Felsic magmatism at subduction initiation in the ensimatic Tannuola–Khamsara island arc (Tuva, Central Asia): geochemical, Sm–Nd and U–Pb data from the Ondum subzone
A.A. Mongush1, N.I. Gusev2, I.V. Karmysheva3, R.V. Kuzhuget1, S.G. Prudnikov1, Ch.K. Oydup1, Ch.O. Kadyr-ool1, E.K. Druzhkova1, F.P. Lesnov3
1 Tuvinian Institute for Exploration of Natural Resources, Siberian Branch of the Russian Academy of Sciences, Kyzyl, Russia
2AII-Russian Geological Research Institute of A. P. Karpinsky, St. Petersburg, Russia
2V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, Novosibirsk, Russia
Keywords: Suprasubduction zone, subduction initiation, ophiolites, plagiorioliths, plagiogranites, geochemistry, zircon, U-PB age, paleogeodynamics
Abstract
The paper examines why voluminous felsic magmas formed during subduction initiation in the Tannuola-Khamsara island-arc system (Tuva, Central Asia), focusing on the Ondum subzone. We report geochemical, Sm-Nd and U-Pb zircon data for plagiorhyolites-plagiodacites, andesibasalts-andesites-dacites, their intrusive equivalents (plagiogranites-tonalites and quartz diorites-diorites), and gabbroic inclusions. Two geochemical types are recognized: depleted Type I (dominant; εNd(t)=+10.4…+6.4, (La/Yb)N=0.3–0.9) and moderately depleted Type II (subordinate; εNd(t)=+7.7, (La/Yb)N=1.0–1.7). Type I felsic rocks are consistent with low-pressure, water-saturated partial melting of a gabbroic source, whereas Type II reflects lower degrees of melting and/or weaker fluid input. SSZ ophiolites (580–569 Ma) occur in the forearc, arc and back-arc domains, implying near-synchronous SSZ crust formation at subduction initiation. Xenogenic zircon (571–663 Ma) and metamorphic blocks suggest involvement and partial preservation of an inherited oceanic substrate, possibly with an unusually thick terrigenous-carbonate sedimentary cover. We model spatially heterogeneous SSZ extension: new crust formed in spreading sectors, whereas non-spreading segments retained pre-subduction blocks where hydration and anatexis of gabbroic basement (potentially aided by deep seawater-driven hydrothermal circulation along faults) produced large volumes of felsic melts. Local slab melting is recorded by an adakitic tonalite (556±3 Ma, εNd(t)=+1.0), consistent with a terrigenous sediment component carrying an evolved crustal isotopic signature.
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