Constraining the Timescales of Magmatic Differentiation with U-Pb Zircon Geochronology

Abstract:

Quantifying the timescales of magmatic differentiation is critical to understand the rate at which silicic plutonic and volcanic rocks form. However, directly dating this process is difficult because locations with both clear evidence for fractional crystallization and the accessory phases necessary for radiometric dating (e.g. zircon) are rare. This study focuses on the Dariv Igneous Complex in western Mongolia where early saturation of zircon in a suite of cogenetic, upper crustal (<0.5 GPa) igneous rocks ranging from biotite-bearing ultramafic cumulates through to evolved granitoids and late stage felsic dikes allows for dating of magmatic differentiation through U-Pb geochronology for the first time. Notably, zircon is an early crystallizing mineral in the Dariv Igneous Complex, first appearing in the ultramafic cumulates, likely due to the high-Zr contents and alkaline nature of the crystallizing parental melt. Detailed textural and petrographic observations confirm that zircon is magmatic in origin and crystallized either before or contemporaneously with the volumetrically dominant mineral phases. Indistinguishable crystallization ages from Th-corrected ²⁰⁶Pb/²³⁸U dates of zircons from 5 samples across the sequence indicate that fractionation from a basalt to high silica (>65 wt.% SiO₂) melt occurred in ≤300 ka between 502-503 Ma. If crystallization rates in crustal intrusions are primarily a function of cooling, rates of fractionation will be strongly dependent on the size and depth of the magmatic system, as well as, the dynamics of magma chamber replenishment. Therefore, the Dariv Igneous Complex, which crystallized at relatively shallow, cool levels in the crust, represents an end-member constraint for timescales associated with fractional crystallization of a basaltic melt. Fractional crystallization of mantle-derived basalts in the lower crust may occur on more protracted timescales due to greater ambient temperatures at depth and repeated influx of hot basalts.