Geochemical and zircon U–Pb–Hf isotopic study of Early Cretaceous A‐type rhyolites in Hong Kong: Implications for Palaeo‐Pacific Plate subduction

Abstract

Hong Kong is located at the southern margin of a Mesozoic igneous belt in Southeast China, where voluminous magmatism formed during the Early Jurassic to Early Cretaceous. The final phase of volcanism formed the rhyolitic High Island Formation, which shows geochemistry similar to A‐type granite. For example, these rocks have high content of SiO2 and alkali (e.g., high K2O + Na2O, with K2O/Na2O ratios greater than 1.0) and are characterized by weakly enrichment in high‐field‐strength elements (HFSEs) and rare‐earth elements (REEs) (except for Eu) and extreme depletion of Ba, Sr, P, Ti, and Eu, with high 10,000× Ga/Al ratios. Zircon U–Pb dating for three porphyroclastic rhyolite samples from the High Island Formation yielded weighted mean 206Pb/238U ages of 140.0 ± 0.8, 139.8 ± 0.7, and 139.1 ± 1.1 Ma. These zircons have εHf(t) values of −9.0 to +0.4, with two‐stage Hf model ages (TDM2) of 1,763–1,172 Ma, indicating a magma source that involved melting of predominantly Palaeoproterozoic to Mesoproterozoic continental crust with a minor juvenile mantle component. The ~140 Ma A‐type magmas in Hong Kong area were probably formed in response to rollback and/or break‐off of the Palaeo‐Pacific Plate. Previous studies indicate that the tectonic transformation from the Palaeo‐Tethys to Palaeo‐Pacific tectonic domain occurred at >165 Ma. Hong Kong underwent a repeated slab subduction and rollback process involving (a) Palaeo‐Pacific Plate subduction formed I‐type granites and continental margin arc volcanic rocks at 165–161 Ma; (b) slab rollback lead to the formation of A‐type granites at 161–159 Ma with minor magmatism formation during the rollback stage at ~159–148 Ma; (c) renewed slab subduction formed continental margin arc rocks at 148–141 Ma; and (d) renewed slab rollback resulted in the formation of A‐type volcanic rocks at 141–139 Ma (e.g., High Island Formation).