Kûngnât, revisited. A review of five decades of research into an alkaline complex in South Greenland, with new trace-element and Nd isotopic data

Abstract

The Kûngnât Complex (1275±1.8 Ma) in the Gardar Alkaline Province, South Greenland, cuts Archaean gneisses and comprises two intersecting syenitic stocks and a gabbroic ring-dyke. The magmas, with increasingly more primitive compositions, were emplaced successively by ring-faulting and roof stoping. The syenites are orthocumulates (cumulus alkali feldspar, olivine, pyroxene, titanomagnetite and apatite; intercumulus phases include alkali amphibole, biotite, quartz and calcite). In the well dissected earlier stock, a 2.2 km-thick layered sequence displays graded modal layering, feldspar lamination and cryptic layering. Modal layering in both stocks is directed mainly inwards at 35° – 50°. Heterogeneous nucleation of the cumulus assemblage, close to steep thermal boundary layers, is inferred. The modal layering is ascribed primarily to gravitational sorting aided by the large density differential between a) feldspar and b) Fe-rich silicates and oxides. Episodic collapse of cumulus + melt slurries contributed to inward-dipping crystal pediments on the chamber floors. The Ring-Dyke (up to 100 m wide) is nearly continuous through 360°. Kûngnât exhibits a compositional nearcontinuum from olivine gabbro through syenite intermediaries to alkali granite, ascribed to protracted assimilation/fractional crystallization processes. The most radiogenic Nd isotope data from Kûngnât (εNdi values between –3.3 and –1.0) point to a lithospheric mantle source, whereas the most unradiogenic values imply enrichment in LREE by crustal contamination of the magmas.

Tectono-magmatic evolution of the younger Gardar southern rift, South Greenland

“…Comparable downdip thickening of mafic/ultramafic layers occurs in other Gardar cumulate bodies, e.g. within the Kûngnât Fjeld syenites (Upton et al 2013). There is a notably inequi-granular (i.e.…”

“…Extremes range from those that lack any discernible marginal border group (e.g. Itillip Saqqaa, Tuttutooq (Upton 1987) and the western stock of the Kûngnât complex (Upton et al 2013) to cases where marginal border groups are well developed (e.g. Asorutit, Krydssø and Itillip Saqqaa).…”

“…The development of stratified magma chambers, analogous to that postulated to explain the relationships in the BFDs, is thought to have been responsible for the sequence of intrusions in the South Qôroq complex (see above). Such stratified magmas may have been generated repeatedly throughout Gardar times as exemplified by the Kûngnât complex in the Older Gardar (Upton et al 2013). It is probable that, with passage of time, the volume and depth of the salic upper component progressively increased, thereby reducing the chances for the underlying mafic magma to ascend.…”

“…Unit 2 (c. 1200 m diameter) is a heterogeneous quartz syenite that contains an abundance of xenoliths. These include: (a) gabbro from the YGDC, (b) basalt lava, ba- It is hypothesised that a raft of roofing rocks (reminiscent of features in the Grønnedal-Íka and Kûngnât complexes (Emeleus 1964;Upton et al 2013) collapsed into the Unit 2 chamber. Whilst this raft, composed of rocks from above and below the Julianehåb granite/Eriksfjord Formation unconformity, underwent disintegration, it retained its overall stratigraphic integrity.…”

“…liquid+crystals) convection developed. The Older Gardar eastern stock of the Kûngnât complex appears to provide the closest analogue in the Gardar Province (Upton 1960;Upton et al 2013).…”