A nonconformity refers to a hiatal surface located
between metamorphic or igneous rocks and overlying sedimentary or volcanic
rocks. These surfaces are key features with respect to understanding the relations among
climate, lithosphere and tectonic movements during ancient times. In this
study, the petrological, mineralogical and geochemical characteristics of
Variscan basement rock as well as its overlying Permian volcano-sedimentary
succession from a drill core in the Sprendlinger Horst, Germany, are analyzed
by means of polarization microscopy, and environmental scanning electron
microscope, X-Ray diffraction, X-ray fluorescence and inductively coupled
plasma mass spectrometry analyses. In the gabbroic diorite of the basement,
the intensity of micro- and macro-fractures increases towards the top,
indicating an intense physical weathering. The overlying Permian volcanic
rock is a basaltic andesite that shows less intense physical weathering
compared with the gabbroic diorite. In both segments, secondary minerals are
dominated by illite and a mixed-layer phase of illite and smectite (I–S). The
corrected chemical index of alteration (CIA) and the plagioclase index of
alteration (PIA) indicate an intermediate to unweathered degree in the
gabbroic diorite and an extreme to unweathered degree in the basaltic
andesite. The
Nonconformities refer to contact surfaces between different lithologies in the geological record that were produced over long-lasting periods of non-deposition and/or erosion and are of paramount importance for the subdivision and correlation of stratigraphic successions (Catuneanu, 1996). They also largely control the geometry of reservoirs for oil, gas and water (Gardner, 1940). Moreover, nonconformities play a key role in understanding changes in past interactions of the atmosphere, hydrosphere and lithosphere as well as in elucidating driving mechanism for the adaption and evolution of life on Earth (Fedo et al., 1995; Nesbitt and Young, 1989; Panahi et al., 2000) According to Catuneanu (1996), stratigraphic sequences and bounding surfaces are assigned to different orders based on their relative importance, which is also known as a sequence hierarchy.
Especially for continental nonconformities, the buried paleo-weathered surfaces provide an ideal opportunity to analyze the weathering and climate conditions during exposure (Jian et al., 2019; Zhou et al., 2017). This includes the alteration and deformation of minerals, such as changes in crystal morphology of primary and secondary minerals during the weathering process, which is also called supergene alteration (Borrelli et al., 2014). After the paleo-weathered surface has been covered by sediments or volcanic rocks, burial commences, leading to an increase in temperature and pressure as well as the passage of diagenetic fluids. This second overprint during deep burial diagenesis is called hypergene alteration and has to be carefully distinguished from the primary supergene alteration (Dill, 2010).
The widespread post-Variscan nonconformity represents an important first-order bounding surface within the central and western European strata. The nonconformity is a result of the denudation of the Variscan orogen which mainly took place from the late Carboniferous to the early Permian (Henk, 1995; McCann, 1999; McCann et al., 2006; Zeh and Brätz, 2004). Locally, however, the contact surface was overlain during the Triassic period. To date, the tectonic evolution in central Europe during post-Variscan times has been well studied (Matte, 1991; Ziegler et al., 2004). However, studies on weathering during the Permo-Carboniferous are fairly scarce. On the other hand, climate and paleoenvironmental conditions are well known from coal-bearing paralic and lacustrine sediments in the sub-Variscan foredeep and post-Variscan intramontane basins which indicate an overall aridification trend from humid conditions in the Westphalian to hyperarid conditions in the Guadalupian (Upper Rotliegend). From the Lopingian, the climate turned back to semiarid conditions (Roscher and Schneider, 2006).
For the reconstruction of the weathering conditions and the paleoclimate, fine-grained sediments such as siltstone or mudstone are usually investigated (Nesbitt and Young, 1982; Singer, 1988). However, caution is needed as these sediments may be multi-sourced, recycled, and/or overprinted during transport and sedimentation (Fedo et al., 1995; Jian et al., 2019). To avoid this, it is feasible to analyze the weathering profile of igneous or metamorphic rocks in the source area itself. With this approach, more accurate in situ information regarding the weathering conditions during a certain period can be acquired. This approach also applies to the diagenetic history of the rocks that are situated in direct proximity to the post-Variscan nonconformity (Dill, 2010). The Sprendlinger Horst (Hesse, Germany) is a key area in southwestern Germany for investigating the rock alteration processes at the post-Variscan nonconformity, as plutonic Variscan basement rocks in this area are widely covered by only a thin layer of Cisuralian volcano-sedimentary rocks and the contact surface has been penetrated by numerous drilling efforts (Kirsch et al., 1988).
For this study, we selected a representative drill core reaching from unweathered basement rock into the volcanic-sedimentary cover that was analyzed at high resolution, in particular near the nonconformity. This drill core allowed not only for the study of the alteration in Variscan basement rocks but also for the study of the subsequent alteration of overlying early Permian basalt. We carried out a detailed petrographical, mineralogical, and geochemical study and applied a new workflow to distinguish the supergene and hypogene alteration processes within a first-order nonconformity. The workflow is based on normalizing mineral types and geochemical weathering indices to un-overprinted conditions and quantifying the observed deviations. With corrected geochemical and mineral information of the weathered profile, the weathering and paleoclimatic condition as well as the alteration scenario were addressed.
The Variscan orogen in central Europe was formed due to the collision of the Gondwana and Laurussia mega-continents and intervening microplates, namely Avalonia and Armorica. The final assemblage of these continents led to the formation of Pangaea between ca. 360 and 320 Ma during the Carboniferous (Powell and Conaghan, 1973; Schulmann et al., 2014). Due to a southward-directed subduction of the oceanic lithosphere below the Armorica microplate, the so-called Mid-German Crystalline Zone (MGCZ) was formed as a magmatic arc at the northern margin of Armorica during the Early Carboniferous; as a highland, this region continuously weathered and eroded until thermal subsidence began to dominate in central Europe (von Seckendorff et al., 2004a; Willner et al., 1991; Zeh and Brätz, 2004; Zeh and Gerdes, 2010). As a consequence, the post-Variscan nonconformity was formed, which represents a diachronous time gap of multiple tens to hundreds of millions years in central Europe (Henk, 1995; Kroner et al., 2007; von Seckendorff et al., 2004b; Zeh and Brätz, 2004). The weathering surface was covered and, hence, preserved by Permian sedimentary or volcanic rocks summarized as the Rotliegend Group (Becker et al., 2012; Korsch and Schzfer, 1991; Stollhofen, 1998).
The Odenwald basement is the largest basement window of the MGCZ and
consists of two major parts that are separated by the Otzberg shear zone:
the Bergsträsser Odenwald in the west and the Böllstein Odenwald in
the east (Zeh and Gerdes, 2010).
The Bergsträsser Odenwald is subdivided into three tectonic units which
are composed of magmatic and metamorphic rocks. These are, ordered from
north to south, Unit I, which includes the gabbro-dioritic Frankenstein
Complex; Unit II with the so-called Neunkirchen Magmatic Suite; and Unit III,
which is dominated by large intrusive bodies of the Weschnitz, Heidelberg
and Tromm plutons (Dörr and
Stein, 2019; Zeh and Will, 2008). The basement rocks of the so-called
Cenozoic Sprendlinger Horst belong to Unit I and represent a northern
extension of the Odenwald basement, consisting of amphibolite, granite,
diorite, gabbroic diorite and gabbro. Geochronological investigations of
crystalline rocks of Unit I yield an emplacement age of 362
The Cenozoic Sprendlinger Horst constituted a structural barrier between the
nearby Saar–Nahe Basin in the west and the Hessian Basin in the east from
the early Cisuralian (McCann, 1999). In the Saar–Nahe Basin, a subsidence rate of approximately 0.26 mm/a has been
revealed by backstripping analyses for the time between the Namurian and the
Saxonian (Schäfer, 2011). The oldest
sedimentary rocks of the Sprendlinger Horst are represented by the Moret
Formation (Becker et al., 2012). The Permian Moret
Formation deposited in an alluvial environment mostly in wadi-like systems
that contain poorly sorted conglomerates, pelites, coarse-grained
sandstones/wackes and breccias. The fluvial sedimentary rocks of the
overlying Lower Langen Formation are interbedded with basalts and basaltic
andesites. These volcanic rocks are the product of a Permo-Carboniferous
volcanism which took place throughout central Europe.
During the Permo-Carboniferous, due to continental climate conditions within the supercontinent Pangaea, the paleoclimate in central Europe graduated from humid to hyperarid conditions (Parrish, 1993). In the Permian and Triassic, only the margins of the supercontinent attracted monsoonal rainfall and showed semiarid to subhumid conditions (Parrish, 1993, 1995). The overall aridization is superimposed by several wet phases, namely the Westphalian C/D, the Stephanian A (303.6 to 301.7 Ma), the Stephanian C to early lower Rotliegend (299.1 to 295.5 Ma) and the early upper Rotliegend I wet phase (291 to 287 Ma), respectively. These “wet phases” can be observed in the whole European region and are thought to be linked to deglaciation events of the Gondwana ice cap (Roscher and Schneider, 2006).
During the Mesozoic, the tectonic activity in central Europe was relatively
low and was accompanied by continuous subsidence and marine transgression. In
this phase, around 1500 m of sediment accumulated, which overlaid the
Variscan basement and/or Permo-Carboniferous sediments and volcanic rocks
(Timar-Geng et al., 2006). The maximum
thickness of overburden is expected for the Jurassic
(Schäfer, 2011). This is also the period
of maximum heating and hydrothermal activity, which overprinted both the
Variscan crystalline basement and the overlain sediments and volcanics
locally. For this period, the formation of hydrothermal ores in central
and southwest Germany is also well documented
(Bons
et al., 2014; Staude et al., 2011; Timar-Geng et al., 2004). Based on
apatite fission track analysis in crystalline rocks of the Odenwald
basement, Wagner et al. (1990) estimated temperatures of more than
130
During the late Cretaceous and the Eocene, coupled with compressional intraplate stress of the Alpine Orogeny, the Upper Rhine Graben system was formed (Behrmann et al., 2005). In conjunction with the formation of the Upper Rhine Graben, the Sprendlinger Horst was formed. The latter is bounded by the Rhine Graben fault system in the west and by the Gersprenz Graben in the east. Most of the sediments that overlaid the post-Variscan crystalline basement in the research area have been eroded since the Cretaceous (Mezger et al., 2013; Schwarz and Henk, 2005). In the nearby southeastern Odenwald region, they are partly conserved and reach a thickness of 500 m for the Lower Triassic Buntsandstein (Marell, 1989). On the Sprendlinger Horst, only Permian volcano-sedimentary rocks remained which decrease in thickness from north to south of Darmstadt from 250 to 0 m (Marell, 1989; Mezger et al., 2013)
Numerous drill cores in the Sprendlinger Horst were acquired by a scientific drilling project between 1996 and 2001. Many of the drill cores expose the post-Variscan nonconformity in shallow depths of up to 80 m below ground surface and, thus, provide a unique chance to investigate this paleo-surface at local scales. Along these drill cores, core GA1 was selected because it exposes three different lithological units, namely the plutonic basement at the bottom, the Permian volcanic lava in the middle and the overlying Permian Rotliegend sedimentary rocks at the top. The sedimentary rocks show a gradual transition from alluvial facies at the base to fluvial facies at the top (Fig. 1) and mainly consist of matrix-rich breccias, wackes and siltstones. Both the top of the basement rock and the top of the volcanic lava constitute paleo-surfaces that faced intense alteration throughout their exposure, which is thought to be significantly shorter for the volcanic rocks. The macroscopic alteration underneath these surfaces is intense; this is expressed by a higher degree of fracturing, bleaching and grain disaggregation. The degree of macroscopic alteration decreases with increasing depth in these parts. The core offers the unique opportunity to study subsequent weathering intervals and to compare typical end-members of rock types with weak (gabbroic diorite) and strong (andesitic basalt) vulnerability to chemical weathering.
Location and geology of the research area, and the lithological section of the GA1 drill core; note the gap between 30 and 54 m drilling depth. Blue stars indicate the sampling locations.
In total, 24 samples were extracted from the GA1 drill core: 11 belonged to the basement, 6 belonged to the overlying volcanic rock (Fig. 1) and 7 belonged to the sedimentary rocks of the Lower Langen Formation. In order to capture the small-scale petrographic and geochemical variations in the weathering zones appropriately, we reduced the sampling interval in the topmost part of the volcanic rock and in the basement to 40 cm; for the rest of the core, the interval was around 1 m in order to avoid the fractures.
The samples were used to prepare thin sections that were analyzed by
polarization microscopy and SEM–EDX (scanning electron microscopy–energy dispersive X-Ray analysis) for their petrographic characteristics.
For mineral composition, trace element and major element analyses, all
samples were crushed and milled into a powder with a diameter of less than
63
The powder samples were sent to the State Key Laboratory of Isotope
Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of
Sciences, for the examination of major elements by X-ray fluorescence (XRF) and the
examination of trace elements by inductively coupled plasma mass
spectrometry (ICP-MS). Before XRF analysis, samples were roasted at
900
For trace and rare earth element analysis, 40 mg of sample powder was
weighed and placed into high-pressure-resistant Savillex Teflon beakers to
which 0.8 mL
The Variscan basement in drill core GA1 consists of a coarse-grained plutonic rock with a conspicuous salt-and-pepper appearance and a phaneritic texture. Fractures are pervasive from 20.6 to 21.4 and from 21.7 to 22.3 m. The width of the fractures is around 1 cm. From 22.3 to 23.4 m, both fracture density and width gradually reduce downwards. Nearly all fractures are filled by secondary minerals (Fig. S1). Under the microscope, the fresh parts of the plutonic basement rock mainly consist of plagioclase (oligoclase and labradorite), quartz, biotite and amphibole (Fig. 2a). With decreasing depth, primary minerals such as plagioclase and biotite were gradually altered and transformed into secondary minerals; fractures are also ubiquitous in the thin sections in the topmost part. Primary grain shapes are distorted and most of them are filled with calcite (Fig. 2b). Moreover, recrystallized quartz coupled with calcite is found filled in the fractures (Fig. 2c). Some of the fractures are filled by dolomite accompanied with quartz (Fig. 2d). Even in the topmost part (20.6 m), the plagioclase grains are only partly altered (Fig. 2e).
Petrographic characteristic of the basement and the
overlying volcanic rock in the GA1 drill core:
The lava has a phaneritic, amygdaloidal texture. Fractures in this part are very limited and occur between 13.8 and 14.7 m with a high angle to the horizon. The width is less than 1 mm, and the fractures are also filled with secondary minerals (Fig. S1). The fresh part of the volcanic rock mainly consists of plagioclase (albite) crystals and amygdaloid bodies that are made up of calcite and zeolite locally accompanied with chalcedony (Fig. 2f). With a decrease in depth, the content of plagioclase gradually decreases under the microscope. However, the thin section in the topmost part of the lava (13.9 m) shows a sudden change compared with the samples from lower parts. Here, nearly all primary minerals were altered to secondary clay minerals, but the primary grain shapes are still relatively intact (Fig. 2g). Some residual grains consist of a kaolinite rim and a core of illite or illite–smectite (I–S) mixed layers (Fig. 2h). In the void, adularia with kaolinite can be observed (Fig. 2i).
The XRD results are listed in the Supplement (Table S1) and are plotted in Fig. 3. The plutonic basement part is composed of plagioclase (oligoclase and labradorite), K-feldspar (only in one sample), quartz, amphibole (mainly Mg-hornblende) and mica phases, with secondary minerals of illite (not separable from micas), vermiculite, I–S mixed layers, minor kaolinite, anatase, hematite, calcite and dolomite. Amphibole abundance decreases in the middle section and disappears in the uppermost part. In the topmost portion (20.6–23.5 m), amphibole is not found. Furthermore, plagioclase decreases in abundance from around 40 % at the bottom to about 8 % at the top. Considering the mineralogical composition of the fresher parts of the plutonic rock, based on the quartz, alkali feldspar, plagioclase, feldspathoid (QAPF) diagram for plutonic rocks, the protolith of the basement rock can be classified as a quartz diorite/tonalite (Fig. S2), which corresponds well with Mezger et al. (2013).
Mineral compositions of both Permian volcanic lava and Paleozoic basement rock in the GA1 drill core (measured by powder XRD).
The fresh samples of the volcanic rock are composed of augitic pyroxene and plagioclase (albite). In the weathered part, most of the phases appear as secondary minerals, such as quartz, hematite and anatase; clay minerals, such as illite, mixed layer illite-smectite (I–S), vermiculite and kaolinite; and carbonate minerals, like calcite and minor dolomite. The uppermost part of the lava is dominated by I–S mixed-layer minerals, the plagioclase content is less than 5 % and pyroxene is absent. The abundance of plagioclase (and pyroxene) gradually decreases from bottom to top, and illite and vermiculite exhibit an increasing tendency (Fig. 3). Based on the QAPF diagram for volcanic rocks, the protolith of the weathered volcanic rock can be classified as a basalt or andesite (Fig. S2).
In order to further verify the lithological type of the plutonic and volcanic rock for the basement part of the GA1 drill core, the geochemical data from comparatively fresh samples (23.5–56.5 m) are plotted in the total alkali silica (TAS) diagram (Middlemost, 1994) (Fig. 4). Here, the results mainly plot in the gabbroic diorite field (Fig. 4a), which is grossly consistent with the results from the petrographic classification (Fig. S2). For the volcanic rock classification, a revised Winchester–Floyd diagram is applied, which is based on immobile trace elements (Pearce, 1996). Most lava samples fall into the andesite/basaltic andesite field (Fig. 4b), which is also in accordance with the petrographic classification result. In the following, we use the result of chemical classification and term the two protolith rocks as gabbroic diorite and basaltic andesite.
Classification of rocks of the GA1 well with a TAS diagram
and revised Winchester–Floyd diagram:
The concentrations of major elements are listed in the Supplement
(Table S2) and are visualized in 1-D profiles for both basaltic andesite (Fig. 5a) and gabbroic diorite (Fig. 5b). Within the lower relatively fresh part
of the gabbroic diorite, major elements such as K
Major element content (in wt %) of the basaltic
andesite
Concerning the basaltic andesite, there are some clear trends with
increasing Al
Trace element data are also given in the Supplement (Table S3). Variations in representative trace elements from both gabbroic diorite and basaltic andesite are shown in Fig. 6. In the gabbroic diorite, except for the topmost part, fluctuations in high-field-strength elements (HFSEs) such as Zr, Hf, Nb, Ta and Th (in parts per million, ppm) are limited or even constant. For large-ion lithophile elements (LILEs), Rb and Cs decrease from top to the bottom, whereas Sr increases from 61 to 348 ppm. Ba fluctuates in the upper part of the gabbroic diorite section but is almost constant in the lower part.
Representative trace element content (in ppm) of basaltic
andesite
Concerning the basaltic andesite part, HFSEs such as Zr, Hf, Nb, Ta and Th (in ppm) all exhibit an increasing tendency from the bottom to the top, with a sharp increase in the topmost sample. LILEs such as Rb and Cs also show an increasing tendency, whereas Sr reveals an inverse trend, with a decrease from 123 ppm at the bottom to 47 ppm in the topmost part. The tendency for Ba is irregular compared with the other elements, but the overall trend is decreasing.
REE concentrations are listed in the Supplement (Table S4) and are
shown as chondrite-normalized patterns
(McDonough and Sun, 1995) in Fig. 7.
The calculation for the anomalies of cerium (Ce) and europium (Eu) are
defined as follows:
The REE pattern of basaltic andesite
During chemical weathering, alkalis and alkaline earth elements contained in
silicates such as feldspar, mica minerals, pyroxene and amphibole will be
gradually depleted, whereas aluminum tends to remain in situ, generating clay
minerals
(Clift
et al., 2014; Nesbitt and Young, 1982; Vázquez et al., 2016). Based on
this mechanism, different types of weathering indices were developed to
evaluate the weathering intensity. Among these, the chemical index of
alteration (CIA) and the plagioclase index of alteration (PIA) were proposed by
Nesbitt and Young (1982) and
Fedo et al. (1995), respectively. They are
defined as
In our case, the CIA and PIA decrease from top to bottom for both gabbroic diorite and basaltic andesite. The PIA values are clearly higher than the CIA values (Fig. 8a). In the topmost basaltic andesite part, the CIA is up to 77, which indicates an intermediate degree of weathering. The results from XRD and backscattered electron microscopy (BSE), however, indicate that the plagioclase is weathered and 74 % of the constituent is I–S. This is well expressed by the PIA, which yields a value of 98; hence, the PIA is more consistent with the mineralogical and petrographic character than the CIA.
Weathering indices with an A–CN–K diagram of both basaltic andesite and gabbroic diorite before and after K correction, based on Fedo et al. (1995) and Nesbitt and Markovics (1997).
To better evaluate the weathering intensity, an A–CN–K ternary diagram is
applied (Fedo et
al., 1995; Nesbitt and Young, 1984). The letter A stands for
Al
With corrected K
The corrected CIA values in the basaltic andesite suggest an extreme to
incipient weathering degree, with the CIA
For quantification of element transfer due to weathering and diagenesis, the
The results for major and trace elements are listed in the Supplement (Table 5) and are plotted in Fig. 9. In the basaltic andesite
part, both Ca and Na are strongly depleted in the topmost section (13.9 m), with
Element characteristics:
The
Based on K
In contrast to Ca and Na, elements closely related to clay formation such as
K, Rb and Cs show significant enrichment (Fig. 6). In the gabbroic diorite
part, the
To search for the origin of this enrichment, correlation diagrams for gabbroic diorite, basaltic andesite and sediments are plotted (Fig. 9c). The linear and close relationships between Cs, Rb and K in the gabbroic diorite and basaltic andesite point to a joint alteration of both, whereas the overlying sediments can be excluded as a source. This is consistent with the conclusion of Molenaar et al. (2015), who claimed that the overlain Permian Rotliegend sediment on Sprendlinger Horst formed a “closed system” and diagenetic fluids did not transfer matter in and out of the system. Palmer and Edmond (1989) claimed that mobile elements such as K, Rb and Cs are very easily extracted by thermal fluids and transferred during hydrothermal activity. In addition to clay transformation, typical minerals formed from hot fluids are observed in thin sections and XRD, such as dolomite accompanied by secondary quartz and adularia (Fig. 3). This observation supports the model of a second alteration that was pervasive through the nonconformity and must have happened during burial diagenesis.
Figure 10 displays the
In both gabbroic diorite and basaltic andesite, the
The high-field-strength elements (HFSEs), Zr, Nb, Hf, Ta and Th, are expected
to be immobile. In the gabbroic diorite, the
In addition to correcting the alteration trend, the A–CN–K diagram can also be used to kinetically predict the primary weathering products of plutonic and volcanic rocks (Nesbitt and Young, 1984; Panahi et al., 2000). In this case, this concept is applied to differentiate surface weathering from burial diagenesis by comparing the remaining secondary minerals in the profile with the theoretical weathering products.
Due to the chemical composition restriction, rock types that contain only minor amounts of K tend to be weathered directly by forming smectite and kaolinite instead of illite regardless of the climate (Nesbitt and Young, 1989). In our case, both gabbroic diorite and basaltic andesite contain minor K. However, secondary minerals mainly consist of illite which makes metasomatic addition of this element during burial diagenesis highly probable (Fedo et al., 1995). As discussed in Sect. 5.2, K metasomatism is possibly promoted by hydrothermal fluids. According to the A–CN–K diagram (Fig. 11), the initial weathering products of the gabbroic diorite should have mainly consisted of smectite, and a small quantity of kaolinite is expected in the top part (20.6–21.5 m). For the basaltic andesite (13.9 m), kaolinite with a small portion of smectite is expected, whereas smectite should be dominant in the lower part. ESEM (environment scanning electron microscopy) indicated kaolinite in two morphologies: vermiform (Fig. 2h) and booklet form (Fig. 2i). According to Chen et al. (2001) and Erkoyun and Kadir (2011), vermiform kaolinite is favored during the in situ formation of kaolinite, whereas the euhedral booklet form is favored during autogenic diagenesis (e.g., Bauluz et al. (2008). Kaolinite formed by chemical weathering is always more anhedral (Bauluz et al., 2008; Varajao et al., 2001); therefore, the influence of subrecent surface-related weathering can be excluded in our case. Based on the XRD results of the gabbroic diorite (Fig. 3), the remaining mineral in the topmost part (13.9 m) of the basaltic andesite is I–S, which can be explained by the conversion of kaolinite and smectite into I–S.
Theoretical weathering trend during burial diagenesis (Fedo et al., 1995): “a” is the observed weathering trend for both basaltic andesite and gabbroic diorite, “b” is the theoretical weathering trend for basaltic andesite, “c” is the K-metasomatism trend of the topmost basaltic andesite, “d” is the K-metasomatism trend of the lower part of basaltic andesite, “e” is the theoretical weathering trend for gabbroic diorite and “f” is the K-metasomatism trend for gabbroic diorite.
Na and Ca, as needed for the I–S formation, may originate from the original
smectite or from diagenesis fluids. The
The occurrence of weathering products, such as illite, smectite and kaolinite, can be applied as a useful tool to assess the paleoclimate (Clift et al., 2014; Raucsik and Varga, 2008; Singer, 1988). However, caution should be used when working with these parameters as clay minerals may be overprinted by transformation or neoformation during burial diagenesis, and there is also the restriction due to the geochemical composition of rock type, as discussed in Sect. 5.3. It follows that, when the clay minerals in the sedimentary rocks are investigated to assess paleoclimatic conditions, the lithology of the source rocks should be considered. In particular, for the weathering profile of the igneous rock, all of the involved processes mentioned above will lead to misjudgments regarding the paleoclimate conditions if working with raw data. Taking this study as an example and interpreting the original data from XRD and SEM analyses, I–S clay minerals would indicate a subhumid climate with prominent dry seasons (Raucsik and Varga, 2008; Singer, 1988). For the deeper parts of both basaltic andesite and gabbroic diorite, the clay minerals are dominated by illite, pointing to a cold or dry climate. According to this information, the profile would suggest that the climate alternated twice from a cold and dry climate to a seasonal and alternating wet and dry climate. However, due to the limited K availability, the dominant illite in gabbroic diorite and basaltic andesite must be a product of a diagenetic overprint. When correcting the A–CN–K diagram, the primary weathering product in the topmost part of the gabbroic diorite must have been smectite with negligible illite and possibly a small quantity of kaolinite (20.6–21.5 m). Furthermore, smectite can also not be applied to evaluate the paleoclimatic conditions in this case due to the K limitation of the lithology.
Although the CIA and PIA values may be misleading sometimes, they display
the alteration intensity of the gabbroic diorite well, with much higher values in
the topmost part (20.6–21.5 m). However, a related tendency of HFSEs is
nonexistent (Fig. 6b). Due to leaching, these immobile elements are
expected to become indirectly enriched, which is not observed. HFSE values
and
Correction of the A–CN–K diagram for the basaltic andesite section suggests
that primary products of the altered basaltic andesite were dominated by kaolinite in the topmost part
(13.9 m) and mainly by
smectite in the lower part. Again, smectite cannot be applied as a climate parameter due to
the restriction of the K content in basaltic andesite. Similar to the gabbroic
diorite section, the CIA and PIA values, the mineral abundances and the
petrographic features significantly change between the topmost part (13.9 m)
and the lower part (14.3–19.3 m). In contrast to the gabbroic diorite,
however, the relative content of high-field-strength elements (HFSEs), such
as Nb, Ta, Zr, Hf and Ti, are all drastically shifted between the topmost
part (13.9 m) and the lower part (Fig. 6a). The abnormal relative
concentrations of these elements in the topmost part indicate more depletion
of other relatively more mobile elements, which is confirmed by the
As mentioned in Sect. 5.2, the weathering environment for the basaltic
andesite was more acidic than for the gabbroic diorite. The acid present in
the weathered profile can be attributed to CO
Reconstructed alteration model of the GA1 well in Sprendlinger Horst before volcanic eruption around 300 Ma; The abbreviations used in the figure are as follows: Anat – anatase, Bt – biotite, Chl – chlorite, Hbl – hornblende, Hem – hematite, Ill – illite, Pl – plagioclase, Sm – smectite and Vrm – vermiculite.
Reconstructed alteration model of the GA1 well in Sprendlinger Horst after volcanic eruption around 290 Ma. Px denotes pyroxene; other abbreviations are given in the caption of Fig. 12.
Reconstructed alteration model of the GA1 well in Sprendlinger Horst during burial in the Jurassic and Cretaceous. Adl denotes adularia; other abbreviations are given in the caption of Fig. 12.
As the weathering process and the paleoclimate is elucidated, the overall
alteration process at the post-Variscan nonconformity can be separated into
three subsequent steps (Figs. 12–14). Approximately at the
Carboniferous–Permian boundary, the gabbroic diorite was firstly weathered
under relatively arid conditions. This included fracturing by physical
weathering and moderate chemical weathering. Plagioclase was transformed to
smectite with negligible illite (Fig. 12, Eqs. 1–2). Other minerals such
as amphibole and biotite were weathered to smectite and chlorite accompanied
by the generation of hematite and vermiculite (Fig. 12, Eqs. 3–5). With the
beginning of volcanism in the early Permian, the nonconformity was concealed
by the basaltic andesite lava flow, which underwent a short but intense
period of chemical weathering. Firstly, pyroxene and plagioclase were
weathered to vermiculite and smectite (Fig. 13, Eqs. 6–7). Thereafter,
more humid conditions initiated increased leaching and smectite was
transformed to kaolinite (Fig. 13, Eq. 8). During these two stages, elements
such as Na, Ca and K were depleted from the system, either by export or
descendent enrichment in the profile. After a relatively short time
interval, the basalt was concealed by sediments and the weathering process
terminated. During burial diagenesis, fluids transformed smectite and
kaolinite into illite in both gabbroic diorite and basaltic andesite (Fig. 13, Eqs. 9–10). The transformation of smectite to illite led to depletion
of Ca and Na as well. This leaching process is accompanied by the formation
of accessory minerals such as quartz, dolomite, calcite and adularia (Fig. 14, Eqs. 11–12), which indicate a temperature of around 200
A combined study of mineralogy, petrography and geochemistry was performed on a drill core that penetrates the post-Variscan nonconformity on the Sprendlinger Horst (southwestern Germany). The aim of this study was to elucidate rock alteration at and across the nonconformity and to disentangle surficial weathering and its overprint by burial diagenesis. The unconformity is covered by a Permian lava flow followed by alluvial sediments of the Rotliegend. This allows for the study of two different lithologies and two subsequent periods of surficial weathering as well as the burial diagenesis affecting both in a later stage. The crystalline basement is composed of a gabbroic diorite, whereas the lava flow is a basaltic andesite.
In the gabbroic diorite, mineralogical and geochemical parameters show a gradual alteration trend with a maximum depth of around 10 m, whereas the andesitic basalt shows a shallower and more intense alteration with complete chemical alteration in the topmost part. Chemical alteration goes along with physical alteration which is evident from the fracture density. The different alteration steps were separated using thin section analysis, clay mineralogy and geochemistry. In addition, the trends of the A–CN–K diagram and element transfer ratios were used to determine pristine rock compositions.
Surficial weathering of both gabbroic diorite and basaltic andesite are all indicated by petrographic characteristics, increasing abundance of secondary minerals, increasing CIA and PIA values, and enrichment of immobile HFSEs due to the leaching process. In the gabbroic diorite, ubiquitous now filled fractures with a width of around 1 cm in the upmost part (20.6–21.4 m) suggest intense physical weathering combined with chemical weathering, preferentially along these fractures. The corrected clay mineral composition yields smectite with negligible illite and possibly a small quantity of kaolinite, pointing to an arid climate. Physical weathering in the basaltic andesite is much weaker compared with the gabbroic diorite. An overprint by burial diagenesis is indicated by K metasomatism, clay mineral transformation and neoformation of minerals, as well as an enrichment of K, Rb and Cs in the alteration zone. In both the gabbroic diorite and andesitic basalt, the primary weathering products such as smectite and kaolinite were transformed into illite, and this will also have some influences on the evolution of weathering intensity.
At both nonconformities, distinct gradients from the top downward demonstrate that surficial weathering is the major alteration process. During burial diagenesis, fluids preferentially percolated along the post-Variscan nonconformity and at the basalt–sediment boundary, due to a higher permeability. This led to clay mineral transformation and some neoformation of minerals, but it did not change the alteration pattern. Deeper parts of both parent rocks are pristine and are not affected by surficial weathering nor by fluids during burial diagenesis.
Our case study shows that surface weathering in the past is a primary control on the petrography and geochemistry and also guides fluids through the system during burial diagenesis. Moreover, we could demonstrate that the formation of the saprolite zone depends on rock composition, climatic conditions and the duration of the process. Our results have implications for paleoclimatic and burial diagenetic studies. In order to separate hypergene and supergene alteration, we provide a workflow for nonconformities and shed light on the use of paleo-weathering surfaces for paleoclimate research.
The PDF-2 and PDF-4 powder diffraction files applied in this paper are available from the International Centre of Diffraction Data at
All of the original data presented in this study are documented in the Supplement related to this paper.
All samples are available at the Institute of Applied Geoscience, TU Darmstadt, and can be requested from liang@geo.tu-darmstadt.de.
The supplement related to this article is available online at:
LF conceptualized and prepared the paper, NJ provided the foundation and contributed to the conceptualization, DS and RP conducted the SEM and XRD measurements, and MH and AL supervised this research.
The authors declare that they have no conflict of interest.
The authors would like to thank the Senckenberg Research Station of Grube Messel, Sonja Wedmann and Bruno Behr, who provided drill cores for this work, and Reimund Rosmann, who provided a lot of help. We highly appreciate the constructive reviews from Reinhard Gaupp and Henrik Friis.
This research has been supported by the University Scientific Research Program of Xinjiang Uygur Autonomous Region Education Department (grant no. XJEDU2019Y070), Innovative Talents Project of Karamay Science and Technology Bureau (grant no. 2019RC002A), and the China Scholarship Council (grant no. 201806400006).
This paper was edited by Johan Lissenberg and reviewed by Reinhard Gaupp and Henrik Friis.