About the genesis of ovoidal K-feldspar porphyry and quartz in rapakivi granites and related rocks

Abstract

A comparative study of structural peculiarities of ovoid K-feldspar porphyry and quartz of rapakivi granite of different arrays (Aland, Vyborg, Salmi batholith rapakivi of the Baltic shield and Bardiaush batholith rapakivi of the South Ural), and hybrid rocks of various genesis allowed us to clarify their role in the formation of rapakivi granite complexes. It was established, that the porphyritic quartz and ovoides of K-feldspar and their plagioclase shell was formed before the crystallization of the residual cotectic melt of rapakivi granite. In the composite hybrid magmatic rocks xenocryst K-spar have undergone a disintegration of solid solution with formation of a secondary cryptoperthite plagioclase. And xenocrystic quartz was subject to decrepitation by primary fluid inclusions and the formation of new secondary high-temperature inclusions. In a hybrid composite rocks formed as metamorphic and basic igneous rocks ovoid-like porphyroblast Kspar (including plagioclase shells) and quartz porphyry (including the ‘ocelly’) was crystallized from solution. In the crystals of quartz primary inclusions (negative crystals) of trigonal symmetry were present, typical for low-temperature α-modification. Refs 24. Figs 9. Tables 2.