DEFINITION OF GREENSTONE-GNEISS CONTACT RELATIONSHIPS

An important problem in many greenstone belt studies is

determining the original structural relationships between

greenstone belts and older gneiss terrains. In pre-1990 studies

the significance of early thrusting along thin “slide” zones

went unrecognized, leading to a widespread view that many

greenstone belts simply rest autochthonously over older

gneisses, or that the older gneisses intruded the greenstone

belt. While this may be the case in a few examples, it is difficult

to demonstrate, and the “classic areas” in which such

relationships were supposedly clearly demonstrable have

recently been shown to contain significant early thrust faults

between greenstones and older sedimentary rocks that rest

unconformably over older gneisses. Such is the case at Steep

Rock Lake in the Superior Province, at Point Lake and

Cameron River in the Slave Province, in the Theespruit type

section of the Barberton greenstone belt, at Belingwe, Zimbabwe,

and in the Norseman-Wiluna Belt.

The Norseman-Wiluna, Cameron River, and Point Lake

greenstone belts contain up to 1,640-feet (500-m) wide

dynamothermal aureoles at their bases. The aureoles contain

upper-amphibolite facies assemblages, in contrast to greenschist

and lower amphibolite facies assemblages in the rest of

the greenstone belts. The aureoles have mylonitic, gneissic,

and schistose fabrics parallel to the upper contacts with the

greenstone belts and are locally partially melted forming

granitic anatectites. The aureole at the base of the Norseman-

Wiluna belt is an early shear zone structure related to the juxtaposition

of the greenstone belt with older gneisses, and it is

overprinted by greenschist facies metamorphic fabrics and

two episodes of regional folding, the second of which is the

main regional deformation event and is associated with a

strong cleavage. A late-spaced cleavage is associated with

upright folds. On Cameron River and Point Lake in the Slave

Province, the aureoles represent early thrust zones related to

the tectonic emplacement of the greenstone belts over the

gneisses. Amphibolite-facies mylonites were derived through

deformation of mafic and ultramafic rocks at the bases of the

greenstone belts, which are largely at greenschist facies. The

broad field-scale relationships in these cases are also similar

to those found in dynamothermal aureoles attached to the

bases of many obducted ophiolites.

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