ABSTRACT
The tectonic and magmatic processes which have shaped the post-Archean continents are dominated by convergent plate tectonics. In a convergent plate setting most primary magmas are generated in the mantle-wedge above the subducting plate. The fluids that migrate into the mantle-wedge are likely to contain variable amounts of elements such as carbon, fluorine, silica, sulphur, chlorine, potassium, rubidium, strontium, thorium and uranium. Basaltic andesites and andesites are the characteristic rocks of the volcanic chains that develop along convergent plate margins. The rocks surrounding porphyry ore deposits are usually fractured or brecciated. Rhyolites and granites are exceptional rocks because silicic magma occupies the principal low temperature trough in petrogeny's residua system. Most volcanic silicic rocks crop out as pyroclastic rocks, obsidian, pumice or pitchstone. A diverse group of extrusive and intrusive rocks can evolve within a subduction system. A middle layer that contains various gabbroic rocks and a lower layer of depleted mantIe peridotite.
