Investigation of the Feldspars of the Skaergaard Intrusion, Eastern Greenland

An optical and single-crystal X-ray study has been made of some thirty feldspars, mostly plagioclases, of the marginal facies, the laminated and unlaminated series, and the late granophyres of the Skaergaard Intrusion. This study was undertaken so that data on the structural states of the feldspars could be used, together with information from the associated minerals, in any interpretation of the cooling history of the complex. It is found that there are appreciable variations in the structural states of the feldspars within the layered series, and at other points in the intrusion, which can be correlated generally with the changes in the ex-solution and inversion textures of pyroxenes and other minerals. Nevertheless, while it is found that this work can confirm and extend a little many of the earlier ideas of the cooling history of the Skaergaard Intrusion, limitations have become apparent in that the present data on the dynamics of the inversion and unmixing processes are far too meager to allow any detailed assessment of the physico-chemical conditions obtaining during any stage of the crystallization. It is clear that, before any major progress in the field of geological thermometry can be made by utilizing processes of this kind, much more experimental data will be needed on the nature of the processes involved and on the factors which control the process at all points within the composition fields of the major mineral groups.