Magmatism in a continental rift: the Paatusoq Syenite Complex, SE Greenland

The Paatusoq region of Greenland is a spectacular, exhumed subvolcanic system with horizontal and vertical (<2.5 km) sections preserved, making it an ideal natural laboratory to study magmatism. Two intrusive centres, a gabbro and syenite, are exposed. The gabbro contains a layered centre and an unlayered margin, with augite monzodiorite and olivine gabbro units. The layering contains channel and slump structures but is gravitationally inverted with plagioclase on the bases of layers grading up into denser mafic minerals at the top. Incompatible element geochemistry and field evidence indicate it is related to the Ketilidian Orogeny. The syenite complex comprises three major syenite units which young from W to E. U-Pb geochronology confirms Paatusoq as the youngest Gardar intrusive centre at 1140 Ma, emplaced over a timescale of <1 Ma, and its proximity to the Paatusoq Gabbro Centre is fortuitous. The mineralogy of each syenite unit is dominated by a primocryst framework of crypto- and micro-perthitic alkali feldspar with interstitial feldspar, pyroxene, amphibole ± quartz. A transsolvus feldspar assemblage of interstitial discrete grains of plagioclase and potassium feldspar alongside quartz and granophyre (i.e., vermicular intergrowth of quartz and K-feldspar) between microperthite primocrysts indicates that water and silica increased significantly in the latter stages of crystallisation. Coalescence and migration of interstitial melt is inferred to have produced irregular alkali granite and biotite syenodiorite bodies which cut the syenites. Hydrothermal alteration of hedenbergite in syenite created aligned lamellae of magnetite in pyroxene and the formation of hydrated ferric oxides which stained some units a characteristic orange colour. Paatusoq is a fascinating, deeply dissected slice through a subvolcanic magma system, providing evidence for crustal contamination and melt migration. Comparison of the Paatusoq Syenite Complex with other Gardar centres provides insights into processes that occur during continental rift magmatism.