DISSOLUTION-REPRECIPITATION REACTIONS IN THE DEADMAN PEAK PLUTON: CRYSTALLIZATION TO SUB-SOLIDUS FLUID PROCESSES RECORDED BY AMPHIBOLE

The Deadman Peak Pluton is a mostly tonalitic magmatic body in the Klamath Mountains (CA/OR). Hornblende from tonalites and quartz diorite in the pluton have brown to olive-green cores with patchy pale green rims. Compositional zoning (major and trace elements), textural, and crystallographic relationships are assessed and interpreted, identifying a sequence of crystallization, resorption, neo-crystallization and coupled dissolution-reprecipitation processes.

Major element analysis identifies dark brown/green cores as magnesio-ferri-hornblende with crystallization temperatures as high as ~830°C. Patchy pale-green rims are actinolite and give calculated crystallization temperatures of <600°C. For trace elements, Zr, Sr, Hf, and Ba have normal zoning distributions from core-to-rim and across sub-domain boundaries characteristic of an amphibole growing in a magma undergoing fractional crystallization. However, sharp increases in MREE and HREE (Y, Yb, Sm), Sc and Nb between the cores and pale-green rims do not conform with expected fractional-crystallization trends.

The geometry of compositional boundaries was investigated using high-resolution electron microscopy, including compositional and EBSD maps post-processed in MATLAB. Sharp, stepwise boundaries between compositional zoning are most visible in line-scans of Mg, Ti, and Al. However, optical extinction is continuous and no crystallographic discontinuities (misorientation angles > 0.5°) are observed across compositional boundaries in EBSD maps.

The compositionally sharp and crystallographically-continuous sub-domain boundaries are characteristic of coupled dissolution-reprecipitation processes that occurred in individual crystals of amphibole on a pluton-scale in the presence of a magmato-hydrothermal fluid. Some primary compositional signatures are preserved in recrystallized sub-domains, but others were modified during alteration. This has implications for the application of trace-element analysis to understanding magmatic histories. Understanding the mechanism of sub-domain growth may aid interpretation of high-resolution analysis of isotope systems in amphibole, particularly Ar-Ar, that may improve the understanding of temperature, time and rates of processes in igneous systems.