Devonian volcanic rocks of the southern Chinese Altai, NW China: Petrogenesis and implication for a propagating slab-window magmatism induced by ridge subduction during accretionary orogenesis

Abstract

Ridge–trench interaction is a common tectonic process of the present-day Pacific Rim accretionary orogenic belts, and this process may facilitate “slab-window” magmatism that can produce significant thermal anomalies and geochemically unusual magmatic events. However, ridge-trench interaction has rarely been well-documented in the ancient geologic record, leading to grossly underestimation of this process in tectonic syntheses of plate margins. The Chinese Altai was inferred to have undergone ridge subduction in the Devonian and a slab-window model is proposed to interpret its high-temperature metamorphism and geochemically unique magmatic rocks, which can serve as an excellent and unique place to refine the tectonic evolution associated with ridge subduction in an ancient accretionary orogeny. For this purpose, we carried out geochemical and geochronological studies on Devonian basaltic rocks in this region. Secondary ion mass spectrometry (SIMS) zircon U-Pb dating results yield an age of 376.2 ± 2.4 Ma, suggesting an eruption at the time of Late Devonian. Geochemically, the samples in this study have variable SiO<inf>2</inf> (43.3–58.3 wt%), low K<inf>2</inf>O (0.02–0.07 wt%) and total alkaline contents (2.16–5.41 wt%), as well as Fe<inf>2</inf>O<inf>3</inf> ^T/MgO ratios, showing typical tholeiitic affinity. On the other hand, the basaltic rocks display MORB-like REE patterns ((La/Yb)<inf>N</inf> = 0.90–2.57) and (Ga/Yb)<inf>N</inf> = 0.97–1.28), and have moderate positive ε<inf>Nd</inf>(t) values (+4.4 to +5.4), which collectively suggest a derivation from a mixing source comprising MORB-like mantle of a mature back-arc basin and subordinate arc mantle wedge. These basaltic rocks are characterized by Low La/Yb (1.26–3.69), Dy/Yb (1.51–1.77) and Sm/Yb (0.83–1.32) ratios, consistent with magmas derived from low degree (∼10%) partial melting of the spinel lherzolite source at a quite shallow mantle depth. Considering the distinctive petrogenesis of the basaltic rocks in this region, the Late Devonian basalts in the southern Chinese Altai is suggested to have witnessed the propagating process of slab-window magmatism that was induced by ridge subduction in a nascent rifting stage of a back-arc basin.