Trondhjemitic Gneisses Research Articles

The geochemistry and isotopic age of eclogites from the Belomorian Belt (Kola Peninsula): evidence for subducted Archean oceanic crust

We present results of geochemical studies and isotope dating of eclogites and associated rocks from the Kuru-Vaara quarry, Belomorian Belt, Northeastern Baltic Shield. The southern and northern eclogites are similar in geochemical features. Their protoliths were primitive, mainly high-Mg basalts of oceanic affinity derived from a primitive mantle source rather than from a depleted mantle source characteristic of modern MORB. The post-eclogitic intrusive rocks show obvious evidence for crustal contamination. The eclogite-hosting tonalitetrondhjemite-granodiorite (TTG) gneisses form a coherent series including high-Al and low-Al varieties. The trace element data show that the TTG series formed through the hydrous partial melting of the southern eclogites in the presence of garnet and amphibole in the field of the rutile stability (>15 kbar). Zircons from the southern eclogites exhibit features of their strong re-equilibration by coupled dissolution–repre-cipitation processes but have locally preserved patches with a primary magmatic zoning. The geochemistry of the patches points to the oceanic provenance of protolithic zircons; their isotope dating (SHRIMP-II) yielded a concordant age of 2821 ± 21 Ma. Zircons from the trondhjemite gneiss with geochemical features of Archean adakite were dated at 2805 ± 11 Ma, which suggests the syneclogitic facies origin of the TTG melts. The concordant age of high-pressure zircons from the northern eclogites is 2722 ± 21 Ma, close to the age of the earlier described Gridino eclogites. The overview of the isotopically dated eclogite bodies show the presence of at least three temporally distinct groups of eclogites in the Belomorian Belt, ∼2.86–2.87, ∼2.82–2.80, and ∼2.72 Ga, which is in a good accordance with the known isotopic ages for major crust-forming events in the belt. This, in turn, implies a close genetic relationship between the eclogites and the TTG origin, which might be consistent with the model of the short intermitted events of subduction of the thickened Archean oceanic crust. The presence of HP/UHP eclogites of different ages and the structural style of the Belomorian Belt permit it to be assigned to megamélange belts.

REPLY ON COMMENTS ON THE PAPER "PETROLOGY AND GEOCHEMISTRY OF THE GRANITOID ROCKS OF THE JOHANNESBURG DOME, CENTRAL KAAPVAAL CRATON, SOUTH AFRICA" BY D.M. VAN TONDER AND H. MOURI (SAJG, 113, 257-286)

The authors of the paper would like to acknowledge the substantial pioneering work done on the Johannesburg Dome (JD) by C.R. Anhaeusser as was referenced in the said paper. Although the subdivision of the JD granitoids proposed in the paper differs from previous work by Anhaeusser (1973), it is based on new and existing geochronological data, field relationships and geochemistry. The proposed suites are broadly similar to those proposed in the new book “The Geology of South Africa” by Robb et al. (2008) of which Anhaeusser was a co-editor: 1. a Granodiorite- to Adamellite Gneiss suite (GAG) with local development of tonalite and trondhjemite gneiss, i.e. probably the ~3.34 Ga rocks of the same composition of Poujol and Anhaeusser (2001) (Lanseria Gneiss, Robb et al, 2008) occurring over much of the northern …

Age and growth of the Archean Kongling terrain, South China, with emphasis on 3.3 ga granitoid gneisses

The North China craton and the Yangtze craton (South China) both contain Archean rocks in eastern China. Unlike the North China craton, where Archean rocks are widespread, in the Yangtze craton the exposed Archean rocks are only known in the Kongling terrain (360 km²). Zircon U-Pb ages and Lu-Hf isotopic compositions of three granodioritic-trondhjemitic gneisses and three metasedimentary rocks from the Kongling terrain were analyzed by LA-ICP-MS and LA-MC-ICP-MS. Igneous zircons in one trondhjemitic gneiss in the north of the Kongling terrain have an age of 3302±7 (1σ) Ma. Evidence from cathodoluminescence imaging, variations in Th/U and degree of U-Pb age discordance suggest that apparently younger zircons in the same population are variably disturbed 3302 Ma grains. Thus, this trondhjemitic gneiss is the oldest known rock in South China and predates the earlier reported ∼2900 Ma granitoid magmatism by 400 Ma. Zircon cores from one granodioritic gneiss in the north of the Kongling terrain also give a concordant age group at 3200 to 3300 Ma. Regardless as inherited or not, these cores crystallized from a magma indistinguishable in age with the trondhjemite. Concordant U-Pb ages for igneous zircons in one granodioritic gneiss in the south of the Kongling terrain yielded a weighted average ²⁰⁶Pb/²⁰⁷Pb age of 2981±13 Ma (2σ, MSWD=9.7, n=21). The zircon age and initial Hf isotopic compositions are similar to those of widespread granitoid gneisses from the north of the Kongling terrain (2903-2947 Ma), and indicate that the south and north of the Kongling terrain are correlative. The results also reinforce that magmatism of the whole Kongling terrain mainly occurred at 2900 Ma. Available Hf isotopic data from the Kongling terrain show that juvenile crustal additions occurred mainly between 3150 and 3800 Ma with a significant peak at 3300 to 3500 Ma. The ∼3300 Ma zircons from the trondhjemitic gneiss have Hf crust formation ages of 3450 to 3730 Ma, some of which have nearly chondritic ε_Hf (t). The whole-rock depleted mantle Nd model age of this rock is 3400 Ma, close to its age of magmatism and consistent with the Hf model age. Its ε_Nd value at 3300 Ma is nearly chondritic (1.26). These lines of evidence suggest that the 3300 Ma trondhjemite represent juvenile crust additions to the pre-existing continental crust.

Archaean fluid-assisted crustal cannibalism recorded by low δ18O and negative εHf(T) isotopic signatures of West Greenland granite zircon

The role of fluids during Archaean intra-crustal magmatism has been investigated via integrated SHRIMP U–Pb, δ18O and LA-MC-ICPMS 176Hf isotopic zircon analysis. Six rock samples studied are all from the Nuuk region (southern West Greenland) including two ~3.69 Ga granitic and trondhjemitic gneisses, a 3.64 Ga granitic augen gneiss, a 2.82 Ga granodioritic Ikkattoq gneiss, a migmatite with late Neoarchaean neosome and a homogeneous granite of the 2.56 Ga Qôrqut Granite Complex (QGC). All zircon grains were thoroughly imaged to facilitate analysis of magmatic growth domains. Within the zircon analysed, there is no evidence for metamictization. Initial εHf zircon values (n = 63) are largely sub-chondritic, indicating the granitic host magmas were generated by the remelting of older, un-radiogenic crustal components. Zircon from some granite samples displays more than one 207Pb/206Pb age, and correlated with 176Hf/177Hf compositions can trace multiple phases of remelting or recrystallization during the Archaean. Model ages calculated using Lu/Hf arrays for each sample indicate that the crustal parental rocks to the granites, granodiorites and trondhjemites segregated from a chondrite-like reservoir at an earlier time during the Archaean, corresponding to known formation periods of more primitive tonalite–trondhjemite–granodiorite (TTG) gneisses. Zircon from the ~3.69 Ga granite, the migmatite and QGC granite contains Eoarchaean cores with chondritic 176Hf/177Hf and mantle-like δ18O compositions. The age and geochemical signatures from these inherited components are identical to those of surrounding tonalitic gneisses, further suggesting genesis of these granites by remelting of broadly tonalitic protoliths. Zircon oxygen isotopic compositions (n = 62) over nine age populations (six igneous and three inherited) have weighted mean or mean δ18O values ranging from 5.8 ± 0.6 to 3.7 ± 0.5‰. The 3.64 Ga granitic augen gneiss sample displays the highest δ18O with a mildly supra-mantle composition of 5.8 ± 0.6‰. Inherited Eoarchaean TTG-derived zircon shows mantle-like values. Igneous zircon from all other samples, spanning more than a billion years of Archaean time, record low δ18O sub-mantle compositions. These are the first low δ18O signatures reported from Archaean zircon and represent low δ18O magmas formed by the remelting and metamorphism of older crustal rocks following high-temperature hydrothermal alteration by meteoric water. Meteoric fluid ingress coupled with crustal extension, associated high heat flow and intra-crustal melting are a viable mechanism for the production of the low δ18O granites, granodiorites and trondhjemites reported here. Both high and low δ18O magmas may have been generated in extensional environments and are distinct in composition from Phanerozoic I-type granitic plutonic systems, which are typified by increasing δ18O during intra-crustal reworking. This suggests that Archaean magmatic processes studied here were subtly different from those operating on the modern Earth and involved extensional tectonic regimes and the predominance of remelting of hydrothermally altered crystalline basement.

Triassic retrograded eclogites and Cretaceous gneissic granites in the Tongbai Complex, central China: Implications for the architecture of the HP/UHP Tongbai–Dabie–Sulu collision zone

The Permo-Triassic Qinling–Tongbai–Dabie–Sulu collision zone between the Sino-Korean and Yangtze cratons hosts a number of terranes; some of them carry evidence for ultrahigh-pressure (UHP) metamorphism. Discovery of retrograded high-pressure (HP) eclogites in the Tongbai Complex, in the middle of the orogen, brings fresh information to correlate segments of the orogen and understand dynamics of the orogeny. The 20 km wide Tongbai Complex is underlying two HP eclogite bearing lithotectonic units (the northern and southern eclogite zones) in the Tongbaishan area. It consists of gneissic granites and subordinate metamorphic enclaves. Among them, retrograded eclogites carry garnet with a compositional profile indicating a three-stage metamorphic evolution, from epidote-amphibolite, through eclogite, to amphibolite facies conditions. A thermobarometric study yields estimates of 660–700 °C and 8.0–10.3 kbar for the amphibolite facies stage. U–Pb geochronology and trace element characterization of zircon produced two important results. First, the dominant gneissic granites are Early Cretaceous (ca. 140 Ma) and not Neoproterozoic as formerly reported. Second, metaigneous enclaves, i.e. retrograded eclogites, garnet amphibolites, and dioritic–trondhjemitic gneisses, formed between 933 and 742 Ma and were metamorphosed at 232–220 Ma. Metasedimentary enclaves contain detrital zircons yielding two age populations of 2960 and 1966 Ma. Consequently, the tectonometamorphic evolution of the Tongbai Complex is quite similar to that of the Northern Dabie Complex, but the Tongbai Complex is likely to be positioned at a higher crustal level in the orogenic edifice. Furthermore, the lower metamorphic P- T conditions in Tongbaishan relative to the Dabie–Sulu region are hypothesized to have resulted from a shallower subduction. The younger metamorphic age (232–220 Ma) for the Tongbai Complex with respect to the overlying eclogite zones (metamorphism at ca. 255 Ma and cooling at ca. 238 Ma) reflects diachronous subduction and exhumation for these two HP slices. A similar process occurred in the Dabie and Sulu terranes.

Continental growth and convergence-related arc plutonism in the Mesoarchaean: Evidence from the Barberton granitoid-greenstone terrain, South Africa

U–Pb zircon ages from trondhjemitic gneisses of the Badplaas domain, located in the southwestern parts of the Mesoarchaean Barberton granitoid-greenstone terrain of South Africa, document a hitherto largely unrecognized plutonic event. The geochronological results indicate the addition of juvenile, felsic crust over a period of 60 Myr between ca. 3290 and 3230 Ma and prior to the main collisional event in the granitoid-greenstone terrain at 3230 Ma. The timing and duration of plutonism, together with the structural and compositional heterogeneity of the Badplaas domain suggest that the Badplaas domain represents part of a convergence-related magmatic arc. On a regional scale, the spatial and temporal relationships between plutonism, metamorphism and deformation are interpreted to preserve an almost complete inventory of a Mesoarchean arc-trench system. This includes (1) the 3290–3230 Ma convergence-related magmatic arc, (2) a largely coeval deposition of the back-arc type volcano-sedimentary 3260–3225 Ma Fig Tree Group of the Barberton greenstone belt, (3) relics of the underplated crust preserved in high-P, low-T rocks to the immediate east of the magmatic arc, juxtaposed against (4) high-T medium-P rocks of the overlying plate along the major crustal structure of the Inyoni shear zone. These findings corroborate the notion that thermal and rheological conditions on the early Earth supported, at least locally, crustal growth and the extraction of buoyant felsic crust along convergence-related magmatic arcs.

Petrology of dioritic, tonalitic and trondhjemitic gneisses from Encantadas Complex, Santana da Boa Vista, southernmost Brazil: paleoproterozoic continental-arc magmatism

The Encantadas Complex is a unit composed of dioritic, tonalitic and trondhjemitic gneisses with minor hornblendite. This complex is intruded by granites of Neoproterozoic age. Major and trace element data indicate metaluminous to slightly peraluminous composition related to the medium-K calc-alkaline series. Compositional parameters are consistent with a common evolution from less differentiated magmas, probably through fractional crystallization. The orthogneisses show LaN/YbN ratios from 10 to 50, K2O/Na2O varying from 1.1 to 3.0, Y contents from 3 to 39 ppm, Yb from 0.3 to 3.7, and Lu with contents in the range 0.06 and 0.54 ppm. Such geochemical features are similar to those of Archaean tonalitic rocks and are usually described in rocks formed by partial melting of mafic rocks under high-pressure conditions leaving an eclogitic residue. The presence of diorites and hornblendites, associated to tonalitic and trondhjemitic gneisses suggests, on the other hand, that tonalitic magmas could be formed by hornblende-controlled fractionation of hydrous basaltic magmas. Tonalitic gneisses show U-PbSHRIMP zircon age of 2, 263 ± 6 Ma for igneous crystallization and 2, 045 ± 10 Ma for the metamorphism. The geochemical parameters, tectonic and geochronological features of Encantadas Complex are consistent with magmas derived from mantle metasomatized by subduction-related fluids in a continental-arc.

Geoquímica de Gnaisses do arco magmático de Goiás na Região Sul do Estado de Goiás

Granitic to trondhjemitic gneisses from the Pontalina region in the southern part of Goiás State, Central Brazil, have calcic to calc-alkaline, metaluminous to peraluminous compositions. They have low concentrations of alkaline elements, and are enriched in Ba, Sr, K, Rb in relation to Nb, Y, Zr and REE, and have negative anomalies of Nb and Ti, features which are similar to those of magmas generated in magmatic arc environments. Those rocks were previously interpreted as part of the basement of the Brasília Belt, attributed to the Archean to Paleoproterozoic, but new Sm - Nd isotopic data indicate a neoproterozoic age (T DM = 0,9 a 1,2 Ga), and the preliminary geochemical data reveal compositions similar to the gneisses of the other regions belonging to the Goiás Magmatic Arc.

Spatial and temporal zoning of hydrothermal alteration and mineralization in the Sossego iron oxide–copper–gold deposit, Carajás Mineral Province, Brazil: paragenesis and stable isotope constraints

The Sossego iron oxide–copper–gold deposit (245 Mt @ 1.1% Cu, 0.28 g/t Au) in the Carajas Mineral Province of Brazil consists of two major groups of orebodies (Pista–Sequeirinho–Baiano and Sossego–Curral) with distinct alteration assemblages that are separated from each other by a major high angle fault. The deposit is located along a regional WNW–ESE-striking shear zone that defines the contact between metavolcano–sedimentary units of the ∼2.76 Ga Itacaiunas Supergroup and tonalitic to trondhjemitic gneisses and migmatites of the ∼2.8 Ga Xingu Complex. The deposit is hosted by granite, granophyric granite, gabbro, and felsic metavolcanic rocks. The Pista–Sequeirinho–Baiano orebodies have undergone regional sodic (albite–hematite) alteration and later sodic–calcic (actinolite-rich) alteration associated with the formation of massive magnetite–(apatite) bodies. Both these alteration assemblages display ductile to ductile–brittle fabrics. They are cut by spatially restricted zones of potassic (biotite and potassium feldspar) alteration that grades outward to chlorite-rich assemblages. The Sossego–Curral orebodies contain weakly developed early albitic alteration and very poorly developed subsequent calcic–sodic alteration. These orebodies contain well-developed potassic alteration assemblages that were formed during brittle deformation that resulted in the formation of breccia bodies. Breccia matrix commonly displays coarse mineral infill suggestive of growth into open space. Sulfides in both groups of deposits were precipitated first with potassic alteration and more importantly with a later assemblage of calcite–quartz–epidote–chlorite. In the Sequeirinho orebodies, sulfides range from undeformed to deformed; sulfides in the Sossego–Curral orebodies are undeformed. Very late, weakly mineralized hydrolytic alteration is present in the Sossego/Currral orebodies. The sulfide assemblage is dominated by chalcopyrite with subsidiary siegenite, and millerite. Pyrrhotite and pyrite are minor constituents of ore in the Sequerinho orebodies while pyrite is relatively abundant in the Sossego–Curral bodies. Oxygen isotope partitioning between mineral pairs constrains temperatures in the deposit spatially and through time. In the Sequeirinho orebody, the early sodic–calcic alteration stage was characterized by temperatures exceeding 500°C and $$ \delta ^{{{\text{18}}}} {\text{O}}_{{{\text{H}}_{{\text{2}}} {\text{O}}}} $$ values for the alteration fluid of 6.9 ± 0.9‰. Temperature declines outward and upward from the zone of most intense alteration. Paragenetically later copper–gold mineralization displays markedly lower temperatures (<300°C) and was characterized by the introduction of 18O-depleted hydrothermal fluids −1.8 ± 3.4‰. The calculated δDH2O and $$ \delta ^{{{\text{18}}}} {\text{O}}_{{{\text{H}}_{{\text{2}}} {\text{O}}}} $$ values suggest that the fluids that formed the early calcic–sodic alteration assemblage were of formational/metamorphic or magmatic origin. The decrease of $$ \delta ^{{{\text{18}}}} {\text{O}}_{{{\text{H}}_{{\text{2}}} {\text{O}}}} $$ values through time may reflect influx of surficially derived waters during later alteration and mineralization events. Influx of such fluids could be related to episodic fluid overpressure, resulting in dilution and cooling of the metalliferous fluid, causing deposition of metals transported as metal chloride complexes.

Timing of magmatism and metamorphism in the Gruinard Bay area of the Lewisian Gneiss Complex: comparison with the Assynt Terrane and implications for terrane accretion—reply

U-Pb zircon data from selected gneiss samples provide estimates of the timing of igneous protolith emplacement and granulite facies metamorphism in the Gruinard Bay, Badcall Bay and Lochinver areas of the mainland Lewisian Gneiss Complex of north-west Scotland. Zircons from the gneisses contain complex zoning patterns making them amenable to SHRIMP ion microprobe analysis, with inherited zircon distinguishable from magmatic growth and metamorphic recrystallisation. From Gruinard Bay, a tonalitic granulite yielded a protolith age of 2,825±8 Ma for oscillatory-zoned zircon cores, with recrystallised low-U rims preserving an age of 2,733±12 Ma. An inherited 2,905±15 Ma age component was recognised in a sample of trondhjemitic gneiss, the protolith of which is estimated to have been emplaced at 2,858±11 Ma. Low-U rims yielding an age of 2,729±20 Ma are also present, which together with the rim age from the granulite are considered to date granulite facies metamorphism in the Gruinard region. For comparison with the Gruinard Bay gneisses, a trondhjemitic partial melt and trondhjemite sheet were analysed from the granulite facies rocks at Badcall Bay in the Assynt terrane. Age data do not show definitive relationships with zoning patterns in the zircons, and show a spread along concordia typical of previous ion microprobe age data from the Scourie gneisses. Both samples do, however, show a significant isotopic disturbance at 2,480 Ma, the most recent estimate for the Badcallian metamorphism. Protolith ages for components of the Assynt terrane range between ca. 2,900 and 3,030 Ma, and appear to be older than the 2,860–2,825 Ma gneiss protoliths from the Gruinard Bay area. The timing of granulite facies metamorphism also appears to be different between the two areas, suggesting that the two granulite regions of the mainland may not be related as previously thought. Instead, they are interpreted in terms of a terrane accretion model whereby the two areas evolved as separate blocks prior to their juxtaposition during the Palaeoproterozoic. The boundary between the two terranes is thought to lie to the south of Lochinver, corresponding with the Strathan Line, because of the observed 2,480 Ma age signature to the north and 2,730 Ma signature to the south. By defining a new mainland terrane, the Gruinard Terrane, the number of mainland blocks, which includes the Assynt (central), Rhiconich (northern) and Southern terranes, has been expanded.

Geochemistry of the NeoArchaean high-Mg basalts, boninites and adakites from the Kushtagi–Hungund greenstone belt of the Eastern Dharwar Craton (EDC); implications for the tectonic setting

Volcanic rocks of the Kushtagi–Hungund greenstone belt from the Eastern Dharwar Craton (EDC) show extreme diversity due to the presence of high-Mg, high Fe-, high Mg–Fe basalts, high Mg-dacites and andesites (boninitic affinities), and sodic–plagioclase–quartz rich felsic rocks (adakites with Na 2O (∼4–6 wt.%). The adakites ( Drummond and Defant, 1990; Martin, 1999) and high-Mg-dacites–andesites are being reported for the first time from greenstone belts of the Dharwar craton. Trace elements in these different types of volcanic rocks have distinct characteristics. For example, the basalts have (1) Al 2O 3/TiO 2 ∼16 (PM value of 17), (2) (La/Yb) N ∼1–3, (3) low order negative anomalies of Nb/Nb*, Zr/Zr*, Hf/Hf* and Ti/Ti*, (4) Zr/Nb ∼4–10 and (5) Ce/Nb ∼3–4. Two samples of andesites having Mg-enrichment and Al 2O 3/TiO 2 ratios of 31–35 are interpreted to have boninitic affinities. Other dacites and andesites are characterized by (1) high-Mg (8–10 wt.%), (2) high Al 2O 3/TiO 2=22–27, (3) Zr/Nb=8–14, (4) (La/Yb) N ∼2–7, (5) negative Nb/Nb*, Zr/Zr*, Hf/Hf* and Ti/Ti* anomalies and (6) Sc/Lu=67–172. The adakites are characterized by (1) a narrow range of SiO 2 (67–70 wt.%), (2) enriched Na 2O (4–6 wt.%) with very high Na 2O/K 2O (2.5–7.7), (3) Low Mg# (0.38–0.48), (4) high (La/Yb) N (13–31) with fractionated patterns, no Eu anomaly and (Gd/Yb) N =2–4, (5) very high Al 2O 3/TiO 2 (50–80) and (6) enriched Ni, Cr and Co (total 50 ppm). These adakites resemble 3.0 Ga tonalite–trondhjemite gneisses of the Dharwar craton, which are interpreted as products of partial melting of the basaltic slab. The available data suggests that partial melting of the different mantle and slab sources generated this collage of compositionally different rocks within an island arc setting.

New UPb zircon ages from Tonga (Cameroon): coexisting Eburnean–Transamazonian (2.1 Ga) and Pan-African (0.6 Ga) imprints

Abstract The central domain of the Pan-African Belt in Cameroon is characterized by abundant porphyritic granitoids, which were emplaced synkinematically and variably orthogneissified in relation with ENE-striking steeply dipping transcurrent shear zones. These plutonic rocks have intermediate to felsic compositions and constitute a high-K calk-alkaline series. Conventional U Pb zircon dating yields an age of 618 Ma for this syntectonic Pan-African magmatism in the Tonga area. The country rocks are made of metabasites (garnet amphibolites) and tonalitic to trondhjemitic gneisses, which suffered two distinct orogenic cycles: the first one is the Palaeoproterozoic Eburnean–Transamazonian cycle at 2.1 Ga and the second one is the Pan-African orogenesis. These new ages confirm the existence of an extensive Palaeoproterozoic crust in Cameroon and question the areal extent of the Congo–Sao Francisco craton towards the north. To cite this article: E.L. Tanko Njiosseu et al., C. R. Geoscience 337 (2005).

Petrological, geochemical and U-Pb isotopic studies of Archaean granitoid rocks of the Makoppa Dome, northwest Limpopo Province, South Africa

The Makoppa Dome, located in the far northwestern part of the Kaapvaal Craton, constitutes one of the least well-exposed Archaean granite-greenstone terranes in southern Africa. For this reason it has received very little attention in the past and its characteristics have largely gone undocumented. This study presents new field, petrological, geochemical and single zircon dating results for a suite of granitoid rocks that comprises foliated trondhjemitic gneisses and homogeneous monzogranitic and granodioritic phases, all of which post-date remnants of Archaean greenstone belts metamorphosed to amphibolite facies. Three separate granitoid types are recognized on the basis of their distinctive field as well as major, trace and rare-earth element characteristics. Confirmation of the three granite types follows from new ID-TIMS and LAM-ICP-MS dating of single zircons extracted from samples collected on the dome. The oldest granitoid rocks so far recognized are represented by the Vaalpenskraal trondhjemitic gneisses, two samples of which yielded ages that are identical within error and are dated at 3013±11 Ma and 3034±64 Ma, respectively. A single sample of homogeneous granitoid, referred to as Makoppa monzogranite, yielded an age of 2886+3/−2 Ma, while a further two samples of homogeneous granodiorite, referred to as Rooibokvlei granodiorite, are representative of the youngest granitic event recorded on the dome and yielded ages of 2777±35 Ma and 2797±2 Ma, respectively. The granitoid rocks of the Makoppa Dome have many features in common with those of the Kraaipan-Amalia terrane on the western side of the Kaapvaal Craton, as well as those of the Murchison-Giyani-Pietersburg terrane on the northern side, supporting suggestions that these rocks developed in, or adjacent to, a crescent-shaped magmatic arc that began forming at ~3.1 to ~3.0 Ga around the northern, northwestern and western rim of the Kaapvaal Craton. Continent-arc collision followed in the period up to ~2.8 Ga, the time of emplacement of the Rooibokvlei granitoids on the Makoppa Dome and the 2791±8 Ma Mosita granitoids in the Kraaipan terrane. These ages closely approximate the age of the ~2.79 Ga Gaborone Granite Complex in neighbouring Botswana and may represent the proto-magmatic stage leading to the development, on thickened crust, of the A-type Gaborone granite suite and Kanye volcanic formation, followed, in turn, by the rift-related ~2.71 Ga volcanism of the Ventersdorp Supergroup.

Geology and geochemistry of a granite-greenstone association in the southeastern Vredefort dome, South Africa

A sequence of mafic to ultramafic chlorite-actinolite-hornblende-talc rocks with komatiitic and komatiitic basalt compositions has been mapped in the southeastern Vredefort dome. A volcanic origin for the rocks is indicated by pillow structures, flow banding, variolites and spinifex textures in low-strain domains. They are intercalated with quartz-sericite-biotite schists. The rocks are metamorphosed to mid-greenschist-facies (~400°C) and are heterogeneously deformed, with a dominant subvertical, northwest-trending schistosity (S3) that developed under peak metamorphic conditions. To the northwest, the greenstones are bounded by a kilometre-wide dip-slip shear zone, which includes poorly-exposed high-grade (~700°C, 5 kbar) amphibolites and migmatitic trondhjemitic gneisses, and pegmatitic granite, and which separates the low-grade rocks from the high-grade gneissic core of the dome. The variation in metamorphic grade across the mapped area is attributed primarily to the displacement across the shear zone (southeast side down), but it may also reflect, in part, deeper levels of exhumation towards the centre of the dome. In contrast to the komatiitic greenstones, the amphibolites within the shear zone have a tholeiitic composition; however, they also show features consistent with volcanic precursors and are interleaved wirh banded ironstones and quartz-sericite-biotite schists. The mafic and ultramafic metavolcanics display close similarities to the komatiites and komatiitic basalts in the Johannesburg dome, but are relatively depleted in Fe2O3, TiO2 and CaO and enriched in Al2O3 relative to the Barberton greenstones. They are compositionally distinct from the dismembered granulite-facies greenstone remnants in the central parts of the dome.

Episodic granitoid emplacement in the Archaean Amalia–Kraaipan terrane, South Africa: confirmation from single zircon U–Pb geochronology

The Amalia–Kraaipan granite–greenstone terrane, located in the western part of the Kaapvaal Craton, South Africa consists of metamorphosed mafic volcanic rocks and interlayered ferruginous and siliceous metasediments (mainly banded iron formations), intruded by a variety of granitoid rocks comprising tonalitic and trondhjemitic gneisses, granodiorites and adamellites. This study presents new single zircon dating for several granitoid rocks in order to define the chronology of magmatic events influencing this terrane. New TIMS and SHRIMP U–Pb data demonstrate episodic granitoid emplacement events, which occurred over a time-span of approximately 250 Ma. The oldest granitoid rocks so far recognized are dated at ca. 3008 Ma and are represented by biotite–trondhjemite gneisses located southwest of the town of Amalia. Homogeneous leuco-trondhjemite dykes intruding these gneisses yield an age of ca. 2940 Ma. Two granodiorite samples from the central part of the Amalia–Kraaipan terrane were dated at 2913±17 and 2915±12 Ma, respectively, while a further two granodiorite samples from the central and northern parts of the study area yielded ages of 2879±9 and 2879±11 Ma. The youngest granitoid body, the Mosita adamellite, which is exposed in the northwest sector of the Amalia–Kraaipan terrane, was dated at 2791±8 Ma. Since the 3008 Ma biotite–trondhjemite gneisses contain conformably interlayered amphibolite xenoliths, at least some of the Kraaipan Group volcano-sedimentary rocks are probably older than 3008 Ma. Based on available age data the 3010–2920 Ma Amalia–Kraaipan granitoids described in this paper could represent a possible source for some of the Witwatersrand sediments 100 km to the east and their contained gold. The ca. 2791 Ma age obtained for the Mosita adamellite permits a possible genetic link between this granitoid body and the ca. 2781 Ma Gaborone Granite Complex exposed approximately 120 km north in neighbouring Botswana.

Metamorphism of the granite–greenstone terrane south of the Barberton greenstone belt, South Africa: an insight into the tectono-thermal evolution of the ‘lower’ portions of the Onverwacht Group

This paper reports new geochronological and metamorphic data from Onverwacht Group greenstone remnants exposed in the granitoid-dominated terrain to the south of the Barberton greenstone belt. The greenstone remnants consist of metamorphosed mafic and ultramafic metavolcanic sequences, with minor sedimentary units that comprise thin chert and banded iron formation layers, interbanded with the (ultra) mafic volcanic units, as well as an up to 8 m-thick clastic sedimentary unit that contains well-preserved primary sedimentary features such as trough cross-bedding. Coarse-grained portions of the clastic sediments contain up to 4.5 wt.% K 2O and represent metamorphosed impure arkoses. SHRIMP and conventional U–Pb dating of detrital zircons reveal dates ranging between ca . 3521 and 3540 Ma, indicating that at least two protoliths for these sediments predate the formation of the bulk of the Barberton greenstone belt. A minimum age of 3431±11 Ma for the formation of the metasediments is given by a trondhjemite gneiss that intrudes the greenstone remnant. Thus, these metasediments were deposited between ca. 3521 and 3431 Ma, contemporaneously with the erosion of spacially associated older, and presumably potassium-rich, granitoid rocks. Other portions of the clastic metasediments have a mafic affinity and are characterised by the peak-metamorphic mineral assemblage diopside+andesine+garnet+quartz. This assemblage, and garnet in particular, is extensively replaced by epidote. Peak-metamorphic mineral assemblages of magnesio−hornblende+andesine+quartz, and quartz+ferrosilite+magnetite+grunerite have been recorded from adjacent amphibolites and interlayered iron formation units, respectively. In these rocks, retrogression is marked by actinolitic rims around peak metamorphic magnesio–hornblende cores in the metamafic rocks, and by a second generation of grunerite that occurs as fibrous aggregates rimming orthopyroxene in the iron formation. PT calculations, using a variety of geothermometers and barometers, for the peak-metamorphic mineral assemblages in all these rock types vary between 650 and 700 °C and 8 and 11 kbar. This implies a tectonic setting comparable to some modern orogenic belts and that the granite–greenstone terrane investigated in this study possibly represents an exhumed mid- to lower-crustal terrane that formed a ‘basement’ to the Barberton greenstone belt at the time of the peak metamorphic event at ca. 3230 Ma.

Variations of sulfur isotopes in metamorphic rocks from Bamble Sector, southern Norway: a laser probe study

Variations of sulfur isotopes in metamorphic rocks from Bamble Sector, southern Norway, are investigated using a Q-switched, pulsed mode, laser combustion technique. The results indicate that laser-related fractionations are reproducible (1 σ=0.2‰), provided uniform conditions are maintained during analysis. Bamble is a Precambrian high-grade metamorphic terrain. A transition from amphibolite- to granulite-facies occurs across the 25-km width of the Sector. Supracrustal rocks of mid-Proterozoic ages were intruded by felsic and mafic magmas and metamorphosed during the Sveconorwegian orogenic episode. Earlier studies indicated that Bamble rocks equilibrated under a relatively high f O 2 , 2–3 log units above QFM buffer, during peak metamorphism [J. Geochemical Exploration 31 (1989) 149; J. Geology 100 (1992) 447; Contrib. Mineral Petrol. 1139 (2000) 180], which led to phase changes in sulfides. Pyrrhotite, or Po mss, has variably been replaced by pyrite±magnetite. Mantling of pyrite by magnetite is common. Metasupracrustal rocks display a large variation in δ 34S CDT (−1‰ to +12‰), consistent with data from sedimentary rocks of similar age. No distinction can be made between amphibolite, transition, and granulite zones. The isotopic signature of sulfides was determined by bacteriogenic reduction of seawater sulfate with an initial δ 34S value of 20–30‰. Tonalitic–trondhjemitic gneisses from granulite zone show a large scatter in δ 34S CDT (0‰ to +8‰), implying that the parent magmas were contaminated by sedimentary materials. This is supported by S/Se ratios. Mafic rocks from various metamorphic zones display a clustering of δ 34S CDT values around 0‰, and S/Se ratios in the range 3×10 3 to 6×10 3. This is consistent with a mantle source for S. Ni–Cu sulfide ores locally associated with the mafic rocks, however, are enriched in 34S, indicating the involvement of sedimentary S in the formation of the ores. Mass and isotopic balance calculations indicate that oxidation of pyrrhotite, or Po mss, proceeded without significant addition or removal of S. Sulfur liberated during mantling of Fe–sulfides by magnetite was in a reduced form and reprecipitated in short distances as secondary overgrowth and fracture filling pyrite. The large variation of δ 34S values for the supracrustal rocks, and the preservation of the original isotopic features for various rock units suggest that no isotopic homogenization occurred in spite of the high-grade metamorphism.

Single zircon U-Pb dating of the Kongling high-grade metamorphic terrain: Evidence for &gt;3.2 Ga old continental crust in the Yangtze craton

Single zircons from two trondhjemitic gneisses and two clastic metasedimentary rocks without Eu anomaly of the Kongling high-grade metamorphic terrain are dated by the in situ SHRIMP U-Pb method. The results show that the trondhjemitic magma emplaced at 2947—2903 Ma. Concordant age of as old as 3.3 Ga is present in the detrital zircons from the clastic metasedimentary rocks. Together with the depleted mantle Nd model age ( T DM =3.2—3.3 Ga) of the clastic metasedimentary rocks, this documents the presence of Paleoarchean continental crust in the Yangtze craton.

The Johannesburg Dome, South Africa: new single zircon U–Pb isotopic evidence for early Archaean granite–greenstone development within the central Kaapvaal Craton

The Johannesburg Dome, located in the central part of the Kaapvaal Craton, constitutes one of the key areas to better understand the Archaean crustal evolution of this part of the craton. The dome comprises a variety of Archaean granitic rocks intruded into mafic–ultramafic greenstone remnants. This study presents new precise U–Pb single zircon dating for seven different granitoid samples and an amphibolite dyke collected from the Johannesburg Dome. A trondhjemitic gneiss sampled on the northwestern part of the dome yielded an age of 3340±3 Ma and represents the oldest granitoid phase recognized so far. This result has important implications with regard to the age of the mafic and ultramafic greenstone remnants scattered throughout the dome as it implies that the greenstone remnants are older than c.3.34 Ga. This initial magmatic episode, involving early greenstone development and the intrusion of trondhjemitic and tonalitic granitoids on the northern half of the dome, was followed by the emplacement of a 3201±5 Ma hornblende–biotite–tonalite gneiss in the south. Following the trondhjemite–tonalite gneiss emplacement a further period of magmatism took place on the dome, which resulted in the intrusion of mafic dykes that are manifest as hornblende amphibolites. The age of these dykes has yet to be determined quantitatively, but they fall within the time constraints imposed by the age of the trondhjemitic gneisses (3340–3200 Ma) and later, crosscutting, potassic granitoids. These rocks, consisting dominantly of granodiorites constitute the third magmatic event and occupy an area of batholithic dimensions extending across most of the southern portion of the dome. The southern and southeastern parts of the batholith consist mainly of medium-grained, homogeneous, grey granodiorites dated at 3121±5 Ma. Their porphyritic granodiorite equivalents in the southwestern part of the dome yielded an age of 3114±2.3 Ma. An age of 3117±12 Ma, from zircons extracted from one of the mafic dykes possessing granitic microveins, provided confirmation of the timing of this third magmatic event. Lastly, pegmatites that crosscut all these earlier granitoid events are younger than 3114 Ma and might be at least 3.0 billion-years old. These new data provide confirmation of the conclusion that the Witwatersrand Basin was deposited after c.3074 Ma on an Archaean basement as young as c.3120 Ma. The data, combined with that from other parts of the Kaapvaal Craton, further supports the view that the evolution of the Craton was long-lived and episodic, and that it grew by accretionary processes, becoming generally younger to the north and west of the c.3.5 Ga Barberton-Swaziland granite–greenstone terrane situated in the southeastern part of the Craton.

Geochronology of the Proterozoic basement of southwesternmost North America, and the origin and evolution of the Mojave crustal province

The Proterozoic Baldwin gneiss in the central Transverse Ranges of southern California, a part of the Mojave crustal province, is composed of quartzofeldspathic gneiss and schist, augen and granitic gneiss, trondhjemite gneiss, and minor quartzite, amphibolite, metagabbro, and metapyroxenite. Sensitive high resolution ion microprobe (SHRIMP) data indicate that augen and granitic gneisses comprise a magmatic arc intrusive suite emplaced between 1783 ± 12 and 1675 ± 19 Ma, adjacent to or through thinned Archean crust. High U/Th rims on zircons in most samples suggest an early metamorphic event at ∼1741 Ma, but peak amphibolite facies metamorphism and penetrative, west vergent deformation occurred after 1675 Ma. The Baldwin gneiss is part of a regional allochthon emplaced by west vergent deformation over a Proterozoic shelf‐slope sequence (Joshua Tree terrane). We hypothesize that emplacement of this regional allochthon occurred during a late Early or Middle Proterozoic arc‐continent collision along the western margin of Laurentia.