EARLY VARISCAN ARC MAGMATISM IN NW TURKEY: EVIDENCE OF GEOCHEMISTRY AND U-Pb ZIRCON AGES Fırat ŞENGÜN1 and Osman Ersin KORALAY2 1
Department of Geology Engineering, Faculty of Engineering, Çanakkale Onsekiz Mart University, 17100, Çanakkale, Turkey 2 Department of Geology Engineering, Faculty of Engineering, Dokuz Eylül University, 35160, İzmir, Turkey E-mail:
[email protected]
ABSRACT
26o22 30
26o30
Neogene sedimentary and volcanic rocks
Massive, fine-grained metavolcanic rocks of the Çamlıca metamorphic unit exposed in the Biga Peninsula, northwest Turkey, have provided new Carboniferous ages and arc-related calc-alkaline volcanics indicating that the Biga Peninsula was possibly involved in the Variscan orogeny. The metavolcanic rocks are mainly composed of metalava- metatuff intercalation and have an andesitic composition. Chondrite-normalized REE patterns within these rocks are fractionated (La N /Yb N ~ 2.2 to 8.9), Europium anomalies are slightly variable (Eu/Eu* 0.6 to 0.7) and generally negative (average Eu/Eu* 0.68). The metavolcanic rocks have a distinct negative Nb anomaly and negative Sr, Hf, Ba and Zr anomalies. These large negative anomalies indicate crustal involvement in their derivation. The crustal influence may be related to either partial melting at the base of continental crust or contamination of mafic magma with crustal material. Tectonic discrimination diagrams show that all metavolcanic rocks were formed within a volcanic arc setting. Zircon ages (LA-ICPMS) of two samples yield 333.5±2.7 Ma and 334.0±4.8 Ma . These ages are interpreted to be the time of protolith crystallization. This volcanic episode in the Biga Peninsula can be correlated with the Central Europian Variscides. Carboniferous calc-alkaline magmatism in the Sakarya Zone can be ascribed to arc-magmatism as a result of northward subduction of the Palaeo-Tethys under the southern margin of Laurasia. Geochemical and U-Pb zircon data indicate that the Sakarya Zone is strikingly similar to that of the Armorican terranes in central Europe. The Biga Peninsula can provide a link between the Sakarya Zone and the Armorican terranes .
32
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Denizgören ophiolite
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{
Salihler formation (phyllite, marble, calcschist) Dedetepe formation (schist, marble, calcschist) Andıktaşı formation (metalava, metatuff) Çanakalan HP tectonic slice 34 sample location
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Denizgöründü
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normal and strike-slip fault
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lineation
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Figure 2: Detailed geological map of the Çamlıca metamorphic unit. The location of the dated metavolcanic rocks (08-50, 08-51) is shown in the map.
Moesian Platform Varna
The common mineral assemblages of the metavolcanic rocks (the Andıktaşı formation) are mainly made up of quartz + chlorite + epidote + albite + actinolite + calcite ± sphene ± zircon (Figure 3a).Their primary porphyritic volcanic textures can still be recognized (Figure 3b). The metavolcanic rocks are mostly massive and fine-grained rocks (Figure 3c,d).
200 08-51 Metatuff 08-52 03-133 08-53 09-83 Metalava 09-84 09-85
10
100
10
Figure 8: Concordia diagrams showing U-Pb isotope ratios and ages derived from LAICPMS analyses for metavolcanic samples (a) 08-50 and (b) 08-51.
1 1
b
.1
Rhodope Massif
Zonguldak
Thrace basin
Florina Thassos 400N Study area Biga Peninsula
Marmara Sea
plg
Sakarya Zone
Kazdag Massif Çamlık granite
Ankara
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Kirsehir Massif
Edremit
plg 200 µ
Lesvos Athens
Peloponnese Hydra
Ionian Sea
360N
İzmir Chios Karaburun
Cyclades Naxos
Menderes Massif Lycian nappes
d Konya
100
Adana
c
Hf/3
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(a)
A
Sea of Crete 0
100
200 km
Crete
Figure 9: (a) Paleogeographic reconstruction of the eastern Mediterranean region for the Mississippian (modified after Stampfli and Borel, 2002; Okay et al., 2006). Paleo-Tethys separated Eurasia from Gondwana and subducted northward beneath the Eurasia during the Mississippian time. However, the northern margin of Gondwana remained passive. (b) Tectonic model for Turkey during the Carboniferous Variscan orogeny. The Sakarya Zone in the northwest Turkey and other continental fragments rifted from Gondwana during early Palezoic time and then drifted northwards till they accreted to the active continental margin of Eurasia (modified from Stampfli and Borel, 2002).
Plotting the immobile elements that preserve their abundance through postformational processes in a Zr vs. Y diagram (Müller et al., 2001), all the samples show arc-related affinity (Figure 6a). The tectonic setting of the metavolcanic rocks is a volcanic arc (Figure 6b), as indicated by the Th-Hf-Ta ternary discrimination diagram (Wood, 1980).
Arabian Platform
Kithira
N
La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
Istanbul
Aegean Sea Evla
0,2
Ba Rb Th U Nb Ta La Ce Sr Nd Pb Sm Zr Hf Eu Y Yb
The Biga Peninsula shows a connection between the Sakarya Zone and the Armorican terranes (Figure 9).
Figure 5: (a) N-type MORB-normalized (normalization values are from Sun and McDonough 1989), and (b) Chondrite-normalized REE patterns of the metavolcanic rocks. Chondrite normalizing values from Taylor and Mc Lennan (1985).
Africa Bistra Mountains
(b)
(a)
Turkey
Black Sea
The weighted average age of 333.5±2.7 Ma (08-50) and 334.0±4.8 Ma (08-51) can be interpreted as the primary crystallization age of this metavolcanic rock (Figure 8).
The metavolcanic rocks have a distinct negative Nb and Ta anomaly, typically seen in arc magmas, with negative Sr, Ba, Hf anomalies (Figure 5a). Chondritenormalized REE diagrams illustrate that the metavolcanic rocks from the Biga Peninsula generally exhibit LREE enrichment with respect to HREE (Figure 5b). 100
3. PETROGRAPHY
a
Europe
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Mediterranea nSea
D-TH
10 Cyprus
The Çamlıca metamorphic unit is the tectono-stratigraphic basement unit in the Biga Peninsula. This basement has a threefold subdivision: metavolcanic association in the lowermost part (Andıktaşı formation), a metapelitic unit in the upper most part (Salihler formation), and a metapelitic and amphibolitic part (Dedetepe formation) as a transition zone between them (Figure 2). Metavolcanic association is a broad distribution of greenschist-facies rocks, predominantly metalavas associated with relatively minor metatuff.
B D-CA C
09-85
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Zr
Figure 3: a) Photomicrograph showing the mineral composition of the metavolcanic rocks (crossed polars, ab: albite, ep: epidote, chl: chlorite, q: quartz, b) photomicrograph showing relic volcanic texture c) photograph of brown, yellowish green-coloured, nonfoliated metavolcanic rocks, d) a close-up view of relict texture reflecting the volcanic origin.
4. GEOCHEMISTRY Major, trace and rare earth element (REE) geochemistry for metavolcanic rocks from the Biga Peninsula has been determined to reveal their origin and tectonic setting.
A: N-MORB B: E-MORB, WPT C: WPA D: VAB CA: calc-alkaline TH: Tholeiitic
Arc-related
08-51 Metatuff 08-52 03-133 08-53 09-83 Metalava 09-84
Figure 1: Simplified tectonic map of the Eastern Mediterranean showing the major geotectonic units, bounding sutures and location of the Biga Peninsula (modified from Meinhold and Frei, 2008; Okay et al., 2006).
2. GEOLOGY
Th/U=0.8
4 km
depleted in HFS 24 E
Th/U=0.4
Th/U=1.2
Figure 7: Cathodoluminescence (CL) images of selected zircons from metavolcanic samples (08-50 and 08-51). Spots on zircons represent areas of LA-ICPMS analyses
Figure 4: (a) Rock classification diagram of metavolcanic rocks from the Biga Peninsula (after Pearce, 1996). (b) Th/Yb versus Ta/Yb diagram (after Pearce, 1982) showing source characteristics for the metavolcanic rocks (c) Th/Yb versus Ta/Yb geodynamic setting discrimination diagram of Pearce (1983) modified by Gorton and Schandl (2000).
39o52 30
hill village
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The northwestern Anatolia is known as a region where different tectonic units meet and forms one of the critical regions in the geology of Turkey. The Sakarya Zone, one of these crustal units, is E-W – trending crustal assemblage bordered by the Istanbul Zone and Thrace Basin in the north, and by the Anatolide-Tauride Platform in the south (Figure 1). The Paleozoic continental metamorphic rocks of the Biga Peninsula located in the westernmost part of the Sakarya Zone have a complex thermo-tectonic history during the Variscan and Alpine events. However, Mississippian magmatic activity is not known from the Biga Peninsula. The present study reports Carboniferous protolith ages and tectonic setting of metavolcanic rocks which provide a better understanding of the Variscan orogeny in the Sakarya Zone and demonstrate Armorican affinity of the Sakarya Zone during the Mississippian. .
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Ahlatoba
1. INTRODUCTION
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Salihler
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Th/U=0.7
354 Ma
.001 .01
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Subduction component
Alkali Basalt
Basalt
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tholeitic lavas
Foidite
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40o00'
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323 Ma
338 Ma
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Ovacık
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shoshonites
Phonolite
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Uludağ
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Figure 6: Tectonic discrimination diagrams of the metavolcanic rocks. Diagrams after (a) Müller et al. (2001); (b) Wood (1980). Abbreviations for all plots: MORB = mid-ocean ridge basalt. WPT = within plate tholeiite; WPA = within plate alkali. VAB = volcanicarc basalt. 5. GEOCHRONOLOGY
The zircons used for age calculations from sample 08-50 and 08-51 have U contents of 184-685 ppm and Th/U ratio ranging from 0.4 to 1.4 and 0.4 to 1.2 respectively, which indicates typical magmatic origin.The internal structure of the zircons, revealed by CL imaging, shows typical oscillatory zoning for the magmatic zircons (Figure 7).
6. CONCLUSIONS Zircons from two metavolcanic samples yielded ages of 333.5±2.7 Ma and 334.0±4.8 Ma (Mississippian). They can be interpreted as the time of protolith crystallization of metavolcanics. Carboniferous metavolcanic rocks occur in the Sakarya Zone and are the products of subduction-related magmatism. The Carboniferous calc-alkaline magmatism in the Sakarya Zone resulted from northward subduction of the PaleoTethys under the southern margin of Laurasia. These metavolcanics from the Çamlıca metamorphic unit are connected with an arc which developed on the continental crust at the subduction-related active continental margin, which is characterized by calcalkaline magma. This magmatic arc comprises the Sakarya Zone in the Pontides, the Rhodope and Strandja Massifs in the Balkans, and the Caucasus which can be correlated with the Armorican terranes in central Europe.
