Siliceous High Magnesian Basalts Research Articles

Geochemistry of 2.45 Ga mafic dykes in northern Finland: Constraints on the petrogenesis and PGE prospectivity of coeval layered intrusions

The Karelian craton contains abundant ~2.45 Ga mafic dykes that are compositionally, temporally and, in some cases, spatially related to PGE-mineralised mafic-ultramafic layered intrusions. The dykes can be sub-divided into four groups, namely siliceous high-magnesian basalts (SHMB), gabbronorites (GBNO), low-Ti tholeiites and Fe-rich tholeiites. In this study, we group the SHMB and GBNO dykes to one group as SHMB group, and the two tholeiitic group dykes as tholeiite group, based on their similar geochemical and mineralogical features. In the SHMB group dykes, plagioclase has initial 87Sr/86Sr ratios of 0.7028 to 0.7036 and initial bulk-rock εNd values vary from −2.5 to −1.0, indicating moderate degrees of contamination with Archaean basement. Tholeiitic dykes show a less-radiogenic Sr isotope composition with an average initial 87Sr/86Sr ratio of 0.7024 and higher initial εNd values ranging from +0.3 to +1.7. Thermodynamic and geochemical modelling suggests that the SHMB group dykes could have formed by crustal contamination of a komatiitic magma at deeper crust followed by fractional crystallisation at shallower depth, whereas the tholeiitic group mainly experienced fractional crystallisation with less crustal contamination. Alternatively, the SHMB dyke s may have been derived from a SCLM mantle whereas tholeiitic dykes from a plume mantle, or the two types of dykes derived from different part of a mantle plume with different melting degrees, though these latter two models are not favoured in this study. Based on trace element and isotope characteristics, the SHMB dykes are suitable candidates for the parental magmas to some of the Finnish PGE-mineralised intrusions (e.g., Penikat and Portimo), whereas the tholeiitic dykes may represent the parental magma of the Tsipringa layered intrusion in Russia. Both the SHMB and tholeiitic dyke types are fertile with regard to PGE, with up to 10–20 ppb Pt and Pd and mantle-like Cu/Pd ratios in their least evolved members, suggesting that the magmas remained sulphide undersaturated during mantle melting and en route to the upper crust. This interpretation is consistent with the fact that most of the ~2.45 Ga Fennoscandian layered intrusions contain PGE mineralisation. Sulphide melt saturation in the dykes and layered intrusions was mostly attained after their final emplacement, likely due to crystal fractionation.

The Role of Siliceous High-Magnesium Basalts during the Formation of a Neoproterozoic Mafic-Ultramafic Intrusion in the Tarim Craton (China)

The Xingdi No.2 intrusion in the Kuluktag Block of northeastern Tarim Craton, which intrudes into the Palaeoproterozoic basement with an exposed area of ca. 12 km2, is an orthopyroxene-rich mafic-ultramafic intrusion in a continental rift setting. It consists of gabbros, pyroxenites, and peridotites, and exhibits a crystallization sequence of the principal rock-forming minerals from olivine, orthopyroxene, clinopyroxene, to plagioclase. The gabbros show a concordant SHRIMP U-Pb zircon age of 752 ± 5.4 Ma. In addition, the olivine grains have forsterite content values of 78–85 mole% and mostly contain low NiO, MnO, CaO, and Cr2O3. The rocks are relatively enriched in large ion lithophile elements and LREE, and depleted in HSFE, have non-radiogenic Pb, low εNd (t) values (−2.8 to −23), initial 87Sr/86Sr ratios (0.7059–0.7130). It could be concluded that the rocks represent an analogue of siliceous high magnesium basaltic magma originated by the partial melting of a hydrous and enriched subcontinental lithospheric mantle and contaminated by the continental crust.

Identification, classification, and interpretation of boninites from Anthropocene to Eoarchean using Si-Mg-Ti systematics

AbstractBoninites are rare, high-Si, high-Mg, low-Ti lavas that have considerable tectonic significance, especially for recognizing and interpreting episodes of subduction initiation in the geologic record. Formal identification and classification of boninites may be carried out using MgO-SiO2 and MgO-TiO2 diagrams to find compositions that satisfy modified International Union of Geological Sciences (IUGS) criteria of Si8 > 52 and Ti8 < 0.5, where Si8 and Ti8 refer to concentrations of the oxides at 8 wt% MgO. However, screening of highly metasomatized rocks and accurate classification require precautions, including normalization to a 100% volatile-free basis. The MgO-SiO2 diagram can also be used for subdivision into low-Si boninites (Si8 < 57) and high-Si boninites (Si8 > 57). Satisfying one but not both of the boninite criteria are rocks with Si8 > 52 but Ti8 ≥ 0.5 (siliceous high-magnesium basalts) and rocks with Si8 ≤ 52 but Ti8 < 0.5 (low-Ti basalts). We tested the classification methodologies using ∼100 low-Ti lava suites dating from the present-day back to the Eoarchean. We conclude that, of those classifying as “boninite series,” Izu-Bonin-Mariana arc–type subduction initiation terranes provide the dominant setting only back as far as ca. 2 Ga, which marks the maximum age of extensive clinopyroxene-undersaturated melting and eruption of high-Si boninites. From 2 to 3 Ga, most boninites formed in intraplate settings by melting of refertilized, depleted cratonic roots. Prior to 3 Ga, hot, depleted mantle plumes provided the main boninite sources. Nonetheless, arc-basin boninites, though rare, do extend back to 3.8 Ga, and, together with the inherited subduction component in intracratonic boninites, they provide evidence for some form of subduction during the Archean.

Composition of the Marginal Rocks and Sills of the Rustenburg Layered Suite, Bushveld Complex, South Africa: Implications for the Formation of the Platinum-Group Element Deposits

The Bushveld Complex contains large ore deposits of platinum-group elements (PGE), V, and Cr. Understanding how these deposits formed is in part dependent on estimates of the compositions of the magmas that filled the Bushveld chamber. Over the past 20 years, estimates for the major oxides and some trace elements in the magmas have been made using the marginal rocks of the intrusion. However, data for most of the trace elements have not been available. This paper presents the results for a full range of trace elements, including the platinum-group elements. The marginal rocks of the Lower and lower Critical zones (B-1 magmas) are tholeiitic Mg-rich basaltic andesites with Mg# 71. It had been suggested that they are boninites but their mantle-normalized incompatible lithophile trace element patterns (spidergrams) resemble those of the upper continental crust and the concentrations of the elements are much higher than those of boninites. The patterns resemble siliceous high magnesium basalts. An unusual feature is that the Pt/Pd ratios are >1.5. The Pt contents of the B-1 rocks (15–25 ppb) are slightly higher than those observed in most primary mantle melts, suggesting that the high Pt/Pd ratio is due to Pt enrichment rather than Pd depletion. The crystallization order and composition of the minerals formed in equilibrium with the B-1 magma matches that of the Lower and lower Critical zones and thus this magma appears to be representative of the parental magma of these zones. The marginal rocks to the upper Critical zone (B-2) are tholeiitic basalts in terms of major element composition, with Mg# 55. The spidergrams show some similarities with E-MORB; however, the B-2 rocks have strong positive Ba and Pb anomalies and negative P, Ti, Hf, and Zr anomalies, and thus they more closely resemble lower continental crust. The B-2 rocks have lower and more variable Pt + Pd contents than the B-1 magma, suggesting that some of the samples have experienced sulfide saturation, but in common with the B-1 magmas, the Pt/Pd ratios are high, in excess of 1.5. The crystallization order of the Upper Critical zone cannot be modelled by the B-2 magma alone. However, mixtures of B-2 magma and B-1 magma satisfy the crystallization order and mineral composition of the upper Critical zone. The marginal rocks of the Main zone (B-3) are also tholeiitic basalts in terms of major element composition but have a higher Mg# (62) than the B-2 rocks. Trace element patterns in part resemble those of B-2 magmas but are depleted in most incompatible elements with large positive Ba, Pb, and Eu anomalies and negative Nb, Ta, Hf, and Zr anomalies, suggesting the rocks contain a plagioclase component. The PGE contents of the B-3 rocks are lower than those of the B-1 magma and less variable than those of the B-2 magma, but in common with both the other magmas, they have high Pt/Pd. The crystallization order and composition of the minerals in equilibrium with the B-3 magma matches that of the Main zone. Two processes have been suggested to explain the compositions of the Bushveld magmas: mixing of primitive mantle melts with partial melts of continental crust and mixing of primitive mantle melts with melts derived from the subcontinental lithospheric mantle (SCLM). The trace element concentrations of the magmas can be modelled by crustal contamination. This interpretation is supported by oxygen isotopes, initial 87Sr/86Sr ratios and ɛNd of the cumulate rocks. However, the high Pt/Pd ratios of all of the magmas and the overall higher than normal Pt concentrations of the B-1 magma are difficult to explain by mixing of primary mantle melt with crustal components. The SCLM has high Pt/Pd ratios and mixing of primary mantle magma with SCLM-derived magma could account for the high Pt concentrations and high Pt/Pd ratios. This interpretation is supported by recent work on Os isotopes of the Kaapvaal SCLM. It should be kept in mind that the two processes are not necessarily exclusive. A magma with a SCLM component could have been emplaced into the crust and subsequently have been contaminated by partial melts of the crust.

Precambrian mafic magmatism in the Bastar craton, central India

The Bastar craton has experienced many episodes of mafic magmatism during the Precambrian. This is evidenced from a variety of Precambrian mafic rocks exposed in all parts of the Bastar craton in the form of volcanics and dykes. They include (i) three distinct mafic dyke swarms and a variety of mafic volcanic rocks of Precambrian age in the southern Bastar region; two sets of mafic dyke swarms are sub-alkaline tholeiitic in nature, whereas the third dyke swarm is high-Si, low-Ti and high-Mg in nature and documented as boninite-norite mafic rocks, (ii) mafic dykes of varying composition exposed in Bhanupratappur-Keskal area having dominantly high-Mg and high-Fe quartz tholeiitic compositions and rarely olivine and nepheline normative nature, (iii) four suites of Paleoproterozoic mafic dykes are recognized in and around the Chattisgarh basin comprising metadolerite, metagabbro, and metapyroxenite, Neoarchaean amphibolite dykes, Neoproterozoic younger fine-grained dolerite dykes, and Early Precambrian boninite dykes, and (iv) Dongargarh mafic volcanics, which are classified into three groups, viz. early Pitepani mafic volcanic rocks, later Sitagota and Mangikhuta mafic volcanics, and Pitepani siliceous high-magnesium basalts (SHMB). Available petrological and geochemical data on these distinct mafic rocks of the Bastar craton are summarized in this paper. Recently high precision U-Pb dates of 1891.1±0.9 Ma and 1883.0±1.4 Ma for two SE-trending mafic dykes from the BD2 (subalkaline) dyke swarm, from the southern Bastar craton have been reported. But more precise radiometric age determinations for a number of litho-units are required to establish discrete mafic magmatic episodes experienced by the craton. It is also important to note that very close geochemical similarity exist between boninite-norite suite exposed in the Bastar craton and many parts of the world. Spatial and temporal correlation suggests that such magmatism occurred globally during the Neoarchaean-Paleoproterozoic boundary. Many Archaean terrains were united as a supercontinent as Expanded Ur and Arctica at that time, and its rifting gave rise to numerous mafic dyke swarms, including boninitenorite, world-wide.

Geochemistry and petrogenesis of Neoarchaean high-Mg low-Ti mafic igneous rocks in an intracratonic setting, Central India craton: Evidence for boninite magmatism

The southern Central India craton (also known as Bastar craton) has experienced several episodes of mafic magmatism during the early Precambrian time. These mafic igneous rocks are emplaced as dyke and volcanic rocks. A set of mafic dyke swarm and few exposures of mafic volcanics, both emplaced in the Neoarchaean time, contain high-SiO2 (>52 wt%), high-MgO (>8 wt%), and low-TiO2 (mostly <0.5 wt%) and classified as boninites on IUGS classification scheme. Al2O3/CaO ratio classifies these high-Mg mafic igneous rocks into high-Ca and low-Ca (Type 3) boninites. High-Ca boninites show typical boninitic geochemical characteristics, whereas low-Ca variety shows geochemical characteristics similar to the high-Mg norites. Siliceous high-magnesium basalts (SHMB) show similar geochemical properties as observed for the high-Mg norites. Early Precambrian high-Mg norites are reported from the rift setting environment but, on the other hand, boninitic magmatism represents an enigmatic style of magmatism and commonly formed at convergent margin settings. Most boninites are Phanerozoic in age and rarely reported from the Archaean terrains. The established geological and tectonic setting of the Archaean Bastar craton clearly suggests existence of a stable continental rift environment in the region since the Archaean time. Thus, report of Neoarchaean boninite-like rocks in an intracratonic setting is an important feature noticed in the Archaean terrain of Indian peninsula. A highly refractory mantle source region is required to form boninitic magma. Refractory lithosphere may be result of extensive extraction of mafic magma during Mesoarchaean. The preservation of refractory reservoir beneath the Bastar craton for long period of time corroborates a high degree of crustal stability during the Meso-Neoarchaean time. Trace element modelling suggests that high-Mg mafic igneous rocks under study are product of different pulses of boninitic magma produced by ∼20-25% melting of a refractory mantle source.

Archean Layered Mafic-Ultramafic Intrusions in the West Pilbara Craton, Western Australia: A Synthesis of Some of the Oldest Orthomagmatic Mineralizing Systems in the World

Considerable exploration interest has been generated by the platinum-group element (PGE) and Ni-Cu potential of the Archean layered mafic-ultramafic intrusions in the West Pilbara Craton, Western Australia. The Munni Munni intrusion contains the largest resource of PGE associated with a layered intrusion in Australia, and the Radio Hill and Mount Sholl intrusions host significant resources of Ni-Cu-Co sulfides. Titaniferous magnetite layers, remobilized sulfides, and structurally controlled hydrothermal polymetallic deposits have also been a focus for exploration in recent years. The ca. 2.9 Ga Munni Munni, Andover, Radio Hill, Mount Sholl, and Sherlock layered intrusions are a cogenetic suite of high-level (<5 kbars) bodies that represent some of the oldest mineralizing systems of their type in the world. Although they display similar field relationships and mineralogical, geochemical, and isotopic features, their contrasting chalcophile metal distribution patterns show that the timing and mechanism of the S saturation event were critical for the development of PGE-enriched sulfide-bearing layers and basal segregations of base metal sulfides. The intrusions form thick (>5.5 km) dikelike bodies or relatively thin (0.5–2 km) sheets and sills emplaced at different levels along major lithological discontinuities in the upper crust. Rhythmically layered ultramafic components are generally thinner than, and occur along the northern sides of, more massive overlying mafic components. The ultramafic zones consist of dunite, lherzolite, wehrlite, olivine websterite, clinopyroxenite, and websterite. Inverted pigeonite gabbronorite, magnetite gabbro, olivine gabbro, anorthositic gabbro, and anorthosite comprise the mafic sequences. Olivine and clinopyroxene were generally the first minerals to crystallize, except in the Andover intrusion, where orthopyroxene preceded clinopyroxene and possibly reflects greater contamination of the parent magma by felsic crustal material. The crystallization of chromite was inhibited in the ultramafic zones by the partitioning of Cr into early crystallizing clinopyroxene, thus downgrading the potential for PGE-chromite associations. The PGE mineralization in the Munni Munni intrusion occurs in the upper levels of a porphyritic plagioclase websterite orthocumulate layer directly below the ultramafic-gabbroic zone contact. Mineral compositional trends and Nd isotope data indicate that a Pd-Pt-Au-enriched S-undersaturated magnesian basaltic magma was frequently injected into a small magma chamber during formation of the ultramafic zone. A major influx of more fractionated, S-saturated tholeiitic gabbroic magma related to the resident magnesian magma, rapidly inflated the chamber, and induced turbulent magma mixing that resulted in the formation of the PGE-bearing porphyritic websterite layer. In contrast, the parent magmas that formed the Mount Sholl, Radio Hill, Andover, and Maitland intrusions were saturated in S before they were emplaced into the magma chambers. In these intrusions gravitational and structural controls were important for the concentration of PGE-poor (5–400 ppb Pt + Pd + Au) massive Ni-Cu-Co sulfides in depressions and structural embayments along the basal contacts beneath the thickest sequences of mafic-ultramafic cumulates. The parent magmas to the West Pilbara intrusions were siliceous high magnesian basalts of Al-depleted komatiitic affinity (Barberton-type) with 9 to 12 percent MgO, 15 to 25 ppm Sc, 12 to 18 ppm Y, low Al2O3/TiO2 (ca. 11 or half chondrite ratios), and light rare earth enrichment (chondrite-normalized La/Sm = 2.7, La/Lu = 9.0). They were generated with garnet in the residual asthenospheric mantle with probable involvement of a pre-3.0 Ga subduction-modified lithospheric mantle. Isotopic and geochemical modeling suggests that the magmas were contaminated by ca. 3.0 to 3.3 Ga Archean tonalitic to granodioritic crust before being emplaced into high-level magma chambers.

Crust–mantle interaction in the genesis of siliceous high magnesian basalts: evidence from the Early Proterozoic Dongargarh Supergroup, India

Siliceous high magnesian basalts (SHMB) represent a rock type chemically distinct from other common volcanic rocks. The petrogenesis of this magma, reported at or near the Archean–Proteorzoic transition across the world, is controversial. In this paper, we present chemical (XRF, INAA) and mineralogical data on a SHMB suite from the Early Proterozoic (2.1–2.5 Ga) Dongargarh Supergroup, Central India, the first of its kind reported from the Indian Precambrian. This suite of basaltic rocks is unusually high in SiO 2 (54 wt.%) and enriched in incompatible elements. The SHMB melts discussed here can neither have formed by partial melting of the Earth's mantle nor by fractional crystallisation of a mantle-derived melt. It is shown here that excess SiO 2 and incompatible elements in SHMB are supplied by a crustal component to a basaltic komatiitic parent magma of mantle origin. Major and trace elements abundances and geochemical mass balance calculations suggest that a basaltic komatiite melt assimilated 15–20% of acid volcanics, the immediately underlying unit to these rocks in Dongargarh, before erupting as SHMB. Mantle-derived rocks have Ta/Th ratios of around 0.5 whereas crustal rocks have ratios of about 0.2. The Ta/Th ratios of Dongargarh SHMB and acid volcanics are nearly identical but both unusually low (<0.1) when compared to the normal upper continental crust (Ta/Th ∼0.2), supporting the view that the acid volcanics are the source for incompatible elements and SiO 2 in SHMB. It is also shown that there is an overall compositional similarity of such temporarily unique but spatially unrelated SHMB magmas occurring in different continents including the Dongargarh SHMB.

Possible komatiitic affinity and potencial PGE mineralization of the archean Ribeirão dos Motas layered sequence, Brazil

The southern portion of the Sao Francisco Craton (SSFC), located in Minas Gerais State, underwent a long crustal development that occurred from Mesoarchean to Neoproterozoic times (Carneiro et al., 1998a; 1998b; Teixeira et al., 1996). The Archean rocks of the SSFC, which formed between 3.5 – 2.5 Ga are essentially gneisses and granitoids (TTG suite), a greenstone belt sequence (Rio das Velhas Greenstone Belt - RVGB), an intrusive ultramafic-mafic layered complex (Ribeirao dos Motas Layered Sequence - SARM), and mafic dikes swarms. Locally, the Archean rocks are covered by Proterozoic intracratonic sedimentary basin sequences containing quartzite-carbonate-shale with minor contribution of the bimodal volcanics-quartzite-arkose and volcanics-graywacke and intruded by swarms of mafic dikes and/or granitoid bodies. The SARM comprises alternate layers of main peridotitic and pyroxenitic rocks with minor gabronorite and amphibolite rocks, outcropping as several disrupted bodies (Carneiro et al., 1997). Olivine, spinel, amphibole and pyroxene (orto- and clino-) constitute the peridotite mineralogy. The pyroxenite is constituted by pyroxenes (orto- and clino-), amphibole and spinel. Besides the typical petrographic features and texture commonly found in layered complexes such as: igneous layering, euhedral crystals and cumulus textures, the studied rocks present some sui generis petrographic aspects such as chadacristal of euhedral amphibole included in euhedral oikocristal of pyroxene. Major and minor elements have been performed on 15 whole rock samples from different SARM outcrops. The SARM ultramafic rocks have MgO contents ranging from 23.8 to 34.9 wt% (MgO = 30.08 average wt%); TiO2 = 0.182 average wt%; CaO/Al2O3 ratio of 0.72 (average); Al2O3/TiO2 ratio of 33 (average). Analyses of REE and PGE have been performed on 6 samples. All samples are low in total REE abundance; Pt and Pd average are 28 and 10.6 ppb respectively. These chemical data suggest that SARM magmas were derived from komatiite magmas similar to siliceous high magnesian basalts (SHMB). Furthermore, the higher Pt and Pd (average ppb) suggest that magmas feeding the SARM could be carrying conspicuous amounts of PGE. Regionally, the geochemistry studies reveal great similarity between the SARM and the komatiite rocks of the RVGB (Padilha et al., 1984; Sichel et al., 1983; Schorscher et al., 1992). However, the SARM ultramafic rocks have higher average contents of MgO, Cr, Ni and lower average contents of SiO2, TiO2, K2O and REE. These geochemical features suggest that the RVGB komatiite may have been generated from more evolved SARM magmas. However, it is also possible that the mantle source region for both ultramafic magmas had been depleted during a melting episode that caused extraction of the RVGB komatiite magma. Thus, in spite of the lack of unequivocal geological relationships between SARM and the komatiite rocks of the RVGB, the two sequences referred to above may have had the same magmatic source and, as a consequence, been tectonically related. Assuming that this is true, the SARM rocks are a fraction of the komatiite magma that was encapsulated in the lower crust forming a layered ultramafic-mafic complex. Another portion of the komatiite magma would have reached the surface forming ultramafic volcanic supracrustal sequences of the SGRV. Finally, the SARM geochemical data when compared with those of the Archean Australian layered complexes (Sun et al., 1991; Hoatson et al., 1992) exhibit higher average Pt, Pd, Al2O3/TiO2 ratio, Cr and Ni, while Ti/Sc ratio, Ti, SiO2 and K2O average (wt%) are lower. The higher Pt and Pd values in comparison with those values of the Archean Australian layered complexes, which were presented by Sun et al. (1991) and Hoatson et al. (1992), are suggestive of a potential PGE mineralization in the SARM.

Boninite-like rocks from the Palaeoproterozoic greenstone belt of Bogoin, Central African Republic: geochemistry and petrogenesis

Abstract The upper pillowed amphibolites of Bogoin (UPAB) are part of the supracrustal series of the Bogoin-Boali greenstone belt (western Central African Republic). They are underlain by Al-depleted komatiites, back-arc tholeiites and arc-related greywackes; they are interbedded with undepleted komatiites and overlain by banded iron formations (BIFs). PbPb and SmNd isotopic data yield an imprecise isochron age of about 2.3 Ga, which is believed to represent the time of greenstone deposition. No magmatic minerals are preserved in the UPAB, but pseudomorphs after olivine, Ca-poor and Ca-rich pyroxenes are present. The secondary paragenesis is related to upper greenschist facies metamorphism. Immobile element geochemistry suggests that UPAB are not siliceous high-magnesium basalts but are very similar to modern boninites. They are depleted in P and Ti, enriched in Sr and Zr and show high Zr/Sm, Sr/Sm, Al2O3/TiO2 and low Ti/V and Ti/Sc values. The REE patterns are U-shaped or LREE-depleted/HREE-enriched. The overprint of a magma mixing with the interbedded komatiites on a partial melting event can explain most of the differences noted between UPAB and modern boninites. UPAB genesis is explained by mixing depleted and enriched components. Sm/Nd isotopic data and major element contents suggest participation of a trondhjemite-like melt rather than sediments as the enriched component. The depleted component can be produced by remelting of a peridotitic residue following extraction of tholeiites similar to the tholeiites of the lower unit. The ascending diapir that yielded the upper komatiites may have furnished the necessary heat input to the remelting event. The presence of back-arc tholeiites, arc-related greywackes and boninite-like UPAB argues strongly for a compressive margin plate boundary setting for the Bogoin greenstone belt.

Relationships between calc-alkaline acidic and basic (mantle-derived) magmas in Late Archaean composite dykes, Kambalda Goldfield, Western Australia

Composite dykes from the North Dome area, Kambalda, comprise basic spessartite (hornblende-plagioclase lamprophyre, ≈56% SiO 2 ), with mg numbers, Cr and Ni contents typical of primitive mantle-derived rocks, and more acidic plagioclase-phyric quartz-andesite (≈ 66% SiO 2 ), together with felsic net-veins and hornblendic patches suggesting local differentiation. The spessartites can be regarded chemically as volatile-, LILE-and incompatible element-enriched versions of siliceous high-magnesian basalts from the local greenstone sequence. A Pb-Pb isochron on one dyke yields an age of 2636±27 Ma, making this the oldest composite dyke so far dated. Field and geochemical evidence, including identical Pb initial-ratios for the constituent rock-types, virtually indistinguishable REE spectra, and tight linear trends on plots such as Y versus Ti and Ce/Cr versus Ti/Cr, suggests that the two magmas (lamprophyric and acidic) were genetically related and were emplaced contemporaneously into the same fracture. Quantitative least-squares modelling of whole-rock and mineral compositions suggests that the acidic components resulted mainly from fractionation of hornblende from parental spessartite magmas, together with a possible contribution from mixing of the same spessartite magma with crustally-derived acidic (e.g., trondjhemitic) magma; the latter is represented by separate dykes and stocks in the Kambalda area. Close similarities between the Kambalda composite dykes and Archaean-Tertiary composite dykes in other parts of the world, imply a persistent role for mantle-derived magmas such as lamprophyres in the genesis of the dominant intermediate-acidic (e.g., granitoid) magmas in many calc-alkaline provinces.

Use of geochemistry as a guide to platinum group element potential of mafic-ultramafic rocks: examples from the west Pilbara Block and Halls Creek Mobile Zone, Western Australia

Integrated chemical and isotope studies of late Archaean mafic-ultramafic layered complexes and associated extrusive volcanic rocks in the west Pilbara Block, Western Australia, suggest that they were derived from komatiite magmas through processes of crustal assimilation followed by fractional crystallization. On the basis of Nd isotope data the contaminants are believed to be felsic basement rocks of mainly early Archaean age. Parent magmas of these layered complexes are siliceous high magnesian basalts (SHMB) and can be regionally correlated with two types of komatiites. The Cooya Pooya Dolerite and the Negri Volcanics could be related to group 1 komatiite, characterized by near chondritic Al 2O 3/TiO 2 (∼21), Ti/Sc (∼74) and flat heavy rare earth element (REE) patterns, whereas the Munni Munni, Mt. Sholl, Dingo, Maitland and Andover complexes could be related to the group 2 komatiites characterized by depletion in Al (Al 2O 3/TiO 2∼10), heavy REE and Sc. Komatiites are products of very large degrees (e.g. 30–50%) partial melting of the convective mantle. They commonly have Pd∼8−10 ppb, Au ∼ ppb, Pt 10−14 ppb ppb and are strongly undersaturated in sulfur (∼ 500 ppb?). Archaean SHMB commonly have higher Pd(∼ 15 ppb), but are still S-undersaturated (< 1000 ppm) unless contaminated by S-rich crustal rocks. Komatiites and SHMB commonly have Ti/Pd (∼2−3 × 10 5), and Pd/Cu ( ∼−3 × −4 ) similar to estimates of the primitive mantle. The west Pilbara SHMB have PGE contents (Pd ∼10−15 ppb) similar to the estimated parent magma of the Bushveld complex, and other late Archaean SHMB. Some late Archaean major layered complexes and early Proterozoic dykes in the Yilgarn Block, Western Australia arc inferred to have similar values. These magmas are believed to have potential for PGE mineralization through sulfide saturation processes in magma chambers and feeder dykes. Early Proterozoic mafic-ultramafic intrusives in the Halls Creek Mobile Zone, have parent magmas with a chemical affinity to tholeiites generated in continental rift environments. Among them, parent magmas of the Woodward Dolerite, Lamboo Complex, and Panton Sill have high inferred abundances of Pd (up to 15 ppb), Pt and Au. Abundances of some incompatible elements (Ti, Zr, Y and P) in these rocks are lower than in mid-ocean ridge basalts suggesting magma generation by large degrees (>/25%) of mantle melting. Their primitive parent magmas, when first scaparated from the mantle, are inferred to have been S-undersaturated and to have had Pd, Pt and Au contents similar to the Archaean SHMB. With the exception of the Woodward Dolerite, they were generally saturated with sulfur prior to emplacement resulting in occurrences of minor basal chromitite layers with PGE-enriched sulfides generated during injection of new pulses of Saturated magma into the chamber (e.g. Panton Sill). Such rocks are therefore not believed to be favourable hosts for Merensky Reef style PGE mineralization. Feeder dykes of these intrusives and the Woodward Dolerite may have potential for mineralization similar to the NiCuCuPGE sulfide deposits in feeder dykes of Permian-Triassic continental flood basalts in west Siberia. The key factor for PGE potential in mafic-ultramafic magmas is S-undersaturation which can be achieved either by high temperature (> 1400°C) and ⩾10% partial melting under high pressure (⩾ 25kb) and/or large degrees (⩾25%) melting of MORB type mantle source under low pressures. Their mantle sources appear to have fairly constant Pd (∼4 ppd), Pt (∼6 ppb), Au (∼1 ppb) and S (∼250 ppb) from Archaean to the present. S-undersaturation may also be achieved by smaller degrees of meltung of residual, refractory mantle poor in Ca, Al, Ti, Fe and S ⪡ 250 ppm) (with Pd, Pt ⪡ 4 ppb, Au ⩽ 1 ppb) of the continental lithosphere or wedge above a subduction zone. Rocks of the first association include komatiites, continental flood basalts and picrites, which are believed by some to be related to mantle plume activity. These PGE rich magmas could be further enhanced by melting of the refractory lithospheric mantle, whereas the second type of rocks include boninites, low-Ti tholeiites, some lamprophytes and shoshonites.

Are Bushveld U-type parent magmas boninites or contaminated komatiites?

Three distinct categories of magmas — Bushveld “U-type” parent magmas, boninites, and siliceous high magnesium basalts from Archaean greenstone belts —share the distictive geochemical characteristics of high MgO (9%–19%), low TiO2(less than 1%) and high SiO2(greater than 52%). Boninites are generally thought to form by hydrous melting of metasomatized, previously depleted upper mantle, while siliceous high magnesium basalts (SHMB) in greenstone belts have recently been recognized as the products of combined fractionation and crustal contamination of komatiites. Both these mechanisms can apparently give rise to similar end products, and both mechanisms have been proposed for the petrogenesis of Bushveld U-type magmas.

Chemical composition of Archaean komatiites: Implications for early history of the earth and mantle evolution

Estimates of the chemical composition of the Archaean mantle, derived from elemental abundance ratios in komatiites combined with ultramafic xenolith data, support a model of a multistage heterogeneous accretion history of the Earth and synchronous core formation, 4.6 Ga ago. Most refractory lithophile element abundance ratios in komatiites and xenoliths are close to chondritic except for V/Ti and Ca/Al. Depletion of vanadium is likely due to its partial incorporation into the iron core; whereas fractionation of Ca/Al observed in Archaean Al-undepleted komatiites (1.20 times chondrites) and in some modern fertile spinel lherzolite xenoliths (1.15 times chondrites) could be due to small amounts of garnet (rich in Al but poor in Ca) segregation into the lower mantle during partial or complete melting of the upper mantle in the very early history of the Earth. However, this process may have had only a small effect on the overall chemical composition of the upper mantle. Simultaneous occurrence of early Archaean Al-undepleted (Al/Ti ∼ chondrites) and Al-depleted (Al/Ti ∼ 0.5 chondrites, and depletion of Sc and heavy REE) peridotitic komatiites in the Barberton area, S. Africa, and late Archaean Newton Township, Canada, argue against derivation of peridotitic komatiites from a circum-global magma ocean. Garnet separation from a mantle diapir which intersects the solidus at great depth (⩾ 200 km) in a hotter early Archaean mantle could explain the chemical characteristics of Al-depleted komatiites. Alternatively, these two types of komatiites could have been derived from different layers in a fractionated mantle. A limited amount of Hf isotope data for Archaean komatiites seems to suggest that both mechanisms are important. This chemically and minerallogically layered mantle, if it existed, was homogenized by mantle convection after early Archaean times. Constant P 2O 5/TiO 2, Ni/Mg, Co/Mg, Fe/Mg ratios (siderophile/lithophile) and PGE abundances, estimated for the mantle sources of komatiites from Archaean to modern times, strongly argue against continuous growth of the Earth's core since the early Archaean. Extensive crustal contamination might have been involved in the generation of Archaean-early Proterozoic siliceous high magnesian basalts with “boninite affinity”. However, involvement of chemically modified ancient continental lithosphere may also be important in the generation of these basalts.