Abstract

The magnetic characteristics of shocked and unshocked anorthosites of the Charlevoix impact structure have been reexamined to assess the effects of high thermal and alternating field (af) cleaning treatments, and to evaluate any shock-induced features of the remanence in terms of mineralogy of the shocked rocks.From 16 sites, 197 specimens were subjected to stepwise thermal treatments to 710 °C, and a further 46 to incremental af cleaning to 290 mT. The 10 sites from the unshocked St-Urbain anorthosite carry a magnetization with unblocking temperatures (TUB) generally between 600–625 °C, with a significant fraction of remanence with TUB > 670 °C. Similarly, most of the remanence has a resistive coercive force (rcf) of 100–175 mT, with a significant fraction exceeding 290 mT. The remanence is carried by two exsolution phases of titano-hematite, with the later and more hematite-rich lamellae having the higher TUB and rcf. Although only one direction exists, at two sites where intensities are somewhat lower and some natural remanent magnetization (NRM) directions scattered, treatment reveals a dual polarity remanence. The pole (154°E, 02°S) obtained from these unshocked St-Urbain anorthosite sites falls on a well established segment of the late Precambrian apparent polar path of suitable age (≈950 Ma). Significant results could not be obtained from three additional sites in the marginal zone due to their ready acquisition of a viscous magnetization following thermal treatments.Data from two sites which display no shock effects and lie near the margin of the central uplift, and from one highly shocked sample from the crater centre show some possible effects of the shock event. All three have intensities substantially lower than in the St-Urbain sites, which is interpreted in the case of at least one of these sites as a shock-diminished remanence. A puzzling feature of the highly shocked samples is that the phase with a texture reminiscent of the ilmenite–hematite exsolution in the unshocked anorthosites, contains no iron. The remaining remanence, whose direction is parallel with that of the unshocked sites, is interpreted as pre-crater rather than shock-produced. Although many instances have been reported of a new shock-induced remanence imparted to rocks of equivalent and even lesser shock grade than those examined at Charlevoix, in all cases the magnetic carrier seems to have been magnetite. The lack of a shock-induced remanence at Charlevoix is attributed to the high TUB and rcf of hematite in these rocks.

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