Rapakivi granite

File:Moro Narvijärvi 3.jpg

Rapakivi is a fairly uncommon type of granite, but has been described from localities in North and South America (Illescas Batholith, Uruguay,{{cite journal |last1=Teixeira |first1=Wilson |last2=D'Agrella-Filho |first2=Manoel S.|last3=Hamilton |first3=Mike A. |last4=Ernst |first4=Richard E. |last5=Girardi |first5=Vicente A.V.|last6=Mazzucchelli |first6=Maurizio|last7=Bettencourt |first7=Jorge S. |date=2013 |title=U–Pb (ID-TIMS) baddeleyite ages and paleomagnetism of 1.79 and 1.59 Ga tholeiitic dyke swarms, and position of the Rio de la Plata Craton within the Columbia supercontinent |journal=Lithos |volume=174 |pages=157–174 |doi= 10.1016/j.lithos.2012.09.006|bibcode=2013Litho.174..157T }} Rondônia, Brazil{{cite journal |last1=Bettencourt |first1=J.S. |last2=Tosdal |first2=R.M. |last3=Leite |first3=W.B. |last4=Payolla |first4=B.L. |date=1999 |title=Mesoproterozoic rapakivi granites of the Rondoˆnia Tin Province, southwestern border of the Amazonian craton, Brazil — I. Reconnaissance U–Pb geochronology and regional implications |journal=Precambrian Research |volume=95 |issue=1–2 |pages=41–67 |doi=10.1016/S0301-9268(98)00126-0 |bibcode=1999PreR...95...41B }}) parts of the Baltic Shield, southern Greenland, southern Africa, India and China. Most of these examples are found within Proterozoic metamorphic belts, although both Archaean and Phanerozoic examples are known.

Finnish Rapakivi granites occur in the Åland, Laitila, Vehmaa, and Wiborg batholiths. The Wiborg batholith dates from 1900-1830 Ma. The age of the Wiborg granites range from 1646 to 1627 Ma, and consist of 80% wiborgite, and smaller inclusions of pyterlite, porphyritic rapakivi granite, even-grained rapakivi granite, and aplitic rapakivi granite.{{cite web |last1=Harma |first1=Paavo |last2=Selonen |first2=Olavi |title=Natural stone production in the Wiborg rapakivi granite batholith in southeastern Finland |url=https://www.researchgate.net/figure/Geological-map-of-the-Wiborg-rapakivi-granite-batholith-on-the-Finnish-side-The-main_fig2_325216283 |website=ResearchGate |publisher=The Finnish Natural Stone Association |access-date=4 March 2025 |date=2018}}

Rapakivi granites have formation ages from Archean to recent and are usually attributed to anorogenic tectonic settings. They have formed in shallow (a few km deep) sills of up to 10 km thickness.{{Citation needed|date=July 2009}}

Rapakivi granites are often found associated with intrusions of anorthosite, norite, charnockite and mangerite. It has been suggested that the entire suite results from the fractional crystallization of a single parental magma.Zhang, S-H., Liu, S-W., Zhao, Y., Yang, J-H. Song, B. and Liu, X-M. The 1.75–1.68 Ga anorthosite-mangerite-alkali granitoid-rapakivi granite suite from the northern North China Craton: Magmatism related to a Paleoproterozoic orogen. Precambrian Research, 155, 287–312.{{refn|name=note 1|group=note|Some geologists of the first half of the 20th century regarded the rapakivi granites as "graniticized" Jotnian sediments, an idea which is now discredited.{{cite journal |last=von Eckermann |first=Harry|author-link=Harry von Eckermann |date=1939 |title=The Weathering of the Nordingrå Gabbro |journal=Geologiska Föreningen i Stockholm Förhandlingar |volume=61 |issue=4 |pages=490–496 |doi= 10.1080/11035893909444616}}}}

Rapakivi is enriched in K, Rb, Pb, Nb, Ta, Zr, Hf, Zn, Ga, Sn, Th, U, F and rare earth elements, and poor in Ca, Mg, Al, P and Sr. Fe/Mg, K/Na and Rb/Sr ratios are high. SiO₂ content is 70.5%, which makes rapakivi an acidic granite.Rämö, T., Haapala, I. ja Laitakari, I. 1998. Rapakivigraniitit – peruskallio repeää ja sen juuret sulavat. In: Lehtinen, M., Nurmi, RA., Rämö, O.T. (Toim.), Suomen kallioperä – 3000 vuosimiljoonaa. Suomen geologinen seura. Gummerus kirjapaino, Jyväskylä. 257–283.

Rapakivi is high in fluoride, ranging 0.04–1.53%, compared to other similar rocks at around 0.35%. Consequently, groundwater in rapakivi zones is high in fluoride (1–2 mg/L), making the water naturally fluoridated. Some water companies actually have to remove fluoride from the water.{{cite journal | last1 = Lahermo | first1 = P. | last2 = Sandström | first2 = H. | last3 = ja Malisa | first3 = E. | year = 1991 | title = The occurrence and geochemistry of fluorides in natural waters in Finland and East Africa with reference to their geomedical implications | journal = Journal of Geochemical Exploration | volume = 41 | issue = 1–2 | pages = 65–79 | doi=10.1016/0375-6742(91)90075-6}}

The uranium content of rapakivi is fairly high, up to 24 ppm. Thus, in rapakivi zones, the hazard from radon, a decay product of uranium, is elevated. Some indoor spaces surpass the 400 Bq/m³ safety limit.Valmari, T., Arvela, H., ja Reisbacka, H. 2012. Radon in Finnish apartment buildings. Radiation Protection Dosimetry, 152, 146–149.Weltner, A., Mäkeläinen, I., ja Arvela, H. 2002. Radon mapping strategy in Finland. In: International Congress Series 1225, 63–69.

File:Rapakivi granite.jpg

File:Pyterlite Suomi.jpg

Vorma (1976) states that rapakivi granites can be defined as:Vorma A., 1976. On the petrochemistry of rapakivi granites with special reference to the Laitila massif, southwestern Finland. Geological Survey of Finland, Bulletin 285, 98 pages.

• Orthoclase crystals have rounded shape
• Most (but not all) orthoclase crystals have plagioclase rims (wiborgite or viborgite type, named after the city of Vyborg){{cite book | title=Igneous Rocks — A Classification and Glossary of Terms | publisher=Cambridge University Press | editor-last=Le Maitre |editor-first=R. W. | year=2002 | location=Cambridge | isbn=978-0-521-66215-4}}{{rp|157}}
• Orthoclase and quartz have crystallized in two phases, early quartz is in tear-drop shaped crystals (pyterlite type, named after the location of Pyterlahti).{{rp|134}}Walter Wahl: Die Gesteine des Wiborger Rapakiwigebietes. Fennia, Band 45/20, Helsingfors (Tilgmann) 1925, p. 24

A more recent definition by Haapala & Rämö states:{{cite journal | last1 = Haapala | first1 = I. | last2 = Rämö | first2 = O.T. | year = 1992 | title = Tectonic setting and origin of the Proterozoic rapakivi granites of southeastern Fennoscandia | journal = Transactions of the Royal Society of Edinburgh: Earth Sciences | volume = 83 | issue = 1–2 | pages = 165–171 | doi=10.1017/s0263593300007859| s2cid = 129835203 }}

Rapakivi granites are type-A granites, where at least in larger associated batholites have granites with rapakivi structures.

Rapakivi is the material used in Åland's mediaeval stone churches.[http://www.kyrkor.ax/churches/eckero-church/ Eckerö church], Retrieved 2012-10-19. In 1770, a rapakivi granite monolith boulder, the "Thunder Stone", was used as the pedestal for the Bronze Horseman statue in Saint Petersburg, Russia. Weighing 1,250 tonnes, this boulder is claimed to be the largest stone ever moved by humans.{{cite journal | last1 = Adam | first1 = Jean-Pierre | year = 1977 | title = À propos du trilithon de Baalbek: Le transport et la mise en oeuvre des mégalithes | url = http://www.persee.fr/doc/syria_0039-7946_1977_num_54_1_6623| journal = Syria | volume = 54 | issue = 1/2| pages = 31–63 | doi = 10.3406/syria.1977.6623 }} Modern building uses of rapakivi granites are in polished slabs used for covering buildings, floors, counter tops or pavements. As a building material, rapakivi granite of the wiborgite type is also known as "Baltic Brown".[http://naturalsciencesresearch.wordpress.com/2012/08/10/quasicrystals/ North Carolina Museum of Natural Sciences blog], Retrieved 2012-10-19.{{cite web | url=https://kivi.info/kivet/baltic-brown-3 | title=Baltic Brown | publisher=KIVI, Finnish Natural Stone Industry Association | work=Finnish Natural Stones | access-date=12 April 2024}}

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Category:Granite

Category:Geology of Finland

Category:Precambrian Europe

Category:Mesoproterozoic