Emplacement Mechanisms Of Sill Complexes: Information From The Geochemical Architecture Of The Golden Valley Sill Complex, South Africa

摘要

The well-exposed Golden Valley Sill Complex, Karoo basin. South Africa, consists of four large sills (ca. 100 m thick; long axes: 13-24 km), 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), plus some minor intrusions. Field mapping shows physical connections between the small sill and one of the large sills, but no other connection between the large sills, or connections between the sills and the large dykes. Geochemical variation diagrams for 327 dolerite samples from the different sills and dykes in the Golden Valley Sill Complex showed similar compositional ranges for the dolerite bodies, but some differences in ratios between strongly incompatible elements. These differences were probed by Forward Stepwise-Discriminant Function Analysis using all major and trace elements as variables, and the height major sills and dykes as populations. The Forward Stepwise-Discriminant Function Analysis identified six different geochemical signatures among the sill-dyke populations. The six geochemical signatures were interpreted as representing six magma batches of distinct chemical characteristics that were involved in the formation of the Golden Valley Sill Complex. Four different magma batches gave rise to two of the large saucer-shaped sills and the two dykes. The exposed large dykes thus do not represent feeders to the main sills. A fifth magma batch gave rise to the two other large saucer-shaped sills as well as the smaller one. The statistical analysis also gave information about the connection between different dolerite exposures that could not be obtained by field, and made it possible to reconstruct the different sills through complex areas where several sills meet, and through widely spaced exposures. On the basis the field mapping and statistical analysis of geochemical data the geochemical architecture of the complex was established. Our study gives support to both the main models proposed earlier to explain the emplacement of sill complexes: (1) each sill in a sill complex is fed by different magma batches, and (2) the different sills in a sill complex feed one another, forming a group of nested sills generated by one single batch of magma. Furthermore, our study indicates that the interconnected sills in the Golden Valley Sill Complex are connected along their outermost edges through lateral overflow. The statistical analysis establishes a link between minor dolerite bodies and the main intrusion, thereby improving the geological mapping of the sill complex. The different units in the Golden Valley Sill Complex show strong geochemical affinity to lavas in the Lesotho Formation, the younger of the two formations in the Lesotho lava remnant. This suggests that these sills were emplaced at the same time as the Lesotho Formation, and that magmas with the same chemical characteristics as the Lesotho Formation were emplaced over a significantly larger area than covered by the remnants of the Lesotho Formation.