Armin Zeh

Hafnium isotope record of the Ancient Gneiss Complex, Swaziland, southern Africa: evidence for Archaean crust–mantle formation and crust reworking between 3.66 and 2.73 Ga

Abstract: Combined U–Pb and Lu–Hf isotope analyses of zircons from 16 tonalite–trondjemite–granodiorite (TTG) gneiss and granite samples from Swaziland reveal that the oldest rocks of the Ancient Gneiss Complex in southern Africa formed by reworking of Early Archaean or perhaps Late Hadean crust at 3.66 Ga, and that new crust was extracted from a depleted mantle source during Palaeoarchaean events between 3.54 and 3.32 Ga. This interpretation is supported by εHft of −1.6 ± 2.0 obtained from 3.66 Ga TTG gneisses, corresponding to hafnium model ages between 3.77 ± 0.18 Ga, for a presumed Hadean–Early Archaean chondritic mantle, and 4.08 ± 0.18 Ga, for a presumed Hadean depleted mantle reservoir, with the first model age being the most likely in the light of recent data from worldwide sources. Furthermore, it is reflected by superchondritic εHft up to +2.2 ± 2.0 for TTGs formed at 3.54, 3.45 and 3.32 Ga. The new datasets additionally show that the Palaeoarchaean crust formed between 3.54 and 3.32 Ga was intensely reworked afterwards, without significant addition of depleted mantle derived material, during orogenic and intracratonic melting processes at 3.23, 3.1 and 2.7 Ga. This is well reflected by an array of decreasing εHft from +2.2 to −7.2 between 3.3 and 2.7 Ga, which can be forced by 176Lu/177Hf of 0.0113, which is similar to that of present-day average continental crust, and might result from lower crust zircon fractionation during Archaean crust reworking. Supplementary material: Results of in situ U–Pb and Lu–Hf isotope zircon analyses and concordia diagrams are available at www.geolsoc.org.uk/SUP18465.

A combined zircon SHRIMP and Sm-Nd isotope study of high-grade paragneisses from the Mid-German Crystalline Rise: evidence for northern Gondwanan and Grenvillian provenance

SHRIMP analyses of detrital zircon cores from high-grade metasediments from the Mid-German Crystalline Rise, which is situated between Eastern Avalonia in the NW and Saxo-Thuringia in the SE, yield mostly ages of c. 550 Ma and c. 2.06 Ga, with minor c. 1.0 and 2.4–2.9 Ga components. The sedimentary protoliths were deposited during the Late Proterozoic to Early Cambrian, probably prior to the break up of the northern Gondwana margin at 460–500 Ma. These data are consistent with the sediments’ high ϵNd values (0.9 to −3.0), which are comparable to those of well-documented Late Proterozoic sediments from other parts of Europe. The combined isotopic data suggest derivation of the sediments from at least three distinct crustal source regions. Dominant sources were the Avalonian–Cadomian orogenic belt (c. 45%), situated at the northern margin of Gondwana during the Neoproterozoic, and the West African and/or eastern Amazonian cratons (c. 45%). The Grenvillian belt was a minor source (c. 10%).

U–Pb and Hf isotope data of detrital zircons from the Barberton Greenstone Belt: constraints on provenance and Archaean crustal evolution

Combined U–Pb and Hf isotope analyses of detrital zircons from the Fig Tree and Moodies Groups of the Barberton Greenstone Belt, South Africa, yield similar Hf isotope compositions and age populations, thus pointing to a similar provenance. Zircon populations of Fig Tree Group greywacke and Moodies Group quartzarenite are both dominated by age clusters at 3.53, 3.47, and 3.28 Ga, and a minor cluster at 3.36 Ga. The Moodies quartzarenite sample additionally contains a younger age population at 3.23–3.19 Ga. Hafnium isotope data indicate that the source area of both sediments was affected by new crust formation from depleted mantle sources at 3.53, 3.47, and perhaps at 3.36 Ga (ϵHft between −1.7 and +4.5), accompanied by partial reworking of an Eoarchaean crustal component as old as 3.75–3.95 Ga. In contrast, crustal reworking was the predominant process between 3.28 and 3.22 Ga (ϵHft between −6.0 and +0.9), probably related to subduction and collision of terranes along the Inyoka Fault system. The zircon U–Pb and Hf isotope datasets favour a southern provenance for the Fig Tree and Moodies sediments, comprising granitoids in the vicinity of the southern Barberton Greenstone Belt and in Swaziland. This finding is in contrast to the sedimentary record of the Moodies Group, which mostly suggests a northern and along-strike provenance. This discrepancy may be due to reworking of sediments during extensive syn- and postorogenic strike-slip faulting and high uplift or subsidence between 3.26 and 3.19 Ga. Supplementary material Results of in situ U–Pb and Lu–Hf isotope zircon analyses are available at www.geolsoc.org.uk/SUP18561.

Petrological evolution in the roof of the high-grade metamorphic Central Zone of the Limpopo Belt, South Africa

In this study we present new petrological results from the Endora Klippe in the Central Zone of the Limpopo Belt, which may result from horizontal tectonics during the Proterozoic at c. 2.0 Ga. Microstructures, assemblages and garnet zonation patterns observed in metapelitic rocks provide evidence that the Endora Klippe rocks underwent a contemporaneous pressure-temperature increase from c. 600 ◦ C/5 kbar to 650 ◦ C/6.5 kbar. This is inferred by the use of conventional geothermobarometry and interpretations based on quantitative phase diagrams in the system MnO- (TiO2)-(CaO)-(Na2O)-K2O-MgO-Al2O3-SiO2-H2O. Thus, the petrological results indicate that this part of the Central Zone only underwent a medium-grade metamorphic overprint during a single orogenic event and was never affected by granulite-facies metamorphism, as reported from other parts of the Limpopo Belt. The inferred P-T path, in combination with previous structural and petrological results, leads to the conclusion that the area surrounding the Endora Klippe forms the roof zone of the c. 2.0 Ga old granulite-facies rocks forming wide parts of the Limpopo Central Zone.

In Situ Sr isotopes in Plagioclase and Trace Element Systematics in the Lowest Part of the Eastern Bushveld Complex: Dynamic Processes in an Evolving Magma Chamber

The Bushveld Complex is the largest mafic layered intrusion on Earth, containing immense mineral resources. Despite its importance there remains uncertainty as to its origin and the source of the magmas that formed it. The lower Bushveld series, made up of the Basal Ultramafic Sequence (BUS), the Marginal Zone and Lower Zone, is of particular importance because it allows detailed insight into the reservoirs that multiply contributed to the formation and modification of the initial magmas. This sequence is represented by three overlapping drill cores (2100 m of stratigraphy) in the eastern Bushveld Complex for which we present the first comprehensive study on intercumulus mineral assemblages, incompatible trace elements and in situ Sr isotopes in plagioclase from over 130 core samples for this section (Clapham area). The intercumulus mineral assemblage that crystallized from the trapped melt comprises plagioclase, K-feldspar, biotite and quartz and a wide variety of accessory phases, including zircon, loveringite and primary magmatic anhydrite, the first time the last mineral has been reported in the layered sequence of the Bushveld Complex. The incompatible trace elements (ITE) show a gradual decrease in concentration upwards through the stratigraphic section, reflecting mainly the decreasing trapped melt component in the cumulates as the chamber became established. ITE ratios and changes in initial Sr/Sr [referred to as (Sr/Sr)i at 2055 Ma] give insight into the various mantle sources and the contaminants, which were derived from both the lower crust and the country-rock sediments of the Pretoria Group (upper crust). (Sr/Sr)i ranges from 0 7042 to 0 7076, with the lowest values associated with the most ultramafic rock units in the BUS and the highest in Marginal Zone norites that formed from evolved magma at the top of the early chamber prior to ingression of Lower Zone magma. The evolved magma was affected by significant assimilation of sediments as evidenced by partly digested (now restitic) metapelite xenoliths, particularly in the upper part of the Marginal Zone. Rare earth element (REE) patterns combined with modelling of trace elements and (Sr/Sr)i confirm the strong crustal contamination derived from both the lower crust of the Kaapvaal Craton and the enclosing sediments. The Lower Zone shows a progressively increasing mantle signature upwards in the sequence with both flat and steep light REE (LREE) patterns indicating primitive (but still crustally contaminated) and more highly contaminated melts. However, the relatively low Mg# for orthopyroxene and olivine (maximum Mg# 0 88) in the Lower Zone requires a melt contribution of lower Mg# than would be associated with an asthenosphere mantle source and at the same time having an LREE-depleted composition. Such a source is indicative of subcontinental lithospheric mantle (SCLM) and a progressively greater melt component of an eclogitic protolith contributing to the parental magmas. The source of heat for this massive, but short-lived, melting event is likely to VC The Author 2017. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com 327 J O U R N A L O F P E T R O L O G Y Journal of Petrology, 2017, Vol. 58, No. 2, 327–360 doi: 10.1093/petrology/egx018 Advance Access Publication Date: 10 May 2017

Timing of Upper Carboniferous-Permian horst-basin formation and magmatism in the NW Thuringian Forest, central Germany: a review

Abstract During Late Carboniferous-Early Permian times dextral transtensional movements along the NW-trending Franconian Fault System and parallel faults caused complex block faulting in the Thuringian Forest region, Germany, accompanied by intense magmatism. This is well constrained by geochronological data (207Pb/206Pb single zircon, SHRIMP, 40Ar/39Ar mica, zircon fission-track ages), field relations, and the sedimentary record from the Ruhla Crystalline Complex (RCC) and surroundings. The combined dataset indicates that the Ruhla Crystalline Complex was faulted into three nearly N-S-trending segments, which underwent different exhumation histories during Late Carboniferous-Permian times. The central segment of the RCC was exhumed by several kilometres as a horst block, while the eastern and western segments subsided simultaneously, forming the basement to the Oberhof and Eisenach molasse basins, respectively. Late Carboniferous-Permian uplift of the central segment is constrained by 40Ar/39Ar cooling ages of 311 ± 3 (muscovite) and 294–288 ± 3 Ma (biotite), a weighted zircon fission-track age of 272 ± 7 Ma and overlying Zechstein sediments. In contrast, the eastern segment shows much older 40Ar/39Albiotite cooling ages between 336 ± 4 and 323 ± 3 Ma, was exposed at c. 300 Ma, and subsequently covered by molasse sediments and volcanic rocks between 300 and c. 275 Ma. A similar Late Carboniferous evolution is inferred for the western segment, as it is also overlain by Lower Permian volcanic rocks and has a 297 ± 29 Ma single zircon fission-track age. Simultaneous horst and basin formation is additionally constrained by granite pebbles in conglomerates of the Oberhof and Eisenach basins. These pebbles can partly be derived from granites in the central segment of the RCC. Age data and the orientation of granitoid bodies and dykes in the Ruhla Crystalline Complex and its surroundings provide evidence for the opening of NE-trending structures between 300 and 294 Ma, and formation or reactivation of W- to NW-trending structures between 290 and 275 Ma. Magmatic activity in the Thuringian Forest region may have been caused by widespread mantle upwelling in central Europe during the Late Carboniferous-Early Permian.

Celebrating the Centenary of “The Geology of Central Minas Gerais, Brazil”: An Insight from the Sítio Largo Amphibolite

One century ago, in 1915, Harder and Chamberlin established the stratigraphy on which the geology of the Quadrilátero Ferrífero of Minas Gerais is based. One of their key observations was the occurrence of metavolcanic rocks in spatial association with the Itabira iron formation, an observation that has been ignored by most authors. Crucial for the understanding of the geology of the Quadrilátero Ferrífero, but equally ignored, is a 400-m-thick unit of amphibolite that rests immediately on the Itabira iron formation, also known as the Cauê Itabirite. Here, we revitalize the Sítio Largo amphibolite by indicating its basaltic affiliation. We present new results of U-Pb dating of zircon and rutile that show that the amphibolite protolith, which covered an erosional surface on the Cauê Itabirite, is ca. 2.18 Ga old and underwent metamorphic overprint at ca. 500 Ma.

Mafic magmatism in the Bakhuis Granulite Belt (western Suriname) : relationship with charnockite magmatism and UHT metamorphism

Abstract The Bakhuis Granulite Belt (BGB) is a metamorphic terrain within the Guiana Shield that experienced ultrahigh-temperature (UHT) metamorphism at 2.07–2.05 Ga. In the southwest of the BGB, the Kabalebo charnockites were emplaced at ca. 1.99 Ga and thus postdate UHT metamorphism by at least 60 Myr. Two generations of gabbroic intrusions have been recognized within the BGB, which could act as a heat source for the two UHT events. A younger generation of tholeiitic “Charlie” gabbros yields a baddeleyite U/Pb age of 1971 ± 15 Ma. The presence of a metamorphic overprint indicates that the hornblende-bearing “Moi–Moi” metagabbros predate the Charlie gabbros. Large zircons with complex zoning patterns are found in a Moi–Moi metagabbro sample. The main growth domains of these zircons give an age of 1984 ± 4 Ma, which is indistinguishable from the surrounding charnockites. Matching trace element and Hf isotope characteristics indicate that the complex zircons are derived from the charnockites. We argue that the emplacement of the metagabbros and charnockite magmatism were contemporaneous and that zircon grains from the charnockitic melt were mechanically transferred to the gabbroic bodies during magma mingling. The new ages for the gabbroic bodies in the BGB confirm that they are contemporaneous with, and the likely heat source for, charnockite magmatism, but that they are not associated with the 2.07–2.05 Ga UHT event. Furthermore, the new ages and recognition of the Moi–Moi metagabbros as an Alaskan-type complex provide the first direct evidence for late Transamazonian subduction zone magmatism in the Guiana Shield.

Post-peak re-equilibration in a mafic gneiss from the KTB main hole: implications for the metamorphic evolution

Abstract Detailed microprobe investigations were carried out on mineral phases of a mafic garnet–biotite–hornblende and garnet–biotite gneiss from the KTB main hole. The results indicate that the complex zonation patterns of garnet porphyroblasts are mainly due to post-growth diffusional exchange, as constrained by theoretical modelling. Moreover, the chemical composition of biotite and hornblende enclosed in garnet were also changed by later diffusion and retrograde net-transfer reactions. Therefore, the garnet zonation in combination with mineral inclusions cannot be used to reconstruct the prograde metamorphic evolution or to estimate the maximum temperatures attained. However, by applying the GRISP geobarometer, minimum pressures of 8.5 kbar (at 700 °C) can be derived for the metamorphic peak, well conforming to previous estimates on the amphibolite-facies metamorphic stage of the KTB metamorphics. Mineral assemblages formed during the retrograde evolution allow to estimate P–T–X conditions of approximately 3 kbar, 450 °C and X(H2O)=0.9.