Re-evaluating pluton/volcano connections and igneous textures in light of incremental magma emplacement

Mills, Ryan Douglas

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March 20, 2019

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Mills, Ryan Douglas
- Affiliation: College of Arts and Sciences, Department of Geological Sciences

Abstract

Zircon U-Pb geochronologic data collected within the past ten years indicate that plutons are emplaced incrementally over time periods of 105 to 106 annum at magma accumulation rates on the order of 10-4-10-3 km3/a. Although incremental emplacement of magmas is now widely accepted, evaluation of the wide-ranging effects on pluton-volcano connections and the generation of magma diversity has just begun. Magma emplacement rates calculated for large ignimbrites (10-2 km3/a) are higher than most estimates for plutons (10-4-10-3 km3/a). Thermal models for magma emplacement in the crust predict this rate disparity and suggest that magma emplacement rates of 10-2 km3/a or greater are needed to produce large ignimbrites. Thus, there is a fundamental difference in the rates of accumulation of ignimbrite and pluton magmas but little is known on how individual magmatic centers behave. Geochemistry and U-Pb zircon data from the Mt. Princeton batholith and spatially associated ignimbrites in central Colorado indicate that the vast majority of the batholith was emplaced between periods of ignimbrite eruption at a rate of 1.6x10-3 km3/a. The temporal disconnect supports the hypothesis that ignimbrites are generated during periods of high magma flux, without significant fractionation in the upper crust and that plutons represent similar magmas that froze in the crust during periods of low magma flux. Thermal models and geochronology also indicate that pluton emplacement must be episodic with only small fractions (<5%) of mobile magma existing at any one time. The episodic emplacement of plutons also leads to temperature cycling of the magma, which is hypothesized to affect crystal textures. Experiments on crystal growth of ammonium thiocyanate in a magma analog at approximately 50ºC and plagioclase and olivine growth in an alkali basalt at approximately 1150ºC indicate that temperature cycling changes the texture of magmas dramatically; creating large crystals and decreasing crystal number density. Also, crystal alignment is observed in the magma analog experiments coincident with the thermal gradient. Together, these results indicate that temperature cycling of magmas can affect the crystal size distribution and fabric of the resultant rock and is a variable that needs to be assessed when interpreting igneous textures.

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