Rock Magnetism and Paleomagnetism of Mafic Dykes of the Sergeevka Terrane (Southern Sikhote-Alin)

The results of petromagnetic and paleomagnetic studies of rocks from mafic dykes of the Sergeevka Terrane that intruded the basement rocks and are widespread in coastal outcrops of the eastern part of Peter the Great Bay of the Sea of Japan between the Anna and Priboinaya bays in the southern part of Primorskii krai are reported. The structural information allows us to conclude that dykes of both the eastern (Srednyaya Bay–Priboinaya Bay) and western (Anna Bay–Cape De Livron) clusters intruded from the igneous feeding channels of the same orientation. Rocks of the dykes were subjected to significant secondary alterations. Inverse magnetic fabric in the dykes is due to high-temperature decomposition and granulation of primary titanomagnetites upon postmagmatic deformations and metamorphism, traces of which are clearly visible in the microscopic and microprobe study of secondary alteration products. This provides evidence for a significant secondary change in rocks of the dykes after their intrusion. This is the secondary heating of the dykes above the temperature of 600°C, i.e., above the Curie point of magnetite (578°C). The nature of the isolated high-temperature NRM component of the dykes is not primary, but secondary, that is, metachronous. The paleomagnetic data constrained the time of the metamorphic event leading to the remagnetization of mafic dykes of the Sergeevka Terrane to about 250 Ma. The calculated paleolatitude of the Sergeevka Terrane at the time of acquisition of the metachronous NRM component by mafic dykes (21.8° ± 4.2° N) is consistent with the paleolatitudes of the northeastern edge of the North China Craton at the Late Permian/Early Triassic boundary.