Igneous layering and magma dynamics in alkaline intrusions: textural evidence for gravitational settling and compaction within cumulates

The mechanisms responsible for igneous layering and the concentration of critical minerals within alkaline intrusions remain a matter of debate. The Ilímaussaq complex, South Greenland, is a layered alkaline intrusion containing economically important deposits of rare earth elements. Based on geochemical and petrological data, the two leading hypotheses for the formation of igneous layering at Ilímaussaq are: (1) repeated magma recharge and the in situ nucleation of minerals; or (2) the gravitational settling of crystal mats within a closed magmatic system. We provide novel field, rock magnetic and crystallographic preferred orientation data from two representative outcrops of igneous layering at Ilímaussaq to test these hypotheses. Our rock magnetic data show that both arfvedsonite and magnetite contribute to the anisotropy of magnetic susceptibility fabrics, with magnetite defining a subvertical foliation across the igneous layering. Our data show that the anisotropy of magnetic susceptibility fabric is inverse to the silicate fabric (i.e. the magnetic foliation is normal to the silicate foliation) and records consistent sub-horizontal mineral foliations and lineations vertically through the layers. Crucially, the silicate fabric is often oblique in both strike and dip to the modal igneous layering. Our measured fabrics best support a closed system crystal mat model and subsequent phases of differential and intrusion-wide compaction.