Highly siderophile element behaviour during flood basalt genesis and evidence for melts from intrusive chromitite formation in the Mackenzie large igneous province

Abstract

The 1.27Ga Coppermine continental flood basalt (CFB) province in northern Canada represents the extrusive manifestation of the 2.7Mkm2 Mackenzie large igneous province (LIP) that includes the Mackenzie dyke swarm and the Muskox layered intrusion. New Re–Os isotope and highly siderophile element (HSE: Re, Pd, Pt, Ru, Ir, Os) abundance data are reported together with whole-rock major- and trace-element abundances and Nd isotopes to examine the behaviour of the HSE during magmatic differentiation and to place constraints on the extent of crustal interaction with mantle-derived melts. Mineral chemistry and petrography are also reported for an unusual andesite glass flow (CM19; 4.9wt.% MgO) found in close proximity to newly recognised picrites (>20wt.% MgO) in the lowermost stratigraphy of the Coppermine CFB. Compositions of mineral phases in CM19 are similar to the same phases found in Muskox Intrusion chromitites and the melt composition is equivalent to inclusions trapped within Muskox chromites. The apparently conflicting elevated HSE contents (e.g., 3.8ppb Os) and mantle-like initial 187Os/188Os (γOs=+2.2), versus stable isotope (δ18O=+12‰) and lithophile element evidence (εNdi=−12.8) for extensive crustal contamination, implicate an origin for CM19 as a magma mingling product formed within the Muskox Intrusion during chromitite genesis. Combined with Nd isotope data that places the feeder for lower Coppermine CFB picrites and basalts within the Muskox Intrusion, this result provides compelling evidence for direct processing of some CFB within upper-crustal magma chambers. The Coppermine CFB defines a 187Re–187Os isochron with an age of 1263 +16/−20Ma and initial γOs=+2.2±0.8. The initial Os isotope composition for the Coppermine CFB is slightly higher than the near-primitive-mantle initial 187Os/188Os for the Muskox Intrusion (γOs=+1.2±0.3). This result is interpreted to reflect greater crustal contamination in extrusive CFB and the sensitivity of Os isotopes, compared with absolute HSE concentrations, for tracking crustal contributions.Modelling of absolute and relative HSE abundances in global CFB reveals that HSE concentrations decrease with increasing fractionation for melts with <8±1wt.% MgO, with picrites (>13.5wt.% MgO) from CFB (n=98; 1.97±1.77ppb) having higher Os abundances than ocean island basalt (OIB) equivalents (n=75; 0.95±0.86ppb). The differences between CFB and OIB picrite absolute Os abundances may result from higher degrees of partial melting to form CFB but may also reflect incorporation of trace sulphide in CFB picrites from magmas that reached S-saturation in upper-crustal magma chambers. Significant inter-element fractionation of (Re+Pt+Pd)/(Os+Ir+Ru) are generated during magmatic differentiation in response to strongly contrasting partitioning of these two groups of elements into sulphides and/or HSE-rich alloys. Furthermore, fractional crystallization has a greater role on absolute and relative HSE abundances than crustal contamination under conditions of CFB petrogenesis due to the dilution effect of continental crust, which has low total abundances of the HSE. Combined data for the basaltic and intrusive portions of the Mackenzie LIP indicate a mantle source broadly within the range of the primitive upper mantle. The majority of Archaean komatiites and Phanerozoic CFB also require mantle sources with primitive upper mantle to chondritic Re/Os evolution, with exceptions typically being from analyses of highly-fractionated MgO-poor basalts.