within larger minerals.Biotite has the greatest tendency to include accessories, therebyindirectly controlling the geochemistry of REE, Y, Th and U.We conclude that REE, Y, Th and U are unsuitable for petrogeneticalmodelling of granitoids through equilibrium-based trace-elementfractionation equations. KEY WORDS: accessory minerals; geochemical modelling; granitoids; REE, Y, Th, U     

The trace element compositions of S-type granites: evidence for disequilibrium melting and accessory phase entrainment in the source

Within individual plutons, the trace element concentrations in S-type granites generally increase with maficity (total iron and magnesium content and expressed as atomic Fe + Mg in this study); the degree of variability in trace element concentration also expands markedly with the same parameter. The strongly peraluminous, high-level S-type granites of the Peninsular Pluton (Cape Granite Suite, South Africa) are the product of biotite incongruent melting of a metasedimentary source near the base of the crust. Leucogranites within the suite represent close to pure melts from the anatectic source and more mafic varieties represent mixtures of melt and peritectic garnet and ilmenite. Trace elements such as Rb, Ba, Sr and Eu, that are concentrated in reactant minerals in the melting process, show considerable scatter within the granites. This is interpreted to reflect compositional variation in the source. In contrast, elements such as LREE, Zr and Hf, which are concentrated within refractory accessory phases (zircon and monazite), show well-defined negative correlations with increasing SiO₂ and increase linearly with increasing maficity. This is interpreted to reflect coupled co-entrainment of accessory minerals and peritectic phases to the melt: leucocratic rocks cannot have evolved from the more mafic compositions in the suite by a process of fractional crystallisation because in this case they would have inherited the zircon-saturated character of this hypothetical earlier magma. Trace element behaviour of granites from the Peninsular Pluton has been modelled via both equilibrium and disequilibrium trace element melting. In the disequilibrium case, melts are modelled as leaving the source with variable proportions of entrained peritectic phases and accessory minerals, but before the melt has dissolved any accessory minerals. Thus, the trace element signature of the melt is largely inherited from the reactants in the melting reaction, with no contribution from zircon and monazite dissolution. In the equilibrium case, melt leaves the source with entrained crystals, after reaching zircon and monazite saturation. A significant proportion of the rocks of the Peninsular Pluton have trace element concentrations below those predicted by zircon and monazite saturation. In the case of the most leucocratic rocks all compositions are zircon undersaturated; whilst the majority of the most mafic compositions are zircon oversaturated. However, in both cases, zircon is commonly xenocrystic. Thus, the leucocratic rocks represent close to pure melts, which escaped their sources rapidly enough that some very closely match the trace element disequilibrium melting model applied in this study. Zircon dissolution rates allow the residency time for the melt in the source to be conservatively estimated at less than 500 years.     

Sources of stream sediments in the granulite terrain of the Walawe Ganga Basin,Sri Lanka,indicated by rare earth element geochemistry

Thirty-eight samples of stream sediments draining high-grade metamorphic rocks in the Walawe Ganga (river) Basin, Sri Lanka, were analysed for their REE contents, together with samples of metamorphic suites from the source region. The metamorphic rocks are enriched in light REE (LREE) compared to heavy REE (HREE) and are characterised by high La/Lu ratios and negative Eu anomalies. The chondrite-normalised patterns for these granulite-grade rocks are similar to that of the average post-Archaean upper crust, but they are slightly enriched with La and Ce. The REE contents of the <63-μm fraction of the stream sediments are similar to the probable source rocks, but the other grain size fractions show more enriched patterns. The <63-μm stream sediments fraction contains lower total REE, more pronouncd negative Eu anomalies, higher Eu_N/Sm_N and lower La _N/Lu_N ratios relative to other fractions. The lower La _N/Lu_N ratio is related to the depletion of heavy minerals in the <63-μm fraction. The 63–125-μm and 125–177-μm grain size fractions of sediments are particularly enriched in LREE (average ΣLREE=2990 μg/g and 3410 μg/g, respectively). The total HREE contents are surprisingly uniform in all size fractions. However, the REE contents in the Walawe Ganga sediments are not comparable with those of the granulite-grade rocks from the source region of the sediments. The enrichment of REE is accounted for by the presence of REE containing accessory mineral phases such as zircon, monazite, apatite and garnet. These minerals are derived from an unknown source, presumably from scattered bodies of granitic pegmatites.     

Tracing Lithosphere Evolution through the Analysis of Heterogeneous G9-G10 Garnets in Peridotite Xenoliths, II: REE Chemistry

Following previous publication of major–minor elementdata, this paper presents rare earth element (REE) data forheterogeneous (chemically zoned) garnets belonging to the peridotitesuite of mantle xenoliths from the Jagersfontein kimberlitepipe, South Africa. The rim compositions of the garnets in thehighest temperature–pressure (deepest) deformed peridotitesshow a typical megacryst-like pattern, of very low light REE(LREE) increasing through the middle REE (MREE) to a plateauof heavy REE (HREE) at c. 20 times chondrite; these compositionswould be in equilibrium with small-volume melts of the mid-oceanridge basalt (MORB) source (asthenosphere). With decreasingdepth the garnet rims show increasing LREE and decreasing HREE,eventually resulting in humped relative abundance patterns.A set of compositions is calculated for melts that would bein equilibrium with the garnet rims at different depths. Theseshow decreasing relative abundance of each REE from La to Lu,and the La/Lu ratio of the melts increases with decreasing depthof formation. Modelling of the effects of crystal fractionationshows that this process could largely generate the sequenceof garnet rim and melt compositions found with decreasing depth,including the humped REE patterns in high-level garnets. Consideringthe behaviour of major–minor elements as well as REE,a process of percolative fractional crystallization is advocatedin which megacryst source melts percolate upwards through peridotitesand undergo fractionation in conjunction with exchange withthe peridotite minerals. The initial megacryst melt probablyincludes melt of lithospheric origin as well as melt from theMORB source, and it is suggested that the process of percolativefractional crystallization may form a variety of metasomaticand kimberlitic melts from initial megacryst melts. Repeatedmetasomatism of the lower lithosphere by such differentiatingmelts is suggested by consideration of garnet core compositions.Such metasomatism would progressively convert harzburgites tolherzolites by increasing their CaO content, and this may accountfor the fact that the Cr-rich diamond–garnet harzburgiteparagenesis is commonly preserved only where it has been encapsulatedin diamonds. KEY WORDS: cratonic lithosphere; garnet zoning; mantle xenoliths; megacryst magma; metasomatic melt     

REE fractionation,mineral speciation,and supergene enrichment of the Bear Lodge carbonatites,Wyoming, USA

The Eocene (ca. 55–38 Ma) Bear Lodge alkaline complex in the northern Black Hills region of northeastern Wyoming (USA) is host to stockwork-style carbonatite dikes and veins with high concentrations of rare earth elements (e.g., La: 4140–21000 ppm, Ce: 9220–35800 ppm, Nd: 4800–13900 ppm). The central carbonatite dike swarm is characterized by zones of variable REE content, with peripheral zones enriched in HREE including yttrium. The principle REE-bearing phases in unoxidized carbonatite are ancylite and carbocernaite, with subordinate monazite, fluorapatite, burbankite, and Ca-REE fluorocarbonates. In oxidized carbonatite, REE are hosted primarily by Ca-REE fluorocarbonates (bastnäsite, parisite, synchysite, and mixed varieties), with lesser REE phosphates (rhabdophane and monazite), fluorapatite, and cerianite. REE abundances were substantially upgraded (e.g., La: 54500–66800 ppm, Ce: 11500–92100 ppm, Nd: 4740–31200 ppm) in carbonatite that was altered by oxidizing hydrothermal and supergene processes. Vertical, near surface increases in REE concentrations correlate with replacement of REE(±Sr,Ca,Na,Ba) carbonate minerals by Ca-REE fluorocarbonate minerals, dissolution of matrix calcite, development of Fe- and Mn-rich gossan, crystallization of cerianite and accompanying negative Ce anomalies in secondary fluorocarbonates and phosphates, and increasing δ¹⁸O values. These vertical changes demonstrate the importance of oxidizing meteoric water during the most recent modifications to the carbonatite stockwork. Scanning electron microscopy, energy dispersive spectroscopy, and electron probe microanalysis were used to investigate variations in mineral chemistry controlling the lateral complex-wide geochemical heterogeneity. HREE-enrichment in some peripheral zones can be attributed to an increase in the abundance of secondary REE phosphates (rhabdophane group, monazite, and fluorapatite), while HREE-enrichment in other zones is a result of HREE substitution in the otherwise LREE-selective fluorocarbonate minerals. Microprobe analyses show that HREE substitution is most pronounced in Ca-rich fluorocarbonates (parisite, synchysite, and mixed syntaxial varieties). Peripheral, late-stage HREE-enrichment is attributed to: 1) fractionation during early crystallization of LREE selective minerals, such as ancylite, carbocernaite, and Ca-REE fluorocarbonates in the central Bull Hill dike swarm, 2) REE liberated during breakdown of primary calcite and apatite with higher HREE/LREE ratios, and 3) differential transport of REE in fluids with higher PO₄³⁻/CO₃²⁻ and F⁻/CO₃²⁻ ratios, leading to phosphate and pseudomorphic fluorocarbonate mineralization. Supergene weathering processes were important at the stratigraphically highest peripheral REE occurrence, which consists of fine, acicular monazite, jarosite, rutile/pseudorutile, barite, and plumbopyrochlore, an assemblage mineralogically similar to carbonatite laterites in tropical regions.     

High-Grade Fluid Metasomatism on both a Local and a Regional Scale: the Seward Peninsula, Alaska, and the Val Strona di Omegna, Ivrea-Verbano Zone, Northern Italy. Part II: Phosphate Mineral Chemistry

This study explores the origin and geochemical evolution ofapatite, monazite, and xenotime along two metamorphic traverses.The first, from the Kigluaik Mountains, Seward Peninsula, Alaska,consists of a localized (85 cm) orthopyroxene–clinopyroxene-bearingdehydration zone. The second consists of orthopyroxene ±clinopyroxene-bearing granulite facies metabasite layers interlayeredwith metapelites over a 3–4 km traverse, along the ValStrona, Ivrea–Verbano Zone, Northern Italy (IVZ). In bothdehydration zones small Th- and U-poor inclusions of monaziteand/or xenotime occur in the apatite. These inclusions are metasomaticallyinduced and nucleated within the apatite via the coupled substitutionsNa⁺ + (Y + REE)³⁺ = 2 Ca²⁺ and Si⁴⁺ + (Y + REE)³⁺ = P⁵⁺ + Ca²⁺.These are not present in apatite from the original amphibolitefacies gneiss. Apatite, in both dehydration zones, also showsa relative increase in both F and Cl compared with apatite fromthe amphibolite facies zone. Granulite facies metabasites inthe IVZ also contain isolated monazite grains, which range fromuniform to complexly zoned in Th the (13–30·1 mol% ThSiO₄). These are the product of breakdown and subsequentmobilization of the lanthanides and actinides from monazite-(Ce)in the metapelite layers into the metabasite layers at the startof granulite facies metamorphism. KEY WORDS: apatite; monazite; xenotime; KCl–NaCl brines; metasomatism; phosphate minerals; charnockite–enderbite; granulite facies metamorphism     

Behavior of accessory phases and redistribution of Zr,REE, Y,Th, and U during metamorphism and partial melting of metapelites in the lower crust: an example from the Kinzigite Formation of Ivrea-Verbano,NW Italy

This study is aimed at understanding the behavior of monazite, xenotime, apatite and zircon, and the redistribution of Zr, REE, Y, Th, and U among melt, rock-forming and accessory phases in a prograde metamorphic sequence, the Kinzigite Formation of Ivrea-Verbano, NW Italy, that may represent a section from the middle to lower continental crust. Metamorphism ranges from middle amphibolite to granulite facies and metapelites show evidence of intense partial melting and melt extraction. The appearance of melt controls the grain size, fraction of inclusions and redistribution of REE, Y, Th, and U among accessories and major minerals. The textural evolution of zircon and monazite follows, in general, the model of Watson et al. (1989). Apatite is extracted from the system dissolved into partial melts. Xenotime is consumed in garnet-forming reactions and is the first source for the elevated Y and HREE contents of garnet. Once xenotime is exhausted, monazite, apatite, zircon, K-feldspar, and plagioclase are progressively depleted in Y, HREE, and MREE as the modal abundance of garnet increases. Monazite is severely affected by two retrograde reactions, which may have consequences for U-Pb dating of this mineral. Granulite-grade metapelites (stronalites) are significantly richer in Ti, Al, Fe, Mg, Sc, V, Cr, Zn, Y, and HREE, and poorer in Li, Na, K, Rb, Cs, Tl, U, and P, but have roughly the same average concentration of Cu, Sr, Pb, Zr, Ba, LREE, and Th as amphibolite-grade metapelites (kinzigites). The kinzigite-stronalite transition is marked by the sudden change of Th/U from 5–6 to 14–15, the progressive increase of Nb/Ta, and the decoupling of Ho from Y. Leucosomes were saturated in zircon, apatite, and (except at the lowest degree of partial melting) monazite. Their REE patterns, especially the magnitude of the Eu anomaly, depend on the relative proportion of feldspars and monazite incorporated into the melt. The presence of monazite in the source causes an excellent correlation of LREE and Th, with nearly constant Nd/Th ≈ 2.5–3. The U depletion and increase in Th/U characteristic of granulite facies only happens in monazite-bearing rocks. It is attributed to enhancement of the U partitioning in the melt due to elevated Cl activity followed by the release of a Cl-rich F-poor aqueous fluid at the end of the crystallization of leucosomes. Halide activity in partial melts was buffered by monazite and apatite. Since the U (and K) depletion does not substantially affect the heat-production of metapelites, and mafic granulites maintain similar Th/U and abundance of U and Th as their unmetamorphosed equivalents, it seems that geochemical changes associated to granulitization have only a minor influence on heat-production in the lower crust.     

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     

云南双沟变质橄榄岩中蛇纹石的稀土元素空间分布

本文探讨变质橄榄岩的ＬＲＥＥ富集型及Ｕ型分布的ＲＥＥ球粒陨石模型，是橄榄石固有的还是后期蚀变形成的。并寻求复原蚀变前橄榄石的ＲＥＥ丰度及型式。我们用蛇纹石颗粒代表蚀变后的橄榄石，用分选法和化学分层剥离法对样品进行予处理。用中子活化分析（ＮＡＡ）手段进行了测试。结果表明：分选法确定了ＬＲＥＥ的主要赋存载体是蛇纹石。分层剥离最后内核的ＲＥＥ丰度及模型，可近似地作为原来（未蚀变前）橄榄石的ＲＥＥ丰度及模型。     

Clinopyroxene REE Geochemistry of the Red Hills Peridotite, New Zealand: Interpretation of Magmatic Processes in the Upper Mantle and in the Moho Transition Zone

The Red Hills peridotite in the Dun Mountain ophiolite of SouthIsland, New Zealand, is assumed to have been produced in a paleo-mid-oceanridge tectonic setting. The peridotite is composed mostly ofharzburgite and dunite, which represent residual mantle andthe Moho transition zone (MTZ), respectively. Dunite channelswithin harzburgite blocks of various scales represent the MTZcomponent. Plagioclase- and clinopyroxene-bearing dunites occursporadically within common dunites. These dunites representproducts of melt–wall-rock interaction. Chondrite-normalizedrare earth element (REE) patterns of MTZ clinopyroxenes showa wide compositional range. Clinopyroxenes in plagioclase dunitesare extremely depleted in light REE (LREE) ([Lu/La]_N >100),and are comparable with clinopyroxenes in abyssal peridotitesfrom normal mid-ocean ridges. Interstitial clinopyroxenes inthe common dunite have flatter patterns ([Lu/La]_N 2) comparablewith those for dunite in the Oman ophiolite. Clinopyroxenesin the lower part of the residual mantle harzburgites are evenmore strongly depleted in LREE ([Lu/La]_N = 100–1000) thanare mid-ocean ridge peridotites, and rival the most depletedabyssal clinopyroxenes reported from the Bouvet hotspot. Incontrast, those in the uppermost residual mantle harzburgiteand harzburgite blocks in the MTZ are less LREE depleted ([Lu/La]_N= 10–100), and are similar to those in plagioclase dunite.Clinopyroxenes in the clinopyroxene dunite in the MTZ are similarto those reported from mid-ocean ridge basalt (MORB) cumulates,and clinopyroxenes in the gabbroic rocks have compositions similarto those reported from MORB. Strong LREE and middle REE (MREE)depletion in clinopyroxenes in the harzburgite suggests thatthe harzburgites are residues of two-stage fractional melting,which operated initially in the garnet field, and subsequentlycontinued in the spinel lherzolite field. The early stage meltingproduced the depleted harzburgite. The later stage melting wasresponsible for the gabbroic rocks and dunite. Strongly LREE–MREE-depletedclinopyroxene in the lower harzburgite and HREE-enriched clinopyroxenein the upper harzburgite and plagioclase dunite were formedby later reactive melt migration occurring in the harzburgite. KEY WORDS: clinopyroxene REE geochemistry; Dun Mountain ophiolite; Moho transition zone; orogenic peridotite; Red Hills     

Petrogenesis of Peraluminous Granites, Monashee Mountains, Southeastern Canadian Cordillera

Two-mica granites that locally contain garnet and sillimaniteoccur as dikes, sills, and sheets up to 50 m thick within thesillimanite zonc of the Monashee Mountains in the southeasternCanadian Cordillera of British Columbia. Syn-kinematic and post-kinematicgranites are recognized. U-Pb dating of zircon demonstrates that the syn-kinematic granitesare 100.4?0.3 Ma old, based on duplicate concordant single abradedzircon analyses. Other zircons have slightly older Pb/Pb dates,indicating small amounts of inherited zircons. Monazites are99?10 Ma old. Post-kinematic granites have 62.5?0.2 Ma zirconages and 634+0.1 Ma monazite ages. High initial 87 ratios (0.71492–0.74181)and evidence of Precambrian Pb inheritance indicates that bothsyn- and post-kinematic granites were derived from a crustalsource. Geobarometric estimates suggest that both generationsof granites equilibrated at 6–8 kb (22–30 km). Zirconand monazite saturation temperatures range from 660–824?Cand indicate that these minerals were liquidus phases earlyin the crystallization history of the granites. Because monazitesaturation temperatures generally exceed those of zircon, itis possible that some monazites may be inherited. Apatite saturationtemperatures in excess of 900?C suggest that both generationsof granites contain source inherited apatite. Syn- and post-kinematic granites have essentially identicalmajor and trace element chemistries. Syn-kinematic graniteshave steep light rare earth element (LREE) enriched patternswith pronounced negative Eu anomalies. The REE patterns of post-kinematicgranites range from steep LREE enriched patterns with negativeNd and Eu anomalies to flat patterns with low LREE contents,negative Nd anomalies, and both positive and negative Eu anomalies.Modelling of REE, Rb, Sr, and Ba contents demonstrates thatsyn-kinematic gramtes could have been generated by a low degreeof partial melting (with 10–25% feldspar fractionationof the melt) of Late Proterozoic Horsethief Creek Group metapelitesleaving a monazite-bearing upper amphibolite facies residue.Post-kinematic granites were produced by partial melting ofa geochemically and isotopically similar metapelitic source.The suite of post-kinematic granites can be related by a smallamount (up to 0.1%) of monazite crystal fractionation.     

Crustal Trace Element and Isotopic Signatures in Garnet Pyroxenites from Garnet Peridotite Massifs from Lower Austria

Gamet-bearing high-temperature peridotite massifs in lower Austriawere exhumed during Carboniferous plate convergence in the Bohemianmassif. The peridotite massifs contain garnet pyroxenite layers,most of which are high-pressure cumulates that crystallizedin the deep lithosphere during ascent and cooling of hot asthenosphericmelts. Many of the pyroxenites have negative Eu anomalies andhigh LREE abundances in pyroxenes and bulk rocks, ⁸⁷Sr/⁸⁶Sr(335 Ma) as high as 0.7089, and Nd (335 Ma) as low as –4.8(leached clinopyroxenes and garnets). These pyroxenites alsoshow strong depletions in Rb, K, Ta, P and Ti compared withthe REE Equilibrium melt compositions calculated from the cumulatecompositions have very high LREE abundances (La_n = 300–600)and show strong LREEfractionation [(La/Sm)_n = 7–47)].Trace element abundances, the Ca–Al-rich composition ofthe cumulates and possible Ti saturation in the melts suggestthat these melts were of primitive carbonatitic–meliliticor lamprophyrt-like composition. Other garnet pyroxenites suchas Al-rich garnet-kyanite clinopyroxemtes with positive Eu anomaliesprobably represent metamorphosed crustal rocks which were subductedand accreted to the lithospheric mantle. The high ⁸⁷Sr/⁸⁶Sr,low Nd (335 Ma) and negative Eu anomalies of the high-pressurecumulates can be explained if their equilibrium melts containeda component derived from subducted upper-crustal rocks. Thehigh equilibration pressures of the host peridotites (3–3.5GPa) and the high equilibration temperatures of the pyroxenites(1100–1400C) indicate that these melts are likely tobe derived from the sub-lithospheric mantle. There, meltingmay have been triggered by small amounts of melt or fluids derivedfrom a subducting slab at greater depth. KEY WORDS: garnet pyroxenites; geochemistry; lower Austria; ultramafic massifs; subduction     

Residence and redistribution of REE, Y, Zr, Th and U during granulite-facies metamorphism: behaviour of accessory and major phases in peraluminous granulites of central Spain

Accessory minerals are thought to play a key role in controlling the behaviour of certain trace elements such as REE, Y, Zr, Th and U during crustal melting processes under high-grade metamorphic conditions. Although this is probably the case at middle crustal levels, when a comparison is made with granulite-facies lower crustal levels, differences are seen in trace element behaviour between accessory minerals and some major phases. Such a comparison can be made in Central Spain where two granulite-facies terranes have equilibrated under slightly different metamorphic conditions and where lower crustal xenoliths are also found. Differences in texture and chemical composition between accessory phases found in leucosomes and leucogranites and those of melanosomes and protholiths indicate that most of the accessory minerals in melt-rich migmatites are newly crystallized. This implies that an important redistribution of trace elements occurs during the early stages of granulite-facies metamorphism. In addition, the textural position of the accessory minerals with respect to the major phases is crucial in the redistribution of trace elements when melting proceeds via biotite dehydration melting reactions. In granulitic xenoliths from lower crustal levels, the situation seems to be different, as major minerals show high concentration of certain trace elements, the distribution of which is thus controlled by reactions involving final consumption of Al-Ti-phlogopite. A marked redistribution of HREE–Y–Zr between garnet and xenotime (where present) and zircon, but also of LREE between feldspars (K-feldspar and plagioclase) and monazite, is suggested.     

Petrogenetic significance of rare-earth element behavior in the basement rocks of southern Obudu Plateau, Bamenda Massif, southeastern Nigeria

Rock samples representing various igneous and metamorphic rocks of southern Obudu Plateau were analyzed for rare-earth element ( REE ) behavior by ICP-MS. Results of the analyses indicate a range of REE abundances and distinctive patterns from highly fraetionated patterns with negative Eu anomalies in granitic rocks to relatively low abundances and less REE fractionated flat patterns with little Eu anomaly in some paragneisses, schists, enderbites and dolerites to unfractionated patterns with positive Eu anomalies in some paragneisses and charnockites. Over all, there are low to high ∑ REE contents with negative to positive Eu anomalies. The ratios of different parameters, especially La/Yb and Ce/Yb, show behaviors consistent with crustal to mantle derivation. The heterogeneity of REE abundances and REE patterns reflects mantle to crustal petrogenetic variations of different rock suites on the Plateau. The LREE content is higher than the HREE content in the highly differentiated rocks, as evidenced by their La/Yb,Ce/Yb and La/Sm ratios, which are normally higher in residual products than in primary melts. The dominantly intermediate nature of the source rock of the orthogneisses is suggested by the generally low ∑ REE. The granites enriched in LREE and depleted in HREE and some of the charnockites with negative Eu anomalies were probably formed by partial melting and crystallization.     

Trace-element distributions in silicates during prograde metamorphic reactions: implications for monazite formation

To assess the petrogenetic relationship between monazite and major silicates during prograde metamorphism, REE were measured across coexisting zoned silicates in garnet through kyanite‐grade pelitic schists from the Great Smoky Mountains, western Blue Ridge terrane, southern Appalachians, to establish REE concentrations and distributions before and after the monazite‐in isograd, and to identify the role major silicates play in the formation of monazite. Results indicate significant scavenging of light rare‐earth elements (LREE) from silicates during the monazite‐in isograd reaction; however, the absolute concentration of LREE hosted in the silicates was insufficient to produce monazite in the quantity observed in these schists. Monazite must have formed mainly from either the dissolution of allanite or some other source of concentrated LREE (possibly adsorbed onto grain boundaries), even though direct evidence for allanite is lacking in a majority of the samples. Laser‐ablation ICP‐MS analyses and theoretical thermodynamic calculations show that monazite may have formed as a result of contributions from both allanite and major silicates. Allanite breakdown initially formed monazite, and monazite production drew LREE liberated from allanite, major silicates and possibly from crystal boundaries. In many rocks the reaction was further promoted by the staurolite‐in reaction, allowing for rapid, isogradic monazite growth.     

Monazite–xenotime thermochronometry: methodology and an example from the Nepalese Himalaya

Monazite-xenotime thermochronometry involves the integration of petrographic, geochronological, and geochemical techniques to explore the thermal evolution of igneous and metamorphic rocks containing these accessory minerals. The method is illustrated in this paper by application to an orthogneiss sample from the Everest region of the Nepalese Himalaya that contains leucogranitic segregations produced by in-situ anatexis. Observations of phase relationships and the internal structure of accessory minerals made using both transmitted light and electron microscopy revealed the existence of multiple generations of monazite and xenotime and guided microsampling efforts to isolate grain fragments of Himalayan (Tertiary) and pre-Himalayan age. Nearly concordant U-Pb isotopic ratios for 13 single monazite and xenotime grains ranged in age from 28.37 to 17.598 Ma, making determination of the timing of anatexis difficult without additional information. Presuming that monazite and xenotime were in equilibrium over that entire interval, temperatures estimated from the yttrium contents of dated monazites range from 677-535 °C. Only the highest temperatures are consistent with experimental constraints on the conditions necessary to produce anatectic melts of appropriate composition, implying that the ~25.4-24.8 Ma dates for the grains with high apparent equilibration temperatures provide the best estimates for the age of anatexis. Two monazite crystals yielded ²⁰⁷Pb/²³⁵U dates that are statistically indistinguishable from the ²⁰⁷Pb/²³⁵U dates of coexisting xenotime crystals, permitting the application of both quantitative Y-partitioning and semi-quantitative Nd-partitioning thermometers as a cross-check for internal consistency. One of these sub-populations of accessory minerals, with a mean ²⁰⁷Pb/²³⁵U date of 22.364ǂ.097 Ma, provides inconsistent Y-partitioning (641ᆻ °C) and Nd-partitioning (515-560 °C) temperatures. We suspect the discrepancy may be caused by the high Th concentration (6.12 wt% ThO₂) in this subpopulation's monazite. The Y-partitioning thermometer was derived from experimental data for the (Ce, Y)PO₄ binary and may be inappropriate for application to high-Th monazites. For the other subpopulation (mean ²⁰⁷Pb/²³⁵U date=22.11ǂ.22 Ma), the Y- and Nd-partitioning temperatures are indistinguishable: 535ᇅ and 525-550 °C, respectively. This consistency strongly suggests that the sample experienced a temperature of ~535 °C at 22.11 Ma. This finding is tectonically important because temperatures at higher structural levels were much higher (by ~100 °C) at the same time, lending support to earlier suggestions of a major structural discontinuity within the upper part of the Himalayan metamorphic core at this longitude. An additional finding of uncertain importance is that inherited monazite and xenotime yielded U-Pb discordia with indistinguishable upper intercept ages (465.5NJ.7 and 470ᆟ Ma, respectively) and application of the Y-partitioning thermometer to the inherited monazites produced a restricted range of model temperatures averaging 470 °C. Whether or not these temperatures are geologically meaningful is unclear without independent corroboration of the assumption of equilibrium between the inherited monazites and xenotimes, but it appears that monazite-xenotime thermochronometry may be useful for "seeing through" high-grade metamorphism to extract temperature-time information about inherited mineral suites.