Download Petrology and Geochemistry of Pillow Lavas from the Western

Petrology and Geochemistry of Pillow Lavas
from the Western Ophiolitic Complex of
Khoy, NW Iran
Banafsheh Lakimahaleh 1,Mohssen Moazzen2 and Robab Hajialioghli3
1
Petrology Student
Associate professor, Department of Earth Sciences, University of Tabriz
3
Associate professor, Department of Earth Sciences, University of Tabriz
2

ABSTRACT
The Khoy ophiolite in northwestern Iran represents
remnants of Neo-Tethys oceanic lithosphere formed in the
Mesozoic. This ophiolite complex consists from bottom to top
of peridotites (dunite, harzburgite) and serpentinized
peridotite, gabbros, sheeted dikes, pillow and massive lava
flows, and pelagic sedimentary rocks, including radiolarian
chert. The investigated pillow lavas of the western ophiolitic
complex of Khoy and the associated rocks have been studied
on the basis of field geology and petrology. Also chemical
composition, magma series and tectonic setting of the
investigated rocks has been determined based on whole rock
chemistry. Petrographic studies of pillow lavas of the western
ophiolitic complex of Khoy show that the main minerals
include plagioclase and clinopyroxene phenocrysts. The main
textures of the rocks are porphyry, microporphyry and
intersertal, glomero-porphyritic, variolitic and vesicular.
Geochemical information indicates that the investigated
samples are basaltic to basaltic-andesite and the magma was
sub-alkaline. Tectonic setting of the basalts was determined as
an IAB environment. Enrichment in LREE/HREE is the most
important characteristics of these rocks.
KEYWORD
Iran, Khoy Ophiolite, Pillow Lavas, Sub-alkaline, IAB
INTRODUCTION
O
phiolitic complexes, which are fragments of
oceanic lithosphere were emplaced on the
continental crust during the tectonic processes. Using
mantle ultramafic sequence and mafic crustal sequence of
ophiolites provide invaluable information on the ocean
environment and geodynamic processes involved in
generation and emplacement of the oceanic crust [1].
The Iranian ophiolites are part of the Tethyan ophiolite
belt of the Middle East. They link the Middle Eastern,
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Mediterranean and Hellenides–Dinaride ophiolites (e.g.,
Turkish, Troodos, Greek and East European) to more
easterly Asian ophiolites (in Pakistan and Tibet) [2].
The Iranian ophiolites have been classified into four
groups [3,4,5,6]: (i) ophiolites of northern Iran along the
Alborz range [e.g. Talash (Rasht) and Virani (Mashhad)
ophiolites]; (ii) ophiolites of the Zagros suture zone, (e.g.
Neyriz and the Kermanshah ophiolites), which seems to be
of the same age of the Semail ophiolite emplaced onto the
Arabian continental margin (e.g. [7, 8]); (iii) ophiolites and
colored melanges of Makran region which are located to the
south of the Sanadaj-Sirjan microcontinental block,
including unfragmented complexes such as Sorkhband and
Rudan; and (iv) ophiolites and colored mélanges that mark
the boundaries of the central Iranian microcontinental
(CIM) block including some of those in the Makran region
(e.g. Band-e- Zeyarat, Dar Anar, Ganj, RemeshtMokhtarabad) and those inside the Sanadaj-Sirjan
microcontinental block and its extension through of the
Makran (Shahr-Babak, Naien, Baft, Sabzevar, Tchehel
Kureh, Iranshahr and Fanunj-Maskutan) (Fig.1).
The Khoy ophiolite in northwestern Iran represents
remnants of Neo-Tethys oceanic lithosphere formed in the
Mesozoic [9].
[10] Recognized two distinct types of ophiolitic
assemblages in the Khoy area: (i) an older, metamorphic
and pre-Cretaceous ophiolitic assemblage, consisting of
huge tectonic slices of mantle tectonites, associated with
lenses and dikes of metagabbros, amphibolites and
metadiabases. The mafic rocks are metamorphosed in the
amphibolite facies, and the 40K/40Ar ages on the
metamorphic minerals have yielded Lower Jurassic to
Upper Cretaceous ages. These dismembered ophiolite
fragments are narrowly associated with the Eastern
metamorphic zone. (ii) a younger, non-metamorphic and
Upper Cretaceous ophiolitic complex (the Khoy ophiolite
sensu stricto). This ophiolite represents the last oceanic
ridge activity in the Khoy basin, obducted over the Arabian
continental platform, or a detached fragment of it. It has the
same age as other well-known ophiolites of western Iran,
Turkey and Oman, belonging to the peri-Arabic ‘ophiolitic
crescent’ [7].
All these ophiolites, devoid of regional metamorphism,
were obducted during Late Cretaceous over the southern
continental margin of the Neo-Tethys ocean (ArabianAfrican platform), or over ‘Gondwanian’ continental
fragments, detached from the Gondwana block during
Permian-Triassic times. The studied ophiolite related to,
western ophiolitic complex of Khoy. The investigated
pillow lavas of the western ophiolitic complex of Khoy and
the associated rocks have been studied on the basis of field
geology and petrology. Pillow lavas in ophiolites represent
the extrusion lavas in the aquatic environment and give
valuable information about the origin of the lava, partial
melting and tectonic environment [11]. The pillow lavas are
one of the main units in many ophiolite complexes which
are placed in Crustal sequence, directly under the
sedimentary section and on the diabase dikes [12, 13].
classification of structural-sedimentary zones of Iran, the
study region is placed between the western Alborz and
Azerbaijan (Central Iran) zone and the coloured melanges
of northwestern zone Iran.
According to the field geology and petrography studies,
the mantle rocks include peridotites (dunite, harzburgite)
and serpentinized peridotite and the crust rocks consist of,
gabbro, diorite, pillow and massive basalts and a sequence
of sedimentary rocks. Pillow lavas are one of the main units
of this ophiolite complex. This unit consists of basaltic lava
flows and lava breccias. Basaltic rocks in hand sample have
dark colors, pillow structure and are cryptocrystalline.
Pillows vary in size from several decimeters to several
meters. Pillow basalt units are developed in rout to Hesar
and Badalan villages (Fig.2.a). Some of the Pillow basalts
are altered to epidote and calcite in the Badalan village area
and have been mostly altered in the green schist facies
(Fig.2.b). There are numerous hydrothermal veins and
veinlets made of calcite and epidote. In some of the
sections, epidote and calcite veins are parallel. Also, due to
tectonic stresses in the studied area, the interlayers of
calcite veins in basalts are strongly folded.
Fig.1. Map of distribution of the ophiolite belts in Iran and location of the
Khoy ophiolite [14]. KH: Khoy; KR: Kermanshah; NY: Neyiz; BZ: Band
Ziarat; NA: Nain; BF: Baft; ES: Esphandagheh; FM : Fanuj-Maskutan; IR:
Iranshahr; TK: Tchehel Kureh; MS: Mashhad; SB: Sabzevar; RS: Rasht;
SM: Samail.
STUDY METHODS
Field studied were conducted in order to find the
geological relations between the rock units. After detailed
petrographic studies, in order to study the whole rock
geochemistry of pillow lavas of khoy, 3 samples were
analysed for whole rock, major and trace elements.
REGIONAL GEOLOGY
The Khoy ophiolites are exposed at the northwest of Iran
and west of Khoy (in west Azerbaijan province), and are
extending to the Turkish border. According to the
Fig.2. Field relations of the rocks outcropping in the study area, a) pillow
basalts of west of Khoy, b) close view of the pillow basalt.
PETROGRAPHY
Basaltic rocks in the study area consist of pillow lavas,
gabbro and diabase. The pillow lavas are discussed here.
Abbreviations used in this paper for minerals name are after
from [15].
Petrographic studies of pillow lavas show that the
mineralogical composition of the investigated samples
indicates basalt to basaltic-andesite as the main rock type.
The main textures of the rocks are porphyry, microporphyry
and intersertal, glomero-porphyritic, variolitic and
vesicular.
Cavities in basalts are filled by secondary minerals such
as calcite, chlorite, zeolite and quartz. Plagioclase,
pyroxene and opaque minerals are observed in the matrix of
microlith (Fig. 3.a). Plagioclase and clinopyroxene are the
main mineral phases, and Fe-oxides and titanite are the
accessory minerals. Secondary minerals include quartz,
sericite, epidote, chlorite, iddingsite, amphibole (actinolitetremolite), calcite and zeolite. Microscope investigations of
clinopyroxene indicated that some of the clinopyroxene
phenocrysts show reverse zonings. Carlsbad twinning is
developed in augite phenocrysts. Pyroxenes is partially
changed to tremolite-actinolite and chlorite (Fig. 3. b).
Glomero-porphyritic texture can be observed with pyroxene
minerals in some samples (Fig.3.c). Plagioclase occurs in
the form of either phenocrysts or microlith and generally is
altered to clay minerals and sericite and also is replaced the
mineral assemblage of calcite, epidote and chlorite. Rare
olivine crystals in some samples have coarse anhedral to
subhedral shapes, rounded and with abundant fractures. In
some of them mesh texture can be seen due to
serpentinization (Fig. 3.d).
Fig.3. Petrographical basic rocks of western ophiolitic complex of Khoy:
a) Cavities filled with calcite (cavity texture), b) conversion pyroxene to
tremolite - actinolite of a result oralitization, c) glomeroporphyritic texture
of minerals pyroxene, d) mesh texture from olivin, and the presence of
secondary minerals serpentine basalt samples.
GEOCHEMISTRY
Geochemical studies on the pillow basalts from the
western ophiolitic complex of Khoy represents values of
MgO 3.56-7.27 Wt%, SiO2 45.26-53.27 Wt%, Fe2O3(T)
9.71-12.81 Wt%, TiO2 1.145-1.748 Wt%, Na2O 3.1- 4.25
Wt% and K2O 0.31 – 0.49 Wt%. The CaO content is 6.97
to 14.62 Wt%. Its high content can be attributed to
existence of calcic plagioclase and clinopyroxene in the
rocks. The presence of secondary minerals and alteration of
the rocks is the reason for high (2.73-4.4 Wt %) LOI. The
major, minor and rare earth elements are used to decipher
the magma type and tectonic setting of the studied pillow
lavas. In order to determine the rock type, (Na2O+K2O) vs.
SiO2 diagram [16] were used. The pillow lavas of the
western ophiolitic complex of Khoy are plotted in the basalt
to andesite basalt fields of this diagram (Fig. 3.a). Using
diagrams to find the magma type [17], samples are
characteristic of a sub-alkaline magma (Fig. 3.b). Fig 4
shows chondrite-normalized REE diagram for the pillow
lavas [18]. The patterns are similar and almost parallel for
the studied samples (Fig. 4). [19] believe similar and semiparallel trends of patterns for samples in spider diagrams
without considering some of the anomalies that are owing
to crustal contaminations, could be taken as reason for same
origin for the rocks. The chondrite-normalized REE
patterns for these rocks show significantly negative slope.
This may be related to the LREE and LILE enrichment
(from La to Nd) strong HREE depletion.These
characteristics show that the magma crystallizing the
volcanic rocks originated by partial melting of the upper
mantle which is moderately modified by digestion of the
crustal rocks and contamination. In this diagram, strong
enrichment in LREE (La, Ce) relative to HREE (Lu) can be
seen. Two possibilities can be postulated: one is LREE
incompatible element enrichment relative to HREE
[20].This can be as a result of magmatic changes in rocks
developed in the area. Another factor is the formation of the
studied rocks at a subduction zone [21].In the normalized
diagram (Fig. 4), enrichment of La, Cs, Sm, Nd, Eu and
depletion of Lu can be seen. Enrichment of La and Cs
indicates mantle metasomatism processes and
Fig.4. Chondrite- normalized diagram of pillow lavas [18].
possibly contamination with the crustal material. Positive
anomaly for Eu may be related to plagioclase accumulation
[22].According to the tectonic setting discriminant
diagrams [23] samples are mainly plotted in the Island Arc
Basalt (IAB) environment (Fig. 5).
Fig. 6. tectonomagmatic diagrams for the pillow lavas of the western
ophiolitic complex of Khoy [24].
CONCLUSION
The study area is a part of the western ophiolitic complex
of Khoy in northwestern Iran. The field geology and
petrography studies reveals that the mantle rocks include
peridotites (dunite, harzburgite) and serpentinized
peridotites and the crustal rocks consist of, gabbro, diorite,
pillow and massive basalts and a sequence of sedimentary
rocks (pelagic limestone and radiolarite cherts).
Petrographic studies show that the main minerals of the
pillow lavas are plagioclase and clinopyroxene.
Plagioclase is generally altered to clay minerals and
sericite and is replaced by the minerals assemblage
including calcite, epidote and chlorite. Clinopyroxene is
changed to termolite-actinolite and chlorite, partially. The
main textures of the rocks are porphyry, microporphyry,
intersertal, glomero-porphyry, variolitic and vesicular.
Geochemical information indicates that the investigated
samples are basalt to basaltic-andesite and the magma was
sub-alkaline. Tectonic setting of the samples was
determined as an Island Arc environment. The chondritenormalized REE patterns for these rocks show significant
negative slope, related to LREE (from La to Nd) and LILE
enrichment and strong depletion in HREE.
These characteristics indicate that the original magma of
the volcanic rocks was generated by partial melting of the
upper mantle source and the original magma changed by
digestion and contamination by the crustal rocks.
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