Ring-structures in the Sara-Fier Younger Granite complex, Northern Nigeria

Abstract

The Sara-Fier complex is composed mainly of granites, but also includes minor acid volcanic rocks and a few small intrusions of basic and intermediate composition. The acid intrusive rocks of the complex form five distinct ring-centres which overlap in a north-south line. An ideal order of events in these ring-centres, applicable to the Nigerian Younger Granite province as a whole is: (i) early volcanic rocks, mainly rhyolitic; (2) ring-fracture, cauldron-subsidence, and the intrusion of a ring-dyke, often of hornblende-granite-porphyry or horn-blende-pyroxene-fayalite-granite; (3) the emplacement of a succession of granites within this ring-fault, generally in a concentric fashion, and including hornblende-biotite-, biotite-, and riebeckite-granites. Intrusive activity seems normally to have been confined within the outer ring-dyke and to have been completed in each centre before the beginning of a new centre. The early ring-dykes are steep structures and since they may be up to a mile wide their emplacement cannot be explained by simple cauldron-subsidence. Later ring-dyke-like bodies and stocks do, however, show outward-dipping contacts, and their emplacement seems to have been controlled by underground cauldron-subsidence beneath dome-shaped fractures.