Decoding polyphase migmatites using monazite petrochronology
Abstract:Unraveling the P–T–t evolution of deep crustal rocks requires the use of multiple high-temperature geochronometers integrated with careful petrography and quantitative phase equilibria modeling. As an example, in situ U–Pb monazite ages and Lu–Hf garnet geochronology are used to distinguish mineral parageneses associated with overprinting suprasolidus metamorphic events in migmatitic paragneisses and orthogneisses from the Fosdick migmatite–granite complex in West Antarctica. Then phase equilibria modeling is used to quantify the P–T conditions for each event.
In the Fosdick complex, U–Pb monazite ages define two populations at 365–300 Ma (minor population; cores of polychronic grains) and 120–96 Ma (dominant population; monochronic grains and rims of polychronic grains). For seven samples, Lu–Hf ages of garnet range from 116 to 111 Ma, which are interpreted to record the approximate timing of peak metamorphism during the overprinting Cretaceous metamorphic event. Phase equilibria modeling constrains peak P–T conditions to 720–800°C at 0.45–1.0 GPa for the older (Devonian–Carboniferous) metamorphic event and 850–880°C at 0.65–0.80 GPa for the overprinting Cretaceous event. This younger metamorphic event is dominant throughout the Fosdick complex; it has extensively reworked evidence of the older metamorphic event as indicated by resorbed Devonian–Carboniferous cores of polychronic monazite grains that are always surrounded by Cretaceous overgrowths.
Within the Cretaceous monazite population, the paucity of ages predating peak metamorphism suggests that prograde monazite growth was limited or prograde monazite was obliterated. Y-enriched overgrowths on monazite spatially associated with cordierite and biotite yield ages of 106–97 Ma, which are interpreted to record growth during breakdown of garnet in the presence of melt in the course of exhumation and cooling of the complex. Most monazite in the Cretaceous population yields ages that range from 106 to 96 Ma with a peak at c. 100 Ma. The c. 100 Ma age, which post-dates the metamorphic peak and corresponds to the timing of regional extension and exhumation of the complex, is interpreted to record crystallization of residual melt along grain boundaries throughout cooling to an elevated solidus.