Modal and element variation in Antarctic tholeiites
Review articleOpen access

AbstractA study of the variations in phase, mode and element concentration across fourteen cross sections of sills in the Antarctic Ferrar Dolerites show them to belong to at least three separate periods of intrusion of olivine tholeiite, hypersthene tholeiite and pigeonite tholeiite. The hypersthene tholeiite sills have a central zone of highly mafic norite transitional to pigeonite dolerite above and below; the bulk composition of these sills is much higher in Cr, Ni and Mg and lower in Ba, Sr, Rb, K, Na, Zr, Cu and Zn than the border zones and chilled margins indicating inhomogeneity of magma or multiple intrusion at the time of formation. The pigeonite tholeiite sills were probably nearly homogeneous initially and show no significant phase layering or modal or major element variation but have a higher concentration of Cr, Ni, Mg and lower K, Rb, Ba, Zr, Fe and Na below the middle of the sills while the reverse situation is found in a pegmatoid layer near the top. An olivine tholeiite sill shows a transition from olivine tholeiite near the supposed base through hypersthene dolerite to pigeonite dolerite and granophyre near the top; a general increase in alkali elements and decrease in Mg and associates occurs from base to top with irregularity due to rhythmic banding.Data presented include 115 modal analyses, 55 chemical analyses for major elements and 50 for trace elements. The accentuated trends shown by the trace elements are of great help in this type of problem.

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