A new look at the geodynamic development of the Ediacaran–early Cambrian forearc basalts of the Tannuola-Khamsara Island Arc (Central Asia, Russia): Conclusions from geological, geochemical, and Nd-isotope data

Abstract

Oceanic igneous rocks throughout the Altai-Sayan Fold Belt (ASFB) in central-southern Siberia are often considered to be late Precambrian–early Paleozoic accreted elements of oceanic crust – often of uncertain paleogeographic or geodynamic origin. We explore the role of suprasubduction zone settings in the formation of different ASFB terranes. Our study offers a non-accretionary perspective on the tectonomagmatic development of basalt-bearing units in the ASFB on the example of the forearc terrane of the Ediacaran–early Cambrian Tannuola-Khamsara island arc (herein termed Sayan-Tuvan forearc zone). We describe the geochemistry, structural geology, and stratigraphic relations of basalts of the Aldynbulak, Uttug-Khaia, and Chingin formations, which are integral parts of the Sayan-Tuvan forearc zone. The Aldynbulak basalt samples mainly fall in the compositional fields of ocean island basalts and enriched mid-ocean ridge basalts (E-MORB) and likely derived from a deep mantle source. The Uttug-Khaia and Chingin basalts are N- and E + T-MORB-like basalts, carrying forearc geochemical signatures. Specifically, the Chingin Formation contains boninite dikes and is associated with a boninite-bearing ophiolite. Boninites are commonly associated with forearc magmatism and thus a forearc formation setting is likely. Tectonic and stratigraphic considerations imply that the Aldynbulak basalts formed first, followed by the Uttug-Khaia and later the Chingin basalts and boninites. A schematic model, involving decompression melting of the mantle, is proposed for the development of the studied forearc basalt suites that are linked with the growth of the Tannuola-Khamsara island arc system 580–540 million years ago.