Geochemistry of volcanic rocks from the (~it:ekdag Ophiolite, Central Anatolia, Turkey, and their

16 Pages·2006·1.52 MB·English

Geochemistry of volcanic rocks from the (~it:ekdag Ophiolite,
Central Anatolia, Turkey, and their inferred tectonic setting
within the northern branch of the Neotethyan Ocean
KENAN M. YALINIZ, 1 PETER A. FLOYD 2 & M. CEMAL GONCOOGLU 3
1Department of Civil Engineering, Celal Bayar University, Muradiye, Manisa, Turkey
(e-mail: mukenan@anet, net. tr)
2Department of Earth Sciences, University of Keele, Staffordshire, ST5 5BG, UK
3Department of Geological Engineering, Middle East Technical University, TR-06531
Ankara, Turkey
Abstract: The Central Anatolian Ophiolites (CAO) comprise a number of little studied
Upper Cretaceous ophiolitic bodies that originally represented part of the northern branch
of the Neotethyan ocean. The ~iqekda~ Ophiolite (CO) is an dismembered example of this
ophiolite group that still retains a partially preserved magmatic pseudostratigraphy. The
following units (bottom to top) can be recognized: )1( layered gabbro; (2) isotropic gabbro:
)3( plagiogranite; )4( dolerite dyke complex; (5) basaltic volcanic sequence; and (6) a
Turonian-Santonian epi-ophiolitic sedimentary cover. The magmatic rock units (gabbro,
dolerite and basalt) form part of a dominant comagmatic series of differentiated tholeiites,
together with a minor group of primitive unfractionated basalts. The basaltic volcanics
mainly consist of pillow lavas with a subordinate amount of massive lavas and rare basaltic
breccias. Petrographic data from the least altered pillow lavas indicate that they were
originally olivine-poor, plagioclase-clinopyroxene phyric tholeiites. Immobile trace
element data from the basalt lavas and dolerite dykes show a strong subduction-related
chemical signature. Relative to N-mid-ocean ridge basalt the ~iqekda~ basaltic rocks
(allowing for the effects of alteration) have typical suprasubduction zone features with
similarities to the Izu-Bonin Arc, i.e. enriched in most large-ion lithophile elements,
depleted in high field strength elements and exhibiting depleted light rare earth element
patterns. The geochemical characteristics are similar to other eastern Mediterranean
Neotethyan SSZ-type ophiolites and suggest that the CO oceanic crust was generated by
partial melting of already depleted oceanic lithosphere within the northern branch of the
Neotethyan ocean. The ~iqekda~ body, along with the other fragmented CAO, is thus
representative of the Late Cretaceous development of new oceanic lithosphere within an
older oceanic realm.
Turkey occupies a critical segment in the CAO) found in the Central Anatolian Crystal-
Alpine-Himalayan orogenic system where rem- line Complex (CACC) have received little
nants of both the Palaeotethyan and Neoteth- attention until now. The CACC represents the
yan ocean basins crop out along broadly linear northern passive margin of the Mesozoic
tectonic belts. Turkey is divided into three main Tauride-Anatolide Platform that faced the
east-west trending belts of separate ocean VIAE ocean (G6nciJo~lu 1986; G6nctio~lu et
basins ($engOr & Yllmaz 1981): (1) a northern .la 1991) (Fig. 1). The CAO are significant in
belt representing remnants of an Intra-Pontide that they were initially generated within the
ocean; (2) a median belt representing alloch- VIAE ocean and emplaced southwards onto
thonous units derived from the Vardar-Izmir- the CACC (Yahnxz & G6nciio~lu 1998). The
Ankara-Erzincan (VIAE) ocean although majority of these ophiolites are tectonically
G6rtir et .la (1984) also consider a separate dismembered, although stratigraphic recon-
derivation for the Inner Tauride Belt; and (3) struction reveals that they originally repre-
a southern belt, variably called the Peri-Arabic sented a complete ophiolitic sequence (Yahmz
Belt or southern Neotethyan ocean ($eng6r & & G6nciio~lu 1998) (Fig. 1, inset b).
Yllmaz 1981) (Fig. 1, inset a). In terms of the pseudostratigraphic relation-
Most of the ophiolites occurring in these belts ship of the magmatic units and their chemical
have been studied to varying degrees, although designation, the CAO exhibit a suprasubduction
the isolated outcrops of allochthonous ophiolitic zone (SSZ) character within the VIAE segment
rocks (termed the Central Anatolian Ophiolites; of the Neotethys (Yahnlz 1996; Yahnlz &
:morF ,TRUKZOB E., ,RETSEHCNIW J. A. & ,REPIP J. D. A. (eds) scinotceT dna msitamgaM ni yekruT dna eht
gnidnuorruS .aerA Geological Society, London, Special Publications, 173, 203-218.1-86239-064-9/00/$15.00
~) The Geological Society of London .0002
402 .K YALINIZ ETAL.
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AL~IKULU NISAB
Fig. .1 Generalized geological map of the Central Anatolian Crystalline Complex and the location of the
~iqekda~ Ophiolite. (a) Neotethyan suture map of Turkey and the location of the Central Anatolian Crystalline
Complex (CACC); IPSC, Intra-Pontide Suture Zone; IAESZ, lzmir-Ankara-Erzincan Suture Zone; SEASZ,
Southeast Anatolian Suture Zone. (b) Simplified columnar section of the Ciqekda~ Ophiolite: ,1 Central
Anatolian Metamorphics; ,2 metamorphic ophiolite-bearing olistostrome; ,3 thrust boundary; ,4 layered gabbro;
,5 isotropic gabbro; ,6 plagiogranites; ,7 dyke complex; ,8 massive and pillow lavas; ,9 epiophiolitic sediments and
lavas; ,01 granitoids; ,11 Lower Tertiary cover sediments.
G6nctio~lu 1998; Yahmz et al. 1996; Floyd et al. picture for the geodynamic evolution of the
1998a). northern branch of the Neotethys.
Objectives Distribution of rock types and
stratigraphy
The ~iqekda~ Ophiolite (CO) is one of the most
complete and best exposed of the CAO bodies. The CO tectonically overlies the CACC base-
Although it has not been studied in detail, little ment along a moderately steep northward dip-
geochemical data being available, its general ping thrust just to the south of the ~iqekda~
character invites comparison with other better Massif. Previous field studies in the ~iqekda~
known CAO, such as the Sankaraman Ophio- area were mainly carried out by Ketin (1959),
lite (SO). New and extensive geochemical data Erdo~an et al. (1996) and Ydmaz-$ahin &
on the least altered volcanic rocks of the CO are Boztu~ (1997). Typical ophiolitic sequences are
presented here. The purpose of this study is to exposed around Akqakent, in the core of a
describe the compositional features of the vol- roughly north-south trending antiform in the
canic rocks of the CO as a basis for determining ~iqekda~ region (Fig. 2). Well-preserved and
their original tectonic setting and a comparison unfaulted successions are rare, although the
with other better known SSZ-type Central section along the Ak~akent-Kilimli road exhi-
Anatolian ophiolites. A further aim is to discuss bits much of the original structure.
the genesis and the palaeotectonic setting of the The CO pseudostratigraphy consists (from
CO within the VIAE ocean to provide a fuller top to bottom) of mainly pink pelagic cherty
GEOCHEMISTRY OF THE ~I~EKDAG OPHIOLITE 502
i ! i i i i i ~,~'+:::+~.,~i:!:!:!:i:~i i i i Aal.a.~l.ll. i ! i i i ! i i i ili ! ! !
• ,,e.'"^ ~ ~ ~^^^^^ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ v ~ ~
SSE N N W
~ Mezargedi~i 6 6 7 ~
54-55 61-64 57 59 ..~~vvvvvv£~'~ .... "~.~ 72
i ,v ,,,., v ~ v v
~ ' ~ ~ ' ~ ~ ~ _ ~ _ ~ ~ 1 O0 rn I 20 m
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1 2 3 4 5 6 7
SE
NW 46-49
42-4 50
J 5 \ 19-2' 16-18
~ 31-3435-411 \ ~5. ~
'/~ #~,'.~~'3.~.~'.~'o.~" o ~ o ~.~ $'t~.¢Jr~J~ 6 ~lg4w~J.~.~6 # ,~'~-'~'6/# o ~# t:::.:.:........-.._
1 2 3 4 5 6 25 m
I I
Fig. .2 (a) Simplified geological map of the ~igekda ~ Ophiolite and surrounding area. ,1 Central Anatolian
Metamorphics; ,2 layered and isotropic gabbros; ,3 dyke complex; ,4 lavas and epi-ophiolitic sediments; ,5
granitoids; ,6 Eocene cover units; ,7 Neogene continental clastics; ,8 locations of geological cross-sections (S-l,
~6kelik section; S-2, Mezar~edi~i section). (b) Generalized geological cross-section from the Mezargedi~i Pass;
,1 spherulitic pillow lava; ,2 pillow lava; ,3 pillow breccia; ,4 massive lava; ,5 lavas interlayered with sediments; ,6
pelagic sediments; ,7 feldspathoidal granitoids; ,8 sample locations (numbers correspond to samples listed in
Table .)1 )c( Generalized geological cross-section from ~6kelik village; ,1 Tertiary cover; ,2 pillow lavas and
volcaniclastics; ,3 pelagic sediments; ,4 Group III dykes; ,5 Group II dykes; ,6 Group I dykes; ,7 sample locations
(numbers correspond to samples listed in Table .)1
602 .K YALINIZ ETAL.
limestone and pillow intercalations of primary mineral phases and magmatic textures
Turonian-Santonian age (Erdo~an et al. 1996), are strongly overprinted by secondary minerals.
pillowed and massive basalts with basaltic As well as the matrix, secondary minerals are
breccias, sheeted dyke complex, isotropic to also abundant in vesicles, veins and pillow
layered gabbros and rare associated plagio- interstices, and include albite, chlorite, epidote,
granites (Fig. ,1 inset b). The lavas, which ex- quartz, calcite, actinolite and iron oxides. Fresh
hibit typical pillow structures, are best clinopyroxene phenocrysts are the only relict
developed at the top of the volcanic sequence igneous phase, although they occasionally show
where volcanoclastic intercalations are also marginal alteration to actinolite. Actinolitic
present. Massive basaltic flows, ranging in thick- amphibole crystals are also identified as needles
ness from c. 1 to 3 m, are also present. The in the groundmass. Throughout the ~iqekda~
volcanic-volcanosedimentary successions are basaltic lavas, plagioclase laths and phenocrysts
known as the ~6kelik Volcanics (Erdo~an et al. are mostly pseudomorphosed by albite. Chlorite
1996), which outcrop on the flanks of the commonly occurs in the groundmass of these
antiformal structure. The areas immediately to lavas and also as massive, radiating fibrous
the northeast of Korkorlu village and southwest aggregates in filling vesicles and veinlets. Epi-
of ~6kelik village on the western flank, and the dote is found in vesicles at the margins of lava
Mezargedi~i area on the eastern flank of the flows, along the walls of fractures and in vein-
antiformal structure provide well-exposed sec- lets. Where it occurs as a vesicle infill, it forms
tions through the basaltic extrusives. Due to the fan-shaped crystal aggregates often reaching
rarity of volcanic rock exposures in the vicinity 2-3 cm in diameter, commonly surrounded by
of ~iqekda~, these locations are particularly subhedral quartz crystals which line the vesicles.
important and were chosen for detailed sam- Quartz is generally deposited together with
pling (Fig. 2a and b). The sheeted dyke complex epidote and forms a vesicle infill. Calcite is
is poorly exposed relative to the gabbros but much less abundant than chlorite, epidote and
locally contains plagiogranite dykes. The gab- quartz in the ~iqekda~ lavas, but similarly
broic sequence is mainly composed of isotropic occurs in rock spaces rather than replacing
gabbros and subordinate layered gabbros. The previous minerals. Pillow breccias show similar
ultramafic part of the ophiolite sequence is not petrography and textures.
observed in the ~iqekda~ area. The dykes display an equigranular texture,
The ophiolitic succession is intruded by early are medium to fine grained, and are character-
monzogranitic, and later syenitic, intrusions. To ized by ophitic, subophitic and intergranular
the east and northeast of Korkorlu village, textures. They are mostly aphyric to sparsely
gabbroic and basaltic rocks are found as roof plagioclase-phyric dolerites. Although the pri-
pendants within the granitoids. Dykes of felds- mary igneous mineralogy of the dykes consists
pathoidal syenites are the last products of the of clinopyroxene, calcic plagioclase and opaque
Late Cretaceous coUision-type felsic magma- minerals, most of these are replaced by a green-
tism and cross-cut the volcanic carapace. The schist facies assemblage made up of actinolite,
earliesffossiliferouscoversedimentsinthe~iqekda~ chlorite, epidote, plagioclase, quartz, sphene
region are Lutetian in age (Ketin 1959). and secondary Fe-Ti oxides and hydroxides.
Rare core relicts of clinopyroxene are also
locally identified at the base of the sheeted
Petrography
dyke complex. Most dykes retain their primary
The basaltic rocks of the CO are weakly to igneous textures but there are some that are
strongly phyric, with plagioclase and subordi- more severely altered to equigranular textured
nate clinopyroxene as phenocryst phases set in a quartz- and epidote-rich epidosites. These are
variably quenched matrix of serrated plagio- often green coloured compared to the more
clase microlites, variolitic fans of crystallites normal grey dykes that are usually characterized
and an opaque matrix. In the ~6kelik area, by dominantly albite-actinolite-chlorite assem-
basalts intercalated with the sediments higher blages.
in the ophiolite sequence are distinguished from
the lower flows by the presence of sparse Sampling and analytical methods
'olivine' phenocrysts, which are usually pseudo-
morphosed by orange-red calcite and hematite Sixty-three samples were collected from basaltic
in these pervasively altered rocks. lavas and dykes, many selected from the Mezar-
Basaltic samples are all hydrothermally ~edi~i and ~6kelik traverses (see location and
altered to varying degrees. In many sections, relative sampling positions in Fig. 2). Mafic
the effects of alteration are so intense that samples were taken from the crystalline
GEOCHEMISTRY OF THE ~I~EKDAG OPHIOLITE 207
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208 K. YALINIZ ETAL.
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GEOCHEMISTRY OF THE ~I~EKDAdJ OPHIOLITE 902
8
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012 .K YALINIZ ETAL.
Table .2 Additional chemical data on ~ifekda~, basaltic rocks
elpmaS 85-C 86-C 97-C 32-C
Rock type Massive lava Massive lava Pillow lava Sheeted dyke
Incompatible elements (ppm)
Os 80.3 76.2 79.0 31.0
Hf 37.0 38.0 37.0 80.1
cS 77.83 33.83 76.64 04.24
Ta 30.0 40.0 40.0 50.0
Th 60.0 11.0 70.0 70.0
U 11.0 02.0 21.0 51.0
Rare earth elements (ppm)
La 95.1 74.1 81.1 34.1
Ce 36.3 49.3 93.3 03.4
Pr 56.0 76.0 75.0 67.0
Nd 46.3 37.3 71.3 93.4
Sm 72.1 43.1 31.1 36.1
Eu 05.0 65.0 54.0 06.0
Gd 38.1 79.1 46.1 24.2
Tb 43.0 73.0 13.0 54.0
Dy 22.2 14.2 50.2 69.2
Ho 94.0 45.0 54.0 56.0
Er 64.1 06.1 23.1 78.1
Trn 52.0 62.0 22.0 23.0
Yb 05.1 16.1 53.1 29.1
Lu 42.0 62.0 22.0 13.0
interiors of pillows, pillow breccias, massive during all but the most severe seafloor or hydro-
lavas and dykes, and selected in order to mini- thermal alteration (e.g. Pearce 1975; Shervais
mize the effect of vesiculation, alteration and 1982). SiO2 and most large-ion lithophile
weathering. All samples were analysed for elements (LILE: Na, K, Cs, Rb, Ba, Sr) are
major and selected trace elements, and a subset almost certainly mobile under these alteration
of four samples for rare earth element (REE), conditions (Smith & Smith 1976; Coish 1977;
and Hf, Ta, Th and U (Tables 1 and 2). Analysis Humphris & Thompson 1978a, b). Thus, chemi-
for major oxides and most trace elements were cal discrimination mainly relies on the HFSE
determined on an ARL 8420 X-ray fluorescence and REE, which tend to be the least mobile in
spectrometer (Department of Earth Sciences, aqueous fluids.
University of Keele, UK) calibrated against
both international and internal Keele standards
Geochemistry
of appropriate composition, whereas the REE
etc. were determined by instrumental neutron
General features
activation analysis (Activation Laboratories
Ltd, Canada). Details of methods, accuracy and On the basis of their Nb/Y ratios (Winchester &
precision are given in Floyd & Castillo (1992). Floyd 1977), all the mafic samples are tholeiitic
(Nb/Y = 0.04-0.20) with generally low HFSE
contents (Table 1). Metabasaltic lavas from the
Chemical discrimination
Mezargedi~i and ~0kelik traverses have over-
As outlined above, all the basaltic lavas and lapping compositions in terms of incompatible
dykes in the CO have undergone greenschist element abundances, similar Zr/Y and Zr/Ti
facies alteration and metamorphism. Thus, for ratios, and depleted REE patterns. These fea-
chemical discrimination of the volcanics, only tures suggest that they represent a similar lava
elements that are immobile during alteration sequence, possibly related to a single chemical
should be used. There is general agreement group, but now separated by tectonic dismem-
that the high field strength elements (HFSE: P, berrnent. However, direct chemical correlation
Ti, Y, Zr, Nb, Hf, Ta; Saunders et al. 1980), Th is not possible between the two lava sequences
(Wood et al. 1979), the transition metals (Sc, V, throughout, as two characteristics found in the
Cr, Ni) and REE are essentially immobile ~6kelik traverse are missing in the Mezargedi~i
GEOCHEMISTRY OF THE ~I~EKDA60PHIOLITE 112
0001
- ! tlasaB aval wolf (C-58)
tlasaB aval wolf )86-C(
tlasaB wollip lava (C-79)
100 -
o
N citlasaB deteehs ekyd (C-23)
-,~
E 10-
O
C
nr
1
0
0.1-
I = )A(
Z
0.01
. . . . . . . . . . . . . . .
CsRbBaTh NbTaLaCeSr P NdSmZr HIEu dG YbLu
100-
I! tlasaB lava wolf )85-C(
tlasaB lava wolf )86-C(
"O
50 tlasaB wollip lava )97-C(
N
citlasaB deteehs ekyd )32-C(
,
E
--
c
N Ol ^ C 0 C C """~""'~ ~
e- 5
O r"
o
)B(
Ca' Ce rP'Nd' nrS uE ' 6d+b' Dy oH ' rE' mT uL
Fig. .3 (a) N-MORB normalized multi-element diagram for ~iqekda~ basalts showing HFSE element depletion
and LILE enrichment. (b) Chondrite-normalized REE patterns of ~iqekda~ basalts showing light REE
depletion. Normalization factors from Sun & McDonough .)9891(
traverse. In particular, the former sequence simple fractionation. Relative to N-mid-ocean
contains numerous feeder dykes towards the ridge basalt (MORB), the normalized incom-
base and a set of primitive lavas with low Zr patible element patterns show the depleted
and Y contents that are missing from the HFSE and enriched LILE features of SSZ
Mezargedi~i sequence. On the grounds that the ophiolites, although the later variable abun-
lower volcanic portions of an ophiolite should dances (apart from Th) are largely governed by
contain a greater density of feeder dykes than alteration effects.
the upper, it is suggested that the ~6kelik se-
quence records a generally lower volcanic sec-
Internal variation
tion than the Mezargedi~i sequence.
Normalized patterns for incompatible ele- Samples from the lavas and dykes of the
ments and REE (Fig. 3) illustrate a number of Mezargedi~i and (~6kelik traverses, and isolated
features typical of the CO. All basaltic samples locations elsewhere, show internal variation that
show broadly parallel and depleted light REE is largely governed by fractional crystallization.
patterns with (La/Yb)N of c. 0.6, suggesting that Using Zr as a fractionation index, increasing
the samples plotted are probably related by FeO°/MgO (Fig. 4a) and decreasing Cr (Fig.
212 .K YALINIZ ETAL.
: 1'°° :
ppm8o (A) n p pCr • " (B)
- -I o
.:' -,
oo
so . ¢..o. . I
"" " "
I :
'f' 1""-' • 'NI ""
40 0"" " ~ 200 F I • " "
II
n ~@~., ,
I I I I I I 0
O0 1 2 3 4 5 6 0 20 40 60 80 100
FeO*/MgO Zr ppm
TiO2 2 y 40
)c(
wt.% ppm (D) ~. +
5.1 30
,w . +e • 4 " .+'1. "
• (oOp.. ,~ °i'
~. ."
1 20 •
• ~o
5.0 P I" saval
10 ~mt
Ik iO lava
Dykes cutting
I+,S"~t~, d~o,
o m I I l 0 I I I I
0 20 40 60 80100 0 20 40 60 80 001
Zr ppm Zr ppm
Fig. .4 Chemical diagrams illustrating internal variation for ~iqekda~ lavas and dykes. (a) and (b), Effects of
mafic fractionation; (c) and (d), linear covariance suggests a single comagmatic suite for the basalts.
4b) implies mafic ffactionation, as indicated by Stratigraphic • Mezargedi~i traverse
the observed relict phenocryst phases. Some of thgieh
)sertem( + C)6kelik traverse
the scatter here is the consequence of alteration,
whereas the covariance of immobile elements OO5
(Fig. 4c and d) indicate that all the samples
probably represent a single, fractionated,
comagmatic chemical group. The very low Zr 400 •
(< 20 ppm) and Y (c. 10 ppm) set of samples,
however, might be a different chemical group
3O0 -- •
from the rest, especially in view of their position
in the ~6kelik traverse magmatic stratigraphy
(see below). 200
D
-+,~.
Chemostratigraphy
Systematic sampling along the Mezargedi~i and
~6kelik traverses allows some monitoring of
• I D l ~ l •
chemical variability with height in the volcanic 0 50 10000.511.50 400 8000 400
portion of the CO. Due to tectonic dismember- Zr ppm TiO2wt.% V ppm Cr ppm
ment, stratigraphic height is only relative, being
measured from the arbitrary base of the traverse Fig. .5 Chemical variation with height in the ~iqekda~
in both cases. As seen in Fig. 5, the basal part of lavas. Height is relative and arbitrary starting at the
base of the traverse in both the Mezargedi~i and
each traverse is highly variable in terms of the
~6kelik sequences.
elements plotted, whereas the uppermost