ORIGIN OF THE LOWER ZONE OF THE KIGLAPAIT LAYERED MAFIC INTRUSION, LABRADOR, CANADA: COMPUTATIONAL CONSTRAINTS ON CRYSTALLIZATION AND MAGMA RECHARGE

The Kiglapait is a 1.305 Gya (DePaolo, 1985) intrusion in coastal Labrador composed of a troctolitic (ol + plag) Lower Zone (LZ) that accounts for ~84% of the intrusion. Ol and plag have forsterite (Fo) and anorthite (An) contents that range from ~Fo₇₀ and An₆₇ (basal LZ) to ~Fo₆₀ and An₅₂ at the top of the LZ; most crystals lack zoning and hence are termed adcumulates ( Wager et al, 1960; Morse, 2015). Several hypotheses have been proposed to explain the origin of this intrusion, including closed system crystallization (FC) (Morse, 2015) and FC accompanied by magma recharge/mixing (DePaolo, 1985). Using the Magma Chamber Simulator (MCS) (Bohrson et al., 2014), a thermodynamic geochemical modeling program, FC and magma recharge/mixing hypotheses were tested using troctolitic parental compositions over a range of pressures (0.05 to 0.8 GPa), initial H₂O contents, and f_O2 buffers (FMQ+2 to FMQ-4). Best-fit models were evaluated via comparison of hundreds of models with observed Fo and An contents versus percent crystallization (e.g., PCS, Morse, 1979). For all tested pressures in the FC models, the Fo of liquidus olivine is higher than observed at around ~Fo₈₀, while liquidus plagioclase is also higher than observed at ~An₇₆. Invoking a higher-pressure stage crystallization event (~0.8 GPa), followed by crystallization at shallower crustal levels (~0.33 GPa) produces initial olivine and plagioclase within the ranges documented for the bottom of the LZ. However, these multi-stage FC models produce Fo versus PCS trends that are inconsistent with observation; FC model olivine drops below the lowest observed Fo in the LZ (Fo₆₀) by 30 PCS, while plagioclase drops below minimum observed An content (~An₅₂) by 65 PCS. Multiple magma recharge events, where the new magma is the same composition as the initial parent, are required to maintain the relatively small range of olivine and plagioclase composition for the LZ. Best-fit models also suggest the Kiglapait parental troctolite magma is characterized by low oxygen fugacity (FMQ-4) and low initial H₂O contents (0.1 wt. %), consistent with previous work (Morse, 2015). This suggests an open magmatic system involving multiple recharge events produced the chemical trends observed in the LZ; a closed system FC scenario is insufficient to reproduce these compositional trends.