Tholeiitic to calc-alkaline and alkaline volcanisms in an extensional arc setting of a Tethyan ophiolite: insights from small-scale compositional and temporal transitions from the Dali sector (Armenia)

Abstract

• Dali's sector shows a transition from island-arc magmatism to more alkaline volcanism. • Alkaline lavas comprise E-MORB, transitional calc-alkaline basalts and atypical OIB. • The transition to alkaline lavas occurred in the Tithonian–Valanginian time interval. • The diversity of magmatic affinities suggests an extensional oceanic arc setting. New field and petrographic observations, combined with whole rock and mineral geochemical analyses are applied on volcanic rocks present in the Dali sector, east of Lake Sevan (Armenia). A small-scale sampling of the volcanic sequence allows us to identify, for the first time in Armenian ophiolites, two groups of lavas (groups B and C1) stratigraphically and geochemically intermediate between previously recognized arc tholeiite (group A) and OIB-like (group C2) basalts. Their age is constrained by two distinct intercalated radiolarite sequences, which were dated as Tithonian – Valanginian. Group B lavas are low-K tholeiitic basalts and basaltic andesites derived from an enriched mantle source (La N /Yb N 1.8–6.1; Sm N /Yb N 1.3–2.4; Nb/Yb 2.0–6.7). The composition of their clinopyroxenes ranges from Ti-poor augite to Ti-rich diopside augite in correlation with higher La/Yb and Nb/Yb ratios in bulk rocks. Group C1 lavas are basaltic andesites containing Ti and Na-rich amphiboles (kaersutites, hastingsites) as major mineral phases. Three analyzed diopside-amphibole-porphyritic samples are alkaline trachybasalts (La N /Yb N 20–21; Nb/Yb 16–18). Both the group B and C1 lavas exhibit variable, although moderate, enrichments in Th/Yb for given Nb/Yb ratios and Nb negative anomalies in normalized multi-elements patterns (Nb/La 0.53–1.02), which are not correlated with the degree of enrichment of their mantle sources. Our results suggest that transitional and OIB-like volcanic lavas were generated by low- (group B) and very low- (groups C1–C2) melting of a heterogeneous mantle source in an oceanic arc–back-arc system in extension, possibly in relation with slab break-off.