Porphyroblastic differentiation in fleck gneiss from Vastervik, Sweden
Titel:
Porphyroblastic differentiation in fleck gneiss from Vastervik, Sweden
Auteur:
Russell, Richard V.
Verschenen in:
GFF
Paginering:
Jaargang 91 (1969) nr. 2 pagina's 217-282
Jaar:
1969-06-30
Inhoud:
Flecks are small scale metamorphic differentiation structures characterized by dark biotite-rich cores (melanosome) surrounded by quartz-feldspar or quartz-muscovite mantles (leucosome) in an intermediate matrix (paleosome). Two flecked meta-sedimentary units in the Kasimirsborg area, Vastervik show a change in metamorphic grade along strike. The higher grade rocks contain andalusite and sillimanite. The general area is called the andalusite-sillimanite zone. Lower grade equivalents form what is defined as the muscovite zone. Whole-rock chemical analyses show a decrease in water and oxidation grade with increased metamorphic grade. The crystallization of andalusite-sillimanite is accompanied by the breakdown of iron-rich muscovite. Triclinicity measurements show that K-feldspar is monoclinic within both metamorphic zones. Flecks from the muscovite zone are characterized by muscovite-quartz leucosomes. Leucosomes in the andalusite-sillimanite zone are reddish, being composed essentially of K-feldspar and quartz. The presence of unaltered cordierite in inner melanosomes, Mg-rich green biotite in melanosomes and concentric melanosome zoning are important arguments in the conclusion that most flecks from Kasimirsborg represent altered cordierite porphyroblasts. “Granitic” leucosomes have resulted through the “concretionary” growth of mafic assemblages. Leucosomes are residues. The porphyroblastic mechanism for the production of “granitic” material is based on the crystallization of mafic phases in regular clusters or zones, by leaching mafic components from the surrounding rock. Operating alone, the process probably accounts for small scale structures, like flecks. Combined with tectonic deformation and mixing, however, the scale of the structures could be greatly increased.
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