Petrology of the Vandfaldsdalen Macrodike, Skaergaard Region, East Greenland

Abstract

The Vandfaldsdalen macrodike is a layered and differentiated gabbroic dike approximately 3·5 km long and from 200 to 500 m wide. It appears to cut the eastern margin of the Skaergaard intrusion and may have served as a feeder for the Basistoppen sill. The macrodike can be divided into three series of rocks: a marginal series of differentiated gabbros adjacent to the walls of the dike; a central series of differentiated and subhorizontally layered gabbros and ferrodiorites in the interior of the dike; and an upper felsic series of granophyric rocks with abundant quartzo-feldspathic xenoliths. The mineral and bulk-rock compositions through both the marginal series and central series show progressive iron enrichment. The most Ca-rich plagioclase (An69) and most magnesian pyroxene (Wo42 En46 Fs12) occur in olivine-bearing rocks of the marginal series about 5 m from the contact with wall rocks. The most Na-rich plagioclase (An39) and Fe-rich pyroxene (Wo38 En24 Fs38) are in olivine-free ferrodiorite of the central series, about 20 m below the contact with the felsic series. Evidence from field observations, bulk-rock chemical compositions, and Sr and Nd isotopic data indicate the felsic series formed as a mixture of the initial macrodike magma and granitic country rock. 87Sr/86Sr ratios of specimens from the felsic series range between 0·7129 and 0·7294. 143Nd/144Nd ratios vary between 0·51208 and 0·51118. Both ratios vary serially with the SiO2 contents of the specimens. We suggest that the felsic series evolved as a separate body of low density liquid which floated on the denser gabbroic magma of the central series. Heat from crystallization of the gabbroic magma must have diffused into the felsic layer, enabling extensive assimilation of the granitic xenoliths, but our data indicate there was very little exchange of chemical components between the two liquids.