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.