Large Layered Intrusions Research Articles

Vein formation of the Monchepluton and the Monchetundra intrusion contact zone

The Monchegorsk region (Kola Peninsula) is distinguished by its platinum metal specialization and is a promising target for prospecting and exploration of valuable, strategically important platinum group metals. In the course of the study, the petrographic, petrochemical, and geochemical features of the vein bodies in the junction zone of large Early Proterozoic layered intrusions (the Monchegorsk pluton and the Monchetundra intrusion) have been characterized. Vein bodies are observed within the South Sopchinsky massif and the Moroshkovoe Lake massif. The veins are located near the contact of fine-medium-grained metapyroxenites (metanorites) and coarse-grained gabbronorites. Two main types of veins are identified: 1) veins of plagioclase-pyroxene and plagioclase-amphibole composition and 2) veins of quartz-feldspar and amphibole-plagioclase composition. Analysis of the content of chalcophilic elements in this area rocks has shown that the increased content of platinum group elements is associated with the first type of vein formations. Vein rocks of plagioclase-pyroxene and plagioclase-amphibole composition (type 1) contain on average Cu - 0.22 %, Ni - 0.11 %, ΣPGE - 1 g/t, Au - 0.042 ppm. Vein rocks of quartz-feldspar and amphibole-plagioclase composition (type 2) contain on average Cu - 0.04 %, Ni - 0.01 %, ΣPGE - 0.05 ppm, Au - 0.012 ppm. The distribution spectra of rare-earth and rare elements in the vein rocks of the South Sopchinsky massif and the Moroshkovoe Lake massif show both the commonality of all types of veins and the regular nature of differences in the composition of the veins.

Intermediate rocks in the Comei large igneous provinces produced by amphibole crystallization of tholeiitic basaltic magma

The origin of intermediate rocks in large igneous provinces (LIPs) is unclear. We investigated this with an integrated petrological, mineralogical, geochronological, and geochemical study of the Chigu Tso tholeiitic intrusive suite (including dolerite, diorite, and tonalitic dykes and sills) in central Comei LIP, SE Tibet. The intermediate rocks (diorites and tonalites) within this suite were dated at c. 133 Ma, coeval with the c. 132 Ma Comei LIP. This suite exhibits a compositional continuum from dolerite through diorite to tonalite (SiO2 = 51.2–62.6 wt%) and similar Nd isotopic compositions, suggesting derivation from a common parental magma. Mass balance calculations of major elements suggest that the tholeiitic intermediate rocks with a high FeOt/MgO ratio (> 3.4) in Chigu Tso can be produced by large degrees (60–85 wt%) of differentiation of primary magmas with a low FeOt/MgO ratio (0.7). The P–T estimates based on clinopyroxene and amphibole compositions suggest a persistent mid-crustal magma chamber (11–19 km deep) beneath Chigu Tso that allowed for cooling (> 350 °C) and extensive magma differentiation (up to 85 wt% fractional crystallization). We attribute the generation of intermediate tholeiitic magmas to the fractional crystallization of a large amount of SiO2-poor and halogen-rich amphiboles at the late stage of differentiation. This process can also explain the generation of “granophyre” in large layered intrusions, based on their similar volatile-rich nature and high FeOt/MgO ratio.

THE ORIGIN AND STRUCTURE OF THE LOWER CRUST OF OCEANS AND BACK-ARC SEAS: EVIDENCE FROM THE MARKOV DEEP (MID-ATLANTIC RIDGE) AND THE VOIKAR OPHIOLITE ASSOCIATION (POLAR URALS)

The Markov Deep (the axial part of the slow-spreading Mid-Atlantic Ridge, 6°N, Sierra Leone oceanic core complex) and the Paleozoic Voikar ophiolite association (Polar Urals) formed in the back-arc sea conditions. In both cases, the lower crust of a close structure was formed on the basements composed ofdepleted peridotites of the ancient lithospheric mantle. The available data show that the composition of the lower crust of the oceans and back-arc seas is dominated by layeredmafic-ultramafic intrusions originating from the MORB melts, and suggest a similar asthenospheric source of magmas. Sills and dykes formed from other magma sources represent the second structural element of the lower oceanic crust: in case of the ocean, mainly ferrogabbroids originating from specific melts with the OIB involvement, and, in case of the back-sea sea, gabbro-norites of the supra-subduction calc-alkaline series. In both cases, the upper crust originates frombasaltic flows that occurred later and are associated with new episodes in the tectonic development. According to [Sharkov,2012], the development of slow-spreading ridges takes place in discrete impulses and non-simultaneously along their entire length. Furthermore, oceanic core complexes (OCC) in their axial parts are the ridge segments, where spreading is resumed. At the OCC stage, newly formed basalt melts move upwards from the magma generation zone into fractures (dykes) through the lithospheric mantle, and the thickness of the lower crust is built up by sills. As spreading develops in this area, the crust becomes thicker from below due to underplating in form of large layered intrusions. The newly formed restites, in their turn, cause an increase in the lithospheric mantle thickness from below. Apparently, the lower crust formed in the back-arc seas according to a similar scenario, although complicated by the processes taking place in the subduction zone.

The genesis of the newly discovered giant Wuben magmatic Fe–Ti oxide deposit in the Emeishan Large Igneous Province: a product of the late-stage redistribution and sorting of crystal slurries

A giant Fe–Ti oxide deposit hosted by the Wuben mafic intrusion has recently been discovered in the Pan–Xi area of the Emeishan Large Igneous Province (ELIP). The evolved compositions of the gangue minerals within the Fe–Ti oxide ores indicate that they formed during later stages of magma differentiation than those within the neighboring Panzhihua intrusion or other ore-bearing intrusions in this area. The rocks from the Wuben intrusion and MZb of the Panzhihua intrusion contain compositionally similar silicate minerals and have similar titanomagnetite/ilmenite ratios, suggesting that the former is related to and probably connected to the latter by subsurface magmatic conduits. This indicates that unconsolidated minerals that formed in the MZb flowed as crystal slurries into the Wuben magma chamber during the later stages of evolution of the parental magma. The later secondary enrichment of Fe–Ti oxides by mechanical redistribution and the sorting of crystals as a result of density and size differences generated the Wuben massive Fe–Ti oxide bodies. The ilmenite was commonly saturated in the magma at late stage of differentiation in the ELIP, thereby the associated deposit contains much higher contents of ilmenite. This indicates that future exploration for Fe–Ti oxide mineralization in the ELIP should not merely focus on the lower parts of large layered intrusions but should also include nearby relatively small intrusions or even the upper parts of large intrusions, especially as ilmenite-enriched Fe–Ti oxide deposits may have greater economic value than ilmenite-poor deposits.

Geology, U–Pb zircon geochronology, and geochemistry of PGE-bearing Neoarchean and Paleoproterozoic gabbroic rocks of the Peter Lake domain, southern Hearne craton, Canada

The Peter Lake domain, a component of the Hearne craton in Saskatchewan, includes several intrusive complexes ranging in age from 2.70 to 1.83 Ga. Two gabbroic complexes (Swan River and Porter Bay) together represent one of the largest accumulations of Precambrian mafic intrusion, both metamorphosed at amphibolite facies. Sulfide-poor PGE occurrences are found in both complexes and share many textural and lithological characteristics with magmatic contact-type PGE–(Ni–Cu) deposits such as the chaotic Lac des Iles pluton and with layered stratiform deposits of large layered intrusions such as Bushveld or Stillwater. Lithologically, both complexes are dominated by leucocratic gabbronorite and gabbro, locally characterized by cumulate layering, cross-bedding, brecciated, and pegmatitic textures. U–Pb zircon crystallization ages between 2562 and 2560 Ma were obtained for the Swan River complex. sulfide formation is interpreted to be of magmatic origin rather than remobilized, as had been speculated by exploration geologists, and is therefore of the same age as the host gabbro. Geochemical data support the interpretation of a mantle plume origin in a subduction environment for the Swan River complex. The Porter Bay complex is much more restricted in areal extent and a leucocratic gabbronorite yielded a U–Pb zircon age of 1913 ± 1 Ma, which is interpreted as the crystallization age and the time of sulfide formation. Major and trace element geochemical data of Porter Bay complex rocks show a considerably more evolved character than the Swan River complex, and is interpreted as indicating emplacement in a continental arc environment.

Immiscible Fe- and Si-rich silicate melts in plagioclase from the Baima mafic intrusion (SW China): Implications for the origin of bi-modal igneous suites in large igneous provinces

The Emeishan large igneous province (ELIP) in SW China is characterized by voluminous high-Ti and low-Ti basalts and spatially associated Fe-Ti oxide-bearing mafic-ultramafic and syenitic/granitic intrusions. The Baima layered mafic intrusion in the central part of the ELIP is surrounded by syenitic and granitic rocks and contains a Lower Zone of interlayered Fe-Ti oxide ores, troctolites and clinopyroxenites and an Upper Zone of isotropic olivine gabbros and gabbros (UZa) and apatite gabbros and Fe-Ti-P oxide ores (UZb). Polycrystalline mineral inclusions, for the first time, were observed in primocryst plagioclase from the basal part of the UZa through to the top of the UZb and consist mostly of clinopyroxene, plagioclase, magnetite, ilmenite and apatite with minor orthopyroxene, sulfide and hornblende. These minerals are commonly anhedral and form irregular shapes. Daughter plagioclase usually crystallizes on the walls of host primocryst plagioclase and has An contents typically 3–6 An% lower than the host plagioclase. Daughter clinopyroxene has similar Mg# but lower TiO2 and Al2O3 contents than primocryst clinopyroxene. These polycrystalline mineral inclusions are considered to crystallize from melts contemporaneous with host plagioclase. The compositional differences between daughter and primocryst minerals can be attributed to equilibrium crystallization in a closed system of the trapped melt inclusions in contrast to fractional crystallization and possible magma replenishment in an open system typical for primo-cumulates of large layered intrusions. Heated and homogenized melt inclusions have variable SiO2 (33–52wt%), CaO (7–20wt%), TiO2 (0.1–12wt%), FeOt (5–20wt%), P2O5 (0.2–10wt%) and K2O (0–2.2wt%). The large ranges of melt compositions are interpreted to result from heterogeneous trapping of different proportions of immiscible Si-rich and Fe-Ti-rich silicate liquids, together with entrapment of various microphenocrysts. The separation of micrometer-scale Si-rich melts at least started from the lower part of the Upper Zone. We thus infer that the Baima high-Ti basaltic magmas evolved into the field of immiscible Fe-rich and Si-rich melts. Prolonged fractional crystallization of olivine, plagioclase and clinopyroxene from the Fe-rich melts formed gabbro and olivine gabbro in the UZa, before the melts became saturated in Fe-Ti oxides and apatite and formed apatite-rich Fe-Ti oxide ores in the UZb. Immiscible Si-rich melts migrated upwards and coalesced to form fayalite syenite around the Baima intrusion. Silicate liquid immiscibility may have played an important role in the petrogenesis of Fe-Ti oxide-rich layered mafic intrusions and syenitic rocks in the ELIP and elsewhere. This mechanism may be common and may explain the traditionally thought bi-modal assemblages of large igneous provinces in intra-continental extensional settings.

An overview of nickel mineralisation in Africa with emphasis on the Mesoproterozoic East African Nickel Belt (EANB)

Nickel production in Africa takes place principally in Botswana, South Africa and Zimbabwe, with much of the South African and Zimbabwean production being a by-product of platinum-group element mining in the Bushveld Complex and Great Dyke. Several large nickel deposits have been discovered elsewhere in Africa but until recently, their development has been hindered by political risk and limitations in energy and transport infrastructure. Most of the continent is significantly underexplored with respect to base metals, including the area covered by the East African Nickel Belt (EANB). The known nickel deposits of the EANB all occur in mafic-ultramafic intrusive rocks of the Mesoproterozoicage Kibaran igneous event. These intrusive bodies take the form of medium to large layered intrusions, small dynamic magma conduits (chonoliths and sills) and dyke swarms. Laterite deposits are developed over exposed dunite and peridotite lithologies in the basal sequence of larger layered intrusions, whereas nickel sulphide deposits are developed at the base of the small chonoliths. Geochronological and geochemical data suggests that all intrusions in the EANB formed in a single magmatic event (1350 to 1400 Ma) and were derived from a picritic parental magma, which was variably contaminated in mid to upper-crustal staging chambers by metasedimentary rocks. As a result, nickel sulphide mineralisation was formed in all of the intrusions, but in most, the grades and tenors are too low to be considered economic in the foreseeable future. In the 1970s, government-led regional surveys identified a large nickel laterite deposit at Musongati in Burundi and a nickel sulphide deposit at Kabanga in the northwest of Tanzania. These deposits have subsequently been explored and delineated by mining companies, but they remain undeveloped due to their distance to the coast and a lack of transport and energy infrastructure. The Kabanga sulphide deposit now comprises a total mineral resource of 58 million tonnes grading 2.6% nickel. The Musongati laterite deposit comprises an overall resource of 122 million tonnes with a grade of 1.4% nickel.

HIGH-PRECISION U-Pb ZIRCON-BADDELEYITE DATING OF THE J-M REEF PLATINUM GROUP ELEMENT DEPOSIT IN THE STILLWATER COMPLEX, MONTANA (USA)

The Stillwater Complex, Montana (USA), is one of the world’s largest layered intrusions and hosts the Johns-Manville (J-M) Reef, a world-class platinum group element (PGE) deposit. In this study, zircon and baddeleyite were successfully extracted from three mineralized samples of the J-M Reef. The samples, which were collected over 15 km of strike length of the deposit, provide high-precision crystallization ages for the reef and the Stillwater Complex. All samples are coarse grained to pegmatitic and range in composition from olivine-bearing gabbronorite to anorthosite and troctolite. Irregular (anhedral) morphology and prominent sector zoning characterize interstitial zircon. U-Pb crystallization zircon ages determined by chemical abrasion thermal ionization mass spectrometry (CA-TIMS) for the three samples are indistinguishable within uncertainty: Frog Pond adit (2709.11 ± 0.56 Ma), East Boulder mine (2709.28 ± 0.32 Ma), and West Fork area (2709.00 ± 0.45 Ma). A U-Pb baddeleyite date from the Frog Pond adit sample (2708.85 ± 0.46 Ma) is identical within uncertainty with the zircon dates. Collectively, the results illustrate that the J-M Reef is an intrusion-wide time marker that formed at ca. 2709 Ma during crystallization of the Lower Banded series of the Stillwater Complex. The recognition that zircon can be successfully extracted from mafic-ultramafic rocks associated with magmatic ore deposits provides new opportunities for testing the timing of mineralization and duration of mineralization processes in layered intrusions worldwide.

Petrogenesis of the 2115 Ma Haicheng mafic sills from the Eastern North China Craton: Implications for an intra-continental rifting

The Rhyacian (2300–2050Ma) is a special era of the Paleoproterozoic represented by large layered intrusions in many cratons. It is well known that there are widespread igneous events at ~2100Ma in the Eastern North China Craton; however, their tectonic environments are under debate: whether they were related to an intra-continental rifting or an arc/back-arc setting along a continental margin. These ~2100Ma igneous events comprise several mafic dykes/sills, with some coeval A-type granites and volcanic events in several rifts; among them, the Haicheng mafic sills in the Liaohe rift are unique as their host rock, the Liaohe Group, bears the world's largest magnesium deposit. Most of the mafic sills are E-W-elongated at present coordinates. Exclusive of superimposition caused by deformation, the widths of the individuals are tens to hundreds of meters and the lengths are hundreds to thousands of meters. They have metamorphosed to an assemblage of plagioclase and hornblende, with minor quartz and accessory chlorite, epidote, apatite, ilmenite, and magnetite. However, relic gabbro and ophitic textures with mainly plagioclase and clinopyroxene are well-preserved. SIMS Pb–Pb dating on baddeleyites from one ~1000m thick sill near Xialiulinzi village yields an average 207Pb/206Pb age of 2115±3 Ma (n=15, MSWD=2.3), representing the timing of crystallization. SIMS U–Pb dating on zircon yields a similar forming age. They are tholeiitic in composition (MgO: 4.36–8.88 wt.%; SiO2: 45.76–53.39 wt.%), enriched in light rare earth elements ((La/Yb)N=1.72–4.37) and large ion lithophile elements (i.e., Cs, Rb, Sr, and K) but depleted in high field strength elements (i.e., Nb, Ta, and Ti). These features were unlikely caused by crustal contamination during their emplacement, as there are little variations in Nb/La and Th/Nb. The rocks have experienced significant plagioclase-plus clinopyroxene-dominating fractional crystallization. Their enriched Sr–Nd isotope characteristics (87Sr/86Srt=0.703~0.705, εNdt=−1.9~0.6) and trace element patterns indicate that their source(s) could be the ancient subcontinental lithospheric mantle; and this source is similar to those coeval sills from other parts of the craton. Their arc-like trace element features could be inherited from their source regions formed via a subduction process at the late Archean rather than at the middle-late Paleoproterozoic. These sill swarms, throughout the craton, might have developed in an integrated intra-continental rift system at ~2100Ma.

Olivine Slurry Replenishment and the Development of Igneous Layering in a Franklin Sill, Victoria Island, Arctic Canada

The Franklin sills and dykes on Victoria Island in the Canadian Arctic represent the sub-volcanic plumbing system to the Natkusiak flood basalts, which are associated with the late Neoproterozoic (c. 723–716 Ma) break-up of Rodinia. The Lower Pyramid Sill (LPS) is the distal end of a sill complex that may be rooted in the Uhuk Massif, a major fault-guided magmatic feeder system. The LPS is unusual for a thin (c. 21 m), shallow, tholeiitic intrusion because it displays well-developed cumulate layering similar to that seen in large layered intrusions. The LPS has an aphanitic, olivine-phyric (c. 5%) Lower Chilled Margin (LCM), a (<1 m thick) dendritic, olivine-phyric Lower Border Zone (LBZ), a (c. 7 m thick) olivine-dominated (up to c. 55%) melagabbro–feldspathic-peridotite zone (OZ), a thin (c. 1 m) clinopyroxene-rich cumulate gabbro (CPZ) containing sector-zoned euhedral clinopyroxene, a (c. 10 m thick) doleritic gabbro zone (DZ), a (<1 m thick) aphyric, dendritic Upper Border Zone (UBZ) and an aphanitic, olivine-phyric (c. 5%) Upper Chilled Margin (UCM). Distinct compositional groups recognized in olivines from the OZ can be associated with specific crystal morphologies, some showing significant reverse zoning. Melt compositions were calculated through application of the olivine–melt Fe ¼ Mg exchange coefficient. The calculations suggest that phenocrystic and primocrystic olivine (Fo88–82) in the LCM–LBZ and lower OZ formed from melts with c. 13–10 wt % MgO. Modeling implies that reversely zoned olivine primocrysts and chadacrysts have rims in equilibrium with melts of c. 10–8 wt % MgO that were saturated only in olivine (þ minor chromite), whereas some olivine cores formed from melts as evolved as c. 6–5 wt % MgO that would have coexisted with a gabbroic assemblage. The presence of multiple olivine populations in the OZ (some reverse zoned) indicates that the LPS did not crystallize from a single pulse of melt that evolved by closed-system fractional crystallization. We propose that the reverse zoning pattern records incorporation of evolved crystals, most derived from the mushy gabbroic host, when an olivine-charged replenishment under- or intraplated the partly crystallized basaltic magma, now preserved as the DZ. The intervening CPZ may also owe its origin to the emplacement of the olivine slurry, possibly as a result of pore-scale melt mixing at this interface. The DZ shows inward differentiation trends that can be explained by in situ differentiation. The data imply that late emplacement of olivine-rich

Settling and compaction of olivine in basaltic magmas: an experimental study on the time scales of cumulate formation

A series of centrifuge-assisted settling experiments of 30 vol % olivine in 70 vol % basaltic melt was conducted to elucidate the formation mechanisms and time scales of gravitational cumulates. The settling experiments were performed in a centrifuging piston cylinder at 200–1,500g, 1,270–1,280 °C, and 0.8–1.1 GPa on previously annealed and texturally equilibrated samples. The mechanical settling of the dense olivine suspension occurs at about 1/6 the speed of simple Stokes settling, resulting in a sedimentation exponent n = 4.1(6) in agreement with predictions from analogue systems. The porosity (φ m ) of the orthocumulate resulting from gravitational settling of crystals is about 54 % and formation times of olivine orthocumulates result to 0.1–10 m day−1 (for an initial crystal content of the melt of 1–5 % and grain sizes of 2–10 mm). After mechanical settling, olivine grains rest on each other, and further compaction occurs through pressure dissolution at grain contacts, olivine reprecipitation where olivine is in contact with melt, and concomitant expulsion of excess liquid from the cumulate layer. With centrifugation at 400g for 50 h, porosities as low as 30.3 vol % were achieved. The olivine content at the bottom of the gravitational cumulate is 1 − φm ~ log(Δρ · h · a · t), where Δρ is the density difference between crystals and melt, h the crystal layer thickness, a the acceleration, and t the time of centrifuging. Compaction is hence proportional to effective stress integrated over time indicating that pressure dissolution is the dominant mechanism for chemical compaction. The compaction limit, that is the lowermost porosity to be reached by this mechanism, is calculated by equating the lithostatic and hydraulic pressure gradients in the cumulate and results to 3–5 % porosity for the experiments. Crystal size distribution curves and a growth exponent n of 3.1(3) indicate that diffusion-controlled Ostwald ripening is the dominant crystal growth mechanism. The above relationship, combined with a linear scaling for grain size as appropriate for reaction-controlled pressure solution creep, allows calculation of formation times of adcumulates. If chemical compaction is dissolution–reprecipitation limited, then single layers of natural olivine adcumulates of ½ m thickness with 70–75 vol % olivine at the base (as observed in the Rhum layered intrusion) would have typical formation times of 0.4–3 years for grain sizes of 2–10 mm. This time scale compares favourably with characteristic cooling times of sills. If a greater than 20-m-thick series of cumulate layers pressurizes a base layer with the porosity still filled by a melt, then compaction proceeds to the compaction limit within a few years. It can thus be expected that in layered mafic intrusions where cumulates are continuously deposited from a large magma chamber and which characteristic cooling times of more than decades, a compaction zone of several tens of metres forms with adcumulates only maintaining porosities in the order of 5 %. In conclusion, gravitational settling and gravitation-driven chemical compaction are feasible cumulate-forming processes for dense mafic minerals in basaltic magmas and in particular in large layered intrusions.

Whole-Rock and Mineral Composition Constraints on the Genesis of the Giant Hongge Fe-Ti-V Oxide Deposit in the Emeishan Large Igneous Province, Southwest China

The Hongge giant Fe-Ti-V oxide ore deposit is hosted by a layered intrusion located in the central part of the Emeishan large igneous province, SW China. The intrusion is relatively small in comparison with other typical oxide-bearing intrusions worldwide; it consists of a lower olivine clinopyroxenite zone, a middle clinopyroxenite zone, and an upper gabbro zone (herein referred to as the lower, middle, and upper zones). Most of the economic Fe-Ti-V oxide ore layers occur within the middle zone. The Hongge oxide ores are depleted in REE and enriched in Zr, Hf, Nb, and Ta as compared to the associated clinopyroxenites. This enrichment of elements that are compatible in titanomagnetite is consistent with the interpretation that the ores formed by accumulation of magnetite and ilmenite. As in the nearby coeval Panzhihua Fe-Ti-V oxide deposit described previously by others, mafic silicates in the Hongge deposit have much higher MgO contents than those in other oxide deposits associated with large layered intrusions in the world. This highlights the importance of relatively primitive parental magma becoming saturated in titanomagnetite at an early stage in the genesis of the giant Fe-Ti-V oxide deposits in the Emeishan large igneous province. Phase equilibrium constraints suggest that the parental magma of the Hongge deposit is similar to that of some of the most primitive high Ti basalts in the Emeishan large igneous province. The ferrobasaltic parental magma and the ferropicritic primary magma of the Hongge intrusion are similar in major and trace element composition to the ferropicritic-ferrobasaltic magma in the Pechenga belt, Kola peninsula, Russia. Depletion of incompatible trace elements in the oxide ores and associated rocks in the Hongge intrusion as compared to the coeval high-Ti basalts suggest that not all the magma involved in the development of the Hongge intrusion has been retained in the intrusion. The occurrence of multiple Fe-Ti oxide layers alternating with Fe-Ti oxide-bearing silicate layers within a single zone and the repetitive appearance of sulfides, olivine, and Cr-rich layers suggest that multiple pulses of magma were involved in the formation of the Hongge intrusion and related Fe-Ti-V oxide deposit. We propose that the middle zone of the Hongge intrusion was a magma stepwise flow-through system and that some of the liquid was lost to the peripheral lavas to form the basalt.

Anorthosite formation by plagioclase flotation in ferrobasalt and implications for the lunar crust

The Sept Iles layered intrusion (Quebec, Canada) is dominated by a basal Layered Series made up of troctolites and gabbros, and by anorthosites occurring (1) at the roof of the magma chamber (100–500 m-thick) and (2) as cm- to m-size blocks in gabbros of the Layered Series. Anorthosite rocks are made up of plagioclase, with minor clinopyroxene, olivine and Fe–Ti oxide minerals. Plagioclase displays a very restricted range of compositions for major elements (An 68–An 60), trace elements (Sr: 1023–1071 ppm; Ba: 132–172 ppm) and Sr isotopic ratios ( 87Sr/ 86Sr i : 0.70356–0.70379). This compositional range is identical to that observed in troctolites, the most primitive cumulates of the Layered Series, whereas plagioclase in layered gabbros is more evolved (An 60–An 38). The origin of Sept Iles anorthosites has been investigated by calculating the density of plagioclase and that of the evolving melts. The density of the FeO-rich tholeiitic basalt parent magma first increased from 2.70 to 2.75 g/cm 3 during early fractionation of troctolites and then decreased continuously to 2.16 g/cm 3 with fractionation of Fe–Ti oxide-bearing gabbros. Plagioclase (An 69–An 60) was initially positively buoyant and partly accumulated at the top of the magma chamber to form the roof anorthosite. With further differentiation, plagioclase (<An 60) became negatively buoyant and anorthosite stopped forming. Blocks of anorthosite (autoliths) even fell downward to the basal cumulate pile. The presence of positively buoyant plagioclase in basal troctolites is explained by the low efficiency of plagioclase flotation due to crystallization at the floor and/or minor plagioclase nucleation within the main magma body. Dense mafic minerals of the roof anorthosite are shown to have crystallized from the interstitial liquid. The processes related to floating and sinking of plagioclase in a large and shallow layered intrusion serve as a proxy to refine the crystallization model of the lunar magma ocean and explain the vertically stratified structure of the lunar crust, with (gabbro-)noritic rocks at the base and anorthositic rocks at the top. We propose that the lunar crust mainly crystallized bottom-up. This basal crystallization formed a mafic lower crust that might have a geochemical signature similar to the magnesian-suite without KREEP contamination, while flotation of some plagioclase grains produced ferroan anorthosites in the upper crust.

Temporal and spatial controls on the formation of magmatic PGE and Ni–Cu deposits

Magmatic PGE and Ni–Cu deposits form in contrasting geologic environments and periods. PGE deposits predominantly occur in large layered intrusions emplaced during the late Archean and early Proterozoic into stabilized, relatively S-poor cratonic lithosphere that provides enhanced preservation potential. The magmas ascend through intracratonic sutures where extension and rifting is limited. Crystallization under conditions of low regional stress, with limited magma-induced sagging due to underlying thick buoyant sub-continental mantle lithosphere, is consistent with their laterally continuous layering. Most of the global resources occur in three large intrusions: Bushveld, Great Dyke and Stillwater. Due to the large size (tens of kilometres) and limited complexity of the deposits, they are relatively easy to locate and delineate. As a result, the search space is relatively mature and few new discoveries have been made in the last few decades. The parental magmas to the intrusions are predominantly derived from the convecting mantle but, in addition, the involvement of the sub-continental lithospheric mantle is suggested by the relative Pt enrichment of most of the major deposits. In contrast to the PGE deposits, Ni–Cu deposits form throughout geologic time, but with the largest deposits being younger than ca. 2 Ga. The sulfide ores are concentrated under highly dynamic conditions within lava channels and magma conduits. The deposits are preferentially located near craton margins towards which mantle plumes have been channelled and where mantle magmas can readily ascend through abundant trans-lithospheric structures. Magma flow is focused and locally enhanced by shifting compressive–extensional tectonic regimes, and abundant S-rich crustal rocks provide an external S source that is required for the majority of deposits. The igneous bodies hosting the deposits tend to be irregular and small, tens to hundreds of metres in width and height, and are difficult to locate. As a result, the search space remains relatively immature. Understanding their tectonic setting helps reduce the prospective search space for world-class examples.

Layered intrusions and volcanic sequences in Central Brazil: Geological and geochronological constraints for Mesoproterozoic (1.25 Ga) and Neoproterozoic (0.79 Ga) igneous associations

A 350-km long belt of layered complexes and associated volcano-sedimentary sequences forms a continental-scale feature exposed along the internal portion of the Neoproterozoic Brasília Belt in central Brazil. This study provides new geochronological results and a critical review of the available data of these igneous associations of central Brazil. Precise age dating combined with geological and petrological studies indicate that this belt consists of two distinct igneous rock associations. The 1.25 and 0.79 Ga igneous episodes are constrained by reliable U–Pb zircon ages (SHRIMP and ID-TIMS) obtained in different regions and lithotypes of this belt. Both igneous associations were affected by high-grade metamorphism and tectonism at ca. 0.76 Ga, which partially disrupted the original igneous stratigraphy of the layered complexes and volcano-sedimentary sequences. The present configuration of this belt results from later events, probably caused by final ocean closure and continental collision between the São Francisco and Amazonian continents at ca. 0.63 Ga. The 1.25 Ga magmatic event represents an igneous rock association comprising extensive bimodal volcanic rocks (Palmeirópolis, Juscelândia and Indaianópolis sequences) and associated large mafic layered intrusions (Serra dos Borges and Serra da Malacacheta complexes). Geochemical data of mafic volcanic rocks indicate compositions similar to MORB, suggesting a tectonic setting where continental rifting led to the opening of an ocean basin. Large layered intrusions coeval to the volcano-sedimentary sequence consist mainly of olivine plus plagioclase cumulates, with abundant troctolite and leucotroctolite, associated with olivine gabbro, gabbro, anorthosite and pyroxenite. These layered intrusions are characterized by moderately primitive olivine compositions (Fo 63–76), positive ɛ Nd(1250Ma) values, low contents of incompatible trace elements and LREE-depleted mantle-normalized-REE patterns. These features suggest an origin from the same depleted mantle source that produced the extensive MORB-type volcanism. Intrusion of layered complexes possibly occurred since the early stages of rifting, being emplaced both at the base of and within the volcanic pile. Different magmatic ages and fractionation trends of layered bodies suggest that they represent a cluster of intrusions that originated during the time span of the continental to oceanic ridge rifting event. The 0.79 Ga igneous association consists of three large layered intrusions (Niquelândia, Barro Alto, and Canabrava complexes). Despite intense tectonism, several striking geological similarities between the three separate bodies have been used to suggest that they originally constituted a single continuous magmatic structure. These layered intrusion are characterized by thick ultramafic zones, highly primitive compositions of olivine (up to Fo 93) and extensive Fe-enrichment trends of pyroxenes. Layered mafic–ultramafic rocks have abundant evidence of crustal contamination, including highly negative ɛ Nd(800Ma) values, as well as older crustal xenoliths and zircon xenocrysts. Cyclic units consisting of successive layers of dunite, harzburgite, orthopyroxenite, websterite and gabbronorite occur throughout the stratigraphy of these layered intrusions. The Niquelandia Complex ranks among the thickest layered intrusions in the world, and together with the Barro Alto and Canabrava layered intrusions represent an enormous volume of mantle-derived magma emplaced into the crust during a continental rifting event. The 1.25 and 0.79 Ga rifting events are not exposed in the Brasília Belt or in the São Francisco Craton. This supports previous interpretations suggesting that these igneous rock associations are part of an exotic block in the Brasília Belt. Equivalent Mesoproterozoic (ca. 1.25 Ga) and Neoproterozoic (ca. 0.79 Ga) extensional events have been reported in other continents (e.g., North America). These igneous associations described within an allochtonous block in the Brasília Belt provide additional constraints, therefore, for continental reconstruction models before the break-up of Rodinia.

Flood basalt-related Fe–Ti oxide deposits in the Emeishan large igneous province, SW China

In the Panzhihua–Xichang region (Sichuan Province, SW China), there are a number of world-class magmatic Fe–Ti oxide deposits. They are hosted as conformable masses in lower parts of layered mafic–ultramafic intrusions that are part of the end-Guadalupian (∼ 260 Ma) Emeishan large igneous province. The ore-bearing Panzhihua, Hongge, Baima, Taihe and Xinjie intrusions are spatially and temporally associated with flood basalts and granitoids in the province. New mineralogical data for Fe–Ti oxide ores in the Hongge and Baima intrusions, combined with an overview of previously published data, provide a better understanding on the formation of these enigmatic deposits. Geochemical and Sr–Nd isotopic data confirm a genetic relation between the intrusions and high-Ti Emeishan flood basalts. Parental magma compositions, estimated using the most Mg-rich cumulus olivine in the intrusions, are characterized by slight to moderate degrees of fractionation (MgO = 7.2–11 wt.%). Occurrences of rare Cr-bearing titanomagnetite (Cr 2O 3 = 1.2–10.7 wt.%) in the Panzhihua, Hongge and Xinjie intrusions are consistent with early crystallization of Fe–Ti oxides. Oxide-silicate equilibria and low V concentration (< 4800 ppm) in magnetite are consistent with relatively high oxygen fugacity. Flood basalt-related origin, early Fe–Ti oxide crystallization and relatively high oxygen fugacity, are characteristic of Fe–Ti oxide deposits in the Panxi region but are fundamentally different from those hosted in highly-differentiated, upper parts of large layered intrusions (e.g. the Bushveld Complex). Similar Fe–Ti oxide deposits have not been documented in other well-studied large igneous provinces to date, but a number of Fe–Ti oxide ore occurrences worldwide are potentially similar to this type of flood basalt-related oxide deposits.

Review of Brazilian chromite deposits associated with layered intrusions: geological and petrological constraints for the origin of stratiform chromitites

Chromitite layers in mafic-ultramafic layered intrusions represent a special case of cumulate rock where chromite is the sole liquidus (cumulus) mineral. Different mechanisms for the formation of chromitite layers have been postulated. In order to provide geological and petrological constraints for these models, chromitites and hosting mafic and ultramafic rocks from the Bacuri, Niquelândia and Ipueira-Medrado layered complexes in Brazil were investigated. The Bacuri Complex (2·2 Ga) is a large layered intrusion of the Amazon Craton. Chromitite layers of the Bacuri mafic-ultramafic complex are restricted to a 30–120 m-thick sequence of ultramafic cumulates overlying mafic cumulates. Most of the chromite is concentrated in a several metres-thick chromitite layer (the main chromitite) located at the base of the ultramafic cumulates. Cryptic variation of olivine with stratigraphic height (from Fo89 at the base to Fo76 at the top) is consistent with extensive fractionation of the ultramafic sequence. The stratigraphic position of the main chromitite supports a model for its origin associated with a major new influx of primitive parental magma. The cause of chromite saturation must therefore result from changes in magma composition resulting from mixing new influxes of primitive magma with a fractionated magma resident in the magma chamber. The Niquelândia Complex (0·8 Ga) is a large layered intrusion in central Brazil. The Niquelaândia Complex has a thick (up to 4 km) Ultramafic Zone consisting mainly of dunite and interlayered harzburgite, websterite and several centimetres-thick discontinuous chromitite layers. A stratigraphic interval hosting relatively thicker and more consistent layers of chromitite occurs in the intermediate portion of the Ultramafic Zone. Compositional variation of olivine composition through this interval indicates that the magma located below the interval where chromitites occur was more primitive than the magma located above them. This feature is consistent with a model in which chromitites were formed by new influx of primitive magma, and mixing with slightly more fractionated resident magma. It is however intriguing to note that chromitite seams in the Niquelaândia Complex are mainly hosted by dunite. Different from chromitites observed in other large layered intrusions (e.g. Bushveld, Great Dyke), chromitite seams in the Niquelaândia Complex do not appear at the base of cyclic units. In the Niquelaândia Complex, the crystallisation of clinopyroxene as a cumulus mineral occurs immediately after the crystallisation of orthopyroxene. Extensive Cr depletion through clinopyroxene crystallisation is suggested as an explanation for the absence of chromitites at the base of cyclic units in the Niquelaândia Complex. The Ipueira-Medrado intrusion (2·0 Ga) is a small sill (7 km-long, 0·5 km-wide) consisting mainly of a basal Ultramafic Zone (∼250 m-thick) and an upper Mafic Zone (∼40 m-thick). At the upper part of the Ultramafic Zone a thick (5–8 m) and continuous chromitite layer (MCL) occurs. Cryptic variations of olivine suggest that the lowered sequence located below the MCL crystallised in a dynamic magma chamber undergoing frequent replenishment with primitive magma. Chromite compositions are very similar and show no systematic fractionation trends over the 5–8 m of the MCL, indicating that a continuous supply of primitive parental magma occurred during the time span of chromite crystallisation. Cryptic variation data for olivine rule out a model for the origin of the MCL from mixing new influxes of primitive parental magma with a fractionated magma resident in the magma chamber. In fact, the most primitive compositions of olivine occur in dunite and harzburgite located immediately below the MCL. Lithogeochemical and isotope data suggest that chromite crystallisation in the MCL of the Ipueira-Medrado sill was triggered by changes in physical–chemical parameters associated with crustal contamination. We propose that an increase in oxygen fugacity resulting from assimilation of carbonate-rich host rocks was the triggering factor leading to crystallisation of chromite as a sole cumulus phase in the Ipueira-Medrado sill. The unusually thick chromitite layer of the Ipueira-Medrado sill is interpreted to result from a large volume of magma passing through a conduit hosted by carbonate-rich crustal rocks. The examples of chromite deposits studied indicate that chromitite layers are associated with changes in the melt composition at stratigraphic horizons of the magma chamber. However, the specific trigger responsible for appropriate phase changes leading to chromite crystallisation may be different for each deposit.

Two independent processes responsible for compositional zonation in mafic dykes of the Åland-Åboland Dyke Swarm, Kestiö Island, SW Finland

Detailed sampling across three dolerite dykes of different size (small, 7 cm; middle, 75 cm; and thick, 675 cm) of the Åland-Åboland dyke swarm has revealed an internal zonation of an anomalous nature. The small, almost glassy dyke exhibits a systematic inward decrease in whole-rock MgO and Mg# (indicating a normal fractionation trend) together with a simultaneous increase in normative An and Cpx and decrease in whole-rock Zr, Y, CaO, TiO 2 (indicating a reverse fractionation trend). The middle dyke shows similar compositional trends across its narrow margins, but in the more crystalline interior whole-rock MgO and Mg# gradually but steadily increase inwards. As a result normal and reverse fractionation trends of the margins grade to exclusively reverse fractionation trends of the interior. The thick, almost totally crystalline dyke exhibits an internal zonation similar to that of the middle dyke, with fractionation trends becoming only much more pronounced in the centre of the dyke. The almost glassy nature of small dyke suggests that its anomalous compositional zonation most likely resulted from temporal changes in the composition of magma as it formed the dyke. The mechanism(s) responsible for such systematic changes in composition of inflowing magma remains, however, unknown. The margins of middle and thick dyke form in a similar way whereas their interiors formed by in situ cumulate growth against dyke sidewalls. This process resulted in a gradual inward increase in the proportion of cumulus phases owing to magma crystallization in progressively less supercooled conditions with increasing distance from cold country rocks. The compositional zonation of these dolerite dykes is thus produced by two independently operating mechanisms: successive changes in composition of inflowing magma ( an external liquid-state process) and an in situ cumulate growth on dyke sidewalls ( an internal crystal-liquid process). Based on the relatively minor development of internal zonation in interiors of middle and thick dykes, the former mechanism appears to be several times more effective in causing magma differentiation than the latter. It remains to be determined whether these two processes are a general reason for the formation of marginal reversals in mafic–ultramafic dykes, sills and large layered intrusions.

Phase Equilibria Constraints on Relations of Ore‐bearing Intrusions with Flood Basalts in the Panxi Region, Southwestern China

Abstract:There are two types of temporally and spatially associated intrusions within the Emeishan large igneous province (LIP); namely, small ultramafic subvolcanic sills that host magmatic Cu‐Ni‐Platinum Group Element (PGE)‐bearing sulfide deposits and large mafic layered intrusions that host giant Ti‐V magnetite deposits in the Panxi region. However, except for their coeval ages, the genetic relations between the ore‐bearing intrusions and extrusive rocks are poorly understood. Phase equilibria analysis (Q‐Pl‐Ol‐Opx‐Cpx system) has been carried out to elucidate whether ore‐bearing Panzhihua, Xinjie and Limahe intrusions are co‐magmatic with the picrites and flood basalts (including high‐Ti, low‐Ti and alkali basalts), respectively. In this system, the parental magma can be classified as silica‐undersaturated olivine basalt and silica‐saturated tholeiite. The equivalents of the parental magma of the Xinjie and Limahe peridotites and picrites and low‐Ti basalts are silica‐undersaturated, whereas the Limahe gabbro‐diorites and high‐Ti basalts are silica‐saturated. In contrast, the Panzhihua intrusion appears to be alkali character. Phase equilibria relations clearly show that the magmas that formed the Panzhihua intrusion and high‐Ti basalts cannot be co‐magmatic as there is no way to derive one liquid from another by fractional crystallization. On the other hand, the Panzhihua intrusion appears to be related to Permian alkali intrusions in the region, but does not appear to be related to the alkali basalts recognized in the Longzhoushan lava stratigraphy. Comparably, the Limahe intrusion appears to be a genetic relation to the picrites, whereas the Xinjie intrusion may be genetically related to be low‐Ti basalts. Additionally, the gabbro‐diorites and peridotites of the Limahe intrusion are not co‐magmatic, and the former appears to be derived liquid from high‐Ti basalts.

Comments on: 'Textural Maturity of Cumulates: a Record of Chamber Filling, Liquidus Assemblage, Cooling Rate and Large-scale Convection in Mafic Layered Intrusions' and 'A Textural Record of Solidification and Cooling in the Skaergaard Intrusion, East Greenland'

Two recent papers by Holness et al. (2007a, 2007b) have introduced an interesting new petrographic technique that could become a valuable tool for judging the textural maturity of mafic igneous rocks. They have shown that variations of the dihedral angles of mineral grains can reflect differing rates of heat loss of large layered intrusions, such as Rum and Skaergaard. Although they say in one paper (Holness et al., 2007b) that these variations record ‘the time-integrated thermal history at sub-solidus temperatures’, in the other paper they attribute all the observed variations to changes in the magmatic regime during active crystallization: ‘the arrival of fresh magma in the chamber, the onset of chamber-wise convection and changes in the liquidus assemblage’ (Holness et al., 2007a). The effects of important sub-solidus processes that are known to have extensively altered the textures, modal proportions, and bulk compositions of these same rocks have been ignored, even though they are conspicuous in the field and have been pointed out repeatedly in the literature (Hunter, 1987; McBirney & Sonnenthal, 1990; Sonnenthal, 1992; McBirney & Hunter, 1995; Sonnenthal & McBirney, 1998; McBirney & Creaser, 2003). For example, a sharp change in the textures found near the middle of the section in the Rum intrusion is said to mark the ‘onset of convection’. This interpretation seems to be based on the idea that a magma can remain static after its initial intrusion and then, at some later time, after crystallizing hundreds of meters of layered rocks, start to convect. This is just backwards. The thermal and compositional gradients that drive convection are most pronounced when the magma is first intruded and decline with time. Similarly, the statement (Holness et al., 2007b) that a textural change might coincide with ‘a significant change in the rate and mode of heat loss from the intrusion as it moves from a convective to a conductive regime’ seems to assume that the rate of heat loss is more rapid if the magma is convecting. As Carrigan (1988) has pointed out, the rate of heat loss is not governed by convective transfer in the interior but by the much more limiting rate of