Large Volumes Of Magma Research Articles

Unusually Cu-rich magmas associated with giant porphyry copper deposits: Evidence from Bingham, Utah

Mass-balance constraints indicate that formation of giant porphyry copper deposits (PCDs) requires either highly efficient collection of Cu from large volumes of magma or unusually Cu-rich parent magmas. Support for the second of these possibilities has been discovered in the form of mafic enclaves with abundant bornite and chalcopyrite in the Last Chance stock, one of the parent intrusions of the giant Bingham PCD, Utah, United States. Mineral assemblages and compositions indicate that the Last Chance enclaves are autoliths consisting of phases that crystallized from the intrusion, and that the intrusion was unusually enriched in Cu. One possible mechanism for generating Cu-rich magmas is fractional crystallization during pyrrhotite undersaturated conditions. The high fO 2 conditions observed for the Last Chance stock may have allowed such an evolution. Alternatively, if the magma has not undergone significant fractionation, early crystallization of chalcopyrite and bornite from the magma would indicate that the lower-crustal source region for the magma probably contained Cu-Fe sulfides. Possible Cu-rich source regions are a subcrustal mafic intrusion with sulfide cumulates, or a deeply buried metamorphic terrane containing Cu deposits such as those in the Curaca Valley (Brazil) or Okiep (South Africa). Heterogeneous distribution of Cu-Fe sulfides in an Okiep-type source terrane would produce local PCDs such as Bingham, or large accumulations of Cu-Fe sulfides, possibly in the form of cumulates in subcrustal intrusions at convergent margins, could produce giant PCD provinces such as those in Indonesia, Papua New Guinea, and central Chile.

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Large caldera collapses represent catastrophic natural events, second only to large meteoritic impacts. In addition, some calderas are densely populated, making the risk extreme, even for moderate eruptions. Understanding caldera mechanisms, unrest and the danger of eruption is therefore a crucial challenge for Earth sciences. Several key features of caldera behaviour have yet to be fully understood. Through a combination of case studies and theoretical modelling, the following topics are addressed in this volume: the conditions required to produce and to release large volumes of magma erupted during caldera formation; how magmatic feeding systems evolve before and after a caldera has formed; the processes that limit the behaviour of precursors to eruptions; how pre-emptive precursors can be distinguished from those that drive unrest without an eruption; and given that post-collapse eruptions may occur across a wide area, the optimum procedures for designing hazard maps and mitigation strategies.

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.

Large phreatomagmatic vent complex at Coombs Hills, Antarctica: Wet, explosive initiation of flood basalt volcanism in the Ferrar-Karoo LIP

The Mawson Formation and correlatives in the Transantarctic Mountains and South Africa record an early eruption episode related to the onset of Ferrar-Karoo flood basalt volcanism. Mawson Formation rocks at Coombs Hills comprise mainly (≥80% vol) structureless tuff breccia and coarse lapilli tuff cut by irregular dikes and sills, within a large vent complex (>30 km2). Quenched juvenile fragments of generally low but variable vesicularity, accretionary lapilli and country rock clasts within vent-fill, and pyroclastic density current deposits point to explosive interaction of basalt with groundwater in porous country rock and wet vent filling debris. Metre-scale dikes and pods of coherent basalt in places merge imperceptibly into peperite and then into surrounding breccia. Steeply dipping to sub-vertical depositional contacts juxtapose volcaniclastic rocks of contrasting componentry and grainsize. These sub-vertical tuff breccia zones are inferred to have formed when jets of debris + steam + water passed through unconsolidated vent-filling deposits. These jets of debris may have sometimes breached the surface to form subaerial tephra jets which fed subaerial pyroclastic density currents and fall deposits. Others, however, probably died out within vent fill before reaching the surface, allowing mixing and recycling of clasts which never reached the atmosphere. Most of the ejecta that did escape the debris-filled vents was rapidly recycled as vents broadened via lateral quarrying of country rock and bedded pyroclastic vent-rim deposits, which collapsed along the margins into individual vents. The unstratified, poorly sorted deposits comprising most of the complex are capped by tuff, lapilli tuff and tuff breccia beds inferred to have been deposited on the floor of the vent complex by pyroclastic density currents. Development of the extensive Coombs Hills vent-complex involved interaction of large volumes of magma and water. We infer that recycling of water, as well as recycling of pyroclasts, was important in maintaining water supply for phreatomagmatic interactions even when aquifer rock in the vent walls lay far from eruption sites as a consequence of vent-complex widening. The proportion of recycled water increased with vent-complex size in the same way that the proportion of recycled tephra did. Though water recycling leaves no direct rock record, the volcaniclastic deposits within the vent complex show through their lithofacies/structural architecture, lithofacies characteristics, and particle properties clear evidence for extensive and varied recycling of material as the complex evolved.

Volcanic stratigraphy of large-volume silicic pyroclastic eruptions during Oligocene Afro-Arabian flood volcanism in Yemen

A new stratigraphy for bimodal Oligocene flood volcanism that forms the volcanic plateau of northern Yemen is presented based on detailed field observations, petrography and geochemical correlations. The >1 km thick volcanic pile is divided into three phases of volcanism: a main basaltic stage (31 to 29.7 Ma), a main silicic stage (29.7 to 29.5 Ma), and a stage of upper bimodal volcanism (29.5 to 27.7 Ma). Eight large-volume silicic pyroclastic eruptive units are traceable throughout northern Yemen, and some units can be correlated with silicic eruptive units in the Ethiopian Traps and to tephra layers in the Indian Ocean. The silicic units comprise pyroclastic density current and fall deposits and a caldera-collapse breccia, and they display textures that unequivocally identify them as primary pyroclastic deposits: basal vitrophyres, eutaxitic fabrics, glass shards, vitroclastic ash matrices and accretionary lapilli. Individual pyroclastic eruptions have preserved on-land volumes of up to ∼850 km3. The largest units have associated co-ignimbrite plume ash fall deposits with dispersal areas >1×107 km2 and estimated maximum total volumes of up to 5,000 km3, which provide accurate and precisely dated marker horizons that can be used to link litho-, bio- and magnetostratigraphy studies. There is a marked change in eruption style of silicic units with time, from initial large-volume explosive pyroclastic eruptions producing ignimbrites and near-globally distributed tuffs, to smaller volume (<50 km3) mixed effusive-explosive eruptions emplacing silicic lavas intercalated with tuffs and ignimbrites. Although eruption volumes decrease by an order of magnitude from the first stage to the last, eruption intervals within each phase remain broadly similar. These changes may reflect the initiation of continental rifting and the transition from pre-break-up thick, stable crust supporting large-volume magma chambers, to syn-rift actively thinning crust hosting small-volume magma chambers.

Magma intrusion and discharge process at the initial stage of the 2000 activity of Miyakejima, Central Japan, inferred from tilt and GPS data

SUMMARY The 2000 eruptive activity of Miyakejima island began with an earthquake swarm and crustal deformation that was clearly observed by continuous observations of ground tilt and GPS on Miyakejima. Based on the crustal deformation data, we estimated the magma migration process at the initial stage (18:30 LT on 2000 June 26—06:00 LT on 2000 June 27) of the activity. The activity in 2000 has been characterized by a collapse-caldera formation and no fissure eruption on the flank of the island, which is markedly different from the style of the recent eruptions in 1940, 1962 and 1983. We constructed a source model that approximately explains the crustal deformation data. The model consists of four dykes, of which three are intruding dykes and the other is a contracting dyke. We can infer the magma migration process at the initial stage from the model as follows. At 18:30 LT on June 26, the magma began to ascend from a dyke-shaped magma chamber and intrude at the southwestern flank of Miyakejima. In the intrusion, the total volume of intruded magma was ∼4 × 10 6 m 3 .A taround 21:00 LT, a relatively large volume of magma began to intrude as a dyke beneath the west coast of Miyakejima. The dyke then propagated laterally towards the northwest. The amount of magma intruded from 21:00 LT to 01:00 LT was ∼40 × 10 6 m 3 . This large intrusion caused a discharge of magma from the magma chamber. The northwestward propagation of the dyke and the contraction of the chamber continued thereafter. The discharge of magma from the chamber beneath Miyakejima probably starved the first intrusion that had been ascending towards the southwestern flank of the island, resulting in the collapse-caldera formation after the initial stage. The style of the 2000 eruptive activity relative to recent activity has primarily been changed by the subsurface discharge of magma towards the northwest.

Contrasting sulphide contents of the Bushveld and Sudbury Igneous Complexes

The Sudbury Igneous Complex (SIC) contains abundant sulphides, especially near the base, and hosts one of the world’s largest nickel and copper deposits. The Bushveld Complex (BC) contains relatively little sulphide, but hosts the world’s largest platinum-group element deposits. The most recent calculations of the sulphur solubility in magmas that produced the BC are based on the sulphur solubility of mid-ocean ridge basalts that have less SiO2 than Bushveld magmas. Such a difference may lead to an overestimation of sulphur solubility by as much as 25%. The revised sulphur solubility curve presented here for Bushveld magmas may also have relevance to the SIC in view of its siliceous nature. Sulphur solubility curves can be used to determine the proportion of sulphide expected in cumulate rocks once sulphur saturation is attained. These models are tested using observed sulphide contents in both intrusions. The observed decreasing sulphur contents (>0.3–0.05% S) from the base of the SIC upward are broadly consistent with these sulphur solubility curves, and are consistent with sulphide saturation through the entire mafic portion. In contrast, the lower half of the BC contains extremely little sulphur (generally <0.02% S), except for two thin layers, which is not consistent with sustained sulphide saturation at any level. Previous interpretations of the sulphur content of Bushveld rocks have suggested that the Lower and Critical Zones were sulphide saturated, but that they had then lost some of the sulphide due to various processes. The present sulphide content of the cumulates of the BC is so low that, if they had once been saturated, over 90% of all the sulphide must have been removed. Mass balance calculations indicate that these large amounts of displaced sulphur remain unaccounted for in such models. Instead, the observed sulphur contents are in reasonable agreement with that expected in a cumulate sequence forming from a sulphur-undersaturated magma. Whereas the Merensky Reef and Bastard pyroxenite contain minor sulphides, the compositions of the immediate hanging wall rocks indicate sulphide undersaturation. Such an abrupt return to sulphide undersaturation is not consistent with models involving sulphide formation from large volumes of magma. One possible explanation for these two observations is that intermittent sulphur degassing occurred through a fractured roof of the BC, so that the magma was never continuously sulphur-saturated with respect to an immiscible sulphide liquid.

Early Palaeozoic magmatism in the English Lake District

SUMMARY The Lower Palaeozoic rocks of the Lake District record the rifting of the microcontinental terrane of Avalonia from Gondwana early in the Ordovician, its drift northwards across the Iapetus Ocean and collision with Laurentia and Baltica in the Silurian, and its Acadian deformation in Early Devonian times. Three discrete episodes of magmatism occurred during these events. In late Ordovician (Caradoc) times large volumes of magma were generated in response to subduction of Iapetus oceanic crust beneath Avalonia. Large-scale eruptions formed the basalt–andesite–rhyolite volcanic fields that are now preserved as the Eycott and Borrowdale volcanic groups. Large bodies of mainly granitic magma were emplaced beneath the Borrowdale Volcanic Group as part of the largely Ordovician Lake District batholith; these include, for example, the Eskdale and Ennerdale intrusions. Less than 10 Ma later, a brief episode of silicic volcanism is recorded within the marine sedimentary strata of the lowest part of the overlying Windermere Supergroup. A period of about 40 million years of apparent quiescence then ensued before further granitic masses, the Shap and Skiddaw plutons were emplaced around the margins of the batholith in Early Devonian times. All of these magmatic episodes were accompanied by minor intrusions. The Eycott and Borrowdale volcanic groups are rare examples in the geological record of the products of subaerial volcanism. Preservation is entirely due to subsidence keeping pace with the emplacement of new material. Subsidence resulted both from extensional tectonic processes and, locally, during the movement

Bubbling Magma Chambers, Cupolas, and Porphyry Copper Deposits

Porphyry copper deposits are the major source of copper and significant sources of molybdenum, gold, and other metals. They are associated with the near-surface intrusion of small stocks of intermediate composition. They can form when H2O-unsaturated magma is emplaced into wall rock that is cool enough that steep lateral thermal gradients create a narrow solidification front. At depths less than ∼4 km, cooling and crystallization cause fluid saturation to occur within sidewall magma that is mobile because it contains less than ∼25% suspended crystals. After a sufficient volume of bubbles forms, mobile sidewall magma buoyantly rises instead of sinking. The bubbles expand as they decompress, and at depths of ∼2 km they become large enough to rise on their own. separate from the upwelled magma, and charge the cupola at the top of the stock with magmatic fluid. The partially degassed magma sinks into the interior of the stock. Upwelling of saturated sidewall magma entrains deeper-seated, nearly saturated magma, which decompresses and saturates as it rises. As the system cools, the depth of H2O saturation and sidewall upwelling increases. Bubbles of copper-rich fluid are generated where the saturation front extends to depths of ∼6 km or more. Overall, the system is cooling, but the upward advection of heat maintains the cupola region at roughly constant position for the life of convective upwelling along the sidewalls. Porphyry copper ore deposits can form where draining of the fluid pocket beneath a cupola is steady and a large volume of magma is cycled through the system. Magma in the stock that escapes to intrude commonly has a porphyritic texture because crystal growth is enhanced, and nucleation is suppressed when the magma is H2O saturated. Porphyry copper deposits of common size can form during the solidification of large stocks. Super-giant porphyry copper deposits can form where the saturation front propagates from a stock into an underlying batholithic chamber with a magma volume on the order of 1000 km3 and a top at depths of 10 to 15 km.

A Gold- and Platinum-Mineralized Layer in Gabbros of The Kap Edvard Holm Complex: Field, Petrologic, and Geochemical Relations

The Lower Layered Series a cumulates of the Kap Edvard Holm Complex contain a stratiform layer, 1 to 20 m thick and > 6 km long, that is mineralized with gold and platinum-group elements (PGEs). The Lower Layered Series a cumulates (>650 m thick) are dominated by modally layered gabbro with lesser volumes of granular anorthosite and troctolite that together form 2- to 30-m-thick layers that are most abundant above the mineralized layer. Minor volumes of poikilitic wehrlite form a few 1- to 2-m-thick layers throughout the section. Field and petrographic relations as well as cryptic variations in cumulus mineral compositions indicate that the Lower Layered Series a formed in an open system magma chamber from repeated magma injections. Within the mineralized layer, average concentrations over 3 m thickness are 250 ppb Pt, 40 ppb Pd, and 50 ppb Au, with individual samples containing up to 5 ppm Pt and 6 ppm Au. The concentrations of Ir, Os, Ru, and Rh are uniformly low (<20 ppb). Stratigraphic zoning of Pt, Pd, and Au is consistent along strike with peak Au concentrations overlying Pt and Pd peaks. Platinum minerals are primary Pt-Fe alloy, sperrylite, and moncheite, with secondary Pt sulfide overgrowths. Gold occurs as alloys with Ag and Cu. These minerals are found in contact with primary and secondary silicates and oxides and with intercumulus base metal sulfides that consist of the assemblage bornite + chalcopyrite + digenite. Deposition of the mineralized layer is associated in space and time with injections of plagioclase-saturated, primitive magma that mixed with more evolved, resident magma. These events caused efficient partitioning of Au and PGE into sulfides derived from a large volume of magma, deposition of these sulfides to form the mineralized layer, and subsequent deposition of the overlying anorthosite layers. The wehrlite layers, however, were formed by postcumulus reactions between gabbro cumulates and hydrous, intercumulus melts, probably derived from the compacting cumulus pile. Subsolidus reactions between primary minerals and meteoric hydrothermal solutions caused extensive alteration of the mineralized gabbro, oxidation and desulfurization of Cu-Fe sulfides and local recrystallization of PGE minerals.

Oxygen Isotope Studies of the Voisey's Bay Ni-Cu-Co Deposit,Labrador, Canada

Oxygen isotope studies of the Voisey’s Bay intrusion and surrounding country rocks were initiated to help assess the importance of magma contamination in the generation of Ni-Cu-Co sulfide mineralization. The δ 18O values of Proterozoic Tasiuyak paragneiss country rocks range from 8.3 to 16.1 per mil, whereas those of enderbitic and mafic orthogneiss range from 5.7 to 8.7 per mil, with most values less than 7.5 per mil. The δ 18O values of the principal units of the Voisey’s Bay intrusion fall in the range of 5.4 to 7.7 per mil, with over 90 percent of the samples less than 7 per mil and not indicative of magma contamination. Strong evidence for country-rock contamination of the mafic magma comes from inclusions of paragneiss found in the basal breccia sequence of the Eastern Deeps and the feeder breccia in the Reid Brook zone. The gneiss inclusions range in δ 18O values from 4.7 to 10.6 per mil and are depleted in 18O relative to parental paragneiss country rocks. Loss of a siliceous, 18O-rich component has accompanied progressive conversion of the xenoliths to residual assemblages rich in plagioclase and hercynite. Elevated δ 18O values (up to 9.3‰) are found in troctolitic and noritic matrix of the breccias but are restricted to distances less than 2 cm from the margins of inclusions. These isotopic exchange profiles have been produced during subsolidus cooling and do not record the transport of the major high 18O component lost from the gneiss. Oxygen isotope evidence in the igneous rocks for assimilation of high 18O material derived from the gneiss is thought to have been destroyed via dilution as large volumes of magma moved from lower to higher chambers through a conduit system. The introduction of the high 18O, SiO2-rich contaminant may have acted in concert with the introduction of country-rock sulfide in promoting sulfide saturation of the Voisey’s Bay magma.

The Evolution of Troctolitic and High Al Basaltic Magmas inProterozoic Anorthosite Plutonic Suites and Implications for the Voisey's BayMassive Ni-Cu Sulfide Deposit

The Voisey's Bay massive Ni-Cu sulfide deposit is contained within troctolitic (plagioclase + olivine) intrusions of the 1.34 to 1.29 Ga Nain Plutonic Suite, coastal Labrador, Canada, and represents a new setting for magmatic sulfide deposits. Proterozoic anorthosite plutonic suites consist of vast quantities (as much as 20,000 km 2) of cumulate anorthosite-leucotroctolite-leuconorite, intrusions of troctolite and Fe-enriched dioritic rocks, and large granitic batholiths. Crystallization ages span nearly 1,200 m.y. within the Middle Proterozoic (2.12-0.92 Ga). Many Proterozoic anorthosite plutonic suites were emplaced into relatively young Proterozoic crust and some were intruded along Archean-Proterozoic boundaries (e.g., Nain, Labrador; Laramie, Wyoming; Korosten, Ukraine). Nearly all of the largest plutonic suites contain significant areas (thousands of km 2) of plagioclase-rich cumulate rocks that are olivine bearing (olivine anorthosite-leucotroctolite). Troctolitic rocks (20-30% modal olivine) occur in Proterozoic anorthosite plutonic suites, principally as cumulates in layered or massive intrusions (e.g., Kiglapait, Hettasch, Jonathon-Nain) intruded into anorthositic rocks. With the notable exception of the Voisey's Bay deposit, nearly all troctolitic rocks crystallized late in the evolution of individual plutonic suites. Many of the troctolitic intrusions have fine-grained olivine-rich marginal rocks that closely represent parental magma compositions. Numerous fine-grained olivine-normative dikes with high Al basaltic compositions also occur within many plutonic suites (Nain, Harp Lake, Laramie, Adirondack) and share compositional characteristics with the troctolitic magmas. These olivine-normative high Al basaltic magmas record the least evolved compositions in Proterozoic anorthosite plutonic suites (Mg = 0.43-0.64; An = 45-60), have high Al 2O 3 (15-19 wt %) and FeO(total) (9-15 wt %) contents, and typically show the least contaminated Sr and Nd isotope compositions in a given plutonic suite. Many dikes and marginal rocks have strong positive Eu anomalies, a feature of the melt composition that is not related to plagioclase accumulation. Except at Laramie, the least evolved troctolitic magmas appear to have about 8 wt percent MgO and corresponding Ni and S concentrations between 100 to 150 ppm and 0.02 to 0.05 wt percent, respectively. Based on closed-system fractionation in the Kiglapait intrusion, sulfide saturation occurs late in the crystallization history of troctolitic magmas, after Ni concentrations have been severely depleted by protracted olivine fractionation. The optimal conditions for formation of a Ni-Cu sulfide deposit associated with a Proterozoic anorthosite plutonic suite require a relatively high MgO parental magma that has not crystallized much prior olivine and an external source of S to promote early sulfide saturation, probably through desulfurization reactions as hot troctolitic magmas ascend through the crust. An economically important massive Ni-Cu sulfide deposit on the scale of Voisey's Bay clearly requires large volumes of magma, probably involving continuous replenishment of relatively Ni rich, unfractionated parental magma in staging chambers or mixing between magmas in conduits during ascent. A model for the formation of troctolite-associated massive Ni-Cu sulfide deposits is proposed. Depending on the relative degrees of sulfide saturation and fractionation of the mixed magmas, mixing between troctolitic magmas in a conduit or magma chamber may either poison the system, stopping sulfide segregation, or enhance sulfide segregation.

40Ar/ 39Ar dating of the Jurassic volcanic province of Patagonia: migrating magmatism related to Gondwana break-up and subduction

The Mesozoic large igneous province (LIP) of Patagonia (southern South America), which is one of the largest silicic provinces on Earth has been investigated by the 40Ar/ 39Ar method. Twenty-seven ages considered as valid, including twenty plateau ages, show that the volcanic activity, ranging from 187 to 144 Ma, occurred between and contemporaneously with the initial break-up of Gondwana (starting with the Karoo-Antarctic-Tasmanian (KAT) flood basalt province) in the east, and a subduction in the west. The data display a regular decreasing of ages from the ENE (187 Ma) to the WSW (144 Ma) along about 650 km, apparently related to the tectonic structure in half-grabens oriented NNW-SSE. The good fitting of this trend with the opening of the Rocas Verdes-Sarmiento marginal basin favors a space time evolution of this continental volcanism culminating towards the SSW in a continental disruption behind the magmatic arc. The observed age progression of volcanism may be the result of the variations of the physical characteristics of the subduction. The spreading and thermal effect of the KAT plume may have an additional effect and also could account for the unusually large volume of magma.

The present state of the magmatic system of the Campi Flegrei caldera based on a reconstruction of its behavior in the past 12 ka

New geochemical and Sr-isotope data have been acquired on samples representative of volcanic units erupted inside the resurgent Campi Flegrei caldera (CFc) over the past 12 ka. These data, integrated with previous published petrological, and with newly acquired geochronological, volcanological and geothermal data, shed light on the nature and timing of the processes that controlled the evolution of the Phlegraean magmatic system. In the past 12 ka, three isotopically and geochemically distinct magmatic components were erupted at the CFc as either homogeneous or mixed magma batches. One component, Campanian Ignimbrite component (CIc) ( 87 Sr/ 86 Sr =0.70735–0.70740), is similar to the trachytic magma extruded during the first phase of the Campanian Ignimbrite (CI) eruption (37 ka). A second component, Neapolitan Yellow Tuff component (NYTc) ( 87 Sr/ 86 Sr =0.70750–0.70757), is similar to the latitic–alkali–trachytic magma batches extruded during the course of the Neapolitan Yellow Tuff (NYT) eruption (12 ka). A third component, Minopoli component (MIc) ( 87 Sr/ 86 Sr ≈0.7086), is similar to the trachybasaltic magma of the Minopoli 2 (MI) eruption (9.7 ka). These components were erupted as either single batches of magma, or mixed CI–NYT or MI–NYT batches of magma, through vents located either along the structural boundary of the NYT caldera or inside the NYT caldera, mainly on portions of the resurgent block under extensional stress. The CI and NYT components represent residual portions of older, large-volume magma reservoirs which have fed eruptions since about 60 and 15 ka, respectively. The least-evolved MI component was erupted only during the 12–9.5 ka and 8.6–8.2 ka epochs of activity, through vents located on a NE–SW regional fault system. This component could represent a deeper reservoir tapped by the NE–SW regional fault system reactivated after the NYT caldera collapse. Deeper MI and shallower CI and NYT magmatic systems interacted by mixing among batches of magma during their rise to surface. Overall, the data suggest that the CFc magmatic system today is characterized by the presence of two larger, independent reservoirs, filled by residual portions of the CI and NYT magmas. These generated many smaller, shallower pockets of evolved magma, that fed most of the eruptions that occurred in the CFc over the past 12 ka. Moreover, a deeper reservoir (MI), tapped by the NE–SW regional fault system, provided batches of less-evolved magma that mixed with magma present in the shallower pockets.

Volcanic rocks as discriminants in evaluating tectonic versus climatic control on depositional sequences, Permo-Carboniferous continental Saar-Nahe Basin

The third- and fourth-order sequential arrangement of continental sedimentary strata in the Permo-Carboniferous Saar–Nahe Basin preserves a detailed record of contemporaneous faulting and volcanism. Transgressive lacustrine facies of the Upper Carboniferous coincide with the stratigraphic position of extrabasinally derived pyroclastic fallout layers and features of synsedimentary tectonism such as unconformities, thickness and facies changes across faults, and seismogenic deformation structures. The tectonic imprint on fourth-order cycle initiation is further documented by abrupt vertical facies transitions during lake transgression, implying rapid and episodic rather than gradual basin subsidence. This interaction becomes much more pronounced within the fluvially dominated Lower Permian, when tectonism controlled both the ascent of large volumes of magma within the basin as well as episodic enlargement of accommodation space and depositional area. Major extrusive volcanic units are preferentially associated with sequences recording transgression that are only preserved in fault-bounded hanging wall traps starved from clastic sediment input. It is suggested that the stratigraphic position of pyroclastic fallout within the sequential framework of basins elsewhere, provides an important analytical tool for the identification of contemporaneous tectonic events and as such form a discriminant between purely climatically induced Milankovitch band cycles versus those involving a tectonic control.

Emplacement and Crystallization Time for the Bushveld Complex

The Bushveld Complex formed by the crystallization of successive basaltic magmatism may be of the order of several million years. For example, the Columbia River Basalts (Hooper, injections of magma, which were sufficiently closely spaced in time that each previous magma had not cooled and differentiated 1988) were erupted in the period 17–12 Ma, with minor eruptions for a further 5 my, although most outpouring significantly before the addition of the next one. To constrain the emplacement and crystallization times, a thermal model is presented occurred within the first 2 my. It is now recognized that large intrusions were not emplaced in a single pulse, but which permits the investigation of the rate of cooling of magma in an intrusion repeatedly subjected to magma addition (and subresult from multiple magma injection. The question is how rapidly were magma chambers, such as the BC, traction). Such modelling indicates that magmas injected into the Bushveld Complex were emplaced within 75 000 years. At that filled and how much magma was involved. Answers are relevant to the dynamics of melt production, storage and time injection into the Complex ceased. The volume of rock in the Eastern and Western limbs is 370 000–600 000 km. However, transport in the mantle and crust. This paper describes a thermal modelling technique (not previously applied a quantitative evaluation of the Cr budget in the formation of chromitite layers indicates that large volumes of magma cannot be to magma chambers) which can be used to analyse this process, and to obtain an estimate of the emplacement accounted for in the preserved rock sequence. Similarly, an evaluation of the incompatible trace-element abundances, such as those for Zr time. To present this model it is necessary to discuss the stratigraphy, size, and connectivity of the different limbs and K, suggests that the chamber was open and that large volumes of differentiated magma escaped. The volume of magma therefore of the Bushveld Complex, and to consider the extent of tapping of the magma chamber as well as its filling. greatly exceeded the preserved volume of cumulate rocks, giving an estimated magma volume of over 1× 10 km. An average The term ‘Bushveld Complex’ has been given several meanings in the literature, and according to the South emplacement rate of 13 km/year is indicated by these calculations. African Commission on Stratigraphy (1980) includes not only the ultramafic–mafic layered rocks, but also the sills beneath the intrusion, volcanic rocks which pre-date the

Mingling in mafic magma chambers replenished by light felsic inputs: fluid dynamical experiments

We investigate, by means of laboratory analogues, the fluid dynamics of magma hybridisation caused by the intrusion of felsic magma into a mafic magma chamber. The ‘input’ felsic magma is modelled by a cold, low density fluid and the ‘resident’ mafic magma by a layer of fluid wax close to its solidification point. The cold, rising input drives convection and solidification in the resident fluid, and the solidification causes significant perturbations to an otherwise simple input flow, greatly enhancing mixing and mingling between the two model magmas. The dimensionless parameters controlling the evolution of the system are determined and different morphologies of the solid mass fit into clearly defined fields in the parameter space. The most important implication of the experiments is that the freezing of mafic chambers due to felsic intrusion may give rise to intense hybridization of large volumes of magma. This potentially important mechanism for hybridization has, to date, received little attention in the literature.

Ni, Cu, Au, and platinum-group element contents of sulphides associated with intraplate magmatism: a synthesis

The tectonic setting of intraplate magmas, typically a plume intersecting a rift, is ideal for the development of Ni – Cu – platinum-group element-bearing sulphides. The plume transports metal-rich magmas close to the mantle–crust boundary. The interaction of the rift and plume permits rapid transport of the magma into the crust, thus ensuring that no sulphides are lost from the magma en route to the crust. The rift may contain sediments which could provide the sulphur necessary to bring about sulphide saturation in the magmas. The plume provides large volumes of mafic magma; thus any sulphides that form can collect metals from a large volume of magma and consequently the sulphides will be metal rich. The large volume of magma provides sufficient heat to release large quantities of S from the crust, thus providing sufficient S to form a large sulphide deposit. The composition of the sulphides varies on a number of scales: (i) there is a variation between geographic areas, in which sulphides from the Noril'sk–Talnakh area are the richest in metals and those from the Muskox intrusion are poorest in metals; (ii) there is a variation between textural types of sulphides, in which disseminated sulphides are generally richer in metals than the associated massive and matrix sulphides; and (iii) the massive and matrix sulphides show a much wider range of compositions than the disseminated sulphides, and on the basis of their Ni/Cu ratio the massive and matrix sulphides can be divided into Cu rich and Fe rich. The Cu-rich sulphides are also enriched in Pt, Pd, and Au; in contrast, the Fe-rich sulphides are enriched in Fe, Os, Ir, Ru, and Rh. Nickel concentrations are similar in both. Differences in the composition between the sulphides from different areas may be attributed to a combination of differences in composition of the silicate magma from which the sulphides segregated and differences in the ratio of silicate to sulphide liquid (R factors). The higher metal content of the disseminated sulphides relative to the massive and matrix sulphides may be due to the fact that the disseminated sulphides equilibrated with a larger volume of magma than massive and matrix sulphides. The difference in composition between the Cu- and Fe-rich sulphides may be the result of the fractional crystallization of monosulphide solid solution from a sulphide liquid, with the Cu-rich sulphides representing the liquid and the Fe-rich sulphides representing the cumulate.

Paleostress reconstruction from kinematic indicators in the Oslo Graben, southern Norway: new constraints on the mode of rifting

The Oslo Graben, southern Norway, is a N-S-trending Carboniferous-Permian rift system, characterized by major mafic to silicic magmatism, N-S-trending faults, reactivation of preexisting Precambrian faults and formation of half grabens. Magmatism is expressed by the presence of lavas, dyke injections, cauldron formation and the intrusion of batholiths. Paleostress analyses, mainly based on slickensides, have been performed in the area, both within and outside the rift structure. Combined with the current tectono-magmatic model of the Oslo Graben area, the analyses show the following stress evolution from the Caledonian Orogeny, with its compressional tectonics in the Silurian to the late stages of the Carboniferous-Permian rifting. NW-SE compression occurred in Silurian times, due to the continental collision of the Caledonian Orogeny. After a long period of a missing geological record, the area was affected by N-S compression during the Late Carboniferous. A shallow sedimentary basin developed, indicative for the pre- and proto-rift phases. The transition from the proto-rift phase to the initial rift phase, is marked by a transition from a transpressional regime into a transtensional regime. During the Early Permian, the stress regime changed from pure extension to radial extension. We suggest that the Oslo Graben can not simply be explained in terms of, or passive, or active rifting, but that a combination of both, evolving through time, is more suitable to explain the observations. We propose a model in which the Oslo Graben is initially triggered by far field stresses in the latest Carboniferous and earliest Permian, which infers opening due to passive rifting. These far field stresses are suggested to be linked to the Hercynian Orogeny, active to the south in central Europe in Carboniferous times and the reorganisation of the Pangea supercontinent. In Early Permian times, the stress regime caused radial extension, indicative for uplift and a change to an active mode of rifting. Simultaneously with the radial extension, large volumes of magma are emplaced at near surface levels. Despite all alternative models, an anomalous thermal gradient is necessary to create these large volumes of magmatic material. We propose therefore that shortly after the onset of rifting the dominant rifting mode becomes active (plume-related), although far field stresses might still be present.

Extensional structures during deposition of the 3460 Ma Warrawoona Group in the eastern Pilbara Craton, Western Australia

Structures in the granite-greenstone terrain of the eastern Pilbara were mapped in both the Coongan Belt greenstones and Shaw Batholith granitoids. Important structures that are coeval with an early magmatic event, in both the granitoids and the greenstones, were identified. In the Coongan greenstones a brittle extensional fault array is syn-kinematic with felsic volcanics of the Duffer Formation, dated at 3452 ± 16 Ma (Pidgeon, 1978a). In the underlying Shaw Batholith a major ductile shear zone was mapped at the contact with the Coongan Belt. Although the foliation in the shear zone forms part of a domal structure, the stretching lineations constantly plunge to the ENE. The sense of shear as recorded in the shear zone is consistently east up with subsidiary sinistral strike slip movement on the EW-trending part and dextral movement on the NS-trending part. A wedge-shaped granitoid intruded into the shear zone during deformation. This granitoid is part of the North Shaw Suite which has been dated at 3467 ± 6 Ma (McNaughton et al., 1988). The structures in the greenstones and in the batholith have therefore formed during the same event at different crustal levels. The structural/kinematic, magmatic, metamorphic and geochronological data are consistent with a model in which these structures represent the downdip part of a dome with a core complex type of geometry. In turn, this implies that the domal geometry of the Shaw Batholith was initiated, at mid crustal levels, at a very early stage (namely during deposition of the Warrawoona Group) in the evolution of the Pilbara craton. As extension occurred during intrusion and extrusion of large volumes of magma, the original distribution and thickness of granitoid bodies and greenstones were probably largely determined by the extensional geometry. The initial domal shape of the Shaw Batholith has been enhanced during later deformation phases, which caused steepening of NS-trending structures.