Punctuated anorogenic magmatism

Abstract

The emplacement of anorogenic magmas, be they mantle-derived or crust-derived and silica-undersaturated or silica-oversaturated, marks a period of rifting or tectonic relaxation and apparent quiescence. In a given area, such magmatism commonly recurs episodically, and can yield even more strongly alkaline products than in the first cycle, in spite of the depletion that resulted from that episode of melting. Anorogenic magmatism is said to be punctuated where it recurs, in response to a triggering mechanism. The second cycle reflects an influx of heat and a fluid phase responsible for the fertilization of the depleted source-rock. In cases of an anorogenic stage after a major collision, the first cycle of magmatism, yielding an AMCG suite, arises by gravity-induced sinking of lithosphere and the diapiric rise of an asthenospheric mantle; renewed magmatism may involve localized and renewed detachment as late as 200m.y. after the collision. Where the hiatus is much longer, as in Nigeria, we appeal to a propagating zipper-like zone of extension, possibly related to rotation of a crustal block. The economic ramifications of punctuated anorogenic magmatism are important; the second-generation magmas may well crystallize products that are mineralized in the high-field-strength elements and any other elements enriched in the source rocks. Such a model would account for the rich deposits of alluvial columbite, zircon and cassiterite associated with the Younger Granites of Nigeria.