The
outermost 50-100 m of the MBS was described as “the tranquil division”
by Wager and Deer (1939) because it was believed to have formed before
the onset of magmatic convection (on picture, LZaT). The unit
displays no banding but has local evidence of rapid cooling, heterogeneous
crystal nucleation on the magma chamber wall, and various effects of assimilation
and metasomatism. Outermost, a chilled margin is exposed against the wall
rocks, which in places show evidence of anatectic melting. The chilled
margin has little resistance to weathering and is mostly eroded out to
form a small gully. The chilled margin was examined by Hoover (1989) to
assess the composition of the initial Skaergaard magma. Despite the abundance
of xenoliths in the MBS, the chilled margin show only little contamination
(Hoover, 1989). Despite this, it is rather inhomogeneous and locally displays
strong evidence of crystal accumulation and hydrothermal alteration. Petrological
screening led Hoover to identify a preferred chilled margin sample on the
basis that it showed no cumulate or recrystallisation textures; liquidus
rather than solidus mineral compositions; concentrations of incompatible
elements consistent with expected partition coefficients for the LZ cumulates;
and was free of alteration (Hoover, 1989). This sample consists of a fine
grained doleritic material with the composition of an evolved tholeiitic
basalt.
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).
Xenoliths