7 U-Pb zircon ages
7.6 Source area
Several recent studies applied detrital zircon age spectra to deduce the ancestry of crustal fragments with Cadomian basement from one of the different cratons/units composing northern Gondwana (e.g.
Fernández-Suárez et al. 1999, 2000, 2002; Friedl et al. 2000, 2004; Gutiérrez-Alonso et al. 2003; Murphy et al. 2004a, 2004b; Linnemann et al. 2004; Martínez Catalán et al. 2004). The presence of c. 0.9–1.2 Ga old and/or Mesoproterozoic zircon in Neoprotero-zoic and PalaeoNeoprotero-zoic rocks is used as an indicator for Ama zonian provenance (e.g., Friedl et al. 2000, Gutiérrez-Alonso et al. 2005). In the – meanwhile well dated – West African craton, 0.9–1.2 Ga old or
· Detritus of Neoproterozoic origin (<900 Ma) is frequent or prevailing in all analysed samples (total 312).
Differences in the age distribution patterns chiefl y concern the proportions of Palaeozoic zircon (total 154; Fig. 7.15).
In the Neoproterozoic rocks (samples BL and Dob) and in the oldest Cambrian sample (PJ1/3) a prominent proportion of detritus from Palaeoproterozoic and older sources (22–37%) is traceable (Fig. 7.14, Fig.
7.15). Younger Lower Cambrian rocks still contain contributions from old crust, which is indicated by their whole rock Pb isotope compositions (see chapters 6.2 and 6.5). From the uppermost Lower Cam brian to the Lower Ordovician input of old zircon decreases. Instead, the contributions from sources younger than 550 Ma increase strongly. The scarcity of pre-Neoproterozoic zircon could be attributed to a dilution effect (the higher the proportion of Palaeozoic detritus, the lower the proportion of Precambrian zircon). However, when ignoring the ages younger than 550 Ma there are still low percentages of pre-Neoproterozoic zircon in these samples (CB3: 15.6%, Oh3: 8.7%, TrTo3+ToĀník: 6.1%). The lack of old zircon combined with common (sample CB3 - 38%) or extremely frequent (samples Oh3 and TrTo3/
Tocnik - 65% each) Palaeozoic detritus in uppermost Lower Cambrian to Early Ordovician rocks might be related to crustal tilting with related change in the drainage system and therefore a change/restriction in provenance.
After the sedimentation of the Cambrian Ohrazenice Formation (sample Oh3) until the deposition of the Tremadocian Tʼnenice Formation (samples TrTo3/Tocnik) up to 1500 m andesitic to rhyolitic volcanics were accumulated. The Upper Cambrian age of the volcanism is confi rmed by U-Pb-SHRIMP dating of zircon from rhyolite sample OKR yielding a crystallisation age of 499±4 Ma.
The Upper Cambrian volcanic complex and products of the older Cambrian magmatism occurring in the Teplá-Barrandian (523–505 Ma: Dörr et al. 1998, 2002; Zulauf et al. 1997; Venera et al. 2002) have to be considered as an important source for late Lower Cambrian to lowermost Ordovician siliciclastics.
Mesoproterozoic events have not been recognized hitherto. Consequently the absence of 0.9–1.6 Ga old detrital zircon is an indicator for northwest African provenance. However, a number of detrital zircon
grains with 0.9–1.2 Ga and Mesoproterozoic ages was discovered in the NE African Arabian-Nubian Shield and its cover sequence (Fig. 7.18). Yet crust-forming events between 0.9 and 1.6 Ga are not known from
Neoproteroz.
Pz Mesoproterozoic Paleoproterozoic Archean
Age (Ma)
Numberofgrains detritalzirconfromNeoprot. toDevoniansiliciclasticsediments(n=612)
Teplá-Barrandian
600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000 3200 3400 3600
0 20 40 60 80 100 120 140 160
Avalonia
2 1
Saxothuringia Israel NW Africa Amazonia Baltica
Fig. 7.16: Summarized frequency distribution plot for detrital zircon ages of Neoproterozoic to Devonian samples and comparison with the age spectra of a) potential source areas (Baltica, Amazonia, NW-Africa) and b) other Gondwanan or Gondwana-derived units (Avalonia, Israel, Saxothuringia). Bars show 50 Ma intervals. Shaded areas highlight time intervals that are represented by a number of zircon ages in the Barrandian samples. Data sources: Baltica – Åhäll et al. (1998), Bingen et al. (2001, 2003); and from the compilations of Gaál & Gorbatschev (1987), Hanski et al. (2001), Romer (in press); Amazonia – from the compilations of Teixeira et al. (1989) and Tassinari & Macambira (1999); NW Africa – see Fig. 7.18 (1–3); Avalonia – detrital zircon ages from (1) Cambro-Ordovician and (2) Silurian to Devonian siliciclastics, Collins & Buchan (2004), Murphy et al. (2004a, 2004b); Saxothuringia – detrital and inherited zircon ages from Neoproterozoic to Ordovician siliciclastic and igneous rocks, Linnemann et al. (2004).
Abb. 7.16: Zusammenfassung der Häufi gkeitsverteilungen der neoproterozoischen bis devonischen Proben und Vergleich mit den Altersspektren von a) potentiellen Liefergebieten (Baltica, Amazonia, NW-Africa) und b) anderen gondwanischen bzw. perigond-wanischen Einheiten (Avalonia, Israel, Saxothuringia). Die Balken entsprechen Intervallen von 50 Mio. Jahren. Datenquellen:
Bal tica – Åhäll et al. (1998), Bingen et al. (2001, 2003); und aus den Zusammenstellungen von Gaál & Gorbatschev (1987), Hanski et al. (2001), Romer (in press); Amazonia – aus den Zusammenstellungen von Teixeira et al. (1989) und Tassinari & Macambira (1999);
NW Africa – siehe Abb. 7.18 (1–3); Avalonia – Alter detritischer Zirkone aus (1) kambro-ordovizischen sowie (2) silurischen und devonischen Siliziklastika, Collins & Buchan (2004), Murphy et al. (2004a, 2004b); Saxothuringia – Alter detritischer und ererbter Zirkone aus neoproterozoischen bis ordovizischen Siliziklastika und Magmatiten, Linnemann et al. (2004).
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found a distinct number of detrital zircon crystals with ages between 0.9 and 1.2 Ga in Cambrian sandstones from southern Israel and conclude that either rocks the Arabian-Nubian Shield, too. Nevertheless, during
Precambrian and Cambrian times there must have been a source for such detritus. Avigdad et al. (2003)
Fig. 7.17: Images of detrital zircon of sample BL (Neoproterozoic Blovice Fm.) showing that many of the pre-Neoproterozoic grains are composed of core and overgrowth zone(s). Particularly the grains yielding Archean ages are not derived from pristine Archean terranes but from reworked Archean crust. The same seems to apply to some of the Palaeoproterozoic zircon crystals: A: CL image of an Archean grain with narrow rim, B: CL image of Palaeoproterozoic core with narrow rim, C: BSE image of a Palaeoprotero-zoic grain showing magmatic zoning, D: BSE image of a grain with Archean core and younger rim – the spot straddles the core and a distinctly younger overgrowth zone resulting in a discordant analyses. However, the core has a 207Pb/206Pb minimum age of 3501±16 Ma, E: CL image revealing Archean core and Palaeoproterozoic rim, F) CL image of two Archean grains with narrow rims, G: CL image of an Archean grain with narrow rim.
Abb. 7.17: Mikrosondenaufnahmen detritischer Zirkone aus Probe BL (Neoproterozoikum, Blovice Formation) zeigen, dass viele der prä-neoproterozoischen Körner aus Kern und Anwachszone(n) zusammengesetzt sind. Vor allem die Körner mit archaischen Altern stammen nicht von unberührten Komplexen sondern von überprägter archaischer Kruste. Das Gleiche scheint für die Paläoproterozoischen Zirkone zu gelten: A: Kathodolumineszenz-Aufnahme eines archaischen Korns mit schmalem Rand. B: KL-Aufnahme eies paläoproterozoischen Korns mit schmalem Rand. C: BSE-KL-Aufnahme eines paläoproterozoischen Korns mit mag-matischer Zonierung. D: BSE-Aufnahme eines Korns mit archaischem Kern und jüngerem Rand – der Spot hat sowohl den Kern als auch den deutlich jüngeren Anwachssaum getroffen, was in einer diskordanten Analyse resultiert. Der Kern hat ein 207Pb/206 Pb-Minimum-Alter von 3501±16 Ma. E: KL-Aufnahme, die einen archaischen Kern mit paläoproterozoischem Anwachssaum zeigt.
F: KL-Aufnahme von zwei archaischen Körnern mit schmalem Rand. G: KL-Aufnahme eines archaischen Korns mit schmalem Rand.
of these ages are present but not recognized hitherto in the Arabian-Nubian shield or that detritus of this age was transported by glaciers over a long distance during the Neoproterozoic and became reworked/
redeposited afterward.
The age spectra of the analysed samples show (Fig. 7.14, Fig. 7.16) prominent input from sources younger than 0.9 Ga and from rocks that were formed between 1.75 and 2.20 Ga. Even though less frequent
– zircon with ages from 2.35 to 2.50 Ga and 2.55 to 2.85 Ga was detected several times. Older Archean grains are rare but present.
Taking sample BL (Neoproterozoic) as an example it can be demonstrated that the analyses yielding Archean ages were obtained from core domains of the grains, which are surrounded by younger overgrowth domains (Fig. 7.17), i.e. there were no pristine Archean terranes in the source area but the
cratonic basement remobilised during Pan-African orogeny
Fig. 7.18: Compilation of detrital, magmatic and metamorphic U-Pb and Pb-Pb zircon ages known from the northern part of the African continent. Data sources: 1 (West African craton) – Barth et al. (2002), Bossière et al., (1996), Doumbia et al. (1998), Egal et al. (2002), Hirdes et al. (1996), Hirdes & Davis (2002), Kouamelan et al. (1997), Oberthür et al. (1998), Peucat et al. (2005), Potrel et al. (1996, 1998), Thiéblemont et al. (2001, 2004); and references therein. 2 (Anti-Atlas, Morocco) – Barbey et al. (2004), Gasquet et al. (2004), Inglis et al. (2004), compilation of Soulaimani & Piqué (2004), Thomas et al. (2002). 3 (Tuareg Shield, Benin-Nigeria Shield) – Affaton et al. (2000), compilation of Caby (2003), Kröner et al. (2001), Peucat et al. (1996, 2003), Paquette et al. (1998);
and references therein. 4 (Arabian-Nubian Shield) – compilation of Abdelsalam et al. (2002), Kröner et al. (1994), Stern et al. (1994), Sultan et al. (1994); and references therein. 5 (detrital zircon from Cambrian sandstone in Israel) – Avigdad et al. (2003).
Abb. 7.18: Zusammenstellung von U-Pb- und Pb-Pb-Altern detritischer, magmatischer und metamorpher Zirkone, die aus dem nördlichen Teil Afrikas bekannt sind. Datenquellen: 1 (Westafrikanischer Kraton) – Barth et al. (2002), Bossière et al. (1996), Doumbia et al. (1998), Egal et al. (2002), Hirdes et al. (1996), Hirdes & Davis (2002), Kouamelan et al. (1997), Oberthür et al. (1998), Peucat et al. (2005), Potrel et al. (1996, 1998), Thiéblemont et al. (2001, 2004); und Referenzen darin. 2(Anti-Atlas, Marokko) – Barbey et al. (2004), Gasquet et al. (2004), Inglis et al. (2004), Zusammenstellung von Soulaimani & Piqué (2004), Thomas et al. (2002).
3 (Tuareg-Schild, Benin-Nigeria-Schild) – Affaton et al. (2000), Zusammenstellung von Caby (2003), Kröner et al. (2001), Peucat et al. (1996, 2003), Paquette et al. (1998); und Referenzen darin. 4 (Arabisch-Nubischer Schild) – Zusammenstellung von Abdelsalam et al. (2002), Kröner et al. (1994), Stern et al. (1994), Sultan et al. (1994); und Referenzen darin. 5 (detritische Zirkone aus einem kambrischen Sandstein Israels) – Avigdad et al. (2003).
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herited component. The simplest and most probable explanation for the unchanged zircon age spectra is that the Teplá-Barrandian did not leave its Gondwanan source area prior to the Variscan continent-continent collision but rather drifted together with Gondwana towards lower latitudes. Alternative scenarios would be that the Teplá-Barrandian rifted from the super-continent and drifted separately towards the north as a constituent of (1) a larger crustal fragment in-cluding Gondwanan basement rocks or (2) as a part of Cadomian/Pan-African crust that contained a subaerial part, which was eroded and supplied the basin with reworked (Neoproterozoic?) material. Both inter pretations are not very likely because of (1) the scarcity of pre-Neoproterozoic basement rocks in the Variscan and Alpine orogens (only two occurrences of small areal extent are known - the Svetlík gneiss in the Moldanubian Zone and the Icartian gneiss in the northern Armorican Massif) and (2) the lack of Cambrian detrital zircon in the Upper Ordovician samples (if the pre-Neoproterozoic detrital zircon should be derived from reworked Neoproterozoic grey wackes, the Cambrian plutons intruding these rocks should also supply detrital material).
Neoproterozoic and Cambrian zircon may have been generated by Cadomian tectono-thermal events at the periphery of Gondwana (in the sense of Murphy & Nance 1989) and/or by Pan-African orogenic processes that amalgamated the individual cratons then forming Gondwana (e.g., Trompette 1994, Dalziel 1997; and references therein). Rocks of appropriate geochemical composition and with suitable formation ages are present in both belts.
Furthermore Neoproterozoic and Cambrian zircon may have been derived from local sources. Although Neoproterozoic igneous rocks of the TBU are not or insuffi ciently dated, their geochemistry and their stratigraphic classification make them suitable source rock candidates, particularly for the Cambrian siliciclastics. Cambrian igneous rocks with ages between 523 and 499 Ma (see chapter 2.2.2 for references) are well known in the Teplá-Barrandian unit. These rocks should have been an important source for the younger Lower Cambrian to Early Ordovician detrital sediments of the TBU.
Archean zircon is derived from Archean complexes that were reworked during the Palaeoproterozoic (Fig. 7.17E) and/or during the Neoproterozoic. Also Palaeoproterozoic terranes may have been reworked during later events. Barbey et al. (1989) described mig matites of the Central Hoggar that yielded zircon crystals with polyphase evolution: granodiorite-protoliths of Palaeoproterozoic age (~2.13 Ga) are overgrown by ~610 Ma old metamorphic rims. The core of zircon BL69 (Fig. 7.17B) matches the age of the protolith and shows a narrow rim of unknown age.
The rim might correspond to metamorphic overprint as suggested by the roundness of the grain. The absence of indicators for pristine Archean crust and the lack of ages between 0.9 and 1.7 Ga provides clear evidence for the position of the Teplá-Barrandian in the proximity of (present) northwest Africa during the Neoproterozoic. The most probable position of the TBU was in the northern part/continuation of the Trans-Sahara Belt (see chapter 8 for further discussion). Furthermore the similarity with the age distribution patterns of detrital and inherited zircon from Saxo-Thuringia (Linnemann et al. 2004; see Fig. 7.16), the northern Armorican Massif and the Iberian Ossa-Morena Zone (Schäfer et al. 1993, Fernández-Suárez et al. 2002b) suggest a common history of these units with the Barrandian. In contrast Ava lonia (Collins and Buchan 2004, Murphy et al.
2004a, 2004b; see Fig. 7.16), the Cantabrian and West asturian-Leonese Zones of Iberia (Fernández-Suárez et al. 1999, 2000, 2002a) and the Moravo-Silesian domain of the Bohemian Massif (Friedl et al. 2000, 2004) are characterised by age spectra con-taining prominent proportions of Mesoproterozoic com ponents.
The detrital zircon age spectra of the samples from the Barrandian remain more or less the same from the late Neoproterozoic to the Mid-Devonian, which implies that the source area of the detrital zircon remains more or less the same, too. This source area must have consisted either of Neoproterozoic active margin/magmatic arc sequences and pre-Meso tero zoic basement complexes (and/)or of Neo pro-tero zoic (and Early Palaeozoic?) siliciclastics and possibly crustally derived igneous rocks with an