THE CALEDONIAN SMØLA-HITRA BATHOLITH, CENTRAL NORWAY

The Late Ordovician to Silurian Smøla-Hitra Batholith (SHB), which is part of the Upper and Uppermost Allochthons in the Scandinavian Caledonides, is exposed along the western coastal areas of central Norway. Along a SE to NW transect, the batholith is subdivided into three units occupying the islands Hitra (unit I) and Frøya (unit II), and the Froan Archipelago (unit III). Unit I has calc-alkaline to low-K calc-alkaline diorites, tonalites and granodiorites with ages ~447 – 440 Ma. Isotope data (eNd=1.1–5.9, Sr(I)=0.7036–0.7045, D18O=5.6–7.8 in diorites, and 6.9–9.1 in tonalite/granodiorite) and chemical signatures indicate a subduction-related source, possibly an arc setting. Unit II contains tonalites and granodiorites, locally with abundant metasedimentary xenoliths. Compositional data (eNd=-1.0 – 4.3, Sr(I)=0.7045–0.7065, D18O=8.3–9.9) are consistent with a source like that of unit I, but with greater crustal influence. Unit III is composed of nine texturally and compositionally distinct plutons; quartz-monzodiorite, granodiorite and various types of granite are common. One U-Pb date of ~435 Ma indicates the rocks are younger than in unit I. They are high-K calc-alkaline to shoshonitic, with ASI<1.05, elevated LILE contents, and partly elevated Zr and P. The fairly large variation in isotope composition (eNd=0– -6, Sr(I)=0.7053–0.7094, D18O=8.4–10.3) is consistent with derivation from mafic lower crust in combination with variable proportions of evolved crustal rocks, possibly in a syn- to post-collisonal setting. During the late Ordovician to Silurian, the allochthons were outboard of Fennoscandia, and their tectonic setting during plutonism is poorly constrained. Important information may, however, be derived from the study of the intrusive rocks. Within the SHB the recorded variation in rock type and composition may reflect the transition from a magmatic arc or subduction-related setting (units I and II) to a syn- to post-collisional setting (unit III).