K–Ar age, whole-rock and isotope geochemistry of A-type granitoids in the Divriği–Sivas region, eastern-central Anatolia, Turkey
Introduction
The İzmir–Ankara–Erzincan (IAE) ocean, the part of the northern branch of the Neo-Tethyan ocean, is located between the Eurasian plate in the north and Tauride–Anatolide platform in the south (Şengör and Yılmaz, 1981). The northward closure of the IAE ocean has left some well-documented arc-related igneous rocks in the active margin, i.e., in the eastern Pontides (e.g., Boztuğ et al., 2006a), collisional-related intrusions in the passive margin, i.e., in central and eastern-central Anatolia (e.g., Boztuğ, 2000, Boztuğ et al., 2003, İlbeyli et al., 2004, İlbeyli, 2005), and some supra subduction zone-type (SSZ-type) ophiolitic rocks (Yalınız et al., 1996, Floyd et al., 2000) in Turkey (Fig. 1). These collision-related central Anatolian granitoids have been studied by many authors (e.g., Boztuğ, 1998, Boztuğ, 2000, Köksal et al., 2004, İlbeyli et al., 2004, İlbeyli, 2005). All these authors report that the central Anatolian granitoids represent a high compositional diversity ranging from S-type, peraluminous leucogranites, through I-type or H-type high-K calc-alkaline granitoids, to high-K alkaline, bi-modal A-type granitoids. Most authors suggest that these rocks were generated in a collision-related geodynamic setting except Kadıoğlu et al. (2003) who have proposed an arc-related origin for the I-type Ağaçören intrusive suite.
The aim of this paper is to present some new amphibole/biotite K–Ar ages and geochemical data consisting of major, trace, REE, radiogenic (Sr, Nd, Pb) and stable (O, S) isotope compositions that bear on the timing and magma sources of the post-collisional, bimodal A-type Dumluca and Murmana granitoids in the Divriği region, SE Sivas province, eastern-central Anatolia, Turkey.
Section snippets
Regional tectonic setting
The Late Cretaceous central and eastern-central Anatolian collisional granitoids intrude the supra subduction zone-type central Anatolian ophiolite of Cenomanian–Turonian age (Parlak and Delaloye, 1999, Floyd et al., 2000, Garfunkel, 2004) and crustal sedimentary and metasedimentary units of pre-Late Cretaceous age, and are overlain by Late Palaeocene–Early Eocene or younger detrital sedimentary rocks. These collision-related central Anatolian granitoids show a wide range of compositions not
Geological setting and petrography
The pre-granitoid basement rocks consist of Palaeozoic very low-grade metasedimentary rocks of Keban metamorphics and supra subduction zone-type (SSZ-type) Divriği ophiolitic mélange (Fig. 2). The ophiolitic mélange consists of ultramafic rocks, serpentinized ultramafic rocks, subophiolitic metamorphic sole rocks comprising amphibolites, epidote–amphibolites and calc-silicate schists, along with the Carboniferous–Early Cretaceous Munzur limestone (Parlak et al., 2006). Dumluca and Murmana
Analytical methods
All the sample preparation procedures including thin sectioning, point counting, crushing–grinding–sieving to get rock powder for chemical analyses and mineral separates were carried out in the laboratories of the Department of Geological Engineering of the Cumhuriyet University in Sivas, Turkey. Amphibole and biotite separates for the K–Ar analyses were extracted through bromoform heavy liquid treatment, and then by magnetic separator at the Geological Survey of Israel–Jerusalem, and quartz or
K–Ar geochronology
Among the 17 mafic mineral separates, extracted from the felsic and mafic rocks of the Murmana and Dumluca granitoids, five of them consist solely of amphiboles, seven of them comprise pure biotite, and five of them include a mixture of biotite and amphibole (Table 2). The biotite percentage in the biotite + hornblende mixtures was estimated using the following equation: K2Ototal = wt.% biotite X (K2O content of biotite) + wt.% hornblende X (K2O content of hornblende) in which the average K2O
Major and trace element characteristics
Whole-rock geochemical data (Table 3, Table 4) reveal that most of the rock samples represent alkaline compositions in a total alkali vs. silica plot (Irvine and Baragar, 1971, Fig. 5a) except a few highly evolved samples with more than 68 wt.% silica which plot in the subalkaline field. They all show high-K compositions (Fig. 5b) and most are metaluminous in character (Fig. 5c). However, the highly evolved felsic rocks plotting in the subalkaline fields in Fig. 5a are peraluminous rather than
Radiogenic isotope geochemistry
Sr, Nd and Pb isotopic compositions of four rock samples are given in Table 5. The mafic samples from the Dumluca and Murmana granitoids are of an apparent mantle origin, whereas the felsic samples from both granitoid units are shifted away from mantle array towards the upper crust composition in the (87Sr/86Sr)i vs. εNd(t) plot (Fig. 12a,b). The mantle-derived mafic samples in Fig. 12 plot in the ocean-island basalt (OIB) subfield (Fig. 12c). The offset of trend of felsic rocks from both
K–Ar cooling and reset or new growth ages
K–Ar age determinations of biotite and of hornblende–biotite mixtures yield cooling ages of ca. 72 to 77 and 68 to 75 Ma, respectively, for both of the Dumluca and Murmana granitoids. Apart from these cooling ages, three pure hornblende separates from the Murmana granitoid yield ages of ca. 62 to 65 Ma. Pure hornblende ages are somewhat younger than those containing biotite. The closure temperature of hornblende is 450 and 550 °C while that of biotite is lower and ranges between 250 and 350 °C
Conclusion
The available geological, geochronological, whole-rock and isotope geochemical data from the bi-modal, A-type Dumluca and Murmana granitoids in the Divriği–Sivas region lead to the conclusions that:
- (1)
these granitoids intruded the already-obducted SSZ-type ophiloitic rocks later than Cenomanian–Turonian and cooled at ca. 77 Ma;
- (2)
two different but co-eval magma sources were involved in the genesis of these rocks, i.e., an EM II type of enriched mantle-derived mafic magma for the mafic rocks, and
Acknowledgements
This paper was supported by the Cumhuriyet University-CUBAP (M-255) and TUBITAK (102 Y 149). Partial support for this research was provided by US National Science Foundation grant EAR0309935 to GBA. Mr. Elmar Reitter (Tübingen University, Germany) is thanked for his efforts during Sr, Nd, and Pb isotopic analyses. Prof. John H. Dilles (University of Oregon, USA) and an anonymous reviewer are kindly thanked for their helpful comments which substantially improved the original manuscript. We thank
References (70)
- et al.
The geochemistry of marine sediments, island arc magma genesis, and crust–mantle recycling
Earth and Planetary Science Letters
(1989) - et al.
Oxygen isotope trends and anomalies in granitoids of the Tibetian plateau
Journal of Asian Earth Sciences
(2002) Do coeval mafic and felsic magmas in post-collisional to within-plate regimes necessarily imply two contrasting, mantle and crustal, sources? A review
Lithos
(2004)- et al.
Geochemical characteristics of the composite Kaçkar batholith generated in a Neo-Tethyan convergence system, eastern Pontides, Turkey
Journal of Asian Earth Sciences
(2006) - et al.
Neodymium isotopes in basalts of the southwest basin and range and lithospheric thinning during continental extension
Chemical Geology
(2000) Origin of the eastern Mediterranean basin: a reevaluation
Tectonophysics
(2004)- et al.
Oxygen and hydrogen isotope geochemistry of S- and I-type granitoids: the Cape Granite suite, South Africa
Chemical Geology
(1997) Chemical differentiation of the Earth. The relationship between mantle, continental crust and oceanic crust
Earth and Planetary Science Letters
(1988)- et al.
Petrogenesis of collision-related plutonics in Central Anatolia, Turkey
Lithos
(2004) - et al.
Sm–Nd isotopic evolution of chondrites
Earth and Planetary Science Letters
(1980)