A closer look at remanence-dominated aeromagnetic anomalies: Rock magneticproperties and magnetic mineralogy of the Russell Belt microcline-sillimanite gneiss, northwest Adirondack Mountains, New York

A large, distinct negative aeromagnetic anomaly of over 2000 nT associated with microcline-sillimanite-quartz gneisses in the Russell area, northwest Adirondack Mountains, was previously shown to be remanence-dominated, altho ugh the carriers of remanence were not well documented. Russell Belt gneiss es have a strong natural remanent magnetization with steep remanence direct ions, D = 263 degrees, I = -58 degrees, an average intensity of 3.6 A/m, an d typical susceptibilities of 10(-4) SI. The remanence is thermochemical in origin, acquired during cooling from peak metamorphic conditions of 650 de grees-750 degrees C during the Ottawan Orogen (1050-1080 Ma). The reversed polarity of remanence reflects a reversed paleofield, rather than self-reve rsed, contrary to earlier suggestions. The gneisses contain up to 3% oxide, predominantly metamorphic titanohematite, which accounts for the low susce ptibility values and highly stable remanence. Optical observations show tit anohematite grains with multiple generations of ilmenite, pyrophanite, ruti le, and spinel exsolution lamellae. Microprobe analyses confirm titanohemat ite compositions ranging from 72 to 97% Fe2O3, with hematite(83) being most typical. In rare samples, inclusions of magnetite were identified. The ubi quitous presence of titanohematite, and the rare occurrence of magnetite, i s supported by thermal and alternating field demagnetization studies, satur ation magnetization measurements, hysteresis properties, temperature-hyster esis studies, and low-temperature remanence measurements. Numerous crustal granulites have titanohematite as part of the oxide assemblage, and this ma y contribute a strong remanent component to what have previously been consi dered to be solely induced anomalies.