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Anorogenic magmatic rocks in the Estonian crystalline basement

2004; Estonian Academy Publishers; Volume: 53; Issue: 3 Linguagem: Inglês

10.3176/geol.2004.3.05

1736-7913

J.M. Haapala, Juho Kirs, O. Tapani Rämö,

Geochemistry and Elemental Analysis

The anorogenic magmatic rock bodies of the Estonian crystalline basement belong to the Fennoscandian Palaeo-Mesoproterozoic Rapakivi Province and include the huge composite Riga batholith (250 km × 230 km in subsurface area, mostly in NW Latvia), as well as at least five granite stocks (Naissaare, Märjamaa and its Kloostri satellite, Taebla, Neeme, and Ereda) and the quartz monzodioritic Abja stock in the Estonian mainland.The Riga batholith contains both mafic and silicic rocks, as several Fennoscandian rapakivi complexes do.The granite stocks appear on geophysical maps as gravity and magnetic minima, and they consist of pink, medium-to coarsegrained, microcline-megacrystic, partly trachytoid biotite (in Märjamaa and Naissaare also with hornblende) granite, locally cut by aplitic and microsyenitic dykes.As an exception, the Märjamaa stock is more differentiated and has an anomalously high magnetic central part composed of hybrid granodiorite with hornblende as the main mafic mineral.The Abja quartz monzodiorite stock is strongly magnetic and consists of a dark grey, massive, medium-grained, partly weakly gneissose rock with abundant accessory apatite and titanomagnetite, and is intersected by veins of fine-to medium-grained, slightly porphyritic plagioclase-microcline granite.The major, REE and other trace element contents of the granitic rocks are close to or overlap those of the typical Finnish rapakivis and are best comparable with the less differentiated granitic phases.The Märjamaa granodiorite and the Abja quartz monzodiorite are enriched in Sr, Ti, and P. In Nd and Pb isotopic composition, the felsic and mafic rocks resemble corresponding rocks of the Finnish rapakivi complexes, indicating an approximately chondritic source for Nd in the mafic rocks of the Riga batholith and the Abja intrusion, and a Palaeoproterozoic (Svecofennian) source for the felsic rocks.The T DM model ages of the felsic rocks range from 1890 to 2100 Ma.