Orbicular quartz diorite in Kemi, NW Finland

Abstract

The orbicular rock in Kemi is exposed in one outcrop 10 m2 in size. The country rock is a quartz diorite of the Archean pre-Svecokarelian basement complex. The orbicules are spheroidal or ellipsoidal, 5-16 cm in diameter, and randomly distributed in a leucocratic quartz diorite matrix. They may have from one to five shells, although the single-shelled varieties are the most common. The contacts of the orbicules with the matrix are sharp, but between the shells and against the core they are gradational. The shells are dioritic. The mafic minerals in them - biotite and hornblende - are radially and tangentially oriented. The majority of the cores are dioritic, but some are ultramafic and rich in biotite. Amphibolite xenoliths have also been found. The Kemi orbicular rock is the first found in Archean bedrock in Finland.

Keywords

• Archean
• orbicular rocks
• Finland