Boron isotope insights into the origin of subduction signatures in continent-continent collision zone volcanism

Sugden, PJ; Savov, IP; Agostini, S; Wilson, M; Halama, R; Meliksetian, K

Abstract

We present the first boron abundance and delta B-11 data for young (1.5-0 Ma) volcanic rocks formed in an active continent-continent collision zone. The delta B-11 of post-collisional volcanic rocks (-5 to +2 parts per thousand) from the Armenian sector of the Arabia-Eurasia collision zone are heavier than mid-ocean ridge basalts (MORB), confirming trace element and isotope evidence for their derivation from a subduction-modified mantle source. Based on the low B/Nb (0.03-0.25 vs 0.2-90 in arc magmas), as well as low Ba/Th and Pb/Ce, this source records a subduction signature which is presently fluid-mobile element depleted relative to most arc settings. The heavier than MORB delta B-11 of post-collision volcanic rocks argues against derivation of their subduction signature from a stalled slab, which would be expected to produce a component with a lighter than MORB delta B-11, due to previous fluid depletion. Instead, the similarity of delta B-11 in Plio-Pleistocene post-collision to 41 Ma alkaline igneous rocks also from Armenia (and also presented in this study), suggests that the subduction signature is inherited from Mesozoic-Paleogene subduction of Neotethys oceanic slabs. The slab component is then stored in the mantle lithosphere in amphibole, which is consistent with the low [B] in both Armenian volcanic rocks and metasomatic amphibole in mantle xenoliths. Based on trace element and radiogenic isotope systematics, this slab component is thought to be dominated by sediment melts (or supercritical fluids). Previously published delta B-11 of metasediments suggests a sediment-derived metasomatic agent could produce the B isotope composition observed in Armenian volcanic rocks. The lack of evidence for aqueous fluids preserved over the 40 Myr since initial collision supports observations that this latter component is transitory, while the lifetime of sediment melts/supercritical fluids can be extended to >40 Myr.

Citation

Sugden, P., Savov, I., Agostini, S., Wilson, M., Halama, R., & Meliksetian, K. (2020). Boron isotope insights into the origin of subduction signatures in continent-continent collision zone volcanism. Earth and Planetary Science Letters, https://doi.org/10.1016/j.epsl.2020.116207

• Acceptance Date: Mar 4, 2020
• Publication Date: Mar 16, 2020
• Journal: Earth and Planetary Science Letters
• Print ISSN: 0012-821X
• Publisher: Elsevier
• DOI: https://doi.org/10.1016/j.epsl.2020.116207
• Keywords: post-collisional volcanism; subduction signature; boron isotopes
• Publisher URL: https://www.sciencedirect.com/science/article/pii/S0012821X20301503?via%3Dihub