Abstract
Magnetic properties are compared for the pillow basalts and sheeted dike complexes for the following three ophiolite suites: Macquarie Island (-27 million years), Troodos Massif, Cyprus (∼middle Cretaceous), and Smartville complex, California (∼Jurassic). The magnetic properties of the pillow basalts strongly depend on their degree of alteration. From the least altered, low-temperatureoxidized, upper pillow basalts of Macquarie Island and the pillow basalts of the Troodos Massif (zeolite facies metamorphism) to the most altered pillow basalts of the Smartville complex (greenschist facies metamorphism) the average intensity of the natural remanent magnetization (NRM) decreases from a maximum of 80×10-4 to 0.50×10-4 gauss; the Koenigsberger ratio (Q) decreases from 6 to 0.3; the contribution due to viscous remanent magnetization (VRM) increases, and the NRM stability with respect to alternating fields decreases. For the most altered pillow basalts of the Smartville complex the NRM is probably predominantly a VRM, acquired during the present polarity epoch. Thermomagnetic analyses for the least altered pillow basalts exhibit the characteristic behavior of low-temperature-oxidized cation deficient titanomagnetites, while the more altered pillow basalts have reversible thermomagnetic curves with magnetite-like Curie points, probably caused in situ by thermally induced unmixing of the low-temperature-oxidized titanomagnetites. The sheeted dike complexes have NRM intensities of about 10×10-4 gauss and Q values of about 0.8, and most of their magnetic properties do not vary much between the three ophiolite complexes. All the sheeted dike samples have reversible thermomagnetic curves with magnetite-like Curie points. If these ophiolite complexes represent material which was originally formed at spreading centers and if their alteration represents ocean floor metamorphism, then with progressive alteration the pillow basalts lose their magnetic recording qualities and become poorer sources for the marine magnetic anomalies, and the underlying sheeted dike complexes become relatively more important. The most probable site for the occurrence of the sea floor metamorphism is in the relatively high temperature, high heat flow regions near spreading centers.
