GEOCHEMICAL ORIGIN OF MOZONITE IN A STRATIFIED PLUTON, NORTH-CENTRAL NORWAY

The 466 Ma. Hortavaer Igneous Complex intrudes marble, calc-silicate, and gneissic rocks off the coast of north-central Norway, northwest of the Leka ophiolite, and comprises a stratified pluton that is part of the Bindal Batholith. It is unusual in that previous studies indicated that it evolved from olivine gabbro through monzonite to syenite and alkali granite by AFC of calc-silicate rocks. The western part of the intrusion is underlain by syenite, whereas the eastern part is underlain by diorite, monzonite, and alkali granite. The middle of the intrusion consists of alternating syenite and diorite sheets with local pods of monzonite. In the layered rocks, escape structures indicate that the pluton has rotated 90° or more, with up to the east. This is consistent with similar block rotation of the Leka ophiolite. The escape structures and associated enclave swarms, which result from the density contrast between syenitic and dioritic magmas, suggest that physical mingling occurred within the pluton. If chemical mixing also occurred, then intermediate (monzonitic) rocks may be the result of mixing of dioritic and syenitic endmember magmas, as opposed to AFC. Major and trace element variation and the use of ratio-ratio plots indicate that intermediate rock compositions resulted from both AFC and magma mixing. The influence of the fractional crystallization aspect of AFC is particularly apparent in the enrichment of lithophile elements and depletion of compatible elements in the monzonites relative to the diorites.