The South Armenian Block: Gondwanan origin and Tethyan evolution in space and time

The geodynamic evolution of the South Armenian Block (SAB) within the Tethyan realm during the Palaeozoic to present-day is poorly constrained. Much of the SAB is covered by Cenozoic sediments so that the relationships between the SAB and the neighbouring terranes of Central Iran, the Pontides and Taurides are unclear. Here we present new geochronological, palaeomagnetic, and geochemical constraints to shed light on the Gondwanan and Cimmerian provenance of the SAB, timing of its rifting, and geodynamic evolution since the Permian. We report new 40Ar/39Ar and zircon U-Pb ages and compositional data on magmatic sills and dykes in the Late Devonian sedimentary cover, as well as metamorphic rocks that constitute part of the SAB basement. Zircon age distributions, ranging from ∼3.6 Ga to 100 Ma, firmly establish a Gondwanan origin for the SAB. Trondhjemite intrusions into the basement at ∼263 Ma are consistent with a SW-dipping active continental margin. Mafic intraplate intrusions at ∼246 Ma (OIB) and ∼234 Ma (P-MORB) in the sedimentary cover likely represent the incipient stages of breakup of the NE Gondwanan margin and opening of the Neotethys. Andesitic dykes at ∼117 Ma testify to the melting of subduction-modified lithosphere. In contrast to current interpretations, we show that the SAB should be considered separate from the Taurides, and that the Armenian ophiolite complexes formed chiefly in the Eurasian forearc. Based on the new constraints, we provide a geodynamic reconstruction of the SAB since the Permian, in which it started rifting from Gondwana alongside the Pontides, likely reached the Iranian margin in Early Jurassic times, and was subject to episodes of intraplate (∼189 Ma) and NE-dipping subduction-related (∼117 Ma) magmatism

The macrolithic industry from Aknashen

International audienc

Ancient Armenia at the crossroads: Natural hazards and Adaptation Strategies in Armenia from 10 000 BCE onwards

International audienc

Ancient Armenia at the crossroads: Natural hazards and Adaptation Strategies in Armenia from 10 000 BCE onwards

International audienc

Editorial: Ancient Armenia at the crossroads: Natural hazards and Adaptation Strategies in Armenia from 10 000 BCE onwards

International audienc

Alkaline magmas in collision zone settings:age and petrogenesis of the Tezhsar Alkaline Complex, Armenia

The occurrence of alkaline igneous rocks in collision zones is a topic of intense discussion with respect to their petrogenesis and prospectivity of rare earth elements (REE) in ore exploration, due to their importance in developing modern technologies. Despite that, alkaline complexes within collisional environments are rarely studied. In this work, we present new petrological and geochemical whole-rock data for the Eocene Tezhsar Alkaline Complex (TAC) in central Armenia, and we provide a new date for the complex of 41.0 ± 0.5 Ma obtained by 40Ar/39Ar geochronology. Two major events of the South Armenian Block and Arabia colliding with the Eurasian Plate in the Late Cretaceous and Oligocene respectively are highlighted in the Lesser Caucasian geo-tectonic history. The aim is to improve our understanding of the formation of alkaline rocks in collisional settings and to integrate the study into a wider context of alkaline magma genesis amid regional Eurasian-Arabian continental convergence along the major Sevan-Akera suture. TAC is a concentric volcano-plutonic complex ∼10km in diameter, representing a remnant of a large stratovolcano located just a few kilometres southward of the suture zone. Meliksetian (1989) accounted for >50 different mineral species within Tezhsar including allanite, monazite, xenotime and rare REE-bearing species like loparite, euxenite and britholite. For the purpose of this work, the TAC rocks are subdivided into three major units: An outer volcanic unit (OVU), an inner plutonic unit (SYU) and a central volcanic unit (CVU), that have been subsequently juxtaposed by ring faulting. The volcanic units are dominated by trachytic-phonolitic rocks, with rare pseudoleucite phonolite. The pluton comprises syenites and nepheline syenites with subordinate pegmatites containing large (up to 3cm) melanite garnets. Further petrographic study has shown apatite zoning with an enrichment of LREE in the rims and reaction replacement of clinopyroxene by amphibole. Whole-rock data show a highly fractionated, metaluminous, alkalic and silica-undersaturated composition of the TAC. The general trace element enrichment and strong fractionation of REEs (LaN /YbN ∼70>) point to a relatively enriched magma source and low degrees of partial melting. Negative Nb-Ta in mantle-normalised diagrams for all TAC units show typical subduction signatures. Other trace element indices also point to variable effects of subduction-related metasomatism. Mantle-like initial 87Sr/86Sr ratios (0.704-0.705) and positive εNd values (+3 to +5) indicate an isotopically depleted mantle source and no crustal influence. In summary, the new data suggest an influence of ancient subduction of the Tethyan oceanic plate beneath the Eurasian continental margin. TAC’s formation is broadly contemporaneous with Lesser Caucasian extension and crustal thinning due to rollback of the Neotethyan slab. Its alkalinity however sets it apart from other calc-alkaline centres in the region indicating further metasomatic enrichment just prior to the magma genesis. Furthermore, the presence of pseudoleucites, amphibolitisation reactions and the remobilisation of LREEs suggest late-stage fluid circulation in the complex’s evolutionary story

Alkaline magmas in collision zone settings:age and petrogenesis of the Tezhsar Alkaline Complex, Armenia

The occurrence of alkaline igneous rocks in collision zones is a topic of intense discussion with respect to their petrogenesis and prospectivity of rare earth elements (REE) in ore exploration, due to their importance in developing modern technologies. Despite that, alkaline complexes within collisional environments are rarely studied. In this work, we present new petrological and geochemical whole-rock data for the Eocene Tezhsar Alkaline Complex (TAC) in central Armenia, and we provide a new date for the complex of 41.0 ± 0.5 Ma obtained by 40Ar/39Ar geochronology. Two major events of the South Armenian Block and Arabia colliding with the Eurasian Plate in the Late Cretaceous and Oligocene respectively are highlighted in the Lesser Caucasian geo-tectonic history. The aim is to improve our understanding of the formation of alkaline rocks in collisional settings and to integrate the study into a wider context of alkaline magma genesis amid regional Eurasian-Arabian continental convergence along the major Sevan-Akera suture. TAC is a concentric volcano-plutonic complex ∼10km in diameter, representing a remnant of a large stratovolcano located just a few kilometres southward of the suture zone. Meliksetian (1989) accounted for >50 different mineral species within Tezhsar including allanite, monazite, xenotime and rare REE-bearing species like loparite, euxenite and britholite. For the purpose of this work, the TAC rocks are subdivided into three major units: An outer volcanic unit (OVU), an inner plutonic unit (SYU) and a central volcanic unit (CVU), that have been subsequently juxtaposed by ring faulting. The volcanic units are dominated by trachytic-phonolitic rocks, with rare pseudoleucite phonolite. The pluton comprises syenites and nepheline syenites with subordinate pegmatites containing large (up to 3cm) melanite garnets. Further petrographic study has shown apatite zoning with an enrichment of LREE in the rims and reaction replacement of clinopyroxene by amphibole. Whole-rock data show a highly fractionated, metaluminous, alkalic and silica-undersaturated composition of the TAC. The general trace element enrichment and strong fractionation of REEs (LaN /YbN ∼70>) point to a relatively enriched magma source and low degrees of partial melting. Negative Nb-Ta in mantle-normalised diagrams for all TAC units show typical subduction signatures. Other trace element indices also point to variable effects of subduction-related metasomatism. Mantle-like initial 87Sr/86Sr ratios (0.704-0.705) and positive εNd values (+3 to +5) indicate an isotopically depleted mantle source and no crustal influence. In summary, the new data suggest an influence of ancient subduction of the Tethyan oceanic plate beneath the Eurasian continental margin. TAC’s formation is broadly contemporaneous with Lesser Caucasian extension and crustal thinning due to rollback of the Neotethyan slab. Its alkalinity however sets it apart from other calc-alkaline centres in the region indicating further metasomatic enrichment just prior to the magma genesis. Furthermore, the presence of pseudoleucites, amphibolitisation reactions and the remobilisation of LREEs suggest late-stage fluid circulation in the complex’s evolutionary story