Na-metasomatism of flood basalt lava flows from Torres, Rio Grande do SuI, Brazil
Dristas, Jorge A.
published: Jun 22, 2001
ArtNo. ESP154017602005, Price: 29.00 €
Tholeiitic flood basalts lava flows with intercalated sandstone and breccias at Torres, Brazil, show Na-metasomatism and secondary mineral formation. In some basal parts of the upper lava flow in contact with sandstone inter-lava beds, the phenomenon is also recognised. In both occurrences the intensity of alteration diminishes progressively towards the lava flow interior. The main changes in the altered tholeiitic wall rocks, close to the contact with sandstone are: 1) the obliteration of primary texture and 2) loss of Si and gain in Na, Fe3+ and H. Na enriched fluids from pre-existent shallow lagoons heated by the lava flow induced the initial formation of Na-pyroxene at high temperature followed by extensive albitisation of plagioclase in the wall rocks. Iron, titanium and calcium were released from the primary mafic minerals (Ti-magnetite, Ca-rich pyroxene). Original Ca-rich pyroxene are characterised by reaction rims of Na pyroxene, associate with melanite and titanite, while aluminium was liberated from magmatic plagioclase resulting in the formation of chlorite minerals, albite, zeolite and smectite as an alteration stage at lower temperatures. The latter stage also includes the formation of calcite, iron oxides and quartz cement in sandstone dykes/lets. Na-rich water is assumed to originate from shallow lagoons at the time of lava eruption. This suggests that the crystallisation and mineral/solution exchange were of short duration because of cooling of lavas in contact with shallow Na-rich waters.