Tectonic Evolution of the Archaean Granulite Facies Belt of Sri Lanka - South India

The Archaean granulite facies belts of Sri Lanka - South India contain abundant metasediments consisting of quartzites, garnet sillimanite gneisses, marbles, diopside scapolite calciphyres with subsidiary iron formations, along with other quartzofeldspathic gneisses, amphibolites and hypersthene 'charnockitic' gneisses of dubious origins. In these ensialic linear belts lithologies can be traced for hundreds of kilometres along the strike and the belts appear to be bounded by major tectonic lineaments.

The quartzites - garnet sillimanite gneiss - marble - calciphyre assemblages were derived from a sandstone- shale - limestone - evaporite sequence, which implies shallow water sedimentation in a shelf or intracratonic basin or trough. The sediments indicate a relatively stable cratonic source, which in Archaean times may have been a component of the earliest crust or protocrust. As these belts are fault-bounded, ensialic, linear zones within an older migmatitic basement they may represent intercratonic aulacogenes, The structural, metamorphic and igneous events indicated for these belts also point to tectonic controls that consist of at least two phases.

1) Tensional tectonic stage which led to the aulacogenes (> 3000 m.y.)

2) Transform tectonic stage which converted the aulacogenes into mobile belts (~2800 m.y.)

Movement and displacement along these crustal transform mobile belts caused high angle-orthogonal external rifting. Thus the transform mobile belts are now acting as small circles of rotation initiating external rifting and spreading in the craton along great circles, as expressed in Eulers Theorem. The thin cratonic crust separated to form narrow basins floored by oceanic crust, the precursors of the greenstone belts. Thus the high grade mobile belt and the low grade greenstone belt are complimentary being formed at the same time and at the same crustal level.