Evidence for upward growth of a layered pluton in the Bindal Batholith, north-central Norway
MetadataShow full item record
The Hortavær Igneous Complex, off the coast of north-central Norway, is a zoned pluton that exhibits evidence of upward growth as a series of stacked layers or sheets. What crops out now is a pluton with, from west to east, heterogeneous zones of syenite, intense sheeting, diorite, monzonite, and alkali granite. Sheets dip steeply west and strike N-S in the south and N30E in the north, defining the limbs of a regional fold. The pluton intruded calc-silicate, carbonate, and migmatitic gneissic host rocks about 466 Ma. Assimilation and fractional crystallization of dioritic magmas produced syenitic and monzonitic magmas, which are interpreted to have migrated laterally due to outward flow of successive injections of magma. Continued injection of dioritic magma produced more syenite and monzonite. New pulses of diorite spread laterally over variably crystal-rich syenitic magmas. The density contrast between syenitic and dioritic magmas caused syenite to escape upward, producing tubes and flames of syenite that intruded into the overlying sheets. Flame structures in the sheets point to the east, the original top of the pluton. At their bases, dioritic sheets quenched against the syenite and were penetrated by the escape structures. Along their upper surfaces, the dioritic sheets mingled with overlying syenitic magmas to produce enclaves. Magmatic foliation is oriented parallel to the sheets and is defined by alignment of elongate minerals and flattened enclaves. This foliation is interpreted to result from overburden pressure. Folding of the pluton produced parasitic folds of the sheets in the limbs of the regional fold. Subsequent block rotation about 90 degrees east resulted in the current orientation of sheets with a steep westward dip.