Showing papers in "Journal of Petrology in 2000"

Episodic Silicic Volcanism in Patagonia and the Antarctic Peninsula: Chronology of Magmatism Associated with the Break-up of Gondwana

Abstract: New SHRIMP U–Pb zircon, Rb–Sr whole-rock, and 40Ar–39Ar data are presented for the Jurassic silicic volcanic rocks and related granitoids of Patagonia and the Antarctic Peninsula. U–Pb is the only reliable method for dating crystallization in these rocks; Rb–Sr is prone to hydrothermal resetting and Ar–Ar is additionally affected by initial excess 40Ar. Volcanism spanned more than 30 My, but three episodes are defined on the basis of peak activity: V1 (188–178 Ma), V2 (172–162 Ma) and V3 (157–153 Ma). The first essentially coincides with the Karoo–Ferrar mafic magmatism of South Africa, Antarctica and Tasmania. The silicic products of V1 are lower-crustal melts that have incorporated upper-crustal material. The geochemistry of V2 and V3 ignimbrites is more characteristic of destructive plate margins, but the presence of inherited zircon still points to a crustal source. The pattern of volcanism corresponds in space and in time to migration away from the Karoo mantle plume towards the proto-Pacific margin of Gondwana during rifting and break-up. The heat required to initiate bulk crustal fusion may have been supplied by the spreading plume-head, but thinning of the crust during continental dispersion would also have facilitated anatexis.

IUGS Reclassification of the High-Mg and Picritic Volcanic Rocks

Abstract: The 1989 IUGS classification of the igneous rocks for the high-Mg and picritic volcanic rocks has been revised. Instead of an 18 wt % MgO minimum limit being applied for all high-Mg and picritic volcanic rocks, that is now applicable only to the high-Mg rocks such as komatiite and meimechite. The minimum MgO requirement for picrite is reduced to 12 wt %. The SiO2 former boundary figure between boninite and komatiite–meimechite–picrite, which was 53 wt %, is reduced to 52 wt %, and the total alkali content for komatiite and meimechite is increased to 2% and for picrite to 3%.

Remobilization of Andesite Magma by Intrusion of Mafic Magma at the Soufriere Hills Volcano, Montserrat, West Indies

Abstract: The 1995–1999 eruption of the Soufriere Hills volcano, MontINTRODUCTION serrat, has produced a crystal-rich andesite containing quenchThe Soufriere Hills volcano, Montserrat, located in the textured mafic inclusions, which show evidence of having been molten Lesser Antilles island arc in the West Indies, began to when incorporated into the host magma. Individual crystals in the erupt on 18 July 1995, after about 350 years of dormancy andesite record diverse histories. Amphibole phenocrysts vary from (Young et al., 1998b). The eruption was preceded by a pristine and unaltered to strongly oxidized and pseudomorphed by period of seismic swarms, which began in January 1992. anhydrous reaction products. Plagioclase phenocrysts are commonly Previous earthquake crises have occurred on Montserrat reverse zoned, often with dusty sieve textures. Reverse zoned rims are in 1896–1897, 1932–1935 and 1966–1967 (Powell, 1938; also common on orthopyroxene phenocrysts. Pyroxene geothermometry Shepherd et al., 1971). Initial phreatic and phregives an average temperature of 858 ± 20 °C for orthopyroxene atomagmatic activity was followed by extrusion of an phenocryst cores, whereas reverse zoned rims record temperatures andesitic lava dome in November 1995. Pyroclastic flows, from about 880 to 1050 °C. The heterogeneity in mineral rim generated by gravitational collapse of the unstable dome, compositions, zoning patterns and textures is interpreted as reflecting have been the dominant form of activity throughout the non-uniform reheating and remobilization of the resident magma eruption, but periods of explosive activity, producing body by intrusion of hotter mafic magma. Convective remobilization pumice flows and falls, have also occurred (Robertson et results in mixing together of phenocrysts that have experienced al., 1998; Young et al., 1998b). Dome growth ceased different thermal histories, depending on proximity to the intruding suddenly in March 1998 and further activity up to May mafic magma. The low temperature and high crystallinity are 1999 has mainly involved gravitational collapses of the interpreted as reflecting the presence of a cool, highly crystalline dome and periods of ash venting, with some small-scale magma body beneath the Soufriere Hills volcano. The petrological vulcanian explosive events. observations, in combination with data on seismicity, extrusion rate The Soufriere Hills volcano is the only active centre and SO2 fluxes, indicate that the current eruption was triggered by on Montserrat. The volcano is a composite of at least recent influx of hot mafic magma. five andesitic lava domes, flanked by pyroclastic deposits. Rea (1974) identified four other volcanic centres, all

The Archaean High-Mg Diorite Suite: Links to Tonalite–Trondhjemite–Granodiorite Magmatism and Implications for Early Archaean Crustal Growth

Abstract: appears to require a subduction environment, many Archaean TTG The 2·95 Ga Pilbara high-Mg diorite suite intrudes the central suites show no clear chemical evidence of having interacted with a part of the Archaean granite–greenstone terrain of the Pilbara mantle wedge, and on that basis are more likely to represent partial Craton, Western Australia, and shows many features typical of melts of basaltic lower crust rather than of subducted slab. Highhigh-Mg diorite (sanukitoid) suites from other late Archaean terrains. Mg diorite suites appear to concentrate in the Late Archaean, Such suites form a minor component of Archaean felsic crust. They suggesting that subduction may have become an important process are typically emplaced in lateto post-kinematic settings, sometimes only after >3·0 Ga. in association with felsic alkaline magmatism, and are either unaccompanied by, or post-date, tonalite–trondhjemite–granodiorite (TTG) magmatism, which comprises a much greater proportion of Archaean felsic crust. The TTG series comprises sodic, Sr-rich

Melting of Refractory Mantle at 1·5, 2 and 2·5 GPa under Anhydrous and H2O-undersaturated Conditions: Implications for the Petrogenesis of High-Ca Boninites and the Influence of Subduction Components on Mantle Melting

Abstract: GPa and in the presence of 2‐3 wt % H2O. The eVect of 2‐3 wt % dissolved H2O produces a liquidus depression in primary olivine liquidus depression (∞ C) = 74·403 · (H2 Ow t %) 0 · 352 . boninite of >112 ‐ 19∞C at a given temperature. The H2O- The equation that describes this empirical relationship is non-linear bearing melts, recalculated to 100 wt % anhydrous, are >2‐6 with an error of >9 relative percent. wt % higher in MgO, >1‐2 wt % higher in SiO2 and >1‐1·5 wt % lower in FeO, compared with nominally anhydrous melts at the same P and T. These diVerences are consistent with a change in the melting reaction, resulting in a higher contribution of or- KEY WORDS: high-Ca boninites; mantle melting; H2O-undersaturated melting; olivine liquidus depression; anhydrous melting thopyroxene to the melt phase, compared with anhydrous conditions.

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

Abstract: characteristics are not as useful as those of other REE-rich accessory Numerical reconstructions of processes that may have operated during minerals as a petrogenetic indicator. igneous petrogenesis often model the behaviour of important trace elements. The geochemistry of these trace elements may be controlled by accessory mineral saturation and fractionation. Determination

Chromite in Komatiites, II. Modification during Greenschist to Mid-Amphibolite Facies Metamorphism

Abstract: Chromite compositions in komatiites are influenced by metamorphic chromite is highly susceptible to modification during processes, particularly above 500°C. Metamorphosed chromite is early hydrous alteration and subsequent prograde metasubstantially more iron rich than igneous precursors, as a result of morphism of host rocks. This modification is the subject Mg–Fe exchange with silicates and carbonates. Chromite metaof this paper. morphosed to amphibolite facies is enriched in Zn and Fe, and Metamorphic modification of chromite has been disdepleted in Ni, relative to lower metamorphic grades. Relative cussed extensively in the literature in the context mainly of proportions of the trivalent ions Cr, Al and Fe are not ophiolitic or ‘alpine ultramafic’ complexes (Onyeagocha, greatly modified by metamorphism up to lower amphibolite facies, 1974; Ulmer, 1974; Evans & Frost, 1975; Hoffman & although minor Fe depletion occurs during talc–carbonate alWalker, 1978; Loferski & Lipin, 1983; Kimball, 1990; teration at low temperature. Significant Al is lost from chromite Burkhard, 1993) and in a few studies of komatiitic rocks cores above 550°C, as a result of equilibration with fluids in (Bliss & MacLean, 1975; Donaldson, 1983; Gole & Hill, equilibrium with chlorite. Elevated Zn content in chromite is 1990), and in a detailed study of the Pechenga intrusions restricted to rocks with low (metamorphic) Mg/Fe ratios, and is (Abzalov, 1998). These studies have highlighted two the result of introduction of Zn during low-temperature alteration, important effects. First, chromites become rimmed and with further concentration and homogenization during prograde progressively replaced by chromian magnetite or ‘fermetamorphism. Cobalt and Mn also behave similarly, except where ritchromit’. Second, chromite core compositions become carbonate minerals are predominant in the metamorphic assemblage. progressively modified during prograde metamorphism Chromite at amphibolite facies is typically extensively replaced by as a result of exchange of components with surrounding magnetite. This is the result of incomplete metamorphic reaction silicate minerals (Evans & Frost, 1975; Abzalov, 1998). between chromite and chlorite-bearing silicate assemblages. MagThis paper examines the nature and magnitude of these netite compositions at the inner chromite–magnetite boundary are effects in komatiites from a variety of localities in the indicators of metamorphic grade. Eastern Goldfields Province of the Archaean Yilgarn Block in Western Australia (Fig. 1). Brief locality descriptions are given in Appendix B.

Oxygen Isotope Geochemistry of Oceanic-Arc Lavas

Abstract: Variations of oxygen isotope ratios in arc-related lavas can constrain the contributions of subducted crustal igneous rocks, sediments, and fluids to the sub-arc mantle. We have measured oxygen isotope ratios in 72 arc and back-arc lavas from five ocean–ocean subduction zone systems using laser-fluorination analyses of olivine and other phenocrysts and glass. Eighty percent of our samples have {delta}18O values for any given phase (olivine, plagioclase, glass, or biotite) within 0·2{per thousand} of the average value for that phase in upper-mantle peridotites and mid-ocean ridge basalt (MORB); the range for each phase is <=1·0{per thousand}. This result contrasts with previous studies of whole-rock samples (which are significantly more variable even after exclusion of samples believed to be altered or fractionated by magmatic differentiation) and demonstrates that most arc-related lavas contain <=1–2% of 18O-enriched crustal oxygen from any source (i.e. assimilation or subducted contributions). Elevations in {delta}18O that do occur in these basic, arc-derived magmas relative to values most common for mantle-derived lavas are associated both with ‘enriched’ radiogenic isotope signatures and, even more strongly, with chemical indices consistent with high integrated extents of melting of their peridotite sources. We interpret these relationships as evidence that melting in the sources of the high-{delta}18O lavas we have studied was fluxed by addition of high-{delta}18O aqueous fluid (or perhaps a hydrous melt) from the subducted slab, such that sources that contain relatively large components of slab-derived fluid or melt are both relatively 18O enriched and also experienced relatively large amounts of melting. We have developed a quantitative model linking the amount of melting to the extents of 18O, radiogenic isotope, and trace-element enrichment in a mantle source being fluxed by addition of aqueous fluid. Comparison of this model with observed variations in the geochemistry of lavas from the Vanuatu–Fiji–New Caledonia region (the suite of related samples showing the greatest range in {delta}18O observed in this study) constrains the amounts and chemical and isotopic compositions of slab-derived phases in the sources of these arc-related lavas. Assuming a {delta}18O value of 20{per thousand} for the slab-derived fluid, 0·5–1·0 wt % is added to the sources of most mantle-derived arc magmas; the maximum amount of slab-derived flux in the sources of arc magmas according to our results is 2·5 wt %.

Magma Mixing, Recharge and Eruption Histories Recorded in Plagioclase Phenocrysts from El Chichón Volcano, Mexico

Abstract: operative throughout the 200 ky history of the El Chichon magma Consistent core-to-rim decreases of Sr/Sr ratios and coincident system. increases in Sr concentrations in plagioclase phenocrysts of varying size (>1 cm to 2 mm) are reported from samples of the 1982 and pre-1982 (>200 ka) eruptions of El Chichon Volcano. Maximum Sr/Sr ratios of >0·7054, significantly higher than the wholerock isotopic ratios (>0·7040–0·7045), are found in the cores

The Cameroon Volcanic Line Revisited: Petrogenesis of Continental Basaltic Magmas from Lithospheric and Asthenospheric Mantle Sources

Abstract: from variable amounts of mixing between melts derived from an The volcanic activity of Mts Bambouto and Oku (Western Highlands) and of the Ngaoundere Plateau, in the continental sector of anhydrous lherzolite source (asthenospheric component) and melts the Cameroon Volcanic Line, Equatorial West Africa, ranges in from an amphibole-bearing peridotite source (lithospheric HSr age from Oligocene to Recent. It is characterized by basanitic, alkali component). New Ar/Ar ages for Mts Oku and Bambouto basaltic and transitional basaltic series. Mineral chemistry, major basalts, combined with previous Ar/Ar and K/Ar ages of and trace element bulk-rock compositions, and geochemical modelling basaltic and silicic volcanics, and with volcanic stratigraphy, suggest suggest that the magmatic series evolved mainly at low pressure a NE–SW younging of the peak magmatic activity in the Western (2–4 kbar) through fractional crystallization of clinopyroxene and Highlands. This SW younging trend, extending from the Oligocene olivine±magnetite, at moderately hydrated (H2O= 0·5–1 wt %) volcanism in northern Cameroon (e.g. Mt Oku) to the still active and QFM (quartz–fayalite–magnetite) to QFM + 1 fO2 conMt Cameroon, suggests that the African plate is moving above a ditions. Basalts from Ngaoundere (Miocene to Quaternary) and deep-seated mantle thermal anomaly. However, the age and location from the early activity (31–14 Ma) of the Western Highlands have of the Ngaoundere volcanism does not conform to the NE–SW incompatible trace element and Sr–Nd isotopic compositions similar younging trend, implying that the continental sector of the Cameroon to those of oceanic Cameroon Line basalts, pointing to a similar Volcanic Line cannot be easily interpreted as the surface expression asthenospheric mantle source. By contrast, the late (15–4 Ma) of a single hotspot system. Western Highlands basanites and alkali basalts have anomalously high concentrations of Sr, Ba and P, and low concentrations of Zr, which are exclusive features of continental Cameroon basalts. The genesis of these latter magmas is consistent with derivation from an incompatible element enriched, amphibole-bearing lithospheric

Genesis of Young Lithospheric Mantle in Southeastern China: an LAM–ICPMS Trace Element Study

Abstract: depleted compositions occur in the upper part of the lithospheric Geological and geophysical evidence indicates that at least 100 km mantle, which now is >100 km thick. Garnet peridotites are of Archaean to Proterozoic lithospheric mantle has been removed essentially undepleted, and Y–Ga–Zr relationships of the garnets from beneath large areas of eastern and southeastern China during are typical of Phanerozoic mantle. The overall highly fertile nature late Mesozoic to Cenozoic time. Mantle-derived xenoliths in Tertiary of the existing lithosphere requires that the Archaean and Proterozoic basalts from several localities across this region have been studied mantle that existed beneath the region in Palaeozoic times has been by X-ray fluorescence, electron microprobe and laser ablation microlargely or completely removed, and replaced by younger, hotter and probe–inductively coupled plasma-mass spectrometry to characterize more fertile material. This probably occurred by upwelling of this thinner lithosphere. Trace element patterns of clinopyroxenes in asthenospheric material during late Mesozoic to Cenozoic time, the peridotites from southeastern China can be divided into four underplating to form new lithosphere. The occurrence of rare depleted groups: fertile garnet lherzolites, fertile spinel (± garnet) lherzolites, xenoliths may show that some older mantle material is residual and and depleted and enriched peridotites. The addition of Nb, Sr, light coexists with younger material beneath southeastern China. rare earth elements, but not of Ti and Zr, suggests a metasomatizing agent containing both H2O and CO2. This study also demonstrates that the negative Ti anomaly commonly observed in clinopyroxene from mantle peridotites cannot be balanced by the Ti in coexisting orthopyroxene, but can be explained by small degrees of partial

Noble Metal Enrichment Processes in the Merensky Reef, Bushveld Complex

Abstract: We have analysed sulphides, silicates, and chromites of the Merensky INTRODUCTION Reef for platinum-group elements (PGEs), Re and Au using laser The enrichment of noble metals in layered gabbroic ablation-inductively coupled plasma mass spectrometry and synthetic intrusions to form stratiform Merensky-type ore horizons pyrrhotite standards annealed with known quantities of noble metals. is an extremely efficient yet poorly understood process. Os, Ir and Ru reside in solid solution in pyrrhotite and pentlandite, Metal enrichment must be related to the formation of Rh and part of the Reef ’s Pd in pentlandite, whereas Pt, Au, Re magmatic layering, as the ore horizons form an integral and some Pd form discrete phases. Olivine and chromite, often part of the magmatic stratigraphies of their host insuspected to carry Os, Ir and Ru, are PGE free. All phases analysed trusions. Metal concentration is also related to the prescontain noble metals as discrete micro-inclusions with diameters ence of magmatic sulphides, because the noble metals are either dissolved in base metal sulphides or occur as typically <100 nm. Inclusions in sulphides commonly have the discrete noble mineral phases intergrown with sulphides element combinations Os–Ir–Pt and Pt–Pd–Au. Inclusions in (Vermaak & Hendriks, 1976; Ballhaus & Ryan, 1995; olivine and chromite are dominated by Pt± Au–Pd. Few inclusion Ballhaus & Ulmer, 1995). spectra can be related to discrete noble metal phases, and few There is considerable controversy as to where the inclusions have formed by sub-solidus exsolution. Rather, some metals came from and how metal enrichment worked. PGE inclusions, notably those in olivine and chromite, are earlyFor a stratiform platinum-group element (PGE) deposit magmatic nuggets trapped when their host phases crystallized. We inside the magmatic stratigraphy of a layered intrusion, suggest that the silicate melt layer that preceded the Merensky Reef there are two possibilities: either the metals were derived was PGE oversaturated at early cumulus times. Experiments from the melt column above the ore horizon, or they came combined with available sulphide–silicate partition coefficients sugfrom the magma equivalent to the cumulate package gest that a silicate melt in equilibrium with a sulphide melt underlying the ore. Owing to the large density contrast containing the PGE spectrum of the Merensky ore would indeed be between silicate and sulphide melt, a popular view is that oversaturated with respect to the least soluble noble metals. Sulphide the metals were enriched by gravitational settling of melt apparently played little role in enriching the noble metals in sulphide melt, i.e. out of the magma column above the the Merensky Reef; rather, its role was to immobilize a pre-existing ore (Campbell et al., 1983; Barnes & Naldrett, 1985; in situ stratiform PGE anomaly in the liquid-stratified magma Naldrett et al., 1987). On the other hand, it is well chamber. established that gravitational crystal settling plays little role in producing magmatic layering and magmatic stratigraphies (McBirney & Noyes, 1979; Morse, 1986; Campbell, 1987), and this cannot remain without consequence for stratiform noble metal deposits. A third alternative is noble metal enrichment by late

Time Scales of Crystal Fractionation in Magma Chambers—Integrating Physical, Isotopic and Geochemical Perspectives

Abstract: A simple heat balance model for an evolving magma chamber is used to make predictions of the time scales for magma differentiation, which are compared with geological and isotopic constraints on the rates of crystallization and differentiation. In a 10 km3 magma chamber releasing thermal energy at a rate of 100 MW, basalt and rhyolite magmas should reach 50% crystallization after 2500 and <1500 years, respectively. The tendency for phenocrysts to remain suspended in a cooling magma increases with melt viscosity and hence the degree of magma differentiation. The time scales of crystallization and differentiation may be estimated on the basis of the U-series isotope compositions of separated crystals and bulk rocks, crystal size distributions, and trace element and Sr isotope profiles in phenocryst phases. The last of these indicate crystal residence ages in the melt of tens of years up to 100 years. Short residence and even differentiation times are also obtained from Ra–Th isotope studies of alkali feldspars, and rocks that experienced alkali feldspar fractionation. However, much older ages of 103–105 years for separated phases have been reported from recent volcanic centres in St Vincent in the Lesser Antilles, Vesuvius in Italy, the Kenya Rift Valley and Long Valley, California. These old crystal ages are all from relatively evolved igneous rocks, as predicted from their higher melt viscosities and simple models of cooling and crystal settling. However, the old ages are also typically obtained for complex minerals that are not in bulk equilibrium with their host rocks, and so, apart from offering a minimum estimate of the age of the particular magmatic system, their significance for models of differentiation of the host magmas is not clear. An alternative approach is therefore to determine the variations in U–Th–Ra isotope compositions of bulk rocks reflecting different degrees of magma differentiation, and such data indicate that differentiation in more mafic magmas takes much longer than in more evolved magmas. For example, 50% fractional crystallization of basanite to produce phonolite on Tenerife took 105 years, whereas a further 50% fractional crystallization to generate the more evolved phonolites occurred within a few hundred years of eruption. On Tenerife the more mafic magmas fractionated at greater depths, and the rates of fractional crystallization were higher in the more evolved magmas studied. This is readily explained by a cooling model in which a large volume of primitive magma deep in the crust has a longer cooling time than a smaller body of differentiated magma at shallower depths in the crust.

Trace Element Residence and Partitioning in Mantle Xenoliths Metasomatized by Highly Alkaline, Silicate- and Carbonate-rich Melts (Kerguelen Islands, Indian Ocean)

Abstract: Mantle xenoliths in alkaline lavas of the Kerguelen Islands consist types (e.g. Nixon, 1987; O’Reilly & Griffin, 1996). of: (1) protogranular, Cr-diopside-bearing harzburgite; (2) poikilitic, Studies of upper-mantle xenoliths in alkali basalts, Mg-augite-bearing harzburgite and cpx-poor lherzolite; (3) dunite kimberlites, lamproites and carbonatites have improved that contains clinopyroxene, spinel phlogopite, and rarely amphibole. our understanding of materials and processes involved Trace element data for rocks and minerals identify distinctive in the geochemical evolution of the mantle (e.g. Downes signatures for the different rock types and record upper-mantle & Dupuy, 1987; Ionov et al., 1993; Chalot-Prat processes. The harzburgites reflect an initial partial melting event & Boullier, 1997). The variation and magnitude of followed by metasomatism by mafic alkaline to carbonatitic melts. geochemical heterogeneities in the lithospheric mantle The dunites were first formed by reaction of a harzburgite protolith reflect the composition of mantle melts and fluids and with tholeiitic to transitional basaltic melts, and subsequently the efficiency of heat and mass transfer. Mantle plumes developed metasomatic assemblages of clinopyroxene + phlogopite are important for initiating such transfer processes. ± amphibole by reaction with lamprophyric or carbonatitic melts. The Kerguelen plume is remarkable because of its We measured two-mineral partition coefficients and calculated volume, the persistence of volcanic activity for at least mineral–melt partition coefficients for 27 trace elements. In most 115 My, and its migration across diverse geotectonic samples, calculated budgets indicate that trace elements reside in environments through time as a result of spreading of the constituent minerals. Clinopyroxene is the major host for REE, the Indian Ocean (Weis et al., 1992). Sr, Y, Zr and Th; spinel is important for V and Ti; orthopyroxene In this paper we report a trace element study of bulk for Ti, Zr, HREE, Y, Sc and V; and olivine for Ni, Co and Sc. rocks and constituent minerals of clinopyroxene-bearing

Cooling History and Exhumation of Lower-Crustal Granulite and Upper Mantle (Malenco, Eastern Central Alps)

Abstract: ago during Early Permian lithospheric extension The intrusion rocks Estimated uplift rates of 1‐2 mm/year indicate a 15‐30 My took place along the crust‐mantle transition zone and caused exhumation related to Jurassic rifting The two-stage retrograde granulite metamorphism of lower-crustal and upper-mantle rocks path of the Malenco granulites separated by >50 My suggests that The magmatic crystallization of the gabbro was outlasted by ductile Permian extension and Jurassic rifting are two independent tectonic deformation, which is also observed in the other rocks of the processes The presence of hydrous, Cl-rich minerals at 600 ‐ crust‐mantle transition Two stages of retrograde metamorphism 50∞C and 0·8 ‐ 0·1 GPa requires input of externally derived followed Mineral parageneses in garnet‐kyanite gneiss, metagabbro, fluids at the base of 30 km thick continental crust into previously and metaperidotite record a first stage of near-isobaric cooling under dry granulites at the onset of Jurassic rifting These fluids were anhydrous conditions The stabilized crust‐mantle transition then generated by dehydration of continental crust juxtaposed during persisted over a period of about 50 My into the Late Triassic rifting with the hot, exhuming granulite complex along a active Exhumation of the crust‐mantle complex began with the onset of shear zone continental rifting during Early Jurassic This stage of retrograde metamorphism is recorded by near-isothermal decompression and partial hydration of the granulitic mineral assemblages The whole crust-to-mantle complex was then exposed in the Tethyan ocean KEY WORDS: subcontinental lithospheric mantle; granulite; continental near its Adriatic margin The magmatic assemblage of the Braccia rifting; retrograde metamorphism; Malenco

Primitive Magma From the Jericho Pipe, N.W.T., Canada: Constraints on Primary Kimberlite Melt Chemistry

Abstract: We report the first estimates of primary kimberlite melt composition INTRODUCTION from the Slave craton, based on samples of aphanitic kimberlite The chemical composition and physical character of from the Jericho kimberlite pipe, N.W.T., Canada. Three samples primary kimberlite magma remains enigmatic because derive from the margins of dykes where kimberlite chilled against of the difficulty of isolating material that unambiguously wall rock ( JD51, JD69 and JD82) and are shown to be texturally represents the melt phase (Foley, 1990; Scott Smith, consistent with crystallization from a melt. Samples JD69 and 1996). There are, for example, no occurrences of JD82 have geochemical characteristics of primitive melts: they have quenched ‘glassy’ kimberlite (Mitchell, 1986). The probhigh MgO (20–25 wt %), high mg-numbers (86–88), and high lem is compounded further by the hybrid nature of most Cr (1300–1900 ppm) and Ni (800–1400 ppm) contents. They kimberlites, which contain a mixture of mantle and also have high contents of CO2 (10–17 wt %). Relative to bulk crustal xenoliths, diverse numbers of large (0·5–10 mm) macrocrystal kimberlite, they have lower mg-numbers and lower phenocrysts or xenocrysts (e.g. macrocrysts), subordinate MgO but are enriched in incompatible elements (e.g. Zr, Nb and amounts of cognate microphenocrysts and groundmass Y), because the bulk kimberlite compositions are strongly controlled material (Mitchell, 1986; Foley, 1990; Scott Smith, 1996). by accumulation of mantle olivine and other macrocrysts. The In the absence of kimberlitic glasses, aphanitic samples compositions of aphanitic kimberlite from Jericho are similar to of kimberlite probably represent the next best apmelts produced experimentally by partial melting of a carbonateproximation to the melt phase. Aphanitic samples (e.g. bearing garnet lherzolite. On the basis of these experimental data, <5 vol. % macrocrysts) of kimberlite are relatively rare, we show that the primary magmas from the Jericho kimberlite could especially in hypabyssal facies kimberlite (Scott Smith, represent 0·7–0·9% melting of a carbonated lherzolitic mantle source 1996). Examples described in the literature include at pressures and temperatures found in the uppermost asthenosphere to samples from the Wesselton mine (Shee, 1986), the the Slave craton. The measured CO2 contents for samples JD69 Mayeng Kimberlite Sill Complex (Apter et al., 1984) and and JD82 are only slightly lower than the CO2 contents of the Benfontein (Mitchell, 1997) from South Africa, the Koidu corresponding experimental melts; this suggests that the earliest kimberlite complex from West Africa (Taylor et al., 1994), hypabyssal phase of the Jericho kimberlite retained most of its and the Aries kimberlite from Western Australia (Edwards original volatile content. As such these samples provide a minimum et al., 1992). Of these examples, only the Wesselton and CO2 content for the primary kimberlite magmas from the Slave Benfontein examples are Group I kimberlite; all others craton. are Group II (orangeite; Mitchell, 1995).

H2O Abundance in Depleted to Moderately Enriched Mid-ocean Ridge Magmas; Part I: Incompatible Behaviour, Implications for Mantle Storage, and Origin of Regional Variations

Abstract: The H2O contents and trace-element abundances are presented for two well-studied suites of mid-ocean ridge basalt (MORB) glasses from the Northern East Pacific Rise (EPR, 9-11°N) and the South East Indian Ridge (SEIR, 127-129°E), Exactly the same region of the glass samples has been analysed for these components using microbean techniques. Our data allow examination of the fine details of H2O geochemical behaviour during MORB genesis. We demonstrate that relative H2O contents [i.e. H2O/ (another incompatible element)] vary systematically with increasing (La/Sm)(N) in MORB glasses from both the EPR and SEIR. This indicates that H2O behaves like other incompatible (in peridotite mineralogies) elements during MORB petrogenesis, and is primarily controlled by solid-melt partitioning. However, the relative H2O contents of MORB glasses from the SEIR are higher than in glasses from the EPR at a given (La/Sm)(N), demonstrating global variations in the H2O contents of MORB. Despite regional differences in relative H2O contents, the incompatible behaviour of H2O is similar in both studied regions. The relative incompatibility of H2O varies systematically with increasing (La/Sm)(N): in depleted MORB, H2O is similar to La whereas in EMORB, H2O is similar to Ce. Similar patterns of varying relative incompatibility (to REE) are displayed by Zr, Hf, and P. Our data are best explained if H2O is stored in the mantle in the same phase with LREE (clinopyroxene?) at sub-solidus. Regional variations in relative H2O contents in EMORB that have more radiogenic Sr, Nd and Pb isotopes might be explained by differences in the nature of enriched components recycled via subduction processes. However, when EMORB have the same radiogenic isotope compositions as NMORB within a segment, relative H2O contents in EMORB probably reflect local processes that lead to enrichment in incompatible elements. Regional differences in relative H2O contents of NMORB may reflect either initial variations in the Earth's mantle or inhomogeneities left after formation of the continental crust.

Petrogenesis of Mafic to Felsic Plutonic Rock Associations: the Calc-alkaline Quérigut Complex, French Pyrenees

Abstract: show that post-collisional wrenching in this part of the Variscides The Querigut mafic–felsic rock association comprises two main was under way by 310 Ma. magma series. The first is felsic comprising a granodiorite–tonalite, a monzogranite and a biotite granite. The second is intermediate to ultramafic, forming small diorite and gabbro intrusions associated with hornblendites and olivine hornblendites. A U–Pb zircon age

Geochemistry of Flood Basalts of the Toranmal Section, Northern Deccan Traps, India: Implications for Regional Deccan Stratigraphy

Abstract: , unpublished data, 1998) andthe southeastern Deccan (particularly the Poladpur andAmbenali formations; Mitchell & Widdowson, 1991;Bilgrami, 1999) for as much as 350 km from their south-western type-sections. In addition, lavas isotopically andchemicallyequivalenttothoseoftheAmbenaliFormation(Fm)arepresentinthenortheastern sectoroftheprovinceas far as 900 km from their southwesternmost counter-parts (Mahoney, 1988; Deshmukh

Volatile Components, Magmas, and Critical Fluids in Upwelling Mantle

Abstract: The phase diagram for lherzolite–CO_2–H_2O provides a framework for interpreting the distribution of phase assemblages in the upper mantle with various thermal structures, in different tectonic settings. Experiments show that at depths >80 km, the near-solidus partial melts from lherzolite–CO_2–H_2O are dolomitic, changing through carbonate–silicate liquids with rising temperatures to mafic liquids; vapor, if it coexists, is aqueous. Experimental data from simple systems suggest that a critical end-point (K) occurs on the mantle solidus at an undetermined depth. Isobaric (T–X) phase diagrams for volatile-bearing systems with K elucidate the contrasting phase relationships for lherzolite–CO_2–H_2O at depths below and above a critical end-point, arbitrarily placed at 250 km. At levels deeper than K, lherzolite can exist with dolomitic melt, aqueous vapor, or with critical fluids varying continuously between these end-members. Analyses of fluids in microinclusions of fibrous diamonds reveal this same range of compositions, supporting the occurrence of a critical end-point. Other evidence from diamonds indicates that the minimum depth for this end-point is 125 km; maximum depth is not constrained. Constructed cross-sections showing diagrammatically the phase fields intersected by upwelling mantle indicate how rising trace melts may influence trace element concentrations within a mantle plume.

The Halogen Geochemistry of the Bushveld Complex, Republic of South Africa: Implications for Chalcophile Element Distribution in the Lower and Critical Zones

Abstract: solidification of the interstitial liquid. The stratigraphic distribution Halogen-bearing minerals, especially apatite, are minor but ubiof S, Cu and the PGE in the Critical Zone cannot readily be quitous phases throughout the Bushveld Complex. Interstitial apatite explained either by precipitation of sulfide as a cotectic phase or as is near end-member chlorapatite below the Merensky reef (Lower a function of trapped liquid abundance. Evidence from potholes and and Critical Zones) and has increasingly fluorian compositions with the PGE-rich Driekop pipe of the Bushveld Complex imply that increasing structural height above the reef (Main and Upper Zones). migrating Cl-rich fluids mobilized the base and precious metal Cl/F variations in biotite are more limited owing to crystal-chemical sulfides. We suggest that the distribution of sulfide minerals and controls on halogen substitution, but are also consistent with a the chalcophile elements in the Lower and Critical Zones reflects a decrease in the Cl/F ratio with structural height in the complex. general process of vapor refining and chromatographic separation of A detailed section of the upper Lower Zone to the Critical Zone is these elements during the evolution and migration of a metalliferous, characterized by an upward decrease in sulfide mode from 0·01– Cl-rich fluid phase. 0·1% to trace–0·001%. Cu tends to correlate with other incompatible elements in most samples, whereas the platinum-group elements (PGE) can behave independently, particularly in the Critical Zone. The decrease in the Cl/F ratio of apatite in the

Processes in High-Mg, High-T Magmas: Evidence from Olivine, Chromite and Glass in Palaeogene Picrites from West Greenland

Abstract: Uncontaminated volcanic rocks from the 60 Ma Vaigat Formation, within the crust, presumably as olivine-plated conduit walls. The conduit systems are similar to the crystal-rich narrow magma West Greenland, contain 6·5–30 wt % MgO, averaging 15·5 wt % MgO. Olivine (mg-number 77·4–93·3) forms diverse chambers suggested for mid-ocean ridges but are of much greater vertical extent. assemblages of zoned phenocrysts and xenocrysts showing evidence for equilibrium and fractional crystallization, oxidation, partial to complete re-equilibration, as well as magma mixing. The olivine crystals contain glass inclusions and have high contents of Ca and

Modelling Diverse Processes in the Petrogenesis of a Composite Batholith: the Central Bohemian Pluton, Central European Hercynides

Abstract: The multiple intrusions making up the Central Bohemian Pluton in the Central European Hercynides have petrographic and geochemical features consistent with the presence of four main granitoid suites. Major-element, trace-element and Sr-Nd isotopic compositions are used to model their petrogenesis. Partial melting of metabasic locks or of a CHUR-like mantle source are interpreted to have produced melts parental to the most primitive calc-alkaline Sazava suite. Interaction of basic with more acidic magmas followed by extensive amphibole- plagioclase-dominated fractionation accounts for the production of trondhjemites. Alternatively, the trondhjemites correspond to small-degree melts of a metabasic source. AFC (assimilation- fractional crystallization) modelling with a paragneiss as a contaminant and increasing D-Nd values simulates the characteristics of the Blatna suite. Closed-system fractionation of strongly enriched mantle-derived magmas or their interaction with leucogranitic melts is deduced for the petrogenesis of the shoshonitic Certovo bremeno suite. Partial melting of a metasedimentary source, followed by K-feldspar- dominated fractionation, accounts for the granites of rite Ricany suite. The progression from relatively primitive calc- alkaline granitoids towards evolved, K-rich calc-alkaline and shoshonitic rocks is interpreted to reflect the increasing enriched mantle input in the petrogenesis of the later suites. The evidence for Hercynian subduction is equivocal and the mantle enrichment could have been significantly older.

Silicic Magma Formation in Overthickened Crust: Melting of Charnockite and Leucogranite at 15, 20 and 25 kbar

Abstract: 50–60 vol. %, accompanied by settling of residualof crustal source rocks at depths >50 km; (2) production of syenite crystals, would lead to a magma of restricted compositional rangemagma by partial melting of quartzofeldspathic rocks at pressures with a heavy rare earth element depleted pattern. Overall, the granitic>15 kbar. Melting experiments at 15, 20 and 25 kbar were melt composition obtained in the experiments diVers from that ofperformed on Archaean biotite-bearing charnockite of opx-bearing A-type granite. However, at degrees of melting below

Petrogenesis and 40Ar/39Ar Geochronology of the Brandberg Complex, Namibia: Evidence for a Major Mantle Contribution in Metaluminous and Peralkaline Granites

Abstract: Anorogenic granites of the Brandberg igneous complex in NW peralkaline granites formed from highly evolved melts (Eu/Eu <0·1) and melt inclusion analysis from arfvedsonite pegmatite Namibia formed during early Cretaceous rifting and continental break-up of Gondwana. A metaluminous series [SiO2 = 62–77 indicate that enhanced solubilities in an F-rich peralkaline residual wt %, molar (Na + K)/Al = 0·8–0·95] includes an early melt could account for observed enrichments of high-field strength monzonite body, major biotite–hornblende granite, late biotite granite elements. Compositional variations within the peralkaline group segregations and peripheral dykes of trachydacite. Volumetrically reflect at least in part late-magmatic mineral segregation and minor peralkaline granites of the Amis complex [SiO2 = 72–77 hydrothermal overgrowth. wt %, (Na + K)/Al = 1·0–1·5] intrude the main granite and adjacent country rocks. Compared with the metaluminous main granite, these are in part highly enriched in Zr, Nb, Y, U and Th.

Geochemical Study of Ultramafic Volcanic and Plutonic Rocks from Gorgona Island, Colombia: the Plumbing System of an Oceanic Plateau

Abstract: The only known post-Archaean komatiites are found on Gorgona, INTRODUCTION a small island off the Colombian coast that forms part of the Surprisingly little is known about the internal structure Caribbean oceanic plateau. Mafic and ultramafic intrusions are and magmatic evolution of oceanic plateaux. This is due located in the interior of the island. To establish the relationship in part to the relative inaccessibility of type examples between intrusive and extrusive phases of ultramafic magmatism, such as Ontong Java and other plateaux in the Pacific, and to help understand how an oceanic plateau is constructed, we which are located far from continents, in deep waters, undertook the first petrological and geochemical study of the intrusive and often beneath a deep layer of sediments. Samples rocks. Rare earth element patterns in gabbros range from almost flat obtained by dredging or drilling the submerged portions, to moderately depleted; in dunites and wehrlites, the depletion is or through field work on the few sections exposed on more pronounced. These patterns fall midway in the range measured islands around the Ontong Java plateau consist prein Gorgona volcanics, whose compositions vary from slightly enriched dominantly of tholeiitic basalt with relatively uniform, to extremely depleted. Nd isotope compositions indicate two distinct magmatically evolved compositions (Mahoney et al., 1993; mantle sources, one highly depleted, the other less depleted. MgO Neal et al., 1997). These compositions are far removed contents of parental liquids are estimated from olivine compositions from those of the primary picritic magmas that formed at 20–25% in ultramafic lavas, and 12–13% in the intrusives. through partial melting of the mantle source at subPetrographic observations and similarities in trace-element contents lithosphere depths (Cox, 1980). The generation of basalt indicate that the two magma types are comagmatic, related through from picrite involves crystallization of large volumes olivine fractionation. Modelling of major and trace elements indicates of olivine, which presumably accumulated in magma that the primary ultramafic magmas formed by advanced critical chambers at deeper levels in the plateau. It is commonly melting at high pressure in a rising mantle plume. The plumbing assumed that these chambers are located at the base of system that fed the Gorgona plateau was complex, being characterized the crust where magma would pool as it passes from by a series of magma chambers at different crustal levels. Mantlederived ultramafic liquids either travelled directly to the surface to dense mantle peridotite to less dense basaltic rocks. This erupt as komatiite flows, or were trapped in magma chambers where assumption is supported by geophysical data—layers with they differentiated into basaltic liquid and mafic to ultramafic high seismic velocities in the lower parts of oceanic cumulates. Gorgona gabbros and peridotites formed in shallow-level plateaux are commonly equated with olivine-rich cuexamples of these intrusions. mulates (Coffin & Eldholm, 1994; Leroy & Mauffret, 1996; Mauffret & Leroy, 1997)—and from numerical modelling. Farnetani et al. (1996) have shown that the