Chemical and K–Ar systematics of a mylonitic mica after natural and experimental interactions with varied fluids

Abstract

Flow-through and batch-leaching experiments combining mineralogical, chemical and K–Ar isotopic analyses of mica separates from a mylonitic sample of the Tyndrum faulting system in Scotland (U.K.) were conducted to evaluate and simulate the natural interaction with H2SO4-loaded river water at pHs of 3–4. The flow-through experiments with H2SO4-loaded and pure deionized water completed by a batch-leaching experiment with 1M HCl at room temperature had varied effects on soluble mineral phases, such as Fe-sulfates and Ca/Mg-carbonates, that were present in the varied mica size fractions, but none caused the mineralogical and K–Ar characteristics of the mica to differ from separates of the natural environment.Despite the limited number of K–Ar ages, size fractionation of the mylonitic mica identified two generations of mica with different REE patterns. The coarser (2–8μm) fraction yielded a high La/Yb ratio of 4.3 and crystallized at 359±6Ma at a temperature of 250–300°C, probably during a major tectonic-thermal activity. The smaller (<0.5μm) fraction yielded a La/Yb ratio of 2.1, a younger K–Ar age at 315±5Ma and a lower crystallization temperature of about 200°C.