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Rapakivi granites and related rocks: an introduction
Precambrian Research 95 (1999) 1–7
Preface
Rapakivi granites and related rocks: an introduction I. Haapala *, O.T. Ra¨mo¨ Department of Geology, P.O. Box 11, University of Helsinki, 00014 Helsinki, Finland
Abstract This article is an introductory chapter for the proceedings of the last, seventh, meeting of the International Geological Correlation Programme (IGCP) Project 315 ‘‘Correlation of Rapakivi Granites and Related Rocks on a Global Scale’’, 1991–1996. Based on current knowledge, progress made during the six years of the project, and the seven papers published in this special issue, a synthesis of the age distribution, geotectonic environment, petrogenesis, geochemistry, and metallogeny of the rapakivi granites and related rocks is presented. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Geochemistry; Isotopic age; Petrogenesis; Rapakivi granite; Rapakivi texture
The classic rapakivi granites of southern Finland have been documented in Swedish and Finnish literature for ca 300 years. In 1891, J.J. Sederholm introduced the Finnish rapakivi granites to the international readership (Sederholm, 1891), and at the end of the nineteenth and beginning of the twentieth century, rapakivi granites were described from the Ukraine and Sweden. After the Second World War, a new period of activity commenced in this field, and numerous rapakivi granite occurrences were described from the Baltic countries, South Greenland, Labrador, mid-continental and western USA, Venezuela, Brazil, Botswana and several other Precambrian shield areas. The formation of Proterozoic rapakivi granites and associated anorthosites was considered as an important tectonomagmatic event or period in the construction of the continental crust (e.g. Bridgwater and * Corresponding author. Tel: +358 01911; fax: +358 0191 23466; e-mail: [email protected]
Windley, 1973; Bridgwater et al., 1974; Emslie, 1978; Anderson, 1983). Rapakivi granite complexes were also recognized as showing traits of the typical tin granites, and discoveries of tin mineralization in Fennoscandia, Brazil and Missouri led to understanding the ore-generating capacity of rapakivi granites (e.g. Haapala, 1977a; Bettencourt and Dall’Agnol, 1987; Kisvarsanyi and Kisvarsanyi, 1990). In 1991, a new project (IGCP-315) was founded within the framework of the International Geological Correlation Programme to correlate research on the rapakivi granites and related rocks on a global scale, now comprehensively including the developing countries where a large number of poorly studied rapakivi complexes were known to occur. The main topics of the project comprised the areal and temporal distribution of the rapakivi granites, their tectonic setting and mechanism of emplacement, their relation to crustal evolution, the bimodal character of the magmatism, petro-
0301-9268/99/$ – see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S0 3 0 1- 9 2 68 ( 9 8 ) 0 01 2 4 -7
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graphic and geochemical characteristics, metallogeny, petrogenesis, physical conditions of crystallization and origin of the rapakivi texture. During its six years of activity (1991–1996), the project arranged seven international symposia and six field trips to key areas, and was joined by researchers from >20 countries. Several new rapakivi complexes were identified and a lot of new information on the topics of the project was acquired. These have so far been documented in two proceedings volumes ( Haapala and Ra¨mo¨, 1994; Dall’Agnol and Bettencourt, 1997), five abstract volumes (Haapala and Ra¨mo¨, 1991; Kisvarsanyi, 1993; Emslie, 1994; Dall’Agnol et al., 1995; Haapala et al., 1996), seven field trip guide books (Haapala et al., 1991; Kisvarsanyi and Hebrank, 1993; Higgins and Woussen, 1994; Berg et al., 1994; Bettencourt and Dall’Agnol, 1995; Ahl et al., 1996, 1997) and >60 original papers including two global reviews (Ra¨mo¨ and Haapala,
1995, 1996). The present issue includes seven selected papers from the last meeting of IGCP-315 held on 24–26 July 1996, at the University of Helsinki, Finland. The papers deal mainly with the geochronology, petrology and metallogeny of rapakivi complexes of Brazil, Finland, Missouri and Sweden. These contributions are briefly reviewed below together with the earlier results of the project. Most of the rapakivi granites are Proterozoic in age (generally 1.0 to 1.8 Ga) but there are also Archaean and Phanerozoic granite complexes that can be considered rapakivi suites (Fig. 1). The current definition of rapakivi granite (Haapala and Ra¨mo¨, 1992) does not restrict the age of the rocks and simply considers them ‘‘A-type granites characterized by the presence, at least in the larger batholiths, of granite varieties showing the rapakivi texture’’. Several periods of Proterozoic rapakivi magmatism, often apparently without any clear
Fig. 1. Global map showing the distribution of rapakivi granites. Age data (in Ga) from Ra¨mo¨ and Haapala (1996) and references therein and articles in this volume. Modified from Fig. 2 in Ra¨mo¨ and Haapala (1996).
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systematic geotectonic or areal control, have been recognized from Brazil (e.g. Dall’Agnol et al., 1999; Bettencourt et al., 1999) and Fennoscandia (Alviola et al., 1999; Persson, 1999). The age groups in different shield areas correspond only partially to each other, but, for instance, the 1.54–1.58 Ga age group is present in Fennoscandia ˚ land, Vehmaa, Laitila and (Salmi, Riga, A Nordingra˚ batholiths), Venezuela (Parguaza, Sucucucu and Mucajaı´ batholiths) and Rondoˆnia (Serra da Provideˆncia batholith and its satellites), and the 1.31–1.41 Ga and 1.02–1.08 Ga age groups are found in Rondoˆnia and in the mid-continental to western USA. As pointed out by Sadowski and Bettencourt (1996) and Bettencourt et al. (1999), the rapakivi age groups can be effectively utilized for continental reconstructions. The rapakivi granites were definitely not produced by a single ‘rapakivi event’, but were rather formed in several magmatic episodes from the Late Archean to the Tertiary, although the magmatism was most voluminous between 1.8 and 1.0 Ga. The magmatic association of the rapakivi granites is clearly bimodal (mafic–felsic); diabase, gabbro and anorthosite are found together with rhyolite, granite and syenite. Interaction of coexisting mafic and felsic magmas has locally produced hybrid intermediate (e.g. monzodioritic) members. Mafic plutonic rocks seem to be abundant in the lower parts of the rapakivi complexes. There are some rapakivi plutons that appear not to be associated with mafic rocks, but this may be due to a relatively high erosion level or lack of ample exposure. In addition, diabase and rhyolite dykes outside the rapakivi plutons may be easily overlooked in routine geological mapping. In this volume, excellent examples of the bimodal character of the rapakivi magmatism are given from southeastern Missouri by Lowell and Young (1999), central Sweden by Persson (1999) and southeastern Finland by Alviola et al. (1999). Since the study of Bridgwater and Windley (1973), incipient or aborted rifting has been suggested as the tectonic environment of many anorogenic granitic suites. Extensional geotectonic setting has been convincingly documented for the rapakivi complexes of Finland by recognition of
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rapakivi-age subparallel diabase and porphyry dykes that transect the Paleoproterozoic crust (Haapala and Ra¨mo¨, 1990), graben structures (Haapala, 1988), thinning of the crust in rapakivi areas (Luosto et al., 1990; Haapala and Ra¨mo¨, 1992) and listric faulting ( Korja and Heikkinen, 1995). Also the 1.70 Ga Shachang and other rapakivi complexes in the Beijing area, China, are accompanied with concurrent graben structures and thinned crust ( Yu et al., 1996). Similar structural features and magmatic associations as in Fennoscandia have been documented for the Basin and Range Province of south-western North America where rapakivi-type granites are related to an extreme Miocene extension (Haapala et al., 1995; Calzia and Ra¨mo¨, 1997). The 1.68–1.79 Ga Dala granitoids of Sweden represent a transition from a compressional to an extensional tectonic regime: the 1.79 Ja¨rna granitoids are the last, postorogenic, expressions of subduction-related Svecofennian magmatism while the 1.68–1.70 Ga Siljan and Garberg granites are the first expressions of extensional magmatism within a stabilized craton and were followed by the ca 1.5 Ga rapakivi granites (Ahl et al., 1999). In terms of age, magmatic association, petrography and geochemistry, the Ja¨rna granitoids are similar to the post-orogenic (post-kinematic) granitoids of southern Finland (cf Nurmi and Haapala, 1986). The A-type geochemistry is so characteristic of the classic, well-documented Proterozoic rapakivi granites that it was included in the new definition of rapakivi granite. The metallogenetically important, highly evolved topaz-bearing alkali feldspar granites that typically constitute the latest intrusive phases of rapakivi granite complexes (Haapala, 1995) deviate markedly from the normal rapakivi granites and show the geochemical and mineralogical characteristics of the Phanerozoic tin granites. Petrological and geochemical studies (e.g. Haapala, 1977b, 1997; Bettencourt et al., 1999) have shown, however, that the topaz-bearing granites are petrogenetically intimately associated with the rapakivi granites. Isotopic data on the silicic and mafic rocks of rapakivi complexes show compositions that comply with the overall composition of the enclos-
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ing crust (e.g. Ra¨mo¨ and Haapala, 1995 and references therein). The silicic rocks are generally considered to represent water-poor, high-temperature partial melts from the lower continental crust. They are restite-poor and were crystallized at a low confining pressure and low activities of water and, commonly, oxygen (see Ra¨mo¨, 1991; Frost and Frost, 1997). The mafic rocks of the rapakivi association originated from the mantle. Their Nd isotopic composition suggests that the subcontinental mantle may vary from chondritic or slightly depleted ( Finland, Grenville Province) to clearly enriched (central Sweden, Russian Karelia, Nain Province). Their isotopic signature may, however, also be explained by crustal contamination of the mantle-derived magmas. The Proterozoic rapakivi granite complexes typically occur in metamorphic terranes that were formed a few hundred Ma before the rapakivi magmatism (e.g. Ra¨mo¨ and Haapala, 1996 and references therein). However, the age gap between the rapakivi granites and surrounding crust varies markedly. The 1.70 Ga Shachang rapakivi complex near Beijing (Ra¨mo¨ et al., 1995) and the 1.88 Ga Jamon and Musa granites in eastern Amazonia (Dall’Agnol et al., 1998, 1999) postdate their metamorphic country rocks by at least 800 and 1000 Ma, respectively. Another extremity is represented by the 1.75 Ga rapakivi-textured monzonites of South Greenland that were emplaced not more than 50 Ma after the youngest peak of regional metamorphism (Brown et al., 1992) and the 0.59 Ga rapakivi granites of the Itu Province in southwestern Brazil that overlap or shortly follow the synorogenic calc-alkaline Braziliano magmatism ( Wernick et al., 1997). The relation of the rapakivi magmatism to orogenic processes has been discussed in a number of papers. The proposed models can be categorized into three main groups: (1) mafic underplating including melting of the crust by mantle-derived mafic magmas (e.g. Bridgwater et al., 1974; Emslie, 1978; Anderson, 1983; Haapala and Ra¨mo¨, 1990; Ra¨mo¨ and Haapala, 1996); (2) melting of a thickened orogenic crust ( Vorma, 1976; Windley, 1991); and (3) intracratonic magmatism related to orogenies
at craton margins (Teixeira et al., 1989; Bettencourt et al., 1999). In the case of the mafic underplate model, the partial melting of the mantle may be related to active or passive rifting, extensional collapse of orogen, deep mantle plumes, or instabilities in the mantle related to plate movements. The tectonic settings, bimodal magmatic association, geochemistry and isotope geology of the rapakivi granites can best be explained by mafic underplating, but the reason for the mantle melting remains largely open. Perhaps the doctrine of uniformitarianism can be applied to study the origin of the rapakivi magmatism: information from the youngest areas of rapakivi-type magmatism (e.g. the Basin and Range Province) may be used to interpret the geological environment and origin of the Precambrian rapakivi complexes which usually are hampered by deep erosion and later tectonic disturbances. The rapakivi texture with plagioclase-mantled alkali feldspar megacrysts and two generations of quartz and feldspar has long been a major challenge for petrologists. Formation of the mantled ovoidal alkali feldspar megacrysts probably requires changes in physicochemical conditions (temperature, pressure, magma composition) that stabilize plagioclase in favour of alkali feldspar, allowing plagioclase to nucleate on the alkali feldspar crystals. Experimental studies and petrographic observations indicate that: (1) mixing of two magmas of different composition (Hibbard, 1981; Wark and Stimac, 1992); and (2) crystallization of a granite melt under conditions involving marked pressure release combined with small change of temperature (Nekvasil, 1991) are the two most realistic mechanisms for the generation of the rapakivi texture. In this volume, Eklund and Shebanov (1999) present physicochemical parameters on the rapakivi granites of southeastern Fennoscandia in support of the latter mechanism. Generalizing, it may be concluded that the A-type character of the rapakivi granites reflects the geotectonic setting and origin of the magmas, while the rapakivi texture reflects conditions of crystallization.
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Acknowledgments The six years of the Rapakivi IGCP Project have been most rewarding and it has been a pleasure for the authors to meet and collaborate with the active and internationally most versatile group of geoscientists gathered together in the framework of the project. In particular, the authors would like to thank Ron Emslie who shared much of his time and experience as a co-leader of the project. The Executive Editors of Precambrian Research, K.A. Eriksson and A. Kro¨ner, kindly accepted publication of this special issue, which is gratefully acknowledged. The authors would also like to thank the external reviewers — U.G. Cordani, O. Eklund, R.F. Emslie, C.D. Frost, M. Lehtinen, G.R. Lowell, T. Lundqvist, H. Nekvasil, L.A. Neymark, M. Nironen, J.S. Scoates, T. Skio¨ld, K. Sundblad, R. To¨rnroos, B.G.J. Upton, M. Vaasjoki, W.R. Van Schmus and R.A. Wiebe — for their invaluable help and F. Wallien for editorial guidance in preparation of this compilation. P. Kosunen assisted in processing the final electronic versions of the manuscripts.
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to the Nain area, August 4–10, 1994, IGCP Projects 290 and 315. Bettencourt, J.S., Dall’Agnol, R., 1987. The Rondonian tinbearing anorogenic granites and associated mineralization. In: International Symposium on Granites and Associated Mineralizations, Excursion Guides, Salvador, pp. 49–87. Bettencourt, J.S., Dall’Agnol, R. (Eds.), 1995. Symposium on Rapakivi Granites and Related Rocks, Excursion Guide: The Rapakivi Granites of the Rondoˆnia Tin Province and Associated Mineralization. Center for Geosciences, Federal University of Para´, Bele´m, Brazil. Bettencourt, J.S., Tosdal, R.M., Leite Jr., W.B., Payolla, B.L., 1999. Mesoproterozoic rapakivi granites of the Rondoˆnia Tin Province, southwestern border of the Amazonian craton, Brazil: 1. Reconnaissance U–Pb geochronology and regional implications. Precambrian Res. 95, 41–67. Bridgwater, D., Windley, B.F., Anorthosites, post-orogenic granites, acid volcanic rocks and crustal development in the North Atlantic Shield during the mid-Proterozoic. In: Lister, L.A. (Ed.), Symposium on Granites, Gneisses and Related Rocks (Spec. Publ.). 1973. Geol. Soc. S. Afr. 3, 307–317. Bridgwater, D., Sutton, J., Watterson, J., 1974. Crustal downfolding associated with igneous activity. Tectonophysics 21, 57–77. Brown, P.E., Dempster, T.J., Harrison, T.N., Hutton, D.H.W., 1992. The rapakivi granites of S. Greenland-crustal melting in response to extensional tectonics and magmatic underplating. Trans. R. Soc. Edinburgh: Earth Sci. 83, 173–178. Calzia, J.P., Ra¨mo¨, O.T., 1997. The granite of Kingston Peak: petrogenesis of a middle Miocene post-subduction granite in the southern Death Valley region, California. Terra Nova 9 (EUG9 Abstracts, Abstract Suppl. No. 1), 458 Dall’Agnol, R., Bettencourt, J.S. ( Eds.), 1997. Proceedings of the Symposium on rapakivi Granites and Related Rocks, Bele´m (Brazil ), August 2–5, 1995. Anais da Academia Brasileira de Cieˆncias 69(3). Dall’Agnol, R., Macambira, M.J.B., Costi, H.T. ( Eds.), 1995. Symposium on Rapakivi Granites and Related Rocks, Abstract Volume. Center for Geosciences, Federal University of Para´, Belem, Brazil. Dall’Agnol, R., Ra¨mo¨, O.T., de Magalha˜es, M.S., Macambira, M.J.B., 1998. Petrology of the anorogenic, oxidised Jamon and Musa granites, Amazonian craton: implications for the genesis of Proterozoic A-type granites. Lithos 46, 431–462. Dall’Agnol, R., Costi, H.T., da Silva Leite, A.A., de Magalha˜es, M.S., Teixeira, N.P., 1999. Rapakivi granites from Brazil and adjacent areas. Precambrian Res. in press. Eklund, O., Shebanov, A.D. 1999. Origin of the rapakivi texture by sub-isothermal decompression. Precambrian Res. in press. Emslie, R.F., 1978. Anorthosite massifs, rapakivi granites, and Late Proterozoic rifting of North America. Precambrian Res. 7, 61–98. Emslie, R.F., 1994. Anorthosites, Rapakivi Granites and Related Rocks. International Geological Correlation Programme, Joint meeting IGCP Nos 290 and 315, Program and Abstracts. McGill University, Montreal.
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Frost, C.D., Frost, B.R., 1997. Reduced rapakivi-type granites: the tholeiite connection. Geology 27, 647–650. Haapala, I., 1977a. The controls of tin and related mineralizations in the rapakivi–granite areas of south-eastern Fennoscandia. Geol. Fo¨ren. Stockholm Fo¨rh. 99, 130–142. Haapala, I., 1977b. Petrography and geochemistry of the Eurajoki stock, a rapakivi–granite complex with greisen-type mineralization in southwestern Finland. Geol. Surv. Finland Bull. 286, 128 p. Haapala, I., 1988. Metallogeny of the Proterozoic rapakivi granites of Finland. In: Taylor, R.P., Strong, D.F. (Eds.), Recent Advances in the Geology of the Granite-related Mineral Deposits. Can. Inst. Mining Metallurgy Spec. Vol. 39, 124–132. Haapala, I., 1995. Metallogeny of the rapakivi granites. Miner. Petrol. 54, 141–160. Haapala, I., 1997. Magmatic and postmagmatic processes in tin-mineralized granites: topaz-bearing leucogranite in the Eurajoki rapakivi granite stock, Finland. J. Petrol. 38, 1645–1659. Haapala, I., Ra¨mo¨, O.T., 1990. Petrogenesis of the Proterozoic rapakivi granites of Finland. In: Stein H.J., Hannah, J.L. ( Eds.), Ore-bearing Granite Systems; Petrogenesis and Mineralizing Processes (Spec. Paper). Geol. Soc. Am. 246, 275–286. Haapala, I., Ra¨mo¨, O.T. ( Eds.), 1991. IGCP Project 315 Symposium on Rapakivi Granites and Related Rocks, Abstract Volume. Geological Survey of Finland, Guide 34. Haapala, I., Ra¨mo¨, O.T., 1992. Tectonic setting and origin of the Proterozoic rapakivi granites of southeastern Fennoscandia. Trans. R. Soc. Edinburgh: Earth Sci. 83, 165–171. Haapala, I., Ra¨mo¨, O.T. (Guest Eds.), 1994. IGCP Project 315 Publication No. 12. Miner. Petrol. 50(1–3). Haapala, I., Ra¨mo¨, O.T., Salonsaari, P.T. (Eds.), with contributions by Amelin, Yu., Beljaev, A., Larin, A., Neymark, L., Stepanov, K., 1991. Salmi batholith and Pitka¨ranta ore field in Soviet Karelia, IGCP Project 315 Symposium Rapakivi Granites and Related Rocks, Excursion Guide. Geol. Surv. Finland, Guide 33, 57 p. Haapala, I., Ra¨mo¨, O.T., Volborth, A., Miocene rapakivi-like granites of southern Newberry Mountains, Nevada, U.S.A.: Comparisons to the Proterozoic rapakivi granites of Finland. In: Brown, M., Piccoli, P.M. ( Eds.), The Origin of Granites and Related Rocks, Third Hutton Symposium Abstracts. 1995. U.S. Geol. Surv. Circ. 1129, 61–62. Haapala, I., Ra¨mo¨, O.T., Kosunen, P. (Eds.), 1996. Symposium on Rapakivi Granites and Related Rocks, Abstract Volume. University of Helsinki. University Press, Helsinki, Finland. Hibbard, M.J., 1981. The magma mixing origin of mantled feldspars. Contrib. Miner. Petrol. 76, 158–170. Higgins, M., Woussen, G., 1994. IGCP 290 and 315, Lac-StJean field trip guide. University of Quebec in Chicoutimi, Canada. Kisvarsanyi, E.B. ( Ed.), 1993. Symposium on rapakivi granites and related rocks on a global scale, March 29–30, 1993,
Rolla, Missouri, U.S.A. Missouri Department of Natural Resources, Contribution to Precambrian Geology 23, 45 p. Kisvarsanyi, E.B., Hebrank, A.W., 1993. 27th Annual Meeting, North-Central Section, The Geological Society of America, Field trip No. 6: Rapakivi granites and Related Rocks in the St. Francois Mountains, southeast Missouri. Missouri Department of Natural Resources, Guidebook 24, 40 p. Kisvarsanyi, E.B., Kisvarsanyi, G., Alkaline granite ring complexes and metallogeny in the Midlle Proterozoic St. Francois terrane, southeastern Missouri, U.S.A. In: Gower, C.F., Rivers, T., Ryan, B. ( Eds.), Mid-Proterozoic Laurentia– Baltica (Spec. Paper). 1990. Geol. Assoc. Can. 38, 433–446. Korja, A., Heikkinen, P.J., 1995. Proterozoic extensional tectonics of the central Fennoscandian Shield: results from the Babel and Bothnian Echoes from the Lithosphere experiment. Tectonics 14, 504–517. Lowell, G.R., Young, G.J., 1999. Interaction between coeval mafic and felsic melts in the St Francois Terrace of Missouri, USA. Precambrian Res. 95, 69–88. Luosto, U., Tiira, T., Korhonen, H., Azbel, I., Burmin, V., Buyanov, A., Kosminskaya, I., Ionkis, V., Sharov, N., 1990. Crust and upper mantle structure along the DSS Baltic profile in SE Finland. Geophys. J. Int. 101, 89–110. Nekvasil, H., 1991. Ascent of felsic magmas and formation of rapakivi. Am. Miner. 76, 1279–1290. Nurmi, P.A., Haapala, I., 1986. The Proterozoic granitoids of Finland: granite types, metallogeny and relation to crustal evolution. Bull. Geol. Soc. Finland 58, 203–233. Persson, A.I., 1999. Absolute ( U–Pb) and relative age determinations of intrusive rocks in the Ragunda rapakivi complex, central Sweden. Precambrian Res., in press. Ra¨mo¨, O.T., 1991. Petrogenesis of the Proterozoic rapakivi granites and related basic rocks of southeastern Fennoscandia: Nd and Pb isotopic and general geochemical constraints. Geol. Surv. Finland Bull. 355, 161 p. Ra¨mo¨, O.T., Haapala, I., 1995. One hundred years of rapakivi granite. Miner. Petrol. 52, 129–185. Ra¨mo¨, O.T., Haapala, I., 1996. Rapakivi granite magmatism: a global review with emphasis on petrogenesis. In: Demaiffe, D. ( Ed.), Petrology and geochemistry of magmatic suites of rocks in the continental and oceanic crusts. A volume dedicated to Professor Jean Michot. Universite´ Libre de Bruxelles, Royal Museum for Central Africa ( Tervuren), pp. 177–200. Ra¨mo¨, O.T., Haapala, I., Vaasjoki, M., Yu, J., Fu, H., 1995. The 1700 Ma Shachang complex, northeast China: Proterozoic rapakivi granite not associated with Paleoproterozoic orogenic crust. Geology 23, 815–818. Sadowski, G.R., Bettencourt, J.S., 1996. Mesoproterozoic tectonic correlations between eastern Laurentia and the western border of Amazonian craton. Precambrian Res. 76, 213–227. Sederholm, J.J., 1891. Ueber die finnla¨ndischen Rapakiwigesteine. Tschermak’s Miner. Petrograph. Mitth. 12, 1–31. Teixeira, W., Tassinari, C.C.G., Cordani, U.G., Kawashita, K., 1989. A review of the geochronology of the Amazonian craton: tectonic implications. Precambrian Res. 42, 213–227.
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1995. 1997. Anais da Academia Brasileira de Cieˆncias 69, 395–413. Windley, B.F., 1991. Early Proterozoic collision tectonics, and rapakivi granites as intrusions in an extensional thrust-thickened crust: the Ketilidian orogen, South Greenland. Tectonophysics 195, 1–10. Yu, J.-H., Fu, H.-Q., Zhang, F.-L., Wan, F.-X., Haapala, I., Ramo, T.O., Vaasjoki, M., 1996. Anorogenic Rapakivi Granites and Related Rocks in Northern of the North China Craton. China Science and Technology Press, Beijing (in Chinese and in English).
Preface
Rapakivi granites and related rocks: an introduction I. Haapala *, O.T. Ra¨mo¨ Department of Geology, P.O. Box 11, University of Helsinki, 00014 Helsinki, Finland
Abstract This article is an introductory chapter for the proceedings of the last, seventh, meeting of the International Geological Correlation Programme (IGCP) Project 315 ‘‘Correlation of Rapakivi Granites and Related Rocks on a Global Scale’’, 1991–1996. Based on current knowledge, progress made during the six years of the project, and the seven papers published in this special issue, a synthesis of the age distribution, geotectonic environment, petrogenesis, geochemistry, and metallogeny of the rapakivi granites and related rocks is presented. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Geochemistry; Isotopic age; Petrogenesis; Rapakivi granite; Rapakivi texture
The classic rapakivi granites of southern Finland have been documented in Swedish and Finnish literature for ca 300 years. In 1891, J.J. Sederholm introduced the Finnish rapakivi granites to the international readership (Sederholm, 1891), and at the end of the nineteenth and beginning of the twentieth century, rapakivi granites were described from the Ukraine and Sweden. After the Second World War, a new period of activity commenced in this field, and numerous rapakivi granite occurrences were described from the Baltic countries, South Greenland, Labrador, mid-continental and western USA, Venezuela, Brazil, Botswana and several other Precambrian shield areas. The formation of Proterozoic rapakivi granites and associated anorthosites was considered as an important tectonomagmatic event or period in the construction of the continental crust (e.g. Bridgwater and * Corresponding author. Tel: +358 01911; fax: +358 0191 23466; e-mail: [email protected]
Windley, 1973; Bridgwater et al., 1974; Emslie, 1978; Anderson, 1983). Rapakivi granite complexes were also recognized as showing traits of the typical tin granites, and discoveries of tin mineralization in Fennoscandia, Brazil and Missouri led to understanding the ore-generating capacity of rapakivi granites (e.g. Haapala, 1977a; Bettencourt and Dall’Agnol, 1987; Kisvarsanyi and Kisvarsanyi, 1990). In 1991, a new project (IGCP-315) was founded within the framework of the International Geological Correlation Programme to correlate research on the rapakivi granites and related rocks on a global scale, now comprehensively including the developing countries where a large number of poorly studied rapakivi complexes were known to occur. The main topics of the project comprised the areal and temporal distribution of the rapakivi granites, their tectonic setting and mechanism of emplacement, their relation to crustal evolution, the bimodal character of the magmatism, petro-
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graphic and geochemical characteristics, metallogeny, petrogenesis, physical conditions of crystallization and origin of the rapakivi texture. During its six years of activity (1991–1996), the project arranged seven international symposia and six field trips to key areas, and was joined by researchers from >20 countries. Several new rapakivi complexes were identified and a lot of new information on the topics of the project was acquired. These have so far been documented in two proceedings volumes ( Haapala and Ra¨mo¨, 1994; Dall’Agnol and Bettencourt, 1997), five abstract volumes (Haapala and Ra¨mo¨, 1991; Kisvarsanyi, 1993; Emslie, 1994; Dall’Agnol et al., 1995; Haapala et al., 1996), seven field trip guide books (Haapala et al., 1991; Kisvarsanyi and Hebrank, 1993; Higgins and Woussen, 1994; Berg et al., 1994; Bettencourt and Dall’Agnol, 1995; Ahl et al., 1996, 1997) and >60 original papers including two global reviews (Ra¨mo¨ and Haapala,
1995, 1996). The present issue includes seven selected papers from the last meeting of IGCP-315 held on 24–26 July 1996, at the University of Helsinki, Finland. The papers deal mainly with the geochronology, petrology and metallogeny of rapakivi complexes of Brazil, Finland, Missouri and Sweden. These contributions are briefly reviewed below together with the earlier results of the project. Most of the rapakivi granites are Proterozoic in age (generally 1.0 to 1.8 Ga) but there are also Archaean and Phanerozoic granite complexes that can be considered rapakivi suites (Fig. 1). The current definition of rapakivi granite (Haapala and Ra¨mo¨, 1992) does not restrict the age of the rocks and simply considers them ‘‘A-type granites characterized by the presence, at least in the larger batholiths, of granite varieties showing the rapakivi texture’’. Several periods of Proterozoic rapakivi magmatism, often apparently without any clear
Fig. 1. Global map showing the distribution of rapakivi granites. Age data (in Ga) from Ra¨mo¨ and Haapala (1996) and references therein and articles in this volume. Modified from Fig. 2 in Ra¨mo¨ and Haapala (1996).
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systematic geotectonic or areal control, have been recognized from Brazil (e.g. Dall’Agnol et al., 1999; Bettencourt et al., 1999) and Fennoscandia (Alviola et al., 1999; Persson, 1999). The age groups in different shield areas correspond only partially to each other, but, for instance, the 1.54–1.58 Ga age group is present in Fennoscandia ˚ land, Vehmaa, Laitila and (Salmi, Riga, A Nordingra˚ batholiths), Venezuela (Parguaza, Sucucucu and Mucajaı´ batholiths) and Rondoˆnia (Serra da Provideˆncia batholith and its satellites), and the 1.31–1.41 Ga and 1.02–1.08 Ga age groups are found in Rondoˆnia and in the mid-continental to western USA. As pointed out by Sadowski and Bettencourt (1996) and Bettencourt et al. (1999), the rapakivi age groups can be effectively utilized for continental reconstructions. The rapakivi granites were definitely not produced by a single ‘rapakivi event’, but were rather formed in several magmatic episodes from the Late Archean to the Tertiary, although the magmatism was most voluminous between 1.8 and 1.0 Ga. The magmatic association of the rapakivi granites is clearly bimodal (mafic–felsic); diabase, gabbro and anorthosite are found together with rhyolite, granite and syenite. Interaction of coexisting mafic and felsic magmas has locally produced hybrid intermediate (e.g. monzodioritic) members. Mafic plutonic rocks seem to be abundant in the lower parts of the rapakivi complexes. There are some rapakivi plutons that appear not to be associated with mafic rocks, but this may be due to a relatively high erosion level or lack of ample exposure. In addition, diabase and rhyolite dykes outside the rapakivi plutons may be easily overlooked in routine geological mapping. In this volume, excellent examples of the bimodal character of the rapakivi magmatism are given from southeastern Missouri by Lowell and Young (1999), central Sweden by Persson (1999) and southeastern Finland by Alviola et al. (1999). Since the study of Bridgwater and Windley (1973), incipient or aborted rifting has been suggested as the tectonic environment of many anorogenic granitic suites. Extensional geotectonic setting has been convincingly documented for the rapakivi complexes of Finland by recognition of
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rapakivi-age subparallel diabase and porphyry dykes that transect the Paleoproterozoic crust (Haapala and Ra¨mo¨, 1990), graben structures (Haapala, 1988), thinning of the crust in rapakivi areas (Luosto et al., 1990; Haapala and Ra¨mo¨, 1992) and listric faulting ( Korja and Heikkinen, 1995). Also the 1.70 Ga Shachang and other rapakivi complexes in the Beijing area, China, are accompanied with concurrent graben structures and thinned crust ( Yu et al., 1996). Similar structural features and magmatic associations as in Fennoscandia have been documented for the Basin and Range Province of south-western North America where rapakivi-type granites are related to an extreme Miocene extension (Haapala et al., 1995; Calzia and Ra¨mo¨, 1997). The 1.68–1.79 Ga Dala granitoids of Sweden represent a transition from a compressional to an extensional tectonic regime: the 1.79 Ja¨rna granitoids are the last, postorogenic, expressions of subduction-related Svecofennian magmatism while the 1.68–1.70 Ga Siljan and Garberg granites are the first expressions of extensional magmatism within a stabilized craton and were followed by the ca 1.5 Ga rapakivi granites (Ahl et al., 1999). In terms of age, magmatic association, petrography and geochemistry, the Ja¨rna granitoids are similar to the post-orogenic (post-kinematic) granitoids of southern Finland (cf Nurmi and Haapala, 1986). The A-type geochemistry is so characteristic of the classic, well-documented Proterozoic rapakivi granites that it was included in the new definition of rapakivi granite. The metallogenetically important, highly evolved topaz-bearing alkali feldspar granites that typically constitute the latest intrusive phases of rapakivi granite complexes (Haapala, 1995) deviate markedly from the normal rapakivi granites and show the geochemical and mineralogical characteristics of the Phanerozoic tin granites. Petrological and geochemical studies (e.g. Haapala, 1977b, 1997; Bettencourt et al., 1999) have shown, however, that the topaz-bearing granites are petrogenetically intimately associated with the rapakivi granites. Isotopic data on the silicic and mafic rocks of rapakivi complexes show compositions that comply with the overall composition of the enclos-
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ing crust (e.g. Ra¨mo¨ and Haapala, 1995 and references therein). The silicic rocks are generally considered to represent water-poor, high-temperature partial melts from the lower continental crust. They are restite-poor and were crystallized at a low confining pressure and low activities of water and, commonly, oxygen (see Ra¨mo¨, 1991; Frost and Frost, 1997). The mafic rocks of the rapakivi association originated from the mantle. Their Nd isotopic composition suggests that the subcontinental mantle may vary from chondritic or slightly depleted ( Finland, Grenville Province) to clearly enriched (central Sweden, Russian Karelia, Nain Province). Their isotopic signature may, however, also be explained by crustal contamination of the mantle-derived magmas. The Proterozoic rapakivi granite complexes typically occur in metamorphic terranes that were formed a few hundred Ma before the rapakivi magmatism (e.g. Ra¨mo¨ and Haapala, 1996 and references therein). However, the age gap between the rapakivi granites and surrounding crust varies markedly. The 1.70 Ga Shachang rapakivi complex near Beijing (Ra¨mo¨ et al., 1995) and the 1.88 Ga Jamon and Musa granites in eastern Amazonia (Dall’Agnol et al., 1998, 1999) postdate their metamorphic country rocks by at least 800 and 1000 Ma, respectively. Another extremity is represented by the 1.75 Ga rapakivi-textured monzonites of South Greenland that were emplaced not more than 50 Ma after the youngest peak of regional metamorphism (Brown et al., 1992) and the 0.59 Ga rapakivi granites of the Itu Province in southwestern Brazil that overlap or shortly follow the synorogenic calc-alkaline Braziliano magmatism ( Wernick et al., 1997). The relation of the rapakivi magmatism to orogenic processes has been discussed in a number of papers. The proposed models can be categorized into three main groups: (1) mafic underplating including melting of the crust by mantle-derived mafic magmas (e.g. Bridgwater et al., 1974; Emslie, 1978; Anderson, 1983; Haapala and Ra¨mo¨, 1990; Ra¨mo¨ and Haapala, 1996); (2) melting of a thickened orogenic crust ( Vorma, 1976; Windley, 1991); and (3) intracratonic magmatism related to orogenies
at craton margins (Teixeira et al., 1989; Bettencourt et al., 1999). In the case of the mafic underplate model, the partial melting of the mantle may be related to active or passive rifting, extensional collapse of orogen, deep mantle plumes, or instabilities in the mantle related to plate movements. The tectonic settings, bimodal magmatic association, geochemistry and isotope geology of the rapakivi granites can best be explained by mafic underplating, but the reason for the mantle melting remains largely open. Perhaps the doctrine of uniformitarianism can be applied to study the origin of the rapakivi magmatism: information from the youngest areas of rapakivi-type magmatism (e.g. the Basin and Range Province) may be used to interpret the geological environment and origin of the Precambrian rapakivi complexes which usually are hampered by deep erosion and later tectonic disturbances. The rapakivi texture with plagioclase-mantled alkali feldspar megacrysts and two generations of quartz and feldspar has long been a major challenge for petrologists. Formation of the mantled ovoidal alkali feldspar megacrysts probably requires changes in physicochemical conditions (temperature, pressure, magma composition) that stabilize plagioclase in favour of alkali feldspar, allowing plagioclase to nucleate on the alkali feldspar crystals. Experimental studies and petrographic observations indicate that: (1) mixing of two magmas of different composition (Hibbard, 1981; Wark and Stimac, 1992); and (2) crystallization of a granite melt under conditions involving marked pressure release combined with small change of temperature (Nekvasil, 1991) are the two most realistic mechanisms for the generation of the rapakivi texture. In this volume, Eklund and Shebanov (1999) present physicochemical parameters on the rapakivi granites of southeastern Fennoscandia in support of the latter mechanism. Generalizing, it may be concluded that the A-type character of the rapakivi granites reflects the geotectonic setting and origin of the magmas, while the rapakivi texture reflects conditions of crystallization.
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Acknowledgments The six years of the Rapakivi IGCP Project have been most rewarding and it has been a pleasure for the authors to meet and collaborate with the active and internationally most versatile group of geoscientists gathered together in the framework of the project. In particular, the authors would like to thank Ron Emslie who shared much of his time and experience as a co-leader of the project. The Executive Editors of Precambrian Research, K.A. Eriksson and A. Kro¨ner, kindly accepted publication of this special issue, which is gratefully acknowledged. The authors would also like to thank the external reviewers — U.G. Cordani, O. Eklund, R.F. Emslie, C.D. Frost, M. Lehtinen, G.R. Lowell, T. Lundqvist, H. Nekvasil, L.A. Neymark, M. Nironen, J.S. Scoates, T. Skio¨ld, K. Sundblad, R. To¨rnroos, B.G.J. Upton, M. Vaasjoki, W.R. Van Schmus and R.A. Wiebe — for their invaluable help and F. Wallien for editorial guidance in preparation of this compilation. P. Kosunen assisted in processing the final electronic versions of the manuscripts.
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1995. 1997. Anais da Academia Brasileira de Cieˆncias 69, 395–413. Windley, B.F., 1991. Early Proterozoic collision tectonics, and rapakivi granites as intrusions in an extensional thrust-thickened crust: the Ketilidian orogen, South Greenland. Tectonophysics 195, 1–10. Yu, J.-H., Fu, H.-Q., Zhang, F.-L., Wan, F.-X., Haapala, I., Ramo, T.O., Vaasjoki, M., 1996. Anorogenic Rapakivi Granites and Related Rocks in Northern of the North China Craton. China Science and Technology Press, Beijing (in Chinese and in English).