Title: Petrology of the hornfels contact zone around the Hrossatungur gabbro in the eroded Hafnarfjall central volcano, W-Iceland

Abstract

This study focusses on a hornfels zone on the southern side of the Hrossatungur gabbro, within the
Hafnarfjall-Skardsheidi central volcano, W-Iceland. The intrusion formed by repetitive injections
of basaltic cone sheets, which were trapped at a lava- pyroclastite boundary within the Hafnarfjall
caldera fillings. During contact metamorphism in response to the 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 the
groundmass clinopyroxene 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,
titanite, minor apatite and hornblende. The orthopyroxene include bronzite (Fs₁₀₋₃₀), hypersthene
(Fs₃₀₋₅₀) and ferro-hypersthene (Fs₅₀₋₇₀). Garnet compositional range goes mainly from andradite to
about 20% grossular.

Loss-on-ignition measurements reveal that the majority of samples located at the inner hornfels
zone experience <1% LOI, while samples that show more extensive LOI are mostly situated at the
outer hornfels zone. A comparison of the hornfels LOI with the LOI of Icelandic rocks in different
alteration zones indicates that the hornfels rocks should have shown LOI >1% to <10% prior to the
gabbro emplacement, which indicates that the water has been driven out of the rock by the
replacement of hydrous minerals by non-hydrous minerals.

For comparison, the mineralogy of the hornfels zone surrounding a dyke intrusion in drillhole HE-42
in Hellisheidi geothermal field was studied. There the clino- pyroxene ranges from augite to salite
in veins and vesicles, with minor orthopyroxene in the groundmass, the plagioclase composition
ranges from labradorite to bytownite and anorthite. The hornfels in the well was more intense than
that found around the gabbro observed by more pronounced granoblastic crystallization, less iron
rich clinopyroxenes and more calcium rich plagioclases.

The study shows that the formation of the hornfels is due to direct heat conduction, pulsion of
water from the rock and preventing a direct water-magma
interaction.