The Batbjerg complex, east Greenland: a unique ultrapotassic Caledonian intrusion

Abstract

Batbjerg is an ultrapotassic igneous complex of Caledonian age (445 Ma) at the western extremity of Kangerdlugssuaq Fiord, east Greenland. The part of the arcuate complex not covered by Kangerdlugssuaq Glacier extends 7 km with a 1000 m relief. The dominant intrusive member is alkaline pyroxenite grading to jacupirangite and ijolite but containing leucite as well as nepheline. Dominant mineralogy is clinopyroxene, magnetite, nepheline, and leucite. No feldspar is present except as fingerprint intergrowths of nepheline with potassium feldspar. Intergrowths of kalsilite with potassium feldspar are thought to be derived from leucite by subsolidus breakdown. Leucite has not previously been reported from a plutonic intrusion, and kalsilite only once. Other intrusive members are olivine gabbro, dikes of syenite to granite, and diatremes of appinitic character cutting the basement gneiss outside the alkaline pyroxenite. A partial screen of limestone is the cauldron-subsided remnant of a former Paleozoic sedimentary cover not previously known in this part of east Greenland. The intrusion extends Caledonian magmatism 250 km south of its previous limit, and is about 150 km west of the previously supposed position of the Caledonian front. It is probably part of a predrift Caledonian petrographic province that includes the Assynt alkaline suite of northwest Scotland, and may imply a westward-dipping subduction zone under this region in Caledonian time.