Post on 18-Mar-2023
Mineral chemistry and whole rock geochemistry of Subalkaline gabbros in South Amlash Gabbroic bodies, South
of Caspian Sea, Iran
Fatemeh Zaeimnia1, Ali Kananian1, Ke-zhang Qin 2,Benxun Su 2
1 School of Geology, College of Science, University of Tehran, Tehran, Iran
2 Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing
100029, China
Corresponding Author’s e-mail address: fzaeimnia@khayam.ut.ac.ir
Abstract South Amlash Gabbroic bodies in Guilan province, according to
recent studies, belong to south Caspian Sea Ophiolite.
Minerallogically consist of Pyroxene, Plagioclase, Olivine and
Biotite. Whole rock geochemistry indicates Sub-alkaline affinity
and Mineral chemistry reveals an Island Arc source for studied
gabbros. Enrichment in LILE in addition to other geochemical
characteristics concludes that they belong to an Island-Arc
setting and derived from a mantle wedge which was affected by
subducted slab fluids.
IntroductionSouth Amlash gabbros, located in Guilan province in south of
Caspian Sea, northern Iran, are considered as a part of Caspian
Sea depression which had been developed as a marginal basin
behind the Mesozoic Transcaucasia-Talesh-western Alborz and the
Paleogene central Iranian Andean arcs (Berberian, 1983; Figure
1). There are a few geological and petrological studies about
this region as a part of south Caspian Sea depression. Earlier
studies by authors suggested two different types of Gabbros in
this region. These two types of Gabbro are not recognizable by
field study and their identification is only possible from
precise microscopic studies and Results of Chemical analysis. By
Mineralogical studies, ICP and Mineral chemistry analysis, the
types and genesis of the gabbros were recognized: 1) In Alkaline
type the dominant mineralogy is Plagioclase, Clinopyroxene
(titan-augite) and Apatite which are also enriched from LREE and
HFSE such as Nb and Ti, virtually shows the characteristic of
typical oceanic island basalt, 2) Sub-alkaline type also have
Plagioclase and Clinopyroxene, minor Olivine and Biotite with
negative anomaly of Nb and Zr suggested the rocks which formed in
an island arc.
In this research, characteristics of Sub alkaline gabbros of
South Amlash will be explained.
Geological background
The studied area, in south of Amlash, is bounded by Caspian Sea
alluvium Quaternary deposits in the north and by northern slope
of Alborz Mountain in the south (Alavi, 1996; fig 1a). The Alborz
mountain system in northern Iran, extends in a sinuous manner for
about 2000 km from the Little Caucasus of Armenia and Azerbaijan
Republics in the northwest to the northern Afghanistan in the
east, and forms a composite polyorogenics belt (Alavi, 1996;
Stöcklin, 1974; Figure 1a). The main gabbroic bodies are found in
the south of Amlash (northern part of study area) and small
outcrops in Javaherdasht (eastern) and Omam (western). The
Gabbroic bodies are existed along with late cretaceous pillow
lavas and in this case they could be belong to the late
cretaceous or maybe later, but given the certain age needs
isotope data.
Mineralogy
South Amlash sub-alkaline gabbros have variable mineralogical
composition, but totally contain of Clinopyroxene, Plagioclase
and olivine. In some areas, Biotite is along with major mineral.
Figure 1 up: Tectonic map
of the middle east- Caucasus,
with main blocks and
structure zones, after
Avagyan et al., 2005.
Modified.
South Amlash sub-alkaline gabbros have granular and cumulated
texture and both of them include the Biotites and compositionally
these are Mg-bearing Biotites.These gabbros mainly contain of subhedral to euhedral Plagioclase (ca.
70%) and anhedral Clinopyroxene (20-25%) with Ophitic (Figure 2a) or
interganular texture (Figure 2b).
Figure 2 Amlash simple gabbros texture in crossed polarized light, a) Ophitictexture, b) Interganular texture. The abbreviations after Kretz, 1983.
Olivine crystals are anherdal and altered to serpentine (Figure 3a) and
enclosed in clinopyroxene. Biotite occurred as small inclusions in
clinopyroxene and as subhedral crystal beside clinopyroxene.
Figure 3 Amlash simple gabbros minerals in crossed polarized light a) altered
Olivine (Ol), b)subhedral biotite (Bt). The abbreviations after Kretz, 1983.
Mineral chemistry
The major chemical compositions for minerals were determined by JXA8100
at the Institute of Geology and Geophysics, Chinese Academy of Sciences
in Beijing.
Figure 4 shows WO-EN-FS triangle diagram (Deer et al., 1991) for sub-
alkaline gabbros as Diopside type clinopyroxene. These clinopyroxenes
have calc-alkaline affinity in Ti against Total Al diagram
(Figure 5a).
Figure 4 a) Plot of clinopyroxene composition in the Wo-En-Fs diagram
(Deer et al, 1991)
b) Chemical trend of clinopyroxene sub-alkaline gabbros (Wilkinson, 1959):
A---A’ Chemical trend of clinopyroxene in alkali olivine-basalt
Chemical trend of clinopyroxene in tholeiitic rocks
Chemical trend of clinopyroxene in the tholeiitic
Skaergaard intrusion.
Figure 5: Ti against Total Al diagram (Leterrier
et al., 1982).
Geochemistry
Gabbroic samples from Amlash intrusive bodies were analyzed for
element concentrations by ICP-MS method. SiO2 and MgO content
range from 48 to 51% and 50 to 60 %, respectively.
Sub-Alkaline Gabbros are enriched in Th, K, Pb and Sr, but are
depleted in Nb, Zr and Ti in primitive mantle-normalized diagram
(Figure 6).
Figure 6: Primitive mantle-normalized
diagram for sub alkaline gabbros (Sun
and McDonough, 1982).
According to Nb-Zr-Y diagram (Meschede, 1986) sub alkaline
gabbros were plotted in Volcanic arc tholeiitic rocks (Figure 7a)
and in Th-Hf/3-Ta diagram (wood, 1980) all samples put at
Volcanic arc basalts field (figure 7b).
Figure 7: a) Nb-Zr-Y
diagram (Meschede,
1986)
b) in Th-Hf/3-Ta
diagram (Wood, 1980).
conclusion
South Amlash sub-alkaline gabbros, located in Guilan province,
are enriched in Th, K, Pb and Sr, but are depleted in Nb, Zr and
Ti. These gabbros represent Island Arc basalt signature.
Enrichment of LREE with negative anomalies of Nb, Zr, and Ti in
company with positive Pb anomalies in the samples is generally
due to two processes: (1) crustal contamination and (2) partial
melting of an enriched mantle source which is metasomatized by
slab-derived fluid/melt. Based on Th/Yb، Ta/Yb، Nb/U، Ta/U and Ce/Pbratios and Mg# and SiO2 content, it seems that the
metasomatization of the mantle by the slab-derived fluid is the
main explanation for this topic.
References
Alavi, M., 1996. Tectono stratigraphic synthesis and structural style of the Alborz mountain system in Northern Iran. Journal Geodynamics 2, 1-33.Avagyan, A., Sosson, M., Philip, M.H., Karakhanian, A., Rolland,
Y., Melkonyan, R., Rebai, S., Davtyan, V., 2005. Neogene to Quaternary stress field evolution in Lesser Caucasus and adjacentregions using fault kinematics analysis and volcanic cluster data. Geodinamica Acta 18, 401–416.Berberian, M., 1983. The southern Caspian: a compressional
depression floored by a trapped, modified oceanic crust. CanadianEarth Science 20, 163-183.Deer, W.A., Howie, R.A., Zussman, J., 1991. An introduction to
the rock-forming minerals- Wiley and sons, London 528p.Kretz, R., 1983. Symbols for rock forming minerals. American
Mineralogist 68, 277-279.
Leterrier, J., Maury, R.C., Thonon, P., Girard, D., Marchal, M.,1982. Clinopyroxene composition as a method of identification ofthe magmatic affinities of paleo-volcanic series. Earth andPlanetary Science Letters 59, 139-154.Meschede, M., 1986. A method of discriminating between different
types of mid-ocean ridge basalts and continental tholeiites withthe Nb–Zr–Y diagram. Chemical Geology 56, 207-218.Stöcklin, J., 1974. Possible ancient continental margin in Iran.
In: Burke, C.A., Drake, C.L. (Eds.). The Geology of ContinentalMargins. Springer, New York, 873-887.Sun, S., McDonough, W.F., 1989. Chemical and isotopic
systematics of oceanic basalts: Implications for mantlecomposition and processes. In: Saunders, A.D., Norry, M.J.(Eds.), Magmatism in the Oceans Basins. Geological Society ofLondon Special Publication, 313-345.Wood, D.A., 1980. The applications of a Th–Hf–Ta diagram to
problems of tectonomagmatic classification and to establishingthe nature of crustal contamination of basaltic lavas of theBritish Tertiary Volcanic Province. Earth and Planetary ScienceLetters 50, 11-30.