The Iddefjord granite: geology and age
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- Artikler 
The Iddefjord granite forms part of the Swedish Bohus batholith and ranges in composition from diorite to granite. The granite consists of 13 individual plutons, and the mechanism of intrusion is related to block subsidence rather than forceful intrusion. There is no evidence of marginal chill, and the temperature of the host gneiss must have been similar to that of the granites. This suggests that the intrusion took place in a regionally pre-heated rock complex under plutonic conditions. The major element trends of the granites are similar to those of recent granite batholiths. Rb-Sr age determinations carried out on two of the granites yield a common isochron age of 918+- 7Ma with an initial Sr isotope ratio of 0.7063 +- 0.0008 suggesting contributions from older crustal components. Mineral analyses from one of the investigated granite samples indicate a Rb\/Sr mineral age in the order of 835 Ma, reflec- ting either a slow cooling of the pluton or a younger thermal effect causing resetting of the mineral system.