Plate tectonic lecture - High grade
gneiss terranes and ensialic zones.
High grade gneiss terranes:
- predominantly quartzo-feldspathic material
(continental) with minor sedimentary or volcanic enclaves.
- pervasive granulite facies metamorphism in
many of them. Identified by the presence of metamorphic pyroxene.
This is very significant because you need thickened continental
crust to form granulites!
- occasional large layered mafic or anorthositic
- depleted in Rb, K, U, Th - suggestive of
removal of these by partial melt episodes, i,.e. the dry and
hot deep residue of crustal melting.
- polyphase deformation.
Minnesota River valley a good example.
Role of tectonic exhumation in their history?
One model for formation: deep crustal roots
of an orogen that has undergone thickening, and partial melt segregation;
i.e. highly reworked continental crust.
- These are very large mobile shear zones.
They are linear zones of very high strain - megafaults or plate
boundaries between older craton fragments.
- Given different histories of rocks on either
side (e.g. Kolar India) - it would suggest a suture or a transcurrent
zone, but much more narrow than those of today.
- The Limpopo belt, that separates the
Transvaal and Rhodesian cratonic blocks and formed some 1800-200
Ma, is a better known example.
- Another is the Kibaran fold belt which formed
- Zone of interior reactivation? More deformation
Course materials for Plate Tectonics, GEOL
3700, University of Nebraska at Omaha. Instructor: H. D. Maher
Jr., copyright. This material may be used for non-profit educational
purposes with appropriate attribution of authorship. Otherwise
please contact author.