Thermochemical remanent magnetization in Jurassic silicic volcanics from Nevada, U.S.A.

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Characteristic magnetizations from Middle Jurassic dacitic to andesitic subaerial volcanics (the Fulstone and Artesia Formations) in the Buckskin Mountain Range, western central Basin and Range Province, are well-grouped, generally display univectorial decays to the origin in demagnetization and have hematite blocking temperatures restricted almost entirely to above 620°C. Petrographic, rock magnetic and electron microprobe investigations confirm that nearly pure hematite is the essential magnetic phase (up to about 10 vol. %) occurring as a replacement of coarse titaniferous magnetite phenocrysts and fine groundmass particles, as a secondary alteration product of ferromagnesian phenocrysts and as a mobilized phase filling cracks and other open spaces. The presence of antipodal directions in each flow unit and in interbedded volcanoclastic units (some having retained magnetite as a major magnetic phase) and magnetite-dominated remanences in time-equivalent intrusives cutting the flows indicates that the volcanics acquired their hematite remanence, a faithful record of the geomagnetic field, in high-temperature, deuteric oxidation during and following their emplacement, not during a later thermal event such as regional metamorphism. The remanence is probably a thermochemical remanent magnetization, although part may be of thermoremanent origin.

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