Evidence for possible climatic forcing of late-Holocene vegetation changes in Norfolk broadland floodplain mires, UK.

Plant macrofossil analyses of five peat cores from undisturbed fens in the flood-plain of the Ant Valley of the Norfolk Broadland show the sequence of vegetation development during the last two millennia. Macrofossil assemblages have been grouped into five regional phases and are interpreted largely in terms of the response of the vegetation to changes in sea level, climate and management. Phase 1 represents pre-Roman fen woodland communities (>2000 cal. BP); phase 2 represents salt-marsh communities formed during an estuarine phase in Romano-British times (c. 2000–1600 cal. BP); phase 3 represents ‘tussock-fen’ and carr communities suggestive of drier conditions in the post-Roman to early Medieval period (c. 1600–800 cal. BP); phase 4 represents aquatic communities indicative of wetter conditions from the late Medieval period to c. 300 cal. BP; phase 5 represents communities comparable with present-day vegetation. The biostratigraphic development of the Ant Valley floodplain mires has analogues in climatically induced humification changes of some British ombrotrophic mires, suggesting a response to similar climatic controls. Widespread human interference and control of the fen vegetation may be a relatively recent phenomenon (beginning possibly,400 cal. BP). Peat-accumulation rates in the undisturbed mire sites suggest that the original Medieval turbaries which later flooded to form the Norfolk Broads may have been at least 0.5 m shallower when dug than their present depth. The wide range of environmental conditions experienced by the mires during the last two millennia is of relevance to the development of strategies for their conservation

Petrology, bulk-rock geochemistry, indicator mineral composition and zircon U–Pb geochronology of the end-cretaceous diamondiferous mainpur orangeites, Bastar Craton, Central India

The end-Cretaceous diamondiferous Mainpur orangeite field comprises six pipes (Behradih, Kodomali, Payalikhand, Jangara, Kosambura and Bajaghati) located at the NE margin of the Bastar craton, central India. The preservation of both diatreme (Behradih) and hypabyssal facies (Kodomali) in this domain implies differential erosion. The Behradih samples are pelletal and tuffisitic in their textural habit, whereas those of the Kodomali pipe have inequigranular texture and comprise aggregates of two generations of relatively fresh olivines. The Kosambura pipe displays high degrees of alteration and contamination with silicified macrocrysts and carbonated groundmass. Olivine, spinel and clinopyroxene in the Behradih and the Kodomali pipes share overlapping compositions, whereas the groundmass phlogopite and perovskite show conspicuous compositional differences. The bulk-rock geochemistry of both the Behradih and Kodomali pipes has a more fractionated nature compared to southern African orangeites. Incompatible trace elements and their ratios readily distinguish them from the Mesoproterozoic Wajrakarur (WKF) and the Narayanpet kimberlites (NKF) from the eastern Dharwar craton, southern India, and bring out their similarity in petrogenesis to southern African orangeites. The pyrope population in the Mainpur orangeites is dominated by the calcic-lherzolitic variety, with sub-calcic harzburgitic and eclogitic garnets in far lesser proportion. Garnet REE distribution patterns from the Behradih and Payalikhand pipes display “smooth” as well as “sinusoidal” chondrite-normalised patterns. They provide evidence for the presence of a compositionally layered end-Cretaceous sub-Bastar craton mantle, similar to that reported from many other cratons worldwide. The high logfO₂ of the Mainpur orangeite magma (ΔNNO (nickel-nickel oxide) of +0.48 to +4.46 indicates that the redox state of the lithospheric mantle cannot be of first-order control for diamond potential and highlights the dominant role of other factors such as rapid magma transport. The highly diamondiferous nature, the abundance of calcic-lherzolitic garnets and highly oxidising conditions prevailing at the time of eruption make the Mainpur orangeites clearly “anomalous” compared to several other kimberlite pipes worldwide. U–Pb dating of zircon xenocrysts from the Behradih pipe yielded distinct Palaeoproterozoic ages with a predominant age around 2,450 Ma. The lack of Archean-aged zircons, in spite of the fact that the Bastar craton is the oldest continental nuclei in the Indian shield with an Eoarchaean crust of 3.5–3.6 Ga, could either be a reflection of the sampling process or of the modification of the sub-Bastar lithosphere by the invading Deccan plume-derived melts during the Late Cretaceous