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The Vehmaa Rapakivi Granite Batholith – an Assemblage of Successive Intrusions Indicating a Piston-Type Collapsing Centre

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The Vehmaa Rapakivi Granite Batholith – an Assemblage of Successive Intrusions Indicating a Piston-Type Collapsing Centre Bulletin of the Geological Society of Finland,Vol. 77, 2005, pp. 65–70 The Vehmaa rapakivi granite batholith – an assemblage of successive intrusions indicating a piston-type collapsing centre Olavi Selonen1)*, Carl Ehlers2) , Hannu Luodes1) and Jouni Lerssi1) 1) Geological Survey of Finland, FIN-70211 Kuopio, Finland 2) Åbo Akademi University, Department of Geology, FIN-20500 Turku, Finland Short Communication Key words: granites, rapakivi, batholiths, intrusions, magmatism, Paleoproterozoic,Vehmaa, Finland 1. Introduction The 700 km2 Vehmaa rapakivi granite batholith is lo- ite in the centre of the batholith is described in more cated in south-western Finland. It has a concentric detail than the other granite types. By careful map- structure and is composed of intrusions of five granite ping and by studying geophysical data, two succes- varieties (we use the terminology of the rock names ac- sive intrusion pulses of almost identical types of the cording to the published geological map sheet, Lind- medium-grained granite, in contact with each other, berg &Bergman, 1993): pyterlite, coarse-grained have been defined. The two intrusive phases have not porphyritic rapakivi granite, medium-grained por- previously been described. We think that the mode phyritic rapakivi granite, even-grained rapakivi gran- of intrusion of these granites is important for the un- ite, and porphyry aplite (Fig. 1). The granite types derstanding the emplacement of the whole batholith, yield a concordant monazite U-Pb age of 1573 ± 8 and suggests a more generally applicable model for Ma (even-grained granite of satellite intrusion) and other intrusions of rapakivi granite in Finland. a zircon U-Pb age of 1582 ± 4 Ma (pyterlite) (Lind- Recently, Haapala & Lukkari (2005) discussed the berg & Bergman 1993). The batholith is surrounded evolution of the small, zoned Kymi stock within the by mica schists, hornblende gneisses, and quartz-feld- Wiborg rapakivi batholith, where both the scale of spar gneisses, as well as syn-collisional tonalites and the intrusion, and the suite of rocks suggested an al- late-collisional K-rich granites and migmatites (Eh- ternative mode of formation and intrusion. A closer lers et al., 1993; Selonen & Ehlers, 1998; Väisänen examination of the geochemistry and petrology of the & Hölttä, 1999). Vehmaa batholith will follow in the near future and is In this short communication we discuss the struc- not included in this short note. ture and emplacement of the main Vehmaa rapakivi granite batholith as an assemblage of successive pulses 2.The Vehmaa rapakivi of intrusions. The medium-grained porphyritic gran- granite batholith The main granite type in the Vehmaa rapakivi gran- * Corresponding author e-mail: olavi.selonen@gtk.fi ite batholith is pyterlite with a typical rapakivi texture 66 O. Selonen, C. Ehlers, H. Luodes and J. Lerssi Fig 1. Geological mapofthe Vehmaa rapakivi granite batholith. Modified from Lindberg &Bergman (1993). Loca- tion of Fig. 5ismarked with arectangle. comprising potassium feldspar ovoids (2–5 cm in di- al ovoids. The principal minerals are potassium feld- ameter), occasionally with plagioclase rims. The col- spar,quartz, plagioclase, and biotite, whereas chlo- our of the pyterlite varies from redtobrown with rite, fluorite, apatite, zircon, and opaques are accesso- greenish varieties.Itisahornblenderapakivi gran- ries (Lindberg & Bergman, 1993). The granite is pale ite, composed of potassium feldspar,quartz, plagi- red to red with an occasional greenish tint; occasion- oclase,biotite, and hornblende(Lindberg &Berg- ally it is transected by aplite dykes. man, 1993). Fluorite, apatite, zircon, and olivine are A medium-grained porphyritic granite is exposed in found as accessories. Composite pegmatite and aplite the centre of the Vehmaa batholith. The porphyrit- dykes (a few centimetres to half a metre wide) with ic texture of the granite is defined by potassium feld- quartz veins occur, as well as felsic and mafic igneous spar phenocrysts ( ≤ 2 cm in length). It is a biotite enclaves, and supracrustal xenoliths. rapakivi essentially consisting of potassium feldspar, The coarse-grained porphyritic granite is charac- quartz, plagioclase, and biotite with accessory chlo- terized by large, mostly euhedral, potassium feld- rite, fluorite, apatite, zircon, muscovite and opaques spar phenocrysts, 2–3 cm in length with occasion- (Lindberg & Bergman, 1993). A few potassium feld- TheVehmaa rapakivi granite batholith… 67 Fig 2. The medium-grained porphyritic granite of the Fig 4. Contact between the medium-grained porphyritic outerzone.The textureischaracterized by clusters of granites of theouter and inner z ones on outcrop.The potassium feldspar phenocrysts. N orth-eastern partof contact is indicated with arrows.The granite of the in- the granite body. ner zone is on the left of the contact. North-eastern partofthe granite body. and the coarse-grained porphyritic granite (see also Lindberg & Bergman, 1993). The medium-grained porphyritic granite is actual- ly composed of two distinct but v erysimilar zones of granite, defined on outcrops and by geophysical data as an inner zone and an outer zone,separated by avisi- ble contact. In the granite of the outer zone the potas- sium feldspar phenocrysts occur as clusters in anet- like texture, with only afew separate euhedral crys- tals (Fig. 2). No preferred orientation of the phenoc- rysts has been observedinthe outer granite and sin- gle potassium feldspar phenocrysts areseldom found Fig 3. The medium-grained porphyritic granite of the in the matrix. In the inner zone of granite the potassi- inner zone.The textureischaracterized by dispersed euhedral potassium feldspar phenocrysts. South-east- um feldspars occur as separate euhedral grains (Fig. 3), ern partofthe granite body. also found dispersed throughout the matrix. Magmatic flow-structures and miarolitic cavities areinplaces vis- spar ovoids occur. Rounded or lens-shaped aplitic fel- ible in the inner zone. The contacts between the out- sic inclusions (< 50 cm in mean diameter) and oc- er and the inner granite zones aresinuous and difficult casional small mafic enclaves occur in the granite. to observe (Fig. 4), which is partly due to their gener- Mafic layer-like schlieren rich in biotite are found at ally gentle dip,and partly because the rocks arevisually the southern and south-eastern margin of the gran- almost identical. On the magnetic map,the two gran- ite body. They strike NE and are subhorizontal or dip ites areclearly distinguished (Fig. 5), obviously due to gently ca 40–50° outwards. adifference in magnetite content. The eastern contact against the porphyryaplite A satellite intrusion, located in the eastern margin and the coarse-grained porphyritic granite dips ca of the Vehmaa batholith is composed of even-grained 60° outwards. At its northern contact the medium- granite. The red granite is a biotite granite with po- grained porphyritic granite cuts the porphyry aplite tassium feldspar,quartz, plagioclase, and biotite as 68 O. Selonen, C. Ehlers, H. Luodes and J. Lerssi Fig 5. Grey scale aeromagnet- ic total intensity mapshowing the area of the twomedium- grained porphyritic granites. The outer contact against the coarse-grained porphy- ritic granite is indicated with ablack line.Magnetic map shading is from the northeast at 60°elevation angle.For lo- cation see Fig. 1. GTK data, Jouni Lerssi. the main constitutes (Lindberg & Bergman, 1993). tacts. Altogether, these facts indicate an assemblage Miarolitic cavities, aplitic dykes and patches, red-col- of roughly concentric successive intrusions of gently oured joints, and greisen-like veins occur. dipping granite sheets, together building up the rapa- The porphyry aplite typically occurs at the margins kivi batholith. of different rapakivi types or at the margins of the The medium-grained porphyritic granite occurs as batholith (Shebanov et al., 2000). It is a fine to me- two distinct intrusions in the centreofthe batho- dium-grained rock with sporadic unmantled ovoids lith, forming almost a concentric pattern. Based on and feldspar phenocrysts. The colour is typically grey field relations the structure of the batholith indicates or red. a normal zoning with the older magmatic phases at the margins and the younger ones in the centre. The 3. Discussion porphyry aplites have intruded mostly as minor bod- ies and seem to be confined to contacts. The satellite The contacts of the Vehmaa rapakivi granite batho- intrusion at the eastern margin of the batholith oc- lith against the surrounding older Svecofennian rocks curs as a separate intrusion of even-grained granite. dip gently to moderately (20°–70°) away from the The relativeages for the intrusions from the old- batholith (Kanerva, 1928). We have observedthat est to the youngest are: pyterlite, coarse-grained por- the eastern contact of the medium-grained porphy- phyritic granite, and the medium-grained porphyrit- ritic granite dips ca 60° outwards and that the ma- ic granite (Lindberg &Bergman,1993). The even- fic layer-like schlieren in the S and SE parts of the grained granite of the satellite intrusion is younger granite are flat-lying or dip ca 40–50° outwards. Fur- than the pyterlite. Ourobservations indicate that the thermore, the sinuous contact between the inner medium-grained porphyritic granites in the centrecut and the outer zones of the medium-grained porphy- both the porphyryaplite and the coarse-grained por- ritic granite suggest a gently outward dipping con- phyritic granite in the northern parts
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