Mafic-ultramafic Magmatism Research Articles

Deposition of 1.88-billion-year-old iron formations as a consequence of rapid crustal growth

Iron formations are chemical sedimentary rocks comprising layers of iron-rich and silica-rich minerals whose deposition requires anoxic and iron-rich (ferruginous) sea water. Their demise after the rise in atmospheric oxygen by 2.32 billion years (Gyr) ago has been attributed to the removal of dissolved iron through progressive oxidation or sulphidation of the deep ocean. Therefore, a sudden return of voluminous iron formations nearly 500 million years later poses an apparent conundrum. Most late Palaeoproterozoic iron formations are about 1.88 Gyr old and occur in the Superior region of North America. Major iron formations are also preserved in Australia, but these were apparently deposited after the transition to a sulphidic ocean at 1.84 Gyr ago that should have terminated iron formation deposition, implying that they reflect local marine conditions. Here we date zircons in tuff layers to show that iron formations in the Frere Formation of Western Australia are about 1.88 Gyr old, indicating that the deposition of iron formations from two disparate cratons was coeval and probably reflects global ocean chemistry. The sudden reappearance of major iron formations at 1.88 Gyr ago--contemporaneous with peaks in global mafic-ultramafic magmatism, juvenile continental and oceanic crust formation, mantle depletion and volcanogenic massive sulphide formation--suggests deposition of iron formations as a consequence of major mantle activity and rapid crustal growth. Our findings support the idea that enhanced submarine volcanism and hydrothermal activity linked to a peak in mantle melting released large volumes of ferrous iron and other reductants that overwhelmed the sulphate and oxygen reservoirs of the ocean, decoupling atmospheric and seawater redox states, and causing the return of widespread ferruginous conditions. Iron formations formed on clastic-starved coastal shelves where dissolved iron upwelled and mixed with oxygenated surface water. The disappearance of iron formations after this event may reflect waning mafic-ultramafic magmatism and a diminished flux of hydrothermal iron relative to seawater oxidants.

Multi-stage origin of Roberts Victor eclogites: Progressive metasomatism and its isotopic effects

Xenolithic eclogites are high-pressure, high-temperature garnet–clinopyroxene rocks brought from the mantle to the surface by kimberlites and other primitive magmas. Their origins have been controversial for decades: do they represent metamorphosed subducted oceanic crust or magmatic rocks originally crystallized in the deep earth? The answer has important implications for the definition of major Earth processes.The extensive eclogite suite from the Roberts Victor kimberlite (South Africa) has previously been divided into two types (Type I and II); this study proposes five subgroups (IA, IB, IK; IIA, IIB) based on mineral assemblages and compositions. All of these eclogites were derived from depths of 170–200km, near the base of the contemporary subcontinental lithospheric mantle.The fresh, microstructurally equilibrated Type IIA eclogites are the protoliths of Type I, which were heavily metasomatized by carbonatitic–kimberlitic melts/fluids to form Types IAIBIK. All the Type I rocks show extensive textural disequilibrium, and have abundant fluid inclusions, secondary phases, and higher δ18O than Type II (IA: 5.0–8.0‰; IB: 4.1–6.8‰; IK:2.2–6.8‰; IIA=3.5‰; IIB:2.2–3.9‰). Type I are richer in LREE and LILE than Type IIA; they also are isotopically more homogeneous, with higher 87Sr/86Sr and less radiogenic Nd and Hf isotopes. Type I eclogites give gnt–cpx isochron ages (SmNd (103±10Ma); LuHf (132±16Ma)) similar to the kimberlite eruption age (128±15Ma). Type IIA and IIB eclogites have unradiogenic Sr and radiogenic Nd and Hf isotopic compositions, and give a range of Proterozoic two-mineral “ages” (SmNd: 738–1143Ma; LuHf: 1148–1544Ma), reflecting some preservation of their original isotopic compositions. Type IIB are broadly similar to Type IIA, but have lower MgO; their mutual relationships are not clear.Neither Type I nor Type II eclogites are similar to modern or Archean oceanic crust when all the chemical evidence is compared. In any case, the data from Type I eclogites cannot be used to support a “subduction model” of origin because their major- and trace-element chemistry, and radiogenic- and stable-isotope compositions, reflect the extensive metasomatism, rather than any primary characteristics.Type II eclogites are the key samples for the study of eclogite origins. Their bulk compositions and microstructures (exsolution of gnt and rutile) suggest that most or all the garnet in Type II samples might have formed by exsolution from high-Al clinopyroxenes, crystallized from mafic–ultramafic magmas in the deep mantle. Therefore, the xenolithic eclogites from Roberts Victor Mine probably were originally magmatic rocks crystallized at depth, rather than subducted oceanic crust; as a suite, they record a major metasomatic event in the deep lithospheric mantle, close to the time of kimberlite eruption.

Magmatic Ni-Cu-(PGE) deposits in magma plumbing systems: Features, formation and exploration

The three most crucial factors for the formation of large and super-large magmatic sulfide deposits are: (1) a large volume of mantle-derived mafic-ultramafic magmas that participated in the formation of the deposits; (2) fractional crystallization and crustal contamination, particularly the input of sulfur from crustal rocks, resulting in sulfide immiscibility and segregation; and (3) the timing of sulfide concentration in the intrusion. The super-large magmatic Ni-Cu sulfide deposits around the world have been found in small mafic-ultramafic intrusions, except for the Sudbury deposit. Studies in the past decade indicated that the intrusions hosting large and super-large magmatic sulfide deposits occur in magma conduits, such as those in China, including Jinchuan (Gansu), Yangliuping (Sichuan), Kalatongke (Xinjiang), and Hongqiling (Jilin). Magma conduits as open magma systems provide a perfect environment for extensive concentration of immiscible sulfide melts, which have been found to occur along deep regional faults. The origin of many mantle-derived magmas is closely associated with mantle plumes, intracontinental rifts, or post-collisional extension. Although it has been confirmed that sulfide immiscibility results from crustal contamination, grades of sulfide ores are also related to the nature of the parental magmas, the ratio between silicate magma and immiscible sulfide melt, the reaction between the sulfide melts and newly injected silicate magmas, and fractionation of the sulfide melt. The field relationships of the ore-bearing intrusion and the sulfide ore body are controlled by the geological features of the wall rocks. In this paper, we attempt to demonstrate the general characteristics, formation mechanism, tectonic settings, and indicators of magmatic sulfide deposits occurring in magmatic conduits which would provide guidelines for further exploration.

Mesoproterozoic juvenile mafic–ultramafic magmatism in the SW Amazonian Craton (Rio Negro-Juruena province): SHRIMP U–Pb geochronology and Nd–Sr constraints of the Figueira Branca Suite

Abstract The Figueira Branca Suite (FBS) comprises a layered mafic–ultramafic complex which together with mafic–felsic plugs makes up a string of NW-trending intrusive bodies that are emplaced into the Jauru domain (Rio Negro-Juruena province; 1.80–1.60 Ga). This domain comprises Orosirian calc-alkaline rocks and coeval metamorphic volcanic–sedimentary associations, intruded by voluminous granitoid plutons resulted from outboard Cachoeirinha (1587–1522 Ma) and Santa Helena (1485–1420 Ma) accretionary orogens that eventually created the Rondonian-San Ignacio province along the SW margin of the proto-Amazonian Craton. SHRIMP U–Pb age in zircon for one cumulatic gabbro from the FBS yielded a concordia intercept age of 1425.5 ± 8.0 Ma (MSWD = 1.11). Another gabbroic plug which crops out to the East gives a similar within error concordia intercept zircon age of 1415.9 ± 6.9 Ma (MSWD = 0.25), whereas a nearby monzogranite yields a concordia intercept zircon age of 1428.9 ± 2.8 Ma (MSWD = 1.30). All these results are crystallization ages and constrain an important intraplate magmatic event within the Orosirian continental crust at the time of outboard Santa Helena orogen. On the other hand, igneous titanite from another gabbro located to the West of the FBS yielded a weighted mean 207 Pb/ 206 Pb crystallization age of 1541 ± 23 Ma (MSWD = 0.74). Therefore this rock is not genetically associated with the FBS, as previously suggested by the field information. Additional Nd–Sr isotopic analyses of the FBS mafic–ultramafic rocks and coeval gabbro showed comparable ɛ Nd(1.42Ga) values (+3.0 to +4.7) and variable ɛ Sr(1.42Ga) ones (−39.1 to −8.1). These data plot in the depleted field quadrant of the Nd–Sr diagram, indicating a significant influence of the MORB end-member reservoir in the magma genesis. This interpretation is similarly supported by comparison of the Nd evolutionary path of the FBS with those that characterize the isotopic evolution of the Jauru crust and the Cachoeirinha and Santa Helena intrusive plutons. The bulk isotopic signature allows the genetic relationship among the Santa Helena Suite and the FBS and coeval rocks. The anorogenic character of the FBS is supported by the geologic framework of the Jauru domain, given that the emplacement took place under an extensional regime, associated with the recognized regional NW-trending structures. 40 Ar– 39 Ar analyses were carried out in mafic rocks that are assigned to the FBS. Biotite from a gabbronorite yielded an ideogram age of 1222 ± 5 Ma while a nearby troctolite yielded plateau biotite ages of 1275 ± 4 Ma and 1268 ± 4 Ma. These ages are minimum estimates of the regional cooling of the FBS. One gabbro of the Alto Jauru Group yields a 40 Ar– 39 Ar plateau age of 1781 ± 15 Ma, interpreted as the time of regional cooling that succeeded continental accretion and metamorphism at Orosirian times. Later on heterogenous crustal thickening and uplift took place from West to East, as response from the Cachoeirinha and Santa Helena orogen dynamics, which is marked by the 1539–1510 Ma and 1452–1322 Ma 40 Ar– 39 Ar age-patterns, respectively. The available apparent ages suggest that Cachoeirinha crust was subjected to a fast exhumation rate (ca. 50 m.y.), as estimated by the zircon–titanite–hornblende–biotite time-path, while the Santa Helena crust seems to display a lower exhumation rate. Particularly, the youngest 40 Ar– 39 Ar biotite age (1322 Ma) available for the Santa Helena crust signals the post-tectonic phase of the Rondonian-San Ignacio province.

Age and significance of voluminous mafic–ultramafic magmatic events in the Murchison Domain, Yilgarn Craton

Mafic–ultramafic rocks in structurally dismembered layered intrusions comprise approximately 40% by volume of greenstones in the Murchison Domain of the Youanmi Terrane, Yilgarn Craton. Mafic–ultramafic rocks in the Murchison Domain may be divided into five components: (i) the ∼2810 Ma Meeline Suite, which includes the large Windimurra Igneous Complex; (ii) the 2800 ± 6 Ma Boodanoo Suite, which includes the Narndee Igneous Complex; (iii) the 2792 ± 5 Ma Little Gap Suite; (iv) the ∼2750 Ma Gnanagooragoo Igneous Complex; and (v) the 2735–2710 Ma Yalgowra Suite of layered gabbroic sills. The intrusions are typically layered, tabular bodies of gabbroic rock with ultramafic basal units which, in places, are more than 6 km thick and up to 2500 km2 in areal extent. However, these are minimum dimensions as the intrusions have been dismembered by younger deformation. In the Windimurra and Narndee Igneous Complexes, discordant features and geochemical fractionation trends indicate multiple pulses of magma. These pulses produced several megacyclic units, each ∼200 m thick. The suites are anhydrous except for the Boodanoo Suite, which contains a large volume of hornblende gabbro. They also host significant vanadium mineralisation, and at least minor Ni–Cu–PGE mineralisation. Collectively, the areal distribution, thickness and volume of mafic–ultramafic magma in these complexes is similar to that in the 2.06 Ga Bushveld Igneous Complex, and represents a major addition of mantle-derived magma to Murchison Domain crust over a 100 Ma period. All suites are demonstrably contemporaneous with packages of high-Mg tholeiitic lavas and/or felsic volcanic rocks in greenstone belts. The distribution, ages and compositions of the earlier mafic–ultramafic rocks are most consistent with genesis in a mantle plume setting.

Nature and evolution of the Slave Province subcontinental lithospheric mantleThis article is one of a series of papers published in this Special Issue on the theme Lithoprobe — parameters, processes, and the evolution of a continent.

A review of the ages determined for mantle material (xenoliths and xenocrysts entrained in kimberlite) derived from the Slave Province continental lithospheric mantle (CLM) indicates that a portion of the central Slave lithosphere may be ancient (3.5–3.3 Ga) harzburgite, but the majority of this lithosphere is much younger (2.9–2.0 Ga). Relying on the most robust chronometers, the majority of Slave lithosphere peridotite formed in the Neoarchean (peak at 2.75 Ga), whereas the majority of eclogite formed in the Paleoproterozoic (2.2–2.0 Ga). The northern Slave lithosphere contains evidence of peridotite xenolith ages that young with depth. The Paleoproterozoic eclogites may have multiple origins including remnants of subducted oceanic crust and mafic–ultramafic magmas that crystallized at great depth (100–200 km). Re–Os studies of sulfide inclusions in diamond indicate that some diamonds currently mined are ancient (∼3.5 Ga), but many Slave diamonds could be considerably younger. Most eclogitic diamonds recovered from the Slave craton are interpreted to be related to the formation of Paleoproterozoic eclogite. There is abundant evidence for Mesoproterozoic modification of the Slave lithosphere (e.g., heating by magma emplacement at great depth and metasomatism) and possible new addition to the lithosphere at that time. The Canadian Slave and African Kaapvaal lithospheres have similar peaks in cratonic peridotite formation ages at about 2.8 Ga, indicating that a large portion of the CLM in these two cratons formed and stabilized in the Neoarchean. One difference is that the Slave peridotites are much less enriched in SiO2, possibly reflecting the more metasomatized nature of the Kaapvaal CLM. The dominance of Paleoproterozoic formation ages for Slave mantle eclogites contrasts with the dominance of Neoarchean formation ages for Kaapvaal mantle eclogites.

Intracontinental strike-slip faults, associated magmatism, mineral systems and mantle dynamics: examples from NW China and Altay-Sayan (Siberia)

Abstract In the orogenic belts of the Central Asian Orogenic Belt (CAOB), many mafic and felsic plutons are temporally and spatially associated with orogen-scale strike-slip faults. The CAOB is a huge and complex orogenic collage of accreted terranes that was formed in the Early to Mid-Palaeozoic. In the CAOB, orogen-scale strike-slip faults extend for 100–1000 s of kilometres marking the boundaries of tectonic units and terranes. I use examples from southern Siberia and NW China to illustrate the important role that strike-slip faults have in localising intraplate magmatism and associated metallogeny. Cases from the Altay-Sayan in southern Siberia, the Altay and Tianshan orogens in NW China, are compelling for providing good evidence of the control that strike-slip structures exert for the emplacement of magmas and related mineral systems. These strike-slip faults controlled the emplacement of mafic-ultramafic intrusions, alkaline mafic and felsic magmatism in the period 280–240 Ma, which coincides with mantle plume(s) activity that led to the emplacement of the Tarim and Siberian large igneous provinces (LIPs). Mineral systems that are associated with these LIPs include magmatic Ni-Cu in sill-like intrusions, concentrically zoned mafic-ultramafic intrusions (e.g. Kalatongke, the second largest Ni-Cu sulphide deposit in China, after Jinchuan), epithermal systems, breccia pipes, polymetallic hydrothermal veins, granitoid-related greisen and rare earth pegmatites, as well as kimberlite fields. In the Altay-Sayan and NW China regions, orogen-scale translithospheric strike-slip faults provided the channels for the emplacement of magmas, resulting from lateral flow of mantle melts along the base of the lithosphere. This lateral flow is interpreted to have resulted from the impingement of mantle plumes to the base of the lithosphere of what was, to all intents and purposes, a stationary plate. Lateral flow from mantle plumes head was sustained or facilitated, during stages of extension and movements along orogen-scale strike-slip faults. In the Altay-Sayan and NW China, decompression melting of the mantle material produced mafic-ultramafic magmas that were emplaced along the comparatively narrow conduits of the strike-slip zones, forming concentrically zoned complexes that locally, where favourable conditions allowed it (e.g. crustal contamination), host magmatic Ni-Cu sulphides. Flow of mantle melts into translithospheric strike-slip structures also caused partial melting of a thinned and metasomatised lithosphere, resulting in alkaline magmatic products and a wide range of related mineral systems, from polymetallic veins to greisens. Partial melting of the lower crust also produced A-type granitic magmas that locally vented to the surface, forming calderas hosting epithermal and porphyry systems, as observed in NW China.

Crustal sulfur is required to form magmatic Ni–Cu sulfide deposits: evidence from chalcophile element signatures of Siberian and Deccan Trap basalts

Process models for ore formation in magmatic Ni–Cu–platinum group element (PGE) sulfide systems require that S saturation is achieved in a mafic–ultramafic magma. Traditional models explain the achievement of S saturation or sulfide saturation either by the addition of crustal S, by the felsification of the magma by crustal contamination, or by mixing between primitive and evolved magmas. Which process matters most is important to industry-oriented exploration models where crustal S sources are believed to be encouraging features of a metallotect. Studies of the Siberian Trap flood basalts at Noril’sk have demonstrated that chalcophile element depletion is linked to assimilation of silica-rich crust, but it is less clear whether this contaminant contained an appreciable amount of S. At Noril’sk, the Ni–Cu–PGE sulfide deposits are associated with subvolcanic intrusions that were emplaced into Permian and Carboniferous sedimentary sequences rich in shales, marlstones, and evaporites. Similar to the Siberian Trap basalts, the Deccan Trap contains a volumetrically important suite of crustally contaminated tholeiitic basalts. We present new PGE data for samples from a stratigraphic sequence of basalts from the southern Deccan province. Two of the formations in this sequence (the Bushe and Poladpur Formations) have geochemical signatures indicative of a wide degree of crustal contamination of a magma type that gave rise to the stratigraphically higher Ambenali Formation (a product of transitional midocean ridge basalt magmatism). There are no known deposits or occurrences of Ni–Cu–PGE sulfides associated with subvolcanic intrusions in the Deccan province. Despite the fact that the Bushe Formation exhibits a stronger crustal contamination signature than the most contaminated Siberian Trap basalt formations, and the Poladpur lavas are also strongly crustally contaminated, the Bushe and Poladpur basalts are undepleted in Ni, Cu, or PGE. This indicates that the contaminated Deccan Trap lavas did not achieve S saturation. This, in turn, places constraints on the potential of the Deccan Trap in southern India to host significant magmatic sulfide deposits. Conversely, this observation also indicates that an S-rich crustal contaminant is required for the genesis of magmatic Ni–Cu–PGE sulfide deposits.

Large igneous provinces (LIPs) and carbonatites

There is increasing evidence that many carbonatites are linked both spatially and temporally with large igneous provinces (LIPs), i.e. high volume, short duration, intraplate-type, magmatic events consisting mainly of flood basalts and their plumbing systems (of dykes, sills and layered intrusions). Examples of LIP-carbonatite associations include: i. the 66 Ma Deccan flood basalt province associated with the Amba Dongar, Sarnu-Dandali (Barmer), and Mundwara carbonatites and associated alkali rocks, ii. the 130 Ma Parana-Etendeka (e.g. Jacupiranga, Messum); iii. the 250 Ma Siberian LIP that includes a major alkaline province, Maimecha-Kotui with numerous carbonatites, iv. the ca. 370 Ma Kola Alkaline Province coeval with basaltic magmatism widespread in parts of the East European craton, and v. the 615–555 Ma CIMP (Central Iapetus Magmatic Province) of eastern Laurentia and western Baltica. In the Superior craton, Canada, a number of carbonatites are associated with the 1114–1085 Ma Keweenawan LIP and some are coeval with the pan-Superior 1880 Ma mafic-ultramafic magmatism. In addition, the Phalaborwa and Shiel carbonatites are associated with the 2055 Ma Bushveld event of the Kaapvaal craton. The frequency of this LIP-carbonatite association suggests that LIPs and carbonatites might be considered as different evolutionary ‘pathways’ in a single magmatic process/system. The isotopic mantle components FOZO, HIMU, EM1 but not DMM, along with primitive noble gas signatures in some carbonatites, suggest a sub-lithospheric mantle source for carbonatites, consistent with a plume/asthenospheric upwelling origin proposed for many LIPs.

THE GEOMETRY AND EMPLACEMENT MECHANICS OF A BUSHVELD COMPLEX PERIDOTITE BODY, SOUTH AFRICA

The Apiesdooringdraai peridotite is a syn-Bushveld Complex, dominantly harzburgitic intrusion of Lower Zone parentage. Detailed mapping and three-dimensional modeling shows the intrusion to comprise several discordant segments linked by concordant, sill-like segments. Deformation in the contact aureole is strongly partitioned adjacent to the discordant segments of the intrusion and is detectable at distances from the contact on the order of the intrusion width. In contrast, areas above and below the intrusion, and at greater distances from the discordant intrusive segments, are effectively undeformed. The geometry of structural features suggests that the peridotite is a variably structurally-evolved stepped-sill system that was emplaced as kilometre-scale magma fingers from the northwest to the southeast, a trend heavily utilised by the mafic-ultramafic magmas of the Bushveld Complex. Deformation in the country rock was affected by the lateral expansion of the magma fingers at high postulated strain rates, and linkage of offset fingers formed prominent steps in the intrusion. Emplacement-related country rock deformation is postdated by a shallow dipping foliation that records compaction and crustal loading during contact metamorphism.

Mafic-ultramafic magmatism of the Early Precambrian (from the Archean to Paleoproterozoic)

Compositional evolution of the Archean mafic-ultramafic volcanics is considered in comparison with evolution of the Paleoproterozoic volcanism using available data on the Baltic shield, Pilbara (Australia) and Superior (Canada) cratons, and the Isua greenstone belt (Greenland). The Archean volcanics of mantle origin are of two major types, represented (a) by komatiite-basaltic complexes (komatiites, komatiitic and tholeiitic basalts) and (b) by geochemical analogs of boninites (GAB) and siliceous high-Mg series (SHMS) of volcanic rocks. As is established, the komatiitic and GAB volcanism ceased in the terminal Archean, whereas the SHMS rocks prevailed in the Paleoproterozoic to become extinct about 2 Ga ago in connection with transition to the Phanerozoic type of tectonomagmatic activity. Geochemical trends of mafic-ultramafic associations occurring in the considered cratons are not uniform, being of particular character to certain extent. With transition from the Paleo- to Neoarchean, rock associations of both types reveal a minor increase in Ti and Fe contents. Comparatively high Fe2O3tot TiO2, and P2O5 concentrations (maximal ones in the Archean), which are characteristic of the Neoarchean (2.75–2.70 Ga) basalts from the Superior and Pilbara cratons or the Baltic shield, represent a result of relatively high-Ti intracratonic magmatic activity that commenced in that period practically for the first time in the Earth history. This magmatic activity of the Neoarchean was not as intense as the high-Mg basaltic volcanism, and the absolute maximum in concentrations of the above components was attained only 2.2–1.9 Ga ago, at the time of appearance in abundance of Fe-Ti picrites and basalts typical of the Phanerozoic intraplate magmatism. The Archean volcanic complexes demonstrate gradual secular increase in concentrations of incompatible elements (LREE inclusive) and growth of Nb/Th ratio that apparently reflected the progressing influence of mantle plumes. In the early Paleoproterozoic (2.5–2.35 Ga), values of that ratio considerably declined in the SHMS rocks and then quickly grew in the Middle Paleoproterozoic volcanics (2.2–1.9 Ga) to attain finally the values typical of the Phanerozoic magmas associated in origin with mantle plumes. The ɛNd(T) parameter was decreasing with time from positive values in the Paleoarchean to negative ones in the SHMS rocks of the Paleoproterozoic most likely in response to grown proportion of ancient crustal material in magmatic melts. Since the mid-Paleoproterozoic, the ɛNd(T) values turn in general into positive again reflecting change in the character of magmatic activity: the SHMS melts gave place at that time to the Fe-Ti picrite-basaltic magmas. The primary crust of the Earth was presumably of sialic composition and originated during solidification from the bottom upward of the global magma ocean a few hundreds kilometers deep, when most fusible components migrated up to the surface to form there the granitic crust. Geological history of the Earth commenced at the appearance time of granite-greenstone terranes and granulite belts separating them, the first large tectonic structures formed under influence of raising mantle superplumes.

Delamination of eclogitized lower crust: Control on the crust–mantle boundary in the central Fennoscandian shield

The crustal structure of the central part of the Fennoscandian shield is dominated by a high velocity lower crust (HVLC) ( V P 7.3–7.5 km/s, at the depth of 40–63 km in an area of ca. 400 km × 600 km) formed in the Palaeoproterozoic Svecofennian orogen at ∼ 1.91–1.87 Ga. In the area of the HVLC the Moho is deeper than 50 km and attains depths of up to 63 km in eastern Finland at the Archaean–Proterozoic boundary zone. We suggest that the present Moho dates back to late Svecofennian time and was determined by delamination of high-density eclogitized lower crust after tectonic collisional thickening of crust. Densities deduced from seismic and gravity data suggest a very small or completely lacking density contrast between lower crust and uppermost mantle. New geological evidence from kimberlite-hosted garnet xenocrysts supports the presence of a garnet population with compositions in agreement with crustal eclogitic garnets. In addition, small clinopyroxene–garnet mineral aggregates (microxenoliths) show omphacitic clinopyroxene compositions. Therefore, the presence of eclogites in the high velocity lower crust is implied, although the eclogite proportion as estimated from seismic velocity data is only a few percent in the present HVLC. With results on near-incidence and wide-angle seismic studies and geological and geochemical data on heat producing elements we modeled the Svecofennian collision and the thermal evolution of the crust to provide temperature constraints for the delamination model. The constructed thermal model simulates the genesis of Svecofennian synkinematic (at 1.89–1.88 Ga) and postkinematic (1.88–1.87 Ga) granitoids, as well as the general metamorphic grade of the surface rocks and lower crustal xenoliths. Moreover, the narrow distribution of the ages of the Svecofennian mafic–ultramafic magmatism (ca. 1.89–1.87 Ga) dispersed around the HVLC area is explained by the homogeneous warming of the lithosphere. The conditions of lower crustal delamination were estimated with a Rayleigh–Taylor convective instability model constrained with lower crustal and upper mantle rheological data. Delamination of a lower crustal eclogite layer 5–30 km thick takes place in about 7–50 Ma if the rock dynamic viscosity is ≤ 1·10 21 Pa s. It requires temperatures of ≥ 1000 °C in the lower crust and upper mantle. In the Svecofennian crust, such temperatures were attained at about 1860–1850 Ma, i.e. after the orogenic peak. We conclude that the Moho of the HVLC area was controlled by delamination of a ca. 5–10 km thick layer of high-density lowermost crust into the mantle. Such a density-controlled process may have controlled the Moho depth also in other Proterozoic shield areas.

Tectonic Framework and Evolution of the Gawler Craton, Southern Australia

The Gawler craton is the major crustal province in the southern Australian Proterozoic and is pivotal in models seeking to describe the evolution of Proterozoic Australia. The craton is host to the Olympic Dam iron oxide Cu-Au-U-REE deposit, as well as a number of other iron oxide copper-gold (IOCG), Au, and iron ore deposits. The evolution of the Gawler craton is dominated by two major phases of tectonic activity, Late Archean and late Paleoproterozoic-early Mesoproterozoic, in total spanning ca. 1 billion years. The Late Archean (2560–2500 Ma) basin development was coeval with arclike felsic magmatism and mafic-ultramafic magmas, including komatiites. Collisional deformation between 2480 and 2420 Ma led to ca. 400 m.y. of tectonic quiescence, conceivably within an early Paleoproterozoic continental interior. The second major phase of tectonic activity was in the middle to late Paleoproterozoic and early Mesoproterozoic (2000–1450 Ma). Paleoproterozoic tectonism was initially dominated by rift-related events, which produced a series of basins over the interval ca. 1900 to 1730 Ma, some of which contain significant iron ore reserves. Transient contractional deformation at ca. 1850 Ma led to batholith-scale granitic magmatism, in part derived from melting of the late Archean basement. Major basin development was finally terminated by the 1730 to 1690 Ma Kimban orogeny, the effects of which are widespread and include the formation of crustal-scale shear systems, granitic magmatism, and low- to high-grade metamorphism. The Kimban orogeny was followed by a renewed period of extension between ca. 1680 and 1640 Ma, leading to local magmatism and sedimentation which appears in part to have been coeval with the high-grade ca. 1650 Ma Ooldean event in the western Gawler craton. At ca. 1620 to 1615 Ma, the generation of the arc-related St. Peter Suite in the southern Gawler craton implies that craton components were located at an active plate margin. The arc environment was superseded by a transition to a continental interior regime, which coincided with the voluminous Gawler Range Volcanics (1595–1590 Ma) and Hiltaba Suite granitoids (1595–1575 Ma). The Hiltaba Suite is dominated by high T fractionated felsic rocks with coeval mafic magmatism confirming a mantle involvement. Emplacement of the Hiltaba Suite coincided with the major mineralizing interval in the Gawler craton. Regionally, two distinct mineral systems have been recognized: the Olympic IOCG province in the eastern part of the craton, and the gold-dominated systems within the central Gawler gold province. The spatial distribution of IOCG versus Au-dominated mineral systems appears to reflect regional variations in crustal composition and Hiltaba Suite petrogeneses. Hiltaba-aged granites in the Olympic IOCG province are isotopically more evolved, richer in U and Th, and oxidized compared to similar aged granites in the Au-dominated province. Modern-day heat flow in the IOCG province is significantly higher (90 ± 10 mWm−2) compared to the Au-dominated province (54 ± 5 mWm−2), suggesting there are important lithospheric compositional differences between the two metallogenic provinces, which may reflect an older phase of craton assembly. Widespread northwest-southeast contractional deformation coeval with the emplacement of the Hiltaba Suite is expressed by the formation and/or reactivation of shear zones ranging up to crustal scale. Within this regime, northwest-trending structures such as those in the vicinity of the Olympic Dam deposit are likely to have accommodated dilation associated with strike-slip movements. Intersection between these structures and northeast-trending contractional faults may have formed suitable structural traps for 1590 to 1580 Ma mineralization. The timing of syn-Hiltaba deformation overlaps with transpression and medium to high-grade metamorphism associated with the ca. 1570 to 1540 Ma Kararan orogeny which dominates the geophysical architecture of the central northern part of the craton. The youngest phase of deformation in the craton is expressed by reactivation of shear zones between ca. 1470 and 1450 Ma and regional cooling.

Lithostratigraphy and Geochemistry of the High-Grade Metasedimentary Rocks in the Thompson Nickel Belt and Adjacent Kisseynew Domain, Manitoba: Implications for Nickel Exploration

Exploration for nickel in the Thompson nickel belt is focused on specific paragneiss units within the Paleoproterozoic sedimentary and volcanic cover succession (Ospwagan Group) on the northwestern margin of the Archean Superior craton. These units host ultramafic bodies and all significant nickel deposits in the Thompson nickel belt. Knowledge of the lithostratigraphy is therefore essential for finding the units with the highest potential for the discovery of new nickel deposits. This is made practical by a distinctive stratigraphic succession with numerous marker units, including metamorphosed quartzite, carbonate, pelite, iron formation, and graywacke, as summarized in this paper. However, the sedimentary rocks are generally highly deformed and metamorphosed, making the recognition of some units difficult and their sedimentary origin obscure. To better recognize the sedimentary protoliths of orehosting and related units, a simple plot of SiO 2 versus Al 2 O 3 is used to estimate the approximate proportions of siliciclastic and chemical sedimentary components. This demonstrates that much of the succession, which was previously considered to be derived from calcareous and ferruginous chemical sediments, is a hybrid containing a substantial siliciclastic component, thus explaining the present mineralogy and geochemistry and assisting in the recognition of units at various metamorphic grades. Extended-element plots, using the most abundant metapelite unit in the Ospwagan Group as the normalizing factor, show a common provenance for the entire group. They also illustrate a clear difference between the Ospwagan Group and the underlying basements gneiss and allow the Ospwagan Group to be distinguished from petrographically similar but economically barren, younger paragneiss units of the Burntwood and Grass River groups. Unlike the Ospwagan Group, the younger units, which predominate in the Kisseynew Domain adjoining the Thompson nickel belt, were derived mainly from an exotic Paleoproterozoic arc-related source and therefore have a different geochemical signature. This geochemical difference can be utilized in areas of sparse geologic information, such as beneath Phanerozoic or glacial cover, where initial exploration drilling tests new geophysical targets. Combined litho- and chemostratigraphy improve insight into basin evolution, which probably occurred on a ca. 2 Ga passive continental margin or shelf. The ca. 1880 Ma maficultramafic magmatism related to the nickel deposits may have formed in a different tectonic environment, possibly in a continental back arc associated with a recently proposed episode of felsic calc-alkaline plutonism.

Early Kibaran rift-related mafic–ultramafic magmatism in western Tanzania and Burundi: Petrogenesis and ore potential of the Kapalagulu and Musongati layered intrusions

The Kapalagulu and Musongati intrusions are differentiated mafic–ultramafic intrusions, more than 1 km in stratigraphic thickness and several 10 s of km 2 in size. They form part of the Kabanga–Musongati belt of intrusions in western Tanzania and Burundi. The intrusions of the Kabanga–Musongati belt were emplaced at ca 1.4 Ga into pelitic sediments of the Burundi and Karagwe–Ankolean Supergroups that accumulated during an early rifting phase of the Kibaran orogeny. The parental magmas to the intrusions were of picritic composition (ca 15% MgO) that assimilated variable amounts of sulfidic sedimentary rocks during emplacement. Modeling suggests that the Musongati magma assimilated ca. 5% of sedimentary material, whereas the Kapalagulu magma assimilated ca. 15% of sediment. Contamination caused enrichment of the magma and the cumulates in incompatible trace elements, the development of negative Nb–Ta–Ti anomalies, and crustal sulfur isotopic signatures ( δ 34S = + 4.5 to + 20). At Kapalagulu, contamination of the parent magma led to the formation of basal olivine melanorite cumulates. In the less contaminated Musongati intrusion dunites and harzburgites formed at the base. Both intrusions are prospective for magmatic Ni and PGE deposits. This is indicated by empirical observations, notably the presence of important Ni sulfide ores at Kabanga and reef-type PGE concentrations at Musongati and Kapalagulu. It is also supported by theoretical considerations, namely the high-magnesian composition of the parental magmas and the abundance of sulfides in the host sedimentary rocks. Weathering of the ultramafic rocks resulted in a thick lateritic crust that contains up to > 4 ppm PGE and, at Musongati, hosts one the world's largest Ni-laterite deposits.

SHRIMP U-Pb zircon age constraints on the Late Archaean tectonostratigraphic architecture of the Eastern Goldfields Superterrane, Yilgarn Craton, Western Australia

Reassessment of Late Archaean tectonostratigraphic architecture of the Eastern Goldfields Superterrane is based on 35 new SHRIMP U-Pb zircon ages of supracrustal sequences. The superterrane comprises at least five terranes with ∼2810 to ≤2657 Ma volcanic and sedimentary rocks amalgamated by ∼2660 Ma. Each terrane has distinctive stratigraphy, volcanic facies, geochemistry and timing of volcanism. The Kalgoorlie Terrane comprises a >2715 to ∼2692 Ma plume-influenced, but deep-marine, mafic-ultramafic succession (Kambalda Sequence) overlain unconformably by a 2686 ± 5 to ≤2666 ± 6 Ma arc-related, deep-marine felsic volcaniclastic-shale succession of tonalite-trondjemite-diorite (TTD) affinity (Kalgoorlie Sequence). Felsic volcaniclastic rocks of the Agnew Domain and northern Yandal Belt are included in the Kalgoorlie Terrane, based on their similar age and geochemistry. The terrane also contains enclaves of >2740 Ma volcanic sequences that are remnants of autochthonous basement to the Kambalda Sequence. Arc-related bimodal and intermediate volcanic rocks of the Gindalbie Terrane, ranging in age from 2693 ± 4 to 2676 ± 5 Ma, are coeval with but geochemically distinct from the Kalgoorlie Sequence. They overlie sequences similar in age and composition to volcanic rocks of the Kurnalpi Terrane. The Kurnalpi Terrane comprises deep-marine, calc-alkalic mafic and intermediate volcanic rocks (Kurnalpi Sequence) ranging in age from 2715 ± 5 to 2704 ± 4 Ma, overlain conformably by mafic to ultramafic lavas (Minerie Sequence). Although of overlapping age, arc-related rocks in the Kambalda and Kurnalpi-Minerie Sequences are geochemically different. SHRIMP dating has identified older volcanic sequences in the Laverton (≤2808 ± 8 Ma) and Duketon-Burtville (2805 ± 5 Ma) Terranes, with komatiite-hosted Ni-sulphide deposits at Windarra, in the Laverton Sequence, linked to mafic-ultramafic magmatism ∼100 million years older than in the Kalgoorlie Terrane. Older sequences in the Laverton Terrane are autochthonous basement to the Kurnalpi Sequence. Synorogenic siliciclastic sequences occur as erosional remnants either in synclinal structures or adjacent to terrane-scale faults, and overlie a regional-scale unconformity. Detrital zircon ages indicate deposition after 2657 ± 4 Ma, placing important age constraints on the principal shortening event (D 2) and peak metamorphism. Detrital zircon ages also indicate older sources at ∼2730, ∼2800, ∼2950, ∼3000 Ma and, less commonly, >3400 Ma. Many zircon age-populations correspond to known supracrustal and crustal rocks in the Yilgarn Craton, but some have no known source in the Eastern Goldfields Superterrane. Superterrane-wide distribution and evidence for erosion of all crustal and supracrustal rocks ≥2660 Ma in age requires deposition of these sequences after terrane amalgamation, most likely synorogenic to collision between the Eastern Goldfields Superterrane and the Yilgarn Craton.

Further refinement to the timing of Mesoproterozoic magmatism, Lake Nipigon region, Ontario

The Midcontinent Rift in the Lake Superior region of North America is one of the best preserved examples of an aborted Precambrian intercontinental rift, one that hosts a diverse suite of rock types in addition to the well-studied and voluminous rift-fill flood basalts. Although there is a growing database of high-precision age information for the main volcanic packages and the largest mafic intrusions, there is relatively little information available on the absolute timing of mafic-ultramafic intrusions, dyke swarms, and alkaline complexes, especially in the Ontario portion of the rift. We report new high-precision U–Pb ages for 29 samples, primarily collected in the Lake Nipigon area, Ontario. From these new age results, it is now possible to expand the known distribution of Geon 15 magmatism in the region, confirm an early stage of Midcontinent Rift mafic magmatism between 1150 and 1130 Ma, provide evidence that significant mafic–ultramafic magmatism occurred in the Lake Nipigon region slightly earlier (~1115–1110 Ma) than the main stage of rift magmatism (1108–1094 Ma), and further document synchronous ~1110–1100 Ma tholeiitic and alkaline magmatism.

The LaRonde Penna Au-Rich Volcanogenic Massive Sulfide Deposit, Abitibi Greenstone Belt, Quebec: Part II. Lithogeochemistry and Paleotectonic Setting

The Au-rich massive to semimassive sulfide lenses of the LaRonde Penna deposit (58.8 Mt at 4.31 g/t Au) are stacked in a steeply dipping, southward-facing homoclinal volcanic sequence forming a continuous, differentiated volcanic succession composed of two main formations: the ca. 2700 Ma Hebecourt Formation and the 2701 to 2698 Ma Bousquet Formation, which corresponds to the uppermost segment of the Blake River Group. The Hebecourt Formation is composed of regionally extensive LREE-depleted ([La/Sm]N ∼0.9) tholeiitic, basaltic to andesitic, massive to pillowed flows that formed a submarine stratum on which the Bousquet Formation was emplaced. The Bousquet Formation is further divided into a lower member and an upper member. The lower member of the Bousquet Formation is composed of feldspar and quartz-phyric tholeiitic felsic (Zr/Y ∼3.4, Zr/TiO2 ∼860) sills and extensive effusive and volcaniclastic mafic to intermediate and tholeiitic to transitional rocks. The upper member is mainly characterized by submarine, coalesced dacitic to rhyodacitic autoclastic flows that are cut and/or covered by rhyodacitic and rhyolitic domes and/or partly extrusive cryptodomes and by intermediate to mafic sills and dikes. Mafic to intermediate and tholeiitic to transitional (Zr/Y ∼2.3–5) rocks of the Bousquet Formation are characterized by a low Zr/TiO2 ratio (<60), moderately enriched chondrite-normalized LREE and MREE ([La/Sm]N ∼2.2–2.7) patterns, flat HREE ([Gd/Lu]N ∼1.2–2) patterns, and negative Nb, Ta, Zr, and Hf anomalies. Felsic transitional to calc-alkaline (Zr/Y ∼5–8) rocks of the upper member of the Bousquet Formation are characterized by a moderate Zr/TiO2 ratio (∼250–615), high incompatible element contents, LREE-enriched patterns ([La/Sm]N ∼3.2–6.6), flat HREE patterns ([Gd/Lu]N ∼1–1.4), pronounced negative Nb, Ta, and Ti anomalies, and positive Zr and Hf anomalies. The Nd isotope signature of six separate LaRonde Penna deposit host units (eNd ∼3–3.4) suggests that they were generated by partial melting of depleted upper mantle and/or juvenile material (mafic crust) or by a combination of those two processes. The sequence is interpreted to reflect the progression from diapirism of depleted upper mantle associated with underplating by mafic-ultramafic magma and assimilation and magmatic differentiation (assimilation-fractional crystallization) at midcrustal levels in subsidiary magmatic chambers within a ca. 2721 Ma, relatively thick, juvenile or immature mafic ± felsic arc–back-arc crust in an intermediate setting between back-arc basin and volcanic-arc environments. This setting, compatible with the inferred geodynamic setting for the southern Abitibi belt, could be responsible, at least in part, for the Au enrichment of the volcanic massive sulfide (VMS) deposits of the Doyon-Bousquet-LaRonde mining camp. This study shows that Archean HREE-depleted and high Th, transitional to calc-alkaline dacite, rhyodacite, and rhyolite, referred to as FI and FII type, such as those associated with the LaRonde Penna deposit, can be important hosts for VMS and Au-rich VMS and may be as prospective as the tholeiitic or FIII-type rhyolite-bearing sequences.

Revisiting the “Yanbian Terrane”: Implications for Neoproterozoic tectonic evolution of the western Yangtze Block, South China

There is a consensus that the nature and evolution of the widespread Neoproterozoic igneous rocks along the western margin of the Yangtze Block are important for understanding the tectonic evolution of South China, but the petrogenesis and tectonic affiliations of these rocks are still highly debated. The Yanbian region in the southwestern Yangtze Block preserves Neoproterozoic igneous packages that provide a rare opportunity for unraveling the regional tectonomagmatic evolution. SHRIMP U–Pb zircon data show two major episodes of mafic–ultramafic magmatism in the region. The elder one is represented by a basaltic suite within the Yanbian Group that formed at ca. 920–900(?) Ma. Geochemical and Nd isotopic data suggest these basalts were derived from a highly depleted asthenospheric mantle in a continental back-arc basin, rather than being part of an ophiolite suite formed in an oceanic setting as previously suggested. A major fold and thrust event took place during the final stage of the Sibao orogenesis at ca. 900 Ma, forming the east-northeasterly trending structures within the Yanbian Group rocks (and regional pre-900 Ma rocks as well). The younger magmatic episode is represented by the non-deformed mafic–ultramafic intrusions dated at 820–800 Ma. Mineral compositions and whole-rock geochemistry suggest that they are affinitive with rift-related, rather than arc-related, magmatism, with some pre-existing arc components being assimilated during their genesis. These mafic–ultramafic intrusions belong to the ca. 830–740 Ma anorogenic igneous provinces along the north-south trending Kangdian Rift in the western Yangtze Block. The igneous rocks in the Yanbian region had thus recorded an important tectonic transition from a back-arc setting during the Sibao Orogeny at ≥900 Ma to a continental rift setting after ca. 825 Ma in the southwestern Yangtze Block. This tectonic transition most likely corresponded to the amalgamation and breakup of the Rodinia supercontinent. We see neither geological nor geochemical evidence for the existence of a mid-Neoproterozoic (830–740 Ma) magmatic arc or an ophiolitic complex in the Yanbian region as suggested by previous studies.

Proterozoic mafic–ultramafic intrusions in the Arunta Region, central Australia: Part 2: Event chronology and regional correlations

The Arunta Region in central Australia records multiple Proterozoic crustal processes over a 1500-million-year period. The timing of mafic–ultramafic magmatism in this evolution is constrained by SHRIMP U–Pb dating of zircon, which is a primary phase in mafic–ultramafic intrusions across the region. The earliest mafic magmatism was part of the 1810–1800 Ma Stafford Event, and a second episode is recognised during the 1790–1770 Ma Yambah Event. Zircon overgrowths in some of these early intrusions record metamorphism during the Strangways Event, a regionally pervasive metamorphic episode whose termination at ca. 1690 Ma coincided with intrusion of dolerite dykes. Gabbro intrusion during the ca. 1640 Ma Liebig Event is related to the geologically distinct Warumpi Province in the south of the region. There is no record of mafic magmatism during the ca. 1590 Ma Chewings Event. Later systems include the ca. 1130 Ma alkaline-ultramafic Mordor Complex; and basaltic bodies are components of the Irindina Province which experienced high-grade metamorphism during the Palaeozoic. The dating associates mafic–ultramafic magmatism with each of these events, which suggests that repeated extensional systems are an important aspect of the tectonic evolution. The example of routinely obtaining zircon for dating from mafic and even ultramafic intrusions, using an integrated approach of field and geochemical criteria to guide sampling, opens the possiblility of dating mafic–ultramafic intrusions elsewhere. Direct dating of mafic–ultramafic magmatism permits its significance in Proterozoic event systems to be evaluated directly, free of assumed or conjectural timing correlations with the felsic plutons that are more often studied for age control.