Authigenic Biotite in Quaternary Sediments from the Contact Zone with a Thick Sill, Hole DSDP 485/485A, Gulf of California

Authigenic finely dispersed biotite was found in a single sample from Hole DSDP 485/485A, in the southern Gulf of California, south of the Tamayo Fracture Zone. The sample was taken from Quaternary terrigenous sediments that overlie and alternate with several massive basaltic sills of varying thicknesses. It is noteworthy that the biotite makes up practically entire sample. The formation of the biotite is related to the metasomatic transformation of terrigenous clay minerals at the base of the thickest basaltic sill (30.95 m) by hydrothermal fluids, which were mobilized owing to the emplacement of hot magma. The study of the structural, crystallochemical, and morphological features of the biotite from this sample revealed characteristics that are almost identical to those of authigenic biotite recently discovered in Pleistocene terrigenous sediments from the Central Hill in the Escanaba Trough in the southern Gorda Ridge (northeastern Pacific Ocean).