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Ti-rich chromite from the Mount Ayliff Intrusion , Transkei : Further evidence for high Ti tholeiitic magma

Four profiles through the lower picritic portion of the Mount Ayliff Intrusion from the Karoo Igneous Province of South Africa have been studied. Many analyses of chromite grains enclosed in olivine show major differences from those found in most tholeiitic rocks, in that they are very high in Ti and Cr and are low in Al. Typical ranges for these elements are (as cation proportions recalculated to 32 O anions) Ti : 0.2-1.8 (0.8-8.8 wto/o TiOr); Cr: 6.3-l l. l (26-51wtolo Cr,O,); and Al :2.2-5.0 (6.7-15.7 wto/o Al'O,). Some of these are the most Ti-rich chromites recorded from any tholeiitic suite. The lowest Ti contents occur in the olivine gabbros immediately adjacent to the floor contact. Their compositions are typical of tholeiitic chromite and cannot be related by fractional crystallization to the Ti-rich chromite trends in the overlying picrite, as that would require an increase in Ti and Cr. Models involving exsolution, postcumulus modification of chromite composition by infiltrating residual magma, or subsolidus reequilibration are rejected for the following reasons. The required Cr content of original olivine would have been unrealistically high. Most analyses are of grains totally embedded in olivine; therefore, the potential for reaction with residual magma is restricted. Analyses of chromite grains interstitial to the olivine and enclosed in postcumulus phases, and therefore exposed to residual magma, do not show trends of increasing Cr and Ti. In examples where reaction with residual liquid have been documented in detail (Jimberlana and Rhum Intrusions), there are no analyses of chromite with high Ti and Cr content comparable to those reported here. Comparison of these Tiand Cr-rich chromite samples with other intrusive and extrusive tholeiitic suites, and experimental studies indicate that these compositions could not form from a typical continental tholeiite, but probably crystallized from a magma with high Ti content. The closest analogues to these chromite compositions in tholeiitic rocks are high-Ti tholeiites found in Hawaiian lava lakes. It is suggested that the Mount Ayliff Intrusion formed from at least two injections of magma of different compositions, one being enriched in Ti. Chromite compositions from the diferent profiles through the intrusion show different trends. In the Mount Evelyn profile, the effects of differentiation and mixing can be identified and distinguished, whereas, in the Siroqobeni and Ingeli profiles, the chromite compositions reflect a prolonged period of slow mixing. Data from a restricted interval of the Waterfall Gorge profile at the base of the picrite indicate that the chromite formed from a typical continental tholeiite. This interpretation of multiple intrusion and mixing of geochemically distinct magmas is consistent with previously published data on Mg-rich ilmenite compositions.