Batbjerg

Situated 15 km beyond the head of Kangerdlugssuaq Fjord, Batbjerg is of Caledonian age, the other intrusions of the Kangerdlugssuaq vicinity all being Tertiary. A section some 7 km long and with an elevation of 1 km is exposed along the eastern side of the Kangerdlugssuaq Glacier, an unknown area being covered by ice. The complex cuts Precambrian gneisses but marble with quartzite at one locality forms a discontinuous screen along the contact. The sediments are probably of Palaeozoic age and the limestone has been extensively thermally and metasomatically altered, chert nodules having been altered to assemblages including Na-rich melilite, pectolite and wollastonite, and a spinel approaching Mg2TiO4 (Gittins et al., 1982). The main part of the intrusion is a complex of pyroxenite and jacupirangite with some ijolite and urtite which is cut by dykes and veins of syenite, quartz syenite and granite. The clinopyroxene of the ultramafic rocks is generally homogeneous, but is occasionally zoned to acmitic rims. Magnetite, phlogopite, biotite, olivine and hornblende are variable in amount. The salic minerals are nepheline, leucite, vermiform intergrowths of nepheline and K-feldspar and patchy intergrowths of kalsilite and K-feldspar, with less commonly natrolite, analcime and thomsonite. Discrete grains of K-feldspar are rare. Fresh leucite is present in interstitial pools, its presence possibly being unique in this geological context. Nepheline is also fresh and forms anhedral to euhedral grains as well as extensive vermiform intergrowths. Leucite and intergrowths of kalsilite and K-feldspar have not been observed in the same rock, but almost every other combination of the salic phases can occur. The pyroxenites are cut in a few places by pegmatites of bright green Cr-bearing diopside, phlogopite partly with reversed pleochroism and olivines up to 1 m across. The northeastern margin of the complex is occupied by dunite, the field relationships of which are unclear, and a narrow band of dunite also lies within the pyroxenites. Although generally composed wholly of forsteritic olivine variants rich in magnetite, phlogopite and clinopyroxene are present. Several pipe-like bodies up to 30 m across cut the dunites and pyroxenites and are filled principally with dunite nodules, but a few of lherzolite and wehrlite nodules of olivine, low Al, Cr-rich clinopyroxene, phlogopite, spinel and minor orthopyroxene are to be found. The late syenite and granite dykes and veins form a dense network in some places, while a few lenses and sills of nepheline syenite also occur. The syenites and granites contain acmitic pyroxenes which are replaced by amphibole as the quartz content increases. Analyses of these rocks indicate a large preponderance of K2O over Na2O. A prominent hill (Brown Mountain) on the north side of the complex consists of a rock composed of 40-50% augite, 10-15% biotite, 0-10% serpentinized olivine, 30 -40% perthite and opaques with biotite hornblendites containing 5 -10% altered sodic plagioclase. Several diatremes up to 100 m across cut the basement gneisses around the complex and contain rounded blocks of appinitic rocks and occasional blocks of two pyroxene-amphibole-phlogopite-olivine rocks. The country rocks are cut by dykes of alkali basalt which are probably of Tertiary age.