Clay minerals and continental-scale remagnetisation: a case study of
South American Neoproterozoic carbonates
Abstract
Carbonate rocks frequently undergo remagnetisation events, which can
partially/completely erase their primary detrital remanence and
introduce a secondary component through thermoviscous and/or chemical
processes. Despite belonging to different basins hundreds of kilometres
apart, the Neoproterozoic carbonate rocks of South America (over the
Amazon and São Francisco cratons) exhibit a statistically
indistinguishable single-polarity characteristic direction carried by
monoclinic pyrrhotite and magnetite, with paleomagnetic poles far from
an expected detrital remanence. We use a combination of classical rock
magnetic properties and micro-to-nanoscale imaging/chemical analysis
using synchrotron radiation to examine thin sections of these
remagnetised carbonate rocks. Magnetic data shows that most of our
samples failed to present anomalous hysteresis properties, usually
referred to as part of the “fingerprints” of carbonate
remagnetisation. Combining scanning electron
microscopy-energy-dispersive X-ray spectroscopy (SEM-EDS), highly
sensitive X-ray fluorescence (XRF), and X-ray absorption spectroscopy
(XAS) revealed the presence of subhedral/anhedral magnetite, or
spherical grains with a core-shell structure of magnetite surrounded by
maghemite. These grains are within the pseudo-single domain size range
(as well as most of the iron sulphides) and spatially associated with
potassium-bearing aluminium silicates. Although fluid percolation and
organic matter maturation might play an important role,
smectite-illitisation seems a crucial factor controlling the growth of
these phases. X-ray diffraction analysis identifies these silicates as
predominantly highly crystalline illite, suggesting exposure to epizone
temperatures. Therefore, we suggest that the remanence of these rocks
should have been thermally reset during the final Gondwana assembly, and
locked in a successive cooling event during the Early-Middle Ordovician.