J. Elis Hoffmann

Generation of Eoarchean tonalite-trondhjemite-granodiorite series from thickened mafic arc crust

The earliest compounds forming Earth9s first continental crust were magmatic rocks with tonalitic-trondhjemitic-granodioritic composition (TTGs). TTGs are widely seen as originating from melting of hydrated oceanic crust in subduction zones. Alternative models argue that they may have formed by melting within thickened mafic oceanic protocrust. To simulate formation of Eoarchean TTGs in different tectonic regimes, we combine for the first time the thermodynamic calculation of residual assemblages with subsequent modeling of trace element contents in TTGs. We compare water-absent partial melting of two hydrated starting compositions, a modern mid-oceanic-ridge basalt (MORB) and a typical Eoarchean arc tholeiite from the Isua Supracrustal Belt that represents the country rock of Earth9s oldest TTGs in southern West Greenland. At 10 kbar, partial melting of MORB-like residues results in modeled TTG compositions that are very different from natural ones. Melting at higher pressures (14 and 18 kbar) leads to a better match, but several key trace element parameters in TTGs are still amiss. A perfect fit for trace element compositions is achieved by melting of Eoarchean arc tholeiites at 10 and 14 kbar. These protoliths contain less Al and Na and more Fe and Mg as compared to present-day MORB and form amphibole-rich and plagioclase-free residues even at low pressures. Formation of Earth9s oldest continental crust is therefore best explained by melting within tectonically thickened mafic island-arc crust.

Decoupled Hf-Nd isotopes in Neoarchean seawater reveal weathering of emerged continents

Marine chemical sediments from the Temagami banded iron formation (BIF) in Canada exhibit nonchondritic Zr/Hf and Y/Ho ratios and seawater-like rare earth element patterns, indicating that their Hf and Nd are not detrital, but derived from seawater. This is confirmed by Sm-Nd and Lu-Hf isochron ages of 2605 ± 140 Ma (initial eNd +0.03 ± 4.1) and 2760 ± 120 Ma (initial eHf +7.2 ± 5.3), respectively, that overlap within error the 2.7 Ga U-Pb age of associated igneous rocks. The Temagami BIF is therefore an excellent archive of the Nd-Hf isotopic composition of Neoarchean seawater. Whereas ![Graphic][1] values cluster around +1, ![Graphic][2] values range from +6.7 to +24.1, substantially more radiogenic than those of ambient Neoarchean mantle and continental crust. Such an eHf-eNd distribution is typical of modern seawater, plotting above the terrestrial array as defined by igneous and clastic sedimentary rocks. The only mechanism known to produce natural waters with decoupled Nd and Hf isotope compositions is the incongruent mobilization of Hf from continental crustal material. Therefore, input of such highly radiogenic Hf into seawater requires substantial amounts of evolved Neoarchean continental crust that was exposed above sea level and available to erosion and terrestrial weathering. [1]: /embed/inline-graphic-1.gif [2]: /embed/inline-graphic-2.gif

Paleo- to Mesoarchean polymetamorphism in the Barberton Granite-Greenstone Belt, South Africa: Constraints from U-Pb monazite and Lu-Hf garnet geochronology on the tectonic processes that shaped the belt

The Barberton Granite-Greenstone Belt (BGGB) of South Africa is an exceptionally well preserved Meso-Paleoarchean metamorphic supracrustal belt, one of only a few in the world. Studies of metamorphism in the BGGB have considerable potential to advance our understanding of tectonic processes in the Archean crust. Two current hypotheses persist to explain the origin of amphibolite-facies metamorphism in the southern BGGB. The first interprets these rocks to be the consequence of accretionary tectonics, while the second proposes a “dome-and-keel” vertical tectonic process driven by sinking of greenstone layers and the doming of the underlying granitoid crust. In this study, metamorphic pressure-temperature ( P-T ) analysis has been combined with garnet Lu-Hf and monazite U-Pb geochronology to directly date the amphibolite-facies metamorphism within the Stolzburg terrane of the BGGB. A garnet-biotite-chlorite–bearing sample yields a Lu-Hf garnet age of 3233 ± 17 Ma and a garnet-staurolite-kyanite–bearing sample produces a U-Pb monazite age of 3191 ± 9 Ma, whereas an andalusite-kyanite–bearing sample produces a U-Pb monazite age of 3436 ± 18 Ma. Phase diagrams and garnet compositional modeling produce a clockwise P-T evolution, with rocks reaching peak P-T conditions of 8.5 kbar and 640 °C for the ca. 3200 Ma event and minimum peak P-T conditions of ∼4.5 kbar and 550 °C for the ca. 3435 Ma event. The duration of metamorphism for the ca. 3200 Ma event is estimated to be ∼50–20 m.y. based on differences in age between U-Pb and Lu-Hf systems and durations needed to fit models of diffusionally modified garnet chemical zoning. Similarly shaped P-T paths over the Stolzburg terrane indicate that the metamorphism occurred in response to crustal thickening due to an accretionary tectonic process. Thus, the Stolzburg terrane constitutes an orogenic core, exhumed along the Komati fault.

Eoarchean within-plate basalts from southwest Greenland: COMMENT

[Jenner et al. (2013)][1] reported the occurrence of, what they interpret as, Earth’s oldest ocean island basalts (OIBs) on the island of Innersuartuut, southwest Greenland. However, this interpretation hinges critically on the incompatible trace element contents of the presented rocks. Compared

Hafnium isotope constraints on the origin of Mesoarchaean andesites in southern West Greenland, North Atlantic Craton

Abstract Numerous supracrustal belts in southern West Greenland host leucoamphibolites, which commonly preserve volcaniclastic textures, and are interpreted as meta-andesites. Such rocks are associated with mesocratic amphibolites of tholeiitic basaltic compositions, which display pillow-lava structures and, thus, support eruption in an oceanic environment. Here we present bulk-rock Lu–Hf isotope data for meta-andesites from the approximately 3071 Ma Qussuk supracrustal belt. Surprisingly, we find evidence for the involvement of a source with near-chondritic Hf-isotope composition in the meta-andesites, whereas the metabasalts display more depleted compositions, with around +4. Trace element modelling indicates that fractional crystallization in combination with crustal assimilation (AFC) is not capable of producing the geochemical compositions of the meta-andesitic rocks from a basaltic melt. Instead, these meta-andesites point to large degrees (c. 50%) of magma mixing, involving mafic and felsic end members. This may either represent: (1) a magma chamber process; (2) mantle-wedge overprinting by a silicic component; or (3) large degrees of melting of primitive mafic crust. Given that there is abundant independent structural and metamorphic evidence for horizontal tectonics in the Archaean crust of southern West Greenland, it is likely that these calc-alkaline meta-andesites and tholeiitic metabasalts were produced by Mesoarchaean subduction zone volcanism.

Sm-Nd and Lu-Hf isotope and trace-element systematics of Mesoarchaean amphibolites, inner Ameralik fjord, southern West Greenland

Abstract Fragmented supracrustal rocks are typical components of Archaean high-grade gneiss terranes, such as those in the North Atlantic Craton. Here we present the first major, trace element and Nd-Hf isotope data for amphibolites collected in the yet poorly studied southern inner Ameralik fjord region of southern West Greenland. In addition, new U-Pb zircon ages were obtained from the surrounding TTG gneisses. Based on their trace-element patterns, two different groups of amphibolites can be distinguished. Following screening for post-magmatic alteration and outlying e values, a reduced sample set defines a 147Sm/143Nd regression age of 3038 Ma ±310 Ma (MSWD = 9.2) and a 176Lu/176Hf regression age of 2867 ± 160 Ma (MSWD = 5.5). Initial εNd2970Ma values of the least-altered amphibolites range from 0.0 to +5.7 and initial εHf2970Ma range from +0.7 to +10.4, indicating significant isotopic heterogeneity of their mantle sources with involvement of depleted domains as well as crustal sources. Surprisingly, the amphibolites which are apparently most evolved and incompatible element-rich have the most depleted Hf-isotope compositions. This apparent paradox may be explained by the sampling of a local mantle source region with ancient previous melt depletion, which was re-enriched by a fluid component during subduction zone volcanism or alternatively by preferential melting of an ancient pyroxenite component in the mantle source of the enriched rocks.

Archaean granulite-facies paragneisses from central Swaziland: inferences on Palaeoarchaean crustal reworking and a complex metamorphic history

We present a petrographic, petrological, geochemical, and geochronological study (U–Pb/Lu–Hf) on granulite-facies paragneisses of the Mahamba Gneiss Complex in central Swaziland, eastern Kaapvaal Craton. Our data suggest that prograde metamorphism occurred at c. 3.07 Ga. Dating of detrital zircons of a metapelite in combination with geochronological and ambiguous structural relationships with granitoid gneisses suggests two possible scenarios: (1) the time of deposition of the sedimentary protoliths is prior to c. 3.58 Ga; (2) c. 3.58 Ga granitoid crust was the basement for the sedimentary protoliths. Furthermore, enrichment in Ni and Cr in the Mahamba metasediments may originate from erosion of a greenstone terrane similar in composition to the dominantly mafic and ultramafic lithologies of the Barberton Greenstone Belt or Dwalile supracrustal belt. Evidence for an older crustal basement is provided by the oldest detrital zircons, which yield ages up to c. 3.7 Ga. Because the metamorphic conditions of metapelitic rocks have no similarity to P–T paths of modern subduction–collision tectonic settings we conclude that the metamorphic history of the Mahamba Gneiss Complex cannot be explained by this model. We propose that deformation and metamorphism occurred through intracratonic contraction and a thermal event possibly related to Mpuluzi batholith emplacement. Supplementary material: Sample localities, mineral chemistry, analytical methods, and mineral equilibria modelling method are available at www.geolsoc.org.uk/SUP18786.