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Anders Lindh

Anders Lindh

Professor emeritus

Anders Lindh

Crystallization of orbicular rocks exemplified by the Slattemossa occurrence, southeastern Sweden

Author

  • Anders Lindh
  • Helena Nasstrom

Summary, in English

The orbicular rock at Slattemossa, southeastern Sweden, has a quartz monzodioritic composition. The cores of the orbicules crystallized directly from the orbicule-forming magma; cores made up of xenoliths have not been observed. Outside the core follow first an inner mafic, a felsic and then an outer mafic shell. The orbicules occur in a matrix, which is similar to the core. They grew simultaneously and show an almost perfect parallelism in evolution. After initial 'normal' crystallization of the magma, superheating probably triggered by a sudden addition of volatiles destroyed earlier formed nuclei and also affected the already crystallized part of the rock. Cooling caused heterogeneous nucleation and rapid crystallization, which formed the inner mafic shell. This is enriched in mafic minerals, especially biotite, compared to the core. At the same time the grain size becomes significantly smaller. Depletion in malic components, possibly intensified by a sudden change in physical conditions, destabilized biotite and amphibole crystallization, causing oversaturation in plagioclase components, forming a felsic shell having a sharp boundary with the mafic shell. Plagioclase is extremely altered. Mafic minerals were then stabilized, probably due to depletion of plagioclase components, and an outer mafic shell formed. With the return to homogeneous nucleation, matrix formation concluded the crystallization. Orbicules might have moved in the magma causing some squeezing of magma surrounding the orbicules, but major movements involved the settling of the whole package of orbicules and matrix in the surrounding non-orbicular magma.

Department/s

  • Lithosphere and Biosphere Science

Publishing year

2006

Language

English

Pages

713-722

Publication/Series

Geological Magazine

Volume

143

Issue

5

Document type

Journal article

Publisher

Cambridge University Press

Topic

  • Geology

Keywords

  • monzodiorite
  • Proterozoic
  • orbicular
  • Sweden

Status

Published

ISBN/ISSN/Other

  • ISSN: 0016-7568