PETROLOGY OF METAMORPHOSED BASALTS AND GABBROS IN THE STØREN NAPPE OF THE UPPER ALLOCHTHON, SCANDINAVIAN CALEDONIDES, NORWAY

The Støren Nappe includes a series of ophiolite fragments that were derived from Ordovician oceanic arcs and back arc basins. These rocks were metamorphosed to epidote amphibolite facies and thrust onto Baltica during the Silurian-Devonian Scandian Orogeny. The rocks are relatively undeformed in eastern Norway and Sweden, where pillow lavas and other primary structures occur. In contrast, in western Norway in the northern part of the Western Gneiss Region, the Støren Nappe is highly deformed and preserved in deep synformal folds. 87 samples were collected from the Moldefjord and Surnadal synform to the southwest of Trondheim, and in the Lensvik synform to the northwest. Of these, 43 and 29 are metamorphosed basalts and gabbros, respectively, spanning a wide range from Ca- and Mg-rich gabbroic cumulates to evolved tholeiitic basalts. The mineral assemblages are broadly similar: hornblende-plagioclase ± epidote, quartz, magnetite, titanite, rutile, actinolite, diopside, biotite, and calcite, in decreasing order of occurrence. Hornblende is pale- to medium-green, and zoned in some samples with cores lighter colored than rims (but also hornblende). Plagioclase spans a wide range of composition across the samples and, in individual samples, tends to have more anorthitic rims. Epidote ranges from pistacite in the comparatively Fe-rich basaltic rocks to clinozoisite in Ca-rich, Fe-poor gabbroic cumulates. In cumulates the two-amphibole hornblende-actinolite assemblage is relatively common. Actinolite is colorless and contains abundant hornblende exsolution lamellae, but visible actinolite exsolution lamellae in hornblende is less common. Adjacent amphiboles have sharply defined contacts. Epidote is most abundant and magnetite most rare in the Surnadal synform closest to Trondheim, with generally more magnetite and less epidote to the northwest and southwest from there, indicating higher metamorphic grades. Though mineral zoning makes choice of compositions difficult for calculations, preliminary Perple_X thermodynamic modeling on the two-amphibole assemblages suggests that the mineral assemblages formed in a narrow temperature range 320-400°C, at pressures of 5-7 kbars. Further work will define P-T estimates more clearly, and examine differences in metamorphic conditions in different regions.