Palaeomagnetism of some late Mesozoic dolerite dykes of South Spitsbergen

Abstract

Summary. Opaque mineralogical studies of late Mesozoic dolerite dykes of Wedel-Jarlsberg Land, South Spitsbergen, show that the dykes have undergone a mild hydrothermal alteration. The alteration probably took place at the time of consolidation of the dykes, but may also have occurred as a side effect of Tertiary orogeny, which affected the western parts of Spitsbergen. Seven of these dykes were sampled during two summer seasons, when samples were collected from different exposures. A conventional procedure based on the minimum scatter principle and study of Zijderveld diagrams applied to 282 specimens demagnetized by alternating fields and thermally reveals several 'characteristic remanence' directions. In three dykes the directions are consistently reversed, but one carries both normal and reversed remanence. The reversed directions can be attributed to a geomagnetic field reversal in the late Mesozoic and K-Ar dating of one of the reversed dykes gives an age of 110 -+ 5 Myr BP in agreement with the known age of early Cretaceous field reversal. Using a least squares method of analysis of step demagnetization, low coercivity components, not separated by the conventional procedure, have been successfully isolated in two of the dykes. The analysis applied to all the other dykes confirms the results of the conventional method, yielding comparable mean directions of magnetization. The conventional method applied to 'characteristic' directions from five dykes gives a mean palaeomagnetic pole at 210.1, 78.9 N, ag5 = 3.5 and the least squares procedure applied to directions from six dykes yields a pole at 222.7, 69.7N, ag5= 7.9. The pole positions obtained agree with those obtained for Spitsbergen dolerites by