Closed-system Fractionation Research Articles

Petrogenesis and Ni–Cu–(PGE) prospectivity of the Mount Ayliff Complex in the Karoo Igneous Province: new insights from the Ingeli and Horseshoe lobes

The Mount Ayliff Complex comprises five cognate mafic–ultramafic bodies emplaced along the southern margin of the Kaapvaal Craton. This study examines the Ingeli and Horseshoe lobes and assesses their magmatic sulfide prospectivity with respect to the Insizwa lobe, which hosts massive sulfides at its Waterfall Gorge occurrence. The Ingeli lobe consists of 465 m of olivine–chromite cumulates overlain by 340 m of (olivine-)gabbronorite, while the Horseshoe lobe consists of 45 m of olivine gabbronorite overlain by 5 m of gabbro. The Ingeli lobe possesses sparsely disseminated sulfides at its mafic–ultramafic transition, whereas disseminated sulfides are present throughout the Horseshoe lobe. A petrogenetic model is proposed, where magma accumulated and fractionated nickeliferous olivine and chromite in an upper-crustal staging chamber hosted by Proterozoic basement rocks. Magma then ascended and deposited olivine–chromite cumulates at the level of the complex. Prolonged magma flux in the staging chamber facilitated the assimilation of basement rocks, triggering sulfide saturation and the crystallization of Ni-poor olivine. Contaminated magma then ascended and interacted with pre-existing cumulates, depositing sulfide melt that may have backflowed as magmatic activity waned. Basaltic magma then flowed over the ultramafic cumulates, depositing disseminated sulfides whilst undergoing closed-system fractionation. Basal depressions and underlying feeder structures are the most prospective locations for magmatic sulfide mineralization.

Mineral chemistry of chrome-diopside bearing lamprophyre from Mesoproterozoic Settupalle igneous complex of Prakasam alkaline province: insights on the magma dynamics at shallow lithospheric mantle beneath the NE margin of the Cuddapah basin, Southern India

The Eastern Dharwar Craton shows considerable variations in the activity of kimberlite, lamproite and lamprophyre and their related lithospheric thickness, thermal structure and magmatic source characteristics from the innermost core to the margin. These variations have been revealed through seismic studies and the analysis of xenoliths, and xenocrysts carried in alkaline magmas, particularly kimberlites. This study reports the occurrence of chrome-diopside phenocrysts from a lamprophyre of the Settupalle pluton, located on the northeastern margin of the Cuddapah basin, Prakasam alkaline province in South India. Studied lamprophyre are characterized by phenocrystic clinopyroxene (Cpx) and biotite, embedded in a groundmass of amphibole, biotite, opaques, calcite, feldspar, nepheline and sodalite, with the latter minerals forming ocellar structures. Clinopyroxene displays normal zoning, with a high Mg–Cr rich diopside core (up to 1.92 wt%) and a Na 2 O-, Al 2 O 3 , FeO and TiO 2 -enriched salite rim. These chemical variations, along with elemental substitutions from core to rim, indicate the significant role of fractionation in the evolution of lamprophyre magma. The lack of chemical disparity among the studied Cpx suggests closed system fractionation, under high oxygen fugacity conditions. Thermobarometry results indicate polybaric crystallization for the Cpx (1206–1269°C, 11.82–18.04 kbar) and biotite (825–856°C, 3.49–6.94 kbar). The gradient inferred from the pressure of the Cpx suggests that its crystallization occurred at a depth of 43–66 km, implying shallow lithospheric magma contribution for the genesis of the lamprophyre magma.

Deep carbon through time: Earth’s diamond record and its implications for carbon cycling and fluid speciation in the mantle

Diamonds are unrivalled in their ability to record the mantle carbon cycle and mantle fO2 over a vast portion of Earth’s history. Diamonds’ inertness and antiquity means their carbon isotopic characteristics directly reflect their growth environment within the mantle as far back as ∼3.5 Ga. This paper reports the results of a thorough secondary ion mass spectrometry (SIMS) carbon isotope and nitrogen concentration study, carried out on fragments of 144 diamond samples from various locations, from ∼3.5 to 1.4 Ga for P [peridotitic]-type diamonds and 3.0 to 1.0 Ga for E [eclogitic]-type diamonds. The majority of the studied samples were from diamonds used to establish formation ages and thus provide a direct connection between the carbon isotope values, nitrogen contents and the formation ages. In total, 908 carbon isotope and nitrogen concentration measurements were obtained. The total δ13C data range from −17.1 to −1.9 ‰ (P = −8.4 to −1.9 ‰; E = −17.1 to −2.1‰) and N contents range from 0 to 3073 at. ppm (P = 0 to 3073 at. ppm; E = 1 to 2661 at. ppm). In general, there is no systematic variation with time in the mantle carbon isotope record since > 3 Ga. The mode in δ13C of peridotitic diamonds has been at −5 (±2) ‰ since the earliest diamond growth ∼3.5 Ga, and this mode is also observed in the eclogitic diamond record since ∼3 Ga. The skewness of eclogitic diamonds’ δ13C distributions to more negative values, which the data establishes began around 3 Ga, is also consistent through time, with no global trends apparent.No isotopic and concentration trends were recorded within individual samples, indicating that, firstly, closed system fractionation trends are rare. This implies that diamonds typically grow in systems with high excess of carbon in the fluid (i.e. relative to the mass of the growing diamond). Any minerals included into diamond during the growth process are more likely to be isotopically reset at the time of diamond formation, meaning inclusion ages would be representative of the diamond growth event irrespective of whether they are syngenetic or protogenetic. Secondly, the lack of significant variation seen in the peridotitic diamonds studied is in keeping with modeling of Rayleigh isotopic fractionation in multicomponent systems (RIFMS) during isochemical diamond precipitation in harzburgitic mantle. The RIFMS model not only showed that in water-maximum fluids at constant depths along a geotherm, fractionation can only account for variations of <1‰, but also that the principal δ13C mode of −5 ± 1‰ in the global harzburgitic diamond record occurs if the variation in fO2 is only 0.4 log units. Due to the wide age distribution of P-type diamonds, this leads to the conclusion that the speciation and oxygen fugacity of diamond forming fluids has been relatively consistent. The deep mantle has therefore generated fluids with near constant carbon speciation for 3.5 Ga.

New light on the geology of the Roundstone intrusion, its inversion and that of the Grampian metagabbro-gneiss complex, Connemara, western Ireland

The northern part of the ~470 Ma syntectonic D2 basic and ultrabasic Roundstone intrusion into the Connemara Dalradian metasediments has been mapped in detail for the first time and consists of metagabbros containing a 180m peridotite xenolith and later injections of Quartz diorite gneiss which silicified and agmatised some of the metagabbros. With the already studied southern (Errisbeg) part, plus over 300 chemical analyses of rocks and over 400 probe analyses of minerals, this enables a synthesis of the geology of the whole intrusion. A tholeiitic magma crystallised olivine Fo83-76, orthopyroxene En84-59, diopside to salite and plagioclase from An96-42, and late stage cumulus magnetite. The exceptionally high An% is indicative of high pH2O with copious water (derived from dewatering of a subduction zone?) as is the abundant hornblende that replaced most of the magmatic minerals, the magma being comparable with that of the Lesser Antilles Island arc. The intrusion was formed by numerous already fractionated magma pulses intruded in upward sequence, closely (~467 Ma) followed by pulses of Quartz diorite to granite gneisses with magmatic hornblende, not by closed system fractionation of a single magma injection. The peridotites were mostly carried in as already solidified xenoliths by pulses of gabbro magma. Only the larger bodies in the Errisbeg part of the intrusion were intruded as magma.

New light on the geology of the Roundstone intrusion, its inversion and that of the Grampian metagabbro-gneiss complex, Connemara, western Ireland

The northern part of the ~470 Ma syntectonic D2 basic and ultrabasic Roundstone intrusion into the Connemara Dalradian metasediments has been mapped in detail for the first time and consists of metagabbros containing a 180m peridotite xenolith and later injections of Quartz diorite gneiss which silicified and agmatised some of the metagabbros. With the already studied southern (Errisbeg) part, plus over 300 chemical analyses of rocks and over 400 probe analyses of minerals, this enables a synthesis of the geology of the whole intrusion. A tholeiitic magma crystallised olivine Fo83-76, orthopyroxene En84-59, diopside to salite and plagioclase from An96-42, and late stage cumulus magnetite. The exceptionally high An% is indicative of high pH2O with copious water (derived from dewatering of a subduction zone?) as is the abundant hornblende that replaced most of the magmatic minerals, the magma being comparable with that of the Lesser Antilles Island arc. The intrusion was formed by numerous already fractionated magma pulses intruded in upward sequence, closely (~467 Ma) followed by pulses of Quartz diorite to granite gneisses with magmatic hornblende, not by closed system fractionation of a single magma injection. The peridotites were mostly carried in as already solidified xenoliths by pulses of gabbro magma. Only the larger bodies in the Errisbeg part of the intrusion were intruded as magma.

Cadmium cycling in the water column of the Kuroshio-Oyashio Extension region: Insights from dissolved and particulate isotopic composition

We measured dissolved and particulate Cd isotopic composition in the water column of a meridional transect across the Kuroshio-Oyashio Extension region in a Japanese GEOTRACES cruise to investigate the relative influence of physical and biogeochemical processes on Cd cycling in the Northwestern Pacific Ocean. Located at 30–50°N along 165°E, the transect across the extension region possesses dramatic hydrographic contrast. Cold surface water and a relatively narrow and shallow thermocline characterizes the Oyashio Extension region in contrast to a relatively warm and highly stratified surface water and thermocline in the Kuroshio Extension region. The contrasting hydrographic distinction at the study site provides us with an ideal platform to investigate the spatial variations of Cd isotope fractionation systems in the ocean. Particulate samples demonstrated biologically preferential uptake of light Cd isotopes, and the fractionation effect varied dramatically in the surface water of the two regions, with relatively large fractionation factors in the Oyashio region. Based on the relationship of dissolved Cd concentrations and isotopic composition, we found that a closed system fractionation model can reasonably explain the relationship in the Kuroshio region. However, using dissolved Cd isotopic data, either a closed system or steady-state open system fractionation model may explain the relationship in the surface water of the Oyashio region. Particulate δ114/110Cd data further supports that the surface water of the Oyashio region matches a steady-state open system model more closely. Contrary to the surface water, the distribution of potential density exhibits comparable patterns with Cd elemental and isotopic composition in the thermocline and deep water in the two extension regions, showing that physical processes are the dominant forcing controlling Cd cycling in the deep waters. The results demonstrate that Cd isotope fractionation can match either a closed or open system Rayleigh fractionation model, depending on the relative contribution of physical and biogeochemical processes on its cycling.

The Skaergaard Intrusion of East Greenland: Paradigms, Problems and New Perspectives

The Skaergaard Intrusion of East Greenland is the quintessential example of low-pressure closed-system fractionation of basaltic magma. Field evidence of extensive layering and associated quasi-sedimentary structures, and the resultant ‘cumulate’ paradigm of crystal settling in magma chambers, has led to many long-standing controversies. Of particular significance is the lack of consensus about the microstructural record and the mechanisms by which interstitial liquid is expelled from solidifying crystal mushy zones. Skaergaard remains a cradle for new insights into igneous processes, with recent work highlighting the importance of separation of immiscible liquids on magma evolution.

Origin of mafic and ultramafic cumulates from the Ditrău Alkaline Massif, Romania

Mafic–ultramafic cumulates enclosed in gabbroic-dioritic rocks form part of the Mesozoic Ditrău Alkaline Massif in the Eastern Carpathians, Romania. The poikilitic olivine- and pyroxene-rich and nearly mono mineralic hornblendite rocks display typical cumulate textures with early crystallised olivine (Fo75–73), diopside and augite. In the early stages of their genesis the amphibole was intercumulus whilst in later stages it acquired cumulus status as the fractionating magma evolved. Using major and trace element compositions of minerals and whole-rock samples the origin of these cumulates is determined and the parental magma composition and depth of emplacement are calculated.Cumulus clinopyroxene has more primitive composition than intercumulus amphibole suggesting closed system fractionation for the evolution of poikilitic olivine- and pyroxene-rich cumulates. The evolution of the amphibole-rich mesocumulates is more clearly the result of closed system crystallisation dominated by the precipitation of clinopyroxene and amphibole cumulus crystals.Lamprophyre dykes of the Ditrău Alkaline Massif are proposed to reflect multiple basanitic parental magma batches from which the cumulus olivine and clinopyroxene crystallised. Relative to these dykes the calculated equilibrium melts for intercumulus amphibole in the cumulates was more primitive whilst that for the cumulus amphibole was more evolved. The calculated crystallisation temperature and pressure of ~1000–1050°C and ~0.7GPa, based on the composition of the amphiboles, indicate crystallisation at lower crustal depths. Rare earth element compositions are consistent with an intra-plate tectonic setting.

Origin of Basalts by Hybridization in Andesite-dominated Arcs

Mafic magmas are common in subduction zone settings, yet their high density restricts their ascent to the surface. Once stalled in the crust, these magmas may differentiate, and assimilate crust and other melts and crystal mushes to produce hybridized intermediate magmas. The Soufriere Hills Volcano on Montserrat is a ‘type locality’ for such hybridization processes and yet, just 3?km south of the crater, voluminous basalts have erupted from the South Soufriere Hills volcano within the same time period as the Soufriere Hills Volcano was erupting hybrid andesites (131–128?ka). Basaltic South Soufriere Hills magmas have 48–53?wt % SiO2 and 4–6?wt % MgO. They were hot (970–1160°C), volatile-rich (melt inclusions contain up to 6·2?wt % H2O) and were stored at 8–13?km depth prior to eruption (based on olivine- and pyroxene-hosted melt inclusion volatile geochemistry). Melt inclusions do not preserve basaltic liquids: they are andesitic to rhyolitic in composition, related to one another by a line of descent controlled by simple closed-system fractionation. Whole-rock compositions, however, are best described by a hybridization model involving ‘back-mixing’ of andesitic to rhyolitic melts with mafic crystal phases such as magnetite, olivine, orthopyroxene and clinopyroxene. Phenocryst zoning illustrates repeated mixing events between evolved melts and mafic phenocrysts; this feature, when coupled with the heterogeneity of crystal compositions, strongly suggests that although the bulk compositions are basaltic (containing Fo80 olivine), they were assembled from disparate ingredients, probably derived from mafic crystal mushes and more evolved melt lenses of variable composition. The mixing events occur days to weeks prior to eruption. We propose that the South Soufriere Hills basaltic magmas, with their higher bulk density relative to andesites from neighbouring volcanoes, ultimately may have been eruptible owing to both the transtensional tectonics imposed by offshore grabens (related to oblique subduction in the Lesser Antilles arc) and surface unloading caused by large-scale edifice collapse. Our observations support the idea that compositional changes in arcs might reflect not only changes in source compositions, but also effects caused by variations in crustal strain and tectonics.

Fe isotopes and the contrasting petrogenesis of A-, I- and S-type granite

Abstract We present new Fe isotope data of 42 S-, I- and A-type (ferroan) granites from the Cambrian Delamerian orogen in South Australia, the Palaeozoic Lachlan Fold Belt and Western USA. Interpretation of these data, together with modelling suggests that magmatic processes do result in quite complex Fe-isotopic differentiation trends and can lead to granites with isotopically heavy iron with δ 57 Fe > 0.35‰. By comparison Mid-Ocean Ridge Basalts (MORBs) have δ 57 Fe = 0.15‰ (Teng et al., 2013). These variations are similar to those previously reported (Poitrasson and Freydier, 2005; Heimann et al., 2008; Telus et al., 2012), but, contrary to some interpretations (Beard and Johnson, 2006; Heimann et al., 2008), heavy values are not necessarily the product of late-stage hydrothermal fluid loss, though this process is undoubtedly also an important factor in some circumstances. A-type (ferroan) granites reach very heavy δ 57 Fe values (0.4–0.5‰) whereas I-types are systematically lighter (δ 57 Fe = 0.2‰). S-type granites show a range of intermediate values, but also tend to be isotopically heavy (δ 57 Fe ≈ 0.2–0.4‰). Our results show that the iron isotopic values and trends are signatures that reflect granite generation processes . A modelling using the Rhyolite-MELTS software suggests that contrasting trajectories and end-points in Fe isotope evolution towards granite depend on: oxidation state of the evolving magma and, whether or not the system is oxygen-buffered. Iron isotopic evolution supports an origin of ferroan A-type granite from protracted, closed magma chamber fractionation of moderately reduced mafic magmas. In these systems magnetite saturation is delayed and the ferric iron budget is finite. I-type systems originate with the supply of relatively oxidised, hydrous, subduction-related magmas from the mantle wedge to the upper plate crust. These then experience oxygen-buffered open-system AFC processes in lower crustal hot-zones. S-type magmas are crustal melts that crystallise under reduced conditions initially imposed at source by sulphidic or graphitic sedimentary protoliths. The composition of the resulting melts reflects the domination of partial melting where conditions are hence buffered (open system) followed by subsequent late-stage, closed system fractionation of these extracted, reduced magmas.

Picroilmenites in Yakutian kimberlites: variations and genetic models

Abstract. Major and trace element variations in picroilmenites from Late Devonian kimberlite pipes in Siberia reveal similarities within the region in general, but show individual features for ilmenites from different fields and pipes. Empirical ilmenite thermobarometry (Ashchepkov et al., 2010), as well as common methods of mantle thermobarometry and trace element geochemical modeling, shows long compositional trends for the ilmenites. These are a result of complex processes of polybaric fractionation of protokimberlite melts, accompanied by the interaction with mantle wall rocks and dissolution of previous wall rock and metasomatic associations. Evolution of the parental magmas for the picroilmenites was determined for the three distinct phases of kimberlite activity from Yubileynaya and nearby Aprelskaya pipes, showing heating and an increase of Fe# (Fe# = Fe / (Fe + Mg) a.u.) of mantle peridotite minerals from stage to stage and splitting of the magmatic system in the final stages. High-pressure (5.5–7.0 GPa) Cr-bearing Mg-rich ilmenites (group 1) reflect the conditions of high-temperature metasomatic rocks at the base of the mantle lithosphere. Trace element patterns are enriched to 0.1–10/relative to primitive mantle (PM) and have flattened, spoon-like or S- or W-shaped rare earth element (REE) patterns with Pb &gt; 1. These result from melting and crystallization in melt-feeding channels in the base of the lithosphere, where high-temperature dunites, harzburgites and pyroxenites were formed. Cr-poor ilmenite megacrysts (group 2) trace the high-temperature path of protokimberlites developed as result of fractional crystallization and wall rock assimilation during the creation of the feeder systems prior to the main kimberlite eruption. Inflections in ilmenite compositional trends probably reflect the mantle layering and pulsing melt intrusion during melt migration within the channels. Group 2 ilmenites have inclined REE enriched patterns (10–100)/PM with La / Ybn ~ 10–25, similar to those derived from kimberlites, with high-field-strength elements (HFSE) peaks (typical megacrysts). A series of similar patterns results from polybaric Assimilation + fractional crystallization (AFC) crystallization of protokimberlite melts which also precipitated sulfides (Pb &lt; 1) and mixed with partial melts from garnet peridotites. Relatively low-Ti ilmenites with high-Cr content (group 3) probably crystallized in the metasomatic front under the rising protokimberlite source and represent the product of crystallization of segregated partial melts from metasomatic rocks. Cr-rich ilmenites are typical of veins and veinlets in peridotites crystallized from highly contaminated magma intruded into wall rocks in different levels within the mantle columns. Ilmenites which have the highest trace element contents (1000/PM) have REE patterns similar to those of perovskites. Low Cr contents suggest relatively closed system fractionation which occurred from the base of the lithosphere up to the garnet–spinel transition, according to monomineral thermobarometry for Mir and Dachnaya pipes. Restricted trends were detected for ilmenites from Udachnaya and most other pipes from the Daldyn–Alakit fields and other regions (Nakyn, Upper Muna and Prianabarie), where ilmenite trends extend from the base of the lithosphere mainly up to 4.0 GPa. Interaction of the megacryst forming melts with the mantle lithosphere caused heating and HFSE metasomatism prior to kimberlite eruption.

Mafic–Ultramafic Sills: New Insights from M- and S-shaped Mineral and Whole-rock Compositional Profiles

The origin of mafic and ultramafic sills exhibiting different wholerock compositional profiles (e.g. I-, C-, D-, M- and S-shaped profiles) remains controversial. We have addressed this issue by revisiting three � 100 m thick Siberian dolerite sills (Vavukansky, Kuz’movsky and Vilyuysky) that display remarkable internal differentiation. The Vavukansky sill has an M-shaped profile with prominent basal and top reversals showing inward increases in whole-rock MgO, Mg-number [100Mg/(Mg þ Fe)] and normative An content [100An/(An þ Ab)], followed by the Layered and Upper Border Series with inward decreases in these indices. The Kuz’movsky and Vilyuysky sills both show S-shaped profiles similar to the Vavukansky sill, but lack a top reversal. These whole-rock M- and S-shaped profiles are accompanied by similar profiles in mineral compositions. Plagioclase and, to a lesser extent, olivine show systematic inward increases in An content and Mg-number, respectively, across basal and top reversals.These compositional trends are followed by inward decreases in these ratios in the interiors of the Vavukansky and Kuz’movsky sills. Currently accepted models attribute whole-rock M- and S-shaped compositional profiles to crystal settling, compositional convection or compaction operating in closed systems. Our observations challenge these traditional interpretations because variations in mineral compositions observed in marginal reversals cannot result from closed-system fractionation. We suggest instead that initially the sills evolved as open systems that were slowly inflated by magmas that became gradually more primitive with time. The inflation was accompanied by in situ crystallization that preserved the preceding fractionation history of the injected magmas by forming basal and top reversals with minerals becoming more primitive inwards. This process culminated with rapid inflation of the sills to their current size owing to a major influx of primitive magma. Subsequently, magma flow through the sills ceased and they evolved as closed systems by fractional crystallization. This resulted in the Layered and Upper Border Series with minerals becoming more evolved inwards.This model can be extended to explain other compositional profiles and petrological features in mafic and ultramafic sills.

Source Plutons Driving Porphyry Copper Ore Formation: Combining Geomagnetic Data, Thermal Constraints, and Chemical Mass Balance to Quantify the Magma Chamber Beneath the Bingham Canyon Deposit

The formation of porphyry copper deposits requires a focused flux of magmatic fluid, expelled from a large reservoir of water-, metal-, and sulfur-rich magma. The dimensions of this usually hidden magma reservoir are difficult to determine but can be constrained by combining geophysical observations with thermal constraints and the mass balance imposed by the chemical enrichment of elements in the deposit. Here we show that an internally consistent scenario can be derived for the world-class Cu-Mo-Au deposit at Bingham Canyon (Utah, United States), which quantifies the essential characteristics, approximate dimension, and temporal evolution of a large pluton that generated the deposit. The mineralized district shows a distinct WSW-ENE–striking magnetic anomaly indicating a large intrusive body underlying the sedimentary host rocks of the Oquirrh Mountains. Modeling the deep body by geomagnetic methods is possible because of the high contrast in magnetic susceptibility between sedimentary host rocks and intrusive rocks and because a former volcanic edifice is largely eroded. Additional constraints from drilled geology and district-wide outcropping rocks, including partial demagnetization by hydrothermal alteration on the mine scale, restrict the range of possible solutions to a broadly laccolith-shaped intrusion with a volume of approximately 1,400 to 3,000 km 3 . From the roof of the laccolith, several smaller subvolcanic stocks and dikes protrude to the present surface, of which a major one is hosting the Bingham Canyon deposit. The roof of the laccolith probably lies between 2 and 3.5 km below the bottom of the present open-pit mine, and the average thickness of the laccolith is constrained between 2 and 3.5 km. Thermal modeling, using pluton dimensions derived from the geologic and geomagnetic modeling, indicates that a single laccolith with a magma volume of ~2,000 km 3 beneath Bingham would have solidified within about 230,000 years or less. Comparison of the thermal models with published high-precision geochronologic data and petrologic constraints suggests a scenario in which about 1,000 km 3 of magma was encapsulated by inward crystallization of the pluton after the preore equigranular monzonite stocks solidified and extrusive volcanism was probably terminated. This encapsulated reservoir was close to water saturation and contained approximately 150 billion metric tons (Gt) of magmatic water for subsequent closed-system fractionation and eventual fluid expulsion driving porphyry copper mineralization. Chemical mass balance shows that the known metal endowment and mapped mass of vein quartz within the deposit can be advected and precipitated by a fluid mass that is slightly smaller than the available 150 Gt of water. A conservative estimate indicates that 115 Gt of water is sufficient to precipitate all the quartz associated with successive Cu-Au-and Mo-stage veins as well as their barren precursors. According to our thermal model, approximately 250 km 3 of quartz monzonite magma with a temperature of about 690°C remained partially liquid some 215,000 years after initial intrusion of the laccolith. At that point, it expelled almost simultaneously the quartz monzonite porphyry and the main mass of accumulated fluid, generating most of the vein quartz in the quenched porphyry and the adjacent older rocks. Petrographic evidence indicates that the ore metals precipitated near the end of individual pulses of quartz veining that followed recurrent but waning pulses of porphyry intrusion. Considering published experimental solubility data as well as ore metal contents in fluid inclusions, a small fraction of the available fluid mass is sufficient to transport and precipitate all the ore metals after an initial fluid pulse precipitated most of the quartz. However, the total amount of sulfur present in the deposit, which includes Cu and Mo sulfides as well as a major addition of pyrite, would be facilitated by addition of a mafic magma input into the residual magma chamber that contained the evolved felsic magma. This magmatic injection probably triggered the emplacement of the mineralized porphyries, consistent with the more mafic composition of some of the latest porphyry dikes and the CO 2 -rich nature of ore-related fluid inclusions.

Open-system processes in the differentiation of mafic magma in the Teide–Pico Viejo succession, Tenerife

Oceanic island basalts are commonly thought to differentiate by fractional crystallization, yet closed-system fractionation models have so far failed to reproduce major and trace element variations observed in mafic lavas from the Teide–Pico Viejo stratovolcano complex on Tenerife. Here, new high-precision plagioclase trace element data are fed into such a fractionation model. The results confirm that fractionation of phenocrysts found in the lavas does not reproduce trace element variations, in particular enrichment of Sr and Zr observed in the Teide–Pico Viejo mafic suite. This enrichment of Sr and Zr is tested by an energy-constrained recharge, assimilation and fractional crystallization (EC-RAFC) model at high T and low Δ T intervals, consistent with previously determined magma storage beneath Tenerife at sub-Moho depths. Published mineral–melt equilibrium relations using the plagioclase anorthite content (0.4 &lt; X An &lt; 0.8) constrain the temperature during differentiation. Gabbroic xenoliths found in Tenerife lavas are assumed as contaminant. Enrichment of Sr and Zr in the Teide mafic suite is reproduced by this combined assimilation and fractional crystallization model, as assimilation causes higher degrees of enrichment in incompatible trace elements than is possible by crystal fractionation alone. Recycling of plutonic roots may thus have significantly enriched trace elements in the primitive lavas of the Teide–Pico Viejo succession. Supplementary material: Detailed average mineral compositions of low and high Sr–Zr lavas, composition of the bulk extracts and full graphs of the recalculated closed-system model are available at www.geolsoc.org.uk/SUP18604 .

Petrology and geochemistry at the Lower zone-Middle zone transition of the Panzhihua intrusion, SW China: Implications for differentiation and oxide ore genesis

A sequence of gabbros showing isotropic, layered and fine-grained textures is exposed in the Nalaqing mine at the southern tip of the ∼260 Ma Panzhihua intrusion, SW China. The field relations, structure, texture and mineralogy of the rocks indicate that the sequence represents the transition between the Lower zone and Middle zone of the intrusion. Isotropic gabbros characteristic of the Lower zone pass upward to layered gabbros of the Middle zone through a ∼5 m-thick microgabbro sheet, within and close to which small-scaled, concordant Fe-Ti oxide ore horizons are identified. Strong fractionation between HFSE and REE in a subset of samples is ascribed to cumulus titanomagnetite into which HFSE are preferentially incorporated over REE, as reflected in the parallel relations between Nb/La, Hf/Sm and Ti/Ti*. Both the isotropic and layered gabbros display cumulate textures and have similar mineral compositions (Mg# of clinopyroxene = ∼76–79 and An59–61), isotopic compositions [(87Sr/86Sr)i = 0.7044–0.7045 and εNd(t) = +2.4 to +3.9] and trapped liquid contents inferred from Zr abundance (∼17–34 ppm). However, there are substantial variations in elemental abundances (V, Cr and PGE) and ratios (Ti/V, La/Yb, Ba/Y and Cu/Pd) between the two types of gabbros, features that cannot be explained by cumulate formation from a common magma in a closed system. The microgabbros generally resemble high-Ti Emeishan basalts in major element compositions, but their low trace element abundances indicate some lost of residual liquid is inevitable despite rapid nucleation and cooling. Combined with available data and observations, we propose a model involving in-situ crystallization, followed by magma recharge and closed-system fractionation to explain the formation of texturally distinctive gabbros at Nalaqing and the evolution of the lower part of the Panzhihua intrusion.

THE UPPER ZONE OF THE BUSHVELD COMPLEX AT ROOSSENEKAL, SOUTH AFRICA: GEOCHEMICAL STRATIGRAPHY AND EVIDENCE OF MULTIPLE EPISODES OF MAGMA REPLENISHMENT

Drill core from a fence of five boreholes located in the vicinity of Roossenekal, in the eastern limb of the Bushveld Complex, South Africa, provides complete coverage of what is probably one of the thickest and most well-developed sections of the Upper Zone. The lowermost portions of the Rooiberg Group felsites that overlie the Upper Zone, and the uppermost portion of the Upper Main Zone, which forms the footwall, were also intersected. Field relationships from floor-attached domes (eastern limb) and gap areas (western limb) reveal a marked angular unconformity at the base of the Upper Zone. The Upper Zone represents a discrete intrusive event and should not be interpreted as a continuum, or the fractionated residue of the underlying components of the Rustenburg Layered Suite (RLS). A new subdivision of the Upper Zone into five subunits (A through E) is proposed on the basis of the cumulus mineralogy and whole-rock geochemistry, the latter based on samples at an average vertical spacing of 20 m throughout the column at Roossenekal. The separation of subzones A and B is based on our geochemical data, as is the identification of subzone E, but recognition of subzones C (olivine) and D (apatite) is broadly consistent with the detailed mapping and stratigraphic column presented by Von Gruenewaldt (1973). Subunits A through D reveal macro-layering for which the intrusion is so well known. More than 40 individual Ti-magnetite layers were identified in the drill-core, which together with the numerous anorthosite layers, are the most prominent feature of this part of the column. The principal lithology is a somewhat monotonous sequence of ferrogabbro or magnetite-olivine gabbro. The absence of Ti-magnetite layers and anorthosite from subzone E, which is dominated by diorite with <5 modal % Ti-magnetite is significant. The whole-rock geochemical data presented here is not consistent with a closed system fractionation hypothesis for subunits A through D, despite this being widely accepted in recent contributions. An upward differentiation trend is preserved only in subunit E. The incremental build up of cumulates from successive pulse of magma is not only consistent with our views on the underlying layered cumulates (periods of closed system differentiation are rare) but is also consistent with field relationships, including each of the principal limbs representing individual sill-like features. The Upper Zone crystallized from batches of iron-rich tholeiitic magma derived from a deep staging chamber. Magma was injected as near-lateral flows, probably in a southward direction in the eastern limb. The introduction of evolved magma into a shallow crustal environment resulted in the early crystallization of Fe-Ti oxides. Ti-magnetite layers crystallized from oxide-rich slurries intruded near the floor of the chamber. Fe-Ti oxide pipes formed from slurries forced downward in areas of structural weakness. The regular, albeit step-like, upward change in composition of the Ti-magnetite layers identified by Molyneux (1974) and others is ascribed to mixing of crystal-rich magma slurries with increasingly differentiated resident magma: the lowermost layers (and pipes) contain the highest concentrations of V2O5. The spatial association of Ti-magnetite layers and anorthosite is emphasized and we suggest the latter formed as a consequence of disequilibrium partial melting of earlier formed silicate cumulates due to heat associated with new pulses of magma.

Magmatic history of Dabbahu, a composite volcano in the Afar Rift, Ethiopia

Dabbahu is a composite volcano at the north end of the active Manda-Hararo segment of the Afar Rift in northern Ethiopia. We present 93 new whole-rock analyses, mineral analyses from 65 samples, and 9 new 40Ar39Ar dates for rocks ranging in composition from mildly alkaline basalt through trachyandesite to peralkaline rhyolite (comendite and pantellerite) erupted from Dabbahu. These data, supplemented by a new geological map, are used to provide insights into the evolution of the volcano. We show that Dabbahu has been active for over 67 k.y., but an apparent hiatus occurred between the eruption of comendite (29 ka) and pantellerite (ca. 8 ka) lavas. Mineral data for olivine, clinopyroxene, plagioclase, and alkali feldspar show a remarkably extensive range of solid solution across the rock suite consistent with protracted fractionation from basalt to rhyolite. The parental basalt is compositionally similar to recent rift-related basalts in the Manda-Hararo rift, with low initial H2O contents (<1 wt%). Closed-system fractionation increased H2O contents of residual liquids sufficiently for some rhyolites to erupt both explosively and effusively. The diverse magma types were erupted from a relatively closely spaced network of vents and fissures. Field evidence indicates that magmas were not erupted in a simple fractionation sequence. Some mixing occurred between magmas of less-evolved compositions and more-evolved compositions shortly prior to, or during, eruption. The differentiation of basalt to rhyolite must have occurred on time scales that were relatively short compared to the lifetime of the volcano, probably due to the small volumes of basalt intruded into the crust and consequently enhanced cooling and crystallization rates. A network of stacked sills or closely spaced dikes in the shallow to midcrust represents the most plausible configuration of the subvolcanic plumbing system. Input of new magma batches into such a system may serve as a key eruption trigger at Dabbahu.

The geochemistry and petrogenesis of the Blue Draw Metagabbro

The Blue Draw Metagabbro (BDM) in western South Dakota, is an 800m thick, layered intrusion, which is interpreted to have been intruded as a rift-related subvolcanic sill during the Palaeoproterozoic. The age and tectonic setting of the BDM are similar to those recorded by the East Bull Lake Suite of layered intrusions in Southern Ontario. These similarities have led previous authors to suggest that the two sets of intrusions are cogenetic. The East Bull Lake Suite intrusions are relatively well studied and are known to host significant contact-type Ni–Cu–PGE sulphide mineralisation, however, prior to this work, relatively little was known of BDM both in terms of its geochemistry and mineralisation potential. Chemostratigraphic profiles through the BDM show that the intrusion is the product of at least two magma pulses which fractionated to produce a sequence of rocks which grade from peridotitic at the base of the intrusion to gabbronoritic at the upper margin. Closed-system fractionation following the intrusion of the second magma pulse caused the magma to become saturated in sulphur and precipitate Ni–Cu–PGE bearing sulphides—now preserved in a low-grade 50m thick zone near the top of the intrusion. Petrological modelling shows that the parental melt of the BDM was a low-Ti tholeiite, with a trace element chemistry defined by enrichments in large-ion lithophile and light rare-earth elements and prominent negative Nb, Ta, and Ti anomalies. This ‘arc-like’ geochemistry recorded by the BDM parent magma is shared with parent magmas of the East Bull Lake Suite and may suggest that the BDM and its potential Canadian relatives share a common magmatic source. However, the relative ubiquity of such geochemical signatures in Archaean–Palaeoproterozoic intracontinental magmatic rocks coeval with the BDM suggests that the geochemical similarities recorded by the BDM and East Bull Lake Suite are non-unique and hence, are not definitive evidence of a genetic link between the two sets of intrusions. Instead, this geochemical signature that is common to many ancient igneous provinces may indicate the presence of a transient and (currently) poorly understood Archaean–Palaeoproterozoic mantle reservoir which was a globally significant magma source.

Seawater contribution to polymetallic Ni–Mo–PGE–Au mineralization in Early Cambrian black shales of South China: Evidence from Mo isotope, PGE, trace element, and REE geochemistry

The Early Cambrian black shale sequence of the Niutitang Formation in South China hosts a synsedimentary, organic carbon-rich, polymetallic sulfide layer with extreme metal concentrations, locally mined as polymetallic Ni–Mo–PGE–Au ore. In combination with previously reported data, we present Mo isotope, platinum-group element (PGE), and trace and rare-earth element (REE) data for the polymetallic sulfide ores and host black shales from four mine sites (Dazhuliushui and Maluhe in Guizhou Province, and Sancha and Cili in Hunan Province, respectively), several hundred kilometers apart. The polymetallic sulfide ores have consistently heavy δ98/95Mo values of 0.94 to 1.38‰ (avg. 1.13±0.14‰, 1σ, n=11), and the host black shale and phosphorite have slightly more variable δ98/95Mo values of 0.81‰ to 1.70‰ (n=14). This latter variation is due to variable paleoenvironmental conditions from suboxic to euxinic, and partly closed-system fractionation in isolated marine sedimentary basins. Both the polymetallic sulfides and host black shales show PGE distribution patterns similar to that of present-day seawater, but different from those of ancient submarine-hydrothermal deposits and modern submarine hydrothermal fluids. The polymetallic sulfide bed has a generally consistent metal enrichment by a factor of 107 compared to present-day seawater. PAAS-normalized REE+Y patterns of the polymetallic sulfide bed are characterized by a remarkably positive Y anomaly, consistent with an origin of the REE predominantly from seawater. Small positive Eu anomalies in some of the sulfide ores could reflect minor hydrothermal components involved. The Mo isotope, PGE, and trace and rare-earth element geochemical data suggest that metals in the polymetallic Ni–Mo–PGE–Au sulfide ore layer were scavenged mostly from Early Cambrian seawater, by both in-situ precipitation and local re-deposition of sulfide clasts.

Geological, Petrological and Geochemical Evidence for Progressive Construction of an Arc Crustal Section, Sierra de Valle Fertil, Famatinian Arc, Argentina

The petrogenesis of calc-alkaline magmatism in the Famatinian arc is investigated in the central Sierra Valle Fe¤ rtil, a major, lower to middle crustal section of the Early Ordovician active margin of West Gondwana. Large-scale field relationships show a gradual and continuous compositional variation of the plutonic sequence, ranging from olivine-bearing gabbronorites to hornblendeand biotite-bearing granodiorites. Distinctive lithostratigraphic units are, however, discernible as one compositional type of plutonic rock dominates over mappable areas.These results allow us to identify a continuous plutonic arc stratigraphy that progressively exposes shallower paleo-depths towards the east. At all the exposed levels, calc-alkaline plutonic rocks are volumetrically dominant, interrupted only by granulite-facies migmatites and leucogranites. The migmatites are interpreted to be refractory remnants of supracrustal sedimentary successions, whereas the peraluminous leucogranites have field relationships and chemical and isotopic compositions suggesting that they were produced via anatexis of metasedimentary packages. Massbalance calculations predict that a parental gabbroic magma after progressive closed-system fractionation would crystallize about 80% of the original mass to yield a granodioritic daughter. Because the crystallizing mineral assemblage comprises hornblende and plagioclase, mass balance suggests a volume of residual amphibole-rich gabbroic rocks much larger than that observed, suggesting that differentiation is significantly driven by open-system processes. Indeed, the combination of field and petrographic observations with bulk-rock geochemistry and petrogenetic modeling demonstrates that most dioritic and tonalitic rocks are hybrids formed by either (1) bulk assimilation of metasedimentary materials into gabbroic magmas, or (2) multi-stage and complex interactions between gabbroic rocks and metasedimentary-derived leucogranitic melts. The source region of the granodioritic magmas is located at the transition zone between a tonalite-dominated intermediate unit and a granodiorite-dominated silicic unit. Typical granodiorites have a hornblende-bearing mineralogy, metaluminous chemical signature and isotopic compositions [Sr/Sr(T)1⁄4 0·7075^0·7100 and eNd(T) 5·0] broadly overlapping those of the tonalites of the intermediate rock unit. These major compositional features of the granodiorites can be best explained if three end-member components contribute to their generation. As field observational data suggest, primitive gabbroic rocks, metaluminous intermediate magmas and anatectic leucogranitic melts mixed to produce the calc-alkaline granodiorites; however, the exact petrological process generating the granodioritic magmas is unclear because the mafic end-member may have been incorporated as mafic inclusions in the intermediate magmas or as syn-magmatic dikes, or both.The polygenetic nature of the intermediate to silicic plutonic rocks, along with the preponderance of parental gabbroic rocks at the inferred base of the plutonic column, suggests an upward growth of the intermediate to silicic crust that involved the complete reconstitution of the pre-existing crustal configuration. The main implication of this study is that intermediate and silicic plutonic rocks in the Valle Fe¤ rtil section formed within a crustal column in which the mass transfer and heat