Migmatite structures in the Central Gneiss Complex, Boca de Quadra, Alaska
Article first published online: 5 MAY 2007
Journal of Metamorphic Geology
Volume 6, Issue 4, pages 517–542, July 1988
How to Cite
McLELLAN, E.L. (1988), Migmatite structures in the Central Gneiss Complex, Boca de Quadra, Alaska. Journal of Metamorphic Geology, 6: 517–542. doi: 10.1111/j.1525-1314.1988.tb00437.x
- Issue published online: 5 MAY 2007
- Article first published online: 5 MAY 2007
- Received 4 February 1987; revision accepted 22 February 1988
- melt segregation;
- migmatite structures;
- shear bands;
- syn-migmatization stresses;
- vein networks;
- volume change
Abstract Migmatite structures in the Coast Plutonic-Metamorphic Complex are well exposed in the inlet of Boca de Quadra, southeast Alaska. Two types of anatectic migmatites are present. Patch migmatites formed by in situ melting and subsequent crystallization of melt. Diktyonitic migmatites comprise a discontinuous veined network of leucocratic material, in which leucosomes enclose boudins of host rock. The margins of these boudins show the development of both melanosomes and shear band fabrics.
Strain analysis of diktyonitic melanosomes indicates that these regions have undergone volume decreases of 20-27%. This volume decrease is attributed to melt extraction into the adjacent fracture-filling leucosomes. Thus, diktyonitic migmatites formed by shear-induced segregation of partial melt, whereas in patch migmatites the lack of shear stresses inhibited melt segregation. The variable structural style of anatectic migmatites in Boca de Quadra is not related to host-rock composition, but may be due to differences in the amount of differential stress during migmatization. These in turn may be controlled by host-rock strength and/or diachroneity of migmatization and deformation.
Determination of volume changes during migmatization using strain analysis is potentially capable of discriminating intrusive and anatectic migmatites and consequently of documenting melt segregation and subsequent migration across crustal levels.