Whole rock Rb–Sr measurements were made on associated basic and acid intrusives and extrusives from the northern and southern ends of the Lebombo monocline with a view to determine if the acid rocks came from the mantle or from the crust. In the south, in Swaziland (now called Ngwame) and Zululand, rhyolites and granophyres (age 202 � 14 m.y.) have an initial Si87−Sr86 ratio of 0.7042 � 0.0005; in the north, in the Nuanetsi syncline, rhyolites (age 206 � 13 m.y.) and granites from ring complexes (age 177 � 7 m.y.) have initial ratios of 0.7081 � 0.0008 and 0.7085 � 0.0007, respectively. The initial ratios of the basic rocks, basalts and gabbros, vary in both areas and have a range from 0.7042 to 0.7125 which can be explained by crustal contamination. The low initial ratios and isotopic homogeneity of the acid magmas over long distances (160 km) in Swaziland and Zululand are interpreted as implying that they were derived from the mantle.
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