Minor Phases as Carriers of Trace Elements in Non-Modal Crystal-Liquid Separation Processes II: Illustrations and Bearing on Behaviour of REE, U, Th and the PGE in Igneous Processes

Minor phases which strongly concentrate selected trace elements,termed here ‘carrier-phases’, release relativelylarge amounts of those elements to the liquid phase when theyare eliminated during partial melting and glean relatively largeamounts of those elements when they first appear during progressivecrystallization. It is characteristic of such relationshipsthat concentrations of the selected trace elements in the bulkresidues of partial melting will rise to a peak somewhat beforethe last of the carrier-phase is eliminated during progressivemelting. In the liquids produced during equilibrium partialmelting a corresponding peak in the concentration of the traceelement occurs at the point where the carrier-phase is eliminated;the corresponding peak in trace element concentration in theliquids produced by accumulated perfect fractional melting isfound somewhat above that point. These peaks become more sharplyaccentuated as the distribution coefficient of the trace elementinto the carrier-phase increases. The highest trace elementconcentration in a partial melt liquid product is found in thesmall drop of liquid produced during perfect fractional meltingat the point where the carrier-phase is eliminated. Still higherconcentrations may be found in the first cumulates containingthe carrier-phase which precipitate during perfect fractionalcrystallization but the corresponding liquids do not containexceptionally high concentrations. Under favourable conditionsa large proportion of the available mass of a trace elementin a magmatic system may be transferred from the solid to theliquid phases or vice versa with only a small change in themass fraction of liquid in and energy content of the system.Within that range, separation of otherwise very similarly behavedtrace elements becomes possible. Further complexities ariseand the opportunities for separation increase when two carrier-phasescompete with differing success for the same group of trace elements. KEY WORDS: platinum; uranium; chromite; sulphide; distribution coefficient     

Relating zircon and monazite domains to garnet growth zones: age and duration of granulite facies metamorphism in the Val Malenco lower crust

Zircon from a lower crustal metapelitic granulite (Val Malenco, N‐Italy) display inherited cores, and three metamorphic overgrowths with ages of 281 ± 2, 269 ± 3 and 258 ± 4 Ma. Using mineral inclusions in zircon and garnet and their rare earth element characteristics it is possible to relate the ages to distinct stages of granulite facies metamorphism. The first zircon overgrowth formed during prograde fluid‐absent partial melting of muscovite and biotite apparently caused by the intrusion of a Permian gabbro complex. The second metamorphic zircon grew after formation of peak garnet, during cooling from 850 °C to c. 700 °C. It crystallized from partial melts that were depleted in heavy rare earth elements because of previous, extensive garnet crystallization. A second stage of partial melting is documented in new growth of garnet and produced the third metamorphic zircon. The ages obtained indicate that the granulite facies metamorphism lasted for about 20 Myr and was related to two phases of partial melting producing strongly restitic metapelites. Monazite records three metamorphic stages at 279 ± 5, 270 ± 5 and 257 ± 4 Ma, indicating that formation ages can be obtained in monazite that underwent even granulite facies conditions. However, monazite displays less clear relationships between growth zones and mineral inclusions than zircon, hampering the correlation of age to metamorphism. To overcome this problem garnet–monazite trace element partitioning was determined for the first time, which can be used in future studies to relate monazite formation to garnet growth.     

Garnet-field Melting and Late-stage Refertilization in 'Residual' Abyssal Peridotites from the Central Indian Ridge

The role of residual garnet during melting beneath mid-oceanridges has been the subject of many recent investigations. Toaddress this issue from the perspective of melting residues,we obtained major and trace element mineral chemistry of residualabyssal peridotites from the Central Indian Ridge. Many clinopyroxeneshave ratios of middle to heavy rare earth elements (MREE/HREE)that are too low to be explained by melting in the stabilityfield of spinel peridotite alone. Several percent of meltingmust have occurred at higher pressures in the garnet peridotitestability field. Application of new trace element partitioningmodels, which predict that HREE are compatible in high-pressureclinopyroxene, cannot fully explain the fractionation of theMREE from the HREE. Further, many samples show textural andchemical evidence for refertilization, such as relative enrichmentsof highly incompatible trace elements with respect to moderatelyincompatible trace elements. Therefore, highly incompatibleelements, which are decoupled from major and moderately incompatibletrace elements, are useful to assess late-stage processes, suchas melt entrapment, melt–rock reaction and veining. Moderatelyincompatible trace elements are less affected by such late-stageprocesses and thus useful to infer the melting history of abyssalperidotites. KEY WORDS: abyssal peridotites; mantle melting; garnet     

电子探针技术研究粤北龙华山岩体中独居石蚀变晕圈的结构与成分特征

独居石是华南产铀花岗岩中常见的含铀副矿物.龙华山岩体是粤北诸广山复式岩体中一个重要的产铀花岗岩,该岩体的独居石具有蚀变晕圈现象.但是,该岩体中独居石蚀变晕圈的结构和成分特征以及对铀成矿的指示意义尚未开展研究.本文利用电子探针(EPMA)对龙华山岩体的独居石蚀变晕圈开展结构和成分研究.测试结果表明:独居石蚀变晕圈是从内到...     

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     

Geochemistry of the Sweetwater Wash Pluton,California: Implications for “anomalous” trace element behavior during differentiation of felsic magmas

Field relations, mineralogy and major and trace element data for the very felsic, peraluminous Sweetwater Wash pluton establish a differentiation sequence dominantly controlled by fractional crystallization processes. Elements Ba and Sr show depletion by factors of 50–60X from the earliest granite unit analyzed to the late-stage pegmatites and aplites. The strong Ba depletion is largely due to the partitioning behavior of this element in K-feldspar, while the Sr depletion is due to the combined effects of the two feldspars. The 4-fold increase in Rb during crystallization is also predictable from mineral/ melt partition coefficients.Coefficients for the light rare-earth elements (LREE) in major mineral species predict that these elements should behave incompatibly during crystallization and increase with fractionation. In fact, the LREE abundances decrease by a factor of 10–20X during crystallization. This anomalous behavior is commonly observed in felsic plutonic and volcanic sequences. In the Sweetwater Wash pluton monazite occurs in minute quantities, but it is sufficiently abundant to govern the partitioning of LREE and Th during crystallization. Petrographic observations indicate that monazite was on the liquidus throughout most of the crystallization. Analyses of silicate mineral separates suggest that the monazite contains more than 75% of the LREE in the whole rocks.Fractionation of REE-rich accessories (in particular monazite) from felsic magmas may be the general cause of REE depletion during differentiation of these melts. Monazite can easily be mistaken for zircon and, because it typically contains 50% LREE, extremely minute and easily overlooked quantities of monazite can control LREE abundances.     

Origin of Low-K Intermediate Lavas at Nekoma Volcano, NE Honshu Arc, Japan: Geochemical Constraints for Lower-Crustal Melts

Extremely low-K basaltic andesite to andesite lavas at Nekomavolcano, situated in the frontal volcanic zone of the NE Honshuarc, were produced from melts that originated in the lower crust.Multiple incompatible trace element model calculations suggestthat extremely low-K basalt found in the same arc is a naturalanalog for the source composition. However, fractional crystallization,magma mixing, and crustal contamination models of primary low-Kbasalt cannot reproduce the Nekoma chemical composition. Derivationof melts from an extremely low-K amphibolitic lower-crustalrock with the residual mineral phases hornblende, olivine, pyroxenes,plagioclase, and magnetite is plausible. Major element compositionsof Nekoma lavas are very similar to those of experimental meltsof amphibolite dehydration melting, which further support theproposal. Light rare earth elements are slightly enriched, buttotal rare earth element abundances are relatively low, suggestinga high degree of partial melting of the source. Ba/Th ratiosare low for frontal arc lavas, reflecting modification of theratio during partial melting. Zr/Hf and Nb/Ta ratios are significantlygreater than is usual for arc lavas, suggesting an anomaloussource composition. Markedly low K, Rb, Cs contents in the extremelylow-K lavas are attributed to an extremely low-K source. Underplatingof an extremely low-K basalt originating from a hydrous depletedmantle wedge could form such an amphibolite. In contrast, Ndand Sr isotope ratios fall close to Bulk Earth values, indicatingan isotopically enriched source. Hornblende-bearing rocks maypredominate in the lower crust of the NE Honshu arc, based onthe observation of crustal xenoliths. The presence of largelow-V_p regions at lower-crustal depths beneath the frontal arcis suggested by geophysical observations. These observationsfurther support lower-crustal melting beneath Nekoma as theorigin of the intermediate low-K lavas. KEY WORDS: amphibolite source; crustal melting; low-K andesite; Sr–Nd isotopes; trace element     

Geochemical and Sm–Nd isotopic study of titanite from granitoid rocks of the eastern Dharwar craton,southern India

Titanite occurs as an accessory phase in a variety of igneous rocks, and is known to concentrate geologically important elements such as U, Th, rare earth element (REE), Y and Nb. The differences in the abundances of the REEs contained in titanite from granitoid rocks could reflect its response to changes in petrogenetic variables such as temperature of crystallization, pressure, composition, etc. Widespread migmatization in the granodiorite gneisses occurring to the east of Kolar and Ramagiri schist belts of the eastern Dharwar craton resulted in the enrichment of the REEs in titanite relative to their respective host rocks. A compositional influence on the partitioning of REEs between titanite and the host rock/magma is also noticed. The relative enrichment of REEs in titanite from quartz monzodiorite is lower than that found in the granodioritic gneiss. Depletion of REE and HFSE (high field-strength elements) abundances in granitic magmas that have equilibrated with titanite during fractional crystallization or partial melting has been modelled. As little as 1% of titanite present in residual phases during partial melting or in residual melts during fractional crystallization can significantly lower the abundances of trace elements such as Nb, Y, Zr and REE which implies the significance of this accessory mineral as a controlling factor in trace element distribution in granitoid rocks. Sm–Nd isotope studies on titanite, hornblende and whole rock yield isochron ages comparable to the precise U–Pb titanite ages, invoking the usefulness of Sm–Nd isochron ages involving minerals like titanite.     

Crustal Evolution of Island-Arc Ultramafic Magma: Galmoenan Pyroxenite-Dunite Plutonic Complex, Koryak Highland (Far East Russia)

Alaskan-type platinum-bearing plutons and potassium-enrichedmafic to ultramafic volcanic rocks are temporally and spatiallyassociated within the Late Cretaceous–Paleocene Achaivayam–Valaginskiiintra-oceanic palaeo-arc system, allochthonously present inthe Koryak Highland and Kamchatka Peninsula (Far East Russia).The compositions of the parental magmas to the Alaskan-typecomplexes are estimated using the Galmoenan plutonic complexas an example. This complex, composed of dunites, pyroxenitesand minor gabbros, is the largest (20 km³) in the system andthe best studied owing to associated platinum placer deposits.The compositions of the principal mineral phases in the Galmoenanintrusive rocks [olivine (Fo_79–92), clinopyroxene (1–3·5wt % Al₂O₃, 0·1–0·5 wt % TiO₂), and Cr-spinel(5–15 wt % Al₂O₃ and 0·3–0·7 wt %TiO₂)] are typical of liquidus assemblages in primitive island-arcmagmas in intra-oceanic settings, and closely resemble the mineralcompositions in the Achaivayam–Valaginskii ultramaficvolcanic rocks. The temporal and spatial association of intrusiveand extrusive units, and the similarity of their mineral compositions,suggest that both suites were formed from similar parental magmas.The composition of the parental magma for the Galmoenan plutonicrocks is estimated using previously reported data for the Achaivayam–Valaginskiiultramafic volcanic rocks and phenocryst-hosted melt inclusions.Quantitative simulation of crystallization of the parental magmain the Galmoenan magma chamber shows that the compositions ofthe cumulate units are best modelled by fractional crystallizationwith periodic magma replenishment. The model calculations reproducewell the observed mineral assemblages and the trace elementabundances in clinopyroxene. Based upon the estimated compositionof the parental magmas and their mantle source, we considerthat fluxing of a highly refractory mantle wedge (similar tothe source of boninites) by chlorine-rich aqueous fluids isprimarily responsible for both high degrees of partial meltingand the geochemical characteristics of the magmas, includingtheir enrichment in platinum-group elements. KEY WORDS: subduction; platinum-group elements; clinopyroxene; trace elements; fractional crystallization; Alaskan-type plutons     

Experimental Melting of Carbonated Peridotite at 6-10 GPa

Partial melting of magnesite-bearing peridotites was studiedat 6–10 GPa and 1300–1700°C. Experiments wereperformed in a multianvil apparatus using natural mineral mixesas starting material placed into olivine containers and sealedin Pt capsules. Partial melts originated within the peridotitelayer, migrated outside the olivine container and formed poolsof quenched melts along the wall of the Pt capsule. This allowedthe analysis of even small melt fractions. Iron loss was nota problem, because the platinum near the olivine container becamesaturated in Fe as a result of the reaction Fe₂SiO₄^Ol = Fe^Fe–Ptalloy + FeSiO₃^Opx + O₂. This reaction led to a gradual increasein oxygen fugacity within the capsules as expressed, for example,in high Fe³⁺ in garnet. Carbonatitic to kimberlite-like meltswere obtained that coexist with olivine + orthopyroxene + garnet± clinopyroxene ± magnesite depending on P–Tconditions. Kinetic experiments and a comparison of the chemistryof phases occasionally grown within the melt pools with thosein the residual peridotite allowed us to conclude that the meltshad approached equilibrium with peridotite. Melts in equilibriumwith a magnesite-bearing garnet lherzolite are rich in CaO (20–25wt %) at all pressures and show rather low MgO and SiO₂ contents(20 and 10 wt %, respectively). Melts in equilibrium with amagnesite-bearing garnet harzburgite are richer in SiO₂ andMgO. The contents of these oxides increase with temperature,whereas the CaO content becomes lower. Melts from magnesite-freeexperiments are richer in SiO₂, but remain silicocarbonatitic.Partitioning of trace elements between melt and garnet was studiedin several experiments at 6 and 10 GPa. The melts are very richin incompatible elements, including large ion lithophile elements(LILE), Nb, Ta and light rare earth elements. Relative to theresidual peridotite, the melts show no significant depletionin high field strength elements over LILE. We conclude fromthe major and trace element characteristics of our experimentalmelts that primitive kimberlites cannot be a direct productof single-stage melting of an asthenospheric mantle. They rathermust be derived from a previously depleted and re-enriched mantleperidotite. KEY WORDS: multianvil; carbonatite melt; peridotite; kimberlite; element partitioning     

Hydrothermal alteration of monazite in the Precambrian crystalline basement of the Athabasca Basin (Saskatchewan, Canada): implications for the formation of unconformity-related uranium deposits

Microanalytical studies of basement rocks below the Athabasca sandstone basin indicate that monazite is the dominant uranium-bearing mineral in the study area. Drill core samples of hydrothermally altered basement show that monazite is commonly altered to a Th–silicate phase, and uranium has been significantly mobilized. On average, 75% of the uranium bound to monazite is leached out during monazite alteration. In contrast, no substantial mobilization of uranium from detrital minerals (e.g. zircon) has yet been observed in the Athabasca sandstones. It is suggested that hydrothermal alteration of granitic rocks (especially potassic pegmatoids and potassic orthogneisses) of the sub-Athabasca basement, represents the most important uranium source for the unconformity-type deposits. Received: 3 December 1999 / Accepted: 24 May 2000     

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.     

Identifying Accessory Mineral Saturation during Differentiation in Granitoid Magmas: an Integrated Approach

Numerical reconstructions of processes that may have operatedduring igneous petrogenesis often model the behaviour of importanttrace elements. The geochemistry of these trace elements maybe controlled by accessory mineral saturation and fractionation.Determination of the saturation point of accessory mineralsin granitoid rocks is ambiguous because assumptions about crystalmorphology and melt compositions do not always hold. An integratedapproach to identifying accessory mineral saturation involvingpetrography, whole-rock geochemical trends, saturation calculationsand mineral chemistry changes is demonstrated here for a compositionallyzoned pluton. Within and between whole-rock samples of the BoggyPlain zoned pluton, eastern Australia, the rare earth element(REE)-enriched accessory minerals zircon, apatite and titaniteexhibit compositional variations that are related to saturationin the bulk magma, localized saturation in intercumulus meltpools and fractionation of other mineral phases. Apatite isidentified as having been an early crystallizing phase overnearly the whole duration of magma cooling, with zircon (andallanite) only saturating in more felsic zones. Titanite andmonazite did not saturate in the bulk magma at any stage ofdifferentiation. Although some trace elements (P, Ca, Sc, Nb,Hf, Ta) in zircon exhibit compositional variation progressingfrom mafic to more felsic whole-rock samples, normalized REEpatterns and abundances (except Ce) do not vary with progressivedifferentiation. This is interpreted to be a result of limitationsto both simple ‘xenotime’ and complex xenotime-typecoupled substitutions. Our data indicate that zircon REE characteristicsare not as useful as those of other REE-rich accessory mineralsas a petrogenetic indicator. KEY WORDS: saturation; zircon; apatite; titanite; magma differentiation; trace elements; REE patterns     

粤北长江铀矿田花岗岩独居石U-Pb同位素定年及其地质意义

长江铀矿田位于诸广山复式岩体中南部,是典型的花岗岩型铀矿田.前人采用锆石U-Pb定年方法对赋矿花岗岩进行了年代学研究,但由于全岩和锆石铀含量较高,锆石往往发生了蜕晶化,可能导致锆石U-Pb定年数据散乱,影响锆石U-Pb年龄的可靠性.独居石是花岗岩中广泛存在的含铀副矿物,铀和钍含量均较高,可达10000×10-6,普通铅...     

Trace Element and Platinum Group Element Distributions and the Genesis of the Merensky Reef, Western Bushveld Complex, South Africa

The Merensky Reef of the Bushveld Complex is one of the world'slargest resources of platinum group elements (PGE); however,mechanisms for its formation remain poorly understood, and manycontradictory theories have been proposed. We present precisecompositional data [major elements, trace elements, and platinumgroup elements (PGE)] for 370 samples from four borehole coresections of the Merensky Reef in one area of the western BushveldComplex. Trace element patterns (incompatible elements and rareearth elements) exhibit systematic variations, including small-scalecyclic changes indicative of the presence of cumulus crystalsand intercumulus liquid derived from different magmas. Ratiosof highly incompatible elements for the different sections areintermediate to those of the proposed parental magmas (CriticalZone and Main Zone types) that gave rise to the Bushveld Complex.Mingling, but not complete mixing of different magmas is suggestedto have occurred during the formation of the Merensky Reef.The trace element patterns are indicative of transient associationsbetween distinct magma layers. The porosity of the cumulatesis shown to affect significantly the distribution of sulphidesand PGE. A genetic link is made between the thickness of theMerensky pyroxenite, the total PGE and sulphide content, petrologicaland textural features, and the trace element signatures in thesections studied. The rare earth elements reveal the importantrole of plagioclase in the formation of the Merensky pyroxenite,and the distribution of sulphide. KEY WORDS: Merensky Reef; platinum group elements; trace elements     

A rare earth element-rich carbonatite dyke at Bayan Obo, Inner Mongolia, North China

Summary ?A carbonatite dyke, extremely enriched in rare earth elements (REE), is reported from Bayan Obo, Inner Mongolia, North China. The REE content in the dyke varies from 1 wt% to up to 20 wt%. The light REEs are enriched and highly fractionated relative to the heavy REEs, and there is no Eu anomaly. Although carbon isotope δ¹³C (PDB) values of the carbonatites (−7.3 to −4.7‰) are within the range of normal mantle (−5±2‰), oxygen isotope δ¹⁸O (SMOW) (11.9 to 17.7‰) ratios apparently are higher than those of the mantle (5.7±1.0‰), indicating varying degrees of exchange with hydrothermal fluids during or after magmatic crystallization. The carbonatite is the result of partial melting followed by fractional crystallization. Primary carbonatite melt was formed by less than 1% partial melting of enriched mantle, leaving a garnet-bearing residue. The melt then rose to a crustal magma chamber and underwent fractional crystallization, producing further REE enrichment. The REE and trace element distribution patterns of the carbonatites are similar to those of fine-grained dolomite marble, the ore-host rock of the Bayan Obo REE–Nb–Fe giant mineral deposit. This fact may indicate a petrogenetic link between the dykes described here and the Bayan Obo mineral deposit. Received November 1, 2001; revised version accepted June 16, 2002     

Adakitic Dacites Formed by Intracrustal Crystal Fractionation of Water-rich Parent Magmas at Nevado de Longavi Volcano (36{middle dot}2{degrees}S; Andean Southern Volcanic Zone, Central Chile)

The mid-Holocene eruptive products of Nevado de Longavívolcano (36·2°S, Chile) are the only reported occurrenceof adakitic volcanic rocks in the Quaternary Andean SouthernVolcanic Zone (33–46°S). Dacites of this volcano arechemically distinct from other evolved magmas of the regionin that they have high La/Yb (15–20) and Sr/Y (60–90)ratios and systematically lower incompatible element contents.An origin by partial melting of high-pressure crustal sourcesseems unlikely from isotopic and trace element considerations.Mafic enclaves quenched into one of the dacites, on the otherhand, constitute plausible parental magmas. Dacites and maficenclaves share several characteristics such as mineral chemistry,whole-rock isotope and trace element ratios, highly oxidizingconditions (NNO + 1·5 to >NNO + 2, where NNO is thenickel–nickel oxide buffer), and elevated boron contents.A two-stage mass-balance crystal fractionation model that matchesboth major and trace elements is proposed to explain magmaticevolution from the least evolved mafic enclave to the dacites.Amphibole is the main ferromagnesian phase in both stages ofthis model, in agreement with the mineralogy of the magmas.We also describe cumulate-textured xenoliths that correspondvery closely to the solid assemblages predicted by the model.We conclude that Nevado de Longaví adakitic dacites arethe products of polybaric fractional crystallization from exceptionallywater-rich parent magmas. These basaltic magmas are inferredto be related to an exceptionally high, but transient inputof slab-derived fluids released from serpentinite bodies hostedin the oceanic Mocha Fracture Zone, which projects beneath Nevadode Longaví. Fractional crystallization that is modallydominated by amphibole, with very minor garnet extraction, isa mechanism for generating adakitic magmas in cold subductionzones where a high flux of slab-derived fluids is present. KEY WORDS: adakite; amphibole; Andes; differentiation; Southern Volcanic Zone     

Timescales of partial melting in the Himalayan middle crust: insight from the Leo Pargil dome, northwest India

The Leo Pargil dome (LPD) in northwest India exposes an interconnected network of pre-, syn-, and post-kinematic leucogranite dikes and sills that pervasively intrude amphibolite-facies metapelites of the mid-crustal Greater Himalayan sequence. Leucogranite bodies range from thin (5-cm-wide) locally derived sills to thick (2-m-wide) crosscutting dikes extending at least 100 m. Three-dimensional exposures elucidate crosscutting relations between different phases of melt injection and crystallization. Combined laser ablation inductively coupled plasma mass spectrometry U–Th/Pb geochronology and trace element analysis on well-characterized monazite grains from nineteen representative leucogranites yields a large, internally consistent data set of approximately 700 U–Th/Pb and 400 trace element analyses. Grain-scale variations in age correlate with trace element distributions and indicate semi-continuous crystallization of monazite from 30 to 18 Ma. The youngest U–Th/Pb ages in a given sample are consistent with the outcrop-scale crosscutting relations, whereas older ages within individual samples record inheritance from partially crystallized melt and source metapelites. U–Th/Pb isotopic and trace element data are incorporated into a model of melting within the LPD that involves (1) steady-state equilibrium batch melting of compositionally homogeneous metapelitic sources; (2) pulses of increased melt mobility lasting 1–2 m.y. resulting in segregation of melt from its source and amalgamation into mixed magmas; and (3) rapid emplacement and final crystallization of leucogranite bodies. Melt systems in the LPD evolved from locally derived, in situ melt in migmatitic source rocks into a vast network of dikes and sills in the overlying non-migmatitic host rocks.     

Trace Element Geochemical Effects of Integrated Melt Extraction and 'Shaped' Melting Regimes

The mixing (integration) of liquids obtained as different massfractions of partial melting from source material of the samebulk composition, travelling along different mantle flow-linesthrough a melting regime, can result in deficiencies in therelative concentrations of those incompatible elements whosebulk distribution coefficients are numerically approximatelyequal to the average mass fraction of melt extracted from thetotal source material involved in the provision of the mixedmelts. These deficiencies can be very substantial, exceeding50% of the concentration which would have been expected to bepresent in the liquid if that same average mass fraction ofmelt had been extracted from the whole melting regime by simpleequilibrium or accumulated perfect fractional partial melting.The size of the deficit varies with the shape and plan-formof the melting region, and can be greatly reduced by subsequentperfect fractional crystallization of that liquid. Discriminationis increased between all elements whose distribution coefficientsare numerically smaller than the average mass fraction of partialmelt extracted from the whole region. These effects can leadto steepening of chondrite-normalized REE patterns and to apparentselective light rare earth enrichment in liquid and source. KEY WORDS: melt integration; shaped melting regimes; trace elements; numerical modelling     

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.