Origin of certain types of small-scale igneous layering from the Fongen—Hyllingen basic complex, Norway

Abstract

Mineral graded rhythmic layering occurs throughout most of the Fongen—Hyllingen basic complex. Non-cumulate, fine-grained, granular rocks are found as a conformal part of the layered gabbros. In one particular locality isomodal coarse-grained layered structures are developed within these non-cumulate granular rocks. These layers range mineralogically from olivine + Ca-rich pyroxene + Ca-poor pyroxene + plagioclase to amphibole + Ca-poor pyroxene + plagioclase. The sudden appearance of high concentrations of amphibole, without indications of amphibole primocrysts in the surrounding granular rocks or in adjacent normal rhythmically layered gabbro, is difficult to interpret in the light of the conventional cumulate theory. Heterogeneous nucleation and growth close to or on the bottom of the magma chamber is preferred for the origin of the layered structures.

Key Words:

• Igneous intrusions
• layered structures
• cumulates
• non-cumulates
• grain-size variation
• mineralogical variation
• heterogeneous nucleation
• bottom crystallization
• Fongen—Hyllingen basic intrusion
• Scandinavian Caledonides
• Norway
• N6310 N6310 E1130 E1130