The chilled margin is followed inwards by a coarse-grained inhomogeneous gabbro with discontinuous units of perpendicular feldspar rock, wavy and patchy pyroxene rocks. Wager and Deer (1939) described the occurrence of perpendicular feldspar rock as “reefs” 10-100 cm wide occurring some 15-30 m from the intrusive margin. A typical transect would display two or three reefs inwards from the margin. The perpendicular feldspar rock is an olivine gabbro that displays highly elongate feldspar crystals orientated at a right angle to the margin of the intrusion. The matrix between the feldspars is fine-grained with occational occurrences of poikilitic augite (Wager and Deer, 1939). The feldspars are oscillatory zoned with cores significantly more calcic than feldspars in the chilled margin, suggesting that they formed during a process of fractional crystallisation (in contrast to the quenched liquid that formed the chilled margin). The popular interpretation of the perpendicular feldspar rock is that it formed by heterogeneous nucleation of feldspar crystals on the walls of the magma chamber. During rapid crystal growth, mass transport would favour growth perpendicular to the nucleation surface (Wager and Deer, 1939; Wager and Brown, 1968). Comparable rocks are found as layers in the Rum complex (the harrisitic cumulates) but are also commonly seen in granite intrusions and mineral veins.
The wavy pyroxene rock (Wager and Brown, 1968) is a medium-grained gabbro
with wavy pegmatitic lenses, typically some 50-100 cm long and 10 cm wide,
oriented perpendicular to the intrusive margin. The lenses are dominated
by large feldspar crystals and poikilitic pyroxene
Pyroxene replacement structures are locally abundant in the outer parts of the MBS, where they form irregular bodies that clearly overprint the primary lithologies. Small (up to metre-sized) bodies are exposed along the south coast of Kraemer Ø, but the structures are by far most abundant above the gneiss-sediment unconformity. The replacement is more pronounced on Ivnarmiut, for example, where extensive areas appear to have been altered (McBirney and Sonnenthal, 1990).