Journal of Conference Abstracts

Archaean Volcanism ­ Extensional or Compressional? A New Look at an Old Issue

M. A. Hunter (mah1003@esc.cam.ac.uk)

Dept. Earth Sciences, Cambridge University, Cambridge, U.K.

Studies of volcanic processes in Archaean greenstone belts are hampered by poor preservation and limited exposure. It is important to understand the tectonic setting in which the mafic and ultramafic lavas were generated, as it directly reflects the evolution and subsequent stability of the early crust. Views are polarised as to whether the volcanics are remnants of oceanic crust, obducted onto an early continent during subduction related processes, or whether they were produced as a result of continental extension. Although studies of the petrology and geochemistry of the melts themselves are vital in understanding their generation, it is not possible to establish the setting for their eruption. Pre- and post-volcanic environments can be inferred from the facies changes of related sedimentary sequences and it is possible to use these to suggest a likely tectonic regime responsible for the generation of the melts.

The Belingwe Greenstone Belt in Zimbabwe contains a late Archaean (~2.7Ga) volcanosedimentary succession called the Ngezi Group. Recently there has been considerable debate in the literature concerning how the volcanic sequence was generated. Was it erupted as ocean crust and later obducted or was it the result of continental rifting associated with a mantle plume? The lavas are underlain by a thin sedimentary unit - the Manjeri Formation. A detailed study of these rocks has provided new and compelling evidence that in Belingwe the volcanics were generated as a result of extension in a continental setting. The generation of mafic melts in the Archaean probably occurred in many different environments and each greenstone belt should be treated individually. Perhaps a closer look at similar sedimentary sequences from other areas will bring us closer to understanding the evolution of the early earth.

Microbial Influence in the Lowest Grades of Metamorphism

C. V. Jeans¹, A. E. Fallick², M. J. Fisher³, R. J. Merriman⁴ & R. M. Corfield⁵

¹ Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK.

² Isotope Geology Unit, SURRC, East Kilbride, Glasgow, G75 0QU.

³ Nevis Associates.

⁴ British Geological Survey, Keyworth, Nottingham NG12 5GG.

⁵ Department of Earth Sciences, Parks Road, Oxford OX1 3PR.

The predominantly silty marine volcanogenic sediments of Triassic age exposed at Kaka Point, New Zealand, are part of the Southland syncline, the type area of zeolite facies metamorphism. Geological and geochemical evidence suggests that the organic matter present in the original sediment controlled the pattern of microbial activity. This influenced the development of zeolites and other authigenic minerals from the earlier stages of diagenesis, close to the depoisitional surface, into the lowest grades of metamorphism.