Petrology and geochemistry of the Kruuse Fjord Gabbro Complex, East Greenland

Abstract

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.