The Formation of Megacrysts in Migmatitic Amphibolites from Halland Province, Southwest Sweden
Dr. Edward Hansen
The growth of large mineral grains (megacrysts) can be promoted by a fluid phase that allows for rapid diffusion. Hence, the presence of megacrysts in metamorphic rocks can be one indicator of the presence of a silicate melt or other fluid phase during metamorphism. We studied megacrysts of hornblende (Hbl), garnet (Grt), clinopyroxene (Cpx) and orthopyroxene (Opx) associated with both tonalitic leucosomes and melanosomes in mafic migmatites from the Stensjӧ Nature Preserve and the area around Bjӧrkasjӧ Lake. Both localities are in the Eastern Belt of the Sveconorwegion Orogen, Halland Province, Sweden. We mapped both localities and examined samples with a polarizing microscope and SEM-EDS. We did quantitative mineral analyses by WDS on an electron microprobe. Whole rock analyses were done by X-ray florescence at Actlabs, Ontario. When the host rock contains Hbl+Pl (plagioclase) +/- Cpx without Grt or Bt (biotite) the megacrysts are either Hbl or Hbl+Cpx. The compositions of the megacrysts are similar to compositions of the same minerals in the host rock. Plagioclase in inclusions within and immediately surrounding Cpx megacrysts can contain up to 46 % more Ab (albite) than Pl away from the megacrysts. When the host rock contains Hbl+Pl+Grt+Cpx without biotite the megacrysts are either Grt or Cpx. When the host contains Hbl+Pl+Grt+Bt the megacrysts are Opx. These megacrysts are surrounded by a reaction rim of Hbl+Qtz+/-Cpx+/-Grt, and Hbl in the rims has distinctly lower Ti and K2O+Na2O concentrations than in the host. The anhydrous megacrysts, (Cpx, Opx, Grt), probably formed by vaporabsent (dehydration) melting. The different kinds of anhydrous megacrysts formed by different melting reactions controlled by the chemistry and mineralogy of the original rock. Hornblende megacrysts may have formed by the retrogression of Cpx during crystallization of the melt. Alternatively they may represent water-assisted melting, without partial melting, by a concentrated brine.
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