Precambrian Potassic Ultramafic Rocks: South Greenland

Small hypabyssal intrusives of biotite pyroxenite and biotiteperidotite are described from part of theGardar alkaline province.The intrusives, composed essentially of diopside, olivine, phlogopiteand Fe-Ti oxides (and numerous accessory minerals) are inferredto have crystallized from silica-undersaturated ultramafic magmasof similar bulk composition. Crystallization occurred at anestimated depth of 3–4 km. Despite relatively rapid consolidation(probably attendant on a partial devolatilization) producingtypically fine-grained and sometimes porphyritic rocks, small-scaledifferentiation nevertheless occurred. The differentiates, asveins, pegmatoidal patches and roofing-facies tend to be olivine-poorphlogopite+ferriandiopside+ Fe-Ti oxide rocks, in which perovskite,andradite, apatite and sphene are significant components. Pyroxenecompositions in the ultramafites are consistent with the thesisthat the rocks formed from soda-deficient magmas in stronglyoxidizing conditions. Relative potassium enrichment (averageK₂O: Na₂O wt. % = 1.6) is expressed modally in the ubiquitouspresence of phlogopite and normatively by an average of 6.9%or and/or Ic. Despite compositional affinity with certain olivinemelilitites, melilite is absent, a fact that may be attributableto the comparatively high hydrostatic pressure (c. 1 kb) pertainingduring crystallization. Petrographic and geochemical evidencesuggests that the ultramafic magmas were residua after fractionationof forsteritic olivine from more magnesian magmas of kimberlitetype. It is proposed that the latter may in turn have been residuafrom eclogite fractionation of Fe-Ti rich primary magmas inthe upper mantle.