Textural and compositional variations in titanite, along with whole-rock geochemistry, provide constraints on the emplacement and cooling histories of two plutons: the Jurassic Notch Peak pluton and the Oligocene Little Cottonwood stock, both in Utah. Titanite textures observed with back-scattered electron (BSE) imaging along with their compositions were used to determine four periods of growth: cores, rims, interstitial overgrowths, and secondary replacements. Brightness in BSE images correlates mostly with rare earth elements (REE). REE patterns in cores and rims are compositionally similar in both plutons, although the Notch Peak intrusion tend to be slightly more enriched in REE. Overgrowths and secondary replacements typically have lower concentrations of REE and Fe and higher Al, Mn, F, and U. They also have similar δ18O values to primary titanite, indicating alteration and recrystallization from exolved magmatic fluids rather than meteoric sources. In the Notch Peak intrusion, titanite grains usually have simple, oscillatory zoned textures that include cores which include bright sector zones. These are overprinted by secondary titanite that grows within and replaces the primary titanite grain. At some localities, Notch Peak titanites have been hydrothermally altered to fine-grained aggregates of rutile or brookite, magnetite, quartz, and plagioclase. These observations indicate a simple cooling path after magmatic intrusion, followed by hydrothermal alteration for the Notch Peak intrusion. The Little Cottonwood stock contains titanite grains that are distinctly different from those in the Notch Peak intrusion. They typically contain a distinct patchy core with rounded, resorbed ilmenite inclusions. Surrounding the core is a mantle of oscillatory zoned titanite. On many grains, narrow rims of secondary overgrowths are observed as well as interstitial titanite growing in between chloritized biotite sheets. The cores of these titanite grains suggest that a more reduced, ilmenite-rich magma mixed into an oxidized felsic magma, destabilizing existing ilmenites and forming its patchy texture. This was followed by hydrothermal overgrowths and interstitial titanite, like Notch Peak, but to a lesser extent. Although both plutons had similar emplacement settings–subduction related intrusion into Paleozoic limestone–their whole rock and titanite chemistries are different. The Notch Peak intrusion is more chemically evolved and less mafic than the Little Cottonwood stock. The patchy cores with Fe-Ti oxide inclusions found in the Little Cottonwood stock, along with the abundance of mafic enclaves in the pluton, provide evidence for magma mixing, while no evidence is observed in the Notch Peak intrusion for magma mixing.



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Physical and Mathematical Sciences



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Titanite, Notch Peak intrusion, Little Cottonwood stock, granite, mafic enclave, δ18O