The McIntosh Layered Troctolite-Olivine Gabbro Intrusion, East Kimberley, Western Australia

• Published in: Journal of petrology 1987-02, Vol.28 (1), p.211-234
• Main Authors: MATHISON, CHARTER I., HAMLYN, PAUL R.
• Format: Article
• Language: English
• Subjects: Crystalline rocks; Earth sciences; Earth, ocean, space; Exact sciences and technology; Igneous and metamorphic rocks petrology, volcanic processes, magmas
• ISSN: 0022-3530; 1460-2415
• DOI: 10.1093/petrology/28.1.211

The well-preserved ?lower Proterozoic McIntosh intrusion consists of 96 macro-layers with a total stratigraphic thickness of about 6 km. The lowermost rocks in this possible cone-shaped intrusion are hidden, and the roof and the upper layers were removed by erosion. The layered sequence is dominated by 40 bimodal cyclic units of troctolite and olivine gabbro. Minor gabbronorite layers occur throughout the sequence, and are more abundant and more fractionated higher in the sequence. Six imperfect megacycic units are developed in the upper 2700 m, each unit consisting of several troctolite-olivine gabbro cyclic units followed by a Fe-Ti oxide-bearing gabbronorite. The overall cumulus crystallization order in each megacyclic unit was plagioclase first, closely followed by olivine, then augite, orthopyroxene, and magnetite successively. Cryptic composition data for troctolites and olivine gabbros show a slight overall decrease of 10 mol per cent An and Fo from the base to the top of the layered sequence (approximate ranges An80–70 and Fo78–68). Several major fluctuations occur however, and are generally associated with the oxide gabbronorites, which are significantly more fractionated than the adjacent layers (plagioclase An53–60, orthopyroxene Mg52–69 Each fluctuation comprises a marked progressive discontinuity (rapid normal fractionation) followed by a gradual to rapid regressive discontinuity (or reversal) in the overlying troctolites and olivine gabbros. Apparently, such marked progressive discontinuities have not been described in layered intrusions. A chilled margin and the overall composition of the intrusion suggest an olivine tholeiite parent magma, inferred to have crystallized at P ≤ 6 kb, relatively low PH2O and high fO2 (> NNO buffer). The troctolite-olivine gabbro cyclic units are inferred to have formed by fractional crystallization of periodic additions of new magma. However, the oxide gabbronorites seem too fractionated relative to the underlying layers to have formed by conventional crystal fractionation mechanisms, and they could have resulted from a ‘liquid fractionation’ process in which fractionated residual magma, instead of rising, periodically became denser and ponded on the temporary floor (a density crossover). Gradual, reversed cryptic trends in the cyclic units above the oxide gabbronorite layers may reflect mixing of this fractionated magma with successive magma additions.