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

摘要

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 similar to 386Ma, and aremainly composed of clinopyroxene (Wo(43-55)En(17-27)Fs(26-34)) and albite (An(0-1)Ab(92-99)Or(0-7)). Although the clinopyroxenes have consistent Mg# values (up to 75) that appear to be in equilibriumwith bulk compositions, they can be divided into two groups based on their differences in other geochemical variables: type vertical bar has high SiO2 and CaO but low TiO2 and Al2O3 contents while type. displays low SiO2 and CaO but high TiO2 and Al2O3 contents. This indicates that the two types of clinopyroxenesmay share a common source but crystallized at different depths under different P-T conditions (e.g., type |: 6.3 kbar (similar to 20 km), 1227 degrees C; type.: 15.5 kbar (similar to 49 km), 1332 degrees C). The dykes have low SiO2 (44.5-48.2 wt%) and K2O (0.17-0.33 wt%), but high (Fe2O3) 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) = 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 (Sr-87/Sr-86(i) = 0.7039-0.7042, epsilon Nd-(t) = +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 degrees 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-likemagmatism 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 thatMORB-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. (C) 2020 Elsevier B.V. All rights reserved.