Textural Maturity of Cumulates: a Record of Chamber Filling, Liquidus Assemblage, Cooling Rate and Large-scale Convection in Mafic Layered Intrusions

摘要

Textural maturity describes the extent to which a rock has evolved from the initial reaction-controlled texture towards textural equilibrium controlled by the minimization of interfacial energy. Solidification in a magma chamber results in the formation of an impingement texture by the random juxtaposition of planar-sided grains. Orthocumulates, in which the initial melt-filled pores are pseudomorphed by later-crystallizing phases, have an ophitic or intersertal texture immediately after complete solidification, which then evolves towards solid-state equilibrium by rounding of initially planar grain boundaries and an increase in the median dihedral angle subtended at the junctions of two primocrystic grains with the interstitial phase. The bulk of the increase in angle occurs just below the solidus temperature in kilometre-scale mafic plutons. Quantification of textural maturity via measurement of dihedral angle populations in troctolitic and gabbroic cumulates from the Rum Eastern Layered Intrusion and the Skaergaard Intrusion demonstrates that the rocks preserve a record of thermal events related to magma chamber replenishment and the onset of chamber-wide convection. Textural maturity is also a function of the liquidus phase assemblage: for systems in which only olivine and plagioclase are liquidus (i.e. cumulus) phases in the main magma body above the crystal mush, the texture is significantly less mature than that in systems in which clinopyroxene is an additional liquidus phase. The difference in textural maturity reflects differences in the cooling and solidification rate, and demonstrates directly that the liquidus phase assemblage plays a role in determining the thermal history of plutons.