Mafic Sills Research Articles

Dating the Gaofan and Hutuo Groups – Targets to investigate the Paleoproterozoic Great Oxidation Event in North China

There are several sedimentary units in North China that are proposed to be associated with the Paleoproterozoic Great Oxidation Event (GOE) and/or subsequent events; however, few of them have been precisely dated. In this study, deposition age of the greenschist facies Gaofan and Hutuo Groups is determined. Zircon grains liberated from a tuff layer (metamorphosed to sericite-quartz schist) in the upper part of the Mohe Formation (the second of the three formations of the Gaofan Group) yield a weighted average 207Pb/206Pb age of 2186±8Ma (n=7, MSWD=1.3), representing time of deposition. This age and the detrital zircon U-Pb ages of the basal feldspar quartzite (meta-siltstone), as well as the initial deposition age of the unconformably overlying Hutuo Group, confine the deposition age of the Gaofan Group to 2350–2150Ma. This result negates the Gaofan Group as one subgroup of the 2560–2510Ma Wutai greenstone belt. Zircons from the Banlaoyao mafic sill (meta-diabase) that intruded the Dongye Subgroup of the Hutuo Group yield an upper intercept U-Pb age of 2057±25Ma (n=14, MSWD=1.3), representing time of crystallization. Considering the age of the basalt in the first formation of the Doucun Subgroup and the tuff in the first formation of the Dongye Subgroup, the deposition age of the Doucun and Dongye Subgroups of the Hutuo Group is confined to 2150–2090Ma and 2090–2060Ma, respectively. These age brackets, as well as the available carbon and nitrogen isotope data indicate that the Zhangxianbu Formation of the Gaofan Group possibly recorded the GOE; whereas the Mohe-Yaokouqian Formations of the Gaofan Group and the Doucun-Dongye Subgroups of the Hutuo Group recorded the subsequent Lomagundi-Jatuli Event (LJE). However, the Lomagundi-Jatuli carbon excursions are hardly distinguishable from the Gaofan Group and the Doucun Subgroup (Hutuo Group) as both units consist of little inorganic carbon but terrestrial clastic turbidites.

Petrogenesis of the~2115 Ma Haicheng Mafic Sills in the Eastern North China Craton and Their Implications for An Intra‐Continental Rifting

Petrogenesis of the~2115 Ma Haicheng Mafic Sills in the Eastern North China Craton and Their Implications for An Intra‐Continental Rifting

Lateral Magma Flow in Mafic Sill‐complexes

The structure of upper crustal magma plumbing systems controls the distribution of volcanism and influences tectonic processes. However, delineating the structure and volume of plumbing systems is difficult because (1) active intrusion networks cannot be directly accessed; (2) field outcrops are commonly limited; and (3) geophysical data imaging the subsurface are restricted in areal extent and resolution. This has led to models involving the vertical transfer of magma via dikes, extending from a melt source to overlying reservoirs and eruption sites, being favored in the volcanic literature. However, while there is a wealth of evidence to support the occurrence of dike-dominated systems, we synthesize field- and seismic reflection–based observations and highlight that extensive lateral magma transport (as much as 4100 km) may occur within mafic sill complexes. Most of these mafic sill complexes occur in sedimentary basins (e.g., the Karoo Basin, South Africa), although some intrude crystalline continental crust (e.g., the Yilgarn craton, Australia), and consist of interconnected sills and inclined sheets. Sill complex emplacement is largely controlled by host-rock lithology and structure and the state of stress. We argue that plumbing systems need not be dominated by dikes and that magma can be transported within widespread sill complexes, promoting the development of volcanoes that do not overlie the melt source. However, the extent to which active volcanic systems and rifted margins are underlain by sill complexes remains poorly constrained, despite important implications for elucidating magmatic processes, melt volumes, and melt sources.

Southern continuation of the Wakeham Group and Robe-Noire mafic suite (eastern Grenville Province) from hydrocarbon-targeted seismic reflection data on Anticosti Island, Quebec, Canada

Hydrocarbon-targeted seismic reflection profiles acquired on eastern Anticosti Island (Quebec) image subparallel reflections with significant continuity below the Paleozoic St. Lawrence Platform. These intra-basement reflections define a seismic unit with a relatively simple geometry characterized by broad open folds, an array of subparallel markers, and east-northeast-dipping faults. The reflective seismic unit likely corresponds to the southern extension of the Mesoproterozoic Wakeham Group and Robe-Noire mafic sills that are exposed on the nearby north shore of the Gulf of St. Lawrence, in the eastern Grenville Province of Quebec.

A c. 1710 Ma mafic sill emplaced into a quartzite and calcareous series from Ighrem, Anti-Atlas – Morocco: Evidence that the Taghdout passive margin sedimentary group is nearly 1 Ga older than previously thought

The Taghdout Group is a passive margin sequence deposited during rifting and break-up of the northern margin of the West African craton (WAC), culminating with the creation of an oceanic basin between the northern edge of the WAC and an unknown terrane. However, the age of this passive margin has been poorly constrained. It was previously thought to be c. 800–1000 Ma on the basis of age of the contact metamorphosed host rocks of the associated mafic dykes (Rb/Sr, 789 ± 10 Ma). However, with the U-Pb dating of numerous dyke swarms in the Anti-Atlas Inliers, at c. 870, 1416-1380, 1650, 1750, and 2040 Ma, it was suggested by Youbi et al. (2013) that the Taghdout Group could be Mesoproterozoic in age, with a preference for an age of 1750 Ma. In order to test this idea, a mafic sill within the Taghdout Group has been dated by the ID-TIMS U-Pb method on baddeleyite, yielding an approximate age of c. 1710 Ma. This preliminary age confirms that the Taghdout Group is much older than previously thought. Further geochronology work is required to determine whether this c. 1710 Ma age represents a new intraplate event in the WAC or whether more concordant data could yield an age closer to the known WAC LIP event of c. 1750 Ma. With this result we propose a new lithostratigraphic framework for the Proterorozoic in the Anti-Atlas.

Mesoproterozoic continental breakup in NW China: Evidence from gray gneisses from the North Wulan terrane

The North Wulan terrane forms part of the micro-continental blocks in the early Paleozoic Qilian and North Qaidam orogenic belts in NW China. LA-ICP-MS dating of magmatic zircons from two gray gneisses from this terrane yielded U–Pb ages of 1519±5Ma and 1497±8Ma, suggesting emplacement of their precursor magmas at ∼1.5Ga. The rocks have high SiO2 (70.6–75.6wt.%) and Na2O (3.96–4.84wt.%), but relatively low Al2O3 (mostly <15wt.%) and K2O (0.90–2.52wt.%) contents. They possess low contents of Ni, Cr, Sc, Sr, Rb and moderate Y, with slightly enriched LREE ((La/Yb)N=3.46–11.3) and flat HREE ((Gd/Yb)N=0.95–2.06) patterns and strongly negative Eu anomalies (Eu/Eu∗=0.22–0.38). Thus, these rocks have trondhjemite compositions and are comparable to the low-Al or low-pressure TTG gneisses worldwide. Zircons from these rocks yielded εHf(t) and TDM2 values of −2.2–5.1 and 1.93–2.39Ga, respectively. Our data suggest that the precursor low-Al trondhjemite magma was generated from partial melting of the early Paleoproterozoic mafic rocks at shallow crust level through possible heat input from upwelling mantle in a continental rifting setting. We suggest that these ∼1.5Ga gneissic rocks were formed in a tectonic setting identical to that of coeval mafic sills and dykes in the northern Tarim Craton and western Yangtze Craton, and can be broadly correlated with the anorogenic magmatism associated with the initial fragmentation of the supercontinent Columbia.

Geological setting and fluid inclusion characteristics of a lead-copper-barium occurrence hosted in a Neoproterozoic mafic sill at Kiatak, Northumberland Island, Northwestern Greenland

We describe a Pb-Cu-Ba occurrence in Northumberland Island, Northwestern Greenland. Mineralization occupies the brecciated upper contact domain of a Neoproterozoic diabase sill belonging to the Franklin-Thule dike swarm (720–716Ma), with pyrite-bearing black shales of the Dundas Group, upper Thule Supergroup. The host tholeiitic diabase sill is of different composition (low TiO2 and P2O5) than the locally crosscutting dikes (high TiO2 and P2O5). Chloritization, carbonatization and silicification are intense in proximity to sulfides. Coarse grained, open space-filling galena and minor chalcopyrite are accompanied by 2 generations of calcite and 2 of barite. Galena contains significant amounts of Ag (av. 400ppm), Sb (av. 700ppm), Se (av. 20ppm), traces of Bi, Cd, and Sn. Fluid inclusions in the gangue of the Kiatak occurrence indicate two fluid types. Prior to galena precipitation, a CaCl2-NaCl-rich aqueous brine (~20wt.%eq NaCl) cooled from temperatures >300°C and was trapped first in early calcite, and with further cooling, in barite together with solid bitumen inclusions. Following galena crystallization, secondary inclusions containing a similar brine, but of lower salinity, higher Ca:Na ratio, and lower temperature, were trapped in calcite. Corrosion of galena was followed by precipitation of lower temperature (~100°C) barite from a second fluid, comprising immiscible water and CH4. Despite its location in the contact between shale and large mafic sill, the low-temperature mineralization postdates the cooling of the sill, and may be related to basinal fluid circulation controlled by regional extensional faults parallel to diabase dikes. Although uneconomic, the Kiatak occurrence may be witness to a larger metallogenic process that could have formed significant SEDEX type metal concentrations in strata within the Thule Supergroup.

Magmatism in Western Cuddapahs: The Mafic Sills and Lava Flows of Vempalle and Tadpatri Formations

Abstract: The western part of Cuddapah basin has been considered as a foreland to the fold-thrust belt represented by eastern Nallamalais and Nellore schist belt. The igneous activities, in the form of lava flows and sills, in western Cuddapahs have been a subject of controversy since tectonogenetic models ranging from a mantle plume to lithospheric extension have been suggested. The present work covers a detailed study of the mafic sills and lava flows of Vempalle and Tadpatri formations in western Cuddapahs, which earlier workers have also noted. A detailed field and laboratory work in this study brings out, for the first time, five distinct petrographic and geochemical varieties of sills. Geochemical studies also suggest that the different sill types have originated from varying degrees of partial melting of a metasomatized sub-crustal lithospheric mantle, the metasomatism of source being attributed to older Archaean convergent settings/greenstone belts that the basin unconformably overlies. The possible contribution of any small plume is only heating and doming of overlying mantle that induced incipient crustal rifting. Our geochemical studies on the lava flows of Vempalle Formation also suggest that the magma was derived from small degree of partial melting of spinel-garnet peridotite mantle in a rift setting.

Palaeoproterozoic banded iron formationhosted high-grade hematite iron ore deposits of the Transvaal Supergroup, South Africa

Banded iron formation-hosted high-grade (>60 weight percent (wt%) iron (Fe)) hematite ore deposits make up the bulk of the world's iron ore production and reserves. They developed mainly through supergene and/or hydrothermal leaching of silica from the iron formation host rock under oxidizing conditions. Early Palaeoproterozoic iron formations of the Transvaal Supergroup host several such high-grade hematite ore deposits. The largest ones are developed in the Asbesheuwels Subgroup iron formation on the Maremane Dome, between Sishen and Postmasburg in the Northern Cape Province of South Africa. These deposits currently produce virtually all of South Africa's annual supply of about 78 million tonnes (Mt) of high-grade hematite ore, to the local and export market from the Sishen, Khumani, Beeshoek and Kolomela Mines. These represent ancient supergene deposits that formed along the approximately 2.2 to 2.0 Ga unconformity at the base of the Gamagara/ Mapedi red bed succession of the Keis (formerly Kheis) Supergroup. Smaller ancient supergene deposits are also developed along the same unconformity such as where it intersects the Rooinekke Iron Formation of the Koegas Subgroup at the Rooinekke Iron Ore Mine to the south of the Maremane Dome and the intersection of the Hotazel Iron Formation of the Voelwater Subgroup of the Transvaal Supergroup, in the Kalahari Manganese Field to the north of the dome. The supergene ores occur in four types, namely laminated, massive, brecciated and conglomeratic ores, the latter forming part of the Gamagara/Mapedi red bed succession. With the exception of the ore at Rooinekke, which is manganiferous, the supergene ores are of high grade (>60 wt%) and they also have low silica and phosphorous contents. Normalized rare earth elements, in contrast to the banded iron formation protolith, show light rare earth element depletion and the oxygen isotopes are slightly depleted to slightly enriched in 18 O. Hydrothermally enriched deposits include: Thabazimbi, situated in the Penge Iron Formation of the Transvaal outcrop area of the Transvaal Supergroup in the metamorphic aureole of the Bushveld Complex; and the Bovenzeekoebaart and magmatic hydrothermal Nauga East deposit, both located in the Kuruman Iron Formation of the Asbesheuwels Subgroup, along the southern extremity of the Griqualand West outcrop area of the Transvaal Supergroup. These hydrothermal iron ores are all developed at the bottom contact between the host iron formation and underlying carbonaceous shale that marks the transition into the Malmani- Campbellrand carbonate platform succession below. In some instances this contact is faulted. The ore grades upwards into oxidized iron formation, with isolated lenses sometimes developed at higher stratigraphic levels. Fault systems intersect the iron formations and the mineralisation is concentrated between mafic sills, higher up in the iron formations and the bottom shale contact. At Nauga East, mineralisation is developed in contact with a steeply dipping zoned syenite-carbonatite dyke. The hydrothermal ores are of high grade (>60 wt%), but can have higher phosphorous contents of up to 0.25 wt%. Normalized rare earth elements are enriched but similar to that of the host iron formations and the oxygen isotopes are generally depleted in 18 O. The mining of iron ore in South Africa dates from between 800 and 1200 AD, with modern exploration and mining starting from 1916. The first production of high-grade banded iron formation-hosted ore commenced in 1931 from the Maremane Dome and Thabazimbi. The first truly world class iron ore mine in South Africa, Sishen, was opened in 1953 and still operates today. The deposits at Khumani and Welgevonden have gone into production as recently as 2008 and 2011 respectively. Current estimates place the life of mine of the larger deposits at between 15 and 40 years.

Lateral magma flow in mafic sill complexes

The structure of upper crustal magma plumbing systems controls the distribution of volcanism and influences tectonic processes. However, delineating the structure and volume of plumbing systems is difficult because (1) active intrusion networks cannot be directly accessed; (2) field outcrops are commonly limited; and (3) geophysical data imaging the subsurface are restricted in areal extent and resolution. This has led to models involving the vertical transfer of magma via dikes, extending from a melt source to overlying reservoirs and eruption sites, being favored in the volcanic literature. However, while there is a wealth of evidence to support the occurrence of dike-dominated systems, we synthesize field- and seismic reflection–based observations and highlight that extensive lateral magma transport (as much as 4100 km) may occur within mafic sill complexes. Most of these mafic sill complexes occur in sedimentary basins (e.g., the Karoo Basin, South Africa), although some intrude crystalline continental crust (e.g., the Yilgarn craton, Australia), and consist of interconnected sills and inclined sheets. Sill complex emplacement is largely controlled by host-rock lithology and structure and the state of stress. We argue that plumbing systems need not be dominated by dikes and that magma can be transported within widespread sill complexes, promoting the development of volcanoes that do not overlie the melt source. However, the extent to which active volcanic systems and rifted margins are underlain by sill complexes remains poorly constrained, despite important implications for elucidating magmatic processes, melt volumes, and melt sources.

Geochronological evidence for an extension of the Northern Lobe of the Bushveld Complex, Limpopo Province, South Africa

We have conducted the first geochronological study of the ultramafic–mafic succession (the Waterberg Project) located in the Southern Marginal Zone of the Limpopo Belt north of the Northern Lobe of the Bushveld Complex using U/Pb dating on zircons extracted from both mineralized reefs as well as on detrital zircons of the Waterberg Group unconformably overlying the mafics. This contribution is the first in a series of publications describing the well-mineralized succession and will therefore give an overview of the local geology. U/Pb baddeleyite ages for the main mafic sill intruding the area supplement the data. The succession is extremely well mineralized and therefore of interest to the wider geological community.The intrusion ages for the Lobe are 2059±3 and 2053±5Ma and within error coeval with the intrusion ages of the Bushveld Complex. Detrital zircon age probability density distributions indicate a maximum age of deposition of c. 2045Ma for the Waterberg Group and show abundant peaks of Archean age. Paleoproterozoic zircons are interpreted to represent zircon from the detritus of the now eroded roof rocks of the lobe, while the Archean zircons are most likely sourced from the Limpopo Complex to the northeast. The major dolerite sill intruding the project area has been confirmed to belong the Umkondo Igneous Province with a baddeleyite age of 1125±5Ma.The combination of these new age dates, local geology and existing literature is used to argue that (i) the ultramafic–mafic succession is related to the Bushveld Complex; (ii) a significant unconformity is present between igneous rocks and the overlying sediments, and (iii) the area was exposed to a level of Paleoproterozoic tectonism not previously observed in the Bushveld Complex.

Structural controls of fluid flow and gold mineralization in the easternmost parts of the Karagwe-Ankole Belt of north-western Tanzania

Gold mineralization in the Biharamulo region of western Tanzania is confined to the sheared, low-angle basement-cover contact between Archaean basement gneisses of the Tanzania Craton and the structurally overlying, low-grade metamorphic metasediments of the Mesoproterozoic Karagwe-Ankole Belt. Regional-scale fluid flow along this detachment is indicated by the pervasive silicification and retrogression of wall rocks to pervasively foliated phyllonites and pyritization of particularly metasediments, commonly graphite-rich, in the hanging wall of the shear zone. Gold mining centres on specific structural sites along the detachment, but also in stratigraphically higher sections in the structurally overlying metasediments. Zones of gold mineralization along the detachment correlate with NE trending ramp structures (dip angles 20°–35°) that are most ideally orientated for slip and reactivation within the low-angle phyllonitic detachment. Repeatedly overprinted auriferous quartz-vein stockworks in quartzofeldspathic gneisses immediately below the detachment indicate brittle fracturing of the competent footwall lithotypes during slip along the weaker detachment. In cases of massive silicification, up to 50m thick quartz blows are formed along the contacts between detachment phyllonites and footwall gneisses. The multiple overprinting relationships of successive quartz-vein generations in these zones of massive silicification suggests that the quartz blows acted as competent blocks in the weak detachment, causing the repeated overprint of earlier silicification by later fracturing and quartz-veining events. Gold mineralization above the detachment and in stratigraphically higher metasediments is closely associated with fold structures that form part of the low-grade metamorphic fold-and-thrust belt. Veining is particularly abundant in competent lithotypes, such as quartzite and chemically reactive ferruginous mafic sills. Overprinting relationships between quartz vein sets illustrate fluid flow during fold amplification and, importantly, the final lock-up stage of folds, during which much of the mineralization was introduced. Oxygen isotope values for quartz veins indicate fluids were likely derived from clastic, mainly metapelitic sedimentary sequences of the Karagwe-Ankole Fold Belt. The data also implies that the partially reworked Archaean granitoid-greenstone basement of the Tanzania Craton has not contributed to the fluid evolution and possibly gold mineralization. The extent (>100km) of the basement-cover detachment and associated alteration is indicative for a regional-scale fluid system. Gold mineralization is, however, controlled by local structures and lithological contrasts that require the detailed mapping and sampling of the regional structure.

Geology and U/PB geochronology of the Gamtoos Complex and lower Paleozoic Table Mountain Group, Cape Fold Belt, Eastern Cape, South Africa

The Cape Fold Belt (CFB) along the southern coast of South Africa contains several tectonic windows that expose low-grade “basement” rocks as inliers that are inferred to be late Neoproterozoic in age subjected to tectono-metamorphism during the Pan African Saldanian Orogeny (ca. 650 to 550 Ma). Whilst a Saldanian tectonic history along the northwest to southeast trending western branch of the CFB has been well documented, such Neoproterozoic deformation along the east-west trending southern branch of the CFB is not established. This work presents new field observations and SHRIMP U/Pb geochronologic data from the Kleinrivier Sequence within the Gamtoos Inlier exposed along the eastern-most known tectonic window, near Port Elizabeth, and which is here renamed the Gamtoos Complex. The study area includes 3 tectonic sequences from the Gamtoos Complex (“pre-Cape”) and 2 sequences from the overlying Table Mountain Group of the Cape Supergroup. The lower sequences of the Gamtoos Complex comprise north-east verging thrust packages within a regional antiform, all of which have a common oriented planar tectonic fabric (S 2 ) coincident with those in the lower Paleozoic rocks. Phyllites from the Kleinrivier and upper Sardinia Bay sequences (spanning the pre-Cape and Cape boundary) are similarly affected by a slaty cleavage (S 1 ), crenulation cleavage (S 2 ) and multiple generations of quartz veins. U/Pb data are from zircons of metasediments and igneous rocks from the Gamtoos Complex and from the overlying rocks of the Table Mountain Group. Meta-greywackes of the Kleinrivier Sequence (youngest zircon: 523 ± 6 Ma) are probably Cambrian in age. However, some units from the Kleinrivier Sequence are intruded by felsic sills with a Concordia age: 530.2 ± 4.4 Ma and therefore require further U/Pb detrital zircon analyses. Mafic sills and dykes also intrude the Kleinrivier Sequence and were subsequently deformed and contain S 2 . Boulder conglomerates in the Sardinia Bay Sequence (youngest matrix zircon: 521 ± 6 Ma) are separated from the Kleinrivier Sequence by an angular unconformity. It also contains granite-boulders (Concordia age: 530.2 ± 4.7 Ma), and is overlain by the feldspathic psammites and quartzites of the Sardinia Bay Sequence with zircons as young as 510 ± 7 Ma (e.g. middle Cambrian). In total, 75% of the detrital zircons from all sequences date between early-Mesoproterozoic and early-Neoproterozoic (ca. 1455 to 852 Ma) and are likely sourced from the gneisses of the Namaqua-Natal province that are known to underlie the CFB, whilst the Neoproterozoic to mid-Cambrian zircons (ca. 828 to 431 Ma) have been sourced from Pan African mobile belts, possibly the Mozambican and/or Saldanian Belts to the east and west, respectively. Such “Pan African” detrital zircons are more prevalent in the Sardinia Bay Sequence and form the dominant component in the Lower Table Mountain Group, suggesting a lesser source influence from the Namaqua Natal Mobile Belt. The conglomerates at Sardinia Bay possibly represent mid-Cambrian rift sequences, similar to the Klipheuwel Group that overlie the Malmesbury Sequence and Cape granites along the western branch of the CFB. The Sardinia Bay Sequence has zircons equivalent to those of the Peninsula Formation (youngest zircon: 516 ± 5 Ma), and therefore should be linked to represent the lower Table Mountain Group.

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.

The Early Cambrian bimodal magmatism in the northeastern Siberian Craton

We present new data on geochemistry, isotopic geochemistry, and geochronology of the Early Cambrian igneous rocks of the northeastern Siberian Craton (Kharaulakh anticlinorium, contact between the Siberian Platform and the West Verkhoyansk sector of the Verkhoyansk fold-and-thrust belt united into an Early Cambrian bimodal complex. This complex comprises trachyrhyolites forming pebbles in conglomerates near the base of the Cambrian succession, overlying trachybasalts, and mafic sills and dikes cutting Neoproterozoic strata. According to chemical composition, the felsic rocks are high-alkali rhyolites and correspond to A-type granites. The high contents of Ta, Nb, Hf, Tb, and Zr in these rocks suggest the presence of enriched mantle material in their magmatic sources. The mafic volcanics are high-Ti trachybasalts and trachydolerites with similar geochemical characteristics corresponding to alkali basalts or OIB. The high (Tb/Yb)PM ratios in these volcanics evidence that their magmatic source was the garnet peridotite mantle located at depths more than 90 km and characterized by a low degree of melting. However, the rhyolites, trachybasalts, and trachydolerites show high positive εNd(T) values (4.2–4.7, 7.5–8.9, and 7.2–8.2, respectively) indicating a depleted mantle source and no crustal contamination. The high (Nb/Yb)PM ratio points to the mixing of magmas from enriched and depleted mantle sources. Mafic magmas might have been generated from a heterogeneous source or interacted with the depleted mantle before intrusion. Both the felsic and the mafic rocks formed in within-plate environments. U–Pb zircon dating yielded concordant ages of 525.6 ± 3.9 and 537.0 ± 4.2 Ma, corresponding to the Early Cambrian age of the rhyolites. The date of 546.0 ± 7.7 Ma obtained for one sample points (with regard to the error) to the Late Vendian–Early Cambrian age. Thus, at the Vendian–Early Cambrian boundary, the northeastern Siberian Platform was subjected to continental rifting accompanied by bimodal magmatism of antidrome evolution. According to paleotectonic reconstructions, this part of the Siberian Craton might have been connected with the eastern margin of Laurentia in the Late Neoproterozoic (Late Riphean–Late Vendian), and continental rifting that started at the Vendian–Cambrian boundary led to their separation. The obtained isotope-geochronological data suggest that the studied bimodal complex began to form at the Vendian–Cambrian boundary and this process terminated no earlier than the end of the Terreneuvian (Tommotian), i.e., the complex formed during rifting for about 20 Myr.

Early Neoproterozoic emplacement of the diabase sill swarms in the Liaodong Peninsula and pre-magmatic uplift of the southeastern North China Craton

Diabase sill swarms are widespread within the Neoproterozoic sedimentary rocks in the Liaodong Peninsula (named as the Dalian mafic sill swarms), eastern North China Craton (NCC). Our new zircon LA-ICP-MS U–Pb and baddeleyite SIMS Pb–Pb dating results on five diabase sill samples emplaced into the Qiaotou, Cuijiatun and Xingmincun Formations in the Liaodong Peninsula indicate their emplacement during the early Neoproterozoic (Tonian) period at 0.92–0.89Ga and pre-magmatic regional uplift prior to ca. 0.92–0.89Ga. The above results provide important constraints on the upper boundary of the Neoproterozoic strata and their macroscopic carbonaceous fossils and organic-walled microfossils in the eastern and southeastern NCC and indicate they are Tonian in age. The early Neoproterozoic Dalian diabase sills belong to the tholeiitic series and are characterized by low contents of SiO2 and K2O, high contents of TiO2, Fe2O3T and MgO, slight light REE (LREE)-enrichment and no Eu anomalies on chondrite-normalized REE patterns, enrichment of high field strength element (HFSE) and lack of negative Nb, Ta, Zr, Hf, P and Ti anomalies on primitive mantle-normalized spidergrams; and exhibit geochemical characteristics of within plate basalt on discrimination diagrams. The newly identified Dalian diabase sills, together with the previously reported Xu-Huai and Sariwon mafic sills and the Dashigou mafic dykes, constitute an early Neoproterozoic (0.92–0.89Ga) large igneous province in the NCC (Sino-Korean Craton). Formation of the early Neoproterozoic diabase sill (dyke) swarms in the NCC is probably related to a continental rifting event that have led to breakup of southeastern NCC from some other continents in the Rodinia supercontinent. Breakup of the NCC from the Rodinia supercontinent at around 0.92–0.89Ga is also supported by pre-magmatic regional uplift of the southeastern NCC prior to ca. 0.92–0.89Ga and evolution trend of the Dalian diabase sills from within plate basalt to mid-ocean ridge basalt on the Zr/Y vs. Zr discrimination diagram.

Baddeleyite U–Pb ages and gechemistry of the 1875–1835 Ma Black Hills Dyke Swarm across north-eastern South Africa: part of a trans-Kalahari Craton back-arc setting?

Eleven new baddeleyite U–Pb crystallisation ages and associated whole-rock geochemistry on NE–NNE-trending tholeiitic dykes cutting across the north-eastern corner of the Archaean Kaapvaal Craton, the overlying Transvaal basin and the Bushveld and Phalaborwa igneous complexes collectively define a 1875–1835 Ma Black Hills Dyke Swarm (BHDS). Dyke ages do not discriminate between dyke trends or geographic location, but subdivide the BHDS into an older set of four more primitive dykes (MgO = 9.4–6.8 wt.%) and a younger set of seven dykes with more differentiated compositions (MgO = 5.6–4.2 wt.%). Despite being emplaced over a c. 40 Myr period, major element compositions are remarkably consistent with a single inversely modelled bulk fractionating assemblage of 57.5% plagioclase, 29.5% augite and 13.0% olivine. This fractionating assemblage requires an additional assimilation of bulk continental crust (at a low r-value of 0.2) for reversed modelling of parental rare earth elements. Even though this crustal assimilation indicates that primary magmas could potentially have been derived from a spinel-bearing ambient primordial and asthenospheric mantle source, anomalously low Nb and high Pb values for the more primitive older dykes may also have been inherited from a sub-continental lithospheric mantle source. The ages for the BHDS bridge a gap between c. 1889 and 1867 Ma mafic sills and c. 1830 Ma rhyodacitic pyroclasts, interbedded in the top of a ~3 km-thick Sibasa basalt sequence, which combine into a continuous c. 1.89–1.83 Ga igneous province. Similar geochemical signatures are consistent with all sills, volcanic rocks and BHDS feeders collectively belonging to a very voluminous and coherent igneous province, which arguably formed behind active Magondi and Okwa-Kheis arcs, along the western margin of the proto-Kalahari Craton.

Reassessment of Aguapeí (Salto do Céu) paleomagnetic pole, Amazonian Craton and implications for Proterozoic supercontinents

The Aguapeí paleomagnetic pole obtained for mafic sills and dykes from Salto do Céu region at the western margin of the Amazonian Craton constrains its links with Baltica and Laurentia in Rodinian reconstructions. A new U–Pb age on baddeleyite at 1439±4Ma for the intrusives, contrasts strongly with a previous Ar–Ar age at 981±2Ma, with important consequences for paleogeographic reconstructions. We report new paleomagnetic and magnetic anisotropy results for sills from the Salto do Céu region and reassess the paleomagnetic data in view of the new geochronological age. A total of 155 samples were collected from thirteen new paleomagnetic sampling sites, five of them corresponding to sedimentary rocks located at the borders of the sills in an attempt to perform baked contact tests. After thermal and alternating field demagnetization, the sills provided a characteristic magnetic component at Dm=208.2°, Im=68.5° (N=8, α95=6.4°), with a corresponding paleomagnetic pole at 46.4°S; 277.0°E (A95=10.2°). Directions obtained in this study are similar to those reported previously for other mafic sills and dykes in the same region. A pole integrating the new results for the sills with those of the previous Aguapeí pole is situated at 56.0°S; 278.5°E (A95=7.9°). This new combined Salto do Céu pole supersedes the previous Aguapeí pole. Magnetic mineralogy studies, optical and electronic microscopy indicates PSD magnetite as the main magnetic carrier in these rocks. The baked contact tests were inconclusive, but the similarity between Salto do Céu pole and other high-quality poles with ages around 1420–1430Ma suggests they carry a primary thermoremanence of that age. The Salto do Céu and other coeval poles are compatible with a connection between Amazonia and Baltica at the Mesoproterozoic in a paleogeographic configuration slightly different from SAMBA (South America-Baltica). At the same time, the new geochronological and paleomagnetic data imply that all paleogeographic interpretations for the position of Amazonia in Rodinia based on the previously published Aguapeí pole (and now renamed as Salto do Céu pole) must be revised.

A numerical approach to sill emplacement in isotropic media: Do saucer-shaped sills represent ‘natural’ intrusive tendencies in the shallow crust?

Saucer-shaped mafic sills are of common occurrences in sedimentary settings worldwide, but have only rarely been reported for onshore igneous settings. Mafic sills, which have intruded the lava pile of the Faroe Islands Basalt Group (FIBG) of the North Atlantic Igneous Province (NAIP), are characterised by gradual and continuous upward-curving geometries from bases to rims. Some of the smaller Faroese sills gradually wedge out to near-zero thicknesses at one of their ends, but these very thin sill sections still display gradual upward-curving geometries, despite the lack of any displacements of their overburdens that can be directly measured or detected with the unaided eye. This is at odds with earlier emplacement models applying for sedimentary settings, predicting sill climbing in response to stress asymmetries at their propagating tips due to wholesale overburden uplifts. In this paper, published values of Young's modulus for wet basaltic crust are utilised in order to calculate elastic displacements of isotropic host-rocks during individual small-scale inflation sequences in embryonic sills immediately behind their propagating tips at various depths and magmatic pressures. The calculations suggest that stress asymmetries at propagating tips and associated upward-curving sill climbing may occur when proportionally larger melt volumes are emplaced above initial single extension propagation fractures than below them. Angles of sill deflection are determined by differences in melt volumes deposited on top of and below initial planes of propagation fractures respectively, in addition to lengths of individual inflation sequences and magnitudes of magmatic pressures. The results obtained in this paper seem to suggest that asymmetrical small-scale inflation sequences at advancing sill tips in relatively isotropic media have the potential to generate saucer-shaped sills.

Evolution of a ∼2.7 Ga large igneous province: A volcanological, geochemical and geochronological study of the Agnew Greenstone Belt, and new regional correlations for the Kalgoorlie Terrane (Yilgarn Craton, Western Australia)

The thick package of ∼2.7Ga mafic and ultramafic lavas and intrusions preserved among the Neoarchean of the Kalgoorlie Terrene in Western Australia provides valuable insight into geological processes controlling the most prodigious episode of growth and preservation of juvenile continental crust in Earth's history. Limited exposure of these rocks results in uncertainty about their age, physical and chemical characteristics, and stratigraphic relationships. This in turn prevents confident correlation of regional occurrences of mafic and ultramafic successions (both intrusive and extrusive) and hinders the interpretation of tectonic setting and magmatic evolution. A recent stratigraphic drilling program of the Neoarchean stratigraphy of the Agnew Greenstone Belt in Western Australia has provided continuous exposures through a c. 7km thick sequence of mafic and ultramafic units. In this study, we present a volcanological, lithogeochemical and chronological study of the Agnew Greenstone Belt, and provide the first pre-2690Ma regional correlation across the Kalgoorlie Terrane. The Agnew Greenstone Belt records ∼30m.y. of episodic ultramafic–mafic magmatism that includes two cycles, each defined by a komatiite that is overlain by units that become more evolved and contaminated with time. The sequence is divided into nine conformable packages, each consisting of stacked subaqueous lava flows and comagmatic intrusions, as well as two sills without associated extrusions. Lavas, with the exception of intercalations between two units, form a layer-cake stratigraphy and were likely erupted from a system of fissures tapping the same magma source. The komatiites are not contaminated by continental crust ([La/Sm]PM∼0.7) and are of the Al-undepleted Munro-type. Crustal contamination is evident in many units (Songvang Basalt, Never Can Tell Basalt, Redeemer Basalt, and Turrett Dolerite), as judged by [La/Sm]>1, negative Nb and Ti anomalies, and geochemical mixing trends towards felsic contaminants. Crystal fractionation was also significant, with early olivine and chromite (Mg#>65) followed by plagioclase and clinopyroxene removal (Mg<65), and in the most evolved case, titanomagnetite accumulation. Three new TIMS dates on granophyric zones of mafic sills and one ICP-MS date from an interflow felsic tuff are presented and used for regional stratigraphic correlation. Cycle I magmatism began at ∼2720Ma and ended ∼2705Ma, whereas cycle II began ∼2705Ma and ended at 2690.7±1.2Ma. Regional correlations indicate the western Kalgoorlie Terrane consists of a remarkably similar stratigraphy that can be recognised at Agnew, Ora Banda and Coolgardie, whereas the eastern part of the terrane (e.g., Kambalda Domain) does not include cycle I, but correlates well with cycle II. This research supports an autochthonous model of greenstone formation, in which one large igneous province, represented by two complete cycles, is constructed on sialic crust. New stratigraphic correlations for the Kalgoorlie Terrane indicate that many units can be traced over distances >100km, which has implications for exploration targeting for stratigraphically hosted ultramafic Ni and VMS deposits.