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.

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