Cordilleran Section - 115th Annual Meeting - 2019

Paper No. 25-4
Presentation Time: 9:00 AM-6:00 PM


MELONE, Alec, Department of Geological Sciences, Central Washington University, 400 E. University Way, Ellensburg, WA 98926-3148, BOHRSON, Wendy A., Department of Geological Sciences, Central Washington University, 400 E. University Way, Ellensburg, WA 98926 and SPERA, Frank J., Earth Science, University of California Santa Barbara, Santa Barbara, CA 93106

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 fO2 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.