فهرست مطالب zahra gharechahi

Many lamprophyric dykes' outcrops are found in Azerbaijan (in the northwest of Iran). These dykes which were the subject of many studies are including camptonite dykes in Misho Mountain, kersantite dykes of Goye-Poshti Mountain of Maragheh, camptonite and sannaite dykes in Horand, minette dykes of Varzeghan, minette dykes of Marand, minette dykes of Khoy, and minette dykes of Saray volcano. For the first time, Amel (1994) reported the occurrence of lamprophyre in the Monavvar region. According to him, this lamprophyre is spessartite and has Calc-alkaline affinities. In this study, we performed a detailed petrographic study of this lamprophyre. Besides, by using clinopyroxene mineral chemistry and whole rock chemistry, we try to investigate the petrogenesis of these lamprophyres from different aspects.

General geology:

Monavvar region is located in the east Azerbaijan province of Iran. Monavvar region is a part of the Alborz-Azarbaijan zone. Field observations show two spessartite dykes intruded in the andesitic lavas of the studied region. The age of andesitic lava and spessartite dykes is Plio-Quaternary because the andesitic lava intruded in Pliocene pyroclastic lava. Spessartite has a blackish-brown color in the hand specimens.

Petrography:

The main petrographic texture of these lamprophyres is the Porphyry texture. The major minerals are plagioclase microliths (10-15 volume %), orthoclase (5-10 volume %), hornblende phenocrysts with burnt rim (40-50 volume %), clinopyroxene (>20 volume %), and biotite (10-15 volume %). The accessory minerals include zircon, sphene, and apatite. The plagioclase has higher content than orthoclase and both of these minerals could be seen only as microlith. Regarding the nomenclature scheme of Le Maître (2002), these features indicate that these lamprophyres are spessartite.

Mineral Chemistry:

The mineral chemistry of amphibole shows a magnesio-hastingsite composition. However, biotite is phlogopite-eastonite and feldspars are orthoclase and oligoclase in composition.

Mineral chemistry of clinopyroxene studies The clinopyroxenes are in the field of Quad in the Q-J diagram and diopside in the En-Fs-Wo diagram. According to the AlVI+2Ti+Cr-AlIV+Na diagram for clinopyroxenes, Monavvar spessartite has occurred in almost the stable and low oxygen fugacity status.  Based on Soesoo (1997), the clinopyroxenes were crystalized under 1100-1200 ℃ and 2-6 kbar. The chemical composition of clinopyroxenes indicates subduction-related volcanic arcs and within-plate tholeiitic environments.

Whole rock geochemistry of Monavvar spessartite:

Most lamprophyre samples are plotted in the trachybasalt field on the total alkali (K2O+Na2O) versus silica (Si2O) classification diagram. They show alkali basalt composition on the Zr/Ti2O-Nb/Y plot.  K2O-Si2O diagram classified them as calc-alkaline lamprophyres.

REE Geochemistry:

In the spider diagram of studied samples, Nb and Ti show a distinctive negative anomaly, and U, La, K, Th, and Ba show a positive anomaly. HFSEs depletion and LILEs enrichment of samples are characteristics of shoshonitic and calc-alkaline magma. Negative Nb and Ti anomalies could be a result of Ti-bearing mineral segregation or high oxygen fugacity. LILEs enrichment could indicate that aqueous fluid is present during magma-forming processes or crustal contamination during magma evolution. Y depletion could happen as a result of amphibole segregation.   All samples show highly fractionated steep REE patterns which means a distinctive enrichment of LREEs relative to HREEs. LREEs enrichment occurs as a result of small degrees of magma partial melting. However, this feature is a character of shoshonitic and calc-alkaline magma. The REE pattern of Monavvar spessartite does not show an Eu anomaly. In the basic rocks, concurrent crystallization of amphibole and plagioclase caused a lack of Eu anomaly.

Tectonic setting of Monavvar Spessartites;

Based on the Zr-Y diagram and Nb-Zr-Ce/P2O5 diagram, Monavvar spessartites are ascribed to an arc-related tectonic setting.

Petrogenesis of Monavvar Spessartites:

Based on the whole rock composition of Monavvar spessartite, Ni=68-92 ppm, Co=1-23 ppm, Cr=59-125 ppm, and Mg#=25-32%. These values mean the lamprophyres could not be considered as the primary magma, but probably they are very close to the primary magma composition. On the other hand, on Dy/Yb-La/Yb diagram, the samples are scattered in the field of garnet-bearing mantle zone. Similarly, the La/Yb-La diagram indicates the garnet presence in the source peridotite, in addition to the 5-15 % of mantle peridotite partial melting for producing Monavvar spessartite melt.

Geodynamics of Monavvar region:

According to Rock (1991), petrographical, mineralogical, and geochemical investigations revealed M6 and M7 magmas for the Monavvar spessartites. M6 was produced by contamination of primary magma by mantle elements and M7 was produced by crustal contamination of primary magma. By considering this, the function of strike-slip dextral faults in Azerbaijan (northwest of Iran) could be responsible for Monavvar spessartites formation. Due to the mentioned faults function, transcurrent basins are made across the faults. Transcurrent basins caused low partial melting degrees of the metasomatized lithospheric mantle and produced alkaline basic magma. Contamination of this magma in different depths could form spessartite magma.