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Zwing, Alexander (2003): Causes and Mechanisms of Remagnetisation in Palaeozoic Sedimentary Rocks - a Multidisciplinary Approach. Dissertation, LMU München: Faculty of Geosciences
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Abstract

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 <0.2µm clay fractions indicates two diagenetic events in the NE Rhenish Massif. The observation of K-Ar isochrons rules out contamination from detrital sources and preferential loss of radiogenic ^40Ar from authigenic illites. Middle Devonian clastic rocks are characterised by an illitisation event at 336 +/- 6.2 Ma, which is probably connected to a major magmatic event at ca. 340 - 330 Ma in the Mid-German Crystalline Rise. The second period of illite formation at 312 +/- 10 Ma is coeval to the northward migration of deformation through the Rhenish Massif and is only recorded by Upper Devonian and Lower Carboniferous rocks. This indicates that the metamorphic conditions were not sufficient to recrystallise the earlier illite generation in the more deeply buried Middle Devonian rocks. The age of the younger illitisation event is not significantly different from the age of the pervasive, syntectonic remagnetisation. However, the remagnetisation event was not restricted to the upper part of the fold and thrust belt and also affected the Middle Devonian strata. A characteristic enrichment in MREE is observed in Fe-oxide leachates, which is more pronounced in the Middle Devonian clastic rocks and which is correlated to the amount of Ba in the leachates. This indicates, that Ba was mobilised during the older diagenetic event and probably originates from synsedimentary (SEDEX), baryte-bearing deposits. The younger illite generation is characterised by lower Gd/La ratios in leachates, which are thought to reflect the formation of Fe-oxides and apatite with flat NASC normalised REE patterns. Consequently, the REE patterns of leachates indicate the interference of two mineralisations of different ages. Furthermore, the REE patterns from different samples show a variation of Eu and Ce, which indicates varying redox conditions in the lithologies studied. This is taken as evidence against a pervasive migration of orogenic fluids on a regional scale as a cause of remagnetisation in the NE Rhenish Massif. While a temporal relationship between clay diagenesis and remagnetisation is observed in Upper Devonian and Lower Carboniferous rocks, the remagnetisation is not related to clay diagenesis in Middle Devonian rocks. Here, the transformation of smectite to illite cannot account for the growth of authigenic magnetic minerals, which must have been triggered by a different mechanism. Since the ages of remagnetisation and main deformation are generally similar, this mechanism could be related to localised pressure solution and changing pore fluid pressure due to tectonic stress. However, this raises the question, why the remagnetisation occurred during different stages of folding in the northern and southern parts of the NE Rhenish Massif. Barrandian: In the Barrandian, Czech Republic, a remagnetisation is identified, which is predominantly carried by magnetite. Fold test analysis and comparison of the resulting palaeolatitude with the palaeomagnetic reference frame yields ambiguous results. Based on palaeomagnetic evidence, the remagnetisation event could have occurred during the late stage of the Variscan deformation in the Late Devonian or the remagnetisation could be postfolding in origin and of Late Carboniferous age. The apparent K-Ar ages of <0.2µm fractions indicate a diagenetic event around 390 - 365 Ma. This age interval is almost identical with the age of deformation in the Barrandian and the illite diagenesis is likely to be related to the deformation. The REE patterns of Fe-oxide leachates show a distinct enrichment in MREE, which is generally correlated to the Ba-content. SEM studies indicate oxidation of framboidal pyrite to magnetite in grey Late Silurian limestones and the formation of Fe-oxides in red Devonian limestones. Magnetite pseudoframboids and authigenic illite form a characteristic parageneses. Since oxidation of framboidal pyrite is a possible mechanism of remagnetisation, the K-Ar ages of illite are thought to constrain the remagnetisation to be Late Devonian in age. The SEM studies also indicate, that authigenesis of magnetic minerals in red Devonian limestones is related to dissolution of baryte. Holy Cross Mountains: The palaeomagnetic results from Late Devonian limestones from the Holy Cross Mountains, Poland, reveal a complex (re-)magnetisation history. Two remagnetisations are identified, which are postfolding in origin and were acquired in Late Paleozoic (component A) and Mesozoic (component C) times. A third component of magnetisation (B) yields synfolding results during fold test analysis. Although remagnetisations A and B are likely to be Late Palaeozoic in age, the exact timing of remanence acquisition remains unclear. Components A and B are carried by magnetite and SEM studies indicate the oxidation of framboidal pyrite to magnetite. The Triassic remagnetisation (C) resides in hematite, which is observed as secondary coatings of spherical pore spaces. The apparent K-Ar ages and isotopic compositions of <0.2µm fractions indicate a mixture of different generations of sheet silicates. These results can be explained by the presence of a illite generation younger than ca. 290 Ma years, which is contaminated by different amounts of detrital material (muscovite) or an older authigenic illite generation. However, diffusive loss of ^{40}Ar from the illite crystal lattice subsequent to illite formation cannot be ruled out. Further studies are needed to better understand the diagenetic history of the Late Devonian limestones in the Holy Cross Mountains. Remagnetisation processes: The results of this study imply, that the processes and mechanism responsible for the remagnetisations in the areas studied are rather complex. The regional migration of orogenic-type fluids, which is thought to be responsible for widespread remagnetisations in Palaeozoic rocks of the Hercynian realm of North America (Stamatakos et al., 1996 and references therein), can be excluded for the NE Rhenish Massif and is not supported by the observations made in rocks from the Barrandian and the Holy Cross Mountains. Chemical changes associated with the smectite/illite transition could be responsible for the remagnetisation of Late Devonian and early Carboniferous rocks from the NE Rhenish Massif. In limestones from the Barrandian and the Holy Cross Mountains, the observation of pseudoframboidal magnetite indicates the oxidation of pyrite to magnetite as a possible remagnetisation mechanism. This process requires the presence of a fluid phase, which could originate from pore fluids or local migration of fluids on fractures and faults. In the Middle Devonian sequences of the NE Rhenish Massif the illite generation and the remagnetisation are not contemporaneous and oxidation of pyrite was not observed. Here, the remagnetisation must be related to a different mechanism. It can be speculated, that the remagnetisation mechanism in the Middle Devonian sequences could be related to pressure solution and changing pore fluid pressure during deformation. The Mesozoic remagnetisations in the NE Rhenish Massif and the Holy Cross Mountains are carried by hematite and are either related to hematite bearing mineralisation events during phases of uplift in the Mesozoic or caused by oxidising fluids percolating from the weathering surface and penetrating zones of enhanced permeability.