Supracrustal Enclaves Research Articles

Neoarchean (∼2.73–2.70 Ga) accretionary history of the eastern Dharwar Craton, India: Lu-Hf and Sm-Nd garnet geochronologic constraints from the Karimnagar granulite-facies supracrustal enclaves

Cratonic mid-crustal plutons may contain supracrustal enclaves that preserve evidence of an earlier growth history. The Eastern Dharwar craton records Neoarchean two-stage accretionary sequential growth (2.70 and 2.55 Ga) and a chronology of their enclaves could refine orogenic models. To test whether the metamorphic history of their enclaves was related to any of these stages, phase equilibria modelling and combined Lu-Hf and Sm-Nd geochronology on garnet were conducted on metapsammite, now preserved as garnet-orthopyroxene-cordierite gneiss. Phase equilibria modelling indicates peak metamorphic conditions, ∼850 °C and ∼8.5 kbar (M1a), were followed by near isothermal decompression to 5–6 kbar (M1b) and isobaric cooling to ∼800 °C (M1c). The thermobaric gradient related to peak metamorphic conditions, ∼30 °C kbar−1, is typical of collisional orogens. Regression of the whole-rock and garnet, for sample S17b, yield Lu-Hf isochron ages of 2733 ± 29 Ma, and for sample S18, 2724 ± 13 Ma. A Lu-Hf weighted mean age for the porphyroblastic garnet suggests growth at 2725.5 ± 11.9 Ma during the M1a-M1b stages. In contrast, the whole-rock sample S17b and the garnet fractions yield a Sm-Nd isochron age of 2696 ± 10 Ma. From sample S18 the whole rock, garnet fractions, and orthopyroxene yield an isochron age of 2683 ± 15 Ma. The garnet Sm-Nd weighted mean age at 2692.0 ± 8.3 Ma constrains the M1b-M1c stages. We suggest that the protoliths to these supracrustal enclaves were deposited in an arc tectonic setting and underwent thickening followed by heating during peeled-back lithospheric convergence. Therefore, the earliest of the craton-forming accretionary stages is preserved as the ∼2.73 Ga granulite-facies enclaves, marginally older than the 2.70–2.65 Ga cratonic greenstone volcanism. Tectonic exhumation of these mid-crustal granulite enclaves was in response to the late-Proterozoic (∼1.7 Ga) Bhopalpatnam orogeny.

Eoarchean subduction-like magmatism recorded in 3750\xa0Ma mafic\u2013ultramafic rocks of the Ukaliq supracrustal belt (Qu\xe9bec)

Our understanding of the nature of crustal formation in the Eoarchean is limited by the scarcity and poor preservation of the oldest rocks and variable and imperfect preservation of protolith magmatic signatures. These limitations hamper our ability to place quantitative constraints on thermomechanical models for early crustal genesis and hence on the operative geodynamic regimes at that time. The recently discovered ca. 3.75 Ga Ukaliq supracrustal enclave (northern Québec) is mainly composed of variably deformed and compositionally diverse serpentinized ultramafic rocks and amphibolitized mafic schists whose metamorphic peak, inferred from phase equilibria modeling, was below 720 °C. Inferred protoliths to the Ukaliq ultramafic rocks include cumulative dunites, pyroxenites, and gabbros, whereas the mafic rocks were probably picrites, basalts, and basaltic andesites. The bulk-rock and mineral chemistry documents the partial preservation of cumulative pyroxenes and probably amphiboles and demonstrates the occurrence of a clinopyroxene-dominated, tholeiitic suite and an orthopyroxene-dominated, boninite-like suite. Together with the presence of negative μ142Nd anomalies in the boninitic basalts, two liquid lines of descent are inferred: (i) a damp tholeiitic sequence resulting from the fractionation of a basaltic liquid produced by mantle decompression; and (ii) a boninitic suite documenting the evolution of an initially primitive basaltic andesite liquid produced by flux melting. Petrographic observations, thermodynamic modeling, bulk-rock and mineral chemistry, and 142Nd isotopic compositions identify the Ukaliq supracrustal belt as the remnant of an Eoarchean arc crust produced by the recycling of Hadean crust in a similar way as modern-style subduction.

Timing and duration of discrete tectono-metamorphic events of the polymetamorphic high-grade Central zone of the Limpopo Belt (South Africa): Insight from in situ geochronology of monazite and zircon

The Central Zone of the Limpopo Belt in southern Africa is a complex polymetamorphic terrane that has experienced three metamorphic events, at 3.22 Ga (M1), 2.71–2.56 Ga (M2) and 2.02 Ga (M3). On the basis of a previous detailed petrological and geothermobarometrical study of a newly discovered supracrustal enclave within the Bulai Pluton (ca. 2.61 Ga granitoid gneiss in the Central Zone), we conducted a geochronological in situ study of texturally and chemically different monazite and zircon grains, which occur respectively as inclusions in garnet, in decompressional cordierite coronas around garnet and as matrix grains in polymetamorphic granulite-facies metapelites. Secondary-ion mass spectrometry (SIMS) in situ analyses of the monazite and zircon grains in thin sections, complemented by analyses of separated zircon grains, allowed the texturally-controlled direct determination of U-Pb and 207Pb/206Pb ages at a scale of < 10 μm. A concordant age of ca. 2.71 Ga derived from zircon included in garnet is interpreted to reflect the age of initial garnet growth during the prograde M2 metamorphism. Zircons in the cordierite coronas around garnet revealed a slightly younger concordant age of ca. 2.56 Ga. Zircon growth in the coronas is related to the retrograde breakdown of garnet (Zr-bearing) through to the decompression reaction garnet + sillimanite + quartz → cordierite + zircon and hence the ca. 2.56 Ga zircon age is interpreted to reflect the timing of post-peak decompressional uplift. The zircon ages constrain the duration of M2 metamorphism to approximately 145 Myr. A concordant 2.71 Ga age of large monazite inclusions in garnet cores corroborates the 2.71 Ga zircon age of initial garnet growth during the prograde M2 metamorphism. Core domains of the largest monazite in cordierite corona revealed ages between 2.5 and 2.0 Ga that are interpreted to reflect (multiple) Pb-loss during the M3 granulite-facies metamorphism. Other monazites in cordierite coronas and matrix showed an age of ca. 2.0 Ga, reflecting complete re-equilibration or new growth during M3 overprinting.Owing to the preservation of the original micro-textures related to the growth of zircon and monazite we were able to link the age data with the P-T record, and refine the timing and duration of the M2 granulite-facies metamorphism (approximately 2.71–2.56 Ga) in the Limpopo Belt that was previously dated at ca. 2.62 Ga by zircon separation techniques. The ca. 145 Ma duration of granulite-facies metamorphism is consistent with a tectonic model of long-lasting asthenosphere upwelling and subsequent continent–continent collision in the late Neoarchean. Decompression triggered by post-collisional extension occurred at 2.56 Ga. Paleoproterozoic M3 granulite-facies metamorphism at ca. 2.0 Ga caused the widespread re-setting of the U-Pb and Pb-Pb data that also affected most zircon and monazite grains. This study reveals that precise dating of the timing and duration of distinct tectono-metamorphic events by texturally-controlled in situ geochronology allows a much better insight into the complex geodynamic evolution of high-grade polymetamorphic terranes.

Geochemical and textural investigations of the Eoarchean Ukaliq supracrustals, Northern Québec (Canada)

Structural, geochronological and geochemical studies of pre-3.75 Ga rocks of volcano-sedimentary protoliths in the Inukjuak domain in the Superior Province in Québec have been mostly focused on the Nuvvuagittuq Supracrustal Belt (NSB). The Porpoise Cove outcrops – at the southwestern limit of the NSB – are the de-facto “type locality” for the supracrustals of the Inukjuak Complex. Yet, it remains unclear whether the NSB rocks are geochemically distinct from, or are in fact common to, a host of other supracrustal enclaves locked in the dominantly gneissic Inukjuak domain. Here, we report detailed textural and geochemical studies for a suite of rocks from the Ukaliq Supracrustal Belt (USB), located approximately 3 km northeast of the NSB. We find that the USB and NSB have a similar protracted metamorphic history; both experienced amphibolite grade metamorphism and contain granitoid gneiss sheets, quartz-magnetite rocks (banded iron-formation s.l.) and quartz-biotite schists within amphibolitized rocks of basaltic affinity with local retrogressions. If the various Inukjuak supracrustal belts were once a part of a larger coherent (now dismembered) terrane, they should show similar emplacement ages and source chemistry. New zircon UPb geochronology from five gneissic units and two quartz-biotite (metasedimentary) schists reveal the oldest emplacement ages across all units of each individual rock type to be 3.68 ± 0.07 Ga (granitoid gneisses) and 3.65 ± 0.06 Ga (quartz-biotite schists). These new ages are similar to those documented as likely minimum emplacement ages of the NSB determined by UPb geochronology. Zircons from the quartz-biotite schist were also analyzed by ion microprobe for their UPb geochronology and were found to yield statistically identical, albeit more precise, ages than those obtained by LA-ICP-MS. Possible detrital zircons from the USB quartz-biotite schists were analyzed by ion microprobe for their coupled δ30SiNBS28 and δ18OVSMOW values with respective values between −0.75 and − 0.07‰ (δ30SiNBS28), and + 5.61 and + 6.59‰ (δ18OVSMOW). The δ18OVSMOW values, which are on average above mantle-derived zircon, indicate contribution of altered, non-mantle, derived material into the source of the rocks that weathered to form the quartz-biotite schists. Zircon mineral inclusions (quartz, feldspar, apatite, biotite, muscovite and other unrecognized Fe/Al/Si rich phases), along with the major- and trace element contents for the rocks were analyzed to substantiate this interpretation. Together with δ30SiNBS28, δ18OVSMOW, our results suggest that lithologies like authigenic silica and serpentinized rocks re-melted to form the parent melts that gave rise to zircons found in the USB quartz-biotite schists. Additional LuHf studies reported here on the same zircons also show similarities with NSB zircons. The εHf values showed a positive correlation with the measured UPb age from −22.7 ± 0.8 to +1.9 ± 1.1. The LuHf system also reveals that the USB, extracted at ca. 3.8 Ga, carries isotopic signatures of an older Hadean reservoir, having been formed from an Eoarchean mafic melt that incorporated them. Taken together, this supports a co-genetic origin for the NSB and the USB.

Widespread poly-metamorphosed Archean granitoid gneisses and supracrustal enclaves of the southern Inukjuak Domain, Québec (Canada)

The ~12,000 km2 Inukjuak Domain in northern Québec is part of the Archean Minto Block in the northwestern Superior Province of Canada. Eoarchean (ca. >3800–3780 Ma) rocks of the Nuvvuagittuq supracrustal belt (NSB) are the best known occurrence of otherwise abundant <1 m to km-scale supracrustal enclaves dispersed throughout the Innuksuac Complex. The supracrustals are dominantly amphibolites, with subordinate intermediate-, mafic- and ultramafic schists, quartzo-feldspathic (trondhjemitic and granodioritic) sills, dikes and sheets, banded iron-formations and quartz-pyroxene±magnetite rocks, and (detrital) fuchsitic quartzites. Supracrustal assemblages are in turn hosted by variably deformed granite-granitoid gneisses metamorphosed to amphibolite facies. Locally, retrogression is expressed as pervasive chloritization and development of jaspilite box veinings. This metamorphic history precludes preservation of original fragile microfossil shapes. Despite its importance as one of the few terranes to host Eoarchean supracrustal assemblages, limited geochronology was previously available for rocks beyond the ~8 km2 NSB. Here, we report new major-, minor-, and trace-element geochemistry and metamorphic petrology coupled with UPb zircon geochronology, from rocks within and surrounding the NSB. These include the little-studied but volumetrically significant Voizel suite gneisses. Results show that intra-NSB fold belt rocks of the Central Tonalitic Gneiss (CTG) preserve mainly ca. 3650 Ma zircons. Beyond the NSB, the Voizel gneisses – previously considered contemporaneous with the CTG – are instead about 100 Myr younger (~3550 Ma). Tonalitic (ortho)gneisses at the margin of the NSB were previously assigned a ca. 3650 Ma age, and the surrounding Boizard suite gneisses may be about 2700 Ma. We find the Boizard rocks contain inherited zircon cores up to ca. 3700 Ma, with younger overgrowths dated at ca. 2700 Ma. A tonalitic gneiss that transects another highly deformed supracrustal enclave north of the NSB – dubbed the Ukaliq Supracrustal Belt – yields maximum concordant zircon ages of 3653 ± 16 Ma (2σ). Detrital zircons from Ukaliq and Nuvvuagittuq quartzites and quartz-biotite schists define a maximum age of ca. 3780 Ma. No indication of UPb zircon ages older than about 3800 Ma exist in this terrane from our geochronological surveys. New reconnaissance sampling of gneisses to the west of the NSB fold belt yielded more zircon-bearing rocks with ages of ca. 3760 Ma. This discovery of more pre-3700 Ma rocks beyond the NSB outcrops calls attention to the existence of hitherto unidentified but widespread scattered occurrences of Eoarchean and Paleoarchean rocks throughout the region.

Late Neoarchean granites in the Qixingtai region, western Shandong: Further evidence for the recycling of early Neoarchean juvenile crust in the North China Craton

The Neoarchean basement rocks of the Qixingtai region in the northwest of western Shandong, eastern North China Craton are divided into western and eastern domains, separated by a ductile shear zone. The western domain is mainly composed of early Neoarchean tonalitic and supracrustal rocks, and the eastern domain is composed of late Neoarchean crustally‐derived granite with diverse early Neoarchean enclaves up to several kilometres long of supracrustal and gneissic TTG‐diorite rocks. This study reports an integrated field, whole‐rock geochemistry and zircon U–Pb‐Hf study on the granites and their enclaves in the eastern domain. The supracrustal enclaves are mainly composed of meta‐basaltic and meta‐ultramafic rocks and are the same in lithological association and formation time as the 2,750–2,700 Ma Liuhang “Formation” in the western area. The TTG‐diorite enclaves can be divided into early and late groups. Magmatic zircons from early TTG‐diorite samples have U–Pb ages of 2,731 ± 17 to 2,748 ± 13 Ma and εHf(t) values of +2.7 to +9.5, whereas trondhjemite samples of the late group have U–Pb zircon ages of 2,588 ± 18 to 2,606 ± 12 Ma and εHf(t) values of +0.6 to +6.5. The Hf isotopic signatures on magmatic zircon from the &gt;2,700 Ma diorite‐tonalites indicate that they are predominantly juvenile crustal additions, probably with minor involvement of older crust in their genesis, as given by tDM (Hf) ages ranging back to 2.92 Ga. The eastern granites range from monzogranite to syenogranite in composition and commonly have negative Eu anomalies, indicating residual plagioclase in the source region. Magmatic zircons from two granite samples have U–Pb ages of 2,516 ± 24 and 2,527 ± 18 Ma and εHf(t) values of +1.0 to +7.0. A fine‐grained syenogranite dyke contains inherited early Neoarchean zircons with εHf(t) values of +1.6 to +7.4. The isotopic signatures of the late Neoarchean granites that dominate the eastern domain show that they were derived from partial melting recycling of older continental crust recognized in the region – early Neoarchean rocks formed as a result of juvenile crustal additions. This is a common signature for late Archean crustal evolution throughout much of the North China Craton, and is the hallmark of some other Archean terranes, such as the Dharwar Craton of south India. The geodynamic explanations for the formation globally of vast amounts of crustally‐derived late Neoarchean granite are discussed.

Metamorphic rocks of the Murmansk domain (Kola Peninsula) as compared with the oldest rock associations of the northeastern Baltic shield, Canada, and Greenland

Rock associations characterized by heterogeneous sets of petrogeochemical parameters were compared by quantifying the degree of their similarity-dissimilarity and searching for discrimination trends between them. Using procedures specially developed for this purpose, it was demonstrated for the first time that the lithotectonic complexes of the Murmansk domain are fundamentally different from those of typical granulite-gneiss terrains and resemble the granite-greenstone terrains of the Baltic shield, Greenland, and Canada. Based on the whole data set, the Murmansk domain can be considered as a deeply eroded Archean granite-greenstone terrain retaining only the tonalite-trondhjemite-gneiss basement with abundant supracrustal enclaves. A trend of the compositional difference between the older and younger rock associations is similar to that between the tholeiitic and boninitic volcanic series. It was suggested that the petrogeochemical “age” trend reflects the initial stage of the compositional evolution of the metamagmatic rocks of the region from metamorphic rocks similar to tholeiites at the early stages to the Paleoproterozoic boninite-like rocks, which are believed to be linked to the unique PGE-bearing province of the northeastern Baltic shield. This implies that the specific metallogenic features of the region emerged already in the Archean, which supported the suggestion on the long duration of geological processes in the Early Precambrian.

Pre-3750 Ma supracrustal rocks from the Nuvvuagittuq supracrustal belt, northern Québec

Geochemistry and U-Pb ion microprobe zircon geochronology guided by high-resolution mapping (1:50 scale) was used to define a minimum age of ca. 3750 Ma for supracrustal rocks from the Nuvvuagittuq supracrustal belt (NSB) in the northern Superior Province, Québec (Canada). Mineralogy and geochemistry of critical field relationships preserved at the Porpoise Cove locality describe a supracrustal succession of (mafic) amphibolites and ultramafic rocks, finely banded quartz-magnetite units with intermixed coarse-grained ferruginous quartz-pyroxene rocks and quartz-biotite schists that superficially resemble polymict meta-conglomerates with large (up to 10 cm) deformed polycrystalline quartz and mafic clasts. All units in the mapped outcrop have sharp lithological contacts. Narrow (dominantly trondhjemitic) orthogneiss sheets locally preserve intrusive contact relationships to the supracrustals. The total extent of the supracrustal enclave is ∼ 8 km 2; it is strongly deformed and the full deformation history appears to be shared by all units with later modifications from leucogranitoid intrusions. The quartz-biotite schists record complex zircon growth at ∼ 3500 and ∼ 2800 Ma, interpreted to reflect metamorphic events. Zircons separated from orthogneisses in the enclave – including one sheet that transects a banded quartz-pyroxene (± magnetite) unit – yield magmatic 207Pb/ 206Pb ages of ca. 3750 Ma. These ages are slightly younger than earlier provisional reports for an NSB orthogneiss from Porpoise Cove. The Nuvvuagittuq supracrustals are the oldest rocks thus far reported for the Minto Block, they overlap in age with the ca. 3.70–3.83 Ga Isua supracrustal belt and Akilia association rocks in West Greenland, and they represent an important new area for exploration of the early Earth.

Geology, Age and Origin of Supracrustal Rocks at Akilia, West Greenland

New mapping, geochronology, and geochemistry of the supracrustal enclave and associated orthogneisses at Akilia, West Greenland, support a ≥3825 ± 6 Ma age for the sequence and a chemical sedimentary origin for the controversial Fe-rich quartz-pyroxene gneisses. Lithologies of the enclave comprise laterally continuous, mappable units of mafic amphibolite, ultramafic rocks, and two Fe-rich quartz-pyroxene units. A minor anthophyllite-garnet rock with chemical characteristics suggesting a sedimentary protolith was identified. The earliest foliation parallels lithologic contacts. This foliation was isoclinally folded about a steep hinge surface, and then tightly refolded about a steep NS hinge plane. The latest folding produced limbs exhibiting contrasting strains. The full deformation history is shared by all lithologies of the enclave. Two previously unidentified orthogneiss sheets preserve deformed magmatic crosscutting relations with amphibolite and ultramafic units in the low-strain limb. The supracrustal anthophyllite-garnet rock experienced zircon growth at ∼3600 and ∼2700 Ma, in common with amphibolites. These ages are interpreted to reflect metamorphism at granulite and amphibolite facies, respectively, consistent with independent regional evidence. The crosscutting orthogneiss sheets preserve a complex history of zircon growth, but only earliest cores indicate Th/U exchange equilibrium with the bulk rock at magmatic conditions. Weighted-mean ²⁰⁷Pb/²⁰⁶Pb ages of cores of 3730 ± 7 Ma and 3825 ± 6 Ma date the crystallization ages of the sheets, indicating that the Akilia supracrustal rocks were deposited ≥3819 Ma. Quartz-pyroxene rocks on Akilia have been interpreted as metamorphosed chemical sediments that may preserve carbon-isotope evidence for life at the time of their formation. An alternative proposal is an origin by metasomatic alteration. Our results show that these rocks experienced a history of deformation and zircon growth identical to other lithologies of the supracrustal body. Metamorphic zircons give a minimum ²⁰⁷Pb/²⁰⁶Pb age of 3589 ± 13 Ma regardless of origin. The two models of formation of quartz-pyroxene rocks were evaluated using field, petrologic and geochemical tests. The distribution of the units, the absence of field or petrologic indicators of metasomatism, and geochemistry (oxygen and nontraditional stable isotopes, immobile elements and REE) together support a sedimentary, but not a metasomatic, origin. Because their protolith was sedimentary, the quartz-pyroxene rocks are a primary part of the supracrustal sequence and have the same minimum age. Despite strong deformation and polymetamorphism, the Akilia supracrustal enclave contains information about terrestrial surficial processes at ≥3819 Ma.

Establishment of a 3.83-Ga magmatic age for the Akilia tonalite (southern West Greenland)

A recurring challenge to ion microprobe U–Pb zircon geochronology has been to discriminate between preservation of original igneous zircon populations and inherited grains. This has proved particularly problematic in studying the polyphase metamorphic rocks that dominate Early Archean gneissic terranes. For example, differing interpretations exist for the origin of complex zircon populations investigated by U–Pb ion microprobe zircon geochronology from a granitoid (orthogneiss) body, previously used to establish a minimum age for a supracrustal enclave on Akilia (island) in southern West Greenland. We describe a method whereby the geochemistry of the Akilia orthogneiss, coupled with a U–Th–Pb vs. age depth profile in a zircon from the same rock, permits direct assessment of zircon inheritance. Results reveal evidence for three phases of concentric zircon growth at 3.83 Ga, ∼3.6 Ga and 2.7–2.5 Ga; zircon growth at both ∼3.6 Ga and 2.7–2.5 Ga is consistent with precipitation from a metamorphic fluid. Depth profile U–Th–Pb data demonstrate that only the >3.8-Ga zircon core could have crystallized from the host rock. We conclude that the magmatic age of this rock is 3.83±0.01 Ga, not ∼3.65 Ga as has been previously proposed. The U–Th–Pb zircon depth profile technique has wider applications to resolving other geochronological debates in high-grade metamorphic terranes where zircons populations are diverse and individual grains record complex overgrowth and dissolution/re-precipitation features.

Sm-Nd ages of two meta-anorthosite complexes around Holenarsipur: Constraints on the antiquity of Archean supracrustal rocks of the Dharwar craton

Whole-rock Sm-Nd isochron ages are reported for two stratiform meta-anorthosite complexes emplaced into the Archean supracrustal-gneiss association in the amphibolite facies terrain around Holenarsipur, in the Dharwar craton, South India. While these metaperidotite-pyroxenite-gabbro-anorthosite complexes are petrologically and geochemically similar, they differ in the intensity of tectonic fabric developed during the late Archean (c. 2.5 Ga) deformation. They also differ in their whole-rock Sm-Nd isochron ages and initial Nd isotopic compositions: 3.285 ± 0.17 Ga,ɛNd0.82 ± 0.78 for the Honnavalli metaanorthosite complex from a supracrustal enclave in the low-strain zone, and 2.495 ± 0.033 Ga, ɛNd = -2.2 ± 0.3 for the Dodkadnur meta-anorthosites from the high-strain southern arm of the Holenarsipur Supracrustal Belt (HSB). We interpret these results as indicating that the magmatic protoliths of both meta-anorthosite complexes were derived from a marginally depleted mantle at c. 3.29 Ga but only the Dodkadnur rocks were isotopically reequilibrated on a cm-scale about 800 Ma later presumably due to the development of strong penetrative fabrics in them during Late Archean thermotectonic event around 2.5 Ga. Our results set a younger age limit at c. 3.29 Ga for the supracrustal rocks of the HSB in the Dharwar craton.

Superposed folding in the Honakere arm of the Chitradurga-Karighatta schist belt in the Dharwar tectonic province, southern India, and its bearing on the Sargur-Dharwar relation

The supracrustal enclave within the Peninsular Gneiss in the Honakere arm of the Chitradurga-Karighatta belt comprises tremolite-chlorite schists within which occur two bands of quartzite coalescing east of Jakkanahalli(12°39′N; 76°41′E), with an amphibolite band in the core. Very tight to isoclinal mesoscopic folds on compositional bands cut across in the hinge zones by an axial planar schistosity, and the nearly orthogonal relation between compositional bands and this schistosity at the termination of the tremolite-chlorite schist band near Javanahalli, points to the presence of a hinge of a large-scale, isoclinal early fold (F1). That the map pattern, with an NNE-plunging upright antiform and a complementary synform of macroscopic scale, traces folds 'er generation (F 2),is proved by the varying attitude of both compositional bands (S0) and axial pranar schistosity (S 1), which are effectively parallel in a major part of the area. A crenulation cleavage (S 2) has developed parallel to the axial planes of theF 2 folds at places. TheF 2 folds range usually from open to rarely isoclinal style, with theF 1 andF 2 axes nearly parallel. Evidence of type 3 fold interference is also provided by the map pattern of a quartzite band in the Borikoppalu area to the north, coupled with younging directions from current bedding andS 0 -S 1 inter-relation.

The origin of the foliations and tectonic style in the charnockitic gneisses, Eastern Hebei, China

Geological and microstructural studies have identified six types of foliations in the charnockitic gneisses in the Archaean high grade terrain, eastern Hebei province, China. The foliations are believed to result from décollement or ductile shearing under granulite facies metamorphism in the lower crust and subsequent folding. The charnockitic gneisses and supracrustal enclaves can be regarded as tectonic units formed in the lower level of the crust, which are marked by “décollement layers” or “tectonic cakes”. The tectonic style represents a pattern of regional fold interference over the “décollement layers”. A qualitative pattern has been established as follows: (1) detachment deformation of the charnockite and enclaves under granulite facies in the lower levels of the crust, yielding “décollement layers” or “tectonic cakes”; (2) tight folding of the “décollement layers” with a nearly vertical axial plane striking north-south; and (3) superposition of the later open folds with east-west striking axial planes, forming domes or oval structures in the region.

A Pan-African zincian staurolite imprint on Namaqua quartz–gahnite–sillimanite assemblages

AbstractQuartz-gahnite-sillimanite asssemblages are described from a supracrustal enclave at Kraaifontein, 45 km south-west of Springbok in the Namaqualand Metamorphic Complex, South Africa. The assemblage formed by prograde reaction in the granulite facies zone during the 1100 Ma Namaqua event, possibly as the result of desulphidation of sphalerite. Subsequent lower-amphibolite-facies retrogression occurred in close proximity to shear zones during an early Pan-African metamorphic event at 700 Ma. Zincian staurolite formed as overgrowths on gahnite in a hydration reaction involving the consumption of gahnite, quartz, and sillimanite. Compositional zoning to more zinc-rich rims in gahnite at Kraaifontein is unrelated to the retrograde reaction, but is interpreted as a result of changing conditions during the prograde Namaqua event.

Genesis and evolution of the ore deposits in the early Precambrian greenstone belts and adjacent high grade metamorphic terrains of Peninsular India — A study in similarity and contrast

Karnataka and the adjacent areas of Tamil Nadu and Andhra Pradesh in Peninsular India are characterized by sublinear ‘greenstone belts’ set in a tonalite-trondhjemite dominated gneissic terrain (mainly 3400-3000 Ma) skirted by a high grade metamorphic (amphibolite-granulite facies) belt in the south and the east. The greenstone belts are divided into the sedimentary-volcanic Sargur-Holenarsipur-Nuggihalli type (> 3000 Ma), the volcanic-dominated Kolar-Ramagiri-Hutti type (⩾ 3000 Ma) and the sediment-dominated Dharwar type (3000-2600 Ma), arranged in order of increasing areal coverage. The granulite facies metamorphism of the Peninsular Gneiss and associated supracrustals took place 2500–2600 Ma ago. The first type of greenstone belt is poor in ore content. These belts contain a few lean base metal deposits, a few chromite deposits and Ti- and V-bearing magnetite deposits in places. Locally banded magnetite deposits are also present. The second type of belt is known for its Au content. The Dharwar type of belt, of course, contains rich deposits of Fe and Mn and rare small deposits of Cu. Ore deposition in the high grade metamorphic terrain is also poor as in the Sargur type of greenstone belt. Whatever metallizations occur there are confined to the supracrustal enclaves. Differential metallization in the different segments is explained in terms of the differential development of the crust, the atmosphere and the biosphere.

Stratabound scheelite and stratiform tourmalinites in the Archaean Malene supracrustal rocks, southern West Greenland

The Malene supracrustal rocks form large enclaves in the late Archaean Nûk gneisses within an area of at least 36 000 km2 on the west coast of Greenland and have been repeatedly deformed and metamorphosed under amphibolite facies conditions. Stream sediment sampling has shown that scheelite occurs in the supracrustal enclaves throughout the area. It occurs stratabound in amphibolites, micaceous quartzites, siliceous cordierite-bearing metasediments and tourmalinites. A detailed geological map has been made of the central part of Store Malene mountain close to Nuuk. Banded metavolcanic Malene amphibolites and siliceous cordierite-bearing metasediments contain stratiform tourmalinites, often with appreciable scheelite, in addition to common accessory tourmaline. The tourmaiinites predate the earliest recognisable phase of deformation as well as prominent simple pegmatites. The tourmaline probably originated as primary sea floor precipitates, and the primary or diagenetic tourmaline crystals were later overgrown with tourmaline during metamorphism. The scheelite and tourmaline are probably syngenetic and of submarine exhalative origin, and boron complexes may have played a role in transporting tungsten in hot brines.

Field relations, petrography and geochemistry of Archaean amphibolite dykes and malene supracrustal amphibolites, northwest Buksefjorden, southern West Greenland

Abstract The Ameralik dyke swarm intrudes Amitsoq tonalitic—granitic gneisses, ca. 3750 Ma, and older supracrustal enclaves that are believed to have formed a basement to Malene supracrustal amphibolites and metasedimentary gneisses in northwest Buksefjorden. Eight varieties of Ameralik amphibolite dykes have been recognised on the basis of field relations and petrography. Malene metabasites, listed in order of decreasing abundance, are subdivided into (a) banded amphibolites derived by deformation of pillows, pillow breccias and agglomerates, (b) homogeneous amphibolites interpreted as lavas and sills, (c) leucocratic amphibolites that resemble many of the leucogabbros associated with the stratiform anorthosite complexes in southern West Greenland, (d) garnetiferous amphibolites regarded as sills, (e) ultramafic rocks forming discontinuous sheets and lenses regarded as picritic flows or sills, and (f) grey plagioclase—quartz rocks which may be related to oceanic plagiogranites. The Malene amphibolites and certain metasediments are intruded by two principal varieties of Malene amphibolite dykes that are similar to two main categories of Ameralik dyke. Rare preservation of primary contact phenomena of Ameralik dykes suggests intrusion into shear fractures, but conditions of intrusion of Malene dykes are unknown. REE patterns and variation of elements considered to have remained immobile during post-magmatic deformation and high-grade metamorphism show that Ameralik and Malene dykes had a similar magmatic history, having formed as low-K tholeiites with enrichment in Fe, Cr and Ni, like tholeiitic rocks in other Archaean terranes. Some Malene dykes are more evolved than Ameralik dykes. Malene homogeneous amphibolites are less evolved than most Ameralik and Malene dykes but lie on similar evolutionary trends. Homogeneous amphibolites have LREE-depleted patterns unlike most Ameralik and Malene dykes with slight LREE enrichment. Certain Ameralik and Malene dykes have LREE-depleted patterns and may have been feeders to parental lavas and sills of the homogeneous amphibolites. Malene garnetiferous amphibolites had an independent magmatic evolution. The age of the Ameralik and Malene dykes and Malene supracrustal amphibolites remains to be established isotopically. On the assumption that they are broadly contemporaneous and related to late Archaean processes, the dykes and supracrustal amphibolites can be integrated into a pattern of events linking the formation of the Malene and related amphibolites with the genesis of the enormous volumes of Nuk tonalitic—granitic gneisses and their equivalents, ca. 3000-2800 Ma, in the Archaean block of southern West Greenland.

Plutonic development of the Ilordleq area, South Greenland. Part I: Chronology, and the occurrence and recognition of metamorphosed basic dykes

Evidence is presented which shows the rocks of Ilordleq to be the products of two distinct episodes of plutonic (Read, 1957, p.165) activity. In the first of these episodes the mise-en-place of a large area of synkinematic granitic rocks was affected. A NE striking belt of pre-granite supracrustal rocks is preserved in the north of the area, and isolated remnants of the supracrustal rocks remain as enclaves in the granite. Cutting the metasediments and metavolcanics of the supracrustal group are numerous basic dykes which pre-date the granite. The first plutonic episode was followed by a further period of emplacement of basic dykes, which cut both supracrustal and granitic rocks. Indications of original chilled margins in these dykes, together with their regular form, suggest a marked change in conditions from those prevailing in the preceding plutonic episode. The second period of plutonic activity which followed resulted in a reactivation of much of the granite previously formed, especially in the northern part of the area where basic dykes were deformed and became either disrupted or were migmatised by kinematically controlled metasomatism. The surrounding granite shows evidence of flow in the solid state and was evidently approaching a state in which diapiric movement was possible. In a less reactivated part of the area, porphyroblastic microcline augen granites were formed in those places where the supracrustal enclaves were most strongly deformed. Deformation during each plutonic episode took place by homogeneous strain with resulting change in shape rather than tectonic transport; minor structures in supracrustal, granitic and dyke rocks are homologous. Principal stress axes were similarly oriented during both plutonic episodes. A further period of emplacement of basic dykes took place in the closing stages of the second period of plutonic activity, while the country rocks were still at elevated temperatures and during the operation, at least intermittently, of compressive forces. A conjugate set of shear fractures controlled the emplacement of these dykes; movement along the fractures and emplacement of dykes were essentially contemporaneous and gave rise to 'differentiation by deformation' of the partially consolidated dyke magma, sigmoid foliation of the dykes, and other distinctive features. Further swarms of basic dykes were subsequently emplaced during the Gardar volcanic episode (ca. 1150 x 106 years). Reference is made to published criteria for the recognition of relict dykes, replacement dykes, and dykes emplaced in hot granite, and the possibility of confusion with dykes in reactivated granites is indicated. The suitability of the orogenic cycle as the primary unit of Precambrian chronology is questioned and a less interpretative unit advocated. The Ilordleq chronology is compared with those established by the writer's colleagues elsewhere in South Greenland and possible alternatives suggested to some current correlations. The similarity of many features of the Ilordleq rocks to those of the Lewisian rocks of Scotland is referred to, and the concept of a migrating front of plutonic activity shown to be not applicable to Ilordleq.