Figure 8. Total alkalis (Na₂O + K₂O) vs. SiO₂ plot (wt% with FeO adjusted to 85% of total iron) for classifying tholeiitic and alka lic basalts. Boundary line separating alkalic and tholeiitic fields is from Macdonald and Katsura (196 4). A. The open fields show the temporal evolution of Kerguelen Archipelago lavas from the ~30 Ma flood basalts, with compositions transitional between tholeiitic and alkalic lavas (Mts. Bureau and Rabouillere), to the slightly alkalic Southeast Lower Miocene Series (LMS) to the highly alkalic Sout heast Upper Miocene Series [UMS]. Data sources are Weis et al., 1993, 1998; Yang et al., 199 8). In contrast, lavas from Kerguelen Plateau ODP Sites 747, 749, and 750 are tholeiitic basalts; however, the above basement lavas from ODP Site 748 are alkalic basalts. B. Expanded panel s howing more detail for basalts drilled and dredged from the Kerguelen Plateau and Broken Ridge. Lavas d redged from the central Kerguelen Plateau and Broken Ridge (open circles) and ODP Site 738 strad dle the boundary line, largely because the total alkali contents of these lavas were increased du ring postmagmatic alteration. As an extreme example, the solid triangle indicates a highly altered sample from ODP Leg 120, Site 750 (Sample 120-750B-19R-1, 47-50 cm). Data from Davies et al. (1989), Storey et al. (1992), and Mahoney et al. (1995).

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