Abstract

We report concentration data for the trace element titanium (Ti) in magmatic quartz in eight samples of leucogabbronorite, anorthosite, olivine ferrodiorite and quartz anorthosite from the Main and Upper Zones of the Bushveld Complex. The results, obtained by electron microprobe analysis, were used with Ti-in-quartz geothermometers (TitaniQ) to constrain the solidus temperatures of these rocks. Magmatic quartz is present sporadically in the mafic cumulate rocks of the 2950 m Bellevue borehole (Northern Limb), and commonly occurs as intercumulus, triangular patches, typically between cumulus feldspars and pyroxenes, suggesting that it crystallized late, from trapped interstitial melts. Quartz grains in rutile-bearing samples (implying TiO2 activity = 1) yield Ti concentrations of 48 to 106 ppm that constrain the solidus temperatures of the Bellevue cumulate rocks at T= 677 to 767ºC, using a calibration of the pressure-sensitive geothermometer at 3 kbar, thought to represent the emplacement pressure of the mafic cumulate rocks. These results imply a mean solidus temperature for Bushveld cumulate rocks of approximately 732 ± 31ºC (n = 7). Cathodoluminescence imagery (CL) of quartz from the studied samples reveals a variety of textures that correlate with Ti content, and hence temperature. These include rim-to-core (RC) zoning, homogeneous CL intensity and some samples show irregular patches of variable CL intensity. The observed zoning can be interpreted as intercumulus quartz grains that crystallized from their rims inward towards their cores.

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