ГЕОХИМИЯ, 2014, № 4, с. 329-349
GEOCHEMISTRY OF DACITIC VOLCANICS IN THE EASTERN PONTIDES (NE TURKEY)
© 2014 Ferkan Sipahia, * and M. Biirhan Sadiklarb
aJeoloji Muhendisligi Bolumu, Gumu shane University, TR-29000 Gumu shane, Turkey e-mail: ferkansipahi@gmail.com bJeoloji Muhendisligi Bolumu, Karadeniz Technical University, TR-61080 Trabzon, Turkey
Поступила в редакцию 20.12.2011 г.
Принята к печати 11.08.2012 г.
Dacitic rocks that crop out around the Zigana Mountain (Gflmfl§hane) in the eastern Pontide (NE Turkey), are mainly composed of quartz, plagioclase, sanidine, amphibole, muscovite, and biotite as the main minerals. Zircon and rutile are the accessory minerals. Pyrite, chalcopyrite, and covellite are the opaque components. On the basis of trace-element data, dacites have been classified as Dacite-I and Dacite-II. Dacite-I is tholeitic-transitional, whereas Dacite-II is transitional-calc-alkaline. The geochemical variation can be explained by the fractionation of the common mineral phases, such as plagioclase, hornblende, magnetite, and apatite. Dacites also show island-arc properties, with negative Nb, Sr, P, and Ti anomalies. The trace-element distrubitions of the dacitic rocks reflect the typical characteristics of rocks from the subduction-related tectonic setting, with enrichment of large-ion lithofile elements and light rare-earth element, but depletion in high-field-strength elements. The dacitic rocks are developed through plagioclase ± hornblende—controlled fractionation from the same parental magma that settled in two successive stages and are derived from an enriched source, probably by the mixing of slab-derived and lithospheric melts.
Keywords: Dacite, eastern Pontide, fractional crystallization, geochemistry, REE, Turkey.
DOI: 10.7868/S0016752514040086
1. INTRODUCTION
The study area, located in the eastern part of the Black Sea region of Turkey and composed of Late Cretaceous volcanic rocks, belongs to the east Black Sea metallogenic province, which lies along the E-W line in the region and includes various mines. This region has been widely studied by various researchers [1—6]. Volcanic rocks are observed extensively in the region and comprise numerous different ore deposits [7, 8] (Fig. 1). The dacitic rocks of the eastern Black Sea metallogenic belt are host to numerous massive sulfide deposits [1, 2, 9, 10] and are widely spread within an area extending about 400 km in the east-west direction and 60 km in the north-south direction within the eastern Black Sea Region of Turkey. Previous studies on dacites in the region are descriptive and of limited use in exploration. Dacites only can be distinguished in a mine as a footwall and hanging wall due to the intensive alterations. In brief, a geochemical criterion has not been found to identify the footwall and hanging-wall dacites or different dacites. However, the current data imply at least two formation stages.
Some elements may have been mobile because dac-itic rocks have undergone polyphase deformations to yield alterations. The problem of element mobility
during alteration processes has been minimized by mainly considering the high-field-strength elements (HFSE: Th, Nb, Zr, Hf, Ta, P, Ti, and Y), rare-earth elements (REEs, La and Lu), and transition metals (such as Cr, Ni, Co, and V), which are considered relatively immobile during alteration and low-grade metamorphism [11, 12]. Thus, the present study is focused on the petrology and geochemistry of Late Cretaceous aged dacites using HFSE and REE analyses and aims to find geochemical criteria that differentiate these dacites that are altered, in addition to being unidentified.
2. REGIONAL GEOLOGY
The eastern Black Sea metallogenic province is located along the Alpine metallogenic belt, which developed as an island arc from the Jurassic to the Miocene periods during the subduction of the Tethyan oceanic crust [13—15] favors the southward subduction, although [14] suggested that subduction of the Tethyan oceanic crust was to the north. The geological setting of the eastern Black Sea region is mainly the result of three main Neo-Tethyan volcanic cycles during the Jurassic, upper Cretaceous, and Eocene eras [6, 8, 16, 17]. These volcanic cycles provide insights into the
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paleoisland arc and the long-term crustal evolution from presubduction rifting, through arc volcanism and plutonism, to postsubduction alkaline volcanism [18, 14, 19].
Three main different geological zones, differentiated by their lithological characteristics, are present in the eastern Pontides: the northern, southern, and axial zones. These zones are separated by fault zones oriented in the E-W, NE-SW, and NW-SE directions, which define the block-faulted tectonic style of the eastern Black Sea region. The northern zone contains extensive upper Cretaceous and Tertiary volcanic rocks cropping out mainly along the Black Sea coast, whereas Paleozoic plutonic and metamorphic rocks and their sedimentary sequences characterize the southern zone [4, 20, 21]. Volcanism in the region commenced during the Liassic, with the formation of the basic rocks, in a rift environment developed on a Precambrian to Paleozoic basement [4, 22, 23]. The succeeding volcanic rocks were predominantly felsic. The dacitic rocks were followed upward by the Late Cretaceous to the Eocene volcanic rocks [24, 25]. The Cretaceous volcanic rocks are tholeiitic to calc-alka-line in composition and comprise dacites, rhyolites and minor andesites, basalts, and their pyroclastic equivalents. Large volumes of granitoidic magma intruded the region during the Late Cretaceous to Late Eocene [26-29].
Volcanogenic massive sulfide (VMS) deposits in the eastern Black Sea Region are situated in the in-traarc rift zone of the Black Sea island arc (Turkey) characterized by a bimodal volcanic rock suite and hosted by the dacitic rocks of the Late Cretaceous age [30-32].
3. GEOLOGY OF THE STUDY AREA
The Eastern Pontide (NE-Turkey) shows a well-protected island-arc characteristic, with effective submarine volcanism during the period from Jurassic to Tertiary [14, 20, 27]. The basement of the study area is formed by the Late Cretaceous basalt, andesite, and their pyroclastics (Fig. 2). The basement rocks are overlain by the Late Cretaceous aged dacitic rocks, namely Dacite-I and Dacite-II, described by [33] according to their trace element contents. Dacitic rocks are composed of lavas, tuff, aglomerates, and breccias and are commonly interbedded with red biomicrite. All volcanic rocks are cut by andesite and porphyric dacite dykes and are conformably overlain by andesite and its pyroclastics. All volcanic rocks, except for the top of the andesite, include local limestone lenses. On the basis of the paleontological evidences (Margin-otruncanapseudolinneiana, Marginotruncana spv Glo-bigerinelloides spv Dicarinella spv and Ticinella sp.), the age of the volcanics has been accepted as Late Cretaceous (Turonian to Santonian), and in addition, the age of illite in Dacite-I and Dacite-II on average were 78.7 ± 2.3 and 75.3 ± 2.4 My (Campanian-Danian), respectively, according to the radiometric K-Ar dating on illites from these dacites [33].
The volcanic rocks have been affected by the tectonic activity in the region. The tectonic structure of the region is determined by N 20°-70° W and N 20°-70° E strikes. According to the measurement on volcanics, N 40°-50° W and N 60°-70° E fracture strikes are appropriate with all lithologic units and show their importance in terms of ore setting. The Late Cretaceous dacitic rocks are also the hosts of VMS and vein deposits in the study area. [33] determined that Dac-ite-I might host massive sulfide deposits because of
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their chemical affinity, volcanic-arc characteristics, alteration types, and bimodal character.
4. MATERIALS AND ANALYTICAL TECHNIQUES
The least altered and completely altered samples (34 dacitic rock samples, 18 basalt and andesite samples) were selected from among 390 samples for chemical analyses after a detailed microscopical investigation. Analyses of the Y, Sc, Nb, Cs, La, Ce, Hf, Ta, Th, U and REE contents in representative rock samples were conducted on the samples (# 345, 86, 131, 142, 164, X1, 246, 255, 431, 494 and 512) using inductively coupled plasma mass spectrometer (ICP-MS; Perkin-Elmer Elan 600) at the commercial ACME Laboratories Ltd in Vancouver (Canada). The major- and As, Ba, Cl, Co, Cu, Cr, Ga, Ni, Pb, Rb, Sn, Sr, Ta, V, W, Zn, and Zr concentrations of these samples were determined by x-ray fluorescence spectroscopy (XRF) at the Department of Mineralogy, Friedrich-Schiller University (Jena-Germany), whereas the major and trace elements of the remaining samples (# 25, 50, 161, T, 232, 260, 275, 285, 288, 301, 304, 311, 359, 382, 388, 397, 419, 492, 496, 528, 560, 565, 247, and G1) were analyzed by a Philips PW 1404 XRF at Technisc
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