High grade gneiss terranes:

General characteristics:

• 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 complexes.
• 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.

Ensialic zones:

• 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 1100-1300 Ma.
• Zone of interior reactivation? More deformation within plates?

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.