Causes and Mechanisms of Remagnetisation in Palaeozoic Sedimentary Rocks - a Multidisciplinary Approach

The present work combines palaeomagnetic and rock magnetic methods
with clay mineralogy, isotope geochemistry of clay minerals and
trace element geochemistry of Fe-oxide leachates to study
remagnetised sedimentary rocks from Palaeozoic outcrops in Middle
and Eastern Europe. Three areas were selected (NE Rhenish Massif,
Barrandian and Holy Cross Mountains), where the causes of Late
Palaeozoic remagnetisations are yet unclear. The results yield
important implications for the processes and mechanisms responsible
for the remagnetisations in the areas studied. NE Rhenish Massif: A
Late Carboniferous remagnetisation (component B) is identified in
Late Palaeozoic carbonate and clastic rocks from the NE Rhenish
Massif. Three individual incremental regional fold tests across the
area show a unique and distinctive variation in timing of
remagnetisation relative to the age of folding. The remagnetisation
is postfolding in the South and of synfolding origin in the North
of the area. Consequently, the timing and the duration of the
remagnetisation event is constrained by the age of folding, which
varies throughout the area and reflects a northward migration of
the deformation front during 325 Ma to 300 Ma. Comparison of the
resulting palaeolatitude of the NE Rhenish Massif with the
palaeolatitudinal drift history for the region yields an estimate
for the age of remagnetisation of ca. 315 - 300 Ma, which is in
good agreement with the age of deformation. The concordance of the
magnetic palaeoinclinations obtained from the entire area indicates
that the rocks were remagnetised during a relatively short period
of only a few My. The thermal stability of the remanence up to
550°C the comparably low palaeotemperatures in the studied region
and the short duration of the remagnetisation event favour a
chemical remagnetisation process. Rock magnetic experiments reveal
a complex magnetomineralogy of the remagnetised Palaeozoic
sediments from the NE Rhenish Massif. The dominant carrier of the
Carboniferous magnetisation component is magnetite, but pyrrhotite
and hematite accompany magnetite as carrier of the NRM in some grey
carbonates and red sandstones or red nodular limestones,
respectively. The hysteresis ratios, magnetic viscosity and low
temperature behaviour of the carbonate rocks give strong evidence
for the presence of very fine grained (superparamagnetic) magnetic
minerals. This material is also thought to be responsible for
similar rock magnetic properties of siliciclastic rocks. This
interpretation, however, is not unique for the siliciclastic rocks,
due to the predominance of detrital MD magnetite and the high
amount of paramagnetic material. The hysteresis ratios from medium
to coarse grained rocks and reef carbonates fall in or close to the
fields of MD magnetite and remagnetised carbonates, respectively.
The fine grained clastic rocks (siltstones) and limestone
turbidites have intermediate hysteresis properties. This implies
the presence of very fine grained magnetic material in all
lithologies of the NE Rhenish Massif, which is indicative for
authigenic growth of magnetic minerals and formation of a CRM.
However, the magnetic fingerprint of SP grains gets increasingly
disguised with increasing amount of detrital MD magnetite in
clastic rocks. K-Ar dating of