Abstract

Measurements of NRM directions in clasts from selected volcanic breccia units of the Gallatin Range of Montana show that magnetization directions may be near-parallel from clast to clast, or near-random. Near-parallel directions are found in clasts from a unit formed by autobrecciation of a flow; this is to be expected in breccias which come to rest while heated above the average of blocking temperatures of the constituent ferromagnetic grains. Near-random directions are present in a breccia exhibiting field and petrographic characteristics of a mudflow or debris flow; such randomization should take place as breccia clasts are rotated while below the blocking temperature. Paleomagnetic measurements thus help distinguish between Gallatin Range volcanic breccias emplaced at relatively high versus relatively low temperature. The technique should have wide application in rocks of these types.

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