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Geochemical and Pb-Sr-Nd Isotopic Constraints Indicating an Enriched-Mantle Source for Late Cretaceous to Early Tertiary Volcanism, Central Anatolia, Turkey

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Geochemical and Pb-Sr-Nd Isotopic Constraints Indicating an Enriched-Mantle Source for Late Cretaceous to Early Tertiary Volcanism, Central Anatolia, Turkey
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   International Geology Review , Vol. 46, 2004, p. 1022–1041.Copyright © 2004 by V. H. Winston & Son, Inc. All rights reserved. 0020-6814/04/769/1022-20 $25.00 1022 Geochemical and Pb-Sr-Nd Isotopic Constraints Indicating an Enriched-Mantle Source for Late Cretaceous to Early Tertiary Volcanism, Central Anatolia, Turkey M USA  A LPASLAN , 1    Department of Geology, Mersin University, 33343 Ciftlikkoy-Mersin, Turkey R OBERT  F REI , Geological Institute, University of Copenhagen, ster Voldgade 10, DK-1350 Copenhagen, Denmark D URMU º  B OZTUG ,  Department of Geology, Cumhuriyet University, Sivas 58140, Turkey M EHMET  A LI  K URT ,  Department of Geology, Mersin University, 33343 Ciftlikkoy-Mersin, Turkey AND  A BIDIN  T EMEL  Department of Geology, Hacettepe University, 06532 Beytepe, Ankara, Turkey Abstract Bulk-rock major, trace, and REE geochemistry and Pb-Sr-Nd isotopic compositions identifymantle sources involved in the genesis of volcanic rocks of the Upper Cretaceous–Lower TertiaryUluki º la Formation, Çamard-Uluki º la Basin, Nigde Province, central Anatolia. Incompatible trace-element patterns exhibit a large Nb-Ta trough and strong enrichment of LILE such as Ba, Th and U,and LREE, which indicate a subduction-zone signature. Trace-element ratios are compatible with asubcontinental lithospheric source. Isotopic data demonstrate the presence of an EMII-like protolith( 87 Sr/ 86 Sr = 0.707242–0.707582, 143 Nd/ 144 Nd = 0.512336–0.512390, 206 Pb/ 204 Pb = 18.70–18.917, 207 Pb/ 204 Pb = 15.716–15.796, 208 Pb/ 204 Pb = 39.157–39.45). These geochemical and isotopicdataindicate the derivation of the studied volcanic rocks from an enriched subcontinental mantlesource, modified by earlier subduction events. This petrogenetic conclusion is compatible with ageodynamic setting of post-collisional extension for the Çamard-Uluki º la Basin. Introduction A NATOLIA   IS   WIDELY  considered to constitute anamalgamation of microcontinents separated by oce-anic realms ( ª engör and Yilmaz, 1981; ª engör andNatl’in, 1996 and references therein). Within thesegeotectonic segments, the intervening Neo-Tethysocean was separated into northern and southernNeotethys oceanic realms. The northern realm wasfurther subdivided into a Northern branch ( ª engör and Yilmaz, 1981; Yilmaz et al., 1997; Okay and ª ahintürk, 1997) and an Inner Tauride oceanictrough (Görür et al., 1984; Görür and Tüysüz, 2001).The evolution of these oceanic domains is docu-mented by igneous and sedimentary records such asarc magmatism in the eastern Pontides (Okay and ª ahintürk, 1997; Yilmaz et al., 1997; Boztug et al.,2003, 2004), syn- to post-collisional magmatism incentral Anatolia (Boztug, 2000; Düzgören-Aydin etal., 2001; Köksal et al., 2001, Boztug et al., 2003;lbeyli et al., 2004). Fore-arc and post-collisionalbasins in central Anatolia resulted from the Neo-tethyan convergence system in Turkey (Görür et al.,1984; Göncüoglu et al., 1992, 1993, 1995; Erdoganet al., 1996; Poisson et al., 1996; Boztug et al.,2003, 2004). The latter group includes the Çamard-Uluki º la Basin described herein.The basin consists of volcanic and sedimentaryrocks of Late Cretaceous + Paleocene + MiddleEocene age, reaching thicknesses of 5 km (Oktay,1982; Nazik, 1989; Görür et al., 1998). Mafic pillowlavas in this basin are suggested to have beenemplaced during an extensional regime in the LateCrateceous (Blumenthal, 1956; Oktay, 1973). 1 Corresponding author; email: malpaslan@mersin.edu.tr    LATE CRETACEOUS TO EARLY TERTIARY VOLCANISM  1023 Formation of this basin has been variably interpretedin terms of geodynamics as an inter-arc basin (Görür et. al., 1998), a back-arc basin (Demirta º li et al.,1984), or an island-arc–related basin (Oktay, 1982;Ba º  et al., 1986; I º ler, 1988). Boztug et al. (2001)pointed out that the Çamard-Uluki º la volcano-sedi-mentary sequence comprises rift-related basin fillthat was intruded by mantle-derived plutons some-time around latest Cretaceous to Early Tertiary time.Clark and Robertson (2002) documented thewithin-plate character and subduction geochemicalsignature of these volcanic rocks. However, there isnot agreement on the genesis and evolution of thevolcano-sedimentary rocks in the Çamard-Uluki º laBasin. This paper presents new geochemical andPb-Sr-Nd isotopic data, all of which yield newinsights into the genesis of this basin. Tectonic Setting There are two different interpretations of tectonicsetting of the Çamard-Uluki º la Basin. The firstinterpretation assumes that an Inner Tauride oceanexisted between the Bolkar Carbonate platform tothe south and Nigde-Kir  º ehir microcontinent to thenorth (Görür et al., 1984). In this model, the Inner Tauride ocean was subducted northward in LateCretaceous–Early Tertiary time, during which theUluki º la Basin was formed in a fore-arc geotectonicsetting along the margin of Nigde-Kir  º ehir micro-continent (Görür et al., 1984). Geological and struc-tural data all indicate an extensional setting. Theseinclude: (1) unconformity between the rocks of theÇamard-Uluki º la basin and the Bolkar Carbonateplatform; (2) lack of a contemporaneous accretion-ary prism related to arc volcanism; (3) a within-platecharacter of the volcanic rocks in this basin; and (4)correlative stratigraphic and sedimentological rela-tionships (Clark and Robertson, 2001, 2002). The second interpretation assumes that only asingle Northern Neotethys ocean existed (Göncüo-glu, 1986; Dirik et al., 1999). In this interpretation,the Central Anatolian Crystalline Complex existedas a promotory of the Mesozoic Bolkar (Tauride)continent to the south, rather than as a microconti-nent, and no suture formed beneath the Uluki º labasin (Özgül, 1976; Göncüoglu, 1986). The LateCretaceous–Early Tertiary basins bordering thesouthern margin of the Central Anatolian CrystallineComplex are considered to have formed as a resultof post-collisional extension following closure of theNorthern Neotethys along the Ankara-Erzincansuture zone (Çemen et al., 1999; Dirik et al., 1999).Clark and Robertson (2002) demonstrated that thelatest Cretaceous (Maastrichtian)–Late EoceneUluki º la Basin is extensional (or transtensional) andformed after that initial closure of the local strand of the Northern Neotethys ocean (Inner Tauride ocean). Geological Setting The study area lies in southern part of CentralAnatolia, bounded by the Central Anatolian Crystal-line Complex (Göncüoglu et al., 1991) to the north,by the Bolkar Carbonate platform to the south, bythe left-lateral Ecemi º  fault zone and associatedOligo-Miocene deposits (Yeti º , 1984; Koçyigit andBeyhan, 1998; Jaffey and Robertson, 2001; Clarkand Robertson, 2002) to the east (Fig. 1). The west-ern boundary of the basin is not clear, because basi-nal sediments are overlain by continental Neogeneto Recent sediments (Clark and Robertson, 2002);however, the Çamard-Uluki º la Basin is consideredto be genetically linked with the Tuzgölü BasinComplex to the northwest (Görür et al., 1984). The volcano-sedimentary rocks of the basin lieuncomformably on top of the Upper CretaceousAlihoca ophiolite (Fig. 2), which was emplaced ontothe Bolkar Carbonate platform (Demirta º li et al.,1984; Lytwyn and Casey, 1995; Dilek et al., 1999;Clark and Robertson, 2002). The rock types in theÇamard-Uluki º la Basin consist of interlayeredconglomerate, sandstone, marl, pelagic limestone,reefal limestone, claystone, and volcanic rocks; thelatter consist of pillow lava, lava flows, and pyro-clastics of the Uluki º la Formation (Fig. 2; Alpaslanet al., 2003). The Uluki º la Formation is intruded bysome crustal thinning–related mantle-derived intru-sive rocks, the Uçurum monzogabbro, the Elmalitrachyte, and the Yaglita º  diorite (Fig. 2) asdescribed by Boztug et al. (2001) and Alpaslan et al.(2003). In the lower portion of the basin, pillowlavas occur at several stratigraphic horizons. Towardthe middle of the sequence, they gradually decreasein abundance and are replaced by massive lavaflows at the top of the sequence (Alpaslan et al.2003). There are no published radioisotopic agedeterminations for any of these units, but fossilrecords indicate that the volcanism occuredbetween the Late Cretaceous and Paleocene (Fig. 2;Demirta º li et al., 1975; Oktay, 1982; Demirta º li etal., 1984; Dellaoglu and Aksu, 1986; Göncüoglu etal., 1991). However, Clark and Robertson (2002)  1024  ALPASLAN ET AL. F IG . 1. A. Location of the study area and Neotethyan sutures of Turkey (after Clark and Robertson, 2002). B. Major sedimentary basins of central Anatolia. Abbreviations: BP = Bolkar Carbonate platform, NKM = Nigde-Kir  º ehir Massif,UB = Uluki º la Basin; TB = Tuzgölü Basin; HB = Haymana Basin; KKB = Kirikkale Basin; CB = Çankiri Basin; YSB =Yozgat-Sorgun Basin; KB = Kizilirmak Basin; YB: Yildizeli Basin: RB = Refahiye Basin; SB = Sivas Basin; SKB = ª arki º la Basin, EFZ = Ecemi º  fault zone (after Clark and Robertson, 2002).   LATE CRETACEOUS TO EARLY TERTIARY VOLCANISM  1025    F    I   G  .   2 .   S   i  m  p   l   i   f   i  e   d  g  e  o   l  o  g   i  c  a   l  m  a  p  o   f   t   h  e  s   t  u   d  y  a  r  e  a ,  a   f   t  e  r   A   l  p  a  s   l  a  n  e   t  a   l .   (   2   0   0   3   ) .  1026  ALPASLAN ET AL. suggested that the volcanism continued until Earlyto Middle Eocene time. Major structural elements of the mapped area arecomposed of nearly E-W–trending thrust faults,NE-SW–and NNE-SSW–trending left-lateralstrike-slip faults and some ENE-WSW folds (Fig.2). A major and presumably early thrust is presentbetween the Alihoca ophiolite and Bolkarda carbon-ate platform. The ophiolitic unit was thrust onto thecarbonate rocks from north to south (Fig. 2), mostprobably before the opening of the Çamard-Uluki º la Basin—i.e., it belongs to the imbricatedbasement. Another later thrust developed justwithin the Uluki º la formation, again from north tosouth, due mainly to a later N-S compressionalevent. Such N-S compression would also have beenresponsible for the ENE-WSW folds and NE-SWfaults, both of which affect only the Uluki º la Forma-tion, as well as the later thrust (Fig. 2). Therefore, allthese structural elements such as E-W thrusts,ENE-WSW folds, and NE-SW faults affecting onlythe Uluki º la Formation are considered to have beenderived from a N-S compressional event that tookplace after Eocene time. The NNE-SSW–trendingfault in the southeast parts of the mapped area ispart of the well-known Ecemi º fault that is one of themajor neotectonic faults of the Anatolian provinceinduced by ongoing convergence between theEurasian and Arabian plates (Bozkurt, 2001). Analytical Techniques Twenty-eight rock samples were selected for geochemical analyses (major and trace elements,REE), and six of them for isotopic (Pb, Sr, and Nd)analyses (Table 1). For major element analyses,fused disks were prepared using six parts of lithiumtetraborate and one part rock powder. The mixturewas fused in crucibles of 95% Pt and 5% Au at1050°C for 60 minutes to form a homogenous melt.The melt then was poured into a preheated mold,and chilled as a thick glass disk. Whole-rock analy-ses were performed at Hacettepe University using aPHILIPS PW 1480 X-ray spectrometer using USGSrock standards. Trace and rare earth element con-centrations were analyzed at ACME laboratories(Vancouver, Canada) by ICP-MS using the fusionmethod; reported accuracy is better than ±3%.Sm-Nd, Pb, and Sr isotopic data and concentra-tions were obtained from 300 mg aliquots of thesame powders. For isotope dilution data of Sm andNd, a mixed 147 Sm- 150 Nd spike was added. Dissolu-tion of the samples was achieved in two successive,but identical steps which consisted of a strong 8NHBr attack followed by HF-HNO 3 , and then bystrong HCl. Lead leaching experiments involveda1N HCl attack for 5 minutes, after which theleachate was pipetted off and processed as a sepa-rate sample.Chemical separation of Sr and REEs fromwhole-rock samples was carried out on conventionalcation exchange columns, followed by separationusing HDEHP-coated beads (BIO-RAD) charged in6 ml quartz glass columns. Purification of the Sr fraction was achieved by a pass over micro-columnscontaining SrSpec resin. REEs were further sepa-rated over HDEHP-coated bio beads (BioRad)loaded in 6 ml glass stem columns. Pb was sepa-rated conventionally in 0.5 ml glass columnscharged with anion exchange resin, followed by aclean-up on 200 µl Teflon® columns. A standardHBr-HCl-HNO3 elution recipe was applied for bothcolumn steps.Total Pb procedural blanks were <125 pg for whole-rock chemistry, and are negligible relative tothe amount of Pb recovered from each sample. Pro-cedural blanks for Nd (<30 pg) and Sr (<100 pg) areinsignificant, and do not influence the measuredisotope ratios beyond their respective precisions.Mass spectrometric analyses were carried out on aVG Sector 54-IT instrument at the Geological Insti-tute, University of Copenhagen.The mean value for our internal JM Nd standard(referenced against La Jolla) during the period of measurement was 0.511115 for 143 Nd/ 144 Nd, with a2 σ  external reproducibility of ± 0.000013 (five mea-surements). Fractionation for Pb was controlled byrepetitive analysis of the NBS 981 standard (valuesof Todt et al., 1993) and amounted to 0.103 ± 0.007%/amu (2 σ ; n = 5). Sr was normalized to 86 Sr/ 88 Sr =0.1194, and repetitive analyses of the NBS 987 Sr standard yielded 87 Sr/ 88 Sr = 0.710248 ± 0.000004(2s, n  = 6) Rock Descriptions When plotted on the total alkali vs. silica diagramof Le Maitre et al. (1989), the volcanic rocks of theUluki º la Formation range from trachybasalt throughbasaltic trachyandesite to trachyandesite in composi-tion (Fig. 3). All these rock types show a moderate tostrong porphyritic texture with a hypocystallinegroundmass, except for some trachybasalts thatalsomay have intersertal texture. Themajor mafic
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