TEXTURAL AND GEOCHEMICAL EVOLUTION OF HIGH-TEMPERATURE EPISYENITES IN THE SUOMENNIEMI RAPAKIVI GRANITE COMPLEX, SE FINLAND

Dike-like bodies of metasomatic alkali-feldspar syenite (episyenite) are hosted by the granites of the 1644 Ma Suomenniemi rapakivi granite complex of southeastern Finland. Novel geochemical and petrographic data from a deep drill hole penetrating alkali-feldspar syenite bodies and their country rock (subalkaline amphibole granite) enhance our understanding of the metasomatic process involved. A ca. 50 cm wide transition zone from fresh granite to syenite is observed. In the initial steps of alteration, plagioclase is mantled by alkali feldspar, hastingsite alters to ferro-actinolite or breaks down to sodic augite, and titanite and magnetite appear. Veinlets of fluorite are abundant in the marginal zone. Quartz becomes strained, anhedral, and disappears abruptly. Granoblastic clusters of albite form after plagioclase and orthoclase perthite replaces primary alkali feldspar. The acmite component in clinopyroxene increases towards the center of the syenite body. Whereas most of the clinopyroxene-bearing alkali feldspar syenites observed on outcrops are peralkaline, all the analyzed drill core samples are subalkaline (NK/A: syenite 0.88–0.91, amphibole granite ca. 0.80). Geochemical data suggest that the metasomatism resulted in decrease of Si and K and increase in Na; other major elements may be introduced or removed to a lesser extent. A 10–20% increase in total REE is observed from granite to alkali-feldspar syenite, mainly caused by mass loss via quartz dissolution. REE are introduced into the marginal, fluorite-rich zone. A sodic (Na₂O/K₂O = 3.1) alkali-feldspar granite aplite cuts an alkali-feldspar syenite body in the drill core. The aplite has a flat CHUR-normalized REE pattern, similar to that of the evolved topaz granites of the Suomenniemi complex and A-type complexes in general. In contrast to the episyenite bodies of the complex, the aplite has a clear cross-cutting relationship with the syenite and may be related to the source of the metasomatic fluids.