ASYMMETRIC ZONING OF A MIDDLE CRUSTAL PLUTON IN THE NORWEGIAN CALEDONIDES: SELECTIVE ASSIMILATION OF HOST ROCKS

The Sausfjellet pluton was emplaced into high-grade metamorphic rocks. Marble, minor calc-silicate, and rare pelitic rocks host the eastern part. The western part intruded pelitic migmatite, which was remelted during emplacement. Stage 1 (southeastern part of the pluton) consists of pyroxene hornblende diorite and gabbro that crystallized from an H₂O-rich andesitic magma. Stage 2 is asymmetrically, gradationally zoned from a central zone of pyroxene diorite and anorthosite, to rocks as evolved as quartz monzodiorite in the western and in an annular zone as much as 200 m wide. The western/annular zone is mineralogically heterogeneous, with variable amounts of quartz, K-feldspar, biotite, and amphibole, none of which are correlated with position in the pluton or proximity to a contact. Stoped blocks of diorite and pyroxene-rich calc-silicates are present in stage 1 and the central zone of stage 2, whereas the western zone has sparse blocks of quartz-rich gneiss. Chemical variation in the central zone is due to accumulation of pyroxenes + plagioclase. These rocks have approximately constant d¹⁸O of +6.6 ± 0.2 ‰ and lack chemical evidence for assimilation. Heterogeneity in the western/annular zone is reflected in variable incompatible element contents and d¹⁸O values (+6.7 ‰ to +8.6 ‰). Petrologic models indicate that stage 2 arose from H₂O-poor andesitic magma. The central zone evolved in a chemically closed system because stoped blocks were too refractory to assimilate. In contrast, stoping of partly-molten pelitic host rocks into the western/annular zone explains its heterogeneity, variable d¹⁸O, and excess of incompatible elements relative to closed-system processes; as much as 20% of the mass of the western/annular zone can be ascribed to assimilated material. Apparently, evidence for stoping cannot be limited to the presence of stoped blocks; soluble contaminants can be completely assimilated, leaving only geochemical evidence of their existence.