Neoproterozoic tectonic transition from subduction to back-arc extension along the western Yangtze Block, South China: Petrological evidence of Nb-enriched basalts and arc-type intrusive rocks

Abstract

Identifying the tectonic transition process from persistent slab subduction to back-arc extension is vital for the systematical study of arc magmatic evolution in a temporal perspective. Although Neoproterozoic subduction setting along the western Yangtze Block has been widely documented, the detailed tectonic transition from subduction to back-arc extension was not systematically constrained. The coexistence of Nb-enriched rocks and normal arc-type rocks can generally provide unique insights into back-arc extension background under subduction setting. Herein we therefore present combined data of petrology, mineralogy, zircon U-Pb geochronology, whole-rock major and trace elements, and Sr-Nd isotopes as well as zircon Lu-Hf isotopes for the Neoproterozoic Nb-enriched basalts and arc-type intrusive rocks (i.e., gabbros and diorites) from the Luding area along the western Yangtze Block (South China), to characterize back-arc extension setting of Neoproterozoic continental magmatic arc. Zircon U-Pb ages suggest that the Nb-enriched basalts and arc-type gabbros-diorites formed coevally at ca. 780 Ma. The Nb-enriched basalts have much clinopyroxene phases and display higher TiO2, P2O5, Nb, Nb/La, and Nb/U values than normal arc volcanics. They are weakly enriched in Rb and Ba, and slightly depleted in Nb, Ta, Zr, and Hf, and show decoupled Nd-Hf isotopes with highly positive whole-rocks εNd(t) (+5.79 ∼ +7.39) and relatively low zircon εHf(t) (+1.09 ∼ +9.05) values, indicating an origination of subduction slab melts-metasomatized mantle wedge source, resembling with the genesis of those typical Nb-enriched mafic rocks in a back-arc setting. Compared with Nb-enriched basalts, the arc-type gabbros and diorites contain more hornblende minerals, and are characterized by variably high MgO (3.10–5.71 wt%) contents and Mg# values (51.5–57.2), and enriched LREEs and LILEs (e.g., Rb, Ba, Sr, and K) as well as depleted Nb, Ta, and Ti. Considering their relatively depleted and coupled Nd-Hf isotopes, these arc-type gabbros and diorites were formed by partial melting of hydrous mantle wedge modified by subduction fluids. The co-occurrence of Nb-enriched basalts and arc-type gabbros-diorites studied here therefore reveal Neoproterozoic back-arc extension background under the subduction setting. Combined with ca. 810–780 Ma mafic rocks showing Nb-enriched signatures and ca. 860–740 Ma normal arc-type mafic-intermediate rocks in the western Yangtze Block, we support that Neoproterozoic geodynamic transition from continuing subduction to back-arc extension initially occurred at ca. 820–810 Ma. The Neoproterozoic back-arc extension is thus significant for geochemical changes of subduction-related metasomatized agents and arc magmatic types.