Deep Magma Chamber Research Articles

Magmatic and structural controls on porphyry-style Cu–Au–Mo mineralization at Kemess South, Toodoggone District of British Columbia, Canada

Kemess South is the only Cu–Au–Mo mine in the Toodoggone district and a major Cu and Au producer in British Columbia. Porphyry-style Cu–Au–Mo mineralization is mainly hosted by the tabular, SW-plunging, 199.6 ± 0.6-Ma Maple Leaf granodiorite, which intrudes tightly folded, SW-dipping, Permian Asitka Group siltstone and limestone and homogeneous Triassic Takla Group basalt. Southwest-dipping 194.0 ± 0.4-Ma Toodoggone Formation conglomerate, volcaniclastic, and epiclastic rocks overlie the granodiorite and Asitka Group rocks. Minor Cu–Au–Mo mineralization is hosted by the immediate Takla Group basalt country rock, whereas low-tonnage high-grade Cu zones occur beneath a 30-m-thick leached capping in supergene-altered granodiorite and in exotic positions in overlying Toodoggone Formation conglomerate. Granodiorite has an intrusive contact with mineralized and altered Takla Group basalt but displays a sheared contact with unmineralized and less altered Asitka Group siltstone. The North Block fault is a deposit-scale, E-striking, steeply S-dipping normal fault that juxtaposes the granodiorite/basalt ore body against unmineralized Asitka Group rocks. Younger NW- and NE-striking normal–dextral faults cut all rock types, orebodies, and the North Block fault with displacements of up to 100 m and result in the graben-and-horst-style block faulting of the stratigraphy and ore body. Both basalt and granodiorite host comparable vein sequence and alteration histories, with minor variations in hydrothermal mineral assemblages caused by differing protolith chemistry. Early potassic alteration (and associated early-stage Cu ± Au ± Mo mineralization) is partly replaced by phyllic and intermediate argillic alteration associated with main-stage Cu–Au–Mo mineralization. Two main-stage veins have Re–Os molybdenite ages of 201.3 ± 1.2 and 201.1 ± 1.2 Ma. These mineralization ages overlap the 199.6 ± 0.6-Ma U–Pb zircon crystallization age for the Maple Leaf granodiorite. Late-stage pyrite-rich stringer veins and related phyllic alteration assemblages are cut by anhydrite-rich, carbonate-rich, and chlorite veins. Fluids and metals associated with early-, main-, and late-stage veins were probably derived principally from the same deep magma chamber as the Maple Leaf granodiorite. These magmatic-derived fluids interacted with Asitka and Takla Group country rocks and possibly with meteoric and metamorphic fluids prior to mineralization.

Excess degassing from volcanoes and its role on eruptive and intrusive activity

Volcanoes emit larger amounts of volcanic gas than can be dissolved in the volume of erupted magma during a variety of volcanic processes, including explosive and effusive eruption and noneruptive continuous degassing. Degassing of unerupted magma with a much larger volume than that of erupted magma caused such a large degassing; erupted magma represents only a small portion of the magma that drives volcanic activity. Evaluation of the magma‐gas differentiation process causing the excess degassing is necessary to understand eruption processes, magma chamber evolution, and crustal growth by magma intrusion. Three mechanisms are proposed to explain various degassing modes, including eruption of bubble‐accumulated magma, degassing of a convecting magma column, and permeable gas transportation from a deep magma chamber. Examples of large degassing in excess of the erupted magma are common in subduction zone volcanism but are rare in rift‐ and hot spot–associated volcanism.

Gold deposits in dikes of the Yana-Kolyma belt

The ore-bearing dikes widespread in the Yana-Kolyma metallogenic belt vary in composition from granite porphyry to lamprophyres, gabbromonzonite, and dolerite. The dikes make up suites tens of kilometers in extent and up to 8–10 km wide or fields a few thousand square kilometers in area. Previously, the ore mineralization was traditionally referred to the gold-quartz type superimposed on dikes. However, isotopic and thermobarogeochemical data indicate that the mineralization is close to an intrusion-related gold deposit. A deep magma chamber could have been a source of sulfur in sulfides from beresites and quartz veins hosted in the Kolyma dikes. The deposits considered in the paper are permissive for large economic reserves with low and medium Au grades.

Geochemical and geochronological study of the Sanshui basin bimodal volcanic rock suite, China: Implications for basin dynamics in southeastern China

Sanshui basin is one of the typical Mesozoic–Cenozoic intra-continental rift basins with voluminous Cenozoic volcanic rocks in southeastern China. Thirteen cycles of volcanic eruptions and two dominant types of volcanic rocks, basalt and trachyte–rhyolite, have been identified within the basin. Both basalt and trachyte–rhyolite members of this bimodal suit have high values of εNd (+2.3 to +6.2) and different Sr isotopic compositions (initial 87Sr/ 86Sr ratios are 0.70461–0.70625 and 0.70688–0.71266 for basalts and trachyte–rhyolite, respectively), reflecting distinct magma evolution processes or different magma sources. The results presented in this study indicate that both of the trachyte–rhyolite and basaltic magmas were derived from similar independent primitive mantle, but experienced different evolution processes. The trachyte-rhyolitic magma experienced significant clinopyroxene and plagioclase fractionational crystallization from deeper magma chamber with significant crustal contamination, while the basaltic magmas experienced significant olivine and clinopyroxene fractionational crystallization in shallower magma chamber with minor crustal contamination. New zircon U–Pb dating confirms an initial volcanic eruption at 60 Ma and the last activity at 43 Ma. Geologic, geochemical, and geochronological data suggest that the inception of the Sanshui basin was resulted from upwelling of a mantle plume. The Sanshui basin widened due to subsequent east–west extension and the subsequent volcanism constantly occurred in the center of the basin. Evidence also supports a temporal and spatial association with other rift basins in southeastern China. The upwelling mantle plume became more active during late Cenozoic time and most likely triggered opening of other basins, including the young South China Sea basin.

Simultaneous inversion of deformation and gravity changes in a horizontally layered half-space: Evidences for magma intrusion during the 1982–1984 unrest at Campi Flegrei caldera (Italy)

A very large uplift (about 1.8 m) occurred in the period 1982–1984 at Campi Flegrei caldera, Italy, without culminating in an eruption. A still-standing controversy accompanies the interpretation of deformation and gravity changes recorded during the unrest, which were interpreted to result from the sub-surface magmatic reservoir by some authors and from the hydrothermal system or hybrid sources by others. Here for the first time we take into account crustal layering while inverting leveling, EDM, and gravity data using uniformly-pressurized sources, namely small vertical spheroids and finite horizontal penny-shaped sources. The weight of EDM data in the misfit function is chosen from a trade-off curve in order to balance the compromise between fitting the leveling and the EDM data well. Models using a homogeneous medium cannot give a good simultaneous fit to leveling and EDM deformation data of the 1982–1984 unrest, whereas incorporating a layered structure (determined from seismically derived estimates of the P wave speed for the crust, and not adjusted to improve the fit in any of the inversions) allows a significantly better fit. Also, layering affects the sub-surface mass redistribution effects on gravity changes, and we show that the retrieved intrusion density is in full agreement with densities for highly evolved magmas expected at the Campi Flegrei caldera for depths of 3 to 4 km, ruling out hydrothermal fluids as the primary cause of the 1982–1984 unrest. The source of the 1982–1984 CF unrest was probably a shallow (about 3-km deep) penny-shaped magma intrusion fed by a deeper magma chamber; source overpressure was few MPa.

Petrology and U–Pb Zircon Geochronology of Amphibole-rich Cumulates with Sanukitic Affinity from Husky Ridge (Northern Victoria Land, Antarctica): Insights into the Role of Amphibole in the Petrogenesis of Subduction-related Magmas

A microanalytical trace element and geochronological study was carried out on mafic amphibole-rich cumulates (quartz diorites) cropping out in northern Victoria Land (Antarctica). Associated tonalites and basement rocks were also investigated. Rock textures and major and trace element mineral compositions reveal the presence in quartz diorites of two mineral assemblages: (1) clinopyroxene-I + brown amphibole ± dark mica; (2) clinopyroxene-II + green amphibole + plagioclase + quartz. Both mineral assemblages contain mafic phases with elevated Mg-number, but their trace element signatures differ significantly. In situ U–Pb zircon geochronology was carried out to support petrogenetic and geological interpretations. Quartz diorites were emplaced in the mid-crust probably at 516 ± 3 Ma. Parental melts of quartz diorites were computed by applying solid/liquid partition coefficients. The melt in equilibrium with the first mineral assemblage (melt-I) is extremely depleted in heavy rare earth elements (HREE), Y, Ti, Zr and Hf (at about 0·2 times normal mid-ocean ridge basalt) and enriched in B, Th, U, the large ion lithophile elements and light REE (LREE). It shares many similarities with sanukitic melts (e.g. Setouchi andesites), which originated by equilibration of subduction-derived sediment melts with a refractory mantle. The melt in equilibrium with the second mineral assemblage (melt-II) is characterized by a steep LREE enrichment (LaN/YbN up to 39), a U-shaped HREE pattern and low Ti, which is depleted relative to HREE. The trace element signature of melt-II can be acquired through amphibole crystallization starting from a sanukitic melt similar to melt-I, probably in a deeper magma chamber. Our results allow us to constrain that melts from the subducted slab were produced on a regional scale, in accordance with literature data, below a large sector of the east Gondwana margin during the mid-Cambrian. Implications for the role of amphibole in petrogenesis of subduction-related magmas are also discussed.

Platinum-group element and sulphide mineralogy in ultramafic complexes at western Andriamena, Madagascar

Small (<2 km diameter), ultramafic intrusive complexes in the Andriamena region of Madagascar carry high concentrations of platinum-group elements (PGE). The PGE mineralisation at Andriamena is unusual in that it is not obviously associated with abundant sulphide or chromitite layers or pods, as in many layered complexes and Alaskan-type intrusions. Mineralogical investigation of rocks from one complex, designated UM2, has revealed a complex array of Pt and Pd-bearing platinum-group minerals (PGM) that tend to be concentrated interstitially between olivine crystals in dunite units. These PGM are dominated by PGE arsenides and stibioarsenides, accompanied by a small population of Pd oxides that occur in unit close to the surface and appear to have formed by supergene alteration. The PGM population displays a notable contrast between Pt-bearing and Pd-bearing minerals. Palladium-bearing PGM are always found entirely within or at least partially enclosed by Fe–Ni–Cu sulphides. In contrast, Pt-bearing PGM are either found on the margins of Fe–Ni–Cu sulphides or more commonly, they are entirely isolated from sulphides in hydrosilicates or as inclusions in olivine. The discovery of these sperrylite (PtAs2), irarsite (IrAsS) and hollingworthite (RhAsS) inclusions in olivine implies that development of these minerals took place at magmatic temperatures, coincident with the growth of olivine, not in late-stage interstitial fluids. This leads to a new magmatic model for the development of the mineralisation. In this model, PGE and As are initially concentrated into a sulphide liquid in a deeper magma chamber and intruded as a slurry of silicate liquid–olivine–sulphide liquid into the dyke-like Lavatrafo intrusions. The high As content of the sulphide promotes early crystallisation of Pt, Ir and Rh arsenide and sulpharsenide PGM, some of which became detached from the sulphide and became incorporated into growing olivine crystals. Palladium arseno-antimonide PGM follow at a later stage. Early crystallisation of Pt and Pd-bearing PGM leaves the sulphide liquid depleted in these elements. Migration of the fractionated sulphide liquid inwards and downwards towards the centre of the intrusion produces a zone of Cu-rich sulphides that is PGE-poor. Such a strong spatial separation PGE from base metal sulphides is uncommon in magmatic PGE deposits and this is suggested to arise from the As-rich nature of the early sulphide liquid which promoted the formation of PGM earlier than would have occurred in an equivalent As-poor system.

Vertical and lateral propagation of radial dikes inferred from the flow-direction analysis of the radial dike swarm in Komochi Volcano, Central Japan

Dikes with lateral and outward intrusion directions (named L-type dike) and dikes with vertical and upward intrusion directions (V-type dike) are recognized in a radial dike swarm of Komochi Volcano, Japan, by the combined analysis of the preferred orientations of deformed vesicles and anisotropy of magnetic susceptibility (AMS). Intrusion directions of magmas were examined for 41 dike outcrops. Among them, 19 dikes were classified as L-type dike and 15 dikes were as V-type. The L-type dikes distribute over the area of the dike swarm radiating from the central conduit, which is named Daikokuiwa stock, and the V-type dikes occur mainly in the peripheral portion of the volcano. The L-type dikes are characterized with evolved whole-rock compositions similar to those of rocks consisting of the central conduit, whereas the V-type dikes have less-fractionated composition and are enrich in mafic phenocryst as compared to the L-type dikes. The outward intrusion directions of the L-type dikes and their petrological similarity to the rock of the central conduit indicate that these dikes intruded from the shallow part of the central conduit where the magmas underwent fractional crystallization and degassing. The petrological characteristics of the V-type dikes suggest that the less-fractionated magma intruded directly from a magma chamber, in which mafic phenocryst crystals accumulated. The larger dike thickness and higher magnetic foliation of the V-type dike as compared to the L-type dike indicate higher magmatic overpressure, which was possibly result of the direct connection to the pressurized magma chamber and vertical-growth of the dikes with buoyant magma. Solidification of the central conduit is favorable for the accumulation of internal excess pressure in the magma chamber to break the wall rock. Fissure eruptions independent of the central conduit have occurred in many volcanoes and these fissure eruptions might be fed by the dikes directly propagated from the deeper magma chamber.

Geochemistry of mafic phenocrysts from alkaline lamprophyres of the Spanish Central System: implications on crystal fractionation, magma mixing and xenoliths entrapment within deep magma chambers

The Permian alkaline lamprophyres from the Spanish Central System (SCS) are highly porphyritic rocks which carry a heterogeneous population of clinopyroxene and kaersutite zoned phernocrysts. Clinopyroxene phenocrysts may show 1) normal zoning (Cpx-I), 2) reverse zoning with Fe-rich green cores (Cpx-II), and 3) reverse zoning with colourless Al-poor, silica-rich cores (Cpx-III). Kaersutite phenocrysts also show a slight reverse zoning. Major and trace element composition of Cpx-I suggests that their compositional variation is related to a crystal fractionation process from melts similar to the host lamprophyres. The Cpx-II cores represent crystallization from highly evolved melts (low Mg-Cr contents and incompatible element enrichment), genetically related with the SCS alkaline magmatism, and the growth or surrounding Mg-rich inner rims points to a magma mixing process. The major and trace element composition of Cpx-III cores supports derivation from a magma which has fractionated plagioclase. This characteristic, together with their similarities when compared to clinopyroxenes from charnockite xenoliths, suggests that they might be xenocrysts from deep calc-alkaline cumulates. The composition of melts in equilibrium with clinopyroxene and amphibole phenocrysts supports a model in which Cpx-II and Cpx-III cores would have been incorporated into a more primitive lamprophyric magma stagnated at lower crustal levels. The low pressure composition of all phenocryst outer rims indicates that they crystallised directly from the host alkaline magma at their subvolcanic emplacement levels.

Anisotropy of magnetic susceptibility studies in Tertiary ridge-parallel dykes (Iceland), Tertiary margin-normal Aishihik dykes (Yukon), and Proterozoic Kenora–Kabetogama composite dykes (Minnesota and Ontario)

Mafic dykes of different ages were collected from three different tectonic settings and analyzed using anisotropy of magnetic susceptibility (AMS) as a proxy for magmatic flow during intrusion. In Iceland, ridge-parallel basaltic dykes were sampled on each side of the active tectonic boundary. The dykes are < 10 m wide along a 1–2 km strike, and are the result of a single intrusion from 1–2 km deep magma chambers in oceanic crust. Thirteen samples were collected (7 N. American plate; 6 European) and 153 cores were analyzed by AMS and preserve a vertical K max orientation indicating vertical emplacement. The Eocene Aishihik dyke swarm intrudes the Yukon–Tanana terrane in the Yukon province, Canada over an area ~ 200 by 60 km. These dykes were intruded normal to the accretionary margin, are porphyritic andesites, and have an intermediate geochemical signature based on major and trace element analyses. Ten dykes were sampled and 111 cores analyzed using AMS, and the dykes preserve a vertical K max orientation, indicating intrusion was vertical through ~ 30 km of continental crust. The 2.06 Ga Kenora–Kabetogama dykes in northern Minnesota and western Ontario crosscut a variety of Archean terranes (thickness ~ 50 km) in a radiating pattern. The unmetamorphosed basaltic dykes are 1–120 m wide, 10–110 km in length, are vertical in orientation and can be grouped as either being single intrusion or multiple intrusion (composite) dykes. AMS data preserve a vertical K max orientation for the southerly locations (2 dykes, n = 53) and horizontal K max for the remainder to the northwest (15 dykes, n = 194). Maximum magnetic susceptibility axes (4 dykes, n = 92) for composite dykes are scattered and yield inconsistent flow directions with regard to the dyke margin. Almost all of our results are “normal” in that, the magnetic foliation (the plane containing K max and K int, normal to K min) is parallel to the dyke planes, which gives us confidence that the magnetic lineations (i.e., K max orientations) are parallel to magmatic flow.

GEOLOGY, MINERALIZATION, ALTERATION, AND STRUCTURAL EVOLUTION OF THE EL TENIENTE PORPHYRY Cu-Mo DEPOSIT--A DISCUSSION

Sir: In their paper concerning the geology, mineralization, alteration, and structural evolution of the El Teniente Cu-Mo deposit in Chile, Cannell et al. (2005) concluded that El Teniente, despite its anomalously large size, is a typical porphyry Cu-Mo deposit with regard to its alteration and sulfide assemblage zonation and the genetic association of mineralization with dacite intrusions. We do not agree that the dacite porphyries were the causative intrusions for copper mineralization. Instead, most of the mineralization was emplaced in conjunction with the formation of multiple hydrothermal breccia pipes and their associated veins that were derived from a large, deep magma chamber (Skewes et al., 2002, 2005; Stern and Skewes, 2005), and this deposit should be classified as a megabreccia rather than a porphyry deposit. At El Teniente most Cu mineralization (80%) is intimately associated with biotitized mafic intrusive rocks (Camus, 1975; Cuadra, 1986; A. Arevalo, R. Floody, and A. Olivares, unpub. report for CODELCO-Chile, 1998, 76 p.). Cu mineralization occurs disseminated in the biotitized mafic rocks, in different generations of highly visible anhydrite and quartz veins, in less visible but abundant biotite veins, and also in the matrices of the multiple hydrothermal breccia complexes that occur in the deposit. A significant proportion of high-grade (>1%; Fig. 1⇓) hypogene Cu mineralization at El Teniente was emplaced both within and in veins around these multiple hydrothermal breccia complexes in direct genetic association with their formation. Alteration and sulfide assemblages in these multiple breccia complexes are similar to those in typical porphyry systems, but at El Teniente these assemblages and mineralization grades are not zoned around felsic intrusions as in a typical porphyry system. Also, the timing of Cu mineralization (Fig. 2⇓) is not correlated with the age of felsic intrusions as suggested by Cannell et …

Insights into magma chamber processes from the analysis of size distribution of enclaves in lava flows: A case study from Vulcano Island (Southern Italy)

The size distribution of latitic enclaves dispersed in a rhyolitic lava flow is studied. Enclave size distribution is self-similar, a feature that can be explained by a fractal fragmentation process, and estimated fractal dimension of fragmentation is 2.50. Fragment size distribution of enclaves generated by disruption of viscous fingering structures in granitoid rocks is analyzed. It is shown that this distribution is fractal with a value of fractal dimension of fragmentation of 2.55, in close agreement with the value estimated for enclaves in the lava flow. It is suggested that the fragmentation process producing the size distribution of enclaves in the lava flow may have occurred in response to the disruption of viscous fingering morphologies generated by the injection of the more mafic magma into the felsic one. Accordingly, the size distribution of enclaves dispersed in the lava flow is considered a feature inherited from deep magma chamber processes and gives insights into magma chamber dynamics.

Hidden Dykes detected on Ultra Long Period seismic signals at Piton de la Fournaise volcano?

Broadband seismic data enable us to test whether Ultra Long Period (ULP) signals can be used to investigate magma chamber pressure state and to monitor volcanic eruptions. By systematically investigating seismic signals at GEOSCOPE station RER, we find ULP signals associated with volcanic events, during the last 2 decades. We tentatively propose to interpret these signals as events highlighting the activity of the upper magma feeding system of the Piton de la Fournaise volcano and as being related to the deep magma chamber. Some of these unusual signals were previously detected and usually interpreted in term of tilt [Battaglia, J., Aki, K., Montagner, J.-.J.-P. Tilt signals derived from a GEOSCOPE VBB station on the Piton de la Fournaise volcano, Geophys. Res. Lett. 27 (5) (2000) 605–608.]. They are detected using STS1 seismometers sensitive in the bandwidth 10 − 3 to 10 − 2 Hz. This alternative model enables to quantify each event in terms of a pressure drop in the upper magma reservoir of the volcano. Two kinds of seismic waveforms can be associated with two types of fracturing events. The first one is related to the fast rise of a magma batch coming from the deep magma chamber and the second is the result of a slow rise in pressure inside a shallow reservoir. In both cases, we are able to detect these events on very long period seismic data. The interpretation of these signals supports the idea of a single magma source at the sea level. Thus, we propose to classify all the eruptions occurring during the last two decades into two groups, corresponding to the two primary pressure states of the magma chamber.

Petrogenesis of silicic peralkaline rocks in the Ethiopian rift: Geochemical evidence and volcanological implications

Major, trace element and isotopic data for mafic to peralkaline silicic volcanic rocks from the northern sector of the main Ethiopian rift are discussed with the aim of placing constraints on processes of magma genesis and evolution and to present models for magma plumbing systems of rift volcanoes. Basalts straddle the subalkaline–alkaline boundary and exhibit important variations of incompatible element abundances and ratios. Silicic rocks consist of dominant pantellerites and minor comendites and trachytes, although some volcanoes along the rift shoulders consist entirely or predominantly of trachytes. Rocks with intermediate compositions are very scarce. Mafic and silicic rocks exhibit similar values as some basalts for many incompatible element and radiogenic isotopic ratios. Geochemical and petrological modelling shows that the most likely petrogenetic process for rift magmatism is a derivation of rhyolites from basalts by dominant fractional crystallisation occurring at shallow depths. Variations of incompatible element ratios and radiogenic isotopes in the basalts suggest heterogeneous sources and significant interaction with the crust. In contrast, the role of crustal assimilation during evolution of silicic magmas is negligible. It is suggested that large amounts of basalts were emplaced both into the lower continental crust, and at shallow depths. Shallow level fractional crystallisation generated zoned magma chambers with rhyolitic melts accumulating at the top, and mafic magmas ponding at the bottom. Volcanic activity was fed preferentially by the upper rhyolitic layer, whereas mafic magmas were erupted only accidentally, when extensional faults intersected the bottom of shallow reservoirs or tapped directly the deep magma chambers. The presence of trachytic volcanoes along the rift shoulders could result from clinopyroxene-dominated high-pressure basalt fractionation, which did not allow melts to reach rhyolitic compositions. Satellite imagery and field studies reveal the occurrence of a large number of caldera collapses in the main Ethiopian rift, suggesting that several magma chambers have been formed at shallow depths, possibly favoured by block tilting and strike-slip faulting. This explains the huge amounts of silicic rocks along the northern Ethiopian rift. The occurrence of huge magma reservoirs is also supported by positive gravity anomalies detected by previous studies beneath several silicic volcanic centres.

Geochemical constraints of the petrogenesis of the O’okiep Koperberg Suite and granitic plutons in Namaqualand, South Africa: A crustal source in Namaquan (Grenville) times

The Namaquan (Grenville) Orogeny (late Mesoproterozoic) in the O’okiep District is characterized by two tectono-magmatic episodes: the O’okiepian Episode (1210–1180 Ma) with the intrusion of batholitic granites, and the Klondikean Episode (1040–1020 Ma), which includes the intrusion of the copper-bearing Koperberg Suite and the Rietberg Granite. This study focuses on the geochemistry (major and trace elements, Sr and Nd isotopes) of intrusive rocks of the O’okiep terrane to provide better constraints on their source rock characteristics and on their petrogenesis. The O’okiepian Granites belong to the K-rich granite kindred, with shoshonitic affinities. The Concordia Granite results from dehydration melting of a pelitic source. The Rietberg Granite shows geochemical similarities with post-collisional magmatic series. The anorthosites and related rocks of the Koperberg Suite are cumulates; their REE distribution is controlled by their apatite content. A new rock type, jotunite, has been identified in the Koperberg Suite; it is analogous to the Rogaland chilled jotunite, a characteristic which gives strong evidence that the Koperberg rocks belong to the massif-type anorthosite suite. Inversion modelling of plagioclase REE compositions from anorthosite permits the reconstruction of melt compositions and places constraints on the melting process and on the characteristics of the source rocks. The occurrence of jotunite in the Koperberg Suite is strong additional evidence for a crustal source, because jotunite is produced by remelting of gabbronorite under dry conditions. The various intrusions in the Koperberg Suite show distinct isotopic signatures, which resulted from isotopic heterogeneities of the crustal source and from minor contamination with the country rocks. The characteristic negative ɛ Nd(1030 Ma) values (−5 to −11) can be explained by remelting at 1030 Ma of a 1900-Ma-old oceanic crust protolith with an enriched MORB REE distribution. The large range in Sr initial ratios (0.709–0.748) may reflect hydrothermal alteration of the oceanic crust, a process which may also explain the Cu enrichment. The Koperberg intrusions were produced by forceful injection of cumulate crystal mush, which were differentiated in deeper magma chambers or in conduits.

The Aguablanca Ni–(Cu) sulfide deposit, SW Spain: geologic and geochemical controls and the relationship with a midcrustal layered mafic complex

The Aguablanca Ni–(Cu) sulfide deposit is hosted by a breccia pipe within a gabbro–diorite pluton. The deposit probably formed due to the disruption of a partially crystallized layered mafic complex at about 12–19 km depth and the subsequent emplacement of melts and breccias at shallow levels (<2 km). The ore-hosting breccias are interpreted as fragments of an ultramafic cumulate, which were transported to the near surface along with a molten sulfide melt. Phlogopite Ar–Ar ages are 341–332 Ma in the breccia pipe, and 338–334 Ma in the layered mafic complex, and are similar to recently reported U–Pb ages of the host Aguablanca Stock and other nearby calc-alkaline metaluminous intrusions (ca. 350–330 Ma). Ore deposition resulted from the combination of two critical factors, the emplacement of a layered mafic complex deep in the continental crust and the development of small dilational structures along transcrustal strike-slip faults that triggered the forceful intrusion of magmas to shallow levels. The emplacement of basaltic magmas in the lower middle crust was accompanied by major interaction with the host rocks, immiscibility of a sulfide melt, and the formation of a magma chamber with ultramafic cumulates and sulfide melt at the bottom and a vertically zoned mafic to intermediate magmas above. Dismembered bodies of mafic/ultramafic rocks thought to be parts of the complex crop out about 50 km southwest of the deposit in a tectonically uplifted block (Cortegana Igneous Complex, Aracena Massif). Reactivation of Variscan structures that merged at the depth of the mafic complex led to sequential extraction of melts, cumulates, and sulfide magma. Lithogeochemistry and Sr and Nd isotope data of the Aguablanca Stock reflect the mixing from two distinct reservoirs, i.e., an evolved siliciclastic middle-upper continental crust and a primitive tholeiitic melt. Crustal contamination in the deep magma chamber was so intense that orthopyroxene replaced olivine as the main mineral phase controlling the early fractional crystallization of the melt. Geochemical evidence includes enrichment in SiO2 and incompatible elements, and Sr and Nd isotope compositions (87Sr/86Sri 0.708–0.710; 143Nd/144Ndi 0.512–0.513). However, rocks of the Cortegana Igneous Complex have low initial 87Sr/86Sr and high initial 143Nd/144Nd values suggesting contamination by lower crustal rocks. Comparison of the geochemical and geological features of igneous rocks in the Aguablanca deposit and the Cortegana Igneous Complex indicates that, although probably part of the same magmatic system, they are rather different and the rocks of the Cortegana Igneous Complex were not the direct source of the Aguablanca deposit. Crust–magma interaction was a complex process, and the generation of orebodies was controlled by local but highly variable factors. The model for the formation of the Aguablanca deposit presented in this study implies that dense sulfide melts can effectively travel long distances through the continental crust and that dilational zones within compressional belts can effectively focus such melt transport into shallow environments.

The rise and fall of axial highs during ridge jumps

We simulate jumps of ocean spreading centers with axial high topography using elastoplastic thin plate flexure models. Processes considered include ridge abandonment, the breaking of a stressed plate on the ridge flank, and renewed spreading at the site of this break. We compare model results to topography at the East Pacific Rise between 15°25′N and 16°N, where there is strong evidence of a recent ridge jump. At an apparently abandoned ridge, gravity data do not suggest buoyant support of topography. Model deflections during cooling and melt solidification stages of ridge abandonment are of small vertical amplitude because of plate strengthening, resulting in the preservation of a “frozen” fossil high. The present‐day high is bounded by slopes with up to a 40% grade, a scenario very difficult to achieve flexurally given generally accepted constraints on lithospheric strength. We model these slopes by assuming that the height at which magma is accreted increases rapidly after the ridge jumps. This increase is attributed to high overburden pressure on melt that resided in an initially deep magma chamber, followed by a rapid increase in temperature and melt supply to the region shortly after spreading began. The high is widest at the segment center, suggesting that magmatic activity began near the center of the segment, propagated south and then north. The mantle Bouguer anomaly exhibits a “bull's‐eye” pattern centered at the widest part of the high, but the depth of the axis is nearly constant along the length of the segment. We reconcile these observations by assigning different cross‐axis widths to a low‐density zone within the crust.

Magma chamber of the Campi Flegrei supervolcano at the time of eruption of the Campanian Ignimbrite

A supereruption that occurred in the Campi Flegrei area, Italy, ca. 39 ka had regional- and global-scale environmental impacts and deposited the Campanian Ignimbrite (CI). We attempt to shed light on critical aspects of the eruption (depth of magma chamber, intensive pre-eruptive magma conditions) and the large-volume magma plumbing system on the basis of information derived from analyzing melt inclusion (MI) data. To achieve these aims, we provide new measurements of homogenization temperatures and values of dissolved H2O within phenocryst-hosted MIs from pumices erupted during different phases of the CI eruption. The MI data indicate that a relatively homogeneous overheated trachytic magma resided within a relatively deep magma chamber. Dissolved water contents in MIs indicate that prior to the eruption the magma chamber underwent radical changes related to differential upward movement of magma. Decompression of the rising trachytic magma caused a decrease in water solubility and crystallization, and trachytic bodies were emplaced at very shallow depths. The proposed eruptive model links portions of the main magma chamber and apophyses with specific eruptive units.

Trace elements in the Merensky Reef and adjacent norites Bushveld Complex South Africa

Trace elements were analysed in rocks and minerals from three sections across the Merensky Reef in the Rustenburg Platinum Mine in the Bushveld Complex of South Africa. Whole rocks and separated minerals were analysed by inductively coupled plasma-mass-spectrometer (ICP-MS) and in situ analyses were carried out by ion microprobe and by laser-source ICP-MS. Merensky Reef pyroxenites contain extremely high concentrations of a wide range of trace elements. These include elements incompatible with normal silicate minerals as well as siderophile and chalcophile elements. For major elements and compatible trace elements, the measured concentrations in cumulus phases and the bulk rock compositions are similar. For highly incompatible elements, however, concentrations in bulk rocks are far higher than those measured in the cumulus phases. In situ analyses of plagioclase have far lower concentrations of Th, Zr and rare earth elements than ICP-MS analyses of bulk separates of plagioclase, a difference that is attributed to the presence of trace-element-rich accessory phases in the bulk mineral separates. We used these data to calculate the trace-element composition of the magmas parental to the Merensky Unit and adjacent norites. We argue that there is no reason to assume that the amount of trapped liquid in the Merensky orthopyroxenite was far greater than in the norites and we found that the pyroxenite formed from a liquid with higher concentrations of incompatible trace elements than the liquid that formed the norites. We propose that the Bushveld Complex was fed by magma from a deeper magma chamber that had been progressively assimilating its crustal wall rocks. The magma that gave rise to the Merensky Unit was the more contaminated and unusually rich in incompatible trace elements, and when it entered the main Bushveld chamber it precipitated the unusual phases that characterize the Merensky Reef. The hybrid magma segregated sulphides or platinum-group-element-rich phases during the course of the contamination in the lower chamber. These phases accumulated following irruption into the main Bushveld chamber to form the Merensky ore deposits.

Origin of micro-layering in a deep magma chamber: Evidence from two ultramafic–mafic layered xenoliths from Puy Beaunit (French Massif Central)

The origin of magmatic layering is still hotly debated. To try to shed some light on this problem, two ultramafic–mafic layered xenoliths from Puy Beaunit (French Massif Central) were investigated in detail. The nodules belong to a stratiform intrusion emplaced in the deep crust during the Permian (257 ± 6 Ma; Féménias, O., Coussaert, N., Bingen, B., Whitehouse, M., Mercier, J.-C., Demaiffe, D., 2003. A Permian underplating event in late- to post-orogenic tectonic setting. Evidence from the mafic–ultramafic layered xenoliths from Beaunit (French Massif Central). Chem. Geol. 199 293–315.). The 3 to 5 cm thick nodules have, in common, a central orthopyroxenite layer; the succession of layers is, respectively, norite–orthopyroxenite–norite (PBN 00-01) and norite–orthopyroxenite–harzburgite (PBN 00-03). The variations of both major (by electron microprobe) and trace, essentially the RE, elements (by LA-ICP-MS) were measured in major mineral phases (orthopyroxene, clinopyroxene, plagioclase, spinel) along cross-section perpendicular to the layering. Strong grain size, chemical and textural variations occur along these sections: they can be continuous or discontinuous, symmetrical or asymmetrical. Such complex variations cannot be solely related to a single magmatic history (fractional crystallisation, mineral sorting). Other processes such as element enrichment by residual liquid channelling along layer boundaries and/or sub-solidus recrystallisation and element redistribution must be invoked. It appears, in particular, that element distribution in the central orthopyroxenite layer could result from the injection of micro-sills of orthopyroxene-rich liquid between previously consolidated layers.