An experimental and geochemical study of the products of extreme metamorphic processes : ultrahigh-temperature granulites and the Roberts Victor eclogites

Ultrahigh-temperature granulites and eclogites are products of extreme metamorphic processes in both the Earth's crust and mantle. The aim of this thesis is to examine the conditions and processes responsible for the creation of such lithologies. Granulites comprise a significant proportion of the midand lower-crust, and being some of the oldest rocks exposed, an understanding of their origins and evolution is essential in understanding crustal evolution. Due to their anhydrous nature, granulites frequently preserve evidence of a polymetamorphic history in the form of arrested reaction textures and compositionally zoned phases, which when interpreted within the framework of a petrogenetic grid can yield a pressure-temperature (PT) path from which a tectonic history can be inferred. The primary aim of this experimental study was to derive and constrain a petrogenetic grid in the KFMASH (K 20-FeO-MgO-Al203-Si02-H20) system applicable to silica-undersaturated (aluminous) granulite facies metapelites. The qualitative grid was constructed by designing and implementing a new chemographic projection, involving phase projection from sil, ksp and L onto the plane S-FB-MB (qtz Febte Mg-bte), and identifying univariant reactions and probable stable assemblages. This grid qualitatively accounts for assemblages involving the phases garnet, cordierite, orthopyroxene, sillimanite, sapphirine, spinel, potassium feldsapr, quartz and melt. In an attempt to position this grid precisely in PT space experiments were undertaken which involved subjecting five different bulk compositions, ranging from XMg 60 to 90, to elevated temperatures (880 to 1080°C) and pressures (7 to 8 kbar) within internally heated gas-apparatus. The representative granulite bulk starting compositions were composed of synthetic biotite and orthoclase, aluminosilicate and silica gels and natural sillimanite, sapphirine and garnet seeds. None of the 17 silica-undersaturated experiments produced texturally or chemically equilibrated phase assemblages comparable to natural lithologies, instead producing a previously unrecognised aluminosilicate solid-solution, copious highly aluminous acicular orthopyroxene and other frequently non-stoichiometric ferromagnesian phases. The experimental study was extended to encompass silica-saturated bulk compositions. They also produced metastable aluminosilicate and orthopyroxene. The virtually instantaneous crystallisation of the aluminosilicate gel resulted in the formation of two domains within the capsule; one composed dominantly of the quartzofeldspathic components, the other composed of ferromagnesian minerals where orthopyroxene had pseudomorphed the synthetic biotite. Several alternative experimental techniques were employed with the aim of achieving experimental success but the metastable phases persisted. The failure of the experiments was attributed to the imbalance in reactivity between the highly reactive gels and the more sluggish synthetic and natural minerals. The emphasis changed to focus on natural samples; a suite of eclogite xenoliths from the Roberts Victor kimberlite, South Africa. The aim of this work was to determine the process or processes responsible for the formation of the eclogites, principally examining the subduction and mantle cumulate hypotheses. Bulk rock compositions were calculated by reintegration of modal mineralogy with individual electron and ion probe phase analyses. Petrographically, the eclogites are divisible into three groups: Group 1 characterised by subhedral garnets, anhedral clinopyroxene and ragged grain edges and the occasional occurrence of kyanite; Group 2a is similar but with smoother curved grain edges, and diamond occurs in two samples; Group 2b possesses an interlocking texture with straight grain boundaries, sometimes bearing a well-developed elongated fabric or clinopyroxene macrocryst. Composite eclogites, composed of interlayered bimineralic and kyanite-bearing eclogites, are classified as Group 1. In terms of alkali and silica content all the eclogites are comparable to basalts or picrites. On variation diagrams, CaO, K 20 and Na20 contents correlate positively with Si0 2 in Groups 1 and 2a. Group 2b does not show such correlations, generally clustering around a similar Si0 2 content. Major element trends indicative of typical olivine-plagioclase ocean-floor basalt fractionation are not observed, and instead an aluminous orthopyroxene fractionation trend can be discerned. Chemographically, equilibrium is preserved within Groups, but slight dis-equilibrium occurs between groups. Large compositional gradients are preserved at the boundaries of the kyanite and bimineralic layers of the composite xenoliths. In terms of CIPW normative mineralogy, the eclogites are split between nepheline (mostly Group 2b) and hypersthene normative (mostly Groups 1 and 2a). When plotted on a diopside projection within the basalt tetrahedron, the majority of eclogites fall on a very well defined trend, suggesting their compositions are comparable with peridotite equilibrium melts at 15 kbar. In three dimensions this trend is a plane, and on a quartz projection three suites can be distinguished. These three suites, which no not correlate with the textural groupings, are interpreted as clinopyroxene cumulates, and lherzolite and harzburgite equilibrium melts. Oxygen isotope data were available for a selection of the eclogites. 6180 values for Group 1 range from 4.93 to 7.04 %. Group 2a range from 5.45 to 6.07 %, and Group 2b range from 2.63 to 5.30 %. This relatively broad range is interpreted to have been generated as a result of a low-temperature alteration process. Relative to oceanic-basalts, the eclogites are severely depleted in HFSEs. Whole rock trace element compositions mirror the petrographic groupings with Group 1 characterised by relatively flat LREE to HREE patterns, Group 2a by slightly LREE depleted patterns, and Group 2b by significantly depleted LREEs. Kyanite-bearing eclogites bear a positive Eu anomaly. The normatively defined eclogite suites do not behave coherently in terms of whole rock trace element patterns. The major and trace element contents of the composite eclogites could not be modelled as recrystallised MORB and plagioclase cumulates. An alternative explanation for their origin is that they represent hybrids between bimineralic eclogite and slab derived melt. This melt may also have metasomatically transferred a heterogeneous 6180 signature from the slab to the eclogite protolith. The theory that eclogites represent restites after removal of a TTG composition melt from a subducted slab was examined, but these results were found to be ambiguous. Although partial melting may be a viable mechanism to account for the absence of primary accessory phases, it is more likely that the eclogites crystallised from a melt undersaturated in minor elements, principally Ti. From consideration of the petrologiaci and geochemical features preserved in the xenoliths it is proposed that the majority of the Roberts Victor eclogites represent recrystallised products of peridotite partial melt which initially crystallised at -50 km as clinopyroxene rich websterites and pyroxenites. They were subsequently translated along the lithosphereasthenosphere boundary, to depths within the diamond stability field. This increase in pressure induced recrystallisation to eclogite. The eclogites resided at the base of the Kaapvaal craton, until entrainment in kimberlite melt, which carried them to the Earth's surface.