Emplacement Mechanisms and Magmatic Differentiation Induced by Magma Flow in Sill Intrusions in Sedimentary Basins

Abstract

Magma transport through the Earth’s crust often ends in sedimentary basins. While it erupts at the surface, a significant part of the magma emplaces in depth in the basin forming sheet complexes dominated by horizontal sills associated with vertical dykes. Their fundamental units are saucer-shaped sills. They are typically ~20-200 m thick with a radius varying ~10-20 km. When a large igneous event occurs, the emplacement of sill complexes possibly affects the whole sedimentary basin. This can have important implications for: (i) global climate change due to large volumes of greenhouse gases released from metamorphic aureole around the sills; (ii) organic matter maturation and fluid migration, and ground-water aquifers. The main objective of my thesis consisted to characterize the geochemical architecture of a representative sill complex in order to give information about the emplacement mechanism and the magmatic differentiation associated processes. My thesis presents a detailed geochemical study of the Golden Valley Sill Complex (GVSC) that is one of the best exposures of sheet complexes exhumed by erosion in the Karoo Basin, South Africa. Eight distinct geographical intrusive-units involving saucer-shaped sills and dykes were identified. The large and dense sampling coverage allowed a statistical analysis among more than 300 whole-rock analyses to distinguish the various geochemical signatures. Analogue experiments reproducing saucer-shaped sills were conducted in order to integrate the results of the produced geochemical architecture for the GVSC. Finally, magmatic differentiation associated processes were studied from the best exposed saucer and through detailed examination of variations along 18 vertical profiles. My results show that (1) the GVSC formed itself via a mixture of two-end member hypotheses. Among the distinct geographical units, some have identical and some have distinct geochemical signatures suggesting that GVSC is intermediate between single and multi-batch emplacement model. (2) Integrated study with analogue experiments suggests that major saucer-shaped sills are centrally fed. Moreover, we conclude that the common elliptic shape of saucers is controlled by the size and orientation of vertical linear individual feeders. Finally, (3) chemical changes taking place in a saucer-shaped sill result from different fractionations processes taking place at different parts and times of the cooling history.