Search

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). [ABSTRACT FROM AUTHOR]

An integrated study of zircon U-Pb geochronology and geochemistry, together with Nd-Hf isotopes, have been carried out on the rhyodacite and rhyolitic tuff of the Jingtan Formation in the eastern part of the Jiangnan orogen. SIMS zircon U-Pb dating of two samples yielded weighted mean 206 Pb/ 238 U ages of 784 ± 6 Ma and 788 ± 6 Ma, respectively. Geochemically, they are peraluminous (A/CNK mostly around 1.2) and are characterized by enrichments in Rb, Th, REEs and HFSEs (e.g. Zr and Y) but depletions in Ba, Sr, P, Eu and Ti. The volcanic rocks show a clear A-type granite geochemical signature with high total alkalis (K 2 O + Na 2 O = 5.3–7.64 wt%), FeO t /MgO ratios and low CaO, MgO and TiO 2 contents. They have negative whole-rock ε Nd (t) (−4.2 to −1) and positive zircon ε Hf (t) (+1.26 to +11.6) values, illustrating decoupled Nd-Hf isotopes which may be genetically related to their petrogenesis process. The positive ε Hf (t) values and juvenile T Hf DM1 (0.89–1.3 Ga) of zircons indicate that the volcanic rocks may have been derived from the partial melting of the Neoproterozoic to late Mesoproterozoic crustal materials. Combined with the occurrence of significant volumes of contemporary bimodal volcanic rocks in eastern section of the Jiangnan orogen, it is inferred that the Jingtan felsic volcanic rocks formed during post-collisional extension shortly after the final amalgamation of Yangtze and Cathaysia blocks. [ABSTRACT FROM AUTHOR]