AUTHIGENESIS OF AMPHIBOLE AND ITS RELATIONSHIP TO THE DIAGENETIC EVOLUTION OF LOWER CRETACEOUS SANDSTONES OF THE POTIGUAR RIFT BASIN, NORTHEASTERN BRAZIL

Authigenic amphibole occurs in the fan-delta-front arkoses and lithic sandstones of the Neocomian Pendencia Formation in the Potiguar rift b asin, northeastern Brazil as overgrowths, fracture-hearing and replace ment of detrital amphibole. The amphibole, which is closely associated with diagenetic albite, chlorite and sphene, has precipitated from K- Na-CI brines enriched in Ti-Fe-Mg, at temperatures less than or equal to 100 degrees C. These ions were derived from the diagenetic alterati on of detrital amphibole and pyroxene, chloritization of volcaniclasti c sediments at the base of the formation, and from brines that ascende d along faults from the underlying basement.