Chromian spinels in the basalts from the Lisbon volcanic complex (Portugal) and their petrogenetic implications

Abstract

Key-words.. Chromite - Ferritchromit - Basalt - Lisbon - Petrogenesis. Chromian spinels are common in the late Cretaceous alkali basalts of the Lisbon volcanic, Complex in Portugal. They occur as unzoned inclusions in magnesian olivines of all basalt types and as large spectacularly zoned grains in the groundmass of porphyritic basalts. Microprobe analysis indicate complex cationic exchange in the groundmass zoned spinels due to simple peritectic reactions and in response to changing composition of the basalt liquid. The variation of cationic distribution in zoned chromian spinels, reflects very accurately the changing chemistry of the cooling silicate melt and the paragenetical relations of mineral oxides and silicates. Crystallization of initial chromian spinels occurred at T-1,200°C and fO210-8.5 atrn. earlier or contemporaneously with magnesian olivine. The titanomagnetite mantles of zoned chromian spinels crystallized at T-1,200°C and much lower fO2