The thickness of the mantle lithosphere and collision-related volcanism in the Lesser Caucasus

The Lesser Caucasus mountains sit on a transition within the Arabia–Eurasia collision zone between very thin lithosphere (<100 km) to the west, under Eastern Anatolia, and a very thick lithospheric root (up to 200 km) in the east, under western Iran. A transect of volcanic highlands running from NW to SE in the Lesser Caucasus allows us to look at the effects of lithosphere thickness variations on the geochemistry of volcanic rocks in this continental collision zone. Volcanic rocks from across the region show a wide compositional range from basanites to rhyolites, and have arc-like geochemical characteristics, typified by ubiquitous negative Nb–Ta anomalies. Magmatic rocks from the SE, where the lithosphere is thought to be thicker, are more enriched in incompatible trace elements, especially the light rare earth elements, Sr and P. They also have more radiogenic 87Sr/86Sr, and less radiogenic 143Nd/144Nd. Across the region, there is no correlation between SiO2 content and Sr–Nd isotope ratios, revealing a lack of crustal contamination. Instead, ‘spiky’ mid-ocean ridge basalt normalized trace element patterns are the result of derivation from a subduction-modified mantle source, which probably inherited its subduction component from subduction of the Tethys Ocean prior to the onset of continent–continent collision in the late Miocene. In addition to the more isotopically enriched mantle source, modelling of non-modal batch melting suggests lower degrees of melting and the involvement of garnet as a residual phase in the SE. Melt thermobarometry calculations based on bulk-rock major elements confirm that melting in the SE must occur at greater depths in the mantle. Temperatures of melting below 1200°C, along with the subduction-modified source, suggest that melting occurred within the lithosphere. It is proposed that in the northern Lesser Caucasus this melting occurs close to the base of the very thin lithosphere (at a depth of ∼45 km) as a result of small-scale delamination. A striking similarity between the conditions of melting in NW Iran and the southern Lesser Caucasus (two regions between which the difference in lithosphere thickness is ∼100 km) suggests a common mechanism of melt generation in the mid-lithosphere (∼75 km). The southern Lesser Caucasus magmas result from mixing between partial melts of deep lithosphere (∼120 km in the south) and mid-lithosphere sources to give a composition intermediate between magmas from the northern Lesser Caucasus and NW Iran. The mid-lithosphere magma source has a distinct composition compared with the base of the lithosphere, which is argued to be the result of the increased retention of metasomatic components in phases such as apatite and amphibole, which are stabilized by lower temperatures prior to magma generation

Alkaline magmas in zones of continental convergence:The Tezhsar volcano-intrusive ring complex, Armenia

Alkaline igneous rocks are relatively rare in settings of tectonic convergence and little is known about their petrogenesis in these settings. This study aims to contribute to a better understanding of the formation of alkaline igneous rocks by an investigation of the Tezhsar volcano-intrusive alkaline ring complex (TAC) in the Armenian Lesser Caucasus, which is located between the converging Eurasian and Arabian plates. We present new petrological, geochemical and Sr–Nd isotope data for the TAC to constrain magma genesis and magma source characteristics. Moreover, we provide a new 40Ar/39Ar age of 41.0 ± 0.5 Ma on amphibole from a nepheline syenite that is integrated into the regional context of ongoing regional convergence and widespread magmatism. The TAC is spatially concentric and measures ~10 km in diameter representing the relatively shallow plumbing system of a major stratovolcano juxtaposed by ring faulting with its extrusive products. The plutonic units comprise syenites and nepheline syenites, whereas the extrusive units are dominated by trachytic-phonolitic rocks. The characteristic feature of the TAC is the development of pseudomorphs after leucite in all types of the volcanic, subvolcanic and intrusive alkaline rocks. Whole-rock major element data show a metaluminous (Alkalinity Index = 0–0.1), alkalic and silica-undersaturated (Feldspathoid Silica-Saturation Index &lt;0) character of the TAC. The general trace element enrichment and strong fractionation of REEs (LaN/YbN up to 70) indicate a relatively enriched magma source and small degrees of partial melting. All TAC rocks show a negative Nb–Ta anomalies typical of subduction zone settings. The initial 87Sr/86Sr ratios (0.704–0.705) and positive εNd values (+3 to +5) indicate an isotopically depleted upper mantle and lack of significant crustal influence, which in turn suggests the TAC magma has formed via differentiation from lithospheric mantle melts. Regionally, the age of ~41 Ma places the TAC amid a Lesser Caucasian Eocene period of dominantly calc-alkaline magmatism. The TAC's arc-like geochemical signatures are interpreted to result from prior subduction of the Tethyan slab beneath the Eurasian continental margin. The alkaline character, distinct from regional trends, is attributed to Neotethyan slab rollback causing extension and inducing small degrees of decompression melting of metasomatised lithospheric mantle.</p

Post-collisional shift from polygenetic to monogenetic volcanism revealed by new ⁴⁰Ar/³⁹Ar ages in the southern Lesser Caucasus (Armenia)

The post-collisional Syunik and Vardenis volcanic highlands, located in the southern Lesser Caucasus mountains (part of the Arabia-Eurasia collision zone) are host to over 200 monogenetic volcanoes, as well as 2 large Quaternary polygenetic volcanoes in the Syunik highland. The latter are overlain by lavas from the monogenetic volcanoes, suggesting there was a transition in the style of volcanic activity from large-volume central vent eruptions to dispersed small-volume eruptions. 12 new high quality 40Ar/39Ar ages are presented here, with 11 ages calculated by step-heating experiments on groundmass separates, and the final age obtained from total fusions of a population of sanidines. All the ages were younger than 1.5 Ma, except for one ignimbrite deposit whose sanidines gave an age of 6 Ma. While the bulk of the exposed products of post-collisional volcanism relate to Pleistocene activity, it is clear there has been active volcanism in the region since at least the late Miocene. All ages for monogenetic volcanoes in the Syunik highland are younger than 1 Ma, but to the north in Vardenis there is geochronological evidence of monogenetic volcanism at 1.4 and 1.3 Ma. An age of 1.3 Ma is determined for a lava flow from one of the polygenetic volcanoes- Tskhouk, and when combined with other ages helps constrain the timing of the polygenetic to monogenetic transition to around 1 Ma. The new ages illustrate a degree of spatio-temporal coupling in the formation of new vents, which could be related to pull-apart basins focussing ascending magmas. This coupling means that future eruptions are particularly likely to occur close to the sites of the most recent Holocene activity. The polygenetic to monogenetic transition is argued to be the result of a decreasing magma supply based on: (i) volume estimates for Holocene eruptions and for all monogenetic volcanoes and their lava flows in Syunik; and (ii) the volcanic stratigraphy of the Lesser Caucasus region which shows late Pliocene- early Pleistocene continental flood basalts being succeeded by a few large andesite-dacite volcanoes and then the most recent deposits consisting of small-volume scoria cones. The Syunik highland has the highest density of monogenetic centres in the Lesser Caucasus, which is taken to indicate this region has the highest magma flux, and was therefore the last location to transition to monogenetic volcanism, which is why the transition is most clearly seen there. There is no evidence from Sr-Nd-B isotope measurements for the exhaustion of fusible slab components in the mantle source, showing that an inherited slab signature can survive for millions of years after the end of subduction. Although volcanism in the Lesser Caucasus is currently waning, a future pulse of activity is possible

Pleistocene - Holocene volcanism at the Karkar geothermal prospect, Armenia

Pleistocene to Holocene volcanic centres north of the Bitlis-Zagros suture in Turkey, Iran, Armenia and Georgia represent both volcanic hazards and potential or actual geothermal energy resources. Such challenges and opportunities cannot be fully quantified without understanding these volcanoes’ petrogenesis, geochronology and magmatic, tectonic or other eruption triggers. We discuss the age and igneous geology of the Karkar monogenetic volcanic field in Syunik, SE Armenia. The ~30 km2 field is beside the location of Armenia’s only geothermal energy test drilling site. Eruptions of fissure-fed trachybasaltic andesite to trachyandesite occurred on a trans-tensional pull-apart segment of the Pambak-Sevan-Syunik Fault and have previously been interpreted to be of Holocene age. We conducted high-resolution duplicate 40Ar/39Ar dating of 7 groundmass separates, providing composite plateau or inverse isochron ages ranging from 6 ± 3 ka to 332 ± 9 ka (2). Each lava flow displays petrographic and geochemical patterns consistent with melting of subduction-modified lithospheric mantle and crystal fractionation involving ol, sp, opx and cpx, amp and plg. Some crystal-scale zoning was observed, implying recharge prior to eruption, and a preliminary estimate of cpx crystallisation pressures indicates storage in the mid- to upper crust, which may be of relevance for geothermal developments. These data indicate that volcanic activity in Syunik and elsewhere in Armenia overlapped with human occupation and that the presence of a substantive heat source for geothermal energy and a lava inundation hazard for local infrastructure should be further considered. Additional geophysical monitoring of the Pambak-Sevan-Syunik Fault is merited, along with detailed determination of the depths of magma storage both here and also at Porak volcano 40 km north of Karkar

Pleistocene - Holocene volcanism at the Karkar geothermal prospect, Armenia

Quaternary volcanic centres north of the Bitlis-Zagros suture in Turkey, Iran and the Caucasus represent both volcanic hazards and potential or actual geothermal energy resources. Such challenges and opportunities cannot be fully quantified without understanding these volcanoes' petrogenesis, geochronology and magmatic, tectonic or other eruption triggers. In this preliminary study, we discuss the age and geology of the Karkar monogenetic volcanic field in Syunik, SE Armenia. The ∼70 km2 field is close to Armenia's only geothermal energy test drilling site. Fissure-fed trachybasaltic andesite to trachyandesite lavas erupted on a trans-tensional segment of the Syunik branch of the Pambak-Sevan-Syunik Fault, where previous studies suggested a Holocene age for the youngest eruptions. Here, high-resolution duplicate 40Ar/39Ar dating of 7 groundmass separates provided inverse isochron ages ranging from 7.4 ± 3.6 ka and 7.9 ± 2.9 ka to 353 ± 20 ka (2σ). Each lava flow displays petrographic and whole rock geochemical patterns consistent with melting of subduction-modified lithospheric mantle and extensive evolution within the crust involving fractional crystallisation and mixing of magma batches. Data confirm that volcanic activity related to the Syunik Fault overlapped with Palaeolithic to Bronze Age human occupation and remains a minor lava inundation hazard. Further geochemical work will allow constraint of the depth and timescales of magma storage. Both Karkar and the area around Porak volcano, which lies 35 km N of Karkar on the Syunik Fault, might be considered for future geothermal energy developments