Description: 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.

Description: 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 Nd +0.03 ± 4.1) and 2760 ± 120 Ma (initial Hf +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 values cluster around +1, values range from +6.7 to +24.1, substantially more radiogenic than those of ambient Neoarchean mantle and continental crust. Such an Hf - Nd 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.

Description: The earliest compounds forming Earth's 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 Earth's 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 Earth's oldest continental crust is therefore best explained by melting within tectonically thickened mafic island-arc crust.

Description: Paleoenvironmental studies and climate models demonstrate that fluvial runoff and moisture availability in the Caribbean hinterland react very sensitively to climatic variations. Late Pleistocene and Holocene climate records document pronounced dry and wet periods over tropical South America mainly caused by shifts of the Intertropical Convergence Zone (ITCZ). However, forcing mechanisms for changes in the ITCZ position remain controversial. Here we present high-resolution foraminiferal Ba/Ca and 18 O seawater records from a core located within the Orinoco River outflow documenting abrupt hydrological changes in the Orinoco catchment area during the deglacial and Holocene. Our data, obtained from the surface-dwelling foraminifera Globigerinoides ruber (pink), show an abrupt increase in Ba/Ca ratios in the early Holocene, starting ~600 yr after the end of the Younger Dryas (YD) cold interval at ca. 10.8 ka and suggesting a massive reorganization of moisture sources in northern South America. In contrast, the salinity dependent 18 O seawater from the same samples shows a gradual decrease starting at the end of the YD. The offset of our Ba/Ca peak excludes meltwater release in conjunction with the northern Andean glacier retreat well before the end of the YD as a forcing mechanism. We suggest that the Ba/Ca record documents an abrupt increase in Ba-rich waters of a northern Andean source caused by the insolation-driven shift of the ITCZ and/or enhanced monsoon activity.