Basement Reactivation Research Articles

Precambrian Crustal Development in Western Nigeria: Indications from the Iwo Region

The rocks of the Iwo region of southwestern Nigeria comprise a migmatite-gneiss-granite complex and a metasupracrustal sequence. Though dominantly of amphibolite facies grade, pyroxene granulite and charnockite remnants are found in the migmatite-gneiss-granite complex. Old supracrustal remnants, best identified in resistant quartzite horizons, are also represented, and a variety of granitic plutonic rocks form well-marked circumscribed associations. The main metasupracrustal belt is associated with a major dislocation zone and has two principal lithologies — dominant amphibolite and pelite west of the fault zone and quartzite and quartzofeldspathic gneiss to the east. Oligoclase-granite gneiss and pegmatite are associated with the schist. All the rocks of the area have been affected to varying extents by late reworking, metasomatism, and granitic activity. On the basis of the indicated field relations and the limited radiometric age data available, a development sequence for the region is proposed. Archean crustal rocks are considered to have been involved in an Eburnian (1,950 ± 250 m.y.) reactivation of Proterozoic mobile belt type. In the Kibaran period (1,200 ± 200 m.y.), activity of greenstone belt–granite type brought about the formation of the Ife supracrustal–granite gneiss association, with extensive basement reactivation limited to the proximity of the downwarping supracrustal sequence. Late, widespread reworking and granitic plutonism is related to the Pan-African (Older Granite) event (600 ± 150 m.y.). The possible relevance of the proposed scheme to the interpretation of the development of the eastern parts of the West African platform is discussed.

Continental Collisions in the Appalachian-Caledonian Orogenic Belt: Variations Related to Complete and Incomplete Suturing

Certain oceans, whose obliteration during Paleozoic time led to climactic deformation of the Appalachian-Caledonian orogenic belt, closed at different times along the belt. Irregularity of the continental margins that collided during Late Silurian, Early Devonian, and Middle Devonian times caused great variations in structural style and igneous and metamorphic associations. The impingement of projections caused intense deformation leading to cryptic sutures and, in places, basement reactivation. Wider zones of less intense deformation are believed to represent embayments in the colliding margins. Late granite and associated high-grade metamorphic aureole rocks, are more common in the narrower, more intensely deformed regions; we suggest that they represent partial melts from lower parts of a thickened continental crust in these areas. “Granite” in embayments, floored by deformed oceanic basement, is suggested to represent the partial melting of that basement and (or) the covering sediment. In both cases, the magmatism reflects depression of crustal material to a depth below a particular critical isotherm and does not necessitate any abnormal heat input from below the crust.

Tibetan, Variscan, and Precambrian Basement Reactivation: Products of Continental Collision

Extensive terranes of basement reactivation are interpreted as resulting from crustal thickening following continental collision. It is suggested that terranes, such as the Grenville Province and much of the Variscan orogenic belt in Europe, have their modern analog in the Tibetan Plateau. The Tibetan Plateau is underlain by a continental crust between 60 and 80 km thick and is characterized by extensive high-potash Neogene vulcanism. Following T. H. Green's arguments that partial melting of a dioritic lower crust may yield potassic granitic liquids and refractory anorthositic residues, we consider that continental collision is followed by crustal thickening, to accommodate further plate convergence, with ensuing partial melting of the lower crust. At high structural levels, silicic-potassic ignimbrites are extruded in intermontane basin-horst terranes, with subjacent granite plutons. At deeper levels, a dry refractory lower crust consisting of pyroxene granulites and anor-thosites is generated.