Évènements

Climate and Earthquakes Recorded in Nepalese Lake Sediments
Amphithéâtre du CRPG

Nico Esteves will talk about Magma emplacement and differentiation in highly differentiated granites: Insights from the texture & composition of Nb-Ta oxides.

The talk and discussion will be in english.

Guillaume Caro will talk about Early Archaean onset of volatile cycling at subduction zones.

The talk and discussion will be in english.

“Zircon and apatite from supracrustal units of the Barberton Greenstone Belt, South Africa: geochemical and isotopic constraints on the compositional diversity of the ca. 3.5 Ga-3.2 Ga Paleoarchean continental crust in the Eastern Kaapvaal Craton.”

Detrital zircons and apatites represent key mineral archives as they allow to investigate the composition of continental crust that has not been preserved in the geological record. This is particularly relevant to the Archean context, where large volumes of the Earth’s earliest crust have been lost or reworked, leaving behind metamorphosed supra-crustal sediments/volcanic rocks and Na-rich plutonic bodies of the TTG suite. We conducted a study of detrital zircon and apatite grains sampled at different levels of the stratigraphy of the Barberton Greenstone Belt, South Africa and analyzed them for U-Pb-TREE (trace and rare earth elements; both zircon and apatite) and Hf-O isotopic compositions (zircon). Detrital zircons from Moodies age (ca. 3.2 Ga) show ages between 3.57 to 3.21 Ga and εHf(t) between 0 to -5.0 throughout that range. The negative εHf(t) matches that of zircons of ca. 3.40 to 3.54 Ga felsic volcanic sequences and from 3.56-3.25 Ga granitic pebbles in conglomerate layers [1]; but contrast with the positive εHf(t) of zircons from the Barberton TTG suite (from 0 to + 5.0). The negative εHf(t) zircons also show higher Ti content and Nb/Yb ratio than TTG zircons, supporting their crystallization from a compositionally different magma. Apatites from ca. 3.4 to 3.2 Ga clastic sediments and greywackes show LREE-Sr-Y contents similar to apatites crystallized in modern calc-alkaline suites or sanukitoïds but distinct from those of apatites from the TTG suite. These data collectively suggest that the Paleoarchean continental crust used to be more diverse in composition than the currently exposed, binary, TTG-mafic supra-crustal rocks association of most Archean terranes suggests. Among the “hidden crustal components” revealed by this study, K-rich rocks of granitic/rhyolitic composition seemed to have represented a substantial part of the Archean uppermost continental crust, as they produced about half the detrital zircons analyzed so far in the Barberton Greenstone Belt. The present work suggests instead that the Paleoarchean crust was vertically zoned with a K-rich upper crust (granitic) and a Na-rich middle-crust (TTG) but that only the later part survived to our time.
Amphithéâtre du CRPG

Julien Boulliung will talk about Standard and reference material development for N analyses in Fe-Ni alloys by EPMA.

The talk and discussion will be in english.