Abstract

The Nebo-Babel intrusion, which hosts a world-class magmatic Ni-Cu-PGE sulfide deposit, is a concentrically zoned, tube-like gabbronorite body. The emplacement of Nebo-Babel at 1068.0 ± 4.3 Ma was part of a major magmatic event, which included the Giles Complex and the Warakurna large igneous province. Field, mineral, chemical, and isotopic data from the Nebo-Babel intrusion indicate that all of the lithostratigraphic units are cogenetic and that the intrusion is the product of three temporally distinct magma pulses. The initial magma pulse, which formed the marginal units of the intrusion, was the most primitive and has assimilated the highest percentage of the country-rock orthogneiss. The subsequent magma pulses, emplaced into the core of the intrusion, were both less primitive and less contaminated. The final and largest magma pulse underwent in situ closed-system crystal fractionation resulting in the observed stratigraphic mineral and whole-rock chemical trends. Emplacement of Nebo-Babel took place at ca. 10- to 12-km depth. Postmagmatic, structural overturning of the intrusion resulted in stratigraphically reversed mineral and chemical trends. All of the Nebo-Babel units can be related by crystal fractionation of parental tholeiitic magma with 8 to 9 wt percent MgO. Mineral δ18O values are consistent with typical mantle values and preclude large-scale crustal contamination of the parental magma with anything other than material with similar values. Although even the least contaminated intrusive units have negative Nb anomalies and enriched 87Sr/86Sr initial ratios compared to bulk earth at the time, the crustal assimilation must have been significantly less than 10 percent based on mantle-like δ18O values and 143Nd/144Nd ratios. Trace elements indicate that primitive Nebo-Babel melts equilibrated with spinel facies mantle implying shallow melting beneath a lithosphere less than 80 km thick.

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