HR: 11:30h
AN: V22A-05    [Abstracts]
TI: Chemical Variations in Granitic Magmas: Source-inherited or Products of Magmatic Processes?
AU: * Clemens, J D
EM: j.clemens@kingston.ac.uk
AF: School of ESG, CEESR, Kingston University, Penrhyn Rd, Kingston-upon-Thames, KT1 2EE United Kingdom
AU: Helps, P A
EM: K0130333@kingston.ac.uk
AF: School of ESG, CEESR, Kingston University, Penrhyn Rd, Kingston-upon-Thames, KT1 2EE United Kingdom
AU: Stevens, G
EM: gs@akad.sun.ac.za
AF: Department of Geology, University of Stellenbosch, Private Bag X1, Stellenbosch, 7602 South Africa
AB: Though exhibiting considerable scatter in the data, geochemical variations in granitic plutons and silicic volcanic deposits are commonly modelled as the products of differentiation of originally homogeneous magmas. However, many silicic igneous bodies are internally heterogeneous in their mineralogy, geochemistry and isotopic ratios, on a variety of scales, down to 1 m or less, and this variation is mainly inherited from the magma source regions. Granitic plutons and silicic volcanic complexes are therefore commonly constructed by the accumulation of numerous (sometimes quite small) batches (pulses) of magma derived from contrasting source rocks. Such pulses result from the nature of the melting reactions and the physical structure of magma source regions. Internal differentiation of these batches can occur, but most probably not on the scales of whole magma chambers. Rather than being created through differentiation or hybridisation processes, at or near emplacement levels, much of the variation observed within such magmatic bodies (e.g., trace-element or Mg&35; variation with SiO₂ or isotopic ratios) is therefore a primary feature. At emplacement levels, the relatively high viscosities and slow diffusion rates of many chemical components in silicic melts probably inhibit processes that would lead to magma homogenisation; this permits at least partial preservation of the primary heterogeneities. These features create difficulties for petrologists trying to date rocks and model chemical variations in igneous bodies. However, these same heterogeneities are a rich source of data on the charcteristics of and processes within the magma source regions.
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2005 Joint Assembly