The significance of Cenozoic magmatism from the western margin of the eastern syntaxis, southeast Tibet

The major and trace-element geochemistry, Sr–Nd bulk-rock isotopes, U–Pb zircon chronology and Lu–Hf isotopic compositions are described for three granitic bodies which intrude the Nyingchi gneisses (Lhasa terrane) along the western margin of the eastern Himalayan syntaxis. The Bayi two-mica granite and Lunan granite–granodiorite were intruded at 22 ± 1 and 25.4 ± 0.3 Ma, respectively, whereas the Confluence biotite granite was emplaced at 49.1 ± 0.4 Ma. All share strong depletions in Y and HREE requiring a garnet-bearing source both during and following the Eocene collision of the Indian plate with the Lhasa terrane. The isotope geochemistry of these intrusives (ε _Nd(t) = −3 to −5, ⁸⁷Sr/⁸⁶Sr_(t) = 0.706–0.707) indicates a crustal source within the Lhasa terrane. Sr–Nd systematics of the garnet-bearing Nyingchi gneisses together with the U–Pb and Lu–Hf isotopic ratios of detrital zircons recovered from this unit identifies it as a potential melt source. The combined element and isotope geochemistry of the plutons indicate a mixed source; the gneisses provide the older component whereas the Gangdese batholith provides a younger, siliceous component. The involvement of garnet-bearing crustal material in melt sources from the Cretaceous (80 Ma) to the Miocene (20 Ma) is consistent with the presence of a thicker continental crust in the eastern Lhasa terrane, as is the presence of magmatic epidote in several plutons which indicates a regional deepening level of exposure eastwards. Post-collision crustal melting is synchronous with proposed slab break-off during the early Miocene, suggesting advective heating by rising asthenospheric melts.     

Fragility of a dark gray shale in northeastern Jamaica: effects and implications of landslip exposure

Rapid weathering of strong, hitherto un-weathered shale rocks was fast enough to have been measurable weekly (>4 mm depth per week). Regular early morning rain showers followed by hot, dry afternoons caused frequent wet/dry cycles which facilitated rapid slaking of the shale. Other landslips occurred concurrently on the south bank of the Wild Cane River, where the rock beds dip towards lower ground. Resistance to impacts in the rock fabric was equally low after decades, or weeks of exposure. It was concluded that the combination of high frequency wet/dry cycles on steeply dipping shale beds, and rock attitude caused very rapid weathering. Rapid slaking of the illitic shales could cause high turbidity levels in contiguous aquatic environments. Building foundations and roads could be rapidly de-stabilized even on such freshly exposed, apparently consolidated sites.