The Thickness of the Mantle Lithosphere and Collision-related Volcanism in the Lesser Caucasus

Sugden, PJ; Savov, IP; Wilson, M; Meliksetian, K; Navasardyan, G; Halama, R

Abstract

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 north-west to south-east 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 south-east, 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 normalised trace element patterns are the result of derivation from a subduction-modified mantle source, which likely 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 south-east. Melt thermobarometry calculations based on bulk-rock major elements confirm that melting in the south-east 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 north-west 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 north-west Iran. The mid-lithosphere magma source has a distinct composition compared to 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.

Citation

Sugden, P., Savov, I., Wilson, M., Meliksetian, K., Navasardyan, G., & Halama, R. (2019). The Thickness of the Mantle Lithosphere and Collision-related Volcanism in the Lesser Caucasus. Journal of Petrology, 199-230. https://doi.org/10.1093/petrology/egy111

• Acceptance Date: Nov 26, 2018
• Publication Date: Feb 1, 2019
• Journal: Journal of Petrology
• Print ISSN: 0022-3530
• Publisher: Oxford University Press
• Pages: 199-230
• DOI: https://doi.org/10.1093/petrology/egy111
• Keywords: collision setting; Lesser Caucasus; magma petrogenesis; mantle lithosphere thickness; Sr-Nd isotopes
• Publisher URL: http://dx.doi.org/10.1093/petrology/egy111