The Kruuse Fjord Gabbro Complex is a composite intrusion oflayered gabbro and troctolite with subordinate ultramafic rocks andminor trondhjemitic bodies. It was emplaced into Archaean continentalcrust of East Greenland during early Tertiary rifting of Greenland fromEurasia. The work to date has identified an outer gabbro series and aninner troctolite series, and these are separated by a narrow zone oftrondhjemitic intrusions. In the southeast, the partially crystallizedcumulates 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 dykescross-cut the plutonic rocks. Structural and topographic featuressuggest that the layered rocks were affected by synmagmatic subsidenceand deformation but not by monoclinal coastal flexure.

The gabbro series is composed of a marginal gabbro unit, about 20 mwide, bordering more than a 2 km thickness of layered olivine andmagnetite gabbro cumulates. The marginal gabbro is interpreted to bechilled magma. The layered cumulates are the product of repeatedinjections of magma that fractionated in an open-system magma chamber.Anorthositic and troctolitic layers in the lower part of the sequencemay represent inputs of magma and suggest that the order of cumulusmineral 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 areversal in mineral compositions at 1.5 km from the base of the succession suggests a major input of magma occurred at this height. Inthe troctolite series, the composition of cumulus minerals, mineralcrystallization sequence and style of emplacement are similar to thosein the gabbro series. The ultramafic pluton is composed ofcoarse-grained wehrlite, olivine melagabbro and troctolite that wereformed by at least three injections of magma. The typical mineralcrystallization 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 containingAu and platinum-group elements.

Comparison of crystallization sequences and the major and traceelement compositions of clinopyroxene suggests that the gabbroic andtroctolitic rocks formed from a magma represented by the chilledmarginal gabbro, a tholeiitic basalt magma similar to E-MORB, whereasthe ultramafic rocks formed from a magma that was relatively enrichedin incompatible trace elements and volatiles. The association ofthese two magma types is an example of bimodal mafic–ultramaficmagmatism in a rifting environment.