Equilibration and reaction in Archaean quartz‐sapphirine granulite xenoliths from the Lace kimberlite pipe, South Africa

Abstract

A B S TR A CT Ultrahigh-temperature quartz-sapphirine granulite xenoliths in the post-Karoo Lace kimberlite, South Africa, comprise mainly quartz, sapphirine, garnet and sillimanite, with rarer orthopyroxene, antiperthite, corundum and zinc-bearing spinel; constant accessories are rutile, graphite and sulphides. Comparison with assemblages in the experimentally determined FMAS and KFMASH grids indicates initial equilibration at >1040°C and 9-11 kbar. Corona assemblages involving garnet, sillimanite and minor cordierite developed on a near-isobaric cooling P-T path as both temperature and, to a lesser extent, pressures decreased. Garnet-orthopyroxene Fe-Mg exchange thermometers record temperatures of only 830-916°C. These estimates do not indicate the peak metamorphic conditions but instead reflect the importance of post-peak Fe-Mg exchange during cooling. Correction of mineral Fe-Mg compositions for this exhange using a convergence approach of Fitzsimons & Harley (1994 ) leads to retrieved P-T estimates from garnet-orthopyroxene thermobarometry (c. 1000°C and 10.5±0.7 kbar) that are consistent with the petrogenetic grid constraints. U-Pb dating of a single zircon grain gives an age of 2590±83 Ma, interpreted as the age of the metamorphic event. Protolith major and trace element chemistries of the xenoliths differ from sapphirine-quartzites typical of the Napier Complex (Antarctica) but are comparable to less siliceous, high Cr and Ni, sapphirine granulites reported from several ultrahigh temperature granulite terranes.

Shear wave velocity structure of the lower crust in southern Africa: Evidence for compositional heterogeneity within Archaean and Proterozoic terrains

“…Lower crustal xenoliths from the Kimberley terrain are dominated by metapelite, and mafic rocks are absent [ Schmitz and Bowring , 2003a]. In contrast, mafic granulite xenoliths are common at all of the other localities [ Dawson and Smith , 1987; Dawson et al , 1997; Schmitz and Bowring , 2003a, 2003b] (Figure 11), which is consistent with the variability in the shear wave velocity structure of the lower crust described in this study.…”

Zircon U–Pb strain chronometry reveals deep impact-triggered flow

“…Shock pressures experienced by the base of the >38 km Kaapvaal crust (Nguuri et al, 2001) beneath the Lace pipe would have been less than the 20 GPa threshold for shock deformation of zircon (Leroux et al, 1999) due to shock wave attenuation of ~4 GPa/km over the radial distance of ~80 km from the original point of impact (Gibson and Reimold, 2005). Microstructural features indicative of shock metamorphism have not been reported in zircon or main phase assemblages in previous petrologic investigations of the Lace lower crustal xenolith suites (Dawson et al, 1997;Schmitz and Bowring, 2003). Shock wave heating is also predicted to have been insignificant at this region of the crust (Henkel and Reimold, 2002) where, after decay of the Ventersdorp thermal pulse, ambient temperatures cooled from 650°C to 350°C (Gibson and Jones, 2002), i.e., consistently above the self-annealing temperature of zircon radiation damage.…”

Zincian Spinel Associated With Metamorphosed Proterozoic Base-Metal Sulfide Occurrences, Colorado: A Re-Evaluation of Gahnite Composition as a Guide in Exploration

“…Compositional fields of zincian spinel from (1) marbles, (2) metamorphosed massive-sulfide deposits, (3) granitic pegmatites, and (4) aluminous metasedimentary rocks (after Spry & Scott 1986a). Individual data-points pertain to zincian spinel compositions derived from this study and worldwide localities, that have appeared in the literature since Spry & Scott (1986a): metamorphosed massive-sulfide deposits from Colorado (Eckel et al 1997), metamorphosed massive-sulfide deposits in Fe-Al-rich aluminous metasedimentary and metavolcanic units (Hicks et al 1985, Appel 1986, Spry 1987b, Spry et al 1988, Froese et al 1989, Zaleski et al 1991, Parr 1992, Araujo et al 1995, Höller & Gandhi 1997, Morris et al 1997, Chattopadhyay 1999, Jain 1999, Toteff 1999, Rosenberg et al 2000, Schwartz & Melcher 2003, unaltered and hydrothermally altered Fe-Al-rich rocks (Hicks et al 1985, Spry 1987b, Schumacher & Robinson 1987, Damman 1989, Moore & Reid 1989, Spry & Petersen 1989, Schneiderman & Tracy 1991, Parr 1992, Soto & Azañón 1993, Pedrick et al 1998, Dasgupta et al 1999, Sawaki et al 2001, aluminous granulites (Sengupta et al 1991, Visser et al 1992, Ouzegane et al 1996, Carson et al 1997, Dawson et al 1997, Raith et al 1997, Osanai et al...…”