Paleomagnetism of the Thetford Mines ophiolites, Quebec.

Abstract

One hundred and twenty-five (125) oriented samples (286 specimens) were obtained from thirty-three (33) sites in metavolcanic rocks and diabase dyke swarms from the ophiolite complex of Thetford Mines in the Appalachians of Southern Quebec (longitude: 71°00′-71°45′W, latitude: 45°45′-46°15′N). The metavolcanic rocks are massive or pillowed (mainly) lavas of andesitic and mostly basaltic composition and the dyke swarms are composed of basaltic and doleritic diabases; both lithological groups are metamorphosed to the subgreenschist (pumpellyite) facies. Pyroclastics, tuffs and volcanic agglomerates are also present in minor quantities.The NRM intensities of the metalavas and diabases are very weak. Magnetic, titanomagnetite and occasional grains of hematite are the magnetic memory carriers. Native iron was also detected in minute quantities. In order to obtain some pertinent information relative to the stability of the NRM component, stepwise alternating field (AF) demagnetization was conducted on 30% of the specimens in the 50-800 Oersteds range and the others were demagnetized at an optimum AF intensity. Approximately 10% of the specimens underwent thermal treatment in the 100-550°C range; thermal demagnetization was judged less efficient than AF demagnetization and for this reason, the remaining specimens were all AF demagnetized.After AF treatment, the paleopole position of the tilted ophiolite complex is 206° to 09°S (dm=16°, dp=10°, K=61), i. e. 26°E, 09°N (normal polarity).Petrological and magnetic studies of the ophiolite complex of Thetford Mines indicate that the basaltic and doleritic diabase dykes which feed the overlying pillow lavas in the Thetford Mines complex have NRM intensities and Koenigsberger ratios which are comparable to those of the layer of pillow lavas. As the NRM intensities of both the pillow lavas and the diabase dyke swarms are low, they ought to be relatively unimportant contributors to magnetic anomalies. The large NRM intensities and Koenigsberger ratios of the gabbroic layer and particularly the harzburgite, dunite, wehrlite and pyroxenite of Thetford Mines suggest that the ultramafic layers are the most significant magnetic sources which might well explain the younger marine anomalies. Surface alteration and low temperature oxidation may, however, be partly responsible for the low NRM intensities of the relatively thin (1km-thick) layer of pillow basalts of this ancient sea floor.The absence of a reliable model of the source(s) of the magnetic lineations is a serious handicap in the solution of numerous geological and geophysical problems such as dyke intrusion models of seafloor spreading geomagnetic reversal time-scale and paleogeographic reconstruction (BANERJEE et al., 1974). This study is concentrated on these two last aspects.