high-Mg Tholeiites Research Articles

Archean to Paleoproterozoic evolution of the Crixás greenstone belt, Central Brazil: Insights from two contrasting assemblies of metaigneous rocks

The Archean-Paleoproterozoic Crixás greenstone belt is one of the most important components of the Crixás-Goiás Domain, the oldest crustal segment of the Central Brazil Goiás Massif. This gold-bearing greenstone comprises amphibolite facies ultramafic-mafic metavolcanic rocks overlaid by a low-grade metasedimentary succession associated with rocks of intermediate composition. We present new integrated geochemical and isotopic data of the metaigneous rocks to characterize the stages of magmatism and delineate the petrogenetic processes and tectonic environment of the different units. The data give important insights into the geological evolution of the Crixás greenstone belt during the Archean and Paleoproterozoic, including that the Crixás greenstone belt consists of genetically and temporal unrelated rocks that are spatially associated in a structural framework marked by stacking and imbrication. Two lithological assemblies were described: i) a Mesoarchean komatiite-tholeiite association formed by heterogeneous mantle plume melting in an oceanic plateau setting, and ii) a Rhyacian intraoceanic arc association. These units are allochthonous and override an Archean crustal basement dominated by granites and gneisses of mostly TTG composition. Chemical composition of the komatiites is similar to the Barberton-type Al-depleted komatiites, whilst the basalts consist of high-Mg tholeiites with flat chondrite-normalized REE patterns and immobile trace element systematics that resemble Phanerozoic oceanic plateau lavas. Eruption age of the ultramafic and mafic lavas is roughly estimated at ~3.0 Ga, based on previously published SmNd and PbPb isotopic studies, whereas whole-rock ReOs isotopic data of samples of komatiite and basalt presented here yielded a 2609 ± 65 Ma isochron age. This younger age may reflect a Neoarchean overprinting event associated with metasomatic Re-addition recorded in the lower ultramafic-mafic metavolcanic rocks, possibly related to the late stages of the cratonization of the Crixás-Goiás Domain during the emplacement of the youngest Archean granites of the region (~2.70 Ga). Newly described andesites and diorites possess a subduction zone fingerprint. LA-ICP-MS UPb zircon dating of an andesite sample provided an upper intercept Concordia age of 2172.2 ± 12.7 Ma, constraining the age of the intermediate magmatism. Therefore, these rocks are correlated with the Rhyacian syn-orogenic poorly-sorted graywackes and carbonaceous phyllites of the greenstone's upper stratigraphy. Andesites and diorites have homogeneous Paleoproterozoic two-stage Nd model ages of 2.28 to 2.26 Ga and positive initial εNd values of +2.34 to +2.69, indicating a juvenile nature for the arc magmatism. ReOs geochronology of gold-related arsenopyrite from a metasedimentary rock-hosted massive sulfide provided an age of 2137 ± 11 Ma, suggesting that the mineralization is also coeval to the Rhyacian collisional stage.

Geochemical and isotopic constraints on the dynamics of the Earth: a Special Issue dedicated to Jan Dirk Kramers and his legacy to geochemistry

Chromium exists in two oxidation states Cr2+ and Cr3+ during high-temperature magmatic processes and changes in Cr redox are often associated with stable isotopic fractionation. Thus, the stable chromium isotope compositions of mantle-derived magmas bear the potential to trace the oxidation states of their mantle sources as well as any post melting changes in Cr redox (e.g. during magmatic differentiation), in a manner similar to magmatic stable Fe isotopic fractionation. However, these stable isotope fractionation effects are less understood for Cr relative to Fe. Komatiites and tholeiitic basalts represent a wide range of mantle-derived partial melts with variable fractional crystallization of phases with different affinity for Cr2+ and Cr3+. Thus, they offer potential archives to better understand high-temperature Cr isotope fractionation processes and mantle redox.Here, we report major and trace elements as well as coupled stable Cr and Fe isotope compositions of two well-characterized Archean komatiite-tholeiite suites from the Barberton Greenstone Belt, South Africa and Eswatini, and Belingwe Greenstone Belt, Zimbabwe. The sample suites range in MgO concentrations from 3.85 to 34.33 wt%, which allows investigation of the impact of large degrees of magmatic differentiation in a komatiite-basalt system. Whole-rock δ53/52CrSRM979 and δ56/54FeIRMM014 values range from −0.390 ± 0.016 to −0.061 ± 0.016‰ and −0.014 ± 0.018 to +0.192 ± 0.018‰, respectively. The komatiites have a very narrow range in their Cr isotopic composition with an average δ53/52CrSRM979 value of −0.122 ± 0.050‰ (2SD; n = 21), which supports previous estimates of the bulk silicate Earth δ53/52CrSRM979 value. However, high-Mg tholeiites and basaltic andesites exhibit significantly lighter δ53/52CrSRM979 and heavier δ56/54FeIRMM014 values than komatiites. These variations can be linked to crystallization and accumulation of mineral phases observed from fractionation trends of the two Archean komatiite-tholeiite suites. In detail, during crystallization and accumulation of olivine the Cr isotope compositions of komatiites stay invariant, whereas at the onset of Cr-bearing spinel and pyroxene crystallization the Cr isotope signatures of komatiitic basalts, high-Mg tholeiites and basaltic andesites become progressively lighter, which is attributed to the preferential incorporation of isotopically heavier Cr3+ in these mineral phases. The gradual increase of δ56/54FeIRMM014 with increasing magmatic differentiation, does not allow identifying the crystallization of particular mineral phases using Fe isotopes alone. Ultimately, this study demonstrates the power of combining stable Cr and Fe isotopic analyses to examine the effects of fractional crystallization on modifying melt source values and thus to ensure accurate mantle redox estimations.

Coupled stable chromium and iron isotopic fractionation tracing magmatic mineral crystallization in Archean komatiite-tholeiite suites

Chromium exists in two oxidation states Cr2+ and Cr3+ during high-temperature magmatic processes and changes in Cr redox are often associated with stable isotopic fractionation. Thus, the stable chromium isotope compositions of mantle-derived magmas bear the potential to trace the oxidation states of their mantle sources as well as any post melting changes in Cr redox (e.g. during magmatic differentiation), in a manner similar to magmatic stable Fe isotopic fractionation. However, these stable isotope fractionation effects are less understood for Cr relative to Fe. Komatiites and tholeiitic basalts represent a wide range of mantle-derived partial melts with variable fractional crystallization of phases with different affinity for Cr2+ and Cr3+. Thus, they offer potential archives to better understand high-temperature Cr isotope fractionation processes and mantle redox.Here, we report major and trace elements as well as coupled stable Cr and Fe isotope compositions of two well-characterized Archean komatiite-tholeiite suites from the Barberton Greenstone Belt, South Africa and Eswatini, and Belingwe Greenstone Belt, Zimbabwe. The sample suites range in MgO concentrations from 3.85 to 34.33 wt%, which allows investigation of the impact of large degrees of magmatic differentiation in a komatiite-basalt system. Whole-rock δ53/52CrSRM979 and δ56/54FeIRMM014 values range from −0.390 ± 0.016 to −0.061 ± 0.016‰ and −0.014 ± 0.018 to +0.192 ± 0.018‰, respectively. The komatiites have a very narrow range in their Cr isotopic composition with an average δ53/52CrSRM979 value of −0.122 ± 0.050‰ (2SD; n = 21), which supports previous estimates of the bulk silicate Earth δ53/52CrSRM979 value. However, high-Mg tholeiites and basaltic andesites exhibit significantly lighter δ53/52CrSRM979 and heavier δ56/54FeIRMM014 values than komatiites. These variations can be linked to crystallization and accumulation of mineral phases observed from fractionation trends of the two Archean komatiite-tholeiite suites. In detail, during crystallization and accumulation of olivine the Cr isotope compositions of komatiites stay invariant, whereas at the onset of Cr-bearing spinel and pyroxene crystallization the Cr isotope signatures of komatiitic basalts, high-Mg tholeiites and basaltic andesites become progressively lighter, which is attributed to the preferential incorporation of isotopically heavier Cr3+ in these mineral phases. The gradual increase of δ56/54FeIRMM014 with increasing magmatic differentiation, does not allow identifying the crystallization of particular mineral phases using Fe isotopes alone. Ultimately, this study demonstrates the power of combining stable Cr and Fe isotopic analyses to examine the effects of fractional crystallization on modifying melt source values and thus to ensure accurate mantle redox estimations.

Convergent margin magmatism and crustal evolution during Archean-Proterozoic transition in the Jiaobei terrane: Zircon U–Pb ages, geochemistry, and Nd isotopes of amphibolites and associated grey gneisses in the Jiaodong complex, North China Craton

Mafic metavolcanic rocks and grey gneisses are the dominant components of many Archean terranes and preserve significant imprints and information of the crustal evolution. Here we present in situ zircon U–Pb geochronology, whole-rock geochemistry and Nd isotopes of the amphibolites and associated grey gneisses in the drill holes from the Jiaobei terrane, North China Craton. The LA-ICP-MS zircon U–Pb data show that the magmatic protoliths of two amphibolites and two grey gneisses crystallized at 2457–2503Ma followed by the metamorphism at ∼1.8–1.9Ga. These rocks exhibit heterogeneous whole-rock Nd isotopic compositions, with the most radiogenic analyses (ɛNd(t)=+3.8 to +6.9) plotting on or close to the depleted mantle evolution array and the most unradiogenic ɛNd(t) extending down to −2.4. The data suggest that both ∼2.5Ga depleted mantle and ancient continental crust at least as old as 3.05 Ga contributed to the magma source. In combination with previous zircon Hf isotopic data, it is suggested that the ∼2.5–2.6Ga magmatic events in the entire Jiaobei terrane were mostly derived from the reworking of the pre-existing ∼2.7–2.8Ga crustal material. However, large variations in Hf–Nd isotopic compositions of abundant rocks at ∼2.5Ga and ∼2.7–2.8Ga in the Jiaobei terrane were likely to have resulted from interaction of mantle-derived juvenile magma with ancient crustal materials in the source region. Thus, the peaks of zircon Hf crustal model ages from these rocks do not represent the actual time of crustal growth, but could be artifacts of interaction of magmas from different sources. Geochemical features show that the protoliths of the amphibolites are tholeiitic basalts and can be subdivided into two groups: high-Mg tholeiite basalts (Group I) and high-Fe tholeiite basalts (Group II). Trace element systematics indicate that the Group I amphibolites show good affinity to subduction-related tholeiitic arc basalts, whereas the Group II amphibolites represent back-arc lavas modified by subduction components. The two groups of amphibolites and associated grey gneisses collectively define an arc-back-arc tectonic setting in the study area at ∼2.5Ga. Integrated with the previous data from other areas in Jiaobei, it can be inferred that Archean-Proterozoic transition in the whole Jiaobei terrane witnessed subduction-related magmatism in a convergent margin.

Paleoproterozoic basaltoids in the North Baikal volcanoplutonic belt of the Siberian craton: age and petrogenesis

The oldest igneous rocks in the Paleoproterozoic (~1.88–1.85 Ga) North Baikal postcollisional volcanoplutonic belt of the Siberian craton are the basaltoids of the Malaya Kosa Formation (Akitkan Group). The youngest are the composite (dolerite–rhyolite) and doleritic dikes cutting the granitoids of the Irel’ complex and the felsic volcanic rocks of the Khibelen Formation (Akitkan Group). The position of Malaya Kosa basaltoids in the Akitkan Group section and published geochronological data on the felsic volcanic rocks overlying Malaya Kosa rocks suggest that their age is ~1878 Ma. The rhyolites from the center of a composite dike were dated by the U–Pb zircon method at 1844 ± 11 Ma, and the dolerites in the dikes are assumed to be coeval with them. Malaya Kosa basaltoids correspond to high-Mg tholeiites and calc-alkaline andesites, whereas the dolerites in the dikes correspond to high-Fe tholeiites. Geochemically, these basaltoids and dolerites are both similar and different. As compared with the dolerites, the basaltoids are poorer in TiO 2 (an average of 0.89 vs. 1.94 wt.%), Fe 2O 3 ∗ (9.54 vs. 14.71 wt.%), and P 2O 5 (0.25 vs. 0.41 wt.%). However, these rocks are both poor in Nb but rich in Th and LREE, ε Nd(T) being negative. According to petrographic and geochemical data, they derived from compositionally different sources. It is assumed that the basaltoids originated from subduction-enriched lithospheric mantle, whereas the dolerites originated from refractory lithospheric mantle metasomatized by subduction fluids. The isotopic and geochemical features of mafic rocks in the North Baikal belt are well explained by their formation during crustal extension which followed subduction and collision in the region. The early stages of postcollisional extension evidenced the melting of subduction-enriched lithospheric mantle with the formation of parent melts for Malaya Kosa basaltoids. At the final stages of the formation of the North Baikal belt, during the maximum crustal extension, Fe-enriched melts rose to the surface and generated the dolerites of the dikes.

Silurian/Ordovician asymmetrical sill-like bodies from La Codosera syncline, W Spain: A case of tholeiitic partial melts emplaced in a single magma pulse and derived from a metasomatized mantle source

A Silurian/Ordovician extensional event in the southernmost sectors of the Central Iberian Zone is inferred from the Sm/Nd isochron obtained (436 ± 17 Ma) after the diabase sills from the La Codosera syncline. From the geochemical and mineralogical points of view, the diabase sills are subalkaline and range between high-Mg tholeiite diabases to tholeiite andesites. LREE enrichment, an Nb negative anomaly, the absence of a Ta trough and a high Nd isotope signature ( ε Nd t = + 6) are the most relevant geochemical features. The diabase bodies are up to 330 m in thickness and were sampled from bottom to top along several different sections, permitting the definition of an accumulation of clinopyroxene, olivine and plagioclase close to chilled margins at the bottom, and abundant pegmatoid layers at the top. Chemical profiles and mass-balance modelling suggest that the bulk rock and chilled margin compositions are not dissimilar, defining an unusual S-type vertical compositional profile for large (> 50 m thick) sills, which in turn strongly suggests a single magma pulse and a probable gravitational settling. Assuming chilled margin samples as the parental magma, as well as Cr-enriched samples as cumulate layers, a two-stage liquid line of descent has been established, the first one consisting of a clinopyroxene-plagioclase-olivine cumulate assemblage. A second stage in relation to the depletion in Ti, Fe and V is accounted for by ilmenite fractionation, along with that of clinopyroxene, plagioclase and olivine fractionation. Thermobarometric estimations reveal that the clinopyroxene (around 1100 °C and 197 MPa) was a late mineral phase, whereas the plagioclase (around 1200 °C) was pre- to syn-emplacement, in agreement with the presence only of plagioclase phenocrysts in the chilled margins and the very abundant positive Eu anomaly. The energy constraint modelling is consistent with the lack of a significant assimilation process owing to the high temperature contrast between the country rock and the magma itself. The compositional characteristics of chilled margins enable them to be ruled out as primary melts in equilibrium with mantle olivine (Fo 91), a certain amount of olivine fractionation being required, which might have occurred in magma conduits en route to shallow emplacement levels in the crust. Spinel lherzolitic xenoliths from the European Cenozoic alkaline magmatism appear to be unsuitable protoliths to account for the chilled margin compositions. Instead, a hybrid mantle source consisting of a small amount of OIB-mantle component (5 wt.%) and a depleted end-member mantle component seems to be a plausible protolith, resulting in a good fit with the fractionation-corrected chilled margin trends for 10% of partial melting.

Platinum-group elements in the Icelandic rift system: melting processes and mantle sources beneath Iceland

Thirty new platinum-group element (PGE) analyses of various basalt types from Iceland are presented in this study. The analysed samples are divided into three groups based on their Mg-contents, high-Mg tholeiites (10–14 wt.% MgO), composed of primitive olivine tholeiites and picrites, evolved olivine tholeiites (7–10 wt.% MgO) and evolved basalts (4–7 wt.% MgO) consisting of FeTi basalts, quartz tholeiites and alkaline basalts. The high-Mg tholeiites have a range of compositions between a relatively Cu-rich and Pd-poor (120 ppm Cu and 6 ppb Pd) end-member and a relatively Cu-poor and Pd-rich (74 ppm Cu and 17 ppb Pd) end-member. There is a positive correlation between the highly siderophile elements, whereas Cu and Pd correlate negatively between the Cu-rich primitive olivine tholeiite and the Cu-poor picrite end-member. Negative correlation between Cu and the PGEs cannot be reconciled in a model where the two end-members form by varying degrees of partial melting of a common source. The sub-primitive mantle Cu/Pd ratio of the picrite end-member (∼4300) could indicate a strongly depleted mantle source, where Pd was efficiently retained in mantle sulphide, relative to Cu, during previous melt extraction episodes. In mantle melting models presented here, the picrite composition can be approximated by 25% melting of a mantle source that previously lost a normal MORB component ( F∼15%). The olivine tholeiite end-member (Cu/Pd∼19000) can be approximated in mantle melting models by assuming derivation from a separate source, one slightly Cu-enriched source undergoing melting beneath a rift. Based on Cu and Pd variations, the majority of high-Mg tholeiites are interpreted to be mixtures between melts derived from highly depleted to slightly enriched sources, which is consistent with previous studies of Sr–Nd isotopic variations. The similar range in Cu/Pd ratios of the high-Mg tholeiites and the evolved olivine tholeiites could infer that the latter evolved from parental liquids similar to the high-Mg tholeiites under S-undersaturated conditions. Relatively high Pd and low Ir concentrations in the evolved olivine tholeiite samples (∼7 wt.% MgO) indicate, too, that these magmas underwent S-undersaturated fractionation during which Pd accumulated in the melt whereas Ir was incorporated into the fractionating assemblage. This is in contrast to the group of evolved basalts with less than 7 wt.% MgO with low PGE contents and high Cu/Pd ratios that strongly suggest that these samples have experienced S-saturation. We propose a scenario for melt generation beneath the Icelandic rift zone, where picritic magma is generated from a highly depleted mantle, whereas primitive olivine tholeiite magma forms from the depleted as well as a Cu-enriched source. It is proposed that Cu-enriched mantle domains were exhausted at depth since their melting products are absent in the most depleted picrite derived from the uppermost part of the central melting region. The olivine tholeiites can be approximated by around 11–12% partial melting in a triangular melting regime beneath the Icelandic rift zone. The olivine tholeiites are interpreted to reflect efficient melt collection from the deepest and most distal parts of the melting triangle. In contrast, the picrite represent a high degree of melting ( F∼25%) without incorporation of melt batches from the Cu-enriched mantle source.

Juxtaposition of India and Antarctica During the Precambrian: Inferences from Geochemistry of Mafic Dykes

There are several geological, geochemical and geophysical evidences, which corroborate reconstruction of Gondwanaland and juxtaposition of India and Antarctica. Petrology of the Precambrian mafic dykes of East Antarctica and Central-East India also support juxtaposition of India and Antarctica. Mafic dykes of different generations are emplaced in the Archaean granite gneisses of these regions. These dykes appear to be an important tool to support juxtaposition of India and Antarctica. Geological and petrological data of the Central-East India Precambrian mafic dykes suggest four episodes of mafic magmatism in the region - three tholeiitic and one noritic (?). Similarly, East Antarctica also comprises four dyke suites, emplaced during three distinct periods. These suites are 2.4 Ga meta-tholeiites, 2.4 Ga high-Mg tholeiites, 1.8 Ga dolerites and 1.2–1.4 Ga dolerites. Geochemical compositions of these mafic dykes are compared and they show good relationships with each other. Similarities in petrological and geochemical characteristics of Precambrian mafic dykes of East Antarctica and Central-East India strongly support juxtaposition of these two continents.

Continental mafic dyke swarms as tectonic indicators: an example from the Vestfold Hills, East Antarctica

The two main types of magma that were emplaced as mafic dyke swarms in the Vestfold Hills are high-Mg tholeiite (HMT) and Fe-rich tholeiite (FRT). The former magma type is less dense than the Vestfold Hills felsic crust, whereas the latter is more dense at deep to intermediate crustal levels. Therefore, FRT dyke emplacement requires a higher deviatoric extensional stress, as is reflected in a more constant orientation. Palaeoproterozoic HMT-FRT bimodal dyke swarms were emplaced around 2.4 Ga and 2240 Ma. The geometry and orientation of 2240 Ma HMT dykes imply a low deviatoric extensional stress field, inconsistent with a mantle plume origin for magmatism. Current petrogenetic models for HMT magmatism suggest generation at active (convergent) plate boundaries. Although this is broadly consistent with emplacement of the bimodal dyke swarms subsequent to a crustal thickening event at 2.5 Ga, the exact mechanisms for melting and the generation of crustal extensional stress are unknown. A possible scenario involves the sinking of gravitationally unstable oceanic crust. A major Mesoproterozoic magmatic—extensional event in the Vestfold Hills started with normal faulting, followed by lamprophyric dyke emplacement and by two distinct episodes of FRT dyke emplacement at 1380 and 1245 Ma. A consistent direction of the stress field during this event suggests control of a linear belt of crustal uplift. By comparison with other mafic dyke swarms, this is interpreted to be due to a sheet of upwelling asthenosphere (elongated plume) around 1.4 Ga, rather than a low-angle detachment. This upwelling led to thermal erosion of the base of the lithosphere that channelled a subsequent magma pulse at 1245 Ma. Erosion during a long period of uplift prior to 1380 Ma was followed by sedimentation during thermal subsidence, as suggested by segmentation of 1245 Ma dykes. The lithospheric structure inherited from the Mesoproterozoic magmatic—extensional event was probably reactivated during two high-grade tectono-metamorphic events around 1.0 and 0.5 Ga. The Vestfold Hills formed part of a relatively stable craton, peripheral to the mobile belt that was formed during these events.

Secular trends in the melting depths of mantle plumes: Evidence from HFSE/REE systematics of Archean high-Mg lavas and modern oceanic basalts

Al-depleted komatiites and associated Fe-tholeiites from Boston Township, Abitibi belt (∼ 2.7 Ga), Canada, are characterized by the conjunction of low Al 2O 3/TiO 2 (4–5) ratios, low Sc and Y contents, and HREE depletion [(Gd/Yb) n = 1.7–2.3], with negative P, Zr and Hf anomalies relative to MREE on primitive-mantle-normalized diagrams (P/Nd = 23–36; Zr/Sm = 13–18; Hf/Sm = 0.3–0.5). Enriched high-Mg tholeiites (EHMT) from the Norseman area ( ∼ 2.7 Ga), Western Australia, are characterized by low Al 2O 3/TiO 2 (4.0–4.7), fractionated HREE [(Gd/Yb) n = 2.3–2.5], and negative P anomalies (P/Nd = 21–26), but near-chondritic Zr/Sm (23–27 vs. 25) and Hf/Sm (0.6–0.8 vs. 0.7) ratios. In both areas there are tholeiites with flat REE patterns derived from a distinct mantle source than the komatiites and EHMT. Different HFSE and REE systematics between the two late Archean high-Mg lavas are interpreted as partial melting of mantle plumes with different residual minerals at different depths; Boston Township Al-depleted komatiites may have been derived from partial melting of a plume with majorite garnet in the residue at pressures between 14 and 24 GPa, whereas Norseman EHMT may have been generated either by low-degree partial melting with a olivine + orthopyroxene + clinopyroxene + garnet residue at pressures < 6 GPa, or more advanced melting with pyrope garnet in the residue at pressures between 6 and 14 GPa. Comparison of HFSE/REE systematics between Archean high-Mg lavas and modern OIB indicates that most modern OIB are featured by low Al 2O 3/TiO 2 (4–7), P/Nd (50–60) and fractionated HREE [(Gd/Yb) n = 2–3], but near-chondritic Zr/Sm (24–25) and Hf/Sm (0.6–0.7) ratios, similar to Norseman EHMT, consistent with low-degree partial melting with garnet in the residue at low pressures. Partial melting of Archean plumes which generated Al-depleted komatiites may have occurred at greater depths than that of modern plumes. HFSE/REE systematics of plume-derived volcanic rocks records decreasing melting depths of mantle plume over time, consistent with the secular cooling of the Earth.

Petrogenesis of the Fiambala Gabbroic Intrusion, Northwestern Argentina, a Deep Crustal Syntectonic Pluton in a Continental Magmatic Arc

The Fiambaláa gabbroic intrusion is an Ordovician pluton, ∼30 km2 in outcrop area, situated in the Sierras Pampeanas of NW Argentina. It was emplaced at a depth of 21–24 km in an actively deforming continental magmatic are, and it represents some of the earliest are magmatism in the southern Cordillera. Its formation was accompanied by the emplacement of voluminous gabbroic sills and dikes, and was synchronous with regional amphibolite facies metamorphism that is locally upgraded to granulite facies along the edges of the intrusion. The intrusion has been tilted to the east and now stands almost vertically on edge, striking NNW-SSE. It has a 200–500-m basal cumulate zone of dunite and websterite along its western margin, overlain stratigraphically to the east by 2.5 km of poorly layered two-pyroxene gabbro and homblende gabbro. Although olivine is present in the basal ultramafic cumulates, nowhere does it coexist with plagioclase. The common differentiation indices, such as An content of plagioclase, 100 Mg/(Mg+Fe) in the pyroxenes and the whole rocks, and the concentrations of various incompatible trace elements, all vary systematically through the intrusion. The parental magma was evidently a hydrous, high-Mg tholeiite with >11 wt.% MgO, 100 Mg/(Mg+Fe)=73–74, and REE abundances approximately 10 × chondrites. Its estimated trace element characteristics were strikingly similar to those of primary island are magmas from occanic regions. Most of the gabbroic rocks represent or approach liquid compositions; they are hypersthene-normative, almost silica-saturated tholeiites. Evolution from the most mafic compositions comparable to primitive island are basalts to derivative compositions with the typical characteristics of continental are basalts occurred through the combined effects of fractional crystallization, multiple magma batch injection, and crustal assimilation. There appears to be no need to invoke a special subcontinental mantle source to produce the trace element signatures that are so characteristic of continental are magmas. These signatures can be developed by limited crystallization and assimilation in the lower crust.

Estimation of emplacement pressure for 2350 Ma high-Mg tholeiite dykes, Vestfold Hills, Antarctica

Emplacement pressures of the 2350 Ma old, high-Mg tholeiite dyke suite in the Vestfold Hills block have been previously estimated to be 7-8 kbar. New data suggest that previous estimates are too high. The depth of emplacement has been estimated using two different approaches: (1) Geobarometry based on aluminium exchange between clinopyroxene and plagioclase and (2) examination of high pressure liquidus mineralogy of the high-Mg tholeiites. Geobarometry applied to groundmass assemblages (clinopyroxene + plagioclase) indicates across-dyke variations. Samples from the chilled margins give higher pressure estimates (max. 7 kbar) than more slowly cooled samples from the dyke interior (1 to max. 5 kbar, corresponding to a depth of ∼ 3-16 km). The variations of the liquidus mineralogy (opx + ol) from chilled margins and from dyke interior samples are consistent with the results derived from the Al-exchange geobarometry. It is inferred that pressure estimates from the dyke interior (1 to max. 5 kbar) represent the ambient emplacement pressures, whereas pressure estimates derived from phenocryst assemblages (ol + opx) in chilled margins may represent crystallization pressures prior to dyke emplacement. The results suggest a much shallower emplacement depth (1-max. 5 kbar) of the 2350 Ma old tholeiites than previously thought (7-8 kbar)

U-Pb zircon dating of mafic dykes and its application to the Proterozoic geological history of the Vestfold Hills, East Antarctica

The Vestfold Hills, one of several Archaean cratonic blocks within the East Antarctic Shield, comprises a high-grade metamorphic basement complex intruded by at least nine generations of Early to Middle Proterozoic mafic dykes. Extensive U-Pb ion microprobe (SHRIMP) analyses of zircons, derived predominantly from late-stage felsic differentiates of the mafic dykes, provide precise crystallisation ages for several dyke generations. These new ages enable constraints to be placed on both the history of mafic magmatism in the Vestfold Hills and the timing of the various interspersed Proterozoic deformation events. In addition to demonstrating the utility of zircons derived from felsic late-stage differentiates for the dating of co-genetic mafic dykes, this study also places doubt on previous wholerock Rb-Sr dating of mafic dyke suites in this and other areas of East Antarctica. The 207Pb/206Pb zircon ages of 2241±4 Ma and 2238±7 Ma for the Homogeneous and Mottled Norites, respectively, provide a younger emplacement age for associated group 2 High-Mg tholeiite dykes than the whole-rock Rb-Sr date (2424±72 Ma) originally interpreted as the age of all high-Mg intrusives in the Vestfold Hills. Zircon ages of 1754±16 Ma and 1832±72 Ma confirm the previously defined Rb-Sr age of the group 2 Fe-rich tholeiites. Two later dyke generations, the group 3 and 4 Fe-rich tholeiites, are distinguished on the basis of field orientations and cross-cutting relationships, and yield zircon emplacement ages of 1380±7 Ma and 1241±5 Ma which also define minimum ages for two suites of lamprophyre dykes. Xenocrystic zircons within both felsic segregations and mafic dykes yield zircon ages of 2478±5 Ma to ∼2740 Ma, indicating the presence of Archaean crustal source rocks of this antiquity beneath the Vestfold Hills.

The geology and gravity field in the central core of the Vredefort structure

The centre of the Vredefort structure in South Africa comprises an uplifted granitic basement with an associated 300 g.u. gravity high ringed by upturned sedimentary strata. Detailed geological mapping of the basement core has delineated a three-fold concentric zoning of Outer Granite Gneisses (OGG), Steynskraal Formation (SF) and Inlandsee Leucogranofels (ILG). An increase in metamorphic grade occurs toward the centre of the structure from greenschist relics on the basement perimeter to upper amphibolite and granulite facies in the SF and granulite facies in the ILG. A marked lithological change occurs within the SF in which perimeter greenschists and inner amphibolites give way to paragneisses. In the ILG, paragneiss and mafic granulite relics predominate. These divisions are consistent with the “standing-on-edge” model for the basement 1. Three regional metamorphic episodes and two thermal events have been recognised. M 1 and M 3 both attained granulite facies at low-medium load pressures in the ILG. M 1 granulite-facies assemblages extended into the OGG, but the M 3 facies was restricted to the ILG. M 2 was a K-metasomatism event limited to the OGG. An extensive thermal episode, M 4 a , is provisionally correlated with the collar metamorphism. Pyroxene hornfels facies ( T = ± 800 ° C) was attained in the centre as opposed to conditions of P = 4–5 kbar and T = 650° C for alkali granite stocks in the sedimentary collar 2. This event was thus pre- or syn-uplift, but not post-uplift. A high-level, high-temperature ( P < 3 kbar and T = 800°–900° C) event, M 4 b , occurred at a late stage in the overall sequence. A suite of amphibole-orthopyroxene rocks of basaltic komatiite type (CaO/Al 2O 3 ⩾ 1) are chemically related to younger high-Mg tholeiites. These tholeiites are correlated with similar rocks in the sedimentary collar and were intruded syn- to post- M 4 a in several episodes of decreasing basicity. “Shock” metamorphic features, granitic breccias and pseudotachylite are linked to the late-stage history of the structure and a case is presented for a dynamic metamorphic “front” advancing outward and upward from the centre in response to unroofing of the basement. The central gravity high 3 is delineated by 1239 new gravity stations at 1 km and 0.5 km intervals. A low-frequency oval-shaped anomaly with a long axis trending NW-SE has eccentrically positioned on it a plateau extending southwards as a spur. A dyke is responsible for the spur, but the source for the plateau is, from a recent borehole, thought to be largely due to sheared altered harzburgite. Modelling of the low-frequency gravity field component was based on the mapped divisions of the OGG, SF and ILG, and a density contrast of 200 kg/m 3 was determined between the OGG and the SF and ILG lithologies. A purely crustal solution for the excess mass distribution in the basement is conclusively demonstrated.

Geology of the East Bull Lake gabbro–anorthosite pluton, Algoma District, Ontario

The East Bull Lake Pluton, a layered gabbro–anorthosite intrusion 90 km west of Sudbury, Ontario, is in an inward-dipping lopolith and is 780 m thick in the centre and elliptical in plan view (13.5 km × 3.5 km). It intruded Archean metavolcanic and metaplutonic rocks of the Superior Province during the Early Proterozoic (2480 Ma).The intrusion consistes of a basal anorthositic unit, succeeded by rhythmic-layered gabbro, troctolite, layered gabbro, and massive and dendritic gabbro units. It is offset by the west-northwest-striking, subvertical Folson Lake fault. South of this fault, only anorthosite and massive and dendritic gabbro are exposed. North of the fault, subsurface lithologies intersected by me boreholes correlate with surface lithologies. Troctolite, the most distinctive marker that can be correlated between boreholes and surface exposures, confirms the general attitude and shape of the layers and lopolith.Chemical composition of the intrusion ranges from high-Mg tholeiite to calc-alkaline high-alumina basalts.Fractures occur in several preferred orientations, the most common being northwesterly, subparallel to the Folson Lake fault, numerous mafic dykes, and topographic lineaments. Complex fracture-filling and alteration mineralogies formed under a wide range of P–T conditions representative of epidote-amphibolite – greenschist facies, pumpellyite–prehnite facies, zeolite facies, and low-temperature rock–water interaction processes.The last movement on the Folson Lake fault was a dextral strike slip of up to 3.0 km that postdates most mafic dykes.

Erratum: The geochemistry of komatiites and basalts from the Deadman Hill area, Munro Township, Ontario, Canada

A sequence of Archean komatiites (> 18 wt.% MgO), komatiitic basalts (10–18 wt.% MgO), high-Mg tholeiites (6–10 wt.% MgO), and high-Fe tholeiites (< 8 wt.% MgO) is exposed in the Deadman Hill area of Munro Township, Ontario, Canada. Major- and trace-element analyses of 28 samples are used to assess their petrogenetic significance. The use of molecular proportion ratio plots shows the samples have maintained their primary SiO2, FeO*, MgO, TiO2, Al2O3, Ni, Cr, Zr, Y, and V contents. Secondary redistribution of Na2O, K2O, Rb, Sr, Ba, and, in some samples, CaO has occurred.Covariation in both major- and trace-element data suggests the komatiites are primary melts that equilibrated with a harzburgite residua at pressures of 3–6 GPa. Garnet did not have a major role in their petrogenesis or in the petrogenesis of spatially related komatiitic basalts and high-Mg tholeiites. Major- and trace-element variation in komatiitic basalts with 17–12 wt.% MgO requires that they be partial melts in equilibrium with clinopy...

The geochemistry of komatiites and basalts from the Deadman Hill area, Munro Township, Ontario, Canada

A sequence of Archean komatiites (&gt; 18 wt.% MgO), komatiitic basalts (10–18 wt.% MgO), high-Mg tholeiites (6–10 wt.% MgO), and high-Fe tholeiites (&lt; 8 wt.% MgO) is exposed in the Deadman Hill area of Munro Township, Ontario, Canada. Major- and trace-element analyses of 28 samples are used to assess their petrogenetic significance. The use of molecular proportion ratio plots shows the samples have maintained their primary SiO2, FeO*, MgO, TiO2, Al2O3, Ni, Cr, Zr, Y, and V contents. Secondary redistribution of Na2O, K2O, Rb, Sr, Ba, and, in some samples, CaO has occurred.Covariation in both major- and trace-element data suggests the komatiites are primary melts that equilibrated with a harzburgite residua at pressures of 3–6 GPa. Garnet did not have a major role in their petrogenesis or in the petrogenesis of spatially related komatiitic basalts and high-Mg tholeiites. Major- and trace-element variation in komatiitic basalts with 17–12 wt.% MgO requires that they be partial melts in equilibrium with clinopyroxene at pressures of &lt; 3 GPa. They are unrelated to the komatiites. Both lower degree partial melting of the same source as lavas with 17–12 wt.% MgO and crystal fractionation of clinopyroxene from liquids with ~12 wt.% MgO can model the evolution of less magnesian komatiitic basalts and high-Mg tholeiites. The Zr/Y and Zr/Ti ratios of the high-Fe tholeiites indicate that they are unrelated to the komatiites, komatiitic basalts, and high-Mg tholeiites and were derived by partial melting of a garnet lherzolite source.

Palaeogene komatiites from Gorgona Island

UNMETAMORPHOSED mafic and ultramafic rocks overlain by Neogene oceanic sediments are exposed on Gorgona Island which is a remnant of the Coastal Cordillera along the Pacific margin of Colombia1. Gorgona is one of the rare places in the world where young pyroxenitic komatiites exhibiting typical quenched spinifex textures (Fig. 1) occur. The spinifex rocks form lenticular bodies (? lava flows) within cumulate ultramafics and troctolitic gabbros and are closely associated with diabases and tholeiitic pillow basalts1. We show here that in textures, rock and mineral chemistry, the Gorgona spinifex rocks are almost identical with the Archean peridotitic to pyroxenitic komatiites from Munro Township (North-east Ontario)2. For this reason the term komatiites is used for the Palaeogene rocks, although one could call them high Mg-tholeiites.

Graphitic andesite tuffs resulting from high-Mg tholeiite and sediment interaction; Nfigssuaq, West Greenland

Tuffs from the Agatdal Formation of Nilgssuaq show a dominance of olivine-graphite-andesites and orthopyroxe'ne-graphite-andesites while rock types similar to the overlying tholeiite basalts of the Vaigat Formation are subordinate. The tuffs were formed when highly magnesian tholeiitic picrite melts were emplaced into high-level magma chambers within carbon-rich sedimentary strata. Explosive reactions were triggered after extensive reactions with- the sediments. Glass grains are well preserved and some tuffs still contain quenched high-temperature sulphide blebs with metallic iron. Due to the high magnesium content the graphite-andesites are very unusual magma types and can be compared to some native iron bearing intermediate lavas from the Vaigat Formation of northern Disko thought to have erupted largely contemporaneously after having evolved through similar processes.