The Vehmaa rapakivi granite batholith - An assemblage of successive intrusions indicating a piston-type collapsing centre

Abstract

The 700 km Vehmaa rapakivi granite batholith is located in south-western Finland. It has a concentric structure and is composed of intrusions of five granite varieties (we use the terminology of the rock names according to the published geological map sheet, Lindberg & Bergman, 1993): pyterlite, coarse-grained porphyritic rapakivi granite, medium-grained porphyritic rapakivi granite, even-grained rapakivi granite, and porphyry aplite (Fig. 1). The granite types yield a concordant monazite U-Pb age of 1573 ± 8 Ma (even-grained granite of satellite intrusion) and a zircon U-Pb age of 1582 ± 4 Ma (pyterlite) (Lindberg & Bergman 1993). The batholith is surrounded by mica schists, hornblende gneisses, and quartz-feldspar gneisses, as well as syn-collisional tonalites and late-collisional K-rich granites and migmatites (Ehlers et al., 1993; Selonen & Ehlers, 1998; Vaisanen & Holtta, 1999). In this short communication we discuss the structure and emplacement of the main Vehmaa rapakivi granite batholith as an assemblage of successive pulses of intrusions. The medium-grained porphyritic granite in the centre of the batholith is described in more detail than the other granite types. By careful mapping and by studying geophysical data, two successive intrusion pulses of almost identical types of the medium-grained granite, in contact with each other, have been defined. The two intrusive phases have not previously been described. We think that the mode of intrusion of these granites is important for the understanding the emplacement of the whole batholith, and suggests a more generally applicable model for other intrusions of rapakivi granite in Finland. Recently, Haapala & Lukkari (2005) discussed the evolution of the small, zoned Kymi stock within the Wiborg rapakivi batholith, where both the scale of the intrusion, and the suite of rocks suggested an alternative mode of formation and intrusion. A closer examination of the geochemistry and petrology of the Vehmaa batholith will follow in the near future and is not included in this short note.