Abstract

Mr. F. J. Fitch asked the author if he could relate the volume of lava being erupted at any particular time and place to the stage in the volcanic cycle attained at that locality. He confirmed that the great bulk of Karroo Volcanism was confined to a short period of about 20 m.y. in duration at the beginning of Mesozoic time, but agreed that the total time span of Karroo and apparently related magmatic activity might be 10 m.y. or more. In collaboration with Dr. J. A. Miller he had recently suggested that the time sequence of Karroo activity viewed on the K-Ar time scale is as follows: Stages of cycle

Waning phases Lupata alkali-basalts (105–115 ± 10 m.y.) 
  Subsidiary maxima of 'Karroo' dolerite magmatism and metasomatism (~ 155 m.y.; 161 m.y.) 
  Lupata rhyolites (166 ± 10 m.y.) 
  Granites of Mateka Hills (167 ± 7 m.y.) 
  Subsidiary maxima of 'Karroo' dolerite magmatism and metasomatism (~ 172 m.y.) 
Culminating phases 
'Karroo' dolerites, Drakensberg basalts of Lesotho and widespread flood basalts of Central Southern Africa (~ 187 ± 7 m.y.) 
   Nuanetsi and Labombo rhyolites (190–194 ± 12 m.y.) 
Waxing phases Lower alkaline and tholeiitic basalts of Nuanetsi and Lebombo districts (~ 190–195 m.y.) 
  Marangudzi igneous complex (~ 195 m.y.) 
  Shawa igneous complex (~ 197 m.y.) 
Waning phases Lupata alkali-basalts (105–115 ± 10 m.y.) 
  Subsidiary maxima of 'Karroo' dolerite magmatism and metasomatism (~ 155 m.y.; 161 m.y.) 
  Lupata rhyolites (166 ± 10 m.y.) 
  Granites of Mateka Hills (167 ± 7 m.y.) 
  Subsidiary maxima of 'Karroo' dolerite magmatism and metasomatism (~ 172 m.y.) 
Culminating phases 
'Karroo' dolerites, Drakensberg basalts of Lesotho and widespread flood basalts of Central Southern Africa (~ 187 ± 7 m.y.) 
   Nuanetsi and Labombo rhyolites (190–194 ± 12 m.y.) 
Waxing phases Lower alkaline and tholeiitic basalts of Nuanetsi and Lebombo districts (~ 190–195 m.y.) 
  Marangudzi igneous complex (~ 195 m.y.) 
  Shawa igneous complex (~ 197 m.y.) 
The Author thanked Mr. Fitch for his contribution to the time relations of Karroo igneous rocks. When referring to stages in the volcanic cycle it is necessary to be clear about the definitions employed. In terms of the thermal

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