Slab roll-back triggered back-arc extension south of the Paleo-Asian Ocean: insights from Devonian MORB-like diabase dykes from the Chinese Altai

To better understand the origin of mid-ocean ridge basalt (MORB)-like mafic magmas and its geodynamic implications for the subduction zone system, we present an integrated study of petrology, mineralogy, geochronology and geochemistry of newly-discovered diabase dykes from the Chinese Altai, southwestern Central Asian Orogenic Belt. The diabases have zircon U–Pb ages of ~386 Ma, and are mainly composed of clinopyroxene (Wo _43–55En _17–27Fs _26–34) and albite (An _0–1Ab _92–99Or _0–7). Although the clinopyroxenes have consistent Mg# values (up to 75) that appear to be in equilibrium with bulk compositions, they can be divided into two groups based on their differences in other geochemical variables: type ǀ has high SiO ₂ and CaO but low TiO ₂ and Al ₂O ₃ contents while type ǁ displays low SiO ₂ and CaO but high TiO ₂ and Al ₂O ₃ contents. This indicates that the two types of clinopyroxenes may share a common source but crystallized at different depths under different P-T conditions (e.g., type ǀ: 6.3 kbar (~20 km), 1227 °C; type ǁ: 15.5 kbar (~49 km), 1332 °C). The dykes have low SiO ₂ (44.5–48.2 wt%) and K ₂O (0.17–0.33 wt%), but high (Fe ₂O ₃) _T (11.2–13.6 wt%) and MgO (7.16–9.90 wt%) contents, placing them within the low-K tholeiitic series. With both N-MORB- and arc-like geochemical characteristics, including weakly fractionated rare earth element (REE) patterns ((La/Yb) _N [dbnd] 1.43–1.62), enrichment in large-ion lithophile elements (LILE) (e.g., Pb) and slightly depletion of high field strength elements (HFSE) (e.g., Nb, Ta and Ti), and highly depleted Sr–Nd isotopic compositions ( ⁸⁷Sr/ ⁸⁶Sr _i [dbnd] 0.7039–0.7042, ɛ _Nd(t) [dbnd] +8.3 to +8.6), the diabase dykes were probably derived by partial melting of a N-MORB-like asthenospheric mantle source metasomatized by subduction-related fluids under spinel-facies conditions. Melting temperatures and pressures for the primary magmas were estimated at 1371–1394 °C and 2.2–2.4 GPa, respectively. The dykes underwent insignificant crustal contamination, but trace-element modeling indicates that minor subcontinental lithospheric mantle (SCLM) components could have been involved in their source region. We suggest that the MORB-like magmatism was triggered by asthenospheric upwelling in a back-arc extensional setting, in response to the roll-back subduction of the Paleo-Asian oceanic slab. Combined with the coeval arc magmatism in the study area, we envisage that an active arc–back-arc basin system developed in the Chinese Altai during Devonian time, linked to the northward subduction of the Paleo-Asian Ocean. This study emphasizes that MORB-like mafic rocks in paleo-subduction zones could be an important tectonomagmatic process-indicator (e.g., back-arc extension), which offers a new perspective in reconstructing past tectonic regimes.