The Kruuse Fjord Gabbro Complex is a composite intrusion of
layered gabbro and troctolite with subordinate ultramafic rocks and
minor trondhjemitic bodies. It was emplaced into Archaean continental
crust of East Greenland during early Tertiary rifting of Greenland from
Eurasia. The work to date has identified an outer gabbro series and an
inner troctolite series, and these are separated by a narrow zone of
trondhjemitic intrusions. In the southeast, the partially crystallized
cumulates of the gabbro series were intruded by a lenticular,
ultramafic pluton 800 m in thickness. Volumetrically minor,
syenite–trachyandesite net-veined dykes and later, diabase dykes
cross-cut the plutonic rocks. Structural and topographic features
suggest that the layered rocks were affected by synmagmatic subsidence
and deformation but not by monoclinal coastal flexure.The gabbro series is composed of a marginal gabbro unit, about 20 m
wide, bordering more than a 2 km thickness of layered olivine and
magnetite gabbro cumulates. The marginal gabbro is interpreted to be
chilled magma. The layered cumulates are the product of repeated
injections of magma that fractionated in an open-system magma chamber.
Anorthositic and troctolitic layers in the lower part of the sequence
may represent inputs of magma and suggest that the order of cumulus
mineral crystallization was (1) plagioclase (An39–85),
(2) olivine (Fo46–82), (3) augite
(Wo28–47En39–58Fs8–18
) and (4) magnetite. The disappearance of cumulus magnetite and a
reversal in mineral compositions at 1.5 km from the base of the
succession suggests a major input of magma occurred at this height. In
the troctolite series, the composition of cumulus minerals, mineral
crystallization sequence and style of emplacement are similar to those
in the gabbro series. The ultramafic pluton is composed of
coarse-grained wehrlite, olivine melagabbro and troctolite that were
formed by at least three injections of magma. The typical mineral
crystallization sequence was (1) cumulus chromite and olivine
(Fo84–88); (2) poikilitic chrome diopside
(Wo29–51En43–63Fs3–13
); and (3) intercumulus plagioclase (An75–90),
phlogopite, apatite and localized disseminated sulphides containing
Au and platinum-group elements.Comparison of crystallization sequences and the major and trace
element compositions of clinopyroxene suggests that the gabbroic and
troctolitic rocks formed from a magma represented by the chilled
marginal gabbro, a tholeiitic basalt magma similar to E-MORB, whereas
the ultramafic rocks formed from a magma that was relatively enriched
in incompatible trace elements and volatiles. The association of
these two magma types is an example of bimodal mafic–ultramafic
magmatism in a rifting environment.
