4 Volcanic rocks
4.3 Magma sources and implications on the geotectonic setting
The geochemistry of the Teplá-Barrandian Neopro-tero zoic to Silurian volcanic rocks clearly varies among samples of different age and refl ects several stages of the geotectonic history of this part of the Bohemian Massif. Different geotectonic conditions are characterised by magmatism from different sources that can be identifi ed by HFSE and REE pat-terns even in altered volcanic rocks.
The overall character of the Neoproterozoic vol-canism in the TBU is manifold (Fiala 1977, 1978; Pelc
& Waldhausrová 1994; Waldhausrová 1997a, 1997b;
Pin & Waldhausrová 2007) and the number of samples taken only from the Neoproterozoic basement close to the unconformably overlying Skryje-Týʼnovice Cam brian deposits is far too limited to adequately constrain the complex Neoproterozoic plate-tec-tonic processes. However, it can be demonstrated
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rhyolites (ƤNd = +0.2 to +1.8) have positive ƤNd values (Pin et al. 2007). The latter demonstrates that an old crustal source is ineligible for generation of the rhyolites. Instead the classifi cation of the felsic rocks as effusive equivalents of A2-type granites (Fig. 4.7C) suggests derivation from the lower crust, which must have been composed of recently added island-arc-related complexes of Neoproterozoic age.
Therefore the calc-alkaline signature that is indicated by some geochemical features (cf. PatoĀka et al.
1993) represents an inherited component. A suitable reason for melting of the lower crust is underplating by basaltic magmas similar to the Upper Cambrian continental tholeiites. Particularly the dominance of the felsic volcanics suggest storage of hot mantle material at the Moho and melting of the lower crust at high temperatures. The latter is supported by the morphologies of zircon crystals from a rhyolite sample indicative of high temperatures (cf. chapter 7.3.1) Furthermore, it cannot be excluded that some of the rhyolites evolved from or were mixed with mantle magmas. Altogether the Upper Cambrian volcanism belongs to an important rifting event at the northern Gondwana margin that is traceable across many peri-Gondwanan terranes (e.g., Ossa-Morena Zone – Sánchez-García et al. 2003, Saxo-thuringia – Kemnitz et al. 2002, Sudetes – Kryza &
Pin 2002).
Ordovician and Silurian volcanics are much more enriched in incompatible elements than Upper Cambrian continental tholeiites and resemble alkaline basalts. High (La/Yb)N and (Gd/Yb)N (Tab. 4.2) of the Ordovician volcanics suggest low degrees of partial melting of a garnet peridotite mantle source, whereas the garnet remained as a residue in the source. Lower concentrations of highly incompatible elements (i.e., lower [La/Yb]N) and similar (Gd/Yb)N in the Silurian alkalibasalts point to a larger degree of partial melting of the same source.
The Teplá-Barrandian did not represent an intra-oceanic island during the Ordovician and Silurian, but contained a basement of continental crust that formed at the latest in the Neoproterozoic (possibly in places the TBU is underlain by distinctly older basement).
A recent equivalent of OIB-type magmatism that that the analysed basaltic andesites 1) largely
re-semble N-MORB, 2) show slightly elevated Th/Nb and Th/La ratios compared to N-MORB and 3) have Th/Yb vs. Ta/Yb ratios (Fig. 4.7A) that are transitional between tholeiitic volcanics of oceanic island arcs and N-MORB. These geochemical features are compatible with a subduction-related modifi cation of the depleted mantle source and generation of the magmatism in a back-arc setting. This is supported by highly radiogenic ƤNd600 values of +7.8 to +9.3 recently acquired by Pin & Waldhausrová (2007) for similar rocks of the Teplá-Barrandian Neoproterozoic.
A (sub)recent analogue of such a back-arc setting is the Sulu Sea NE of Borneo, whose opening oc-curred during a relatively short time interval in the Early Miocene. The basin is surrounded by island arcs and continental fragments (Silver & Rangin 1991).
The basalts from the fl oor of the Sulu Basin show an affi nity to N-MORB, whereas the mantle source was modifi ed by subduction-related geochemical components. The latter is indicated by enriched LILE and Th contents (Spadea et al. 1991). Although LILE had to be excluded due to their mobility, Fig. 4.3 highlights the similarity of the incompatible element patterns of the Teplá-Barrandian Neoproterozoic vol-canics with the sample from the Sulu Basin.
Upper Cambrian mafi c volcanics are clearly dif-fer ent from the Neoproterozoic volcanism and largely resemble continental tholeiites (cf., Pin et al. 2007). They are similar to continental tholeiites of the Permian rifting-stage in the Central Western Car pathians (Fig. 4.4: samples C-3 and C-4 from Dostal et al. 2003). However, in contrast to the TBU, where intermediate rocks and rhyolites dominate the volcanic suite (Waldhausrová 1971) they are only repre sented by rare volcaniclastics and dykes in the Slovakian Carpathians. ƤNd500 values of +6.1 to +6.7 deter mined on fi ve of the analysed mafi c samples (Pin et al. unpublished data) are within the compositional range of the subcontinental lithosphere (compilation of Rollinson, 1993). The same is true for the initial Sr isotope relationship (87Sr/86Sri ~ 0.704) reported by Vidal et al. (1975). Besides the andesites also intermediate rocks (ƤNd = +4.5 to +5.3) and even
is not restricted to oceanic islands on oceanic crust is the Cameroon Volcanic Line (CVL; Fig. 4.6) ex-tending from the continental interior of equatorial
West Africa to Annobón (formerly Pagalu) Island in the South Atlantic (actually the CVL is the NE’ part of a volcanic chain that commences at the oceanic
Zr/4 Y
Nb*2
AI AII
B C
D N-MORB...D
P-MORB...B WP th...C, AII WP Alk...A VAB...C, D
1 10
10 100 1000
Zr
10 *4 Ga/Al
I&S-types
A-type
Y
Y 3*Ga3*Ga
Nb Nb
A1
A2
10 100 1000
1 10 100 1000
Rb
Y+Nb
syn-collisional
granite within-plate granite
volcanic arc granite
ocean ridge granite
0.01 0.1 1 10 50
0.01 0.1 1 10 50
Ta/Yb
Th/Yb
DEPLETED MANTLE
SOURCE
ENRICHED MANTLE
SOURCE
MORB
intra-plate
basalts active continental
margins
oceanic island arcs S
CA
CA TH CA TH
A A B B C C D D
Fig. 4.7: Discrimination diagrams for Neoproterozoic – Silurian volcanics. Symbols as in Fig. 4.2. Mafi c rocks: A: Ta/Yb vs. Th/Yb diagram after Pearce et al. (1983). Grey triangle stands for sample DB6/13, where Ta was not detected. Therefore the Ta content was estimated by Nb/14 due to Nb/Ta=14 in sample DB1/0. B: Nb-Zr-Y diagram after Meschede (1986). Rhyodacites and rhyolites:
C: Granite discrimination diagram after Whalen (1987), inset: distinction of A-type granites after Eby (1992); D) Y+Nb vs. Rb diagram after Pearce et al. (1984).
Abb. 4.7: Diskriminationsdiagramme für neoproterozoische bis silurische Vulkanite. Symbole wie in Abb. 4.2. Mafi sche Gesteine:
A: Ta/Yb vs. Th/Yb-Diagramm nach Pearce et al. (1983). Das graue Dreieck steht für Probe 6/13, in der Ta nicht nachgewiesen wurde. Der Ta-Gehalt wurde aber mittels Nb/14 abgeschätzt, da Nb/Ta = 14 in Probe DB1/0. B: Nb-Zr-Y-Diagramm nach Meschede (1986). Rhyodazite und Rhyolithes: C: Granit-Diskriminationsdiagramm nach Whalen (1987), Einschub: Unterscheidung der A-Typ Granite nach Eby (1992); D: Y+Nb vs. Rb-Diagramm nach Pearce et al. (1984).
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therefore the geotectonic setting of their source rocks.
Subsequent processes such as weathering, transport and diagenesis modify the primary provenance signals of the resulting sediments (e.g., Bhatia 1983, Johnsson 1993). Particularly duration and intensity of source rock weathering, duration and mechanisms of transport, and the energy of the depositional en-vironment control the extent of these modifi cations.
Since land plants did probably not play an important role until the Devonian2, their effects (e.g., slope stability, acid degradation products, capacity of soil to hold water, retain of sediment in the weathering profi le or in temporary storage) on weathering are assumed to be negligible for the analysed Late Neo-proterozoic to Ordovician detrital sediments.
When keeping in view possible interactions of these various factors, reliable information on source area and modifying conditions may be derived from geochemical compositions of clastic sediments.
In this chapter the analysed rocks are subdivided according to grain size and stratigraphic age into Neoproterozoic siliciclastics (marin, n=13), Lower Cambrian sandstones and conglomeratic sandstones (continental and/or transitional, n=21), Lower Cam-brian shale (transitional, n=1), Middle CamCam-brian sand stones (marine, n=15), Middle Cambrian shales (marine, n=16), Ordovician sandstones (marine, n=14) and Ordovician shales (marine, n=28). Ana-lytical data are given in Tables A5 to A7 of the Appendix. For diagrams using major elements the analyses were recalculated to 100% by compensating the LOI. Sample preparation and analytical methods are described in the Appendix. Most of the diagrams refer to PAAS, which is the post-Archean average Australian shale of Taylor & McLennan (1985) as well as to UCC and BCC, which are upper and bulk continental crust as given by Rudnic k & Gao (2003). The averages of Teplá-Barrandian Palaeozoic Island of St. Helena close to the Mid-Atlantic Ridge
[e.g., Halliday et al. 1988, Fairhead & Wilson 2004]).
Fitton & Dunlop (1985) showed that the alkaline volcanics of the continental part of the Cameroon Line are geochemically and isotopically indistinguishable from those of the oceanic sector and hence, were not substantially affected by interaction with the continental lithosphere. A further similarity with the Barrandian alkaline volcanism is the long magmatic history (~65 Ma to recent in the CVL) during which there is lack of evidence for considerable migration of the volcanism with time. The latter is interpreted to be indicative of a relatively shallow mantle source (Fitton & Dunlop 1985). In the Barrandian Palaeozoic alkaline magmatism is known from the Ordovician and Silurian and even from the Devonian1 (adds up to a total of ~90 Ma), whereas the thickness of the volcanic deposits and the duration of the volcanic activity decrease with time (cf., Štorch 1998). However, this is not necessarily a result of migration but may be related to the decreasing present-day spatial extent of the Palaeozoic successions, i.e., Devonian rocks are only preserved in the core of the Prague syn-form (Fig. 2.3). Such repeated, “stationary” intraplate volcanism can be explained by stress-related frac-tures that form within the lithosphere as a result of changes in absolute plate motions (e.g., Foulger 2003, Fairhead & Wilson 2004, Smith 2004). These frac tures allow the formation and ascent of OIB-type magma.