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Firstly, two of the many orbiculite boulders found at Sääkslahti are described. The matrix of the orbiculites varies from diorite to quartz monzonite. Orbicule are compositionally heterogeneous (from essentially plagioclasic through all kinds of intermediate types to hornblendic), and often they are protoclastically deformed and disrupted. Mineral paragenesis, structures and textures clearly indicate a magmatic origin. Later metamorphic events are manifested by the formation of uralite, secondary biotite and titanite, chlorite, sericite and prehnite. A mechanism for orbicule formation in magmas is presented. An orbiculeinclined magma possesses a low oxygen pressure, which is indicated in the low ratio of ferric to ferrous iron in the orbicule occurrences analyzed. The low oxygen pressure inhibits the nucleation of early spinels, which are detrimental to the evolution of delicate orbicules. In a fluid, turbulent magma, proto-orbicules were generated through synneutic accretion of phenocrysts (glomerophyric structure). These groupings, and sometimes individual single phenocrysts and xenoliths, were centres for the precipitation of shells, by creating local temperature and attraction gradients. From a zone around these local centres an excess phase (in relation to the cotectic or eutectic) diffused and crystallized on the proto-orbicule. This, in turn, caused the enrichment of the components of the other phases in the diffusing front, until they became saturated and crystallized, forming the next zone. Then, again, the first phase began to crystallize. Supercooling prevailed during the process, causing the formation of one-phase shells. Now and then granular »shock-layers» formed. The rhythmic zoning represents a special case of some types of rhythmic igneous layering. Radial arrangement of minerals was formed by the principle of crescumulate growth. Free space was necessary for the radial mineral growth which is shown by many natural examples and experimental silicate synthesis and zone melting.

Petrography and protoclastic structures of the orbiculite boulders from Sääkslahti, Toivakka, Finland and the magmatic genesis of orbiculites