Vol.3, No.9, 750-767 (2011) Natural Science http://dx.doi.org/10.4236/ns.2011.39100 Copyright © 2011 SciRes. OPEN ACCESS K-Ar age, geochemical, and Sr-Pb Isotopic compositions of keban magmatics, elaziğ, Eastern Anatolia, Turkey Sevcan Kürüm Engineering Faculty, Department of Geology, University of Fırat, Elazig, Turkey; skurum@firat.edu.tr Received 9 August 2011; revised 11 September 2011; accepted 27 September 2011. ABSTRACT Keban magmatics consist of plutonic rocks of acidic and intermediate compositions with diffe- rent phases. They are the equivalent of surface rocks. In the current study on plutonic rocks, general petrographic features, disequilibrium textures such as skeletal formation in minerals, poikilitic texture, oscillatory zoning, and mineral fragmentation, and growth states are observed. Besides these microscopic properties, the exi- stence of rounded mafic enclaves of various sizes, petrographic syn-plutonic dykes, and field data support the idea that mafic and felsic magmas are mixed. Keban magmatics have I-type, metaluminous-peraluminous characteri- stics. Diorites and quartz diorites have low-K tholeiitic features, whereas tonalites have low-K calc-alkaline features. Compared with diorites, tonalites are richer in terms of LREE (Rock/ Chondrite); Rb, Sr, and Ba (LILE); and Hf, Zr, Th, and U (HFSE) elements. LILE enrichment, which signals the crustal contamination of mantle-ori- ginated magmas, is particularly observable in tonalites. In both rock groups, the negative ano- maly of Nb is a sign of similarity of pluton to the subduction zone magma series. Based on the K-Ar geochronology dating of amphibole mi- nerals, the ages of these rocks are found to be 75.65 ± 1.5 and 59.77 ± 1.2 Ma in tonalites and 84.76 ± 1.8 and 84.35 ± 1.7 Ma in diorite and quartz diorites. The 87Sr/86Sr isotope ratios in tonalites are 0.705405 and 0.706053, whereas these ratios are 0.704828 and 0.704754 in diori- tic rocks. Pb isotope ratios are similar in both rock types. Keywords: Keban Magmatics, K-Ar Age; Pb-Sr Isotopes; Geochemistry 1. INTRODUCTION Within the Southeastern Anatolia orogenic belt and the Neotethys convergent system developed during the tectonomagmatic evolution of the southern Neotethys, Keban magmatics form the easternmost branch of the Göksun-Afşin, Doğanşehir, and Baskil granitoids [1-8] (Figure 1). These granitoids of the Cretaceous age [3,6] have intrusive contact relationships [3-7,11-13] with platform carbonates (Malatya-Keban metamorphic), op- hiolites (Göksun, İspendere, Kömürhan, Guleman), meta- morphic rocks related to ophiolites (Berit) [6,7,14], and ensimatic island-arc units present in the orogenic belt (Elazığ-Yüksekova) [3,4,6,7,11,12,15]. The objective of the current paper is to present the field relations, petrography, geochemical and isotopic (Sr-Pb) composition, and K-Ar hornblende ages of Ke- ban magmatics. The study contributes in determining the location of these rocks within Cretaceous plutonic rocks spread throughout southeast Anatolia and in establishing geodynamic evolution in future regional studies. 2. ANALYTICAL TECHNIQUES Thin section sample preparation and crushing and grinding to obtain whole-rock powders were performed at the laboratories of the Department of Geological En- gineering, Firat University, Elaziğ, Turkey. Whole-rock chemical analyses have been performed at the ACME laboratories and at the ACT-labs by ICP- AES (major and some trace elements) and ICP-MS (some trace and rare earth elements, REE) in Canada; Mineral separates for K-Ar analyses were extracted by conventional procedures including grinding, sieving and heavy liquid separation. K-Ar age determination of mi- neral separates consisting of amphibole ± biotite and pure amphibole has been performed at the K-Ar Geo- chronology Laboratory, Geological Survey of Israel, Jerusalem, Israel. For K determination, two aliquots of ca. 0.25 g were taken from a sample and dissolved using
S. Kürüm / Natural Science 3 (2011) 750-767 Copyright © 2011 SciRes. OPEN ACCESS 751 Figure 1. Simplified map of the Cretaceous units of the area between Göksum (Kahramanmaras) and Kovancilar (Elazig) (from [9,10]). lithium metaborate (LiBO2). Potassium concentrations were measured on ICP-AES (Perkin Elmer OPTIMA 3300) along with repeated determinations of three of the international standards SO-3, BE-N, BHVO-1, SCo-1, NIM-L, NIM-G. The 1 uncertainty for the K concentra- tion of duplicates was less than 3%. The argon analysis for K-Ar determination was performed using the stan- dard isotope dilution procedures routinely used in the geochronological laboratory at the GSI [16,17]. About 0.03 g sample was loaded into the glass arm of a metal extraction line and heated overnight at 120˚C. Argon was extracted in a molybdenum crucible using RF induction heating. Gases were scrubbed through liquid nitro gen and Zr Al getters. Argon was measured on a VG MM- 1200 mass spectrometer. Measured intensities were cor- rected for linear extrapolation of the 40Ar peak and then iAr/39Ar ratios (i = other isotopes). Argon was measured in duplicates and uncertainties are reported at the 1 level. Sr and Pb isotopic analyses were performed at the Mineralogical Institute of Heidelberg University, Ger- many. 3. GEOLOGICAL SETTING The study area is situated in the Southeast Anatolia Orogenic Belt. This belt was formed by the collision of the Afro-Arabian and Eurasian plates following the oce- anic closure of the south Tethyan in the Cretaceous- Miocene era [6,7]. This orogenic belt, which stretches from east to west, consists of three different zones [6,12, 18] and is divided into two nap zones: lower and upper [12]. The lower nap zone is made up of ophiolitic units, and the upper nap zone consists of Malatya Keban meta- morphic massives [6,12,19]. The granitoids in Maraş, Malatya, and Elazığ regions, which formed during the evolution of southern Neotethys, have intrusive contact relationships with metamorphic massives (Malatya-Ke- ban metamorphites), ensimatic island-arc units (Elazığ magmatic rocks/Yüksekova complex), ophiolitic rocks (Göksun, İspendere, Kömürhan, Guleman), and meta- morphic rock units related to ophiolites (Berit) [7,8,10]. Malatya-Keban metamorphic massives and ophiolitic units were tectonically placed prior to the intrusions formed in the late Cretaceous period [3,13,20]. The geological units in the study area begin with the Keban metamorphic rocks from the Paleozoic-Mesozoic age. Keban metamorphic rocks are composed of marble, schist, and phyllites [21,22]; they crop out along the south-north direction in the western parts of the study area (Figure 2). One of the researcher [21] suggested that these rocks of carbonate and pelitic origin have low P-T conditions and that they underwent metamorphism during the Jurassic-lower Cretaceous era. The other re- searchers [23,24] pointed out that the metamorphism of the Malatya-Keban platform limestones is related to tec- tonism and asserted that these tectonic events caused by subduction still emerged on the edge of an active conti- nent during the Senonian era. Thus, he concluded that metamorphism of limestones and tectonism is contem- poraneous. Conversely, some researchers [2,4] proposed that the P and T conditions that caused the metamor- phism of the Keban metamorphics are related to the northerly subduction of the oceanic crust located south of the Keban unit in the upper Cretaceous rocks and to the formation of the Elazığ magmatic rocks formed above this subduction zone. Contact metamorphism (skarniti- zation) is observed along intrusive contacts between me- tamorphic and plutonic rocks [24,25]. Palaeocene-Oligo- cene sedimentary rocks unconformably overlaid all the metamorphics, ophiolites, intrusive, and volcano-sedi- mentary rocks in the study area.
S. Kürüm / Natural Science 3 (2011) 750-767 Copyright © 2011 SciRes. OPEN ACCESS 752 Figure 2. Geological map of the Keban magmatic rocks (taken from [4]) and location map of the study area. M: Malatya; E: Elazig; K: Keban; Erz: Erzincan; DSFZ: Dead Sea Fault Zone; EAFZ: East Anatolian Fault Zone; NAFZ: North Anatolian Fault Zone.
S. Kürüm / Natural Science 3 (2011) 750-767 Copyright © 2011 SciRes. OPEN ACCESS 753 The thrust fault between the Keban metamorphics and the Keban pluton forms the main tectonic structure in the study area (Figure 2). 4. FIELD OBSERVATIONS AND PETROGRAPHY Late Cretaceous-Palaeocene Keban magmatic rocks were mapped as tonalite, diorite/quartz diorite, and ba- salt/andesite (Figure 2). Nevertheless, among these rocks, only tonalite and diorite/quartz diorites were examined within the context of this study. Diorites are generally medium grained, hard, black, and less widespread than tonalities, which are medium to coarse grained with in- tensive alteration resulting in soft topographies. The to- nalities contain syn-plutonic mafic dykes and mafic mi- crogranular enclaves of various sizes and shapes, indi- cating the contemporaneous existence of mafic and felsic magmas [26,27]. Tonalites also contain aplitic dykes. Most of the enclaves found in tonalites are round and elliptical in shape, and their size may reach up to 50 cm. Volcanic rocks in the Keban magmatic province, which covers a large area, are basaltic and andesitic in compo- sition [4], dark in color, fragile, and contain frequent cracks. Mineralogical studies reveal that Keban magmatics are diorite, quartz diorite, tonalite, and granodiorite in com- position. Geochemical data indicate a composition of tonalites, quartz diorites, and diorites. In the geoche- mical nomenclature diagram [28] (Figure 3), two sam- ples are quartz diorite (SK-23 and SK-24), four are dio- rite (SK-2,-3,-5, and SK-25), and the rest are tonalite in composition. Medium- to fine-grained diorites and quartz diorites have different granular and poikilitic textures, and are mainly made up of plagioclase and amphibole. In some cases, amphiboles are more dominant than plagioclases. In addition to the main mineral phases, quartz (more dominant in quartz diorite), biotite, pyroxene (as relict), opaque minerals, and secondary minerals such as chlo- rite, calcite, and epidote are usually observed. Plagio- clase generally shows polysynthetic twinning and oscil- latory zoning in dioritic rocks. Amphibole generally shows different-sized green pleochroism subhedral and skeletal in structure. The presence of relic pyroxene in some amphibole crystals indicates that these amphiboles are formed by uralitization. Tonalites and granodiorites are hypidiomorphic, gra- nular textured, and composed of plagioclases, quartz, biotite, amphibole, K-feldspar, zircon, apatite, and opa- que minerals (magnetite). Plagioclase, which forms the main felsic mineral, shows albite twinning (An15-25) and zoning. Crystals with overgrowth texture display a sieve texture in some cases. Anhedral quartz varies in size and shows wavy extinction. Minor biotite is generally opaci- fied along the edges, whereas K-feldspar turns to clay. Zircon forms an accessory phase and is generally found with biotite, chlorite, and some quartz, whereas apatite occurs in quartz and plagioclase. Reaction textures in amphiboles, transformation of amphiboles into biotites, zoning, and sieve textures in plagioclases indicate disequilibrium crystallization in some diorites and tonalites. The enclaves and syn-plutonic dykes in the tonalities are dark and fine grained. Although the mineralogical composition of these enclaves and the syn-plutonic dykes resembles that of the host-rock, change in the mineral proportions and variation of grain sizes of the plagio- clases (seriate texture) are important differences. How- ever, twinning and oscillatory zoning are observed in the plagioclase phenocrysts of these rocks. Such disequilib- rium textures observed in phenocrysts are important in determining open system processes such as magma mix- ing [29]. 5. K-Ar HORNBLENDE AGE K-Ar analysis was conducted on four amphibole se- parates extracted from fresh rock samples of Keban in- trusive rocks. The results are presented in Table 1. All four samples (SK-23,-25,-27, and SK-29) solely consist of amphibole grains. Grain size fractions (+212 micron) of the rock samples were extracted. K/Ar ages of 84.8 ± 1.8 and 84.4 ± 1.7 Ma for diorite and quartz diorite sam- ples and 75.7 ± 1.5 and 59.8 ± 1.2 Ma for tonalite sam- ples were obtained. The ages obtained are in accordance with those reported for Baskil granitoids [3,6,30] and Göksun-Afşin granitoids [7,8]. 6. GEOCHEMISTRY 6.1. Major and Trace Element Characteristics Tables 2 and 3 present the results of whole rock che- mical analyses of the samples taken from diorites, quartz diorites, and tonalities of the Keban magmatics. Based on the alkali-silica (Figure 4(a)), AFM (Figure 4(b)) [31], and K2O-silica [32,33] diagrams (Figure 4(c)), the rocks have low-K sub-alkaline characteristics. However, whereas the tonalities show calc-alkaline properties, the diorites and quartz diorites are tholeiitic in character. Examination of the Shand index (Al2O3/(CaO + Na2O + K2O) versus Al2O3/(Na2O + K2O) [34] reveals that both diorites and quartz diorites are located in the metalu- minous and peraluminous regions (Figure 4(d)), whereas the tonalites are located solely in the peraluminous region. The Aluminum Saturation Index (ASI = Al2O3/(CaO +
S. Kürüm / Natural Science 3 (2011) 750-767 Copyright © 2011 SciRes. OPEN ACCESS 754 Table 1. Data for K-Ar Age determinations in the amphibols of the intruzif rocks. Sample Rock Size m Age ± 1σ Ma *40Ar cc STP/ K † % *40Ar % 36 Ar cc STP/ *40K/36Ar *40Ar/36Ar SK-23 Q.diorite +212 84.76 ± 1.8 6.06E−07 0.18 39.84 3.10E−09 43089 491 SK-25 Diorite +212 84.35 ± 1.7 5.36E−07 0.16 71.75 7.14E−10 166092 1046 SK-27 Tonalite +212 75.65 ± 1.5 1.47E−06 0.49 75.70 1.59E−09 227668 1216 SK-29 Tonalite +212 59.77 ± 1.2 1.60E−06 0.68 80.75 1.29E−09 389714 1534 Table 2. Whole-rock major element (wt%) and trace element (ppm) chemical analysis results of the intruzif rocks. Diorite/Quartz diorite Tonalite Sample SK-2 SK-3 SK-4 SK-5 SK-23 SK-25 SK-1SK-6 SK-7SK-8SK-9SK-10 SK-11 SK-12 SK-27 SK-28SK-29 SiO2 45.33 46.82 47.83 42.28 51.85 47.26 69.91 69.82 72.3173.12 70.1669.90 69.97 69.40 71.75 71.81 72.86 Al2O3 17.29 19.02 19.50 18.92 18.33 19.64 13.68 14.7113.75 13.6215.05 14.7714.81 14.95 14.03 14.1513.80 Fe2O3 13.06 13.28 11.82 16.77 8.27 9.53 3.763.953.392.393.403.903.95 3.23 3.47 3.452.46 MgO 7.30 5.18 5.00 6.53 5.40 6.75 0.981.231.15 0.47 0.88 1.17 1.13 1.20 0.89 0.92 0.55 CaO 12.23 10.94 11.56 12.43 11.84 13.65 4.334.61 4.01 2.713.97 4.00 4.27 4.21 3.80 3.87 3.17 Na2O 2.05 2.33 1.98 1.51 2.01 1.44 4.033.81 3.29 4.05 4.10 4.09 4.02 3.93 3.89 3.61 4.02 K2O 0.15 0.14 0.11 0.08 0.31 0.11 0.500.91 1.12 1.61 0.99 0.67 0.23 0.22 0.69 1.190.84 TiO2 0.79 0.82 0.80 1.00 0.49 0.50 0.320.41 0.30 0.210.300.410.35 0.34 0.31 0.30 0.22 P2O5 0.07 0.11 0.09 0.09 0.04 0.04 0.080.100.07 0.05 0.07 0.090.09 0.09 0.05 0.05 0.03 MnO 0.22 0.22 0.18 0.24 0.15 0.14 0.070.100.10 0.04 0.11 0.10 0.04 0.05 0.05 0.08 0.03 LOI 1.55 0.94 0.98 0.75 1.4 1.1 2.150.690.84 1.25 1.151.551.54 2.11 0.9 0.4 1.8 ASI 0.88 1.04 1.05 0.99 0.95 0.95 1.111.131.17 1.15 1.19 1.21 1.26 1.30 1.20 1.16 1.23 Total 100.00 99.83 99.85 100.60 100.11 100.18 99.81 100.30100.3099.52 100.20100.60100.40 99.73 99.86 99.8799.82 Ni 20 <20 <20 <20 5.9 12.7 <20<20 <20 <20 <20 <20 <20 <20 10.0 10.210.3 Co 37.0 35.0 33.0 52.0 29.0 38.8 5.07.0 5.0 3.0 4.0 5.0 5.0 6.0 5.4 5.5 3.3 V 424 351 463 556 221 303 5771 57 25 41 60 59 54 67 68 47 Cu 20.0 40.0 120.0 120.0 23.5 39.1 10.0130.0<10<1010.0<10<10 <10 3.3 3.9 2.7 Pb <5 <5 <5 <5 0.3 0.6 <55 <5 7 <5 <5 <5 <5 0.6 1.8 1.1 Zn 80 70 70 70 16 9 3040 50 <3040 30 <30 <30 12 27 7 Bi <0.4 <0.4 <0.4 <0.4 <0.1 <0.1 <0.41 <0.4<0.4<0.4<0.4<0.4 <0.4 <0.1 0.1 <0.1 Sn <1 <1 <1 <1 <1 <1 <1<1 <1 <1 <1 <1 <1 <1 <1 <1 <1 W <1 <1 <1 <1 0.9 0.3 <1<1 <1 <1 <1 <1 <1 <1 2.8 3.3 3.0 Mo <2 <2 <2 <2 3.5 1.5 <2<2 <2 <2 <2 <2 <2 <2 15.8 16.816.8 As <5 <5 <5 <5 <0.5 <0.5 <5<5 <5 <5 <5 <5 <5 <5 <0.5 <0.5<0.5 Sb <0.5 <0.5 <0.5 <0.5 <0.1 <0.1 <0.5<0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 0.1 0.1 <01 Hg - - - - <0.01 <0.01 - - - - - - - - <0.01 <0.01<0.01 Rb <2 <2 <2 <2 4.0 <0.5 10.021.025.044.019.09.0 4.0 3.0 20.3 35.530.5 Cs <0.5 <0.5 <0.5 <0.5 <0.1 <0.1 <0.5<0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 0.3 0.8 0.5 Ba 26.0 34.0 29.0 18.0 89.9 23.7 120.0 213.0134.0255.0241.0151.090.0 73.0 169.6 174.5204.5 Sr 182.0 214.0 218.0 204.0 205.7 194.6 186.0 231.0147.0 152.0189.0 156.0266.0 213.0 199.4 214.5192.7 Tl <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.10.1 0.1 0.2 0.1 <0.1<0.1 <0.1 <0.1 0.1 <0.1 Ga 16.0 18.0 17.0 19.0 14.4 15.2 12.014.0 12.0 12.0 13.0 13.0 13.0 12.0 13.4 13.4 12.4 Ta <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 0.20.2 0.2 0.4 0.2 <0.10.2 0.2 0.1 0.2 0.3 Nb <1 <1 <1 <1 0.8 <0.5 3 3 2 4 2 1 2 2 2.6 3.0 3.8 Hf 0.9 1.0 0.8 0.8 1.0 0.8 3.03.4 2.5 2.9 2.4 6.0 3.0 3.0 3.1 3.2 3.0 Zr 23.0 23.0 21.0 17.0 30.4 15.6 109.0 143.090.0104.086.0226.0110.0 101.0 101.7 119.096.4 Y 18.0 28.0 18.0 21.0 13.7 13.3 17.017.0 17.011.0 15.0 21.0 16.0 17.0 17.2 15.1 15.2 Th 0.2 0.6 0.2 <0.1 0.7 <0.1 2.02.7 2.2 6.3 2.4 1.0 1.8 2.0 2.5 2.3 5.2 U 0.2 0.2 0.1 0.2 0.4 <0.1 0.90.8 1.1 1.9 1.1 0.5 0.8 0.9 1.1 0.9 1.4 Rb/Sr 0.01 0.01 0.01 0.01 0.02 0.00 0.050.09 0.17 0.29 0.10 0.06 0.02 0.01 0.01 0.17 0.16 K2O/ P2O5 2.14 1.27 1.22 0.89 7.75 2.75 6.259.1 16.0 32.214.149.572.55 2.44 14.0 24.0 28.0 Ba/Nb 28.9 37.8 32.2 20.0 112.37 51.52 40.071.0 67.0 63.7120.5151.045.0 36.5 62.23 58.1753.81 La/Nb 14.0 34.0 25.0 13.0 2.8 2.8 2.73.0 3.9 3.6 3.8 5.9 3.5 3.1 2.6 2.6 3.0 Zr/Nb 28.7 28.7 23.3 18.9 38.0 39.0 36.347.7 45.0 26.0 43.0226.055.0 50.5 39.1 39.7 25.4 Nb/U 4.5 3.0 4.5 3.0 2.0 4.4 3.33.8 1.8 2.1 1.8 2.0 2.5 2.3 2.4 3.3 2.7
S. Kürüm / Natural Science 3 (2011) 750-767 Copyright © 2011 SciRes. OPEN ACCESS 755 Table 3. Whole-rock REE element (ppm) chemical analysis results of the intruzif rocks. Diorite/Quartz diorite Tonalite Sample SK-2 SK-3 SK-4 SK-5 SK-23 SK-25 SK-1SK-6SK-7 SK-8SK-9 SK-10 SK-11 SK-12 SK-27 SK-28 SK-29 La 1.4 3.4 2.5 1.32.2 1.3 8.19.07.8 14.47.55.9 6.9 6.2 6.8 7.9 11.3 Ce 4.5 8.8 5.9 4.35.6 3.8 15.917.114.624.914.2 13.614.012.5 14.6 15.821.5 Pr 0.92 1.57 1.00 0.940.84 0.65 2.122.211.942.831.812.041.921.76 1.77 1.842.34 Nd 5.7 8.9 5.5 5.94.7 3.2 8.58.97.69.67.19.4 7.8 7.5 7.3 7.6 9.0 Sm 2.1 3.0 1.8 2.21.4 1.2 2.22.32.02.01.92.6 2.0 2.1 1.9 1.7 1.7 Eu 0.85 0.93 0.72 0.900.57 0.46 0.650.770.610.490.680.780.680.69 0.59 0.620.53 Gd 3.0 4.3 2.8 3.3 1.72 1.61 2.62.82.61.92.33.2 2.5 2.5 2.03 1.891.72 Tb 0.6 0.8 0.5 0.6 0.37 0.33 0.50.50.50.30.40.6 0.5 0.5 0.43 0.380.32 Dy 3.5 5.0 3.3 3.9 2.23 2.12 2.83.02.91.92.53.6 2.8 2.9 2.49 2.261.82 Ho 0.7 1.1 0.7 0.8 0.47 0.52 0.60.60.60.40.50.8 0.6 0.6 0.57 0.510.46 Er 2.2 3.1 2.0 2.41.45 1.38 1.92.02.01.31.72.5 1.9 2.0 1.83 1.491.43 Tm 0.34 0.48 0.30 0.370.20 0.20 0.320.330.350.230.290.430.310.32 0.29 0.240.25 Yb 2.2 3.1 1.9 2.4 1.35 1.41 2.22.22.41.72.03.2 2.2 2.3 1.81 1.601.63 Lu 0.35 0.46 0.28 0.350.23 0.22 0.370.360.360.300.320.540.350.39 0.31 0.260.28 La/YbCN 0.46 0.79 0.94 0.391.17 0.66 2.642.932.336.082.691.322.251.93 2.69 3.544.97 Eu/Eu* 1.03 0.79 0.98 1.021.12 1.01 0.830.930.820.770.990.830.930.92 0.92 1.060.95 Figure 3. Nomenclature diagram for the Keban magmatic rocks [28]. di: Diorite, dq: Qurtz diorite, to: Tonalite. Na2O + K2O)) [35] values are greater than 1 (Figure 4(d)), which is in accordance with mafic mineral asso- ciation, such as biotit + amphibol. Zr, Nb, and Y-SiO2 (Figures 4(e)-(g)) diagrams indicate that these rocks have I-type granitoid character. Compositional differences in these rocks are also ob- served in the main and trace elements versus silica dia- grams. These units undergo different fractional crystal- lization processes (Figure 5). Diorites have higher Al2O3 than tonalites (Figure 5(a)), indicating that the frac- tionation process is effective in differentiation. Depend- ing on the mafic phase composition, CaO, FeO, MgO, MnO, and Ti2O are higher in diorites than tonalites (Fig- ures 5(b)-(f)). However, tonalites have richer con- tent of main oxides, such as K2O and Na2O, depending on feldspars rich in K and Na (Figures 5(g) and (h)). The same degree of P2O5 enrichment (Figure 5(i)) in both rock groups indicates the presence of apatite in the rocks. The negative correlation of the main elements, such as Al2O3, CaO, FeO*, MgO, MnO, and Ti2O, with SiO2 and its
S. Kürüm / Natural Science 3 (2011) 750-767 Copyright © 2011 SciRes. OPEN ACCESS 756 Figure 4. Major element geochemical discrimination diagrams of the Keban magmatic rocks. (a) Total alkalis vs silica ([31], dividing line between alkaline and sub-alkaline fields [32]); (b) AFM triangular diagram [31]; (c) K2O vs silica [33]; (d) Shand Index ([34], ASI: Aluminum Saturation Index = molar Al2O3/molar (CAO + Na2O + K2O [35]) and (e-g) Plot of thr Keban magatic rocks on the I/A-type gran- ites diagram [36]).
S. Kürüm / Natural Science 3 (2011) 750-767 Copyright © 2011 SciRes. OPEN ACCESS 757 Figure 5. Magor oxides and some trace elements vs. SiO2 variation diagrams for rocks samples from the Keban mag- matic rocks. positive correlation with Na2O and K2O in both rock groups indicate fractional crystallization during magma evolution. The variations of the trace elements, Hf and V, with SiO2 (Figures 5(j) and (k)) are further indications of fractional crystallization. Fractionation phases are mainly plagioclase, pyroxene, and hornblende. The chondrite-normalized spider diagram (Figure 6(a)) demonstrates the differences between the tonalites and diorites in terms of LREE. In general, tonalites show higher LREE [(La/Yb)CN = 6.08 – 1.32], whereas dio- rites are more depleted in these elements [(La/Yb)CN = 1.17 – 0.39]. Except for the negative Eu anomaly (Eu/Eu* = 0.79) in Sample SK-3, the other samples from the diorites do not show negative Eu anomalies (Eu/Eu* = 1.12 – 0.98). All samples of the tonalites display nega- tive Eu anomalies (Eu/Eu* = 0.99 – 0.77) except sample SK-28 (Eu/Eu* = 1.06). In addition, the upward concave distribution from LREEs to HREEs, especially in the to- nalities, emphasizes the importance of the feldspars in tonalites during fractionation and melting [37]. In the MORB-normalized spider diagrams, tonalites are richer in LREE, similar to previous diagram (Fig- ures 6(b) and (c)). However, the distributions of some elements remarkably vary in tonalites and diorites. For instance, whereas Hf and Zr are depleted to the extent of negative anomaly in diorites, these elements are enriched in tonalites. Crustal-origin elements, such as K, Rb, Ba, Th, and Y, are found with fewer amounts in diorites than in tonali- ties. Ti, which represents the mantle, shows remarkable negative anomaly. These findings confirm that these ele- ments are compatible with each other. Although such
S. Kürüm / Natural Science 3 (2011) 750-767 Copyright © 2011 SciRes. OPEN ACCESS 758 Figure 6. Chondrite (a) and MORB (b-c) normalized spider diagrams for the Keban magmatic rocks. MORB and Chondrite-normalizing values [38].
S. Kürüm / Natural Science 3 (2011) 750-767 Copyright © 2011 SciRes. OPEN ACCESS 759 distribution coincides with the mature period of arc plu- tonism [39], acidic magma can be said to have formed as a result of the contamination of magma with basic-inter- mediate composition or its assimilation. The HREE be- tween Gd-Lu is more enriched in tonalites. 6.2. Sr and Pb Isotope Geochemistry The Pb and Sr isotope ratios of the samples are pre- sented in Table 4. Sr isotope ratios in diorites are SK-23 = 0.704828 and SK-25 = 0.704754. They are higher in tonalities, with SK-27 = 0.705405 and SK-29 = 0.706053. The Pb isotope ratios display similar distribution in to- nalites and diorites (Table 4). The distributions of the samples were analyzed in dif- ferent isotope variation diagrams (Figure 7). According to these analyses, the samples in the 87Sr/86Sr - 206Pb/ 204Pb diagram are positioned close to the Upper Conti- nental Crust and Lower Continental Crust (Figure 7(a)) but are mainly found in the Ocean Island Basalt (OIB) zone. Three samples in the 208Pb/204Pb - 206Pb/204Pb dia- gram are found in Enriched Mantle II (EM II), and one sample from the acidic rocks is in the enriched oceanic sedimentary zone very close to the EM II zone (Figure 7(b)). Such distribution indicates that the samples have compositions similar to those of enriched mantle sources and that the differentiation between I-type granitoids and crustal materials plays an active role in the formation of the magma. The intensification of samples in the 207Pb/ 204Pb - 206Pb/204Pb diagram (Figure 7(c)) around the EM II or pelagic sediment zone may indicate the presence of a sedimentary rock in the mantle source or the contami- nation of large magma masses resulting from a used-up under-continent lithospheric mantle. However, the greater concentration in our samples around EM II can be better interpreted as the presence of continental crust traces in the source rock or the presence of continent-derived sediments [40,41]. 7. DISCUSSIONS Based on the evaluation of mineralogical-petrographic and geochemical studies (Figures 3-6), these intrusive rocks indicate two different phases with I-type granitoid property. Thus, utilizing these data is important to de- termine the crystallization processes [i.e., fractional cry- stallization (FC) and accumulation fractional crystalliza- tion (AFC)] of the magma forming the Keban magmatics, magma mixing, and source characteristics. These issues will be discussed within this context. 7.1. Interpretation of Amphibole K-Ar Ages Two amphibole separates extracted from diorites and quartz diorites yielded similar K-Ar ages of about 85 Ma (Table 1). This finding indicates synchronous cooling below 500˚C based on the blocking temperature of ra- diogenic Ar in amphibole minerals [42]. The two am- phibole ages from the tonalites are not consistent with each other (Table 1). Samples SK-27 and SK-29 yielded ages of about 75 and 60 Ma, respectively. This age dif- ference may have resulted from amphibole minerals, which were crystallized at different stages during the solidification of magma; i.e., the older K-Ar age data may come from early-stage amphibole minerals first crystallized during FC. Another reason could be the loss of radiogenic Ar, which could result in low K-Ar age of the rock sample SK-29. This sample shows an alteration effect under microscopy, i.e., chloritization and epidoti- zation of biotites, sericitization and saussuritization of feldspars, and opacitizations of amphibole minerals. In this circumstance, an age interval between approxima- tely 85 and 60 Ma is suggested as the most reliable am- phibole K-Ar cooling age for Keban magmatics. 7.2. Crystallization Processes The main processes of magma crystallization in Keban magmatics are FC, magma mixing, and AFC. Based on the Rock/Chondrite diagram (Figure 6(a)), FC is deve- loped as the FC of different magmatic phases in acidic and intermediate magmas. The negative correlation of CaO, FeO*, MgO, MnO, Ti O2, and P2O5 composition with increasing silica and its positive correlation with Na2O and K2O in the Harker diagrams (Figure 5) indicate the FC effect, especially in basic/intermediate rocks. The 5.00 - 7.30 variation of the MgO content in diorite and quartz diorites further indi- cates the predominance of olivine and pyroxene in frac- tionation phase. This variation is also observed in the Table 4. Sr and Pb isotope geochemical data. Sample Rock Sr (ppm)Rb (ppm) 87Rb/86Sr (87Sr/86Sr)0 ( 87Sr/86Sr)i 206Pb/204Pb 207Pb/204Pb 208Pb/204Pb SK-23 Q diorite 205.7 4.0 0.0562 0.704828 0.70476 19.0597 15.6553 38.8229 SK-25 Diorite 194.6 0.5 0.0074 0.704754 0.70475 18.5931 15.6144 38.4978 SK-27 Tonalite 199.4 20.3 0.2945 0.705405 0.70509 18.9903 15.6472 38.8670 SK-29 Tonalite 192.7 30.5 0.4579 0.706053 0.70566 19.0272 15.6678 38.9609
S. Kürüm / Natural Science 3 (2011) 750-767 Copyright © 2011 SciRes. OPEN ACCESS 760 Figure 7. (a) 206Pb/204Pb vs. (87Sr/86Sr)i, (b) 206Pb/204Pb vs. 208Pb/204Pb and (c) 206Pb/204Pb vs. 207Pb/204Pb variation diagrams of the rock samples from the Keban magmatic rocks [40]. OIB (Oceanic Island Basalts), UCC (Upper Continental Crust), LCC (Lower Continental Crust), NHRL (Northern Hemisphere Reference Line), HIMU (mantle with High U/Pb ratio), PREMA (frequently observed PREva- lent Mantle composition), BSE (Bulk Silicate Earth), EMI and EMII (Enriched Mantle), DMM (Depleted Mantle). change in LILE and HFSEs with silica, enabling the formation of some major rock-forming minerals (Figure 8). The increase in Rb content caused by the silica dia- gram in diorite and quartz diorites (Figure 8(a)) indi- cates AFC processes, whereas Ba and Sr-SiO2 variation diagrams signify pyroxene (orthopyroxene and clinopy- roxene) fractionation for these rocks (Figures 8(b) and (c)). There is no evident trend in tonalites in Rb and Ba com- positions, and biotite and K-feldspar fractionation is an observed trend in Sr (Figure 8(c)). The Y-silica variation diagram shows the effect of amphibole in diorites and tonalites (Figure 8(d)). The effect of amphibole on the
S. Kürüm / Natural Science 3 (2011) 750-767 Copyright © 2011 SciRes. OPEN ACCESS 761 Fiure 8. Rb, Ba, Sr and Y vs. silica (a-d) semi-logarithmic variation diagrams of various I-type granites from the Keban magmatic rocks, Arrows were taken after [43], and they indicate theoretical Rayleigh fractionation vectors modelled for crystallization of indicidual mineral phases. Theoretical vectors for the likely crystallizing phases are for 50% fractiona- tion of single phases. Partition coeddicients used for the modelling are from the compilation [37]. (e) Sm vs Rb variation diagram [44]. (f) Y vs Rb variation diagram [45]. (g) MgO-87Sr/86Sr cariation diagram. Opx, orthopyroxene; cpx, cli- nopyroxene; amp, amphibole; plg, plagioclase; bio, biotite; K-feld, K-feldspar; hb,hornblende; gt, garnet; zr, zircon; ol, olivine. crystallization process of the tonalites is also visible in Sm-Rb and Y-Rb variation diagrams (Figures 8(e) and (f)). A weak effect of amphibole on diorites is also ob- served. Different from the above-mentioned diagrams, the 87Sr/86Sr-MgO variation diagram demonstrates the effect of FC on diorites and the effect of source con- tamination/mixing (SC/M) on tonalites (Figure 8(g)). In addition to all these data, the ratios among the highly incompatible elements are used in petrogenetic processes, such as partial melting and fractional crystal- lization. For example, Zr/Y is not greatly affected by fractional crystallization in basaltic system, but it changes
S. Kürüm / Natural Science 3 (2011) 750-767 Copyright © 2011 SciRes. OPEN ACCESS 762 during partial melting [46]. In case of a low melting de- gree, the Zr/Y rate is high. This finding corresponds to the changed rate of Y, which is higher than Zr. Accord- ingly, Zr/Y rates in the samples (Figure 9) markedly show that the partial melting processes changed during the formation of rocks. This change can be clearly ob- served between tonalites and diorites. Tonalites are also influenced by different melting degrees. Rocks have dif- ferent Fe contents, depending on the differences in their mantle source compositions. As a result, high Fe content and Zr/Y rate indicate high pressure or low melting rates during magma formation processes. Aside from the field data on the presence of MME and syn-plutonic dykes in Keban magmatics, the existence of special textural properties, such as skeleton structure in the minerals, oscillator zoning, and acicular apatites, indicates the mixing of mantle and crustal magma [47]. This interaction between acidic and intermediate-com- position rocks emerges from the different distributions of the elements in the geochemical data (Figures 8(a)-(g)). In these data, AFC processes were observed in both rock groups. Therefore, the co-development of crust assimila- tion and FC is more effective in the crystallization of the felsic magma. 7.3. Source Characteristics Ni content is an important indicator of whether the source material in the plutonic rocks is primitive or de- pleted mantle. Low Ni content (5.9 - 19 ppm) in Keban magmatics demonstrates that the source material is not primitive mantle but depleted mantle melt that under- went significant fractional crystallization [48]. The dio- rites seem entirely derived from the mantle in the Na2O- K2O diagram (Figure 10(a)), whereas it is generally con- centrated in the region of depleted mantle in the Zr/Yb-Nb/Yb diagram (Figure 10(b)). The diorites also demonstrate a passage to the E-MORB. The same dia- gram demonstrates that the tonalites are concentrated in the E-MORB region, whereas the Sm/Yb-Ce/Sm dia- gram (Figure 10(c)) shows that the diorites are in MORB, and the tonalites are in the interaction site of MORB-OIB. This kind of concentration can be created by subduction zone enrichment or crust contamination [53]. Crustal interaction is evident in the Rb/Y-Nb/Y and Ba/La-Ce/Pb diagrams (Figures 10(d) and (e)). Further- more, the effect of subduction is apparent in the Th/Yb- Ta/Yb (Figure 10(f)) diagram used in magmatic petrol- ogy [53]. The FC effect is also visible in this diagram. Although Keban magmatics generally display MORB property in the diagrams (Figures 10(a)-(f)), they also show eastern Hebei granulites, compositional property of arc volcanites, and classic continental sedimentary in the Figure 9. Distribution of the Zr/Y-(La/Yb)N diagram for the Keban magmatic rocks. Normalizing values [38]. Ba/Nb-La/Nb diagram (Figure 10(g)). This effect is due to enrichment of the mantle material by the upper crust sediments before partial melting. As previously mentioned, a clear indicator of the ef- fectiveness of crustal contamination is the increase in Rb/Sr and K2O/P2O5 depending on SiO2 (Table 2) [55]. However, this indicator should be considered together with AFC and partial melting [56]. The high values of some element ratios, such as Ba/Nb (diorite = 20 - 112; tonalite = 40 - 151) and Zr/Nb (dio- rite = 19 - 39; tonalite = 26 - 226; Table 2), in Keban magmatics, which are observed to have similar composi- tional properties with the mantle wedges in some of the diagrams presented, denote that these rocks were sub- jected to mantle-derived depletion [57]. The La/Nb ratios used to differentiate between asthenospheric and litho- spheric mantle sources are higher than 1 (La/Nb > 1) in sub-continental lithospheric mantle sources and lower than 1 (La/Nb < 1) in asthenospheric mantle sources. La/Nb value > 1 in all the samples (1.4 - 5.9) is another indicator of the lithospheric mantle property of these rocks [58]. However, some researchers suggest that rela- tive depletion, especially in Nb and Ta, could be caused by the interaction between the sub-continental litho- spheric mantle and the asthenospheric melt [59]. Similarly, frequent modification of sub-continental lithospheric mantle caused by dehydration in the sub- duction zone and its sediment content [38] causes rela- tive depletion of Ti, Nb, and Ta and the enrichment of Ba. The remarkably negative anomaly of Nb and Ti in Ke- ban pluton diorites and tonalites indicates that apatite and Fe-Ti oxides play an important petro-genetic role in the formation of rocks [60]. These data on magma origin show that Keban mag- matics were formed from a single source. However, in this petro-genetic process, the two rock groups were de- veloped by FC, AFC, and magma composition processes. Accordingly, they were formed in two different phases.
S. Kürüm / Natural Science 3 (2011) 750-767 Copyright © 2011 SciRes. OPEN ACCESS 763 Figure 10. (a) Na2O vs K2O, PRB: batolites of the arc-magmatism in the subduction zones. PFB: lower crust of fractional—crystallization granitoides. M-type: subfield derived-mantle granitodes [49]. (b) Zr/Yb vs Nb/Yb and (c) Sm/Yb vs. Ce/Sm [50]; (d) Rb/Y VS Nb/Y [51]; (e) Ba/La vs Ce/Pb [52]; (f) Th/Yb vs. Ta/Yb; (g) Ba/Nb vs. La/Nb plots of the rock samples from the Keban magmatic rocks.
S. Kürüm / Natural Science 3 (2011) 750-767 Copyright © 2011 SciRes. OPEN ACCESS 764 Crustal interaction and hybrid magma textures in to- nalites are indicative of this. In the samples, 87Sr/86Sr in diorites/quartz diorites is 0.704, which shows that this magma formed from an isotopically derived lithospheric mantle. The tonalites, with values of 87Sr/86Sr = 0.705 - 0.706, are derived from the mixing of the upper mantle material of the same properties. Typical mafic and magma originated from the lower crust. 7.4. Geodynamic Interpretation In many studies conducted in the region, various tec- tonic models have been suggested for the tectonomag- matic units in southeast Anatolia. One premise is that the Neotethyan oceanic was closed in the late Cretaceous era [24]. However, in this model, the existence of the plat- form carbonates (Malatya-Keban) and arc-type intrusive rocks such as Baskil granitoids is difficult to explain [7]. Another suggestion is that southern Neotethyan was closed in middle Miocene [61]. Some authors suggest that the closing of the southern Neotethyan was com- pleted with the emplacement of ophiolites in late Creta- ceous [7,62-65]. Despite these controversies related to the closure of the Tethyan oceanic basin and its relevant formations, the tectonomagmatic/stratigraphic units in the southeast Anatolia orogenic system are undisputable: metamorphic massifs (Malatya-Keban platform carbon- ates), SSZ ophiolites (Göksun, İspendere, Kömürhan, Gule- man), ophiolite-related metamorphic rocks (Berit meta- ophiolite), and granitoides (Göksun, Doğan şehir, Baskil). All granitoids in this belt are intruded into metamor- phosed platform carbonates and ophiolites. Keban magmatics, which have volcanic-arc magma- tics characteristics (Figure 11), are in harmony with all these granitoids in terms of petrographic, chemical, and radiogenic isotope ages and geologic positioning. There- fore, in the regional scale, they must be discussed within the context of the Malatya-Keban platform and Baskil arc magmatics. 8. CONCLUSIONS Keban magmatics, which widely crop out between Elazığ and Keban, represent two different phases in the composition of diorites/quartz diorites and tonalites. The two units show subduction zone VAG and I-type grani- toid properties as well as different K-Ar amphibole ages of 84 - 85 Ma in diorites/quartz diorites and 60 - 75 Ma in tonalites. According to the age data on Keban mag- matics, rocks with acidic composition have been intruded later than the basic ones. These results are in accordance with the field data. Major and trace element variations indicate the effect Figure 11. Rb (ppm) vs Y + Nb (ppm) geotectonic discrimina- tion diagrams for the Keban magmatic rochs. Syn-COLG, syn- collisional granitoids; WPG, within-plate garnitoids; VAG, vol- canic-arc granitoids; ORG, ocean-ridge granitoids. of mineral fractionation during the formation of both rock groups, especially fractionation of plagioclase, horn- blende, pyroxene, and olivine, aside from apatite and Fe-Ti oxides. The enrichment in LIL elements, as observed in the tonalities, can be explained by the enrichment of mantle- derived magmas by crustal contamination [66]. Very strong negative anomalies of Ti and Nb in tonalites are apparent, whereas those in diorites only deplete Nb. This feature confirms that aside from the similarity with the subduction zone series, diorites may also be mantle de- rived. In light of all the data, this pluton can be inferred to be composed of diorites, which developed in the pre-colli- sion environment. These plutons are formed by a man- tle-derived mafic magma source and tonalites derived from the mantle and crust developed in the post-collision environment. The tonalites are affected by the mafic and felsic magma mixture formed by the magma melting the crust during its injection into the crust or its ascent through the crust. The frequent modification of the sub-continental litho- spheric mantle caused by dehydration in the subduction zone and the subduction sediments causes a proportional depletion in Ti, Nb, and Ta and enrichment in Ba. The strongly negative anomalies of Nb and Ti in diorites and tonalites in the Keban pluton can be considered indica- tors of subduction sediments, as observed in the dia- grams. The negative Ti anomaly indicates that apatite and Fe-Ti oxides petrogenetically play an important role in the generation of this magma. According to the petrographic, geochemical, and ra- diogenic isotope data, Keban magmatics have petro- graphic, geochemical, geochronologic, and geotectonic
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