Geochemical Characteristics and Petrogenesis of Pre- to Post-collisional Igneous Rocks in Armenia and Caucasian regions
Date Issued
2010
Date
2010
Author(s)
Lin, Yu-Chin
Abstract
Armenia is located in the Arabia-Eurasia continental collision zone that has also been considered as the product of the “Turkic-type” orogeny involving accretion of a number of terranes. Cenozoic magmatism in this zone, named CIA (Caucasus-Iran- Anatolia) province in this study, took place in two main stages that, respectively, pre- and post-date the Arabia-Eurasia collision. Whereas the pre-collisional magmatism has been generally ascribed to the Neotethyan subduction, how was the cause or mechanisam of the voluminous post-collisional volcanism formed has long been an issue of debates.
This study reports new ages and geochemical data of the pre- and post-collisional igneous rocks from Armenia. All the studied rocks are calc- alkaline and characterized by enrichment in LREE and other highly incompatible trace elements (e.g., Rb, Ba, Th, U), and depletions in the high field strength elements (e.g., Nb, Ta, Ti). These geochemical features, similar to those of coeval magmatic rocks from the CIA province, support the existence of a subduction- modified mantle that prevails throughout the Cenozoic. In Armenia, however, post-collisional rocks are more enriched in potassium and highly incompatible trace elements than pre-collisional ones. Post-collisional basalts [La=24-63 ppm; (La/Yb)N =5.8-20], for example, are more LREE-enriched than pre-collisional basalts [La=15-28 ppm; (La/Yb)N of 3.5-7.9]. All the Armenian rocks show rather uniform Sr-Nd isotopic ratios (87Sr/86Sr ≈ 0.7040 to 0.7047; 143Nd/144Nd ≈ 0.5127 to 0.5129), similar to the isotopic compositions reported in other CIA magmatic provinces. REE modeling suggests that Armenian pre- to post-collisional basaltic magmas were derived from a common mantle source that is located in spinel- to garnet-lherzolite transition region at ~60-80 km depth, with melting degrees being larger in the former (8-10 %) and smaller in the latter (3-6 %).
The Armenian results, combined with our data from other parts of the CIA province and literature information from E. Anatolia, allow us to better constrain the temporal, spatial and geochemical variations in the CIA province. The post-collisional volcanism began at ca. 11 Ma, and it shows change in time and space, prevailing during 9-6 Ma in E. Anatolia or the southwestern part of the CIA volcanic province and then migrating eastward. No volcanism occurred in the southwestern CIA province since ~2 Ma. Along with the predominant calc-alkaline rocks, adakites and ultrapotassic rocks are observed in the CIA volcanic province. The adakites are small-volume but widespread, erupting with a northeastward-younging trend from E. Anatolia to the Greater Caucasus. They have uniform Sr-Nd isotope ratios (87Sr/86Sr ≈ 0.7041 to 0.7050 and 143Nd/144Nd ≈ 0.5127 to 0.5128), similar to those of the other CIA post-collisional volcanics, suggesting a common mantle source. The ultrapotassic rocks that were emplaced in Saray, NW Iran, as one of the earliest eruptions (~11 Ma), have more “radiogenic” Sr-Nd isotope ratios (87Sr/86Sr ≈ 0.7078; 143Nd/144Nd ≈ 0.5125). The adakites are interpreted as partial melts of eclogitized lower crust, formed by basaltic underplating during the Neotethyan subduction and thickened by the collision, and the ultrapotassic rocks as small-degree melts of the metasomatized lithospheric mantle.
The driving force of the CIA post-collisional volcanism may be attributed to roll-back and then break-off of the subducted Neotethyan slab that, assuming an oblique/diachronous collision between Arabia and Eurasia, may have started from the northwest, i.e., beneath the southwestern CIA province, and propagated southeastward. Volcanism thus produced may later be ceased owing to the formation of new lithospheric mantle from below, as the melting residue, and subsequent crustal/lithospheric thickening caused by the continued collision. Under this framework, it is predictable that the post-collisional magmatism will eventually migrate southeastward along the Zagros suture zone.
Subjects
Armenia
Arabia-Eurasia collision zone
geochemistry
pre- and post-collisional volcanism
adakites
ultrapotassic rocks
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