STRUCTURAL AND MAGMATIC EVOLUTION IN THE LOIMAA AREA, SOUTHWESTERN FINLAND
MIKKO NIRONEN
NIRONEN. MIKKO 1999. Structural and magmatic evolution in the Loimaa area, southwestern Finland. Bulletin of the Geological Society of Finland 71, Part 1, 57-71. Within the Loimaa area there is a junction of the general E-W structural trend of southern Finland and a NW-N-NE curving trend. The structure of the area is dominated by ductile D, and D4 deformations with E-W and N-S axial traces, respectively. The typical semicircular structures in the study area are interpret- ed as F,-F4 fold interference structures. The predominant plutonic rocks in the Loimaa area are penetratively foliat- ed tonalites and granodiorites which probably intruded during D2 deformation. Peak regional metamorphism at upper amphibolite facies and emplacement of the Pöytyä Granodiorite ca. 1870 Ma ago occurred during D, deformation. The ductile style of D4 deformation in the Loimaa area is probably related to the high-grade metamorphism at 1850-1810 Ma in the late Svecofennian granite- migmatite (LSGM) zone immediately south of the study area. The Oripää Granite was emplaced during D4 deformation. The structural evolution in the Loimaa area may be correlated with the evolution further to the northwest (Pori area) and north (Tampere-Vammala area) whereas correlation to the south and west is problematic. A transpressional model presented for the LSGM zone is not applicable to the Loimaa area.
Key words: structural geology, structural analysis, granites, absolute age, U/Pb, Svecofennian, Proterozoic, Loimaa, Finland
Mikko Nironen: Geological Survey of Finland, P.O. Box 96, FIN-02151 Espoo, Finland. E-mail: [email protected]
INTRODUCTION area. Moreover, one of the highest positive mag- netic anomalies in southern Finland exists within The Loimaa area, although rather poorly exposed, the study area. The Loimaa area, formerly mapped is geologically interesting because the general in the late 1940's and early 1950's (Salli 1953a, E-W structural trend of southern Finland shifts 1953b; Huhma 1957, 1959), was remapped by the into a curving trend (Fig. 1). The late Svecofen- author as a reconnaissance work for the Global nian granite-migmatite (LSGM) zone of southern Geoscience Transects Project. Within the limited Finland extends to the southern part of the study time (2Vfe months) emphasis was set on structures, 58 Mikko Nironen especially on structural setting of certain intru- ite gneisses, and in the southwestern part sions. migmatitic garnet-cordierite gneisses. The north- ern margin of the LSGM zone near Aura, marked as granite in Fig. 1, consists of garnet-bearing LITHOLOGY pegmatite and granite with relics of supracrustal rocks (mostly garnet-cordierite gneiss). The com- Supracrustal rocks position of garnets within the gneiss xenoliths in- The mineral assemblages in the Loimaa area in- dicates granulite facies conditions (Pentti Hölttä, dicate generally upper amphibolite facies with personal communication 1996). slight differences in metamorphic grade. Salli (1953b) divided the metavolcanic rocks of the area Plutonic rocks into two groups according to lithological differ- ences and preservation of primary structures. This Equigranular, medium-grained tonalites and grano- distinction still appears to hold. The rather well- diorites are the most voluminous and probably the preserved metavolcanic rocks in the southern part oldest plutonic rocks in the Loimaa area. They of the study area (Fig. 1) continue further east as typically contain mafic magmatic enclaves, and a part of the Forssa group of the Häme Belt (see penetrative foliation can be seen in these rocks Hakkarainen 1994). In the southeastern part ma- (Fig. 2d). The quartz dioritic body in the north- fic-intermediate pyroclastic rocks containing ural- ern part of the study area grades into tonalite and ite or less commonly plagioclase clasts predomi- may thus be coeval with the latter. The E-W trend- nate. Agglomerate and flow breccia (Fig. 2a) are ing coarse-porphyritic granite cross-cuts tonalite common. Plagioclase porphyry (Fig. 2b) occurs in at some outcrops. an E-W trending, dike-like setting in the southeast- The two oblate bodies in the central part of the ern part of the study area. Layered intermediate study area (NW and SW of Loimaa) consist of to felsic tuffites become predominant towards the granodiorite and granite. At the margins of these south. bodies migmatitic gneisses grade into a moderate- The metavolcanic rocks of the other group con- ly foliated plutonic rock containing wall-rock frag- sist mainly of hornblende gneisses. The origin of ments. This rock is cross-cut by a slightly foliated these rocks is not as obvious as for the first group, homogeneous granite. but lithological layering with alternating amphi- The large area southwest of Alastaro, mapped bolitic and feldspar-rich layers suggests a volcani- as granite, is lithologically heterogeneous. A mag- clastic origin. Massive uralite porphyry occurs in netite-bearing granite apparently causes the high places as interlayers within the hornblende gneiss- positive magnetic anomaly in Fig. 3. The granite es. The faintly layered quartz-feldspar-biotite is deformed and variable in grain size, ranging gneisses devoid of Al-rich porphyroblasts are in- from a fine-grained type to pegmatite. It contains terpreted as tuffite interlayers. In many places, abundant gneissic and tonalitic xenoliths, and the especially in the western part of the study area, vague appearance of some xenoliths suggests as- migmatization and assimilation have been so in- similation by the granite. Numerous pegmatitic tense that it is difficult to assess whether the rock and aplitic dikes cross-cut both the xenoliths and is a paragneiss or an orthogneiss (Fig. 2c). More- the host granite. over, the supracrustal rocks are cross-cut by The southwestern part of the study area is dom- ubiquitous pegmatite and granite dikes, therefore inated by the large (160 km2) Pöytyä Granodior- the area denoted as "hornblende gneiss" in Fig. 1 ite. The main part of the pluton is equigranular, is very heterogeneous in character. medium-grained and granodioritic to tonalitic in The metasedimentary rocks are in the northern composition. A granitic, porphyritic phase with K- part of the study area migmatitic, mostly K-feld- feldspar phenocrysts (up to 15 mm in length) spar-cordierite gneisses with some garnet-cordier- makes up the central part of the pluton. The gra- Structural and magmatic evolution in the Loimaa area, southwestern Finland 59
10 km
fxft>3 Granite I Porphyritic granite I Granodiorite and tonalite 50 km lllljl Quartz diorite and diorite Svecofennian rocks j ~ I Migmatitic sedimentary rock Potassium granite [ ] Hornblende gneiss Rapakivi granite Felsic and intermediate tuffite Jotnian sandstone Mafic pyroclastic rock Regional structural trend line
Fig. 1. Geological map of the Loimaa area. PG = Pöytyä Granodiorite, OG = Oripää Granite. The small map shows the general structural trends in southern Finland, location of the study area (thick solid line) and the area of Fig. 7 (thick dotted line). LSG M = late Svecofennian granite-migmatite zone, Sk = Skåldö, K = Kemiö, M = Mustio, Su = Suomusjärvi, So = Somero. 60 Mikko Nironen
Fig. 2. a) Flow breccia in amphibolite. x = 6744.66, y = 2455.83. Length of code bar 15 cm. b) Plagioclase- phyric rock, possibly a dike rock, x = 6741.14, y = 2456.66. c) A felsic gneiss of uncertain origin cross-
cut by felsic dikes. Both rock types are folded by F4. x= 6753.85, y = 2425.42. d) Granodiorite with a pen- etrative foliation (marked by solid white line) that is folded (fold axial trace is marked by broken line), x = 6752.10, y = 2449.62. e) K-feldspar-cordierite gneiss.
The penetrative foliation (Sj-S2) parallel to the litho-
logical layering is folded by F3 (axial trace is marked by broken line). Migmatite neosome occurs as a vein along the axial plane and as irregular patches in the Structural and magmatic evolution in the Loimaa area, southwestern Finland 61
dational contact between the rock units suggests porphyroblasts contain an internal foliation de- a close temporal relationship in their emplacement. fined by inclusion trails of small biotite flakes Mafic magmatic enclaves and pegmatitic dikes are (Fig. 2g). This foliation is microfolded, and axial fairly common in both phases. traces of these folds are parallel to the orientation The Oripää Granite in the western part of the of coarser biotite in the matrix. Because no mi- area is a pinkish rock with highly variable grain crofabric could be identified parallel to the litho- size (usually pegmatitic). The granite contains logical layering and the inclusion trails define the abundant fragments of foliated garnet-bearing bi- oldest observed microstructure, lithological layer- otite gneiss and hornblende gneiss. In many places ing is interpreted as bedding (S„), internal folia-
the rock shows schlieren migmatitic structures tion as Sl5 and matrix foliation as S2. Presumably
consisting of lensoid fragments of biotite gneiss K-feldspar grew at an early stage of D2, preserv- in pegmatitic granite. In the central part of the plu- ing the early microstructure (S,) while subsequent ton granite brecciates a medium-grained, foliated coarsening of the matrix and the development of
tonalite. S2 obliterated S! outside the porphyroblasts (cf. Passchier & Trouw 1996, Fig. 7.4). Lithological layering in the metasedimentary STRUCTURES rocks in the northern part is folded by open to iso-
clinal F3 folds with steep, approximately E-W Low-altitude aeromagnetic maps were used to sup- striking axial surfaces. Leucosome in K-feldspar- port the structural and lithological interpretation. cordierite gneisses occurs as veins subparallel to
The structural pattern in the study area is charac- S2 (Fig. 2f), as veins parallel to F3 axial plane, and terized by semicircular structures (Fig. 3). Another as irregular patches at the hinges of mesoscopic notable feature is the E-W trending structural pat- F3 folds (Fig. 2e). These features indicate migma- tern in the northern and central part, NE trending tization during D3. In microscale S3 is expressed pattern in the southwestern part, and N trending as differentiated crenulation cleavage, associated pattern in the northwestern part. with reorientation and recrystallization of coarse
The northern part of the study area, occupied (syn-D2) biotite into small grains, minor growth by K-feldspar-cordierite gneisses, is the key area of biotite parallel to F3 axial plane, pinitization of for the structural interpretation. The predominant cordierite, and sparse formation of muscovite. foliation (preferred orientation of mica, and dif- Commonly cordierite has overgrown coarse biotite ferentiation into mica-rich and quartz-feldspar lay- that has been reoriented during F3 folding. In ar- ers in micaceous lithologies) is subparallel to the eas of intense S3 crenulation cleavage, clusters of lithological layering (Figs. 2e and 2f). K-feldspar fibrous sillimanite within cordierite express two generations of microfolding, the older with axial
plane parallel to S2 in the matrix (not shown) and the younger parallel to S (Fig. 2g). Provided that fold hinge area, x = 6767.68, y = 2445.48. f) K-feld- 3 spar-cordierite gneiss with leucosome veins subparal- sillimanite microfolds are related to regional fold- lel to the composite layering (S/—S2). The hinge of iso- ing episodes, the microstructures described above clinally folded bedding between the two leucosome veins suggest that cordierite grew during syn-D, peak is shown by the white arrow. The composite layering of metamorphism when most of the leucosome and leucosome veins are folded openly by Fs. The drill was formed, preserved F2 and F, microfolded sil- hole in upper right part is the sample site for the thin limanite as relics, and continued to grow after F section shown in Fig. 2g. x = 6769.19, y = 2444.35. g) 3 folding. Relics of sillimanite in cordierite are quite F2 microfolding in K-feldspar porphyroblasts (marked common in pelitic rocks of southern Finland, and by white line) and leucosome folded by F3. The cluster of fibrous sillimanite (Sil), preserved in undeformed microfolding in sillimanite seems to be related to cordierite, is microfolded with an axial trace parallel regional folding at least in some areas (e.g. Niro- to that of mesoscopic F3 folding. nen 1995). 62 Mikko Nironen
Geological Survey of Finland
anomaly map
Fig. 3. Low-altitude aeromagnetic map of the Loimaa area.
Regional D2 deformation was complex in south- foliation in the plutonic rock is parallel to the com- ern Finland (Koistinen et al. 1996) but on the ba- posite (S0-S2) layering in the gneisses, hence it is sis of the structures in the northern part the pene- interpreted as contemporaneous with S2. In areas trative foliation elsewhere in the study area, where the penetrative foliation in the tonalites and marked by coarse biotite and hornblende, is prob- granodiorites is folded (Fig. 2d) the fold axial sur- ably S2; S0 and S, have largely been transposed face is parallel to S3 in supracrustal rocks. These to be subparallel to S2. Upright F3 folding trans- features imply that the tonalites and granodiorites posed D2 and earlier structures to generally steep were emplaced before D2 or, more probably, at the orientations (Fig. 4). F4 folding exists in the study early stage of D2 deformation. Also the porphy- area as open to tight, mesoscopic to macroscopic ritic granite in the north as well as the magnetite- folds with approximately N-S trending axial bearing granite in the central part of the study area traces. In microscale older fabrics are crenulated contain a D3 fabric. in mica-rich layers showing minor growth of bi- The NE trending foliation in the southwestern otite in the axial plane. The large semicircular part of the study area is a composite foliation: structures, including the oval-shaped domal struc- migmatitic garnet-cordierite gneisses exhibit dif- ture in the center of the Loimaa area, are inter- ferentiated layering in the mesosome that in places
peted as type 1 (dome-and-basin) F3-F4 fold in- is tightly folded (Fig. 5a). The fold style and the terference structures. occurrence of leucosome are similar to those in Where the tonalites and granodiorites are in the northern part of the study area (cf. Figs. 2e and contact with supracrustal gneisses the penetrative 5a). Leucosome garnet is devoid of inclusions Structural and magmatic evolution in the Loimaa area, southwestern Finland 63
Fig. 4. Structural interpretation of the Loimaa area, based on geological data and aeromagnetic signatures. whereas garnet in the mesosome is riddled with are deformed. In the northern part of the pluton inclusions. Inclusion trails in the mesosome gar- deformation increases in intensity outwards from net curve and continue as a crenulated foliation the center and the foliation is parallel to the mar- in the matrix (Fig. 5b). Microfolding in the ma- gins. In places the penetrative foliation is de- trix is generally tighter than the curvature of the formed by ductile shear zones that are commonly inclusion trails. The matrix foliation is interpret- filled with granitic material. In a sample from the ed as S2, and the overprinting folding is F3. The granodioritic-tonalitic main phase close to the microstructures indicate that garnet in the meso- boundary of the porphyritic variety, feldspars are some grew when F3 crenulation had initiated and dynamically recrystallized whereas strain-free that the tightening of microfolds in the matrix is quartz is an evidence of post-deformation static the result of continued D3 deformation after gar- recrystallization (Fig. 5c). Penetrative foliation as net growth. well as ductile deformation of feldspars indicate The Pöytyä Granodiorite is pervasively de- deformation at temperatures above ca. 500°C formed, and also the cross-cutting pegmatite dikes (Gapais 1989). In contrast, feldspars in the por- 64 Mikko Nironen
2 mm
Fig. 5. a) Migmatitic garnet-cordierite gneiss (F, axial trace is marked by broken line). Length of code bar 15 cm. x = 6742.80, y = 2421.00. b) Microfolding in garnet-cordierite gneiss. Note curvature of inclusion trails in gar- net (right) and plagioclase-quartz rim around garnet, x = 6742.56, y = 2421.25. c) Microstructure of the equi- granular granodiorite of the Pöytyä batholith. Feldspars are largely recrystallized whereas quartz (Q) is unde- formed and has straight or gently curving quartz/quartz grain boundaries (e.g. at the upper right corner), x = 6742.00, y = 2431.46. d) Microstructure of the porphyritic granite of the Pöytyä batholith. Feldspars are subhe- dral and rather undeformed. The high-angle boundaries within strained quartz (Q) indicate subgrain rotation recrystallization, and irregular margins indicate grain boundary migration recrystallization. x = 6740.50, y = 2424.04. phyritic phase are subhedral and weakly deformed The composite foliation in the supracrustal (Fig. 5d). Strain concentrated in quartz which de- gneisses is parallel to the contact of the Pöytyä formed by combined subgrain rotation recrystal- Granodiorite. Close to the northern contact S2 is lization and grain boundary migration recrystal- folded in the gneisses, and the fold axial planes lization, indicating deformation at high tempera- are parallel to the contact. Hence the granodior- tures (cf. Passchier & Trouw 1996, p. 48). These ite deformed D2 structures during emplacement, microstructural differences suggest that the gran- i.e. the Pöytyä Granodiorite was emplaced after odioritic-tonalitic phase was emplaced earlier and D2 deformation. At the southern margin S3 con- deformed during cooling while the porphyritic tinues across the contact into the pluton - this is phase - emplaced later - was still close to melt- also evident from the aeromagnetic map (Fig. 3). ing temperature during coeval deformation. Such At the southwestern margin of the Pöytyä Grano- conditions would also result in a thermal pulse diorite S3 cuts across a weak contact-parallel fab- causing the observed static recrystallization of ric. Since the microfabrics in the two plutonic quartz in the surrounding granodiorite-tonalite. phases indicate deformation at high temperatures, Structural and magmatic evolution in the Loimaa area, southwestern Finland 65
and since syntectonic pluton emplacement com- The ages were calculated using the decay con- monly involves geometric continuity between the stants of Jaffey et al. (1971). The precision is giv- country rock foliation and pluton fabric (e.g. Brun en at the 2-sigma level. & Pons 1981), the emplacement of the Pöytyä A total of 15 zircon fractions from the porphy-
Granodiorite probably took place during D3 defor- ritic phase of the Pöytyä Granodiorite yielded an mation. age of 1869 ± 8 Ma (Fig. 6a). An age of 1870 ±
F4 folding becomes regionally dominant to- 5 Ma was obtained from eight zircon fractions of wards the northwest. The fairly symmetric style the granodioritic-tonalitic main phase, i.e. the
in the zone of intense F4 folding suggests that the same as from the porphyritic phase within error zone is not a shear zone but a zone of shortening. limits (Fig. 6b). There is no indication of an older This is consistent with the large variation in zircon population except xenoliths that yield dis- plunge of mineral lineations in the zone. Howev- tinct isotopic ratios (Fig. 6a, fraction L). The 1784 er, in many places it was impossible to assess the ± 8 Ma and 1780 ± 12 Ma titanite ages from the foliation-lineation relationships in the composite porphyritic phase and granodioritic-tonalitic phase, foliation planes. respectively, date cooling through the ca. 650°C The pegmatites of the Oripää Granite are in closure temperature of titanite (Scott & St-Onge places slightly deformed, especially in the eastern 1995, Pidgeon et al. 1996). part, but a medium-grained granitic type is unde- The zircons of the Oripää Granite consist of red formed. Undeformed magnetite-bearing granite/ and pale brown varieties. There are several differ- pegmatite dikes, similar to the undeformed Oripää ences between the two populations (Table 1, Fig. Granite, are emplaced along axial surfaces of 6c): 1) uranium content is higher in the red vari-
mesoscopic F4 folds in the western part of the ety in the same density fraction; 2) the red zircons study area. are less discordant than the pale brown ones; and Late shear zones and faults are clearly visible 3) the two populations form separate regression in low-altitude aeromagnetic maps as distinct neg- lines, 1850 ± 27 Ma (red) and 1860 ± 41 Ma (pale ative anomalies cross-cutting the lithological lay- brown). The large error limits show that there is ering. An E-W trending anomaly in the southern a considerable dispersal in the two zircon popu- part of the study area is a subvertical mylonite lations suggesting mixing between them. The 1794 zone, 100-300 m in width. In places an S-C fab- ± 10 Ma age of monazite dates cooling below ca. ric is developed, indicating predominantly hori- 700°C (Parrish 1990, Mezger et al. 1991). zontal (sinistral) sense of movement (Fig. 4). In- tense retrogressive alteration (e.g. epidotization) characterizes the mylonite zone. STRUCTURAL AND MAGMATIC The NNW trending anomaly in the northwest- EVOLUTION ern part of the study area (Fig. 4), diverging into two anomalies towards the south, has been stud- In addition to the age data of the Pöytyä and ied in detail by geophysical methods (Geologian Oripää granitoids referred above, a few U-Pb dat- tutkimuskeskus et al. 1993). It is a brittle fault that ings of rocks in southwestern Finland help con- is younger than the mylonite zone and locates cir- straining the timing of the structural and magmatic culation of ground water. evolution in the Loimaa area. A ca. 1890 Ma age has been obtained from a felsic metavolcanic rock of the Häme Belt (Vaasjoki 1994). This age de- U-Pb ISOTOPIC DATA fines the upper limit for the earliest deformation in the Loimaa area. Gabbroic pegmatoids with a U-Pb isotopic analyses were made on zircon, ti- ca. 1885 Ma pooled zircon age record the earliest tanite and monazite from the Pöytyä Granodior- deformational episodes in the Skåldö area (Fig. 1) ite and the Oripää Granite (Fig. 1 and Table 1). whereas a ca. 1875 Ma monazite age from the 66 Mikko Nironen
Table 1. U-Pb isotopic data on zircon, titanite and monazite from the Loimaa area. Error values for each ratio point are available on request.
Sample, fraction (l) Concentration 206Pb/204Pb Isotopic composition of lead Atomic ratios and (ppm) measured 206Pb = 100 radiometric ages, Ma
238JJ 206pb 204pb 20?pb 208J>b 206Pb/238U 207pb/235U 207pb/206pb
A121 Pöytyä Granodiorite, porphyritic phase A d>4.5 458.1 119.10 1968 .04650 11.936 9.670 .3005 4.684 .11306 1693 1764 1849 B titanite; 3.5
Table 1 (continued)
A1357 Oripää Granite A 4.2 same area has been interpreted as the age of re- tains tonalitic xenoliths it is younger than the to- gional metamorphism (Hopgood et al. 1983). nalites and granodiorites, but the relationship be- D] and D2 deformations and syn-D2 emplace- tween emplacement of the granite and D2 is un- ment of the oldest tonalites and granodiorites in known. The Pöytyä Granodiorite was emplaced at the Loimaa area probably took place during 1890- ca. 1870 Ma during D3 deformation. Because the 1880 Ma. Since the magnetite-bearing granite con- structural setting of the E-W trending coarse-por- i i 1— 1 1 1 a 0.35 Pöytyä Granodiorite, 0.35 _ Oripää Granite porphyritic phase 1869±£ (Oripää AI 357) (Kolkkinen Al 21) 1784±£ 1794±10OJ^1860±41 13 0.20 - 0,20 /F + O ZIRCON E a MONAZITE — 1 1 1 i_ i i i 1 1 1 30 207 236 3.0 Pb/ U 5.0 207Pb/235U 5.0 b 0.35 . Pöytyä Granodiorite, main phase 1870±5^--" (Järvenmäki A296) 1780+12 n Q_ - Fig. 6. Concordia diagrams of U-Pb isotopic data from 0.20 granitoids of the Loimaa area, a) Pöytyä Granodior- • o + ZIRCON ite, porphyritic phase, x = 6740.50, y = 2424.04. ^ 244 ±22 • TITANITE b) Pöytyä Granodiorite, main phase, x = 6742.00, y = — iii. 3.0 207PI3/235U 5.0 2431.46. c) Oripää Granite, x = 6752.20, y = 2425.80. 68 Mikko Nironen phyritic granite in the northern part of the area is of F3 folding (NW to WNW striking axial sur- apparently controlled by D3, the granite may be faces) on earlier structures. These examples show coeval with the Pöytyä Granodiorite. that dome-and-basin structures similar to those in Ductile F4 folding has influenced the structur- the Loimaa area are common in southwestern Fin- al pattern in the Loimaa area. In the north, E-W land, but interpretations of the folding episodes striking F3 axial surfaces are in places deformed related to their formation and of the orientation of by large-scale F4 folding. The strike of S3 planes fold axial surfaces vary considerably. shift into NE towards the southwestern part of the Approximately N-S striking late Svecofennian study area. There is few data on the orientation foliation is visible all over southern Finland, es- of foliation within the weakly deformed granites pecially in the eastern part (Gaål 1982 and refer- in the two domal F3-F4 fold interference struc- ences therein). Hopgood (1984) revealed the struc- tures, therefore the relationship between emplace- tural sequence in the migmatites of the Skåldö area ment and folding episodes is unclear. The struc- (Fig. 1), including N-S striking planar structures. ture occupied by the Oripää Granite is also inter- Schreurs and Westra (1986) described mesoscopic preted as an F3-F4 interference structure. The and macroscopic folding (their F3) with subverti- structures within the granite suggest dominantly cal N-S striking axial surfaces in the Suomusjärvi- in situ melt segregation. The medium-grained Mustio area (Fig. 1) and concluded that granulite granite may represent a melt fraction that segre- grade metamorphism preceded F3 folding. gated from the host rock. The lack of deformation Pietikäinen (1994) studied the Pori Shear Zone in the granite and the occurrence of the granitic northwest of the Loimaa area and concluded that dikes at F4 axial surfaces suggest emplacement at dilatational brecciation of tonalites to form veined a late stage of D4. Unfortunately, zircons from the gneisses took place during regional D,. He further Oripää Granite yield minor information of mag- inferred that mylonitization at the northeastern matism and deformation, but the monazite dating margin of the zone (within the steeply NE dipping indicates that the Oripää Granite was still hot ca. Main Fault Zone; Fig. 7) occurred during late D3 1795 Ma ago. and that the zone is extensional with normal off- sets down to the north and a minor sinistral hori- zontal component. The S, compositional banding in the tonalites of the Pori area is the predominant DISCUSSION foliation and therefore similar to the penetrative S2 foliation in the tonalites and granodiorites of the Domal structures have been described in several Loimaa area. An L2 lineation is the most promi- areas of southwestern Finland. Van Staal and Wil- nent structural element in the Pori area, while L3 liams (1983) interpreted such a structure in the is the predominant lineation in the Loimaa area. Kemiö area (Fig. 1) as an F3-F4 interference struc- Hence it seems that D, structures in the Pori area ture in which F3 axial traces generally trend NNE may be correlated with D2 structures in the Loimaa or NE (see also Verhoef & Dietvorst 1980) where- area; possibly S1; visible as inclusion trails in por- as the F4 axial plane is E-W trending. The domal phyroblasts, did not develop in the Pori area. Ac- structure in the Mustio area having a granitic core cordingly, D3 structures in the Pori area are cor- was interpreted by Bleeker and Westra (1987) as related with D4 structures in the Loimaa area an F2-F3 interference structure (F2 and F3 axial (Fig. 7). However, this correlation is ambiguous surfaces striking E-W and NNE, respectively) because in the Pori area late D3 deformation was whereas Veenhof and Stel (1991) considered the shearing whereas in the Loimaa area F4 folding structure as the result of a single-phase progres- was probably the result of shortening. sive event (D,). Stel et al. (1989) interpreted the Kilpeläinen (1998) concluded that the D2 and two domal structures in the Somero area (both D3 (possibly also D,) structures in the Tampere- with granitic cores) as the result of superposition Vammala area, north of the Loimaa area, devel- Structural and magmatic evolution in the Loimaa area, southwestern Finland 69 oped due to N-S compression. The D3 structures in the Loimaa area are compatible with N-S com- pression but the original attitude of the D2 struc- tures remains unrevealed because they have large- ly been transposed by younger deformation. Kil- peläinen (1998, Fig. 49) interpreted also the NNW-SSE and N-S trending structures as D3 structures; these are interpreted as D4 structures in the present study (Fig. 7). The structural pattern in the Turku-Uusikau- punki area, southwest and west of the Loimaa area, is characterized by folds with vergence to- wards the northwest (Selonen & Ehlers 1998, Väisänen & Hölttä 1999). The granites within the the LSGM zone occur as sheets in generally gen- tly dipping planar structures (Ehlers et al. 1993, Selonen et al. 1996). Ehlers et al. (1993) present- ed a model for the granites of the LSGM zone in- cluding emplacement 1840-1830 Ma ago along t pre-existing subhorizontal structures and simultan- ' ~ eous westward translation; i.e. emplacement with- Fig. 7. A model to correlate the structures in the in a transpressional zone with vergence to the NW. Loimaa, Pori (P), and Vammala (V) areas. Interpreta- The granites were subsequently refolded in open tion is based on low-altitude aeromagnetic map and folds with E-W trending fold axes. Lindroos et al. field observations by the author in the southern part, (1996) dated a pegmatite in the Kemiö area and Pietikäinen (1994) in the Pori area, and Kilpeläinen (1998) in the Vammala area. The relative sense of concluded that transpressional deformation lasted movement in the Main Fault Zone (MFZ) is shown by until 1803 Ma. As the overall structural pattern in arrows and block movement directions (U = up, D = the LSGM zone differs from that in the Loimaa down). The study area is shown by a dotted line. area, where the E-W to NE-SW trending structures were developing already 1870 Ma ago, it is diffi- cult to apply this model to the Loimaa area. Metamorphism culminated between 1850 Ma and 1810 Ma in the LSGM zone (Korsman et al. ed a 1869 + 5 Ma age (van Duin 1992). These data 1984). U-Pb ages between 1860 Ma and 1820 Ma strongly suggest that the 1860-1820 Ma ages of have been obtained from pyroxene-bearing grani- the pyroxene granitoids are the result of high- toids around Turku, southwest of the Loimaa area grade metamorphism, as concluded by Väisänen (Suominen 1991, van Duin 1992; see inset in et al. (1994), and that the true age of these plu- Fig. 1). The granitoids are located within a tons is ca. 1870 Ma. The high-grade metamor- granulite facies area and contain zoned zircons phism in the LSGM zone presumably caused a with secondary overgrowths. A study of zircon thermal event with temperature exceeding the clo- from a pyroxene granitoid suggests that the 1862 sure temperature of the U-Pb system for titanite Ma age is a mixed age of two populations, one in the adjacent Loimaa area. Therefore the 1780 giving primary crystallization ca. 1880 Ma ago Ma age of titanite in the Pöytyä Granodiorite and the other giving the high-grade metamorphism records the decrease in the temperature of regional at ca. 1843 Ma (Suominen 1991, Appendix 5). metamorphism below ca. 650°C. This late meta- Moreover, a tonalite outside the granulite facies morphic peak in the LSGM zone is probably also area, 30 km SW of the Pöytyä Granodiorite, yield- the reason for the ductile style of D4 deformation. 70 Mikko Nironen SUMMARY Geologische Rundschau 71, 158-170. Gapais, D. 1989. Shear structures within deformed gran- ites: Mechanical and thermal indicators. Geology 17, The oldest tonalites and granodiorites in the 1144-1147. Loimaa area were emplaced 1890-1880 Ma ago, Geologian tutkimuskeskus, Turun vesi- ja ympäristöpiiri, Turun Seudun Vesi Oy & Varsinais-Suomen Seutukaa- before the end of regional D2 deformation. The valiitto 1993. Virttaankankaan - Oripäänkankaan - Su- Pöytyä Granodiorite intruded 1870 Ma ago dur- lajoen harjujakson geofysikaaliset ja geologiset tutki- ing D3 deformation. The Oripää Granite was em- mukset 1992. (report, in Finnish) Gillot, Turku. 20 p. placed in an F3-F4 fold interference structure dur- Hakkarainen, G. 1994. Geology and geochemistry of the ing D4. Peak metamorphism was attained during Hämeenlinna-Somero Volcanic Belt, southwestern Fin- land: a Paleoproterozoic island arc. In: Nironen, M. & D deformation. E-W compression at some inter- 3 Kähkönen, Y. (eds.) Geochemistry of Proterozoic su- val between 1850 Ma and 1800 Ma caused duc- pracrustal rocks in Finland. Geological Survey of Fin- tile D4 deformation, coeval with medium/high land, Special Paper 19, 85-100. grade metamorphism in the adjacent LSGM zone. Hopgood, A.M. 1984. Structural evolution of Svecokare- lian migmatites, southern Finland: a study of Protero- The location of the Loimaa area at the intersec- zoic crustal development. Transactions of the Royal So- tion of different structural trends in southwestern ciety of Edinburgh: Earth Sciences 74, 229-264. Finland restricts structural and metamorphic cor- Hopgood, A.M., Bowes, D.R., Kouvo, O. & Halliday, A.N. relation with adjacent areas. The D, structures in 1983. U-Pb and Rb-Sr isotopic study of polyphase de- formed migmatites in the Svecokarelides, southern Fin- the Loimaa area may be correlated with D, struc- land. In: Atherton, M.P. & Gribble, C.D. (eds.) Mig- tures in the Pori area and the D3 structures in the matites, Melting and Metamorphism. Nantwich: Shiva, Loimaa area may be correlated with D3 in the 80-92. Tampere-Vammala area whereas correlation to the Huhma, A. 1957. Marttila. Geological map of Finland 1:100 000, Pre-Quaternary rocks, Sheet 2022. Geologi- south (the LSGM zone) and west is problematic. cal Survey of Finland. Evidently the overall structural correlation in Huhma. A. 1959. Geological map of Finland 1:100 000, southwestern Finland needs further work. Sheet 2022 Marttila. Explanation to the map of rocks. Geological Survey of Finland. 38 p. Jaffey, A.H., Flynn, K.F., Glendenin, L.E., Bentley, W.C. ACKNOWLEDGEMENTS: I am grateful to Olavi & Essling, A.M. 1971. Precision measurement of half- Kouvo, Matti Vaasjoki and the staff of the Unit for lives and specific activities of 235U and 238U. Physical Isotope Geology, Geological Survey of Finland, Reviews C4, 1889-1906. for providing the U-Pb isotopic data, and to Maija Kilpeläinen, T. 1998. Evolution and 3D modelling of struc- tural and metamorphic patterns of the Palaeoproterozo- Kurimo for processing the aeromagnetic map. ic crust in the Tampere-Vammala area, southern Fin- Håkan Sjöström and Pekka Tuisku made valuable land. 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