Ernst Hegner

High-potassium, calc-alkaline I-type plutonism in the European Variscides: northern Vosges (France) and northern Schwarzwald (Germany)

Abstract Early Carboniferous high-K, calc-alkaline I-type plutonic rocks from the northern Vosges and Schwarzwald were studied for their chemical and Sr–Nd isotopic compositions. Intrusion relationships and mineralogical and chemical characteristics allow to distinguish four suites. The oldest intrusions are diorites (1), followed by a granodioritic (2) and a granitic (3) suite. These older granitoids (OG) and their contact metamorphic country rocks are cut by younger high-K to shoshonitic granitic plutons (YG) (4). Still later, peraluminous S-type granitic magmas intruded (not included in this study). Diorites (1) have SiO2 between 46 and 61 wt.% and are characterized by relatively high Mg# of 62–38, low contents of Na2O (2.3–4.0 wt.%), high abundances of incompatible elements (LILE, Nb, and P) and enriched Nd–Sr initial isotopic signatures [eNd(I)=−1.7 to −2.8; 87 Sr / 86 Sr (I)=0.7046–0.7061]. Chondrite-normalized (cn) REE patterns are relatively flat [(La/Yb)cn=5.1–7.8; (Tb/Yb)cn=1.2–1.8] and show small negative Eu anomalies (Eu/Eu*=0.73–0.90). All these characteristics suggest an origin of the diorites from enriched lithospheric mantle sources. Compared with the diorites, the granodiorites (2) show higher eNd(I) (+0.5 to −0.4) but similar 87 Sr / 86 Sr (I) (0.7051–0.7053). High values of molar CaO/(MgO+FeOtot) combined with low Al2O3/(MgO+FeOtot) and K2O/Na2O ratios suggest an origin by dehydration melting from a metabasaltic to metatonalitic source. Radiogenic isotopic signatures of the older granites (3) are similar to those of the diorites [eNd(I)=−1.8 to −2.5; 87 Sr / 86 Sr (I)=0.7048–0.7058]. Compared with the granodiorites the older granites show similar values of CaO/(MgO+FeOtot), but significantly higher ratios of Al2O3/(MgO+FeOtot) and K2O/Na2O pointing to a metagreywacke source. REE patterns of both the granodiorites and the granites are characterized by relatively low (Tb/Yb)cn ratios (1.2–1.7) excluding substantial amounts of garnet as a fractionating phase. Instead, the residues were probably dominated by amphibole and plagioclase, and possibly also pyroxene. The YG (4) have elevated abundances of large ion lithophile elements (K, Rb, Th, U, Ba, and Sr) and of some high field strength elements (Nb and P). Their isotopic signatures [eNd(I)=−1.5 to −3.4; 87 Sr / 86 Sr (I)=0.7046–0.7060] are similar to those of the older granites. Relative to all OG, their REE patterns are characterized by higher ratios of (La/Yb)cn (11.8–38.9) and (Tb/Yb)cn (1.3–2.6) but lower values of Eu/Eu*. Combined with higher Mg# and lower abundances of Y, these characteristics point to an increasing role of garnet in the residues of the partial melts. Relatively low values of molar Al2O3/(MgO+FeOtot) and K2O/Na2O in combination with variable molar CaO/(MgO+FeOtot) ratios suggest that these magmas were derived from heterogeneous metasedimentary sources.

The oldest part of the Barberton granitoid-greenstone terrain, South Africa: evidence for crust formation between 3.5 and 3.7 Ga

Many stratigraphic and age relationships in the southern part of the Barberton greenstone belt (BGB) remain unresolved due to strong deformation including thrusting, nappe stacking and strike-slip faulting. A relatively undisturbed sequence from the lower Onverwacht Group (∼ 3.48-3.45 Ga), followed by the upper Onverwacht (∼ 3.42-3.3 Ga), the Fig Tree Group (∼ 3.26-3.23 Ga) and the Moodies Group (> 3.22 Ga) was established by single zircon dating using various techniques, but the position of the Theespruit Formation is still uncertain. Using the single zircon evaporation and the vapour digestion techniques we obtained remarkably uniform 207Pb/206Pb and UPb ages of 3544 ± 3 to 3547 ± 3 Ma for felsic rocks mapped as Theespruit in the Steynsdorp Anticline of the southeastern BGB, some 100 Ma older than all other dated greenstone units. These rocks were intruded by the 3502–3511 Ma old Steynsdorp TTG pluton containing zircon xenocrysts as old as 3553 ± 4 Ma. A 3.5 Ga granodiorite plug intrusive into metavolcanics of the Komati Formation (lower Onverwacht Group) contained two 3702 ± 2 Ma zircon xenocrysts, the oldest so far measured in the Barberton-Swaziland area and testifying to the presence of very ancient crust in the region. The Theespruit felsic metavolcanics have ϵNd(t) values between + 1.1 and − 1.1 and Nd TDM model ages between 3.5 and 3.7 Ga, suggesting variable contamination of their protoliths with older continental crust. The above zircon ages extend the history of the BGB back to ∼ 3.55 Ga. We suggest that the area of the Steynsdorp Anticline constitutes the oldest nucleus of the BGB onto which successively younger units were tectonically and magmatically accreted. We also speculate that the mafic-felsic volcanic units of the southern BGB may perhaps represent distinct oceanic plateaux, rather than ocean floor material, which amalgamated between 3.55 and 3.42 Ga ago. Our data support the concept that the BGB consists of a number of discrete, fault-bounded terranes, and that large-scale lithological correlations are therefore not justified.

Mineral ages and P-T conditions of Late Paleozoic high-pressure eclogite and provenance of mélange sediments from Atbashi in the south Tianshan orogen of Kyrgyzstan

Ages derived from various isotope systems in high-pressure (HP) rocks of the western Tianshan orogen of NW China have been interpreted as evidence for late Carboniferous and/or Triassic collision of the accretionary margin of the Central Asian Orogenic Belt (CAOB) with the Tarim Craton. In order to elucidate this controversy, we present new P-T data as well as Sm-Nd and 40Ar/39Ar cooling ages for an eclogite sample from Atbashi in the accretionary mélange of the South Tianshan suture in Kyrgyzstan, some 500 km along strike to the west of the controversial locality in the upper Akeyazhi River Valley in NW China. A clockwise P-T path for the eclogite with peak pressures of 18 to 24 kbar at 520 to 600 °C is consistent with near-isothermal decompression and exhumation in a subduction zone before collision of the CAOB with the Tarim Craton. Geochemical data and an initial εNd value of ∼ +9 suggest an N-MORB protolith for the eclogite. The high-pressure mineral assemblage of the eclogite yielded a statistically robust Sm-Nd isochron age of 319 ± 4 Ma (2σ, 5 data points, MSWD = 0.4) for equilibration and closure of the Sm-Nd system during HP metamorphism. 40Ar/39Ar dating of phengite from the same sample yielded a cooling age of 316 ± 3 Ma (2σ) implying rapid exhumation. Docking of the Tarim Craton with the southern margin of the Middle Tianshan-North Tianshan blocks in Kyrgyzstan during the late Carboniferous is supported by widespread emplacement of A-type granitoids of early Permian age that suggest a setting of consolidated crust. An unmetamorphosed and little deformed molasse-type conglomerate of latest Carboniferous age, overlying the HP rocks, indicates that HP metamorphism, exhumation, and exposure of the HP mélange occurred from 320 to ∼300 Ma. The detrital zircon age spectrum of a metagraywacke sample from the accretionary mélange suggests sources in the Tarim Craton and/or from the Middle and North Tianshan that possibly comprise rifted blocks from Tarim.

Fast tectonometamorphism and exhumation in the type area of the Barrovian and Buchan zones

The beginning of the Grampian episode of tectonometamorphism in Scotland is dated by collision and obduction of the Ballantrae Ophiolite Complex at 478 ± 8 Ma. For the first time we have used zircon and garnet to radiometrically date the peak of the Caledonian Buchan and Barrovian tectonometamorphism in their type areas as contemporaneous at 467 ± 2.5 Ma. Detrital garnet from the neighboring Southern Uplands flysch terrane has the same radiometric age and occurs in 465 ± 2.5 Ma molasse in the Midland Valley. We show that the entire Grampian episode lasted 15 ± 2.5 m.y. and that it took fewer than 7.6 m.y. to expose high-grade rocks following the peak of metamorphism. Those who study Precambrian orogeny should note the brevity of this Scottish tectonometamorphic episode and the speed of exhumation.

Geochemistry, single zircon ages and Sm–Nd systematics of granitoid rocks from theGóry Sowie (Owl Mts), Polish West Sudetes: evidence for earlyarc-related plutonism

Granitoid gneisses as well as their migmatitic and anatectic derivates were investigated from theGóry Sowie (Owl Mts) Massif of SW Poland in the central West Sudetes. The gneisses and migmatites aretectonically interlayered with paragneisses, and experienced several consecutive tectono-metamorphicevents. Geochemically, the granitoid gneisses are calc-alkaline and similar to orogenic granite suites, whichtherefore lends support to a subduction-related origin. Single zircon Pb–Pb evaporation ages suggest that the gneiss precursors were emplaced between 473 and488 Ma, and most samples analysed contain zircon xenocrysts with minimum ages between 1124 and2620 Ma. An early phase of high-grade metamorphism is documented by an age of c. 440 Ma for ananatectically derived granite. Two late Variscan granites have an age of c. 333 Ma. Nd isotope systematicssuggest variable involvement of crustal material in the generation of the granitoids. We speculate that the Góry Sowie Massif was part of Avalonia in early Palaeozoic times, derived fromthe northwestern margin of Gondwana, probably northern South America. Granitoid emplacement in theOrdovician took place along a Japan-type active continental margin while Avalonia drifted towardsBaltica and the Tornquist Ocean was consumed. Collision with Baltica occurred in mid-Silurian times. Our data are compatible with an active magmatic arc extending from southern England to thesoutheastern Sudetes in Cambrian and Ordovician times.

Chemical and isotopic variations along the superfast spreading East Pacific Rise from 6 to 30°S

Chemical data of 39 fresh basaltic glasses from the East Pacific Rise (EPR) between 6 and 30°S and Pb, Sr, and Nd isotopic compositions of 12 basalt glasses are presented. Major and trace element data indicate a wide compositional range, including primitive basalts (Mg#=0.67) and highly evolved FeTi-basalts (Mg#=0.34) [molMg/(Mg+Fe2+)]. The compositional range can be attributed to low-pressure fractional crystallization. Fractionation-corrected major element concentrations provide evidence for varying mantle melting conditions. Calculations of the melting conditions suggest melt generation in a rising upper mantle column between 20 and 10 kbar, at temperatures between 1430 and 1280°C, and total degrees of partial melting between 17 and 20% by weight. Leached and hand-picked basalt glasses display large variations in 87Sr/86Sr (0.70235–0.70270), 143Nd/144Nd (0.51312–0.51323), and 206Pb/204Pb (18.064–18.665), but are similar to other N-type MORB from the EPR. The isotopic ratios of basalts from 13 to 23°S show strong correlations and delineate two systematic trends. From 23 to 17°S, 87Sr/86Sr and Pb isotope ratios increase and 143Nd/144Nd decrease in agreement with previous results (Mahoney et al. 1989). A reverse trend is indicated by basalts from 17 to 13°S. However, K/Ti and (La/Sm)N continuously increase from 23 to 13°S. This opposite behavior indicates a recent decoupling of isotopic and minor element ratios in the mantle between 13 and 17°S. North of 13.5°S (Garrett Fracture Zone), isotopic data show no systematic variation with ridge location and display an overall weaker covariation. The results suggest that the isotopic variations and ridge segmentation appear to be unrelated and that major ridge offsets apparently coincide with changes in mantle melting conditions (P, T, F) (F, degrees of melting). There is no evidence for a systematic relationship between calculated melting conditions and second order ridge segmentation. Our isotopic data provide further evidence for regionally confined chemical variations in the mantle at 5 to 30°S. We interpret the isotopic trends as reflecting melting of distinct smallvolume and old enriched mantle components. In contrast, variations in trace elements are attributed to young mantle differentiation processes.

Isotopic composition of recent shark teeth as a proxy for environmental conditions

Abstract The O, C, and Sr isotope compositions of teeth from ten species, belonging to five families, and three orders of sharks were measured to determine the influence of habitat, diet, and possible species-specific fractionation effects on the isotopic composition of biogenic phosphate from fish. The sharks were recently caught in subtropical waters off the KwaZulu-Natal (KZN) coast of South Africa, as well as from cold waters in Prince William Sound (PWS), Alaska, and Victor Bay (VB), Nunavut, Canada. δ 18 O values of tooth phosphate (δ 18 O P ) range from 20.9 to 23.5‰ for the KZN sharks. For most species the range in measured δ 18 O P values is about 0.6‰, but it may be as high as 1.1‰ for different teeth from a single shark. Dentine and enameloid within individual teeth have no apparent differences in δ 18 O P values. The δ 18 O P values of the KZN shark teeth reflect the typical habitat of the studied species, primarily the thermal structure of the water column off KZN at depths between 20 and 280 m. The δ 18 O P values of teeth from different Greenland sharks from VB and Pacific sleeper sharks from PWS are very homogeneous, averaging 25.8 and 24.7‰, respectively. These values appear to be in equilibrium with deep (>500 m) ocean waters in each case at temperatures of about −0.3°C or less. There is little discernable evidence for species-specific fractionation effects for the oxygen isotope composition of phosphate in the studied marine fish. The oxygen isotope composition of carbonate in apatite averages about 9.1‰ higher than corresponding δ 18 O P values, in agreement with equilibrium fractionation between carbonate and phosphate, but with a large variance (1σ = ±1.5‰). δ 18 O C values also vary by up to 1‰ between enameloid and dentine within single teeth, but in a non-systematic way. Differences in δ 13 C values between carbonate in enameloid and dentine is also large (up to 8‰) but the δ 13 C values vary systematically. Enameloid is always enriched in 13 C compared to dentine and the 13 C content increases with developmental stage of the teeth. δ 13 C values measured for enameloid (1.6 to 4.8‰) appear to approach equilibrium with dissolved inorganic carbon in seawater. In contrast, δ 13 C values for dentine range from −6.4 to −2.3‰ for KZN sharks, and −9.0 to −10.8‰ for the cold-water sleeper sharks, and are compatible with a predominantly dietary carbon source. The 87 Sr/ 86 Sr ratios of teeth from KZN sharks as well as those from PWS and VB are uniform, averaging 0.709167. Sr content varies from 1270 to 2100 ppm, a range that is similar to that in well preserved fossilized teeth. Seawater Sr is thus clearly incorporated in vivo. Concentrations of Sm and Nd are in the ppb range and contrast the ppm range in fossilized teeth, indicating a postmortem incorporation of rare earth elements in apatite of the teeth.

Single zircon ages, PT evolution and Nd isotopic systematics of high-grade gneisses in southern Malawi and their bearing on the evolution of the Mozambique belt in southeastern Africa

The high grade gneiss assemblage of central and southern Malawi belongs to the Neoproterozoic Mozambique belt of East Africa, and reached peak metamorphic conditions at 900±70°C and 9.5±1.5 kbar, followed by an isobaric cooling path. We report single zircon U–Pb and Pb–Pb ages and Nd isotopic data for orthogneisses and metapelites collected around Lilongwe and farther south in the region around Blantyre and Zomba. The ages document three distinct events, (1) a Kibaran-age period of intrusion of calc-alkaline granitoids around 1040–929 Ma; Nd isotope data indicate overall juvenile compositions consistent with a magmatic arc environment, or emplacement into thinned continental crust with little involvement of older basement; (2) a Pan-African period of intrusion of calc-alkaline granitoids around 710–555 Ma; Nd isotopes for most samples indicate crustal residence ages of 1.0–1.5 Ga and suggest either remelting of ∼1.0–1.5 Ga (Kibaran) protoliths or mixing of juvenile material with subordinate amounts of older crust; (3) a long-lasting thermal peak of Pan-African high grade metamorphism around 571–549 Ma. The chemical composition of the dated orthogneisses is compatible with an origin of their protoliths in an Andean-type active continental margin setting. Penetrative deformation occurred prior to the thermal peak of metamorphism and also affected the youngest intrusions. It must, therefore, have been of Pan-African age as was the late, retrograde amphibolite facies overprint. Rocks with presumably rather different histories prior to the thermal peak were stacked together. An earlier Kibaran granulite metamorphism, postulated by previous authors, could not be verified. Rather uniform and widespread maximum PT-conditions of the granulite facies in central and southern Malawi, followed by near-isobaric, slow cooling indicate a transient metamorphic gradient of about 26°C/km and suggest stacking of a relatively hot crust and subsequent slow exhumation. The late Kibaran and Pan-African magmatic events in Malawi may be linked to granitoid magmatism of similar ages in northwestern Mozambique and the central Zambezi belt of northern Zimbabwe, but the geodynamic relationships between these terrains remain obscure. The variation in age of peak metamorphism between ∼640 and 520 Ma across the Mozambique belt from Malawi to Antarctica suggests that this belt was assembled from a number of terranes that accreted at different times over a period of ca. 100 million years. The available data, therefore, suggest that East Gondwana was not a coherent block colliding with West Gondwana but consisted of individual terranes before being amalgamated into the supercontinent Gondwana some 550–530 Ma ago.

Age and evolution of late Mesozoic metamorphic core complexes in southern Siberia and northern Mongolia

Numerous Cretaceous metamorphic core complexes (MCCs) extend from Transbaikalia in Russia to northern Mongolia within the Central Asian Orogenic Belt. We investigated the Buteel and Zagan MCCs in detail. Shear sense indicators in mylonitized rocks show footwall-to-the-NW tectonic transport. Single zircon dating of footwall rocks in the Buteel MCC establishes the emplacement of granitoid orthogneiss precursors at 240–211 Ma, a felsic metavolcanic rock at 265.0 ± 1.2 Ma, a syenite at 265.5 ± 1.2 Ma and a metarhyolite of the pre-granitoid basement at 553.6 ± 2.9 Ma. A peralkaline granite intruding orthogneisses of the Zagan MCC has a new U–Pb zircon age of 151.6 ± 0.7 Ma. 40Ar/39Ar ages of 133.5 ± 1.8 Ma of hornblende from amphibolite and 122.6 ± 1.8 Ma of biotite from mylonitized gabbro–dolerite of the Buteel MCC are interpreted as cooling ages representing the time of deformation in the footwall. Geological data suggest that the MCCs in Transbaikalia and northern Mongolia formed as a result of extension in a crust that had previously been thickened by abundant calc-alkaline magmatism in an Andean-type setting on the border of the closing Mongol–Okhotsk ocean, by widespread collisional to post-collisional thrusting, and by extensive alkaline–peralkaline magmatism.

Ca. 750–1100 Ma magmatic events and Grenville-age deformation in Sri Lanka: relevance for Rodinia supercontinent formation and dispersal, and Gondwana amalgamation

Abstract Large volumes of ∼880–1100 Ma calc-alkaline granitoid rocks in the Wanni and Vijayan crustal provinces of Sri Lanka make it likely that these domains were produced in active margin settings, probably Grenville-age magmatic arcs. We report new single zircon evaporation ages and Nd isotopic systematics for dioritic to granodioritic gneisses of the Kadugannawa Complex of central Sri Lanka which record a period of magmatic arc activity between 1006 and 881 Ma and show this complex to be part of the Wanni domain. Both provinces were probably generated in arc-related settings at the outer margin of Rodinia, but this cannot be ultimately proven on the basis of the currently available data. In spite of a strong Pan-African tectono-thermal overprint at ∼600–550 Ma that we relate to involvement of Sri Lanka in the amalgamation of the Gondwana Supercontinent, rare ∼1000–900 Ma structures are preserved in the gneisses of the Kadugannawa Complex that may constitute remnants of the accretion history of Rodinia. We further suggest that the basement rocks of Sri Lanka do not correlate with crustal domains in southern India or in East Africa, since these were not part of Rodinia.