Crustal warping-a possible tectonic control of alkaline magmatism

Abstract

Alkaline and peralkaline magmatism is peculiar to the epeirogenic continental environment; it is characterized by unusual concentration of alkalis, volatiles, and rare elements, the large volumes of alkaline rock contrasting with the small amounts of such rocks in alkali basalt provinces. Probably the most outstanding region of alkaline and related carbonatite activity is in east and central Africa, where localization of the activity along the rift valleys has long been recognized. Recent discoveries tend to establish this relationship. The restriction of major alkaline provinces to the stable continents has led to the suggestion that long undisturbed periods must be necessary for slow accretion beneath the crust; the implications are that the continental plate has a passive role and that any association of alkaline magmatism with rifting is fortuitous. It is here argued that the regional tectonics control the generation of alkaline magmas and their localization along the rift zones. The African shield shows a distinct ‘basin and swell’ pattern, and many of the rifts occupy the axial zones of the long, narrow upwarps, separating the basins. A series of erosion surfaces, dating at least as far back as the Jurassic, shows increasing elevation and separation toward the crests of the swells and indicates uplift of these zones in several distinct stages: the continuity of such erosion surfaces shows that the continental plate behaved competently during these uplifts. Upwarping of a rigid continental plate would relieve the pressure lower in the crust, allowing partial melting and the concentration of fugitive constituents from the underlying mantle. In east and central Africa, similar alkaline magmas of salic differentiate character, rich in volatiles, and often with associated carbonatites, were produced in different regions during different periods of rifting; the unusual volume of such rocks can more reasonably be explained by partial melting of crystalline source rocks and collection of juvenile fugitive constituents than by the usually favored process of fractional crystallization, which would require a vastly greater volume of parent magma.