Zircon U-Pb-Hf Isotopic and Trace-Element Geochemistry Constraints on the Late Jurassic-Early Cretaceous Magmatic Evolution of Southeastern Zhejiang, South China

摘要

The Late Jurassic-Early Cretaceous magmatism at the margin of the South China Block (SCB) provide ideal targets to investigate magma petrogenesis and tectonic evolution of the SCB during the Mesozoic Era. Laser ablation ICP-MS U-Pb dating of zircons from Late Jurassic and Early Cretaceous granitic rocks in southeastern Zhejiang yielded emplacement ages of 144 +/- 2 to 150 +/- 2 and 133 +/- 1 to 134 +/- 2 Ma, respectively. Geochemical analyses show that both the Late Jurassic and Early Cretaceous granitic rocks have high values of A/CNK (Al2O3/[CaO+K2O+Na2O]) and A/NK (Al2O3/(K2O+Na2O)), are enriched in Rb, Th, Ba, Zr, and Hf, and are depleted in Sr, P, Ti, Nd, and Ta. Total rare earth element (REE) contents range from 95 to 216 ppm, and the samples are enriched in light REEs (light REEs/heavy REEs: 3.0-14.8). Values of (La/Yb)(N) range between 2.1 and 14, and all samples show negative Eu anomalies (Eu=0.1-0.7). The Late Jurassic granitic rocks have epsilon(Hf)(t) values of -11.4 to -3.2 and yield two-stage depleted-mantle Hf model ages of 1.37-1.92 Ga, while the Early Cretaceous granitic rocks have epsilon(Hf)(t) values of -13.2 to +4.3 and yield two-stage depleted-mantle Hf model ages of 0.91-2.02 Ga. The data suggest that the Late Jurassic and Early Cretaceous granitic rocks crystallized from magmas with ancient crust sources and that a significant injection of juvenile crust material occurred during the Early Cretaceous. The results, combined with regional magmatic information, indicate that the southeastern SCB experienced a regional tectonic switch from a Late Jurassic transpressive regime to an earliest Cretaceous extensional one. This change was most likely caused by rollback of paleo-Pacific plate subducted slab from the Late Jurassic to the Early Cretaceous.