Other Pre-Tertiary Granitoid Examples
The rapakivi granites (1.7 to 1.55 Ga) of southern Finland occur as epizonal batholiths (e.g., the Wiborg, Åland or Ahvenanmaa, Vehmaa, and Laitila batholiths) and stocks cutting the medium- to high-grade metamorphic Svecofennian (1.9 to 1.8 Ga) crust. Emplacement of the granites was associated with faulting and the intrusion of coeval sets of mainly west-northwest- (some north-northeast) trending diabase and quartz porphyry dikes, indicating an extensional continental tectonic regime.
The rapakivi granite batholiths and stocks are multiple intrusions, several of which also contain minor anorthositic and gabbroic bodies. Granites of the early intrusive phases commonly crystallized from water-deficient magmas and contain biotite and hornblende (± fayalite) as dark constituents. The younger intrusive bodies contain biotite as the only ferromagnesian silicate, whereas the youngest, water-saturated intrusive phases are topaz-bearing granites, in which the dark mica is lithium-bearing siderophyllite. Fluorite, zircon, allanite, apatite, anatase, magnetite, and ilmenite are typical accessory minerals in the granites of the early and main intrusive phases. The biotite granites contain monazite instead of allanite, and the late-stage granites contain topaz, monazite, ilmenite, Nb- and Ta-rich cassiterite, and columbite as common accessory minerals. Topaz-bearing quartz porphyry dikes and greisen-type tin-polymetallic mineralization are often associated with the last intrusive phases.
The rapakivi granites are metaluminous to slightly peraluminous rocks characterized normally by high K, K/Na, Fe/Mg, F, Ga, Rb, Zr, Hf, Th, U, and REE. The early and main intrusive phases are enriched in LREE and show deep Eu anomalies. The last minor intrusive phases show flattened normalized REE patterns with still deeper Eu minima. As a result of extreme differentiation and superimposed alteration, they are anomalously enriched in F, Ga, Rb, Sn, and Nb, and are impoverished in Ti, Ba, Sr, and Zr. The rapakivi granites exhibit geochemical characteristics of subalkaline A-type granites and within-plate granites.
Nd isotopic studies from the northern part of the Wiborg rapakivi area indicate that the rapakivi-age diabase dikes [∊Nd(T) values +1.6 to −1.0] crystallized from mantle-derived magmas that had experienced variable degrees of crustal contamination. The rapakivi granite-quartz porphyry magmas [∊Nd(T) values −0.8 to −1.9] most probably originated by partial melting of the Svecofennian crust formed 0.2 to 0.3 b.y. earlier. Heat flow from the mantle-derived magmas contributed to the partial melting.