PROTOLITH NATURE AND TECTONOMAGMATIC FEATURES OF AMPHIBOLITES FROM THE QUSHCHI AREA, WEST AZERBAIJAN, NW IRAN

Amphibolites from the Qushchi area in west Azerbaijan province, NW Iran are metabasites containing hornblende, plagioclase, epidote, garnet, relict igneous clinopyroxene and titanite, apatite and opaque minerals as accessory phases. They are spatially associated with an ophiolitic melange but their relationship is not clear. Based on whole rock geochemistry of the amphibolites, they are formed from sub-alkaline andesite-basalt with a tholeiitic affinity. TiO2 content of the analyzed amphibolite samples is mainly less than 1%, indicating an EMORB original character for the magma. Major and trace element geochemistry of the studied rocks indicate a volcanic arc setting for the rocks. Chemistry of relict igneous clinopyroxene shows that they are diopside in composition with Mg# of 86.75-88.78 and indicating tholeiitic magma type derived from volcanic arc setting, which is in agreement with the results from the whole rock chemistry. Low Ti content of the clinopyroxene points to a depleted mantle source for the magma of the protoliths of Qushchi amphibolites. There is no isotopic age constrains on the studied amphibolites, therefore their relation to the ophiolitic melange of the area is uncertain especially that the melange is allochthonous. Three possibilities can be proposed for the formation of the studied amphibolites. If these rocks are Late Cretaceous- Paleocene in age, they might have been formed as parts of a volcanic arc in the Neotethyan oceanic crust. In this case, the ophiolitic complex and the volcanic arc rocks all are metamorphosed at amphibolite facies following the Neotethys ocean closure and the continental collision. Based on field relations and comparing the studied amphibolites with similar amphibolites from the adjacent Khoy area, alternatively the amphibolite formation can be consider to predate the formation of Neotethys-related ophiolite melange. Since the serpentinite in the ophiolitic melange is not metamorphosed, the second explanations can be valid for the formation of the amphibolites. The third possibility is that the protolith of the amphibolites was contemporaneous with ophiolite formation, but this protolith is metamorphosed within the accretionary prism but the obducted ophiolitic rocks (including serpentinite) not subjected to metamorphism.

Keywords:

Amphibolite, subalkaline magma, relict clinopyroxene, volcanic arc, W Azerbaijan NW Iran,

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