Mehmet ARSLAN, Zafer ASLAN

9439

Rapakivi granitlerinin jeolojisi ve petrolojisi; Finlandiya Rapakivi granitleri

İri alkali feldispatın plajiyoklas tarafından çevrelenmesiyle oluşan dokuyu gösteren granitler "Rapakivi Granitleri" olarak adlandırılmaktadır. Çoğunlukla Proterozoyik (1.8-1.0 milyar yıl) yaşlı olan rapakivi granitleri, Ukrayna, Baltık ülkeleri, güney Grönland, orta-kıtasal ve batı ABD, Venezuella, Brezilya, Bostwana ve diğer birçok Prekambriyen kalkanında bulunmakla birlikte Güney Finlandiya tip bölge olarak kabul edilmektedir. Rapakivi granitleri sığ seviye, çok fazlı batolitleri ve stokları şeklinde olup, 10 km'den daha az kalınlıkta yatay levhasal kütleler şeklindedir. Rapakivi granitlerinin magmatik birlikteliği bimodal olup mafik üyeler; diyabaz daykları, gabroyidler ve anortozitler; felsik üyeler ise A-tipi granitler, siyenitler ve riyolitlerdir. En yaşlı rapakivi granitleri genelde hornblend-biyotit granitlerden, daha genç olanlar ise topazlı alkali feldispat granitlerden oluşmaktadır. Plajiyoklas (andezin veya oligoklas), ortoklas, mikroklin, kuvars, hornblend ve biyotit gibi ana minerallerin yanında flüorit, anataz, zirkon ve ilmenit aksesuar mineral olarak bulunur. Rapakivi granitler, genellikle metalümin veya kenar zonlarında az derecede peralümin kayaçlar olup, yüksek Fe/(Fe+Mg) oranına sahiptirler. Levha içi granitleri ve A-tipi granitlerinin kimyasal özelliklerini gösteren rapakivi granitleri yüksek K ve Na içermektedir. Ayrıca Si, K, F, Rb, Ga, Zr, Hf, Th, U, Zn ve NTE içerikleri ile K/Na, Ga/Al, Fe/Mg oranları granitik kayaçlardan daha yüksek, Ca, Mg, Al, P ve Sr içerikleri ise daha düşüktür. Finlandiya'daki rapakivi granitlerinin oluşumu mafik magmanın kıta kabuğu altına yerleşmesi (underplating) modeliyle açıklanmaktadır. Buna göre, mantodan türeyen mafik magmajar manto-kabuk sınırında alt kabuğa yerleşerek kabuğun yoğun bir şekilde kısmi ergimesine neden olarak rapakivi granitlerini oluşmaktadır. Rapakivi granitlerinin tektonik ortamları, bimodal magmatik birliktelikleri, jeokimya ve izotop bileşimleri mafik magmanın kıta kabuğu altına yerleşmesi modeli ile çok iyi açıklanabilmesine rağmen mantonun kısmi ergimesinin nedeni büyük ölçüde tartışmalıdır.

Geology petrogarphy and petrology of the Rapakivi granites; The Rapakivi granites of Finland

The granites showing textures of plagioclase-mantled alkali feldspar megacryst are known as "Rapakivi Granites". Rapakivi granites are generally Proterozoic (1.8 to 1.0 Ga) in age, and Southern Finland is the type area of the rapakivi granites although they are present in Ukraine, the Baltic countries, South Greenland, mid-continental and -western USA, Venezuela, Brazil, Botswana and several other Precambrian shield areas. Rapakivi granites are shallow level, multi-phase batholiths and stocks, having less than 10 km thick horizontal sheet-like bodies. The magmatic association of rapakivi granites is bimodal. The mafic members are represented by diabase dykes, gabbroids and anorthosites and the felsic members by A-type granites, syenites and ryholites. The oldest rapakivi granites are generally hornblende-biotite granites whereas the youngest ones are topaz-bearing alkali feldspar granites. In these rocks, plagioclase (andesine or oligoclase), orthoclase, microcline, quartz, hornblende and biotite are present as well as accessory fluorite, anatase, zircon and ilmenite. The rapakivi granites are generally metaluminous or marginally peraluminous rocks, and have high Fe/(Fe+Mg). They show chemical characteristics of within-plate granites and A-type granites, and have high K and Na contents. Furthermore, they have higher Si, K, F, Rb, Ga, Zr, Hf, Th, U, Zn and REE contents and K/Na, Ga/Al, Fe/Mg ratios, and lower Ca, Mg, Al, P and Sr abundances than granitte rocks in general. The formation of rapakivi granites of Finland can best be interpreted by the mafic uhder-plate model. Mantle-derived mafic magmas intruded at the mantle-crust boundary and into lower crust, and caused extensive partial melting of the deep crust, thus forming the rapakivi granites. 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.
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Jeoloji Mühendisliği Dergisi-Cover
  • ISSN: 1016-9172
  • Başlangıç: 1977
  • Yayıncı: TMMOB Jeoloji Mühendisleri Odası
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