The origin of rapakivi texture

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Authors:Dempster, T. J.; Jenkin, G. R. T.; Rogers, G.
Author Affiliations:Primary:
University of Glasgow, Department of Geology and Applied Geology, Glasgow, United Kingdom
Other:
Scottish Universities Research and Reactor Centre, United Kingdom
Volume Title:Journal of Petrology
Source:Journal of Petrology, 35(4), p.963-981. Publisher: Clarendon Press, Oxford, United Kingdom. ISSN: 0022-3530
Publication Date:1994
Note:In English. 46 refs.; illus., incl. 1 table, sketch maps
Summary:The mantling of large ovoids of K-feldspars by a rim of plagioclase in the Rapakivi granites from the mid-Proterozoic Wiborg batholith, SE Finland, has been studied by isotopic analyses of mineral separates and CL combined with EPMA methods. A pulsed development of the mantles involved growth of successive plagioclases of composition An30, An25, the last being in optical continuity with perthitic plagioclase exsolved from the K-feldspar. Plagioclase mantles have high Δ18O and 87Sr/86Sr signatures relative to K-feldspar, indicating the presence of a late, low-T component thought to represent albite exsolved from the K-feldspar and redistributed onto the ovoid margin. Oligoclase components of the mantles are formed by a similar, but higher-T magmatic process, involving the subsolvus re-equilibration of alkali feldspar compositions with evolving melt conditions. The redistribution of the exsolved plagioclase from the alkali feldspar phenocrysts is considered to be linked to high F contents of rapakivi-type magmas, this major reconstruction of the phenocrysts generating their distinctive ovoidal shape. [R.A.H.]
Subjects:Alkali feldspar; Feldspar group; Framework silicates; Granites; Igneous rocks; K-feldspar; Mesoproterozoic; Plutonic rocks; Precambrian; Proterozoic; Rapakivi; Silicates; Textures; Upper Precambrian; Europe; Finland; Scandinavia; Western Europe; Southern Finland; Wiborg Batholith
Abstract Numbers:95M/944
Record ID:1996007885
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom
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