Episodic magmatism in the Lianhuashan tectonic belt: Implications for late Mesozoic crustal reworking in SE South China

Abstract

The Lianhuashan tectonic belt preserves abundant evidence of episodic magmatism and deformation that is crucial for understanding the late Mesozoic crustal reworking in SE South China. Here, we report zircon U-Pb ages, geochemical, and Sr−Nd−Hf isotopic data for representative granitoids and volcanic rocks to clarify the episodic magmatism in the Lianhuashan tectonic belt and its association with regional tectonic evolution. Our results define three episodes of magmatism. The Late Jurassic (160−150 Ma) magmatic rocks show high CaO/Na2O and molar CaO/(MgO + FeOt) (CMF) ratios and enriched Sr−Nd−Hf isotopes that are consistent with high-temperature melting of psammitic sources. The Early Cretaceous (144−131 Ma) magmatic rocks display variable CaO/Na2O ratios and zircon εHf(t) values (−12.0 to −2.5), which implies binary mixing of melts from pelitic and basaltic sources. The Late Cretaceous (ca. 94 Ma) magmatic rocks show high A/CNK and low CaO/Na2O and CMF ratios, which are characteristics of pelite-derived melts. They have zircon Hf isotopes (−4.4 to −1.9) that are slightly higher than those of the Early Cretaceous rocks, which is consistent with partial melting of diverse sources with more involvement of juvenile crust. These magmatic records, coupled with regional structural observations, suggest that the Lianhuashan tectonic belt underwent alternating crustal shortening and extension during the Late Jurassic to Cretaceous time. The Late Jurassic magmatism possibly marks the initiation of crustal extension following Middle Jurassic retro-arc crustal shortening related to the advancing Paleo-Pacific subduction. In the Cretaceous, the two magmatic flare-ups were separated by a magmatic lull associated with crustal shortening. The magmatic flare-ups occurred coevally with two stages of regional extension, as evidenced by widespread generations of basins and extensional domes. The time consistency implies that the magmatism was associated with or facilitated crustal extension that occurred at a regional scale and may have resulted from slab rollback during subduction retreat. We suggest that the magmatism and extension provide essential evidence of the crustal reworking in SE South China governed by the Paleo-Pacific subduction. Changes in the slab dynamics (steepening and shallowing) may control the flare-up, lull, and resumption of magmatism, as well as alternating episodes of crustal extension and shortening.