HR: 0800h
AN: V11A-0372    [Abstracts]
TI: Geochemical Architecture of the Golden Valley Sill Complex, South Africa: Implication for the Emplacement of Saucer-Shaped Sills in Sedimentary Basins
AU: Galerne, C
EM: chrisgal@fys.uio.no
AF: PGP, University of Oslo, P.O.Box 1048, Oslo, 0316, Norway
AU: * Neumann, E
EM: e.r.neumann@geo.uio.no
AF: PGP, University of Oslo, P.O.Box 1048, Oslo, 0316, Norway
AU: Planke, S
EM: planke@vbpr.no
AF: PGP, University of Oslo, P.O.Box 1048, Oslo, 0316, Norway
AB: Saucer-shaped sills and dykes are common features in sedimentary basins worldwide and represent important parts of the plumbing system in such areas. In spite their common occurrence, the emplacement mechanisms that lead to the formation of sill complexes are poorly understood. Two main emplacement models have been proposed, based on field observations, seismic imaging, and mathematical modeling: (1) Each sill in a complex is produced by an individual magma batch emplaced through a dyke or a pipe, ending in the formation of a sill. (2) A single batch of magma gives rise to a series of saucer-shaped sills which feed one another, thereby forming a nested sill complex in which the different sills are interconnected. We report on a detailed geochemical study of a complex of nested, saucer-shaped dolerite sills, the Golden Valley Sill Complex in the Karoo Large Igneous Province, South Africa. This well-exposed sill complex consists of four large sills (ca. 100 m thick; long axes: 13- 24 km) emplaced at slightly different stratigraphic levels, one small sill (55-80 m thick; long axis: 4 km; forming an appendix to one of the large sills), and two large dykes (15-20 m thick; 25 and 70 km long). The field observations show no physical connections between the large sills, or between the sills and the dykes. The compositions of basalts and dolerites in the Karoo basin are very similar. However, small chemical differences do exist. In order to discriminate between sills and dykes of different geochemical signature in the Golden Valley Sill Complex, that might have formed from different magma batches, we used a Forward Stepwise-Discriminant Function Analysis (FS-DFA). Sample groups which, based on field observations, were found clearly to belong to the same sill or dyke were defined as populations, the variables were forty-seven major and trace elements. The FS-DFA showed that several magma batches of distinct chemical characteristics were involved in the formation of the GVSC. Four different magma batches gave rise to two of the large sills and the two dykes. The exposed dykes thus do not represent feeders to the main sills in the GVSC. One additional, chemically distinct magma batch gave rise to the two other large sills plus the small sill. Our study of the GVSC thus gives support to both the main models proposed earlier to explain the emplacement of sill complexes. Some sills in a sill complex may be fed by different magma batches (model 1); other sills in the same complex may overflow and feed new sills, forming a group of nested sills formed from one single batch of magma (model 2). An important difference between model (2) and our observations is the way sills feed one another. Model (2) proposes that each of the nested sills in a complex is fed from its center, connecting the distal part of one sill to the center of the next sill which floor will be located at a higher stratigraphic level. In the GVSC the nested sills are connected along their edges through lateral overflow, the nested sills are thus located at approximately the same stratigraphic level. The GVSC includes complex areas where two or more sills appear to meet, and where two sills are located above one another. Using the FS-DFA parameters for the main populations, we were also able to assign samples in these areas to specific sills, thus improving the geological mapping of the area.
DE: 1065 Major and trace element geochemistry
DE: 3642 Intrusive structures and rocks
DE: 9305 Africa
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting