5.2. Major elements
Major elements in ultramafic and gabbroic rocks are plotted and compared with reference compositional fields in the variation diagrams of Fig. 9. These reference fields correspond to: the mafic metavolcanic rocks of the Puerca Gorda Schists; SSZ mantle pyroxenites of Solomon Islands (Berly et al., 2006), plutonic rocks from the Early to Middle Jurassic Talkeetna Arc section (Greene et al., 2006); primitive (Mg#>66) low-, intermediate- and high-Ca boninites from the ODP Leg 125 (Pearce et al., 1992; Taylor et al., 1994; Pearce & Peate, 1995; Crawford et al., 1989); and the experimentally obtained liquid line of descent of anhydrous, mantle derived, tholeiitic liquids by fractional crystallization at 0.7 and 1.0 GPa (Villiger et al., 2004, 2007).
The clinopyroxenites and websterites display high Mg# values of 77-86 [Mg# = Mg/(Mg+Fe)×100, calculated as cation wt.%] for a wide range in the Al2O3, FeOT and CaO contents. In comparison, the gabbroic rocks show a smaller range and define a more regular trend with Mg# values of 64-81 in the gabbronorites, 65-79 in the troctolites, and 42-66 in the oxide gabbronorites. Therefore, if we consider the decreasing Mg# as an indicator of the degree of magmatic fractionation, there is a clear order from the most primitive compositions of the clinopyroxenites and websterites, to the more evolved gabbronorites, troctolites and oxide gabbronorites. The <70 Mg# values in many gabbroic rocks indicate that they are already evolved melts, and are out of equilibrium with upper mantle peridotite (Müntener & Ulmer, 2018). However, these pyroxenites and gabbroic rocks display cumulate textures, products of solid-liquid separation processes. Therefore their whole-rock compositions are strongly controlled by the cumulate phases. Thus, they do not likely represent liquid compositions.