Abstract

The Mulcahy Gabbro (2.73 Ga) is an exceptionally well-preserved tholeiitic layered intrusion located in the Wabigoon Subprovince of the Superior Province. The intrusion has a 900 m thick marginal zone of weakly layered gabbronoritic cumulate rocks characterized by a trend of upward Mg-enrichment. The overlying 1100 m thick lower zone consists of well-layered cumulate rocks that display a prominent trend of upward Fe-enrichment. The order of appearance of cumulus minerals in the lower zone is orthopyroxene + plagioclase, augite, apatite, pigeonite, Fe-rich olivine, magnetite.The upward increase in the XMg in marginal-zone cumulus low-Ca pyroxene and augite from 0.44 to 0.72 and 0.57 to 0.79, respectively, is primarily attributed to the injection of successively less fractionated batches of magma into the chamber, rather than to the effect of trapped liquid on cumulus mineral compositions. In the lower zone, crystal fractionation control results in an upward decrease in XMg in low-Ca pyroxene and augite from 0.72 to 0.32 and 0.79 to 0.45, respectively. The change from weakly developed layering in the marginal zone to the well-layered lower zone may represent the transition from a regime of cooling through the chamber floor to one of cooling predominantly through the chamber roof and walls. Rhythmic layering on a scale of centimetres to metres in the lower zone is largely attributed to magmatic density currents. Sulphur saturation occurs near the marginal zone – lower zone transition from reverse to normal fractionation and is attributed to mixing of the older fractionated magma with an injection of buoyant younger primitive magma.Based on cumulus mineral compositions, the most primitive parental magma for the lower zone is determined to have had an Mg# equal to or less than 0.46 and weak enrichment of light rare-earth elements ((Ce/Yb)N = 1.4). The cumulates are derived from a hydrous, high-Al2O3 basaltic magma with a moderately high silica activity. These constraints and the close temporal and spatial association with calc-alkaline granitoid rocks suggest that the Mulcahy Gabbro is a cumulate assemblage from a high-Al tholeiite magma and is part of a late Archean calc-alkaline, arc-related plutonic complex.

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