[1] On the basis of new demagnetization and rock magnetic experiments, a scattered, twopolarity remagnetization extending throughout the contact metamorphic aureole surrounding the Jurassic Notch Peak granitic stock in western Utah is now thought to reside in metamorphic pyrrhotite rather than in fine-grained hematite. In part, the pyrrhotite formed by in situ reduction of magnetite by pyrite and organic compounds; it could not have been formed by the large-scale introduction of reducing fluids, as isotopic evidence indicates that these metalimestones remained impermeable. At higher grades, some pyrrhotite probably also formed by the reaction of pyrite and Fe-bearing silicates. As magnetite destruction and pyrrhotite formation extends into rocks that are not obviously metamorphosed, the reactions responsible could proceed at very low (less than $200°C) temperatures, and the feasibility of such low-temperature reactions in a closed system is shown by detailed thermodynamic calculations. This study demonstrates that under reducing conditions in the presence of sufficient pyrite, pyrrhotite can form at the expense of magnetite under very mild conditions; indeed, magnetite may be destroyed merely by deep burial. Such a mechanism may help explain the paucity of primary magnetite in ancient sedimentary rocks, particularly since the single-domain and pseudo-single-domain grains capable of retaining a stable paleomagnetic signature over geologic time are extremely fine-grained and hence particularly vulnerable to destruction.

No abstract

In the Desert Range of southern Nevada, miogeoclinal sedimentary rocks, mostly shallow‐water limestones of latest Precambrian through Early Ordovician age, yield three components of magnetization having different blocking temperature spectra: (1) a low blocking temperature component near the direction of the present axial‐dipole field; (2) an intermediate blocking temperature component with northerly declination and inclination ∼+60°; and (3) a characteristic component with southeasterly declination and inclination ∼−20°. Combinations of alternating field and thermal demagnetization indicate that the intermediate and characteristic components reside in magnetite. The intermediate magnetization probably reflects a viscous partial thermoremanent magnetization (VPTRM) imposed between the Late Cretaceous and mid‐Cenozoic; it was probably acquired when the strata were dipping slightly eastward. The characteristic magnetization is probably a VPTRM imposed during regional uplift in the Late Permian. The degree of heating required to have destroyed any primary magnetization is consistent with the conodont color alteration index observed in the Ordovician rocks; additionally, the characteristic magnetization in those rocks is younger than bedding disruption caused by major, late stylolitization. Red‐purple mudstones from the middle member of the Wood Canyon Formation, in which a directionally similar characteristic magnetization resides in fine‐grained hematite, also appear to have been remagnetized; in these rocks, the remagnetization probably reflects partial recrystallization, as the blocking temperatures are too high to have been reset by burial heating. The sampled sections have undergone relative tectonic rotation about a vertical axis, consistent with late Tertiary oroflexural bending that had been proposed on independent geologic evidence. The characteristic magnetization probably provides a reliable estimate of the magnitudes of the vertical axis rotations, as the regional geology suggests the Desert Range strata were essentially horizontal throughout the Paleozoic. The total observed relative rotation is 44°±5°, representing absolute counterclockwise rotation (27°±4°) of the northern part of the Desert Range, and absolute clockwise rotation (17°±5°( of the southern part. The only unit in the Desert Range sequence that may retain a primary magnetization is the late Precambrian Rainstorm Member of the Johnnie Formation. The characteristic magnetization of this unit exhibits two polarities and probably resides in specular hematite; after correction for 18° of counterclockwise rotation, the resulting pole (5°N, 151°E) is near other late Hadrynian poles from North America.

The Notch Peak pluton, a Late Jurassic granitic stock, intrudes early Paleozoic miogeoclinal strata in the House Range, western Utah. As part of a detailed study to determine textural and chemical changes related to the intrusion, a paleomagnetic reconnaissance has been made of the pluton and its contact aureole. Steep, scattered magnetizations with both polarities present are found in the pluton upon alternating field (af) demagnetization, and they may reflect an original thermoremanence in magnetite that was acquired over at least one polarity reversal. Limestones and argillites in the Big Horse Canyon Member of the Orr Formation (Late Cambrian) were sampled at four stratigraphically controlled sites ranging from unaltered country rock to marble. The site farthest from the pluton is well behaved upon progressive thermal demagnetization and yields a low‐inclination, reversed Paleozoic direction by 400°C. The sites nearer the stock generally have a diffusely defined, two‐polarity, high‐inclination magnetization that reflects remagnetization by the pluton. This magnetization is removed above 300°C, and af demagnetization suggests that it resides in fine‐grained hematite. Of these sites, one appeared nearly unaltered in thin section, whereas the others range from hornfels to skarn. The remagnetization resulted from a late oxidation event related to the intrusion, perhaps from mobilization of groundwater; thus, the country rock has been remagnetized chemically rather than thermally. These results suggest that paleomagnetism may be a useful tool for detecting altered zones around plutons. In addition, no consistent, high‐blocking‐temperature magnetization was found in the aureole rocks; the magnetization of the hornfels was easily measurable at higher steps, but directions changed erratically, and this behavior in part results from the presence of pyrrhotite.