Petrology of the hornfels contact zone around the Hrossatungur gabbro, W-Iceland

Abstract

This study focusses on a hornfels zone at the southern side of the Hrossatungur gabbro, W-Iceland. During contact metamorphism in response to this intrusion emplacement a hornfels contact zone was created by a recrystallization process of previously hydrothermally altered basalt. Here, the hydrothermal alteration and chemical composition of minerals is closely studied to evaluate the development during the contact metamorphism event forming the hornfels around the gabbro. The hornfels mainly contains clinopyroxene compositions ranging widely from augite, salite, ferrosalite to hedenbergite in vesicle fillings, with minor orthopyroxene, while in the groundmass it ranges from diopside, augite to salite. The plagioclase composition ranges from andesine, labradorite to bytownite and occasionally to anorthite within the vesicles and veins, while the groundmass plagioclase ranges from labradorite to anorthite. Other minerals found in the hornfels are iron-titanium oxides (magnetite, ilmenite and titano-magnetite), garnet (andradite to about 20% grossular) and sulphides. Loss-on-ignition measurements of hornfels compared with the LOI of Icelandic rocks in different alteration zones indicates that the water has been driven out of the hornfels rock by the replacement of hydrous minerals by non-hydrous minerals. The study shows that the formation of the hornfels is due to direct heat conduction, the expulsion of water from the rock and preventing a direct water-magma interaction.