LINKING TEXTURAL CHARACTERISTICS OF TUOLUMNE AMPHIBOLES TO COMPOSITION AND TEMPERATURE

Compositional diversity and wide P-T stability give amphiboles high potential for tracking magmatic processes and identifying distinct magmatic units within plutons (e.g. Barnes et al., 2016). However, these records may be complex due to open system processes and subsolidus recrystallization, as exemplified by textural characteristics and compositions of amphiboles from the Tuolumne Intrusive Complex. Major element data indicate that amphiboles range from magnesiohornblende to actinolite in the Kuna Crest (KC), equigranular Half Dome (eHD), and porphyritic Half Dome (pHD) units and that MnO generally increases from the outer KC unit to the inner pHD unit. Patchy zoning is characterized by varying olive green, olive brown and yellow pleochroism and by blueish green-colorless pleochroism. Amphiboles from the Half Dome units display prominent patchy zoning, whereas such zoning is uncommon in KC amphiboles. Some KC amphiboles instead are characterized by a pale-colored core containing inclusions of bleby quartz ± pyroxene. Each unit displays bimodal distributions in Si and Ti (all apfu). Amphiboles with Si > 7.3 correspond to Ti < 0.09 in KC samples, < 0.05 in eHD samples, and < 0.04 in pHD samples.

A new amphibole thermometer (Putirka, 2016) allows these variations in pleochroism and composition to be characterized in terms of magmatic conditions. Amphiboles with Si > 7.48 range from actinolite to magnesiohornblende, display blueish green – colorless pleochroism and yield temperatures ranging from 655-710°C, suggestive of solidus to subsolidus recrystallization. The lowest temperatures (655-680°C) are all from actinolite. Amphiboles with Si < 7.48 are magnesiohornblende, display olive green, olive brown and yellow pleochroism, and yield temperatures of 697-813°C. Greenish blue-colorless patches within these amphiboles correspond to lower temperatures of 697-713°C. Our results suggest that patchy color zoning in Tuolumne amphiboles is linked to both composition and temperature and that bimodal gaps in Si and Ti may separate magmatic amphiboles from solidus to subsolidus amphiboles.