Episodic bimodal magmatism at an active continental margin due to Paleo-Pacific Plate subduction: A case study from the southern segment of the Tan-Lu Fault Zone, eastern China

Abstract

Abstract Volcanic rocks in the southern part of the Tan-Lu Fault Zone (TLFZ), eastern China, provide key evidence for the connection between spatially and temporally related pulses of Cretaceous magmatism, activity on the TLFZ, and the dynamics of the Paleo-Pacific Plate. We identify two stages of bimodal volcanism in the Chaohu–Lujiang segment, southern TLFZ, based on lithological assemblages and precise isotopic ages. The Early Cretaceous (125–121\u202fMa) volcanic rocks are fractionated aluminous A-type rhyolites and basalts, whereas the Late Cretaceous volcanic rocks (97–83\u202fMa) range in composition from porphyritic andesites and trachydacites to rhyolites. The Early Cretaceous rhyolites display strongly negative Nd-Hf isotopic compositions (eNd(t)\u202f=\u202f−16.3 to −16.1, eHf(t)\u202f=\u202f−24.1 to −17.9). The most likely source rocks of the fractionated aluminous A-type rhyolites are metaigneous rocks of the Feidong Complex. The Early Cretaceous potassic basalts (K2O/Na2O\u202f=\u202f1.5–3.4, eNd(t)\u202f=\u202f−9.0) exhibit more enriched isotopic compositions than the Late Cretaceous sodic porphyritic andesites (K2O/Na2O\u202f=\u202f0.6–0.8, eNd(t)\u202f=\u202f−5.6, eHf(t)\u202f=\u202f−15.1 to −6.8), indicating derivation from heterogeneously enriched lithospheric mantle. This heterogeneity is ascribed to derivation from more fertile and isotopically enriched mantle sources with lower solidus temperatures. Modeling of selected elements reveals that the Late Cretaceous trachydacites (eNd(t)\u202f=\u202f−12.2 to −8.2, eHf(t)\u202f=\u202f−16.5 to −6.4) originated by mixing between coeval mafic and felsic magmas. The most negative (eNd(t)\u202f=\u202f−18.8 to −17.1, eHf(t)\u202f=\u202f−20.5 to −4.7) isotopic compositions, along with Neoproterozoic inherited zircons within the Late Cretaceous rhyolites, indicate derivation from lower crust of the South China Block. We suggest that the Early Cretaceous bimodal volcanic rocks formed in an extensional backarc setting induced by retreat of the subducting Izanagi slab. However, the TLFZ became a focus of Late Cretaceous bimodal volcanism due to weak far-field extension caused by the slow northward motion of the Paleo-Pacific Plate. Moreover, the high velocity and low angle of subduction of the Izanagi slab were the main driving mechanisms behind an Early Cretaceous transpressional event that separated the two stages of bimodal volcanism.