Anorthosites as Sources of Magnetic Anomalies

Abstract

Magnetic anomalies provide information about location, size and composition of earth structures, ore bodies and tectonic features even in bodies containing only a few percent magnetic minerals. Here we investigate the magnetic properties and oxide mineralogy of anorthosites, rocks rich in plagioclase (>90%), and compare their magnetic signatures to aeromagnetic anomaly maps of the regions. Two of the anorthosite complexes have large negative anomalies associated with them; both have low susceptibility and high remanence related to hemo-ilmenite mineralogy and remanent directions antiparallel to the present field. One complex has appreciable natural remanent magnetization quasi-parallel to the present field, and strong susceptibility, creating an enhanced positive anomaly. The fourth anorthosite has little or no magnetic anomaly over much of its area, in accordance with the weak remanence, low susceptibility and variable magnetic mineralogy observed. The anorthosite samples producing significant anomalies, and maintaining strong and stable natural remanent magnetization over geologic time all contain oxides of the hematite-ilmenite series. This study adds support to ‘lamellar magnetization’ whereby exsolved phases in the ilmenite-hematite system produce strong and stable magnetization with only minor amounts of oxide material.