Electron microscopic and rock magnetic study of remagnetized Leadville carbonates, central Colorado

Abstract

Thermal demagnetization of limestone and dolomite samples from the Lower Carboniferous Leadville Formation of central Colorado reveals four magnetization components, of which three (A, B, and C) are ancient. The syn-folding A component is found both in limestone and dolomite below about 350°C and yields a paleopole at 71.6°N, 203.3°E, near the Late Cretaceous to early Tertiary segment of the North American APWP. The B component is found only in the limestone and has a pole position at 68.5°N, 138.1°E in situ, and 51.6°N, 136.9°E after tilt correction. The in-situ pole and partially tilt-corrected poles (up to about 30% unfolding) resemble Middle–Late Jurassic paleopoles of cratonic North America. The C component is found only in the dolomite with paleopoles at 45.7°N, 110.0°E (in situ) and at 50.7°N, 112.8°E (after tilt correction). These poles fall near the Permian segment of the apparent polar wander path. The corresponding ages of these three magnetizations therefore are all younger than Early Carboniferous, indicating that they are all secondary magnetizations. A rock magnetic and electron microscope study was successful in linking these multiple remagnetizations to identifiable magnetic mineral carriers, which helps to infer the authigenesis in the carbonates. Unblocking temperatures of the natural remanent magnetizations (NRM) and three-component isothermal remanent magnetizations (3D-IRM) indicate that pyrrhotite and magnetite are the major magnetic carriers of the NRM. Electron microscopy of magnetic extracts revealed magnetite spherules and fine-grained magnetite growing on quartz, albite, and chlorite, whereas electron microscopy of thin sections revealed goethite, hematite, pyrite and pyrrhotite in addition to magnetite. Demagnetization characteristics of NRM and rock magnetic experiments are inconsistent with hematite or goethite carrying a detectable ancient remanent magnetization, and the magnetite spherules are unlikely to carry any magnetization other than the recent magnetic overprint. Pyrrhotite is a likely chemical remanence carrier of the Late Cretaceous–early Tertiary A magnetization in both dolomite and limestone samples. Non-spheroidal iron oxides are observed in scanning and transmission electron microscopy in three forms: (1) as grains on the boundary of residual dolomite and dedolomite, inferred to be magnetite and carrying the B magnetization; formation of this magnetite appears to be related to Jurassic dedolomitization; (2) as fine-grained iron oxide inclusions within calcite, dolomite, or bitumen veins; or (3) as fine-grained magnetite growing on secondary minerals such as quartz, albite, and chlorite. The inclusions in dolomite have been shown by selected-area electron diffraction (SAED) to be magnetite and are inferred to carry the C magnetization acquired during late Paleozoic dolomitization; the iron oxide inclusions in calcite and bitumen resemble those in the dolomite and may also be magnetite, but they could not be analyzed by SAED. The fine-grained magnetite growing on secondary minerals, surprisingly, contains small amounts of titanium and manganese; this observation has to our knowledge not been reported before for authigenic magnetite. The ulvöspinel content ( x) in these titanomagnetites (Fe 3− x Ti x O 4) ranges from 0 to 0.15. This study shows that electron microscope observations can successfully identify some of the magnetic carriers in multiply remagnetized carbonates, even if they are submicrometer in size.