Abstract
Precise initiaJ 87Sr ratio data (2a~0.00006) for five xenolithic anorthosites and five host gabbros from the late phase (~ 1170 m. y.} of activity in the Narssaq area of the Gardar Province, South Greenland, show that the two rock-types had an· identical mean initial B7Sr/B6Sr ratio of 0.70297 at 1170 m.y. The samples de1me a line of 1150 ± 142 m. y. which is in agreement wtih their known age. The concordance of initial B7Sr/B6Sr ratios strongly suggests a related origin and favours models for differentiation of Gardar magmas which involve plagioclase fractionation. Initial B1Sr/B6Sr ratio data for rocks and minerals from small potassic ultramafic bodies belonging to the same magmatic cycle and intruding the gabbros are also uniformly low (~ 0.703). These values axe similar to the low initial B7Sr/B6Sr ratios obtained for other, uncontaminated Gardar intrusions and make any significant crustal contamination of the ultramafic magmas improbable-
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