The timing of sub-solidus hydrothermal alteration in the Central Zone, Limpopo Belt (South Africa): Constraints from titanite U–Pb geochronology and REE partitioning

In the Central Zone of the Limpopo Belt (South Africa), Palaeoproterozoic granulite-facies metamorphism was superimposed on an earlier Archaean orogenic history. Previously determined ages of  2030–2020 Ma obtained from high-temperature chronometers (zircon, garnet, monazite) are generally thought to provide the best estimate of the peak of Palaeoproterozoic granulite-facies metamorphism in the Central Zone, whereas ages as young as  2006 Ma from late melt patches suggest that temperatures remained above the wet solidus for an extended period. We present a new MC-ICP-MS ²⁰⁷Pb–²⁰⁶Pb age of 2030.9 ± 1.5 Ma for titanite found in amphibolite- to greenschist-facies alteration zones developed adjacent to quartz vein systems and related pegmatites that cut a strongly deformed Central Zone metabasite. This age could potentially date cooling of rocks at this locality to temperatures below the wet solidus. Alternatively, the titanite could be inherited from the metabasite host, and the age determined from it date the peak of metamorphism. Integration of the geochronology with LA-ICP-MS trace element data for minerals from the metabasite, the hydrothermal vein systems and comparable rocks elsewhere shows that the titanite formed during the amphibolite-facies hydrothermal alteration, not at the metamorphic peak or during the greenschist-facies phase of veining. This suggests that high-grade rocks in the Central Zone have cooled differentially through the wet solidus, and provides timing constraints on when Palaeoproterozoic reworking in the Central Zone began. This study illustrates the potential of combined geochronological and high-resolution geochemical studies to accurately match mineral ages to distinct crustal processes.     

From partial melting to retrogression in the Pointe Geologie migmatitic complex: a history of heterogeneous distribution of fluids

In the Pointe Géologie area (66°40 S; 140°00 E; Terre Adélie, East Antarctica), the Paleoproterozoic basement consists in a migmatitic complex of metasedimentary origin. Metasediments underwent a thermal event, leading to the high-grade amphibolite facies assemblages biotite–cordierite–sillimanite and to dehydration melting reactions at 4–6 kbar and 700±50 °C, followed by retrogression in greenschist facies.

In most of the archipelago, K-feldspar gneisses (KFG) are characterized by a Sil+Crd+Kfs+Bt assemblage and many K-feldspar-rich leucosomes. Locally, a spectacular rock type occurs as North dipping bands of about 10 m thick and consists in nodular gneisses (NG) that display less abundant, K-feldspar-poor leucosomes.

Commonly, the retrograde imprint facies is quite weak in KFG and only expressed by sporadic Bt–Ms±And equilibrium assemblage, whereas it developed more extensively in NG. A pseudosection calculated at constant P=4 kbar shows that the differences between NG and KFG assemblages can be considered to be mainly driven by difference in H₂O proportions and much less by differences in FeO/MgO or K₂O/MgO ratios. The hydrated assemblage (Bt–Ms nodules) in NG requires at least 10–20% more H₂O than the Crd+Kfs+Sil/And assemblage does in KFG. Parageneses and mineral compositions indicate that this difference in H₂O occurred early in the history, at least as early as the anatectic stage. Therefore, differences between NG and KFG are related to the variation in partial melting features (water distribution, proportion of melt extraction), which appears to be spatially controlled by cryptic tectonic structures. The particular shape and orientation of NG bands are interpreted as a complex history of melt extraction in the Pointe Géologie area which could involve a two stage melting process.     

Retrogression characteristics of landslides in fine-grained permafrost soils, Mackenzie Valley, Canada

Thirteen landslides (retrogressive thaw flows) were investigated to study the behavior of thaw retrogression in permafrost in the Mackenzie Valley, Northwest Territories (NWT), Canada. Those landslides are all in fine-grained ice-rich permafrost soils. Such landslides usually start from small-scale slope failures followed by retrogressive thaw flows when ice-rich permafrost soils are exposed to the atmosphere. The landslides were marked with survey stakes to measure their retreat rates for the thawing season of 2007. Two correlations are presented: one is between scarp wall height and retreat rate; another is between overall slope angle and retreat rate. It was found that thaw flow retrogression rate increases with increase in scarp wall height and slope angle up to a certain limit. It was also confirmed that thaw flow retrogression is not influenced by slope orientation.