Gabbroic, Syenogabbroic and Syenitic Cumulates of the Tugtutôq Younger Giant Dyke Complex, South Greenland

Abstract

The Tugtutôq Younger Giant Dyke Complex comprises an interconnecting suite of dykes, individually up to 800 m broad, traceable for some 145 km through a Proterozoic granitic terrain. The intrusion is thought to have resulted from one major influx of high-alumina mildly alkaline basalt and hawaiite magma at c. 1150 Ma. It has well-chilled margins grading in to coarse troctolitic gabbros. At intervals along their length the inner parts of the dykes are composed of orthocumulates in which rhythmic layering and/or feldspar lamination defines synformal pods, up to 3 km long. Among the features present are modally graded layering, isomodal layering, intermittent rhythmic layering, cross-bedding and magmatic erosion channels, including slump structures, slump breccias, down-dip thickening of mafic layers and high-temperature (annealed) faults. Cryptic and phase layering is also evident. Crystal fractionation occurred in a closed system (or systems) and the sequence of cumulus assemblages was: (i) pl+ol (ii) pl+ol+mt+il+ap (iii) pl+ol+mt +il+ap+cpx (iv) AF+ol+mt+il+ap+cpx. ‘Conventional names’ for the cumulatė rocks are troctolite (with picritic and anorthositic facies), ferro-syenogabbro, ferro-syenite and syenite. The parent magma was critically undersaturated and both quartz syenites and nepheline syenites. are represented among the residual differentiates. Marginal gabbroic facies to the layered synforms can display near-vertical mafic layering, perpendicular feldspar rock and approximations to Skaergaardstyle ‘wavy pyroxene’ facies. Large xenoliths of layered anorthosite cumulates in the intrusion indicate the presence at depth of further cumulate suites crystallised from high-alumina, mildly alkaline, basaltic magma.