Ii I&S

233 Pages·2010·10.92 MB·English

Sr, Nd, Pb ISOTOPE AND TRACE ELEMENT
GEOCHEMISTRY OF CALC-ALKALINE AND ALKALINE
VOLCANICS, EASTERN TURKEY
by
LEVENT GULEN
B. Sc. (Hons), Hacettepe University, Ankara
(1976)
Submitted in partial fulfillment of the requirements for the
degree of
DOCTOR OF PHILOSOPHY
at the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
June. 1984
Massachusetts Institute of Technology
/ n
Signature of Author
Center(for Geoalchemy, Department of Earth,
111 Atmospheric, and Planetary Sciences June 18, 1984
Certified by 1-. 1wri
Stanley R. Hart
Thesis Supervisor
Accepted br ~ 9 Theodore R. Madden
Chairman, Department Committee on Graduate Students
Wil N' Ly T
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Sr, Nd, Pb ISOTOPE AND TRACE ELEMENT
GEOCHEMISTRY OF CALC-ALKALINE AND ALKALINE
VOLCANICS, EASTERN TURKEY
by
LEVENT GULEN
Submitted to the Department of Earth, Atmospheric, and Planetary
Sciences on June 18, 1984 in partial fulfillment of the
requirements for the degree of Doctor of Philosophy
ABSTRACT
A Tertiary to Quaternary volcanic province, which includes both calc-alkaline and
alkaline volcanics, covers wide areas just to the north of the Bitlis Suture Zone in East-
ern Turkey. Investigations have been undertaken to provide constraints on the nature of
the spatially and temporally coexisting calc-alkaline and alkaline volcanics and the
magmato-tectonic evolution of this region. Two types of approaches have been utilized:
1-To study isotopic systematics (Sr, Nd and Pb), major and trace element abundances of
carefully collected, representative volcanic rock samples. 2-To analyse tectonic defor-
mations caused by the Miocene continental collision and still continuing convergence
between the Arabian and "Turkish" plates, based on LANDSAT, SIR-A image analyses, field
studies and existing geological data. Four major volcanoes of this volcanic province have
been chosen to carry out geochemical investigations. Ararat and Suphan are calc-alkaline,
Tendurek and Nemrut are alkaline volcanoes.
The calc-alkaline rocks of Ararat and Suphan include high-alumina basalts, andesites
dacites and rhyolites. The results of this study show that the four volcanostratigraphic
Ararat suites, distinguished by field studies, also form four coherent geochemical groups.
Isotopic compositions of the Ararat suites are within the observed range for island-arc
basalts. However a careful examination of their isotope systematics reveals a very limited
involvement of a lower crustal component in their petrogenesis. In contrast to Ararat,
Suphan lavas exhibit significant crustal contamination signatures. The obtained geochemical
data indicate that various two-component magma mixing processes involving six
end-members, that are derived from two distinct mantle-derived magmas by the interplay
of fractional crystallization, limited crustal contamination and cumulate assimilation can
account for the major, trace element and Sr, Nd, Pb isotopic composition of the Ararat
lavas.
Alkaline products of Tendurek, Nemrut volcanoes and fissure lavas of the Lake Van
region consist of rocks ranging in composition from alkali basalts, through hawaiites,
benmoreites, mugearites and sodic trachytes to peralkaline commendites and pantellerites.
Alkali volcanism is basically sodic and Al-rich in character and the most primitive lavas
are transitional to tholeiites. All of the alkaline suites studied have been variously
affected by lower and/or upper crustal contamination. Derivation of the alkaline lavas by
partial melting of a recently metasomatized, heterogeneous, hydrous phase-bearing
(amphibole), depleted peridotite mantle source followed by variable degrees of crustal
contamination and fractional crystallization is consistent with their overall geochemical
characteristics.
Based on the geochemical compositions of the most primitive basalts as best exempli-
fied by the Ararat high-alumina basalt suite, two distinct mantle sources can be inferred
for the Eastern Turkish volcanics. The first one represents a mantle that has had a
time-integrated depletion in Rb/Sr and Nd/Sm and similar to that of transitional-MORB
and continental arc basalts. The second mantle source is characterized by relatively less
depletion but quite radiogenic Pb 207/204 isotope composition. Models involving both an
ancient subducted oceanic crust source or a segment of depleted subcontinental mantle
that has been contaminated and metasomatized by a component carrying sediment signa-
tures, during a previous subduction event, are isotopically plausible. However, considering
all the geological and geochemical facts, the latter appears to be the most satisfactory
petrogenetic model.
It is not possible to establish a direct link between the subduction of the
Bitlis-Zagros ocean crust and the volcanism in Eastern Turkey. However, the assumption
of a detached, sinking slab following Miocene continental collision along the Bitlis-Zagros
Suture Zone, can be viewed as a plausible trigger for the generation of the calc-alkaline
magmas and their emplacement within the continental crust. The calc-alkaline volcanism
may be maintained to present by the continued sinking and dehydration of this detached
slab, as well as by the continued tectonic deformations caused by continental collision.
This creates mantle upwelling which not only initiates the alkali volcanism, but also keeps
the calc-alkaline volcanism alive.
The consideration of the detailed geology, tectonic structures and their trends along
with regional seismicity permits the identification of a number of deformational domains
in eastern Turkey bounded by major shear zones, in which coherent deformational styles
are displayed. The Van and the surrounding deformational domains formed and evolved
under the influence of a continental collision, following the Late Miocene elimination of
the Bitlis-Zagros ocean along the Bitlis-Zagros suture zone between the Anatolian-Iranian
and Arabian blocks. Mainly three different, but nevertheless related deformational styles
take up the still continuing continental convergence that is the result of the northward
motion of the Arabian plate. These are: 1- Folding and thrusting within the Arabian plat-
form and along the Caucasuses, 2- Displacements along NW-SE and NE-SW trending sets
of oblique-slip faults, causing lateral escape, and 3- Tilting and bending of crustal blocks
and thrusting of those blocks over a decollement surface within the continental crust.
The contemporaneous development of extensional and compressional tectonic regimes
along with calc-alkaline and alkaline volcanism can best be explained by limited thinning
as opposed to thickening of the continental crust, as a result of wide scale compression
in eastern Turkey. This is accomplished by the tilting and bending of slab like crustal
blocks and thrusting of these blocks over a decollement surface within the continental
crust, providing that there is an available "escape space" in the direction of the tilting
and thrusting. It is suggested that the combined Black and Caspian Sea back-arc
basins(marginal sea?) through the Caucasuses provided the required "escape space" in
this region. However, the consumption of oceanic crust along the Caucasuses by the
overthrusting continental margins, from both north and south, prevented the further
development of an extensional regime in this region. The slab-like crustal blocks of the
Van deformational domain, trapped between the converging Arabian and Scythian blocks
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escape eastwards away from the maximum compression region along the NW-SE trending
set of oblique-slip faults instead of thickening the crust by overthrusting onto each oth-
er. This model may offer plausible alternative explanations for the origin of an exten-
sional regime, either immediately following or contemporaneously developing with a
compressional regime in an adjacent region.
Thesis supervisor: Stanley R. Hart,
Professor of Geology and Geochemistry
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ACKNOWLEDGEMENTS
I feel very lucky and honored by being a student of Stan Hart who is not only a
very fine scientist, but also a very fine person. His constant encouragement and enthusi-
asm has been a prime source of inspiration. I would like to thank him also for taking his
time patiently correcting the spelling and English of this thesis.
Special thanks to Fred Frey for help, advice and providing free access to his INAA
facilities.
I would like to thank Nobu Shimuzu for advice, discussions, and for constructive
critisism of Chapter 1.
Special thanks to Nafi Toksoz for help, advice, providing LANDSAT images, and for
initiating my interest in SIR-B project, as well as in Cabernet Sauvignon.
I would especially like to acknowledge Clark Burchfiel, Peter Molnar, Wiki Royden, and
Dave Walker for advice, discussions and their constructive critisism of Chapter 3.
The engineering expertise of Ken Burrhus has been extremely helpful in maintaining
equipment of a high level performance. He taught me a lot about mass spec as well as
Wall Street.
Special thanks to Donna Hall for being a good friend who cares and shares. She
willingly took responsibility for graphics.
I would like to thank M. K. Roden for teaching me various analytical techniques.
Gile Beye and John Zannos helped with graphics.
I would like to gratefully acknowledge Gurol Ataman who provided continuous support
during my college years and initiated my interest in geochemistry.
I gratefully acknowledge receipt of a Graduate Fellowship from the Scientific and
Technical Research Council of Turkey (TUBITAK).
I acknowledge the logistic support by MTA (Institute of Mineral Research and Explora-
tion) during the field work. I would like to thank E. Arpat, Y. Guner, and F. Saroglu of
M.T.A. for introducing me to the geology of Eastern Turkey and for discussions.
Special thanks to my parents, who not only influenced my personal growth by giving
me a certain "vision of life", but also unknowingly made me determined to be a geologist
by telling a real life story when I was eight years old.
I am particularly grateful to my wife, Suna, for her constant encouragement, support,
and patience. She also skillfully handled word-processing. This thesis is dedicated to
Suna, without whom the world would have been dark for me.
Table of Contents
ABSTRACT - - ..- .- ..--.. 1I.
ACKNOWLEDGEMENTS .................. - - - - - - - - . - - - - - - - - 4
CHAPTER-1 Geochemistry of the calc-alkaline volcanics
1.1 Introduction ..................
... .- - - - - - - - - - - - - - - - 12
1.2 Ararat Volcano . . . . . . . . . . . . . . . .
1.3 Petrography - - - - - . 15 .--
1.4 Analytical Techniques . . . . . . . . . . . . ......- - - - - - ........ 16
1.5 Results . . . . . . . . . . . . . . -.- - .- -. - -.- -.. -. . ... 18
1.5.1 Major and Trace Element Variation ......... .... .... 18
.. . . . . . . . . . . . . . .. 7
1.5.2 Rare Earth Elements
1.5.3 Isotope Geochemistry . . . . . . . . .
... ................. 58
1.5.3.1 Sr and Nd Isotope Variation
.. .................. 70
1.5.3.2 Pb Isotope Variation . . . .
... ..- - - ............ 75
1.6 Discussion . . . . . . . . . . .
1.6.1 Crustal Contamination .
1.6.2 Fractional Crystallization ... ................. 78
1.6.3 Magma Mixing . . . . . -..-. -.- -..-. -.- -..-. -.- ... 8 1
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1.6.4 Mantle Source Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . 102
1.7 Conclusions . . . . . . . . . . . . . . . . .. . . . . . . . . . - - - - - - -. . . . 106
CHAPTER-2 Geochemistry of the alkaline volcanics
2.1 Introduction . . . . . . . . . . . . . . . . . .. . . . . . .. . .. - - - . . - - . .. 109
2.2 Description of major volcanoes and fissure lavas . . . . . . . . . . . . . . . . . . 112
2.2.1 Tendurek Volcano . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112
2.2.2 Nemrut Volcano . . . . . . . . . . . . . . . . . . .. . . . . .. . . . . . . . 112
2.2.3 Fissure Lavas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
2.3 Petrography . . . . . . . . . . . . . . . . . . . . . . .. . . - - - - - - -. .. - . . 113
2.4 Results . . . . . . . . . . . . . . . . . . - - - - . -. .. . - - - - - - - - - - - - - - 114
2.4.1 Major and trace element variation . . . . . . . . . . . . . . . . . . . . .. . . 114
2.4.2 Rare Earth Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
2.4.3 Isotope Geochemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
2.4.3.1 Sr and Nd Isotope Variation . . . . . . . . . . . . . . . . . . . . . . . 131
2.4.3.2 Pb Isotope Variation . . . . . . . . . . . . . . . . . . . . . . . . . . . 142
2.5 Discussion . . . . . . . . . . . . . . . . . . . . .. . . .. . - -. - - - - - - - - - - . 147
2.5.1 Crustal Contamination . . . . . . . . . . . . . . . . . .. . . . . . . .. . .. . . 147
2.5.2 Mantle Source Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . 152
2.6 Conclusions . . . . . . . . . . . . . . . . . . . - - -. - - - - - - - - - - -. .. . . . . 156
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CHAPTER-3 Deformational domain continental tectonics in the Lake Van region,
Eastern Turkey: implications on the origin of an extensional regime
associated with compression
3.1 Introduction: . . . . . . . . . . . . . - - - - - - - - - - - - - - - - - - - - - - 158
3.2 Geological Setting . . . . . . - - . . . 162
3.3 Tectonic Setting . . . . . . . -...-..- . . -- - . . . 168
3.3.1 North Anatolian Fault ... .. .... . . . . . 168
3.3.2 East Anatolian Fault .. . . . . . . . . . . . . . . 170
3.3.3 Main Recent Fault -- - . . . - --- - - . . 173
3.4 Van Deformational Domain .-.---.-.--- - . . . 173
3.4.1 Pasinler Fault . -. . . . -- --- --- - 178
3.4.2 Ararat Fault . . . . - . . . . -- --- - . . . 181
3.4.3 Hakkari Fault . . . . . --. --- --- -- - 181
3.4.4 Agri Fault . . . . . . - -.- - - - - - - - - . . . 185
3.4.5 Caldiran-Tutak Fault . .-....- -. ..- 185
3.4.6 Patnos and Malazgirt Faults .. . . . . . .. .- . 188
3.4.7 Adilcevaz Fault -- -. .------ --- - 188
3.4.8 Lake Van Basin - -- .- - - - - - - . . 189
3.5 Discussion and tectonic model . . . . . . . - . . - - - - 195
REFERENCES: ................. ...... - - - - 214
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CHAPTER 1
GEOCHEMISTRY OF THE CALC-ALKALINE VOLCANICS
1.1 Introduction
A Tertiary to Quaternary volcanic belt which covers wide areas between the
Caucauses (U.S.S.R) and Lake Van (Eastern Turkey) extends about 900 km. towards the
Bijar region of northwest Iran (Figure-1.1). This volcanic belt is situated to the north of
the Bitlis-Zagros suture zone. As a result of the overall convergence between the
Arabian and Eurasian plates, the existing southern branch of Neo-Tethys was consumed
by northward subduction; continental collision took place in the Miocene forming the
Bitlis-Zagros suture zone (Rigo de Righi & Cortesini, 1966; Dewey et al., 1973; Hall,
1976; Stocklin, 1974). However, continental convergence is still taking place as mani-
fested by active deformation and diffuse seismic activity in this region (Nowroozi, 1971;
McKenzie, 1972; Rotstein & Kafka, 1982; Jackson & McKenzie, 1984). This volcanic
province has certain peculiarities by which it can be distinguished from typical continental
volcanic arcs. First of all, calc-alkaline and alkaline volcanics spatially and temporally
coexist. Some of the volcanic centers that produced alkali volcanics are located just to
the north of the presumed trench zone, which is represented by the Bitlis-Zagros Suture.
Moreover, this volcanic belt attains it's maximum width in Eastern Turkey, where volcanic
rocks extend northward, for about 350 km., from Lake Van area (Turkey) to the
Caucasuses (U.S.S.R.). The existing volcanoes do not line up to form an elongate volcanic
zone, subparallel to the presumed continental margin in Eastern Turkey, as they do in
most continental arcs. Finally, major volcanoes were formed in Pliocene to Quaternary
times, long after the Miocene continental collision.
Although the volcanics of this province have not been systematically dated (except
the ones in the immediate vicinity of Lake Van and northwestern Iran), calc-alkaline
volcanism appears to be represented by two main phases of activity. K/Ar dating by
Innocenti et al.,(1976) indicates that the first phase of calc-alkaline, high-potassic activity
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