EXPLORING IGNEOUS ACCESSORY MINERAL TEXTURES: TITANITE FROM NOTCH PEAK GRANITE AND LITTLE COTTONWOOD STOCK, UTAH
In the Little Cottonwood Stock (LCS), primary magmatic titanite has sector, oscillatory, and mottled core zoning. Titanite in LCS mafic enclaves exhibit poikilitic textures, where titanite grew surrounding plagioclase. Notch Peak (NP) primary titanite exhibits similar sector and oscillatory zoning, but does not include mottled cores. Secondary titanite in both the LCS and NP consists of partial replacement crystallization in existing grains or interstitial overgrowth that extends into surrounding minerals during hydrothermal activity. Titanite in both granites were analyzed using ESEM-BSE, MC-LA-ICP-MS, SIMS and electron microprobe to compare textural and compositional characteristics.
In both granites, primary titanite formed euhedral crystals with distinct bright and dark growth zones under BSE imaging, reflecting rare earth element (REE) concentration. Primary titanite usually contains high REE concentrations (0.02-0.03 apfu) and magmatic δ18O values (~5.5). Mottled cores formed during magma mixing events have slightly higher temperatures based on Zr in titanite thermometers but have similar δ18O and REE content. Systematic similarities and differences in the LCS of δ18O, textures, and composition indicate multiple phases of intrusion.
Secondary titanite from both granites are chemically and texturally distinct from primary titanite; it often crystallizes on irregular dissolution surfaces or interleaves with sheets of chloritized biotite. Secondary titanite is typically much darker in BSE images and contains significantly lower LREEs (0.01-0.02 apfu). Replaced titanite temperatures reflect a late crystallization, but have similar δ18O to primary titanite in LCS samples. Interstitial titanite has lower δ18O ratio (~1.0-3.0) which is consistent with precipitation from hydrothermal fluid with a meteoric water component.