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Reconnaissance paleomagnetic investigation of the ophiolitic complex, Thetford Mines, Asbestos, Quebec —Discussion
Tecto~o~h~sics, 53 (1979) 135-138 0 Else&r Scientific Publishing Company,
135 Amsterdam
- Printed in The Netherlands
RECONN~SS~~E P~EOMAGNBTIC ‘I~ESTIGATION OF THE OPHIOLITIC COMPLEX, THETFORD MINES, ASBESTOS, QUEBEC - DISCUSSION
JAIME URRUTIA-FU~UGAUCHI ~epar~meni of geophysics and Planetary Physics, School of Physics, The University, Newcastle-upon-Tyne (Great Britain) (Submitted
January 9,1978;
accepted
January 31,1978)
Seguin (1976) in a recent paper presented the results of a prelimit magnetic and paleomagnetic study of the ophiolitic complex of the Asbestos and Thetford Mines, Quebec, Canada. It is intended to point out in brief some inconsistencies in data and arguments. Although the study seems to involve a total of 41 selected orientated hand samples from an original set available of 60 samples (section on “sampling procedure and measurements”, and figures), unfortunately the magnetic and paleomagnetic results published correspond to eleven (twelve) samples only. Both AF and thermal demagnetization processes were used in order to investigate the paleomagnetic properties and to insolate the primary remanence. It has been suggested in paleomagnetism of igneous rocks that AF and thermal demonetization treatment often produce similar results (Kono and Ueno, 1977). Studies which include both treatments applied to specimens from the same sample and/or samples from the same site are not abundant in the literature (e.g., Spall, 1968; Larson and Mutschler, 1971; Evans and Bingham, 1973), thus it is very interesting t6 look at the results given by Seguin. AF DEMAGNETIZATION
According to the text a minimum of two specimens from each site was subjected to step-wise AF demagnetization. A stability criterion was then used in order to select three AF fields in which the remaining specimens from a site were also demagnetized. Seguin stated “the dispersion within and between sites generally decreased . . .“, however, the within sample dispersion (only a95’s were given) increased in seven (70%), remained in one and decreased in two, of the samples. Unfortunately from the results given in the paper, it is not possible to compare the dispersions as Seguin did it (unit weight to specimens), however, the dispersions after treatment still seem too high.
136 THERMAL D~~AGN~TIZATIGN
From the partial results given (tables IV and V), it seems that the directional behaviour during thermal demagnetization was not very stable, even for samples regarded as stable. Also it is interesting to note that the five samples given as examples of unstable behaviour during thermal treatment gave results (four?) which were accepted in AF analysis and a sample regarded as unstable after AF cleaning, gave an accepted result after thermal treatment. DISCUSSION
Although the pole positions given by Seguin are not very different from one another, it seems that in this case the AF and thermal treatments were not equivalent enough to isolate the primary remanence of the rocks investigated in this study. Due to the poor-directional stability during AF and thermal treatments and the different results displayed for specimens from the same sample, it is very difficult to establish either the pole positions corresponding to the AF and thermal demagnetized data or the pole position for the total data. I feel that a more complete analysis is indispensable in order to obtain a reliable pole position. Also it is more difficult to understand why the pole position (without any statistical parameters) determined in this study is discussed in tectonic terms. REFERENCES Evans, M.E. and Bingham, D.K., 1973. Paleomagnetism of the Precambrian Martin Formation, Saskatchewan. Can. J. Earth Sci., 10: 1485-1493. Kono, M. and Ueno, N., 1977. Paleointensity determination by a modified Thellier method. Phys. Earth Planet. Inter., 13: 305-314. Larson, E.E. and Mutschler, F.E., 1971. Anomalous paleomagnetic pole from isotopically dated Cambro-Ordovician intrusives in Colorado. Geol. Sot. Am. Bull., 82: 16571666. Seguin, M.K., 1976. Reconnaissance paleomagnetic investigation of the ophiolitic complex, Thetford Mines, Asbestos, Quebec. Tectonophysics, 34: 231-243. Spall, H., 1968. Psleomagnetism of basement granites of southern Oklahoma. Okla. U.S. Geol. Surv., Notes, 28: 65-80.
135 Amsterdam
- Printed in The Netherlands
RECONN~SS~~E P~EOMAGNBTIC ‘I~ESTIGATION OF THE OPHIOLITIC COMPLEX, THETFORD MINES, ASBESTOS, QUEBEC - DISCUSSION
JAIME URRUTIA-FU~UGAUCHI ~epar~meni of geophysics and Planetary Physics, School of Physics, The University, Newcastle-upon-Tyne (Great Britain) (Submitted
January 9,1978;
accepted
January 31,1978)
Seguin (1976) in a recent paper presented the results of a prelimit magnetic and paleomagnetic study of the ophiolitic complex of the Asbestos and Thetford Mines, Quebec, Canada. It is intended to point out in brief some inconsistencies in data and arguments. Although the study seems to involve a total of 41 selected orientated hand samples from an original set available of 60 samples (section on “sampling procedure and measurements”, and figures), unfortunately the magnetic and paleomagnetic results published correspond to eleven (twelve) samples only. Both AF and thermal demagnetization processes were used in order to investigate the paleomagnetic properties and to insolate the primary remanence. It has been suggested in paleomagnetism of igneous rocks that AF and thermal demonetization treatment often produce similar results (Kono and Ueno, 1977). Studies which include both treatments applied to specimens from the same sample and/or samples from the same site are not abundant in the literature (e.g., Spall, 1968; Larson and Mutschler, 1971; Evans and Bingham, 1973), thus it is very interesting t6 look at the results given by Seguin. AF DEMAGNETIZATION
According to the text a minimum of two specimens from each site was subjected to step-wise AF demagnetization. A stability criterion was then used in order to select three AF fields in which the remaining specimens from a site were also demagnetized. Seguin stated “the dispersion within and between sites generally decreased . . .“, however, the within sample dispersion (only a95’s were given) increased in seven (70%), remained in one and decreased in two, of the samples. Unfortunately from the results given in the paper, it is not possible to compare the dispersions as Seguin did it (unit weight to specimens), however, the dispersions after treatment still seem too high.
136 THERMAL D~~AGN~TIZATIGN
From the partial results given (tables IV and V), it seems that the directional behaviour during thermal demagnetization was not very stable, even for samples regarded as stable. Also it is interesting to note that the five samples given as examples of unstable behaviour during thermal treatment gave results (four?) which were accepted in AF analysis and a sample regarded as unstable after AF cleaning, gave an accepted result after thermal treatment. DISCUSSION
Although the pole positions given by Seguin are not very different from one another, it seems that in this case the AF and thermal treatments were not equivalent enough to isolate the primary remanence of the rocks investigated in this study. Due to the poor-directional stability during AF and thermal treatments and the different results displayed for specimens from the same sample, it is very difficult to establish either the pole positions corresponding to the AF and thermal demagnetized data or the pole position for the total data. I feel that a more complete analysis is indispensable in order to obtain a reliable pole position. Also it is more difficult to understand why the pole position (without any statistical parameters) determined in this study is discussed in tectonic terms. REFERENCES Evans, M.E. and Bingham, D.K., 1973. Paleomagnetism of the Precambrian Martin Formation, Saskatchewan. Can. J. Earth Sci., 10: 1485-1493. Kono, M. and Ueno, N., 1977. Paleointensity determination by a modified Thellier method. Phys. Earth Planet. Inter., 13: 305-314. Larson, E.E. and Mutschler, F.E., 1971. Anomalous paleomagnetic pole from isotopically dated Cambro-Ordovician intrusives in Colorado. Geol. Sot. Am. Bull., 82: 16571666. Seguin, M.K., 1976. Reconnaissance paleomagnetic investigation of the ophiolitic complex, Thetford Mines, Asbestos, Quebec. Tectonophysics, 34: 231-243. Spall, H., 1968. Psleomagnetism of basement granites of southern Oklahoma. Okla. U.S. Geol. Surv., Notes, 28: 65-80.