Abstract
M.Sc. (Geology)
This treatise presents the first comprehensive investigation in petrography and geochemistry of a mantle-derived eclogite xenolith suite from the Balmoral kimberlite. Eclogites form a minor component of the Earth’s mantle however they play a vital role in our understanding of geodynamic processes, i.e. the subduction of oceanic crust (Jacob, 2004) and the crystallization of diamond within the sub-cratonic lithosphere.
A large portion of eclogites from the Balmoral kimberlite pipe is comprised of bimineralic (garnet and clinopyroxene) rocks with the rest being corundum-bearing. Mica with average modal abundances ≤10 vol% is observed as an accessory phase in bimineralic xenoliths. Modal abundances of corundum in corundum-bearing samples range between 1 and 6 vol%. Textures are ambiguous in Balmoral eclogites and thus chemical criteria of McCandless and Gurney (1989) places all Balmoral eclogites into Group II.
As typically observed in garnets from eclogites (Hills and Haggerty, 1989; Jacob, 2004), garnets from Balmoral eclogites are chromium- and manganese-poor. They have a general trend from pyrope-rich towards grossular-rich compositions, with some almandine. Garnets from the bimineralic eclogites have disparate suites of low- and high-MgO samples. High-MgO bimineralic garnets are pyropic in composition with averages at Pyr63Gros22Alm15, whereas garnets from the low-MgO suite are widespread from relatively less pyropic towards grossular-rich compositions with average compositions of Pyr49Gros40Alm11. Garnets from the corundum-bearing eclogites are homogeneous and characterised by the highest grossular component (averaging at Gros47Pyr28Alm25).
The clinopyroxenes for Balmoral eclogites are omphacitic in composition. Jadeite content is highly variable (ranging between 8 and 58 wt%) in these clinopyroxenes. The clinopyroxenes in bimineralic eclogites are characterised by a wide variation from diopside-rich towards jadeite-rich compositions. Clinopyroxenes in corundum-bearing eclogites have the highest jadeite levels.