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 potassium feldspar phenocrysts occur as clusters in anet- like texture, with only afew separate euhedral crys- tals (Fig. 2). No preferred orientation of the phenocrysts 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 granite 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 porphyritic 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 of the batholith. tact. The geophysical data support the field observa- It is suggested that the Vehmaa batholith was em- tions and correspond to the observed geological con- placed in two stages (Lindberg & Bergman, 1993). TheVehmaa rapakivi granite batholith… 69

The pyterlite and the subsequent coarse-grained porphyritic granite intruded in the fi rst stage, whereas the second stage comprises the medium-grained porphyritic granite followed by the satellitic even-grained granite. The porphyry aplites in the pyterlite are the last intrusions of the first phase, whereas the porphyry aplite in contact with the coarse-grained porphyritic granite and medium-grained porphyritic granite be- gan the second phase of intrusion. The U-Pb zircon and monazite ages overlap, but generally agree with the geological observations regarding the sequence of intrusions (Lindberg & Bergman, 1993). Fig 6. Asketch model forintrusion mechanism of the Glazner et al. (2004) show examples of how large, Vehmaa rapakivi granite batholith. Successivepulses of fractionated magma intrude in connection with the de- seemingly homogeneous plutons can be formed velopment of acollapse caldera, forming subhorizontal by amalgamation of numerous smaller intrusions sheets of subvolcanic magma with steeper margins.The through time. The growth of a pluton proceeds with shape and dimensions of the magma chamber and col- intrusion of individual increments of subhorizontal lapsing blocks arehypothetical. The magma pulses are younging upwards. or subvertical sheets. The gently dipping intrusive contact between the two medium-grained porphyritic granites of the Vehmaa batholith indicates that Asimilar mode of intrusion has b een described they are distinct magmatic phases even if they are pet- earlier for the small Fjälskär rapakivi granite in SW rologically almost similar in composition. The con- Finland, where conspicuous gently outward dipping centric occurrence of the two zones suggest that the ring dykes forms an umbrella-like pattern around the inner phase is younger than the outer phase, and in- rounded stock (Ehlers & Bergman, 1984). A lack of dicate a build up of the batholith by successive addi- brecciated contacts and often steeply to moderate- tions of smaller intrusions. If the amalgamated small- ly dipping outer contacts coupled with subhorizon- er intrusions are petrologically and chemically similar tal multiple intrusions in the central parts of many to the previous pulses, and if the contacts (in the pre- rapakivi batholiths also supportthis type of intru- vailing flat topography) are gently dipping, this mode sion mechanism. The presence of superficial volcan- of formation can be effectively camouflaged and over- ic varieties of rapakivi rocks along the fringes of, e.g. looked. the Åland and Wiborg batholiths (Sederholm, 1890; The general lack of observed feeder dykes to the Bergman, 1986; Eklund et.al.,1996), and their gen- successive intrusions of rapakivi granite could be ex- eral character of multiple overlapping intrusion com- plained with amodelofanintrusionwith aplate plexes, together suggest an emplacement mechanism or piston-type collapsing centre (Cole et al., 2005). related to the formation of syn-intrusional volcan- This mechanism implies the progressive subsidence ic calderas, eroded today to expose their subvolcan- of blocksofolder gneisses andsuccessive repeated ic batholiths. pulses of granite intrusion. The subsidence of gneiss blocks and subhorizontal sheet intrusions of rapakivi Acknowledgements granites could have been partly controlled by the gen- The geological map of the Vehmaa rapakivi granite tly dipping schistosity in the surrounding supracrus- batholith was compiled by Mrs. Terttu Muraja, which tal Svecofennian gneisses. A tentative model for in- is gratefully acknowledged. Tapani Rämö and Mikko trusion of the Vehmaa granite batholith is present- Nironen reviewed the manuscript, which considerably ed in Fig. 6. improved the text. 70 O. Selonen, C. Ehlers, H. Luodes and J. Lerssi

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