B. Auvray

Late Archaean (2550-2520 Ma) juvenile magmatism in the Eastern Dharwar craton, southern India : Constraints from geochronology, Nd-Sr isotopes and whole rock geochemistry

The results of field, geochronologic, geochemical and isotopic studies are presented for the granitoids that occur east of the Closepet batholith up to the Kolar schist belt (KSB). Field data, such as common foliation, strong shear deformation occasionally leading to mylonitization, together with petrographic data, including reduction in grain size with corroded borders, show characteristics of the syn-kinematic emplacement of the granitoids. Single zircon evaporation ages define a minimum age of 3127 Ma for the tonalitic–trondhjemitic–granodioritic (TTG) basement and 2552–2534 Ma plateau ages for the emplacement of the granitoids, which slightly predate (20–30 Ma) the emplacement of the 2518 Ma Closepet batholith.Major and trace element data, together with isotopic data, suggest at least four magmatic suites from Closepet batholith to the east, which have independent magmatic evolution histories. The observed data are compatible with magma mixing for the Closepet batholith, melting of TTG and assimilation–fractional crystallization processes for Bangalore granites, either melting of heterogeneous source or different degree of melting of the same source for the granitoids of Hoskote–Kolar and fractional crystallization for the western margin of the KSB. Isotopic (Nd–Sr) and geochemical data (LREE and LIL elements) suggest highly enriched mantle and ancient TTG crust for the Closepet batholith, enriched mantle and TTG crust for the Bangalore granites, c.a. chondritic mantle source for the granitoids of Hoskote–Kolar and the quartz monzonites of the western margin of the KSB and slightly depleted mantle for granodiorites of the eastern margin of the KSB.We interpret all these geochronologic, geochemical and isotopic characteristics of granitoids from the Closepet batholith to the east up to the KSB in terms of a plume model. The centre of the plume would be an enriched ‘hot spot’ in the mantle that lies below the present exposure level of the Closepet batholith. Melting of such an enriched mantle hot spot produces high temperature magmas (Closepet) that penetrate overlying ancient crust, where they strongly interact and induce partial melting of the surrounding crust. These magmas cool very slowly, as the hot spot maintains high temperatures for a long time; thus they appear younger (2518 Ma). On the contrary, to the east the plume induces melting of c.a. chondritic or slightly depleted mantle that produces relatively colder and less enriched magmas, which show less or no interactions with the surrounding crust and cool rapidly and appear slightly older (2552–2534 Ma). This plume model can also account for late Archaean geodynamic evolution, including juvenile magmatism, heat source for reworking, inverse diapirism and granulite metamorphism in the Dharwar craton.

3.5 Ga old amphibolites from eastern Hebei Province, China: Field occurrence, petrography, Sm-Nd isochron age and REE geochemistry

Amphibolite blocks frequently occur in association with grey gneisses within a small area near the villages of Caozhuang and Huangbaiyu in eastern Hebei. They are interpreted as remnants of an earlier bimodal suite now engulfed in late Archaean K-rich granitic magma and are further cut by pegmatite dykes. They are considered together as giant enclaves within late Archaean granite plutons (U-Pb zircon ages = 2.5–2.6 Ga). The amphibolites have been dated by the Sm-Nd isochron method. The results yielded: T = 3470 ± 107 Ma, INd = 0.50827 ± 11, and ϵNd (T) = +2.7 ± 0.6. This age represents the time of protolithic basalt eruption and possibly suggests a period of formation of primitive mafic crust. This is the oldest reliable age so far obtained for the Archaean rocks of China. The Sm-Nd results of grey gneiss samples yielded model ages (TDM) of about 3.24 Ga. It is not clear whether this ‘younger’ age in comparison with that of the amphibolites indicates the intrusive nature of these rocks or is a result of contamination by later granite about 2.5 Ga ago. The Rb-Sr and common Pb isotopic systems have been highly disturbed, and no isochron ages could be established. The common Pb data show retarded growth of radiogenic Pb and hence imply reduction of U-Pb ratios in the past. This is consistent with the petrographic study, which suggests that the amphibolites have undergone an earlier period of granulite facies metamorphism and were subsequently retrogressed to the present amphibolite facies mineral assemblages. The positive ϵNd (T) value of +2.7 for the amphibolites implies that the protolithic basalt magmas were generated from mantle sources which had undergone long-term depletion in light rare earth elements (LREE). The depletion event must have taken place prior to 4 Ga ago if their source had a chemical nature similar to the modern oceanic mantle; it could have occurred as early as 4.5 Ga ago if their source was less impoverished in LREE. If the mantle depletion was related to extraction of continental or sialic material, the Sm-Nd data of the Caozhuang amphibolites lend support to the existence of sialic crust, if not of a continental size, before 4 Ga ago.

Archean crustal evolution in China: The Taishan complex, and evidence for juvenile crustal addition from long-term depleted mantle

Abstract The main framework for crustal evolution of the late Archean Taishan Complex in the Shandong Province of China is constructed using data from field investigation, petrographic examination, geochemical analysis and isotopic age determination. The history of the complex began with emplacement of basic and tonalitic magmas about 2.7–2.75 Ga ago, which were metamorphosed (designated as metamorphism I) to amphibolites and grey gneisses shortly after their emplacement. These rocks constitute a metamorphic basement termed the Wangfushan Gneisses. The basement gneisses were later intruded by a plutonic association including (1) alkali- and light rare earth element (LREE) enriched dioritic rocks and their differentiates ∼ 2.6 Ga ago, (2) granites and trondhjemites, collectively termed the Hushan granites, also ∼ 2.6 Ga ago, and (3) the Aolaishan-type granites (sensu stricto) 2.4–2.5 Ga ago. The plutonic association, in conjunction with the older basement gneisses, was later subjected to a second period of deformation and metamorphism (metamorphism II) ∼ 2 Ga ago as inferred from published K-Ar biotite dates. The chronological order of all the above events has been determined by a multichronometric approach, and the age results (in Ma, with 2σ uncertainties) are summarized as follows: The relatively low I Sr (initial 87 Sr: 86 Sr ratio) and high positive ϵ Nd ( T ) values for the Wangfushan Gneisses and the diorite group provides strong evidence for continental growth by addition of juvenile material ultimately separated from long-term depleted mantle sources. By contrast, the Sr and Nd isotopic data for the Aolaishan-type granites coherently suggest that they are likely to be derived by anatexis of the basement gneisses at shallow depths. Crustal evolution of the Taishan Complex offers an additional example supporting the concept of a continental accretion-differentiation superevent: that during such a worldwide tectonothermal episode, particularly in the Archean, important differentiation of the upper mantle has contributed to the growth of the continental crust by addition of juvenile material.

Archean crustal evolution in China: U-Pb geochronology of the Qianxi Complex

Abstract The Qianxi Complex (formerly the Qianxi Group) in eastern Hebei Province is a key terrain for studying the Archean crustal evolution of the Sino-Korean Craton in China. The evolution of the complex involved several stages of supracrustal deposition and magmatic intrusion. Except for a few post-tectonic intrusives, all rock assemblages have been metamorphosed to high grade granulite- and amphibolite-facies. The evolution of the Complex has spanned more than 1 Ga since about 3.5 Ga ago. In the present work, two areas in the Qianan region (Qianan County) have been investigated for U-Pb age study: (1) the Caozhuang area in the south, and (2) the Shuichang-Yangyashan area in the north. Important tectonothermal events and magmatic episodes of the Qianxi Complex were identified by the U-Pb isotopic analyses of zircons extracted from various rock types. The results of age determination and the most likely interpretation in terms of temporal evolution of the Complex are summarized schematically. The oldest zircons (3.65-3.67 Ga) of China have been found in a fuchsite-bearing quartzite in association with the 3.5 Ga (Sm-Nd isochron age) amphibolites in the Caozhuang area. This emphasizes the existence of early Archean sialic crust in the Qianxi Complex. However, no ages older than 2.9 Ga have been revealed for grey gneisses enclosing the ca. 3.5 Ga amphibolite enclaves in the Caozhuang area, but a leptynite sample of dacitic composition gave a minimum 207Pb/206Pb age of 3.3 Ga obtained by a single zircon stepwise evaporation analysis. This age and the 3.5 Ga Sm-Nd isochron age for amphibolites suggest that the earliest deposition of supracrustal rocks took place about 3.5-3.3 Ga ago. The deposition of fuchsite quartzite could have taken place contemporaneously or in an earlier period (up to 3.6 Ga), but its precise stratigraphic positions remains to be determined. Nevertheless, the presence of very ancient zircons clearly advocate for the existence of sialic crust at least 3.6 Ga in China. In the Shuichang-Yangyashan area, a supracrustal sequence containing abundant banded iron formations was intruded by a granitic gneiss dated at 2.96 Ga by the conventional multigrain zircon method. Sm-Nd model ages of about 3.3 Ga were obtained for two amphibolite enclaves in a charnockitic gneiss. It appears that supracrustal deposition of the Qianxi Complex might have migrated to the north and took place in the period of 3.3-3.0 Ga. In addition to supracrustal rocks, the Qianxi Complex is characterized by polyphase granitoid intrusions. Except for the contaminated grey gneisses (Sm-Nd TDM=3.2−3.3 Ga) in the Caozhuang area and the ca. 3 Ga granitic gneiss in the Yangyashan area, most granitoid intrusions are of late Archean ages. The period of 2.7-2.5 Ga has undoubtedly recorded the most important tectonothermal events in the evolution of the Sino-Korean Craton. The present zircon age study and available age data in the literature demonstrate that the Archean crustal development in China culminated towards the end of the Archean.

Mantle heterogeneity in northeastern Africa: evidence from Nd isotopic compositions and hygromagmaphile element geochemistry of basaltic rocks from the Gulf of Tadjoura and southern Red Sea regions

Abstract Basaltic rocks from the Gulf of Tadjoura and southern Red Sea regions have been analysed for their Nd isotopic compositions and major and trace element concentrations. The wide variation in isotopic and geochemical compositions of the basaltic rocks is best explained by the mixing phenomenon involving a variety of mantle source components. To test the mixing hypothesis, a combined use of Nd isotopes and hygromagmaphile elemental ratios is proven very powerful. Three reservoirs have been identified as minimum components in their petrogenesis: (1) DMM (depleted MORB mantle), a mantle source depleted in light rare earth elements (LREE), which is the principal component of the N-MORB type basalts of this region; (2) REC (Ramad enriched component), equivalent to the hot-spot type of source detected in the south of Red Sea; (3) TEC (Tadjoura enriched component), a rather unique component located in the region of Tadjoura Gulf; it is characterised by a relative depletion in Rb, K, Th and U in a primitive mantle- or chondrite-normalised incompatible element pattern; this component could have been produced by mantle metasomatism of an originally depleted mantle. Mixing in various proportions of the above components is considered to be the principal mechanism for the formation of basalts with such diverse isotopic and trace element compositions.

The Archaean Greenstone Belts of Karelia (Eastern Finland) and their Komatiitic and Tholeiitic Series

Abstract The geological and geochemical features of three greenstone belts of Eastern Finland (Suomussalmi, Kuhmo and Tipasjarvi) have been studied. We have analysed about 70 metavolcanic rocks from the low greenstone sequence for their major and trace element compositions. The field relationships between various volcanic rocks are rather obscure; but the chemical data allow us to distinguish two general magmatic series, namely, the komatiitic and the tholeiitic series. We have concluded from this preliminary geochemical study that most volcanic rocks in each may have been derived from fractional crystallization of some parental magma. The REE data, mainly presented for the rocks from the Tipasjarvi belt, provide a strong evidence for a “depleted” mantle source, a feature very similar to that of Abitibi, Canada. The REE data also suggest that not all rocks are formed by fractional crystallization; a mechanism of different degree of partial melting is called to account for some rock types. Although the Baltic Shield is one of the first regions in which the Archaean rocks were described (Sederholm, 1897, 1932), it is only recently that the existence of greenstone belts has been clearly demonstrated (Blais et al., 1976, 1977 and in print a and b; Bowes, 1976; Gaal et al., 1976 and in print; Lobach-Zhuckenko et al., 1976; Mutanen, 1976; Suslova, 1976). In this contribution, the principal characteristics of the belts, which we have studied in Finland, are briefly described and we expose the present state of our petrological and geochemical research in this context.