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Scanning electron microscope and Scanning transmission electron microscope observations of thin sections and separates of the Devonian Onondaga and Helderberg limestones and Ordovician Trenton limestone in New York state allow us to identify three types of magnetite: pseudoframboids, nonspherical magnetite, and fine‐grained magnetite. Magnetite was observed replacing pyrite occurring as crystals in spherical pseudoframboids or with nonspherical shapes. Fine‐grained magnetite, consisting of aggregates of one or more rounded single crystals, approximately 2000 Å (200 run) in diameter, could not be observed in thin sections of New York carbonates due to its small size but was found in magnetic extracts. Hysteresis measurements of magnetic extracts verify that fine‐grained magnetite is capable of carrying remanent magnetizations. However, pseudoframboidal magnetite and nonspherical magnetite are polycrystalline and consist of assemblages of single to pseudosingle domain‐sized crystals that also can be carriers of the remagnetization. These data, in combination with observations from other localities, collectively imply that the Alleghenian remagnetization is carried by the fine‐grained magnetite, although pseudoframboidal and nonspherical magnetite may also contribute. Thus the Alleghenian remagnetization is a chemical remanent magnetization due to authigenic magnetite. Crystallization of magnetite was mediated by fluids, with dissolution and crystallization activated by stress during the Alleghenian Orogeny. The origin of such fluids is unknown, but they may have originated through crustal scale fluid migration. Tectonically induced brine migration due to emplacement of thrust sheets is preferred over a meteoric source of the fluids.

ABSTUCT Many normal and a few reversed characteristic directions of magnetization have been obtained by predominantly thermal demagnetization from ten sites of the Late Permian Emeishan Basalts collected near Kunming, Yumran Province. The normal ma~etization directions pass a fold test at the 99% confidence level and yield d~ination/~clina~on = 26"/ -12", k = 46, qg = 6" and a paleopole at 50"N, 241"E. However, the reversed-polarity directions, with d~lination/inc~ation at 244"/+ 3" are not antipodal to the normal ones, which is also noted in other studies of the Emeishan Basalts of the Yangtze Paraplatform of the South China Block. Speculations about the cause of this lack of antipodaiity center on: (1) local relative rotations, (2) incomplete demagnetization, (3) unusually large non-dipole fields, secular variation or asymmetric reversals in the Late Permian, (4) errors in sample orientation, or (5) later remagnetization represented by one or the other polarity group. The first two causes are ruled out by our obsetvations, and the third cannot be tested with data from China alone but is unlikely because it has not been observed elsewhere. Errors in sample o~en~~n may be present in those studies that used a magnetic compass, because sample intensities of magnetization are on tire high side. However, for the normal-polarity directions, a solar compass has been used in some of the studies and ail normal-polarity directions observed in the area are in agreement. Thus, sample orientation errors can be invoked only for the reversed-polarity directions. In order to test cause (5), we have examined the magnetic carriers in eight samples with scanning electron microscopy. Titanium-poor to titanium-rich magnetite, commonly cruciform in crystal habit, indicates primary igneous crystallization in two samples that have NE and WSW declinations. In contrast, six no~~-~Iari~ samples with NNE declinations show pervrsive replacement of original titanom~etite by titanic-me magnetite and sphene, indicating a high degree of alteration. We ascribe this alteration to late hydrothermal circulation and argue that it has caused remagnetization in post-Permian times. This study suggests, therefore, that the NE-SW directions are more likely to be representative of the Late Permian paleomagnetic field than the NNE directions.

Previous paleomagnetic observations for the carbonates of the Lower Ordovician Knox Group have indicated that ancient magnetizations in these rocks are of the same age as the late Paleozoic Alleghenian Orogeny. Rock magnetic properties strongly suggest magnetite as the carrier of the magnetization, but the textural and crystalline characteristics, sizes, morphologies, and mineral associations of these magnetites are poorly known. We have examined magnetic extracts and iron oxides in thin sections with scanning (SEM) and scanning/transmission (STEM) electron microscope techniques to determine whether the observed iron‐oxide grain textures match the rock magnetic properties and paleomagnetic inferences about the mode of formation of the magnetic carriers. Several different forms of magnetite in limestones and dolomites, which in places are host to Mississippi‐Valley type deposits, are documented by imaging and energy‐dispersive analysis using SEM and STEM, by X ray diffraction and electron diffraction patterns using STEM. The magnetite is either spherical with a dimpled surface or nonspherical in the form of void‐filling single grains or grain aggregates. Most of the iron oxides have the composition of pure end‐member magnetite, but occasional titanomagnetite and hematite, including rare zincian hematite, have been observed (only in limestone). Wherever found in thin section, nonspherical magnetites occur in association with secondary dolomite, potassium‐feldspar, illite, and quartz. Some iron oxides have, in fact, inclusions of K‐feldspar and quartz. Some of the magnetite (spherical and nonspherical) is polycrystalline; this implies that the larger observed grains may consist of single domains or pseudo‐single domains. This provides an explanation of the observed rock magnetic properties that apparently reflect the presence of single‐domain (but interacting?) subgrains, on the basis of remanent coercivities and blocking temperatures. We interpret the pure end‐member magnetite to be authigenic, having formed at approximately the same time as the K‐feldspars, which in nearby areas have yielded late Paleozoic radioisotopic ages (278–322 Ma). The Knox carbonates therefore are inferred to carry a chemical rémanent magnetization. Iron‐rich clays or original iron‐titanium oxides in the carbonates may have been the source materials for at least some of the secondary magnetite as it formed through complete dissolution‐precipitation processes. These processes require rock‐fluid interactions which are thought to be related to migrating connate brines during the Alleghenian Orogeny.

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