Mesozoic tectonic transition from the Tethyan to Paleo-Pacific domains in South China: New insights from Mesozoic igneous rocks in the western Nanling Range

Abstract

• The studied Mesozoic igneous in the Nanling Rang include post-orogenic ultramafic–mafic rock, subduction-related mafic rocks and other calc-alkaline intermediate-felsic rocks. • Those igneous rock assemblages in the Nanling Rang reflect the tectonic transition of the SCB. • The Nanling Rang was a Tethyan domain before ca. 170 Ma but a Paleo-Pacific domain after that. • The Mesozoic evolution of the SCB involved four stages: post-orogenic extension, slab foundering, and tectonic transition from subduction to rollback, followed by slab rollback. Mesozoic tectono-magmatic evolution of the Nanling Range in the South China Block (SCB) remains controversial, especially about the role of the Tethyan and Paleo-Pacific dynamics in the generation of Mesozoic igneous rocks. The Mesozoic magmatic events have produced a diverse variety of rocks, dominantly granites and spatially associated mafic–ultramafic rocks, lamprophyres and diorites. We present new zircon U-Pb ages and in-situ Hf isotope data, along with whole-rock elemental and Sr-Nd isotopic data for the Mesozoic igneous rocks from the western Nanling Range. The two ca. 200 Ma ultramafic–mafic rocks have low SiO 2 contents (40.00–44.94 wt%) and typical ocean island basalt (OIB)-like trace elemental affinities with positive Nb and Ta anomalies, positive ε Nd (t) (4.47 to 4.88) and variable ε Hf (t) values (–3.65 to 11.91), suggesting that they were generated by partial melting of an asthenospheric mantle source. The ca. 165 Ma Jiangyong lamprophyre dyke has relatively high SiO 2 content (51.70 wt%) and arc-like trace elemental affinities with significantly negative Nb and Ta anomalies and variable ε Hf (t) values (–9.10 to 2.65). They were derived from a lithospheric mantle source previously modified by subducted component, along with a contribution from the asthenospheric mantle. Granites from the ca. 146–137 Ma Taibao granitic pluton have high SiO 2 contents (63.41–66.66 wt%), variable (La/Yb) N and Sr/Y values, and negative to positive ε Hf (t) values (−4.16 to 0.86). These granites have compositions transitional from adakite to arc-type rocks and were likely generated by partial melting of Mesoproterozoic meta -igneous rocks, with an addition of a mantle-derived component. Compared with those granites, diorites of the ca. 99 Ma Eryue pluton exhibit moderate SiO 2 contents (59.90–60.30 wt%), a wide range of zircon ε Hf (t) values (−12.04 to 0.50) and negative ε Nd (t) values (–8.61 to –8.57), and they were probably formed by magma mixing of crust-mantle materials associated with back-arc extension. Our new data, together with other geological evidence, provide further insights for the evolution of the Tethyan and Paleo-Pacific tectonic domains during the Late Triassic to Late Cretaceous in the SCB. In the early stage, the Tethyan tectonic domain, ultramafic–mafic rocks from the Ludong and Fuchuan plutons in the Nanling Range was related to the post-orogenic extension. This domain was gradually changed to the Paleo-Pacific domain during the period from Late Triassic to Early Jurassic and the tectonic transition was accomplished at ∼ 170 ± 5 Ma. In the later stage, the Paleo-Pacific tectonic domain, the Jiangyong lamprophyre, Taibao granites and Eryue diorites were produced via the crustal thickening, localized extension, and large-scale lithospheric extension due to the advancing, slab-foundering, and rollback of the oceanic slab during the period from Middle Jurassic to Late Cretaceous.