Geochemical and age of collision related volcanism following the closure of the Neotethys Ocean (Lesser Caucasus, Armenia)

Description

In the Lesser Caucasus in Armenia, collision of the SouthArmenian Block (SAB) and Eurasia started during the Paleocene and was forming the Amasia-Sevan-Akera Suture zone (ASASZ). Magmatism covering the suture zone occurred during this collision and is particularly widespread since Middle Eocene. Moreover magmatism occurred after the Arabian plate collision with Eurasia since Miocene. In order to add constraints to the geodynamic context of these magmatisms, an extensive geochemical study (major & trace elements, isotopes) and geochronological study has been developped on 19 magmatic rocks. Only scarce geochemical and geochronological data are available on the Middle to Late Eocene volcanism which is associated to calc-alkaline to alkaline mildly alkaline compositions.12 zircon grains extracted from a rhyodacite sample from SAB have been dated by U-Pb laser ICP-MS ablation which gives a well-concordant middle Miocene age of 14.6±0.2 My. The rocks are overall characterized by enrichment in large ion lithophile elements (LILE) and show significant enrichment in light rare earth elements (LREE) compared to heavy rare earth elements (HREE) with (La/Sm)N evolving from 2.2-7.3 and (La/Yb)N ratios ranging between 2.5 and 16 and up to 47 for the rhyodacite. Extended rock-patterns of the ASASZ samples show positive anomalies in Pb, Sr, Ba associated to negative Nb and Ta spikes typical of subduction or back-arc environments. A more continental character is evidenced for trachyandesites of late Eocene and rhyodacite of middle Miocene ages due to possible slab retreat, a break off and continental crust heating by rising of the astenospheric mantle (Sosson et al. 2010).

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