Identification of remagnetization processes in Paleozoic sedimentary rocks of the northeast Rhenish Massif in Germany by K-Ar dating and REE ...
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Identification of remagnetization processes in Paleozoic
sedimentary rocks of the northeast Rhenish Massif in
Germany by K-Ar dating and REE tracing of authigenic
illite and Fe oxides
A. Zwing, N. Clauer, N. Liewig, V. Bachtadse
To cite this version:
A. Zwing, N. Clauer, N. Liewig, V. Bachtadse. Identification of remagnetization processes in Paleozoic
sedimentary rocks of the northeast Rhenish Massif in Germany by K-Ar dating and REE tracing of
authigenic illite and Fe oxides. Journal of Geophysical Research : Solid Earth, American Geophysical
Union, 2009, 114, pp.B06104. �10.1029/2008JB006137�. �halsde-00510540�
HAL Id: halsde-00510540
https://hal.archives-ouvertes.fr/halsde-00510540
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CopyrightJOURNAL OF GEOPHYSICAL RESEARCH, VOL. 114, B06104, doi:10.1029/2008JB006137, 2009
Identification of remagnetization processes in Paleozoic sedimentary
rocks of the northeast Rhenish Massif in Germany by K-Ar dating and
REE tracing of authigenic illite and Fe oxides
A. Zwing,1,2 N. Clauer,3 N. Liewig,4 and V. Bachtadse1
Received 4 October 2008; revised 11 February 2009; accepted 26 February 2009; published 13 June 2009.
[1] This study combines mineralogical, chemical (rare earth elemental (REE)) and
isotopic (K-Ar) data of clay minerals as well as chemical compositions (major and REE)
of Fe oxide leachates from remagnetized Palaeozoic sedimentary rocks from NE Rhenish
Massif in Germany, for which the causes of remagnetization are not yet clear. The
dominant carrier of the syntectonic, pervasive Carboniferous magnetization is magnetite.
The Middle Devonian clastic rocks record an illitization event at 348 ± 7 Ma probably
connected to a major magmatic event in the Mid-German Crystalline Rise, whereas a
second illitization episode at 324 ± 3 Ma is coeval to the northward migrating deformation
through the Rhenish Massif, being only detected in Upper Devonian and Lower
Carboniferous rocks. The age of that younger illitization is not significantly different from
that of the remagnetization, which, however, is not restricted to the upper part of the
orogenic belt, but affects also the Middle Devonian strata. The REE patterns of the
Fe-enriched leachates support two mineralization episodes with varied oxidation-reduction
conditions outlined by varied Eu and Ce anomalies. This is not compatible with a unique,
pervasive migration of orogenic fluids on a regional scale to explain the remagnetization in the
studied region. While clay diagenesis and remagnetization are time-equivalent in Upper
Devonian and Lower Carboniferous rocks, they are not so in Middle Devonian rocks.
Transformation of smectite into illite cannot, therefore, account for the growth of associated
authigenic magnetite, which must have been triggered by a different process. Since
remagnetization and deformation ages are similar, the mechanism could relate to local
physical conditions such as pressure solution and changing pore fluid pressure due to tectonic
stress as well as to chemical conditions such as changing composition of the pore fluids.
Citation: Zwing, A., N. Clauer, N. Liewig, and V. Bachtadse (2009), Identification of remagnetization processes in Paleozoic
sedimentary rocks of the northeast Rhenish Massif in Germany by K-Ar dating and REE tracing of authigenic illite and Fe oxides,
J. Geophys. Res., 114, B06104, doi:10.1029/2008JB006137.
1. Introduction [3] It is interesting to recall that secondary magnetic
overprints are observed in almost all rock types, including
[2] Understanding of the processes by which rocks be- the major sedimentary, magmatic and metamorphic litholo-
come magnetized is most important for studies of ancient gies. While rocks of all ages appear to have been subjected
magnetization processes, regardless of their application to to remagnetization events, the Proterozoic and Paleozoic
plate tectonic modeling, stratigraphic application, or unrav- lithologies are affected even more often than the younger
eling mechanisms that control the Earth’s magnetic field. ones. The main handicap in the interpretation of such
Such processes are reasonably well known for primary secondary magnetic overprints is the uncertainty associated
magnetization recorded during rock formation, but they with their timing and duration, as timing, for instance, is
are often partly or completely erased by secondary magnetic most often only estimated by comparing the magnetic
overprints acquired any time after rock formation, which directions to existing paleomagnetic data of the same units.
impacts are still poorly understood [Elmore et al., 2001; Beside the large uncertainties associated with those age
Stamatakos et al., 1996]. estimates, the age distribution of the magnetic overprints
1
Department of Earth and Environmental Sciences, Ludwig-Maximi- reveals that most remagnetization events in Paleozoic rocks
lians-Universität, Munich, Germany.
2
occurred between 350 and 250 Ma, including the climax of
Now at Ludwig-Maximilians-Universität, Munich, Germany. the Variscan orogeny. This age span coincides with the
3
Centre de Géochimie de la Surface, Université Louis Pasteur, CNRS,
Strasbourg, France.
Permo-Carboniferous Reversed Geomagnetic Superchron
4
Département Ecologie, Physiologie et Ethologie, Institut Pluridiscipli- (PCRS), which is a long period of reverse polarity of the
naire Hubert Curien, Université Louis Pasteur, CNRS, Strasbourg, France. Earth magnetic field, approximately spanning from Namur-
ian to Late Permian [Opdyke and Channell, 1996]. Both, the
Copyright 2009 by the American Geophysical Union. long period of reverse polarity during the PCRS and the
0148-0227/09/2008JB006137$09.00
B06104 1 of 19B06104 ZWING ET AL.: IDENTIFICATION REMAGNETIZATION PROCESSES B06104
Variscan orogeny are only two among many factors that are unit of the Central European Variscides as defined by
thought to have caused Late Paleozoic remagnetization Kossmat [1927]. In more recent plate tectonic concepts,
[e.g., Stamatakos et al., 1996]. the Rhenohercynian belt is designated as the foreland belt
[4] Since the 1980s, many detailed studies were under- thrusted onto the southern margin of the Avalonian micro-
taken on remagnetized rocks to better understand the continent during the Variscan orogeny [Franke, 2000, and
inducing processes. Two types were mostly reported: references therein]. The sedimentary basin of the Rhenish
(1) physical processes during which magnetization of exist- Massif evolved from shallow marine and deltaic conditions
ing minerals is reset by temperature [e.g., Dunlop and on the southern shelf of the Old Red Continent during Early
Özdemir, 1997; Kent, 1985], its direction changing by either Devonian. Thick clastic deposits as well as red beds, root
a rotational deformation [Kodama, 1988], or a new magne- horizons and conglomerates characterize the Lower and
tization imprinted by a differential stress [e.g., Borradaile, Middle Devonian strata. During subsidence of the shelf,
1994; Hudson et al., 1989; Robion and Borradaile, 2001], neritic facies gave way to hemipelagic and pelagic environ-
and (2) chemical processes favoring the growth of new ments with localized growth of biohermal structures. Clastic
magnetic minerals or the dissolution of existing magnetic sedimentation was almost completely terminated by a trans-
grains [Hirt et al., 1993]. In many published studies, links gressive event at the Devonian-Carboniferous boundary,
between remagnetization and regional fluid flows, hydro- which led to the formation of a carbonate platform further
carbon or clay diagenesis were suggested [e.g., Brothers to the north. At the same time, the closure of the Rhenoher-
et al., 1996; Elmore et al., 2001; Jackson et al., 1988; Lu et cynian basin induced the formation of a flysch trough, which
al., 1990, 1991; McCabe and Channell, 1994; McCabe et propagated northwestward during the Early Carboniferous
al., 1983; Oliver, 1986; Suk et al., 1990, 1993; Sun and [Franke, 2000; Walliser, 1981]. Southward subduction of the
Jackson, 1994; Xu et al., 1994]. Even knowing that forma- Rhenohercynian oceanic basin beneath the Mid-German
tion of authigenic mica-type sheet silicates is common Crystalline Rise and the final collision of the bordering
during fluid-triggered hydrothermal alteration or burial continental blocks resulted in a 50% shortening (200 km)
diagenesis [Gill et al., 2002; Katz et al., 2000; Woods et of the Rhenohercynian zone [Oncken et al., 1999].
al., 2002], and although the isotopic and geochemical [7] The kinematic evolution of the Rhenohercynian fold
signatures of clay minerals can potentially constrain the belt has been studied by a variety of methods in the recent
timing of, and conditions during such events (see, for decades, including strain measurements, metamorphic pe-
example, the review by Clauer and Chaudhuri [1995]), trology and geochronology. Isotopic ages of synkinematic
systematic investigations of clay minerals in remagnetized phyllosilicates (white mica, illite) from eastern Rhenish
sedimentary rocks remain scarce [Elliott et al., 2006; Tohver Massif and outlining a northward propagation of the defor-
et al., 2008]. Furthermore, despite tests of correlative mation front, were first published by Ahrendt et al. [1983]. In
chemical remagnetization and clay authigenesis, no direct rock types with distinct cleavage, mica formation and recrys-
evidence for cogenetic growth of magnetic minerals and tallization processes are related to peak deformation and
mica-type sheet silicates has yet been reported, to the best of cleavage formation [Reuter, 1985]. More recent data [e.g.,
our knowledge. Reuter, 1987; Reuter and Dallmeyer, 1989] show that defor-
[5] The main goal of the present work is therefore a mation started at around 320 Ma to the south and migrated
contribution to a better understanding of diagenetic pro- northward until 300 Ma [Plesch and Oncken, 1999].
cesses occurring during remagnetization events, by gener- [8] Conodont alteration indices (CAI; see Epstein et al.
ating isotopic and geochemical determinations of authigenic [1977]) from Devonian and Carboniferous carbonate rocks
minerals that formed during such events. The NE Rhenish [Königshof, 1992] and vitrinite reflectance data (Rmax%; see
Massif was chosen as the geologic area, because Late Paproth and Wolf [1973]) indicate a general increase in
Paleozoic remagnetization overprinting the primary mag- paleotemperatures from diagenetic and very low-grade
netic information and occurring during very low-grade conditions (300 – 350°C). To the north, vitrinite reflectance
[1983], Wilken [1995] and Zwing et al. [2002]. In addition, is correlated with the stratigraphic layering and Rmax iso-
its geodynamic evolution and deformation history is well lines are folded and offset across faults, indicating preser-
known [Ahrendt et al., 1983; Franke, 2000; Oncken et al., vation of a thermal state related to maximum burial near the
1999; Plesch and Oncken, 1999], while the origin and the onset of deformation. In the southern Rhenish Massif, Rmax
reported remagnetization mechanisms are still unclear. Clay isolines are not folded and crosscut the stratigraphic units.
minerals from different types of remagnetized rocks previ- The average vitrinite reflectance is relatively high there
ously studied by Zwing et al. [2002] for their magnetic (5– 7% Rmax), even in the youngest pretectonic sediments.
properties, were separated and characterized for their min- Consequently, a major tectonic overburden induced synki-
eralogical, geochemical and isotopic (K-Ar) signatures. For nematic resetting of maximum paleotemperatures [Plesch
comparison of the geochemical signatures of authigenic and Oncken, 1999, and references therein] in the southern
clay and magnetic minerals, Fe oxides were leached from part of the Rhenish Massif.
clay fractions and analyzed for their major and rare earth
elemental (REE) contents. 3. Late Carboniferous Remagnetization
[9] A paleomagnetic study of Devonian and Early Car-
2. Geological Setting and Tectonic Evolution boniferous sedimentary rocks from NE Rhenish Massif
[6] The eastern Rhenish Massif is part of the Rhenohercy- outlines a Late Carboniferous remagnetization obtained
nian fold belt, forming the northernmost tectonic-stratigraphic by thermal demagnetization experiments up to 550°C
2 of 19B06104 ZWING ET AL.: IDENTIFICATION REMAGNETIZATION PROCESSES B06104
Figure 1. Geological sketch map of the NE Rhenish Massif with sampling locations. The main
structural units where sampling was carried out include: Paffrath Syncline (PS), Remscheider Anticline
(RA), Luedenscheider Syncline (LS), Attendorner Syncline (AS), and Wittgensteiner Syncline (WS). Site
numbers refer to Table 1.
(see component B in the work of Zwing et al. [2002]). Three taken at locations where paleomagnetic and rock magnetic
individual incremental regional fold tests across the studies had been carried out before (Figure 1). As in the
Remscheider Anticline (RA), Luedenscheider Syncline paleomagnetic and rock magnetic studies [Zwing et al.,
(LS), Attendorner Syncline (AS) and Wittgensteiner Syn- 2002, 2005], sampling was restricted to the northern part
cline (WS in Figure 1) show a unique and distinctive of the Rhenish Massif, where paleotemperatures are below
variation in timing of remagnetization relative to the age 300°C (CAIB06104 ZWING ET AL.: IDENTIFICATION REMAGNETIZATION PROCESSES B06104 Table 1. Location, Description, and Stratigraphic Age of the Selected Samples From NE Rhenish Massif Sample Latitude (°N) Longitude (°E) Location Lithology Stratigraphic Unit Stratigraphic Age LET 51.375 7.561 Henkhausen, near public pool cherty carbonate Kieselkalk-Horizont Tournaisian HAC 51.373 7.976 old quarry at B229, west of Hachen bituminous limest. Kulm-Plattenkalk Visean BRU 51.209 7.578 roadcut at B54, south of Brügge sandstone Honseler Schichten Givetian HAM 51.257 7.644 old quarry at L530, opposite of factory sandstone Honseler Schichten Givetian NEU 51.272 7.778 old quarry near B229, south of Neuenrade quartzitic sandstone Honseler Schichten Givetian OLP 51.366 8.164 roadcut at A46, south of Olpe greywacke Arnsberger Schichten Namurian ALB1 51.106 7.823 roadcut SW of Albringhausen siltstone Wiedenester Schichten Givetian ALB2 51.111 7.818 roadcut at L539, west of Albringhausen sandstone Selscheider Schichten Eifelian ATT 51.111 7.879 roadcut at L708, south of Attendorn gray/red sandstone U. Newberrien Schichten Givetian STC 51.137 7.986 old quarry, east of Sankt Claas limestone Massenkalk Frasnian BOH 51.024 8.387 active quarry (Fa. Böhl) near Raumland quartzitic sandst. Raumländer Schichten Eifelian LER 50.954 7.152 old quarry in park, Hotel Lerbach bituminous limest. Hombacher Schichten Frasnian UTH 51.010 7.196 old quarry at L286 near Unterthal limestone Unterer Plattenkalk Givetian the Paffrath Syncline (PA), where the paleotemperatures evaluate the degree of the diagenetic to very low-grade were found to be minimal (CAI = 1.5; Rmax = 0.9%). metamorphic impact. For quantification, the illite crystallin- Limestones and dolomites (STC) from carbonate buildups ity index (ICI) was used by measuring the full width at half (bioherms) are mainly authochtonous biolithites and dolo- maximum (FWHM) of the first illite basal reflection (10 Å) micrites [Folk, 1959]. All carbonate rocks are characterized [Kübler, 1966]. The boundaries between nonmetamorphic/ by low amounts of clay minerals. At site BOH, a green tuff anchizone and anchizone/epizone are defined at 0.58 and turned out to be much more suitable for geochemical and 0.35° 2Q, respectively, for the
B06104 ZWING ET AL.: IDENTIFICATION REMAGNETIZATION PROCESSES B06104 of the pilot samples, following a procedure close to that terized by high compaction and very low porosity. The described by Bonhomme et al. [1975]. The samples were intergranular pore spaces in the sandstones as well as the preheated to 80°C for several hours to reduce the amount of intraparticulate and vug porosities in the carbonates are atmospheric Ar absorbed on the mineral surfaces during filled with calcite and dolomite cements. Small volumes sample preparation and handling. The results were con- of free intergranular porosity were only observed in the trolled by repetitive analysis of the GL-O standard averag- greywackes. ing 24.67 ± 0.18 10 6cm3/g STP (2s) of radiogenic 40Ar [20] Two illite morphologies could be identified by SEM for 12 independent determinations during the course of the and TEM observation. Subeuhedral and irregular particles study. The atmospheric 40Ar/36Ar ratio was also measured occur as overgrowth of detrital and authigenic minerals. In periodically and averaged 300.8 ± 7.9 (2s). The K-Ar ages carbonates, illite coats large grains of authigenic kaolinite were calculated using Cox and Dalrymple’s [1967] formula (
B06104 ZWING ET AL.: IDENTIFICATION REMAGNETIZATION PROCESSES B06104
Figure 2. Scanning electron micrograph (SEM) and transmission electron micrograph (TEM). (a) SEM
image of sample LET. Abbreviations are as follows: ill, illite; kao, kaolinite; cal, calcite. (b) Backscattered
electron image of a polished section of sample OLP. Abbreviations are as follows: qz, quartz; fsp,
feldspar; Fe-ox, Fe oxide (magnetite). (c) TEM image of theB06104 ZWING ET AL.: IDENTIFICATION REMAGNETIZATION PROCESSES B06104 Table 2. Mineral Composition of the Clay Fractions and ICI Data of the Air-Dried and Glycolated
B06104 ZWING ET AL.: IDENTIFICATION REMAGNETIZATION PROCESSES B06104
Figure 3. XRD patterns of the air-driedB06104 ZWING ET AL.: IDENTIFICATION REMAGNETIZATION PROCESSES B06104
Figure 4. Plot of K-Ar apparent ages versus FWHM (illite crystallinity index) of the 10 Å illite peak.
Except for sample LER, the samples follow a general trend of increasing K-Ar apparent age with
decreasing ICI (=increasing crystallinity). See section 5.1 for additional information.
The 40Ar/36Ar intercept of the mixing line is likely to be and it yields an age of 324 ± 3 Ma, a MSWD of 3.5 and an
different from initial 40Ar/36Ar ratio of mineral parageneses, initial 40Ar/36Ar of 280 ± 35. The observation of two
which is considered to be similar to the present-day atmo- isochrons A and B with 40Ar/36Ar ratios close to the
spheric 40Ar/36Ar ratio (295.5; see Nier [1950]). As stated, present-day atmospheric value and the comparable low
meaningful isochron arrays should also have Mean Square MSWD values can be considered to reflect the presence
Weight Deviates (MSWD) close to unity, as the scatter of two generations of illite: an older in Middle Devonian
around the line should be limited. Owing to the limited clastics (identified as illite generation A) and a later in the
number of samples in a normal data set, the MSWD is often Upper Devonian and Carboniferous carbonates and grey-
larger than 1.0, and Brooks et al. [1972] believe that 2.5 is wackes (identified as illite generation B).
an acceptable cutoff for the definition of an isochron. [29] Four samples do not follow the observed trends and
However, it should be kept in mind, that natural inhomo- were discarded in the isochron calculations. The clay
geneity of sediments may also cause data scatters, and fractions of two sandstone samples (ALB1b, HAM; see
consequently higher MSWD values. Figure 5) plot clearly above isochron A. This could be
[28] Here, the K-Ar data of most samples fall on or close caused either by the occurrence of an older paragenesis of
to one of two regression lines in a 40Ar/36Ar versus K-bearing minerals or by excess 40Ar during illite formation
40
K/36Ar plot. Both regression lines yield low MSWD [Clauer, 2006, and references therein]. However, the clay
values and initial 40Ar/36Ar ratios close to the present-day fraction ALB1, which was separated from the same rock
value (295.5), which are given by the intercept of the sample as ALB1b, fits isochron A and shows no evidence
regression line with the ordinate. These regression lines for excess 40Ar during illite formation. Furthermore, such a
can, therefore, be interpreted as isochrons (isochrons A and mechanism is expected to affect other samples having a
B; see Figure 5). Isochron A consists in the data points of similar lithology such as BRU, or provenance such as
theB06104 ZWING ET AL.: IDENTIFICATION REMAGNETIZATION PROCESSES B06104
Figure 5. K-Ar isochron plot for the data of theB06104 ZWING ET AL.: IDENTIFICATION REMAGNETIZATION PROCESSES B06104 Table 4. REE Contents of the Untreated
B06104 ZWING ET AL.: IDENTIFICATION REMAGNETIZATION PROCESSES B06104
Figure 7. Relation between REE and Fe contents in the leachates.
LER was added to the group of the Upper Devonian and the variation in the Ce and Eu anomalies, which may occur in
Lower Carboniferous carbonates and greywackes defining two valence states, is not significant. However, the ATT1,
isochron B (324 ± 3 Ma) on the Figure 5. However, it should ALB1 and UTH leachates yield negative Eu anomalies, while
be remembered that the LER size fraction plots slightly above leachate LET has a negative Ce anomaly. This is thought to
isochron B in a 40Ar/36Ar versus 40K/36Ar plot. reflect different reduction/oxidation conditions during pre-
[33] The REE patterns of the leachates are characterized by cipitation of REE-carrying minerals, such as the Fe oxides
varying degrees of enrichment in MREEs (Figure 8), and and apatite.
they show significant differences among the samples char-
acterized by the two generations of illite A and B. The 5.3. Leaching Experiments of the Fe-Bearing Mineral
enrichment in MREEs is more pronounced in the leachates Phases
of the Middle Devonian clastics (generation illite A of 348 ± [34] For efficiency control of the leaching procedure, the
7 Ma), while the patterns of the leachates from Upper amount of magnetite in the samples was estimated using the
Devonian and Lower Carboniferous carbonates and grey- anhysteretic susceptibility (kan) as a proxy. While the initial
wackes are generally flatter (generation illite B of 324 ± 3 susceptibility k0 is controlled by all magnetic (ferro(i) -,
Ma). This fractionation, which can be quantitatively de- para-, and dia-magnetic) material of the sample, kan reflects
scribed by the ratio of Gd/La, is directly correlated with the only the amount of ferromagnetic minerals. It depends also
Ba content of the leachates. In the samples of group B with on grain size and is about one order of magnitude greater in
low MREE enrichment and low Ba contents, the ratio of Gd/ the 0.1 mm magnetite grains than in 1 mm grains [Dunlop
La is negatively correlated to Fe2O3 and CaO + P2O5. The and Özdemir, 1997]. Consequently, the mass-normalized
relationship in the leachates is interpreted as reflecting two anhysteretic susceptibility of a whole-rock sample is a good
mineralization processes: an early crystallization of MREE- estimation for the magnetite content in theB06104 ZWING ET AL.: IDENTIFICATION REMAGNETIZATION PROCESSES B06104
Figure 8. NASC-normalized REE patterns of the leachates fromB06104 ZWING ET AL.: IDENTIFICATION REMAGNETIZATION PROCESSES B06104
Table 6. Major and Some Trace Elemental Compositions of the Leachatesa
Sample SiO2 (wt %) Al2O3 (wt %) MgO (wt %) CaO (wt %) Fe2O3 (wt %) MnO (ppm) TiO2 (ppm) K2O (ppm) P2O5 (ppm) Ba (ppm) Th (ppb)
After Crushing and Treatment With Acetic Acid
BRU 0.10 0.64 0.10 0.25 2.16 151 15.8 903 236 189 238
HAM 0.23 0.45 0.09 0.17 0.66 97.8 2.88 818 121 41.5 113
NEU 0.25 0.48 0.13 0.24 0.87 114 16.1 601 175 41.9 161
OLP 0.23 0.75 0.10 0.13 0.65 48.4 1.54 1026 86.5 59.5 50.8
ALB2 0.25 0.36 0.08 0.21 0.76 67.2 9.40 653 236 69.1 125
STC 0.16 0.29 0.04 0.16 4.61 609 198 283 811 24.0 46.6
BOH 0.17 0.49 0.14 0.24 0.06 12.3 5.74 1422 BDLb 266 165
LER 0.38 0.34 0.17 0.48 1.38 11.7 13.8 769 215 23.4 271
UTH 0.15 0.29 0.08 0.46 3.34 539 43.5 361 885 19.8 269
After Freezing/Thawing and Crushing
LETn 0.20 0.10 0.08 0.35 2.31 114 0.09 276 1112 47.6 153
HACn 0.25 0.26 0.02 0.13 1.48 10.8 15.5 858 2826 97.2 362
ALB1n 0.32 0.44 0.06 0.26 1.84 67.7 3.58 617 1096 52.5 217
ATT1n 0.20 0.25 0.07 0.23 3.56 91.0 97.3 519 714 58.3 490
a
Amounts are in wt %, ppm, and ppb (relative to the untreatedB06104 ZWING ET AL.: IDENTIFICATION REMAGNETIZATION PROCESSES B06104 acid during the preparation phase, before leaching. Con- and the short duration of the remagnetization event, favor sequently, it can be assumed that the leachates represent a the occurrence of a chemical remagnetization process mixture of Fe oxides and other soluble minerals in the [Zwing et al., 2002]. Authigenesis of magnetic minerals
B06104 ZWING ET AL.: IDENTIFICATION REMAGNETIZATION PROCESSES B06104 is probably inherited from those synsedimentary mineral was caused by oxidizing fluids percolating from weathering deposits. The second illite formation at 324 ± 3 Ma is coeval surface in zones of enhanced permeability. to the northward migration of deformation through the Rhenish Massif, being only recorded by Upper Devonian 6.2. Interpretation of the REE Patterns From and Lower Carboniferous rocks. This indicates that the Leachates metamorphic conditions were not sufficient to recrystallize [47] The occurrence of Ba and the enrichment in MREEs the earlier illite generation in the more deeply buried Middle (Figure 8), which is most pronounced in Middle Devonian Devonian rocks. It is supported by the high ICI of the Upper clastics, indicate that early illitization was accompanied by Devonian and Lower Carboniferous rocks. However, it mineralization of Ba-rich minerals, probably barite. Ther- needs to be kept in mind that this boundary was folded mally driven fluid flows could have mobilized Ba from during the Variscan deformation, which is outlined by the sedimentary-exhalative deposits that were frequently de- vitrinite reflectance being correlated to the stratigraphic scribed along synsedimentary fault zones in the NE Rhenish units and by the Rmax isolines being folded and set off Massif (e.g., Meggen ore deposit; see Werner [1989]). The across the faults [Paproth and Wolf, 1973]. Consequently, characteristic enrichment in MREEs in the leachates with the first illite generation and the vitrinite reflectance pre- high Ba contents is probably inherited from those synsedi- served a thermal state related to the maximum burial and the mentary mineral deposits. The negative correlation of magmatic event in the Mid-German Crystalline Rise. MREE-enrichment and Fe2O3 and CaO + P2O5 contents [44] The second illitization event is not significantly in leachates of samples affected by the later illitization different from timing of the pervasive and syntectonic indicates mineralization of Fe oxides and possibly apatite remagnetization. In contrast, remagnetization was not only with flat REE patterns. This diagenetic event appears to be restricted to the upper part of the fold and thrust belt, but it coeval to the deformation and could have caused the affected also the Middle Devonian strata, erasing any chemical remagnetization in the Upper Devonian and Lower previous remagnetization possibly caused by the earlier Carboniferous units. thermal event. The younger illite generation is characterized [48] The REE patterns of the leachates from different by lower Gd/La ratios in the leachates, which are thought to lithologies in the NE Rhenish Massif show varying occur- reflect crystallization of Fe oxides and apatite with flat rences of Eu (and Ce) anomalies. This indicates varying NASC-normalized REE patterns. The regional pervasive oxidation-reduction conditions during mineralization of migration of fluids is expected to homogenize the REE soluble mineral phases such as barite, Fe oxides and apatite. signatures of the soluble minerals, but the REE patterns of A pervasive migration of fluids on a regional scale is the leachates rather suggest an interference of two mineral- expected to homogenize the REE signatures of soluble izations possibly of different ages. The REE patterns from minerals. The observation of two mineralization events of different samples also show variations of the Eu and Ce different ages, and the different oxidation-reduction con- contents indicating varied oxidation-reduction conditions in ditions during diagenesis, are against regional fluid flows in the different lithologies. This observation is against a the NE Rhenish Massif. pervasive migration of orogenic fluids on a regional scale for remagnetization in the NE Rhenish Massif. In the 7. Conclusions Ardennes Massif, an Early Permian remagnetization was related to fluid migration during formation of Mississippi [49] The present work combines mineralogy, REE chem- Valley – type deposits [Zegers et al., 2003]. The absence of istry and K-Ar isotope dating of clay minerals as well as such syn- to late-orogenic deposits in the NE Rhenish REE chemistry of Fe oxide leachates to study remagnetized Massif further supports the evidence against a regional-size sedimentary rocks from Paleozoic outcrops in the NE migration of orogenic fluids. Rhenish Massif. The results yield important implications [45] A temporal relationship between clay diagenesis and for the processes and mechanisms responsible for the Late remagnetization is observed in Upper Devonian and Lower Paleozoic remagnetization of the studied area. Carboniferous rocks. On the other hand, remagnetization is [50] 1. In the predominantly Late Devonian and Early not related to clay diagenesis in the Middle Devonian rocks, Carboniferous carbonates, clay diagenesis and remagnetiza- since the latter preserved an older diagenetic event. There- tion are coeval at 324 ± 3 Ma with respect to the main phase fore, the transformation of smectite into illite cannot account of deformation in the Late Carboniferous (320 –310 Ma). for the growth of authigenic magnetic minerals, which was In Middle Devonian clastics, authigenic illite preserved an probably triggered by another process. Since the ages of older diagenetic event at 348 ± 7 Ma, while the age of remagnetization and main deformation are similar, this remagnetization is not different from that of the remagne- mechanism could relate to local pressure solution and/or tization in the younger sequences. Formation of secondary changing pore fluid pressure induced by tectonic stress. magnetite might be linked to chemical processes associated However, this raises the question of why did remagnetiza- with the smectite-to-illite transition in the Late Devonian tion occur during different stages of folding in northern and and Early Carboniferous rocks, while a different mechanism southern NE Rhenish Massif. seems to have induced the growth of magnetite in the [46] A second remagnetization predominantly carried by Middle Devonian clastics. hematite is recorded in rocks from cores with steeply [51] 2. Leaching experiments of the
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A. Zwing, Ludwig-Maximilians-Universität, Geschwister-Scholl-Platz 1,
D-80539 Munich, Germany. (azwing@lmu.de)
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