Brilha J.B., Sequeira Braga M.A., Proust D., Dudoignon P., Meunier A.
Apresentado no International Volcanological Congress, Ankara, Turquia, 1994
Resumo
The outcrop constitutes a chimney whose lava extruded about 55±18 my ago (estimation by Rb/Sr method). It presents typical columnar jointing nevertheless in some areas of the outcrop the alkaline basalt is massive. The columns present cross polygonal sections with ±50cm diameter, the majority of them with hexagonal or pentagonal tendency. One column with about 60cm length and 50cm in diameter was sampled and submitted to transversal cuts in order to obtain slices with 2-3cm thickness. We identified 5 different types of fissures in this column:
Fissures A - present about 1mm width and crosscuts obliquely all the prism. They can be identified by naked eye.
Fissures B - present about 0.5mm width and their extension is always limited to one slice of the prism. It is common the observation of the beginning and the end of the same fissure. They can be identified macroscopically.
Fissures C - present about 0.4mm width and occur near the prism border. They can also be identified by naked eye.
Fissures D - with about 0.01mm width, these type of fissure can only be identified with optical microscopy in thin sections. They occur in all the slices of the prism in the border and in its interior.
Fissures E - are the thinnest ones (less then 0.01mm). They can only be identified in optical microscopy using reflected light and occur in all the thin sections studied.
In spite of the different type of fissures, the mineralogical composition of the infilling is the same: natrolite and analcite - as dominant minerals; calcite (very rare in fissures A) and pectolite (rare and only in fissures A). Fissures E constitute an exception because most often presents no infillings; other times, it is impossible to characterize, even chemically, the material infilling. We observed that the fluids have identical compositional characteristics though the fissures may have been formed at different temperatures.