5 Geochemistry of sedimentary rocks
5.1 Major elements
5.1.2 Early Palaeozoic sandstones
CIA values generally increase from Neoproterozoic to Ordovician, whereas several Cambrian sand-stones differ from this trend having low CIA (Fig. 5.2). Furthermore 6 Ordovician sandstones shale sample from the Lower Cambrian sequence
is similar to the Ordovician shales with respect to its major element composition (Fig. 5.1, Fig. 5.2, Fig. 5.3). The general increase in Al2O3/SiO2ratios from Neoproterozoic to Ordovician (Fig. 5.3A) sug-gests diminished importance of mechanical grain size reduction but increased degradation of pri-mary minerals accompanied by formation of clay minerals towards the Ordovician. The Ordovician rocks with Al2O3/SiO2ratios between those of UCC and PAAS represent rather siltstones, whereas these with distinctly elevated Al2O3/SiO2ratios are shales.
Increasing K2O/Na2O and Al2O3/(CaO+Na2O) ratios of the pelites (Fig. 5.3B, C) are controlled by both
PAAS UCC
PAAS UCC PAAS
UCC
400 450 500 550 600
0.0 0.1 0.2 0.3 0.4
0.5 103
102 101 100 10-1 10-2 102 101 100 10-1
102 101 100 10-1
Al O SiO
2 3 2
K O Na O
2 2
Al O /(CaO+Na O)2 3 2
A B
C
450 500 550 600
Stratigraphic Age (Ma)
Stratigraphic Age (Ma)
Average of Ordovician volcanics
Average of Upper Cambrian silicic volcanics
Average of Upper Cambrian mafic volcanics
Fig. 5.3: Stratigraphic Age vs. Major Element Ratio plots for Cambrian and Ordovician shales and siltstones. For the late Neo-proterozoic greywackes (red triangles) and shales/siltstones (pink triangles) are plotted to highlight the geochemical similarity of the different lithologies. Synsedimentary volcanism does not notably infl uence the shale/siltstone composition. A) Al2O3/SiO2 ratios. B) K2O/Na2O ratios. C) Al2O3/(CaO+Na2O) ratios.
Abb. 5.3: Stratigraphisches Alter vs. Hauptelementverhältnis – Diagramme für kambrische und ordovizische Ton- und Siltsteine. Für das späte Neoproterozoikum sind sowohl die Grauwacken (rote Dreiecke) als auch die Ton-/Siltsteine (rosa Dreiecke) geplottet, um die geochemische Ähnlichkeit beider Lithologien hervorzuheben. Die chemischen Zusammensetzungen der Proben sind offenbar nicht durch synsedimentären Vulkanismus modifi ziert. A: Al2O3/SiO2-Verhältnisse. B: K2O/Na2O-Verhältnisse. C: Al2O3/(CaO+Na2 O)-Verhältnisse.
did not yield reasonable CIA values due to their car bona ceous matrix. Similarly low K2O/Na2O and Al2O3/(CaO+Na2O) ratios of the low-CIA Cambrian sandstones and the carbonaceous Ordovician sili-ciclastics (Tab. 5.1, Fig. 5.4B, C) are indicative of con tributions from fresh volcanic material.
The majority of the Lower Cambrian and Ordo-vician psammites as well as few Middle Cambrian quartzitic siliciclastics of the Skryje-Týʼnovice area are highly enriched in SiO2 and correspondingly depleted in other major element oxides entailing low Al2O3/ SiO2ratios. CIA values of these rocks are distinctly higher than those of the Neoproterozoic siliciclastics (Tab. 5.1, Fig. 5.2). Similarly, high K2O/Na2O and
Al2O2/(CaO+Na2O) ratios, respectively, (Fig. 5.1, Fig. 5.4) are indicative of intense weathering, since Na and Ca are most rapidly removed by fl uids during decomposition of plagioclase and other unstable minerals (Nesbitt et al. 1980). With regard to their major element compositions most of the Middle Cambrian sandstones show more similarities with the Neoproterozoic detrital sediments than with the highly mature Lower Cambrian and Ordovician sandstones.
400 450 500 550 600
0.0
Average of Upper Cambrian silicic volcanics
Average of Upper Cambrian mafic volcanics
Sandstones with carbonaceous matrix
450 500 550 600
Stratigraphic Age (Ma)
Stratigraphic Age (Ma)
Fig. 5.4: Stratigraphic Age vs. Major Element Ratio plots for Neoproterozoic to Ordovician greywackes and sandstones indicate a general increase in chemical maturity with time. Averages of volcanic rocks of the TBU are given for an estimate of potential input from synsedimentary volcanism. A: Al2O3/SiO2 ratios. B: K2O/Na2O ratios – four Lower Cambrian and two Middle Cambrian sandstones are not plotted because Na2O was not detected. C: Al2O3/(CaO+Na2O) ratios.
Abb. 5.4: Stratigraphisches Alter vs. Hauptelementverhältnis – Diagramme für neoproterozoische bis ordovizische Grauwacken und Sandsteine zeigen prinzipiell eine Zunahme der chemischen Reife vom Neoproterozoikum zum Ordovizium. Außerdem sind die Durchschnittswerte für vulkanische Gesteine des Teplá-Barrandiums angegeben, da diese eine potentielle Quelle für die silizi-klastischen Sedimente darstellen. A: Al2O3/SiO2-Verhältnisse. B: K2O/Na2O-Verhältnisse; vier unterkambrische und zwei mittel-kambrische Sandsteinproben sind nicht geplottet, da ihr Na2O-Gehalt unterhalb der Nachweisgrenze ist. C: Al2O3/(CaO+Na2 O)-Verhältnisse.
Journal of Central European Geology 54 (2008) 1–168
GEOLOGICA SAXONICA
have a low residence time in sea water (summarized by Bhatia 1985, Bhatia & Crook 1986, Elderfi eld et al. 1990). Therefore, these elements on the one hand may represent direct provenance indicators, e.g., high Sc is characteristic for a provenance from mafi c rocks, as this compatible element substitutes 5.2 Trace elements
HFSE (e.g., Zr, Hf, Nb, Th, Sc) and REE are considered to be immobile during weathering and transport and
La/YbN La/SmN Eu/Eu* Ce/Ce* Zr/Hf Th/U Th/Sc ȸREE [ppm]
Neoproterozoic
greywackes (n=9) 5.6 –9.7 3.0–4.1 0.72–0.87 0.93–1.00 33.0–39.5 1.0–4.7 0.28–0.70 94–165 shales/siltstones (n=4) 6.1–8.7 2.6–3.6 (0.64)
0.73–1.01
0.85–0.96 30.9–38.9 2.1–2.9 0.29–0.46 107–155 Lower Cambrian
sandstones - high CIA (n=11) 4.2–15.3 2.4–7.1 0.75–0.87 0.94–1.18 29.6–41.5 1.6–5.0 0.42–1.03 (2.17)
25–97 (147) sandstones - low CIA (n=10) 5.0–11.2 2.5–5.2 0.82–1.03 0.86–1.03 36.5–41.7 2.4–3.7 0.35–0.90 49–171
shale (n=1) 7.2 3.6 0.76 0.94 34.1 3.1 0.48 187
Middle Cambrian
sandstones (n=15) 5.7–10.2 2.7–4.5 0.80–0.92 0.85–1.04 36.0–44.4 2.0–3.6 0.26–0.73 20–149 shales/siltstones (n=16) 5.5–8.8 3.0–4.1 0.72–0.82 0.91–1.04 34.6–39.0 2.5–3.3 0.38–0.59 140–188 Ordovician
sandstones (n=8) 4.9–10.7 1.3–4.0 0.67–0.81 0.96–1.12 31.4–36.4 3.0–6.9 0.82–1.71 77–199 sandstones, carbonaceous (n=6) 4.9–13.8 2.6–3.7 0.68–0.83 0.97–1.04 32.2–36.1 5.0–6.3 0.88–2.55 112–222 shales/siltstones (n=28) 8.2–15.0 3.1–5.7 0.65–0.77 0.87–1.08 30.8–34.9 (2.9)
4.5–7.8
0.57–1.48 186–329
PAAS 9.1 4.3 0.64 1.01 42.0 4.7 0.91 183
UCC 10.5 4.1 0.69 0.99 36.4 3.9 0.75 148
BCC 7.1 3.2 0.87 1.02 35.7 4.3 0.26 106
Table 5.2: Trace element characteristics1 of the analysed samples of the individual age groups. Trace element ratios of PAAS, UCC and BCC are listed for comparison.
Tab. 5.2: Spurenelementcharakteristika1 der analysierten Proben der einzelnen Altersabschnitte. Spurenelementverhältnisse von PAAS, UCC und BCC sind zum Vergleich angegeben.
) (
5 . 0
* N N
N
Gd Sm
Eu Eu
Eu
+
= and
) Pr ( 5 . 0
* N N
N
La Ce Ce
Ce
+
=
1 All REE-related values referring to a normalisation (e.g., La/YbN, Eu/Eu*) were calculated with the chondritic abundances given by Boynton (1984). The anomalies of Eu and Ce were calculated by the equations:
On account of the steep LREE patterns of the analysed siliciclastics the application of the geometric mean did not yield reasonable values, particularly for Ce/Ce*. The values obtained by using the arithmetic mean are closer to the visual impression derived from Fig. 5.5.
into early crystallising phases, such as pyroxenes and amphiboles (e.g., McLennan 1999), high LREE and Th may be related to monazite or allanite, which point to a felsic igneous and/or to an amphibolite facies to high-grade metamorphic provenance, respectively.
On the other hand HFSE and REE distributions may identify weathering processes, which in turn may be indicative of the geotectonic setting of the sedimentary basin, e.g., high LREE and Th may also be an indicator of recycled sedimentary provenance, as monazite is relatively stable during sedimentary and weathering processes and may be concentrated in siliciclastic source rocks (Mange & Maurer 1991, and references therein), high Th/U ratios may result from intense source rock weathering in a low-energy/
passive margin setting, as Th is retained in particles during sedimentary and weathering processes but U is more soluble and mobile (McLennan et al. 1993).
Since HFSE and REE patterns of siliciclastic sedi-ments are predominantly controlled by heavy min-erals, analyses with some obviously elevated HFSE and/ or REE are to be considered with caution because mechanical sorting during transport and sedi -mentation may concentrate heavy minerals in parti-cular levels/lenses of the sedimentary pile and may therefore modify the provenance signal in favour of the respective source(s).
LILE (Cs, Rb, Ba, K, Sr) abundances in detrital sediments are largely ruled by weathering processes, because all these elements are highly soluble in aqueousfl uids and therefore controlled by the system:
residual phases – newly formed minerals – released solutions (Mittlefehldt 1999). However, different response to weathering processes gives information on the intensity of source rock weathering, i.e. Sr is most rapidly removed, whereas Rb, Cs and Ba are fi xed onto secondary clay minerals (Nesbitt et al.
1980). In contrast to the HFSE and REE, signifi cant
amounts of LILE are incorporated in rock-forming minerals, such as plagioclase, alkali-feldspar, biotite and muscovite. Consequently, if sandstones and conglomerates contain fresh grains/pebbles, these may refl ect the LILE composition of the source rocks.
LILE behave incompatibly during mantle melting and are therefore enriched in the upper continental crust (Rudnick 1999).
As was already indicated by the major elements, Neoproterozoic greywackes and shales are largely similar to each. This also applies to their trace element compositions (Tab. 5.2). Therefore they are treated together in 5.2.1.
5.2.1 Neoproterozoic siliciclastics and