Igneous and metamorphic perspectives of amphibole: Crystallization to sub-solidus fluid processes recorded in the Deadman Peak pluton, and thermodynamic modeling of amphibolites interlayered with schists from Cerro Colorado, New Mexico

Amphiboles are a group of common hydrous chain-silicate minerals that host all of the major elements of earth’s crust, and are stable over a wide range of P-T space in both igneous and metamorphic rocks. In igneous systems, major and trace elements record processes such as fractional crystallization, chamber recharge, magma mixing and melt extraction, but signatures may be overwritten depending on the process of alteration. In metamorphic systems, amphibole is an index mineral for metamorphic facies in metabasites, such as amphibolite and blueschist facies. However, the many potential compositional vectors for amphibole make thermodynamic modeling of amphibolites challenging. Successful interpretation of metamorphic amphibole requires accurate and thermodynamically consistent solution models and databases. This study targeted igneous amphibole from the Deadman Peak Pluton, and metamorphic amphibolite samples from the Vadito Group in Cerro Colorado, New Mexico. All samples were analyzed with standard petrographic microscopes, BSE/SEM textural analyses, and EDS and EPMA major element spot analyses. Deadman Peak igneous amphiboles were also analyzed for trace elements in LA-ICPMS, crystallographically in EBSD, and prepared by FIB Milling for TEM an diffraction patterns. Metamorphic samples from Cerro Colorado were thermodynamically modeled using Perple_X. 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. Reversals in temperature zoning between pale brown grain cores and dark brown mantles, as well as mineral vs. whole rock relationships suggest the pluton was a cumulate. Patchy zoning of amphibole with continuous extinction, crystallographic continuity demonstrated by EBSD misorientation maps and TEM diffraction, and preserved fractional crystallization in the majority of trace elements suggest that the zoning textures were generated by coupled dissolution-reprecipitation. However, HSFE and HREE were modified in altered domain, challenging the notion that trace elements are not mobilized during alteration. Thermodynamic modeling of amphibolites from Cerro Colorado yielded P-T estimates of 575-650°C at pressures of 4-6 kbar, which is consistent with the decompression trend proposed by Barnhart et al. (2012). Estimations of Fe3+ in amphibole ranged between 0.24 and 0.31 in the models, which agreed with mass balance calculations for EPMA spot analyses; and suggests that conditions varied due to different fluid compositions during metamorphism. Accuracy of solution models could be improved, as tschermak-type substitutions resulted in overestimates of Al in modeled amphibole compositions. Significantly improved resolution to the P-T path would permit using multi-mineral-chronometer techniques to significantly improve the resolution of P-T-time paths, with improved understanding of rates of geologic processes.