Qushchi (kb İran, bati Azerbaycan) amfibolitlerinin tektonomagmatik özellikleri ve protolit tipi

Kuzeybatı İran, Batı Azerbaycan Provensinde yer alan Qushchi Bölgesindeki amfibolitler, hornblend, plajiyoklas, epidot, granat, kalıntı magmatik klinopiroksen ve aksesuvar mineral olarak da titanit, apatit ve opak mineral içeren metabazitlerdir. Amfibolitler mekansal olarak ofiyolitik melanj ile ilişkilidir ancak bu ilişki açık olarak gözlenmemektedir. Tüm kayaç jeokimyasına göre, amfibolitler, toleyitik karakterli sub-alkalin andezit-bazalttan oluşmuştur. TiO2 içerikleri genellikle %1den küçük olup, magmanın E-MORB (zenginleşmiş okyanus ortası sırtı bazaltları) bileşiminde olduğuna işaret etmektedir. Çalışılan kayaçların ana ve iz element jeokimyası, kayaçların volkanik yay bölgesinde yer aldığını ortaya koymaktadır. Kalıntı magmatik klinopiroksenler üzerinde gerçekleştirilen mineral kimyası çalışmaları, bunların diyopsit bileşiminde olduğunu ve yüksek Mg#:86.75-88.78 değerlerinin de volkanik yay bölgesinden türeyen toleyitik karakterli magmaya işaret ettiğini göstermektedir. Bu durum, tüm kayaç jeokimyasından elde edilen sonuçlarla da uyumludur. Klinopiroksen minerallerinde gözlenen düşük Ti içeriği, Qushchi amfibolitini oluşturan protolitin (ilksel kayaç) tüketilmiş bir manto kaynağından türediğine işaret etmektedir. Çalışılan amfibolitlere ait izotopik yaş verisi bulunmadığından, ofiyolitik melanj ile olan ilişkisi, özellikle melanjın da allokton olduğu düşünüldüğünde, belirsizdir. Amfibolitlerin oluşumu ile ilgili 3 olasılık önerilebilir. Bu kayaçlar, Geç Kretase-Paleosen yaşında ise, Neotetis okyanusal kabuğundaki volkanik yayın bir parçası olarak oluşmuş olabilirler. Bu durumda, ofiyolitik kompleks ve volkanik yay kayaçları, Neotetis okyanusunun kapanması ve kıtasal çarpışmayı takiben amfibolit fasiyesi metamorfizmasına maruz kalmıştır. Bunun dışında, arazi gözlemleri ve çalışılan amfibolitlerin Khoy bölgesindeki benzer amfibolitlerle karşılaştırılmasına göre ise, amfibolitlerin oluşumunun, Neotetis ilişkili ofiyolit melanj oluşumundan önce geldiği düşünülmektedir. Çünkü, ofiyolitik melanj içinde yer alan serpantinitler metamorfizmaya uğramamıştır ve böylece bu olasılık, amfibolitlerin oluşumu için geçerli olabilmektedir. Üçüncü olasılık ise, amfiboliti oluşturan protolitin ofiyolit oluşumu ile eş zamanlı olduğudur. Ancak, protolit, yığışım prizması içinde metamorfizmaya uğramıştır fakat üzerleyen ofiyolitik kayaçlar (serpantinit de dahil olmak üzere) metamorfizma olmamıştır.

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 mélange 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 E-MORB 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 mélange of the area is uncertain especially that the mélange 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 mélange. Since the serpentinite in the ophiolitic mélange 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.

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