W. Todt

Origin of continental crust of 1.9-1.7 ga age: nd isotopes in the svecofennian orogenic terrains of sweden

Initial Nd isotopic ratios are reported for 57 samples representing all crustal components in the central and southern parts of the 1.9-1.7 Ga old Svecofennian orogenic terrains of Sweden and the 1.8-1.7 Ga old Transscandinavian Granite-Porphyry Belt (Transscandinavian Igneous Belt). U-Pb zircon ages are determined where the magmatic ages were not sufficiently constrained. The Nd isotopic results show that there are no Archaean crustal blocks within the studied 1.9-1.7 Ga old crustal segment. This is consistent with results from Finland. The main input of recycled Archaean crust to the Svecofennian was in the form of sediments, which are concentrated in the Central Svecofennian Subprovince of north central Sweden. This is consistent with the vicinity of this subprovince to the Archaean craton of the Baltic Shield. These turbidite sediments contain up to 40% Archaean material, but S-type granitoids associated with them have a smaller Archaean component, around 20%. Most of the quantitatively predominant I-type granitoids and volcanics, as well as all the basaltic rocks studied, have ϵNd from −1 to +3, and consist of 90% material newly derived from the mantle 1.9-1.7 Ga ago. We argue that the Archaean material was introduced in the form of sediments, but we cannot distinguish between pelagic sediments delivered on subducting oceanic crust and turbidite accumulations like the ones seen in north central Sweden. In either case, an overall 15% Archaean contribution for the Swedish Svecofennian was reasonably well homogenized with mantle-derived magmatic products. For this to occur, an active near-surface tectonic environment, such as a complex of subduction zones, must have been necessary.

Age and tectonic setting of Late Archean greenstone-gneiss terrain in Henan Province, China, as revealed by single-grain zircon dating

The authors report precise U-Pb zircon ages for single grains of a metarhyodacite from the Late Archean Dengfeng greenstone belt in Henan Province, China, near the southern margin of the North China craton. Most grains belong to an igneous population whose U-Pb isotopic systematics define a straight line intersecting concordia at 2512 +/- 12 Ma, and this is interpreted as the time of crystallization of the original greenstone volcanics. Several grains are distinctly older, between 2576 +/- 9 and 2945 +/- 44 Ma, and the authors interpret the older grains as xenocrysts of pre-greenstone continental crust that provide evidence for crustal derivation or crustal contamination of the original rhyodacitic lava. The xenocrysts suggest evolution of the Dengfeng greenstone belt in a continental environment that may be represented by the Taihua high-grade gneisses bordering the Dengfeng greenstones and for which the authors obtained ages of 2806 +/- 7 and 2841 +/- 6 Ma. The data add evidence to the now widely held concept that most Archean greenstones developed on or near older continental crust and were therefore prone to crustal contamination. In such cases, conventional zircon dating may not always record the precise age of rock formation.

The Gabal Gerf complex: A Precambrian N-MORB ophiolite in the Nubian Shield, NE Africa

We report geochemical and isotopic data for tectonically dismembered units of the Cabal Gerf mafic-ultramafic complex, the largest Neoproterozoic (Pan-African) ophiolite in the Arabian-Nubian Shield and located near the Red Sea in the border region between Egypt and the Sudan. The complex consists of basaltic pillow lavas, sheeted dykes, isotropic and layered gabbros and an ultramafic melange, all in tectonic contact along thrust sheets. Major- and trace-element data, including REE, for the pillow lavas and sheeted dykes are indistinguishable from modem high-Ti N-MORB. Chemical variations in the various rock types can be ascribed to fractionation and accumulation involving olivine, clinopyroxene and plagioclase. A comparison with chemical data from ophiolites of the Arabian-Nubian Shield and elsewhere in the world shows the Cabal Gerf complex to be the only Precambrian ophiolite with N-MORB chemistry, and we suggest that its basalts and sheeted dykes originally formed in a major ocean basin. Sm and Nd isotope analyses combined with published zircon data suggest an age of -750 Ma for the time of igneous crystallization of the Gabal Gerf complex. Ed,, initial values vary between + 6.5 and + 8.8, some of the highest yet reported for Neoproterozoic mantle-derived rocks. Pb isotopic data for the basalts and sheeted dykes are similar to modem N-MORB, while the gabbros are more akin to island arc and back-arc basin rocks. We ascribe their elevated Z07Pb/2MPb ratios to mixing of a small amount of pelagic sediment with the magma source of the gabbros during subduction and subsequent melt generation above a subduction zone. The pillow basal&, sheeted dykes and gabbros were brought together by tectonic stacking during the abduction process when collision of island arc complexes with the active margin of the African continent occurred during an accretion event - 600-700 Ma ago.

Prolonged magmatism and time constraints for sediment deposition in the early Archean Barberton greenstone belt: evidence from the Upper Onverwacht and Fig Tree groups

The single zircon evaporation, SHRIMP ion-microprobe and conventional dissolution techniques were used to determine 207Pb/206Pb and UPb ages on samples from the Upper Onverwacht and Fig Tree groups of the early Archean Barberton greenstone belt, South Africa. Zircons from dacitic rocks of the upper Hooggenoeg Formation yield ages of ∼ 3445–3452 Ma. A tuff in the basal Kromberg Formation has a mean age of 3416 ± 5 Ma. A tuffaceous band, 5 cm thick, in the uppermost Kromberg Formation contains igneous zircons with a mean age of 3334 ± 3 Ma. The 1700 m section of Kromberg Formation between these two samples is composed of basaltic lavas, minor komatiites and cherty metasediments. The overlying Mendon Formation is composed of interbedded komatiitic lavas and metasediments with a minimum thickness of 600 m. A cherty, stromatolitic metasediment 300 m above the base contains several thin ash layers with a mean zircon age of 3298 ± 3 Ma. The basal Fig Tree Group has units as old as 3259 ± 3 Ma, and upper units in the Fig Tree are as young as 3225 ± 3 Ma. Xenocrystic zircons in the Upper Onverwacht and overlying Fig Tree Group samples suggest that successive igneous units inherited zircons from underlying units and that, over several hundred million years, episodes of intermediate to felsic igneous activity took place at 20–40 Ma intervals. Structural repetition by isoclinal folding and thrust faulting are important components of late greenstone belt evolution, but should not obscure the importance of the prolonged interval of magmatic evolution represented by the thick pile of volcanic rocks observed in the Barberton greenstone belt.

Dating of late Proterozoic ophiolites in Egypt and the Sudan using the single grain zircon evaporation technique

Abstract Zircons from gabbro and plagiogranite in late Proterozoic ophiolites of the Arabian-Nubian Shield (ANS) in Egypt and the Sudan, as well as post-ophiolite granites have been dated using the stepwise evaporation method. Zircons from a plagiogranite in the Wadi Ghadir ophiolite, Eastern Desert of Egypt, yielded a mean 207Pb/206Pb age of 746±19 Ma, while a gabbro and diorite associated with the Abu Swayel ophiolite nappe ∼ 250 km southwest of Wadi Ghadir provided zircon 207Pb/206Pb ages of 729±17 and 736±11 Ma, respectively. Zircons from layered gabbro along the western margin of the Jabal Gerf ophiolite just north of 22°N were dated at 741±21 Ma, indistinguishable from the Wadi Ghadir and Abu Swayel results. A plagiogranite sample from the Onib ophiolite in the northern Red Sea Hills, Sudan, contains zircons with a 207Pb/ 206Pb age of 808±14 Ma, while granites which crosscut layered gabbro in this complex have zircon ages of 713±12 and 714±5 Ma respectively. The youngest granite at Onib was dated at 646±10 Ma and may reflect a phase of postaccretionary intraplate magmatism. Our zircon data document ocean crust generation between ∼ 810 and ∼ 730 Ma ago in the Nubian segment of the ANS, and the age data compare well with ophiolite generation in Saudi Arabia. However, some of our data cast doubt on large-scale correlations of suture belts between the Arabian and Nubian segments of the shield.

U-Pb zircon and Sm-Nd model ages of high-grade Moldanubian metasediments, Bohemian Massif, Czechoslovakia

We report single grain U-Pb ion-microprobe as well as conventional bulk size fraction ages for zircons from 3 metasediment samples of the Moldanubian Complex, Bohemian Massif, one of the largest crystalline complexes of the Hercynian foldbelt in Europe. These are complemented by whole-rock Sm-Nd model ages. The metasediments are of upper amphibolite to granulite grade and come from the Bory Massif in Moravia, NW of Brno (sample AA-1) and from the Varied Group (AA-2) and Monotonous Group (AA-3) in the Ceske Budejovice region of SW Bohemia.Ion-microprobe data for detrital zircons yielded 207Pb/ 206Pb ages between ca. 1750 Ma and 2680 Ma and reflect chronologically heterogeneous source terrains. One grain in sample AA-1 of the Bory granulite massif may be as old as 2684±14 Ma, and this constitutes the oldest reported zircon age for the Hercynian belt of central Europe. The single grain data are much less discordant than previously published conventional U-Pb analyses from bulk zircon samples and suggest a significant early Proterozoic crust-formation event between 2 and 2.2 Ga ago. The size fraction data are compatible with the single grain ages and give a fairly precise definition of the Hercynian event between 347 and 367 Ma ago while their upper Concordia intercept ages between ≈1700 and ≈2050 Ma represent the mean of the respective grain populations and are probably geohronologically meaningless. The Nd whole-rock model ages between 1.7 and 3.0 Ga confirm mid-Proterozoic to Archaean source terrains for the dated metasediments.

Age of Palaeozoic granites and metamorphism in the Tuvino-Mongolian Massif of the Central Asian Mobile Belt: loss of a Precambrian microcontinent

Abstract The Tuvino-Mongolian Massif (TMM) was previously interpreted as a Precambrian block within the Central Asian Mobile Belt. According to this idea, it consists of tectonic slices composed of metamorphic rocks of pre-Mesoproterozoic basement that experienced two episodes of regional metamorphism, and Mesoproterozoic ‘cover rocks’ that were reworked together with the basement during high-grade metamorphism. Zircon U–Pb dating of granitoids from all metamorphic complexes demonstrates that the earliest metamorphic event occurred at 536±6 Ma, significantly later than the deposition of the cover rocks. Regional upper amphibolite-facies metamorphism, which affected all metasedimentary units of the TMM, occurred in the short time interval of 497±4 to 489±3 Ma. We propose that there is no simple basement-cover relationship in the Tuvino-Mongolian Massif. Instead, the massif consists of tectonic domains, composed of heterogeneous metasedimentary successions with distinct pre-metamorphic and pre-tectonic histories that were juxtaposed by thrusting prior to 497 Ma and then metamorphosed under upper amphibolite facies conditions. We suggest that this thrusting event was associated with early Palaeozoic collisional processes related to amalgamation of the Central Asian Mobile Belt. Our new model for the evolution of the TMM implies derivation of Neoproterozoic terrigeneous metasediments from ∼800–900 Ma granitoids formed in an Andean-type active continental margin setting.

U-Pb Isotopic Systematics of Zircons from Prograde and Retrograde Transition Zones in High-Grade Orthogneisses, Sri Lanka

We present U-Pb zircon isotopic data from locally restricted prograde (arrested in situ charnockitization) and retrograde metamorphic transition zones, which are well exposed in Proterozoic orthogneisses tectonically interbanded with granulite facies supracrustal rocks of the Highland Group in Sri Lanka. These granitoid rocks yield apparent ages of

U-Pb zircon and Sm-Nd dating of Moldanubian HP/HT granulites from South ohemia, Czech Republic

1942 \pm 22 Ma