The origin of the high-K latites from Camp Creek, Arizona: constraints from experiments with variable fO₂ and $a_{{\text{H}}_{\text{2}} {\text{O}}}$

Abstract

Near-liquidus melting experiments were performed on a high-K latite at fO₂'s ranging from iron-wustite-graphite (IWG) to nickel-nickel oxide (NNO) in the presence of a C-O-H fluid phase. Clinopyroxene is a liquidus phase under all conditions. At IWG $(a_{{\text{H}}_{\text{2}} {\text{O}}} = 0.17)$, the liquidus at 10 kb is about 1,150° C but is depressed to 1,025° C at NNO and $a_{{\text{H}}_{\text{2}} {\text{O}}} = 1$. Phlogopite and apatite are near-liquidus phases, with apatite crystallizing first at pressures below 10 kb. Phlogopite is a liquidus phase only at NNO and high $a_{{\text{H}}_{\text{2}} {\text{O}}}$. Under all conditions the high-K latites show a large crystallization interval with phlogopite becoming the dominant crystalline phase with decreasing temperature. Increasing fO₂ affects phlogopite crystallization but the liquidus temperature is essentially a function of $a_{{\text{H}}_{\text{2}} {\text{O}}}$. The chemical compositions of the near-liquidus phases support formation of the high-K latites under oxidizing conditions (NNO or higher) and high $a_{{\text{H}}_{\text{2}} {\text{O}}}$. It is concluded from the temperature of the H₂O-saturated liquidus at 10 kb, the groundmass: crystal ratio and presence of chilled latite margins around some xenoliths that the Camp Creek high-K latite magma passed thru the lower crust at temperatures of 1,000° C or more.