Givetian-Frasnian rift-related volcanism of the Shipunikha Depression, NW Rudny Altai, Central Asia: Tectono-magmatic significance and new constraints on whole-rock geochemistry, Nd-isotopic data and UPb ages

Abstract

The paper presents new original data on the Devonian rift-related volcanism from the Rudny Altai, a marginal terrane of the Siberian continent (Altai-Sayan Fold Belt, western Central Asian Orogenic Belt). The extensional regime in relation to tectono-magmatic reactivation of the continental margin was accompanied by an intense flare up of bimodal-type shallow-marine volcanism and injection of subvolcanic intrusions. On the first stage basic magmatism evolved from (i) injection of “More LREE- and HFSE-enriched” dolerite (ε_Nd(t) = +5.1) to (ii) eruption of their effusive analogues (ε_Nd(t) = +1.4), and following “Less LREE- and HFSE-enriched” basalt (ε_Nd(t) = +6.1). This stage reflects initial opening of the rift. On the second stage, volcanism evolved from (iii) “LREE- and HFSE-depleted” (ε_Nd(t) = +5.4) to (iv) “LREE-enriched and HFSE-depleted” (ε_Nd(t) = +4.1 to +4.7) types. This stage correspond to the prolonged opening of the pull-apart basin, as there is no geological evidence for island-arc uplift during this period. Variations in indicator HFSE ratios (Th/Yb, Nb/Yb and Zr/Y), enrichment of Nb relative to La and Th in combination with moderate Nd isotopic values resemble those for basalts from the behind-arc region with close spatial-temporal relation to rifting. The trace element variation indicates a change in the partial melting degree of the mantle source, from lower to higher, consistent with progressive rifting. The petrogenesis of magmas has been interpreted as shallow melting of a lithospheric mantle, metosomatised by a previous subduction event, and inherited geochemical signatures of relict mantle. The voluminous felsic magmatism was formed by crustal melting during lithospheric extension when basic magmas reached high crustal levels. The dacites yield zircon UPb ages of ca. 375 Ma, reflecting the peak of volcanic activity. The tectonic record bear evidence of strike-slip deformation, which could been induced by translation of arc and back-arc lithospheric plates.

Summary

The Devonian evolution of the Altai Convergent Margin (NW Rudny Altai) of the Siberian continent was accompanied by the generation of basalt-dacite volcanism (∼380-375 Ma). Based on the sequence of volcanic phases and their geochemical variations in the time range, two stages corresponding to the initial and long-term opening of the pull-apart basin are distinguished. The petrogenesis of the magmas is interpreted as shallow melting of lithospheric mantle metasomatised by a previous subduction event and inheriting geochemical signatures of the relict mantle. The initial and subsequent magmatic phases can be reproduced by varying the degree of partial melting of the magma source from lower to higher, consistent with the evolution of the pull-apart basin. Rift-related magmatism is caused by arc and back-arc plate translation, with a minor role for subduction factor.