Showing papers in "Journal of Petrology in 2001"

A geochemical classification for granitic rocks

Abstract: This geochemical classification of granitic rocks is based upon three INTRODUCTION variables. These are FeO/(FeO + MgO) = Fe-number [or Although granitoids are the most abundant rock types FeO/(FeO + MgO) = Fe∗], the modified alkali–lime index in the continental crust, no single classification scheme (MALI) (Na2O + K2O – CaO) and the aluminum saturation has achieved widespread use. Part of the problem in index (ASI) [Al/(Ca – 1·67P + Na + K)]. The Fe-number granite classification is that the same mineral assemblage, (or Fe∗) distinguishes ferroan granitoids, which manifest strong iron quartz and feldspars with a variety of ferromagnesian enrichment, from magnesian granitoids, which do not. The ferroan minerals, can be achieved by a number of processes. and magnesian granitoids can further be classified into alkalic, Granitoids can form from differentiation of any hyalkali–calcic, calc-alkalic, and calcic on the basis of the MALI persthene-normative melt and from partial melting of and subdivided on the basis of the ASI into peraluminous, metamany rock types. Furthermore, granitic melts may be luminous or peralkaline. Because alkalic rocks are not likely to be derived solely from crustal components, may form from peraluminous and calcic and calc-alkalic rocks are not likely to be evolved mantle-derived melts, or may be a mixture peralkaline, this classification leads to 16 possible groups of granitic of crustal and mantle-derived melts. Because of this rocks. In this classification most Cordilleran granitoids are magnesian complexity, petrologists have relied upon geochemical and calc-alkalic or calcic; both metaluminous and peraluminous classifications to distinguish between various types of types are present. A-type granitoids are ferroan alkali–calcic, although granitoids. Approximately 20 different schemes have evolved over the past 30 years [see Barbarin (1990, 1999) some are ferroan alkalic. Most are metaluminous although some are for a summary thereof]. Most of these schemes are either peraluminous. Caledonian post-orogenic granites are predominantly genetic or tectonic in nature. This paper is an attempt magnesian alkali–calcic. Those with <70 wt % SiO2 are domto present a non-genetic, non-tectonic geochemical clasinantly metaluminous, whereas more silica-rich varieties are comsification scheme that incorporates the best qualities of monly peraluminous. Peraluminous leucogranites may be either the previous schemes, and to explain the petrologic magnesian or ferroan and have a MALI that ranges from calcic to processes that makes this scheme work. alkalic.

The Range of Spinel Compositions in Terrestrial Mafic and Ultramafic Rocks

Abstract: Compositional fields for spinels from a wide variety of mafic– a wide range of conditions from mafic and ultramafic ultramafic igneous rock types and tectonic environments have been magmas and, in the case of chromites, are often among determined from a global database of over 26 000 analyses. the first phases to crystallize. They also exhibit a wide These fields are defined using contoured data density plots based range of solid solution, the thermodynamics of which has on the spinel prism, and plots of TiO2 vs ferric iron, for mantle been studied extensively (O’Neill & Wall, 1987; Mattioli xenoliths, ophiolitic rocks, continental layered intrusions, alkalic & Wood, 1988; Wood, 1990; Sack & Ghiorso, 1991; and lamprophyric rocks, tholeiitic basalts, Alaskan ultramafic Poustovetov, 2000). They are relatively refractory and complexes and komatiites. Several trends appear regularly in the resistant to alteration, particularly compared with other various environments: a trend of widely variable Cr/(Cr + Al) high-temperature igneous minerals such as olivine. They at low Fe/(Mg + Fe) (the Cr–Al trend); increasing Fe, occur in a high proportion of terrestrial mafic and ultraFe/(Mg + Fe) and TiO2 at constant Cr/(Cr + Al) mafic rocks, and a very large volume of microprobe data (Fe–Ti trend); a trend found primarily in kimberlites, similar is available on their compositions. to Fe–Ti but at constant Fe/(Mg + Fe); and an unusual Publications on spinels (particularly chromites) have trend of increasing Al found only in layered intrusions. The routinely used compositional fields based on the spinel Cr–Al and Fe–Ti trends are both found to varying degrees in prism to compare populations of analyses. Some of tholeiitic basalts. The Cr–Al trend is prevalent in rocks that these fields have remarkable longevity, particularly those have equilibrated over a range of pressures, whereas the Fe–Ti defined for layered intrusions and Alpine peridotites by trend is dominantly due to low-pressure fractionation. The most Irvine (1967, 1977). Many new data have become availCr-rich chromites found in nature occur in boninites, diamondable in the last 20 years, however, and it has become bearing kimberlites, some komatiites and ophiolitic chromitites. apparent that a new compilation of data is necessary to Exceptionally reduced chromites are found in some komatiites reflect this. The prime purpose of this paper is to analyse and in ophiolitic chromitites. Detrital chromites from the a global database of about 26 000 spinel analyses from Witwatersrand conglomerates are of komatiitic provenance. terrestrial igneous and metamorphosed igneous rocks, and to extract from this database the characteristic compositional fields of spinels from the wide variety of magma types and tectonic environments in which they occur.

Factors Controlling Chemistry of Magmatic Spinel: an Empirical Study of Associated Olivine, Cr-spinel and Melt Inclusions from Primitive Rocks

Abstract: Compositions of ~2500 spinel-olivine pairs and 400 melt inclusion-spinel pairs have been analysed from 36 igneous suites from oceanic, arc and intraplate tectonic settings. Our data confirm that Cr-spinel mg-number is largely controlled by melt composition, but also influenced by octahedral site substitutions, and rate of cooling. Lavas quenched in submarine environments tend to have higher mg-number at a given cr-number than slowly cooled subaerial lavas and peridotites. Unlike mg-number, Cr-spinel Al2O3 and TiO2 contents show good correlations with melt composition, with only limited post-entrapment modifications. Out data suggest that increased activity of Al2O3 decreases the partitioning of TiO2 into spinels. The Al2O3 content of Cr-spinel is a useful guide to the degree of partial melting of mantle peridotites; however, this same relationship is obscured in volcanic rocks. Al2O3 contents of volcanic Cr-spinels are mostly determined by melt composition rather than mantle source composition. The data also suggest that most spinels from residual mantle peridotites can be readily differentiated from those hosted in volcanic rocks. Mantle peridotite spinel tend to have lower TiO2 and higher Fe2+/Fe3+ ratios than spinel from volcanic rocks. The spinel compositions in our database can be subdivided on the basis of tectonic setting and mode of occurrence using an Al2O3 vs TiO2 diagram. A total of seven fields can be distinguished with varying degrees of overlap. This diagram can then be used to determine the tectonic setting of spinel from altered mafic igneous rocks such as serpentinites or meta-basalts, or detrital spinel in sandstones.

High-Pressure Granulites (Retrograded Eclogites) from the Hengshan Complex, North China Craton: Petrology and Tectonic Implications

Abstract: Both highand medium-pressure granulites have been found as because of the absence of modal minerals. The combination of petrographic textures, mineral compositions, metamorphic reaction enclaves and boudins in tonalitic–trondhjemitic–granodioritic gneisses in the Hengshan Complex. Petrological evidence from these history, petrogenetic grids and thermobarometric data defines a nearrocks indicates four distinct metamorphic assemblages. The early isothermal decompressional clockwise P–T path for the Hengshan prograde assemblage (M1) is preserved only in the high-pressure granulites, suggesting that the Hengshan Complex underwent initial granulites and represented by quartz and rutile inclusions within crustal thickening, subsequent exhumation, and cooling and retrothe cores of garnet porphyroblasts, and omphacite pseudomorphs gression. This tectonothermal path is considered to record a major that are indicated by clinopyroxene + sodic plagioclase symplectic phase of collision between two continental blocks, which resulted in intergrowths. The peak assemblage (M2) consists of clinopyroxene the final assembly of the North China Craton at >1·8 Ga. + garnet + sodic plagioclase + quartz ± hornblende in the high-pressure granulites and orthopyroxene + clinopyroxene + garnet + plagioclase + quartz in the medium-pressure granulites.

Energy-constrained open-system magmatic processes I : General model and energy-constrained assimilation and fractional crystallization (EC-AFC) formulation

Abstract: recharge mass and Teq, the mass of wallrock heated to Teq is Geochemical data for igneous rocks provide definitive evidence for computed and the geochemical trajectory of melt is determined as the occurrence of open-system processes in magma bodies, including magma temperature approaches Teq from its starting value, Tm°. Replenishment by intrusion of primitive magma, Assimilation of The effects of imperfect extraction of anatectic wallrock melt may anatectic wallrock melt and cumulate formation by Fractional be accounted for by introduction of an extraction efficiency factor. Crystallization. A general class of models (Energy ConservedMathematical details of a simpler model, EC-AFC (no reRAFC or EC-RAFC) can be constructed, which allow simulation plenishment) are provided. Energy-constrained models have the of geochemical paths for trace elements and isotopic ratios for magma advantage of linking thermal and chemical properties of magma undergoing simultaneous replenishment, assimilation and fractional chambers. Compared with ‘classical’ models that conserve mass crystallization during the approach to thermal equilibration. The and species only, they represent more complete assessments of general problem of EC-RAFC is formulated as a set of 3 + t the complex physicochemical dynamics governing the geochemical + i + s coupled nonlinear ordinary differential equations, where evolution of open-system magma bodies. Results of EC-AFC the number of trace elements, radiogenic and stable isotope ratios simulations demonstrate that geochemical trends can differ sigsimultaneously modeled are t, i and s, respectively. Partial melting nificantly from predictions based on ‘classical’ AFC even when of wallrock, modeled as fractional melting, is incorporated, as are recharge plays no role. Incorporation of energy conservation and sensible and latent heat effects. Temperature-dependent partition partial melting into geochemical models allows important coupled coefficients are used to describe trace element distributions. Solution effects to play their natural role. of the set of differential equations, with magma temperature (Tm ) as the independent variable, provides values for the average temperature of wallrock (Ta ), fraction of melt within the magma body (Mm ),

Monazite–Xenotime–Garnet Equilibrium in Metapelites and a New Monazite–Garnet Thermometer

Abstract: Prograde suites of pelitic rocks were examined with electron microprobe INTRODUCTION and laser ablation inductively coupled plasma mass spectrometry to Monazite [(Ce,La,Th)PO4] plays an important role in determine the systematics of element partitioning between coexisting studies of igneous and metamorphic petrogenesis. Foremonazite, xenotime, and garnet. Monazite grains that grew in most, monazite is used to date specific events in a equilibrium with xenotime are enriched in Y and Dy compared with petrogenetic sequence (e.g. Parrish, 1990; Harrison et al., monazite that grew in xenotime-absent assemblages. Y and heavy 1997; Hawkins & Bowring, 1999; Foster et al., 2000). rare earth element contents of monazite coexisting with xenotime Monazite may contain a large percentage of the sample increase with rising temperature. Monazite–xenotime Y–Gd and rare earth element (REE) budget and, thus, exert an Y–Dy partitioning is systematic within a metamorphic grade, and important influence on the evolution of melt composition increases slightly with increasing metamorphic grade, suggesting (Wark & Miller, 1993; Bea, 1996). The thermobarometric that monazite–xenotime pairs approached partitioning equilibrium. potential of monazite (coexisting with xenotime) has been Garnet and monazite in both xenotime-bearing and xenotime-absent recognized, leading to calibration of monazite geoassemblages show a strong ( R = 0·94) systematic relationship thermometers (Gratz & Heinrich, 1997, 1998; Heinrich between inverse temperature and ln( KEq ) for the net-transfer equiet al., 1997; Andrehs & Heinrich, 1998). In addition, the librium YAG+OH-Ap+ (25/4)Qtz= (5/4)Grs+ (5/4)An recognition that nearly all lead in monazite is radiogenic + 3YPO4-Mnz + 1/2H2O, suggesting that garnet and monazite (Parrish, 1990; Montel et al., 1994) has led to development crystallized in compositional equilibrium. The following temand application of electron microprobe monazite dating perature–KEq relationship for the equilibrium above has been derived: techniques (Suzuki & Adachi, 1991; Montel et al., 1996; Williams et al., 1999). T(°C)= −1·45P(bars)+447772(±32052) 567(±40)−Rln( KEq ) −273·15 Full realization of monazite as a thermochronometer requires detailed understanding of the specific reactions responsible for its formation. Previous studies of monazite with a precision of some ±30°C for temperature estimates. Our petrogenesis have focused on monazite compositional observations suggest that major and accessory phases interact in a zonation (Zhu & O’Nions, 1999a, 1999b), monazite coupled fashion during metamorphism, and also approach a state growth kinetics (Ayers et al., 1999), behavior of monazite of compositional equilibrium as reactions proceed. during hydrothermal alteration (Poitrasson et al., 1996; Crowley & Ghent, 1999) and melting events (Watt, 1995), or, to a limited extent, the influence of major-phase mineral assemblage in monazite reactivity (Zhu &

Calculation of Phase Relations Involving Haplogranitic Melts Using an Internally Consistent Thermodynamic Dataset

Abstract: A simple thermodynamic model is developed for silicate melts in in the book by Johannes & Holtz (1996). Thermodynamic approaches to calculating melting relations for basic the system CaO–Na2O–K2O–Al2O3–SiO2–H2O (CNKASH). The Holland & Powell ( Journal of Metamorphic Geology, 16, magmas and wet granitic systems have also been made (e.g. Burnham, 1975; Nicholls, 1980; Berman & Brown, 289–302, 1998) internally consistent thermodynamic dataset is extended via the incorporation of the experimentally determined 1987; Stolper, 1989; Nekvasil, 1990; Ghiorso & Sack, 1995). melting relationships in unary and binary subsystems of CNKASH. The predictive capability of the model is evaluated via the exMetaluminous and peraluminous bulk compositions, primarily in the silica-saturated portion of the haploperimental data in ternary and quaternary subsystems. The resulting dataset, with the software THERMOCALC, is then used to granitic system, CaO–Na2O–K2O–Al2O3–SiO2–H2O (CNKASH), will be considered here. These include comcalculate melting relationships for haplogranitic compositions. Predictions of the P–T stabilities of assemblages in water-saturated positions made up of combinations of the end-member components albite (NaAlSi3O8), K-feldspar (KAlSi3O8), and -undersaturated bulk compositions are illustrated. It is now possible to make useful calculations of the melting behaviour of anorthite (CaAl2Si2O8), aluminosilicate (Al2SiO5), quartz (SiO2) and water (H2O). The purpose of this study is to appropriate composition rocks under crustal conditions. extend the thermodynamic dataset of Holland & Powell (1998) to allow prediction of melting relations involving water-saturated and -undersaturated granitic melts. The

Volatiles in Basaltic Glasses from Loihi Seamount, Hawaii: Evidence for a Relatively Dry Plume Component

Abstract: New H2O, CO2 and S concentration data for basaltic glasses from ‘hotspot’, with temperature differences of 200°C or more between hotter upwelling plumes and the ambient mantle Loihi seamount, Hawaii, allow us to model degassing, assimilation, adiabat (e.g. White & McKenzie, 1989; Campbell, 1998; and the distribution of major volatiles within and around the Davies, 1998). In contrast, it has long been known that Hawaiian plume. Degassing and assimilation have affected CO2 ocean island basalts (OIB) are enriched in volatiles relative and Cl but not H2O concentrations in most Loihi glasses. Water to depleted mid-oceanic ridge basalts (MORB), leading concentrations relative to similarly incompatible elements in Hato speculation that the excess magmatism associated with waiian submarine magmas are depleted (Loihi), equivalent (Kilauea, plumes is related to a mantle ‘wet spot’ (Schilling et al., North Arch, Kauai–Oahu), or enriched (South Arch). H2O/Ce 1980) or a ‘not-so-hot-spot’ (Bonatti, 1990). Thus, there ratios are uncorrelated with major element composition or extent or is still lively debate over the relative importance of ‘hot’ depth of melting, but are related to position relative to the Hawaiian and ‘wet’ in the generation of mantle plumes. plume and mantle source region composition, consistent with a zoned The knowledge that OIB are wetter than MORB, plume model. In front of the plume core, overlying mantle is however, does little to answer the question of the origin metasomatized by hydrous partial melts derived from the Hawaiian of volatiles in plume basalts. If the enrichments of volatile plume. Downstream from the plume core, lavas tap a depleted source elements in OIB are proportional to those of nonvolatile region with H2O/Ce similar to enriched Pacific mid-ocean ridge incompatible elements, then their higher concentrations basalt. Within the plume core, mantle components, thought to can be accomplished through simple mineral–melt fracrepresent subducted oceanic lithosphere, have water enrichments tionation processes. In contrast, if the volatile elements equivalent to (KEA) or less than (KOO) that of Ce. Lower H2O/ are decoupled from major and trace elements, then more Ce in the KOO component may reflect efficient dehydration of the complex processes must take place, including involvement subducting oceanic crust and sediments during recycling into the and possible migration into or out of the plume of a deep mantle. separate C + H + O fluid phase, mixing of source regions having different volatile contents, or shallow-level processes such as assimilation or degassing. In particular,

Formation and Modification of the Shallow Sub-continental Lithospheric Mantle: a Review of Geochemical Evidence from Ultramafic Xenolith Suites and Tectonically Emplaced Ultramafic Massifs of Western and Central Europe

Abstract: The petrology and geochemistry of shallow continental lithospheric Extended mantle-normalized incompatible trace element patterns for whole rocks show enrichment in Rb and Ba in peridotites considered mantle (SCLM) can be studied via (1) tectonically emplaced ultramafic massifs and (2) mantle xenoliths entrained in alkaline to have been subduction-metasomatized, whereas those considered to be carbonate-metasomatized have strong negative anomalies in magmas. Data from these two separate sources are used to identify processes that have formed and modified the SCLM. In western Zr, Nb and Hf. Mantle amphiboles are strongly enriched in LREE when found in veins, but can be LREE depleted if they are and central Europe where the continental crust consolidated in Phanerozoic times, both sources of information are available for interstitial. Radiogenic isotope ratios for xenoliths and massifs largely overlap, although the xenoliths show a significant clustering study. Rock types found in ultramafic massifs in Europe are generally similar to those found in ultramafic xenolith suites. The most around a ‘plume-component’ identical to the Neogene alkaline magmatism of Europe. This component is lacking in the massifs, frequent lithology is anhydrous spinel lherzolite, grading towards most of which were emplaced into the crust before the onset of harzburgite. Massifs reveal pyroxenite layering, harzburgite bands Neogene plume activity. Infiltration of carbonatite melts is observed and cross-cutting mafic and ultramafic dykes. The Phanerozoic petrographically in some xenoliths and evidenced by low Ti/Eu European SCLM xenoliths and massifs show broad mineralogical ratios in bulk rocks, but is very rare. The effect of passage of and chemical similarities to Phanerozoic continental spinel peridotites hydrous fluids from subducting slabs is also seen in some suites and world-wide. The main process that controls the geochemistry of the massifs, being exhibited mainly as unusual Sr and Pb isotope SCLM is depletion by removal of basaltic melt. Differences from ratios, although enrichment in K, Rb and Ba, and the presence of this norm reflect significantly different processes in the SCLM, such modal phlogopite, may also point to subduction-metasomatism. as interaction with melts and fluids. Such processes probably gave rise to hornblendite veins and pyroxenite layers, although the latter have also been interpreted as recycled oceanic crust. Rare earth

Origins of Large Volume Rhyolitic Volcanism in the Antarctic Peninsula and Patagonia by Crustal Melting

Abstract: Voluminous rhyolitic volcanism along the palaeo-Pacific margin of Gondwana was marked by three principal episodes of magmatism. The first of these ( V1) is essentially coincident with the main episode of Karoo–Ferrar magmatism at ∼184 Ma. A younger ( V2) episode occurred at ∼168 Ma, and a third episode ( V3) occurred in the interval 157–153 Ma. We evaluate the origin of V1 and V2 rhyolites from the Antarctic Peninsula using major and trace element and isotopic (Sr, Nd, O) data. An isotopically uniform (87Sr/86Sri ∼0·707; eNdi ∼ −3) andesite–dacite magma was generated as a result of anatexis of ‘Grenvillian age’ hydrous mafic lower crust, linked to earlier, arc-related underplating. The lower-crustal partial melts would have mixed with fractionated components of the mafic underplate, followed by subsequent storage and homogenization. Early Jurassic ( V1) rocks of the southern Antarctic Peninsula are interpreted as melts of upper-crustal paragneiss, which have mixed with the isotopically uniform magma in upper-crustal magma chambers. The V2 rhyolites are the result of assimilation–fractional crystallization of the isotopically uniform magma. This occurred in upper-crustal magma chambers involving assimilants with similar isotopic composition to that of the magma. A continental margin setting was crucial in developing hydrous, readily fusible lower crust. Lower-crustal anatexis was in response to mafic underplating associated with the Discovery–Shona–Bouvet group of plumes, thought to be responsible for the Karoo magmatic province. The progression (old to young) of volcanism from NE to SW in Patagonia and south to north in the Antarctic Peninsula is consistent with migration away from the mantle plumes towards the proto-Pacific margin of Gondwana during rifting and break-up.

Energy-Constrained Open-System Magmatic Processes II: Application of Energy-Constrained Assimilation–Fractional Crystallization (EC-AFC) Model to Magmatic Systems

Abstract: data from natural systems indicates that some of the criteria currently Evidence for open-system magmatic processes is abundant in igneous used to demonstrate the efficacy of AFC require modification. Finally, rocks from most tectonic settings and with ages spanning most of comparison of ‘classical’ AFC and EC-AFC results for data from geologic time. Accurately documenting these processes is critical for well-documented volcanic centers demonstrates that EC-AFC does understanding magma reservoir dynamics, including the processes a superior job of tracking the compositional trends, provides a that lead to compositional diversity in igneous rocks, and for plausible physical context for the process of AFC, and allows deciphering the thermochemical evolution of the crust and mantle. geologically relevant predictions to be made about particular magmatic Quantitative models describing open-system processes such as assystems. similation–fractional crystallization (AFC) have provided significant insight into all of these, but, nevertheless, suffer from several serious deficiencies. Foremost among these are the absence of energy conservation and the lack of consideration of country rock partial

Low-δ18O Rhyolites from Yellowstone: Magmatic Evolution Based on Analyses of Zircons and Individual Phenocrysts

Abstract: The Yellowstone Plateau volcanic field is one of the largest centers exchange time to form zoned zircons is between a few hundred and a few thousand years, which reflects the residence time of lowO of rhyolitic magmatism on Earth. Major caldera-forming eruptions are followed by unusual lowO rhyolites. New oxygen isotope, magmas after formation and before eruption. petrologic and geochemical data from rhyolites belonging to the 2·0 my eruptive history of Yellowstone are presented, with emphasis on the genesis of lowO magmas erupted after the Huckleberry Ridge Tuff (2·0 Ma, 2500 km) and Lava Creek Tuff (0·6 Ma,

Nd, Pb and Sr Isotopic Compositions of East African Carbonatites: Evidence for Mantle Mixing and Plume Inhomogeneity

Abstract: New Pb isotopic data are presented for 10 young Mesozoic to their variation in age can provide valuable insights into Cenozoic (0–116 Ma) carbonatites from a 1400 km long segment the nature of the sub-continental mantle over a period of the East African Rift. Patterns observed in Pb vs Pb, Sr vs Pb of >2·7 by. On the basis of stable isotope data (Deines, and Nd vs Pb isotope diagrams define unusual, nearly linear, trends 1989; Keller & Hoefs, 1995), Pb, Sr and Nd isotope that are interpreted as mixing between two components that are compositions (Bell et al., 1982; Nelson et al., 1988; Bell broadly similar to the two mantle end-member components, HIMU & Blenkinsop, 1989; Kwon et al., 1989) and noble gas and EM1, which were first recognized from ocean-island basalts. data (Sasada et al., 1997; Marty et al., 1998), there seems The two plutons with isotope signatures closest to HIMU and little doubt that the parental melts to carbonatites are EM1 crop out within 140 km of each other. From these data, generated within the mantle. One of the most intriguing EM1 and HIMU are now known to occur in both continental features to emerge from the studies of young carbonatites and oceanic settings that are associated with plumes or rifts. is that they share isotope similarities with many oceanic Moreover, these isotopic signatures tend to occur in regions where island lavas (Basu & Tatsumoto, 1980; Bell et al., 1982; seismic tomography indicates prominent low-velocity zones in the Bell & Blenkinsop, 1987a, 1987b; Nelson et al., 1988; lower mantle. For these reasons, we favour a model for the origin Kwon et al., 1989), thus raising the question as to whether of the East African Rift carbonatites that involves melting and parts of the sub-continental mantle and the sub-oceanic mixing of HIMU and EM1 components contained within an mantle are essentially the same in terms of their isotopic isotopically heterogeneous mantle plume. We consider the HIMU signatures. and EM1 sources to be stored within the deep (lower 1000 km) Evidence from melting studies has shown that carmantle, possibly the core–mantle boundary. The role that continental bonatites can be produced in three different ways: (1) lithosphere plays in carbonatite generation is probably one of direct partial melting of a metasomatized mantle source concentrating volatiles at the upper levels of an ascending mantle (e.g. Wyllie & Huang, 1975; Wallace & Green, 1988; plume. Wyllie & Lee, 1998); (2) derivation by immiscible separation at low or high pressures from carbonated silicate

Sources and Fluids in the Mantle Wedge below Kamchatka, Evidence from Across-arc Geochemical Variation

Abstract: Major and trace element and Sr–Nd–Pb isotopic variations in of melting changes from >20% at the arc front to <10% below mafic volcanic rocks hve been studied in a 220 km transect across the back-arc region. The rocks from volcanoes of the northern part the Kamchatka arc from the Eastern Volcanic Front, over the Central of the Central Kamchatka Depression—to the north of the transect Kamchatka Depression to the Sredinny Ridge in the back-arc. considered in this study—are significantly different in their trace Thirteen volcanoes and lava fields, from 110 to 400 km above the element compositions compared with the other rocks of the transect subducted slab, were sampled. This allows us to characterize spatial and their source appears to have been enriched by a component variations and the relative amount and composition of the slab derived from melting of the edge of the ruptured slab. fluid involved in magma genesis. Typical Kamchatka arc basalts, normalized for fractionation to 6% MgO, display a strong increase in large ion lithophile, light rare earth and high field strength

Ilmenite as a Source for Zirconium during High-grade Metamorphism? Textural Evidence from the Caledonides of Western Norway and Implications for Zircon Geochronology

Abstract: distance around rutile, and locally these zircons show a prismatic The Proterozoic Lindas Nappe, part of the Caledonides of western overgrowth. A specific low-Th zircon growth event is related to Norway, was affected by penetrative Sveconorwegian granulite-facies eclogite-facies forming reactions, involving breakdown of a twometamorphism, followed by a fluid-driven eclogiteand amphibolitepyroxene + garnet + plagioclase + ilmenite assemblage to form facies Caledonian overprint, spatially restricted along fractures and a garnet + omphacite + rutile assemblage in the presence of a shear zones. In mafic granulites and amphibolites, a luminescent fluid. The low Th content of this zircon probably stems from the anhedral zircon overgrowth, which gives an average age of 924 ± coeval precipitation of clinozoisite. This oscillatory zoned zircon 58 Ma (Th/U = 0·52; secondary ion mass spectrometry data), records fluid infiltration and coeval eclogitization in the crust. surrounds a magmatic zoned core with an age of 952 ± 32 Ma (Th/U = 1·27). In the granulites, a continuous rim of zircon or a discontinuous corona of >10 m rounded to flat zircon crystals is observed at the outer margin of ilmenite grains. Baddeleyite and

Phase relations of peralkaline silicic magmas and petrogenetic implications.

Abstract: The phase relationships of three peralkaline rhyolites from the Kenya Rift have been established at 150 and 50 MPa, at oxygen fugacities of NNO - 1·6 and NNO + 3·6 (log fO2 relative to the Ni–NiO solid buffer), between 800 and 660°C and for melt H2O contents ranging between saturation and nominally anhydrous. The stability fields of fayalite, sodic amphiboles, chevkinite and fluorite in natural hydrous silicic magmas are established. Additional phases include quartz, alkali feldspar, ferrohedenbergite, biotite, aegirine, titanite, montdorite and oxides. Ferrohedenbergite crystallization is restricted to the least peralkaline rock, together with fayalite; it is replaced at low melt water contents by ferrorichterite. Riebeckite–arfvedsonite appears only in the more peralkaline rocks, at temperatures below 750°C (dry) and below 670°C at H2O saturation. Under oxidizing conditions, it breaks down to aegirine. In the more peralkaline rocks, biotite is restricted to temperatures below 700°C and conditions close to H2O saturation. At 50 MPa, the tectosilicate liquidus temperatures are raised by 50–60°C, and that of amphibole by 30°C. Riebeckite–arfvedsonite stability extends down nearly to atmospheric pressure, as a result of its F-rich character. The solidi of all three rocks are depressed by 40–100°C compared with the solidus of the metaluminous granite system, as a result of the abundance of F and Cl. Low fO2 lowers solidus temperatures by at least 30°C. Comparison with studies of metaluminous and peraluminous felsic magmas shows that plagioclase crystallization is suppressed as soon as the melt becomes peralkaline, whatever its CaO or volatile contents. In contrast, at 100 MPa and H2O saturation, the liquidus temperatures of quartz and alkali feldspar are not significantly affected by changes in rock peralkalinity, showing that the incorporation of water in peralkaline melts diminishes the depression of liquidus temperatures in dry peralkaline silicic melts compared with dry metaluminous or peraluminous varieties. At 150 MPa, pre-eruptive melt H2O contents range from 4 wt % in the least peralkaline rock to nearly 6 wt % in the two more peralkaline compositions, in broad agreement with previous melt inclusion data. The experimental results imply magmatic fO2 at or below the fayalite–quartz–magnetite solid buffer, temperatures between 740 and 660°C, and melt evolution under near H2O saturation conditions.

Mantle Processes and Sources of Neogene Slab Window Magmas from Southern Patagonia, Argentina

Abstract: 0·51261; 206 Pb/ 204 Pb = 18·3–18·8; 207 Pb/ 204 Pb = 15·57– basalt petrogenesis; mantle chemistry; back-arc processes 15·65; 208 Pb/ 204 Pb = 38·4–38·7) of these Patagonian slab window lavas contrast with the mid-ocean ridge basalt (MORB)like, depleted mantle signatures of slab window lavas elsewhere in the Cordillera (e.g. Antarctic Peninsula; Baja California). The

Calculation of Peridotite Partial Melting from Thermodynamic Models of Minerals and Melts, IV. Adiabatic Decompression and the Composition and Mean Properties of Mid-ocean Ridge Basalts

Abstract: Composition, mean pressure, mean melt fraction, and crustal thickness of model mid-ocean ridge basalts (MORBs) are calculated using MELTS. Polybaric, isentropic batch and fractional melts from ranges in source composition, potential temperature, and final melting pressure are integrated to represent idealized passive and active flow regimes. These MELTS-derived polybaric models are compared with other parameterizations; the results differ both in melt compositions, notably at small melt fractions, and in the solidus curve and melt productivity, as a result of the self-consistent energy balance in MELTS. MELTS predicts a maximum mean melt fraction (~0·08) and a limit to crustal thickness (<=15 km) for passive flow. For melting to the base of the crust, MELTS requires an ~200°C global potential temperature range to explain the range of oceanic crustal thickness; conversely, a global range of 60°C implies conductive cooling to ~50 km. Low near-solidus productivity means that any given crustal thickness requires higher initial pressure in MELTS than in other models. MELTS cannot at present be used to model details of MORB chemistry because of errors in the calibration, particularly Na partitioning. Source heterogeneity cannot explain either global or local Na–Fe systematics or the FeO–K2O/TiO2 correlation but can confound any extent of melting signal in CaO/Al2O3.

The Recrystallization Front of the Ronda Peridotite: Evidence for Melting and Thermal Erosion of Subcontinental Lithospheric Mantle beneath the Alboran Basin

Abstract: with a process involving partial melting, kilometre-scale migration of Evidence for a major heating event accompanied by decompression melts by diffuse porous flow and limited melt extraction (2·5–6·5%). was recently reported from crustal rocks drilled in the Alboran basin. Hence, the Ronda recrystallization front is interpreted as the The metamorphic evolution recorded by these rocks implies complete narrow boundary of a partial-melting domain (the coarse-granular removal of lithospheric mantle during the Cenozoic, a process that peridotites) formed at the expense of subcontinental lithospheric is confirmed by geophysical modelling indicating thin lithosphere mantle (the spinel tectonites). The existence of melt-consuming beneath the Alboran domain. In this region, the Ronda lherzolite reactions in the transitional peridotites, a few hundred metres ahead massif (Betic Cordillera, southern Spain) provides a unique opof the melting front, demonstrates that the front was thermally portunity for the observation of mantle processes associated with controlled. This implies that a smooth thermal gradient existed across lithospheric thinning. A striking feature of the Ronda peridotite is the Ronda massif during the development of the recrystallization front. a narrow recrystallization front, which has been ascribed to kilometreDifferences in pyroxene compositions on either side of the front may scale porous melt flow through the massif. The front separates the be explained by a transient heating event at [1200°C (>1·5 spinel tectonite domain, interpreted as old, veined lithospheric mantle, GPa) coeval with partial melting. Consistent with the geodynamic from the granular domain where the lithospheric microstructures, scenario proposed for the Alboran domain during the Cenozoic, the mineralogical assemblages and geochemical signatures were obevolution of the Ronda recrystallization front is considered as an literated by grain growth coeval with pervasive infiltration of basaltic example of thermal erosion and partial melting of lithospheric mantle melts. On the basis of trace-element abundances in peridotites above upwelling asthenosphere. collected over a distance of 12 km along the recrystallization front, our study confirms that the front is a relatively sharp (Ζ400 m) geochemical discontinuity at the scale of the Ronda massif. Compared with the spinel tectonites, the coarse-granular peridotites are more homogeneous, more refractory in terms of major elements and more

Petrogenesis of Migmatites in Maine, USA: Possible Source of Peraluminous Leucogranite in Plutons?

Abstract: high-T–low-P facies series conditions during deformation within a Devonian crustal-scale shear zone system, defined by kilometer-scale straight belts of apparent flattening strain that anastomose around lozenges of apparent constrictional strain. At upper amphibolite facies grade, metapelites are partially melted, the onset of which is INTRODUCTION

Plume–Lithosphere Interactions in the Generation of the Basalts of the Kenya Rift, East Africa

Abstract: Major and trace element and Sr–Nd–Pb isotopic data for mafic volcanic rocks are used to assess the number of mantle plumes contributing to the Tertiary–Holocene magmatism of the Kenya Rift Valley, current estimates of which vary from none to three. Rocks ranging in composition from nephelinite to hypersthene-normative basalt have been sampled from three lithospheric zones: the Tanzanian craton, the craton margin reworked during the late Proterozoic, and the Mozambique mobile belt. The magmas are interpreted as the products of variable degrees of partial melting within the spinel–garnet peridotite transition zone. Trace element and isotopic compositions from all three zones are broadly similar to those of oceanic island basalts, but there is considerable compositional variation, which is related to a strong overprint from the lithosphere on plume-derived melts. Sr and Nd isotopic ratios provide the only clear distinction between magmatic rocks from the three lithospheric domains. Within each setting, mafic magmatism has tended to become less silica undersaturated with time, and at any one locality magmatism has migrated towards the centre of the rift. Magmas may have formed as a result of the infiltration of plume-derived melts into the base of the lithosphere. The extent of interaction of inferred plume melts with the lithosphere has not varied systematically in time or space. The plume component appears to be similar to the source of oceanic island basalts.

Plume–Lithosphere Interaction and the Origin of Continental Rift-related Alkaline Volcanism—the Chyulu Hills Volcanic Province, Southern Kenya

Abstract: Geochemical data are presented for primitive alkaline lavas from INTRODUCTION the Chyulu Hills Volcanic Province of southern Kenya, situated The geochemistry of mafic, alkaline volcanic rocks erupsome 100 km east of the Kenya Rift Valley. In addition to their ted in continental areas can potentially yield valuable primitive compositions, a striking and ubiquitous feature is a strong information about the nature of large parts of the Earth’s but variable depletion in K relative to other highly incompatible interior that are otherwise inaccessible. In combination elements when normalized to primitive mantle values. Semi-quanwith geophysical evidence and clues from mantle-derived titative models are developed that best explain the petrogenesis of xenoliths, such geochemical data hold the key to underthese lavas in terms of partial melting of a source that contained standing the composition and evolution of the Earth’s residual amphibole (but not phlogopite). The presence of amphibole mantle. The purpose of this contribution is to investigate implies a source in the subcontinental lithosphere rather than the aspects of the geochemistry and mineralogy of the mantle asthenosphere. It is suggested that the amphibole is of metasomatic source region of the relatively primitive, rift-related, origin and was precipitated in the lithospheric mantle by infiltrating alkaline lavas of the Quaternary Chyulu Hills Volcanic fluids and/or melts derived from rising mantle plume material. A Province (CHVP) of southern Kenya (Fig. 1). raised geotherm as a consequence of the continued ascent of the The voluminous alkaline magmatism commonly asplume material led to dehydration melting of the metasomatized sociated with continental rifting, of which the East African mantle and generation of the Chyulu Hills lavas. It is proposed Rift System (Fig. 1) is a type example, has been the that the Chyulu Hills Volcanic Province represents an analogue for subject of geochemical investigations for decades (e.g. the earliest stages of continental rift initiation, during which Williams, 1970; Baker et al., 1971; Baker, 1987). One interaction between a plume and initially refractory lithosphere may problem frequently addressed by recent studies is the lead to the generation of lithospheric melts. identification of the source regions of primitive, mafic, rift-related magmas. A primary concern is centred around whether they are derived from the subcontinental lithospheric mantle (SCLM) or from sublithospheric sources in the asthenosphere or in mantle plumes. Some workers have argued that the large volumes of lava

Metasomatism and Melting in Carbonated Peridotite Xenoliths from the Mantle Wedge: The Gobernador Gregores Case (Southern Patagonia)

Abstract: Fil: Laurora, Angela. Dipartamento Di Scienze Della Terra, Universita Di Modena e Reggio Emilia, Modena ; Italia

Almandine garnet in calc-alkaline volcanic rocks of the northern Pannonian Basin (eastern-central Europe): Geochemistry, petrogenesis and geodynamic implications

Abstract: of the Pannonian Basin and the tensional stress field may have Almandine garnet-bearing andesites and dacites occur frequently in enhanced their fast ascent from lower-crustal depths, allowing the Neogene calc-alkaline volcanic series of the northern Pannonian preservation of early-formed almandine phenocrysts. Basin (Hungary and Slovakia). They were erupted during the early stage of volcanism and occur along major tectonic lineaments. On the basis of petrographic and geochemical characteristics, garnets from these rock types are classified into (1) primary phases,

Melt Generation at Very Slow-Spreading Oceanic Ridges: Constraints from Geochemical and Geophysical Data

Abstract: We show that there is a strong and consistent correlation between geochemical and geophysical estimates of the amount of melt generated in the mantle beneath oceanic ridges. This correlation holds across all spreading rates and on scales down to the size of individual ridge segments. There is an abrupt decrease in the amount of melt generated at full spreading rates below ~20 mm/a. Our observations are consistent with the conclusion that <10% of the melt is frozen in the mantle before it reaches the crust and that serpentine probably represents only a small percentage of the material above the Moho. The melt is well mixed on a ridge segment scale, probably in high level magma chambers, but the melts remain distinct between segments. The rare earth element concentrations of basalts from very slow-spreading ridges are higher than those from normal oceanic ridges, which is directly indicative of reduced mantle melting, and they show characteristic light rare earth element enrichment, interpreted as caused by a deep tail of small percentage wet melting. The decrease in melt production at rates below ~20 mm/a points to the importance of conductive cooling inhibiting melting of the upwelling mantle at very slow-spreading centres.

Formation of Diatexite Migmatite and Granite Magma during Anatexis of Semi-pelitic Metasedimentary Rocks: an Example from St. Malo, France

Abstract: Petrological and geochemical variations are used to investigate the formation of granite magma from diatexite migmatites derived from metasedimentary rocks of pelitic to greywacke composition at St. Malo, France. Anatexis occurred at relatively low temperatures and pressures (<800°C, 4–7 kbar), principally through muscovite dehydration melting. Biotite remained stable and serves as a tracer for the solid fraction during melt segregation. The degree of partial melting, calculated from modal mineralogy and reaction stoichiometry, was <40 vol. %. There is a continuous variation in texture, mineralogy and chemical composition in the diatexite migmatites. Mesocratic diatexite formed when metasedimentary rocks melted sufficiently to undergo bulk flow or magma flow, but did not experience significant melt–residuum separation. Mesocratic diatexite that underwent melt segregation during flow generated (1) melanocratic diatexites at the places where the melt fraction was removed, leaving behind a biotite and plagioclase residuum (enriched in TiO 2 , FeO T , MgO, CaO, Sc, Ni, Cr, V, Zr, Hf, Th, U and REE), and (2) a complementary leucocratic diatexite (enriched in SiO 2 , K 2 O and Rb) where the melt fraction accumulated. Leucocratic diatexite still contained 5–15 vol. % residual biotite (mg-number 40–44) and 10–20 vol. % residual plagioclase (An 22 ). Anatectic granite magma developed from the leucodiatexite, first by further melt–residuum separation, then through fractional crystallization. Most biotite in the anatectic granite is magmatic (mg-number 18–22).

Phase Relations in the Fe–Ni–Cu–PGE–S System at Magmatic Temperature and Application to Massive Sulphide Ores of the Sudbury Igneous Complex

Abstract: fractionation temperatures, and magmatic fractionation paths of Experiments in the Fe–Ni–Cu–S system were performed to identify these deposits. the role of the metal/S atomic ratio on monosulphide–melt partition coefficients and closed-system fractionation paths. In accord with previous work, DCu is >0·2 at all temperatures and all metal/S ratios. DNi is highly sensitive to temperature and metal/S, and

The Palaeoproterozoic Komatiite-Picrite Association of Finnish Lapland

Abstract: The large range of chemical variation within intimately associated highly magnesian volcanic rocks in the Palaeoproterozoic Central Lapland Greenstone Belt prompted the construction of a new classification scheme for MgO-rich volcanic rocks, based on an [Al 2 O 3 ] vs [TiO 2 ] diagram where the axes are the Al 2 O 3 and TiO 2 contents (in mole proportions) of the rocks projected from the olivine composition. This diagram places the Lapland rocks in the fields of Ti-enriched komatiites and picrites. Komatiitic rocks are depleted in both light and heavy rare earth elements (LREE and HREE) relative to middle REE (MREE) and possess relatively high TiO 2 even in the most LREE-depleted varieties, whereas picritic rocks approach geochemically Hawaiian picrites. Seven clinopyroxene and whole-rock pairs analysed for Sm–Nd isotopes yield an average age of 2056 ± 25 Ma for the komatiites. Uncontaminated komatiites and picrites have similar positive e Nd values (+4) indicating generation from a mantle source with a long-term depletion in LREE relative to MREE. Geochemical characteristics of the komatiite–picrite association, including REE and Nb/Y–Zr/Y systematics, indicate chemical heterogeneities in the source region, which seem to have been created by complex depletion and enrichment processes shortly before or related to a dynamic melting process. The high MgO contents of the rocks coupled with chemical similarity between the Lapland and Hawaiian picrites supports a mantle plume model for their genesis. Nevertheless, the geotectonic evolution appears to have proceeded without significant regional uplift shortly before volcanism.