Syn-Magmatic Deformation Structures in the Slaufrudalur Pluton, East Iceland

Abstract: Structures and deformation mechanisms of magmatic rocks are controlled by rheology. An increase in melt fraction can weaken a rock body and localises significant amount of strain. This interplay between rheology, melting and deformation in a magma reservoir, however, is not always clearly documented in literature. Therefore, this thesis illustrates the Syn-Magmatic Deformation Features (SMDFs) discovered in the Slaufrudalur pluton in Iceland. The goal of this thesis is to test whether these features formed during the emplacement of the Slaufrudalur pluton. Field mapping, microscopy, geochemistry analysis, electron microprobe analysis (EMPA) and Raman spectroscopy were employed to document and investigate the formation of these features and their relationships with the pluton. The SMDFs bear sharp contacts with the host granite and are divided into two types according to their morphology: 1) Type I SMDFs resemble shear zone with internal deformation fabric. 2) Type II SMDFs resemble magmatic dykes. The Type I SMDFs are interpreted as melt-assisted shearing structures while Type II SMDFs are interpreted as magmatic dykelets. Previous studies postulated that amalgamation of magma batches was responsible for the emplacement of the large plutons. Frequent injection of magma batches built up internal pressure and increased internal temperature of the emplacing pluton, which allowed the pluton to stay partially molten and mechanically weak. This allowed the mixing and mingling of different magma batches, resulting in the occurrence of various structures within the pluton. Solidified magma batches could also be re-mobilised to form mush or magma. The difference in formation mechanisms hints different rheology heterogeneity within the Slaufrudalur pluton. Hence, the occurrence of SMDFs is directly linked to the Slaufrudalur pluton during its emplacement.