DETRITUS OF GRANITES 



97 



Under such a reducing atmosphere one would expect that sands of 

 all sorts of composition would be formed — coarser and finer sands 

 of the lighter minerals, such as quartz plus felspar (not quartz alone), 

 or sands of medium-weight minerals such as hornblende and augite. 

 But also sands of the heavy ore minerals like sulphides and metal- 

 oxides. The size, form and specific weight of the individual grains, 

 their physical properties, would determine their ultimate place of 

 sedimentation, not their chemical properties. 



STUDIES BY RANKAMA: DETRITUS OF GRANITES 



A first attempt to use the difference in exogenic processes under an- 

 oxygenic or oxygenic atmospheres, was made by Rankama (1955) 

 in Finland. He tried to evaluate the character of the atmosphere by 

 studying an ancient deposit of detrital rocks around a granitic rock 

 from which the sediments were thought to be derived. The granitic 

 body, actually a quartz diorite, contains about four per cent ferrous 

 iron and two per cent of ferric iron, measured by weight of the 

 oxides. Now in oxidative weathering, the relative amount of ferric 

 iron will normally always be greater in sediments derived from a 

 granite, because of oxidation of the ferrous iron. In the Finland 

 example, this is not the case, the ratio Fe203/FeO in the sediments 

 is even lower than that of an original diorite pebble included in the 

 sediments, and it is comparable to that of neighbouring quartz 

 diorites (Table V). 



Rankama concluded that this particular quartz diorite weathered 

 under reducing atmospheric circumstances. This gives us a fixed date 

 for the existence of the early anoxygenic atmosphere if the age of the 

 sediments could be established. 



The pre-Cambrian stratigraphy of the Fennoskandian old shield, 

 however, is very much under study at the moment, and absolute 

 dating tends to reverse many of the earlier correlations. The quartz 

 diorite and the derived sediments studied by Rankama were formerly 

 grouped with the Botnian period. This was thought to be somewhat 

 earlier than the Svecofennian period, which was dated at around 

 1800 my. Nowadays, Botnian and Svecofennian are sometimes lump- 

 ed together, and the 1800 my age is then thought to be that of a 

 younger metamorphosis, and not to give the real age of the rocks. 

 More facts will undoubtedly become available in the near future. 



