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Crustal contamination, sulphide mineralization, and compaction during formation of the marginal zone of the Muskox intrusion, Nunavut, and implications for the evolution of the 1.27 Ga Mackenzie magmatic event

University of British Columbia

The age, petrology, geochemistry, and Hf-Nd-S isotopic compositions of basal marginal rocks at two locations (West Pyrrhotite Lake and Far West Margin) along the western margin of the large Muskox layered mafic-ultramafic intrusion, Nunavut, are used (1) to constrain the petrogenesis of the marginal zone and associated sulphide mineralization, and (2) to evaluate the genetic relationship of the Muskox intrusion with the Mackenzie dikes and Coppermine River flood basalts during the 1.27 Ga Mackenzie magmatic event. The marginal zone (1269 ± 2 Ma; U-Pb baddeleyite) consists of a lower 10 m-thick gabbronorite and upper 100-150 mthick peridotite subzone. A shift in incompatible trace element ratios and isotopic compositions at the transition from peridotite (low Th/Yb; high Nb/La; E[sub Hf(t)] = -3 to +2; E[sub Nd(t)]= -4 to 0; δ³⁴S = +6) to gabbronorite (E[sub Hf(t)] = -5 to -15; E[sub Nd(t)] = -1 to -13; δ³⁴S = +7 to +10) and the corresponding early crystallization of orthopyroxene and appearance of granophyre within the gabbronorite indicates that the effects of crustal contamination by the adjacent country rocks (high Th/Yb; low Nb/La; E[sub Hf(t)] =- -24 to -29; E[sub Nd(t)] = -14 to -16; δ³⁴S = +7 to +11) is restricted to a thin (10 m) boundary zone along the contact. Silica and sulphur addition promoted local sulphide saturation within the contaminated magmas along the outer wall of the magma chamber. The low metal content of sulphides throughout the intrusion indicates that in general early solidification of this zone inhibited segregated sulphide liquid from interacting with subsequent pulses of magma (low R-factor). The overlying peridotites contain increasing proportions of postcumulus phases relative to cumulus olivine towards the base of the marginal zone, a feature that is related to the competing effects of compaction and rapid heat loss through the base of the intrusion. The marginal zone peridotites and overlying layered series rocks of Muskox intrusion and the lowermost Coppermine River basalts have the same range of Nd isotopic compositions, suggesting that they both were derived from a common relatively enriched mantle source and that this portion of the flood basalts were fed by magma that temporarily resided and fractionated within the Muskox magma chamber during their final ascent through the crust.

Text

2010-01-07

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http://hdl.handle.net/2429/17703

Geological Sciences

University of British Columbia

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