ORIGIN OF MAGNETIC FABRICS IN GRANITES FROM THE CADILLAC MOUNTAIN INTRUSIVE COMPLEX, MAINE

The Silurian Cadillac Mountain Intrusive Complex (CMIC) is part of the coastal plutonic and volcanic belt of Maine. It contains three major intrusive rock units: fine-grained granites, coarse-grained granites, and gabbro-diorite. Preliminary lab work on both types of granites shows a high magnetic susceptibility before demagnetization in all of the measured sites. Demagnetization behavior, given by the results of alternating-field demagnetization, indicates that the predominant magnetic mineral in the granites is multi-domain magnetite. The alternating-field demagnetization results do not show a believable Silurian paleomagnetic direction. Moreover, the granites have a strong anisotropy of magnetic susceptibility (AMS), although rock fabrics defined by the shape preferred orientation (SPO) of the major minerals are absent or weak, which is a seeming contradiction. The goal of this study is to identify the origin and significance of the AMS in the CMIC granites. Since the AMS signal appears not to relate to the SPO of major minerals, it may instead arise from one of three sources: (1) the SPO of the magnetic minerals, (2) a distribution anisotropy of the magnetic minerals, or (3) a preferred orientation of magnetic domains within magnetite grains. Because there is no significant change in the orientation or grouping of the AMS of samples after demagnetization of the granites, the AMS probably does not arise from the third source (i.e. orientation of the magnetic domains). Further research using image analysis on oriented slabs will test the distribution anisotropy versus the SPO hypotheses for the origin of the AMS.