Insights Into the Mechanism for Orogen-Related Carbonate Remagnetization from Growth of Authigenic Fe-Oxide: A Scanning Electron Microscopy and Rock Magnetic Study of Devonian Carbonates from Northern Spain
A rock magnetic and SEM study of Devonian carbonates from the Cantabria-Asturias Region, northern Spain, was undertaken to further our understanding of the pervasive remagnetization of carbonate rocks during the Late Paleozoic, and the mechanism by which these remagnetizations occur. These rocks contain three ancient Late Paleozoic magnetizations. The rock magnetic properties of mineral extracts were compared with those of whole rock chips and "nonmagnetic'' residue to deduce magnetic carrier(s) and grain sizes. Hysteresis measurements for rock chips show "typical'' wasp-waisted loops, whereas extract shows typical pseudosingle-domain-like (PSD) unrestricted loops. Within all sites, there is a noticeable contribution of superparamagnetic (SP) grains seen in hysteresis properties and low-temperature magnetization measurements of whole rock chips, whereas a trend away from a strong SP contribution is seen when hysteresis properties of whole rock are compared with those of residue and extract. Consequently, our extraction process (predictably) removes SP grains, while preserving the characteristic fraction of remanence-carrying material, which behaves like a typical mixture of single-domain (SD) and PSD magnetite. Paradoxically, the typical "fingerprint'' of remagnetized carbonates, as seen in the whole rock data, seems to be a response to abundant SP grains associated with the acquisition of chemical remanent magnetizations (CRM), and not the actual remanence carrying population itself. Scanning electron microscopy (SEM) observations of magnetic extract reveal abundant authigenic Fe-oxides, characterized as either 10-100 mum Ni-free spherules or individual 0.1-10 mum euhedral grains. SEM observations of thin sections reveal abundant evidence of fluid flow driven chemical reactions that resulted in formation of new Fe oxide. Such reactions occurred along cracks and grain boundaries and within void space, and are associated with Fe-rich clay and calcite-dolomite reactions or as oxidation of Fe-sulfide framboids. Together, the SEM observations and rock magnetic experiments reveal that the three Late Paleozoic remagnetizations experienced by Cantabria-Asturias Paleozoic carbonates are CRMs facilitated by the presence of fluids activated during Late Paleozoic Variscan deformation.