Geochemical and geochronological constraints on Late Jurassic volcanic rocks at Tuen Mun, Hong Kong, with implications for the Palaeo-Pacific subduction

Abstract

Widespread Mesozoic igneous rocks in Hong Kong form part of the giant Mesozoic igneous province in Southeastern China. The Tuen Mun Formation in the Northwestern New Territories is the oldest volcanic unit known in Hong Kong. This formation contains a lower unit composed of dominantly volcanic breccia, minor dacite, rhyolite, and tuff, and an upper unit of basaltic andesitic to andesitic lava. The volcanic rocks from the lower unit contain zircon grains with a mean U-Pb age of 163.4 ± 0.9 Ma, synchronous with the adjacent Tsuen Wan Volcanic Group in the region. Good inter-correlations between different elements in the Harker diagrams indicate that the volcanic rocks with high SiO 2 (e.g. >65 wt.%) are essentially physical mixtures of intermediate magmas and various xenoliths of country rocks, characterizing the palaeo-volcanic plug facies. Preservation of large amounts of xenoliths of sandstone and marble in the volcanic rocks, which is revealed both in field and under microscopic observation, further supports this hypothesis. The original andesitic magma, represented by the intermediate volcanic rocks with SiO 2 <65 wt.%, exhibit enrichments of LILEs and LREEs compared with N-MORB, but have negative Nb-Ta-Ti anomalies in the primitive-mantle normalized diagram, similar to those derived from metasomatized mantle wedge in an arc environment. Large ranges of SiO 2 , MgO, and CaO contents, and negative Eu and Ce anomalies are consistent with significant fractional crystallization of likely olivine/pyroxene and plagioclase. We propose that during the Late Jurassic subduction of the Palaeo-Pacific Plate underneath the coastal continental arc in Southeast China generated prolonged and cyclic volcanism in the northern part of Hong Kong.

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

“…Magmatism in Hong Kong during the Middle Jurassic–Early Cretaceous can be divided into three stages: 165–159, 152–146, and 144–139 Ma (Figure and Table ; Davis et al, 1997; Campbell et al, ; Sewell et al, ; Li et al, ; Jiang et al, ; Zhao et al, ). The distribution of magmatism during these stages was controlled by the NE–SW‐trending Lianhuashan Fault Zone (Figure b).…”

“…can explain some characteristics of the porphyroclastic volcanic rocks, such as the high Si and low Ca contents, enrichment in Ga and Pb, and depletion in Sr and Eu (Wu et al, 2007). Data for the porphyroclastic volcanic rocks plot in the field for partial melts of metapelite or and 144-139 Ma (Figure 1 and Table 1; Davis et al, 1997;Campbell et al, 2007;Sewell et al, 2012;Li et al, 2014;Zhao et al, 2017). The distribution of magmatism during these stages was controlled by the NE-SW-trending Lianhuashan Fault Zone (Figure 1b).…”

“…The peak period of Mesozoic volcanism in Hong Kong was at ~165–139.5 Ma (Middle Jurassic–earliest Early Cretaceous; Figure b), significantly earlier than those in Zhejiang, Fujian, and Guangdong provinces (Campbell, Sewell, Davis, & So, ; Sewell, Campbell, & Davis, ; Li, Zhou, Lai, Chan, & Chen, ; Jiang, 2016; Zhao, Wang, Tian, & Wu, ). The volcanic activity was controlled by the NE–SW‐trending, strike‐slip Lianhuashan Fault (Campbell & Sewell, ) and formed the calderas of Lantau Island, Kowloon, Yim Tin Tsai–Shing Mun, Sai Kung, and Long Harbour (Figure a).…”

“…Intense magmatism occurred throughout the region during the Early Jurassic-Early Cretaceous (mainly Late Jurassic-Early Cretaceous) and formed the extensive Yanshanian volcanic and minor intrusive rocks, which comprisẽ 85% of the basement rocks of Hong Kong (Sewell, Campbell, Fletcher, Lai, & Kirk, 2000). The volcanic rocks of the High Island significantly earlier than those in Zhejiang, Fujian, and Guangdong provinces (Campbell, Sewell, Davis, & So, 2007;Sewell, Campbell, & Davis, 2012;Li, Zhou, Lai, Chan, & Chen, 2014;Zhao, Wang, Tian, & Wu, 2017). The volcanic activity was controlled by the NE-SW-trending, strike-slip Lianhuashan Fault (Campbell & Sewell, 1998) Figure 1 and Table 1).…”

Late Cretaceous lithospheric extension in SE China: Constraints from volcanic rocks in Hainan Island

“…Different models have been proposed to explain their petrogenesis. A subduction model suggests that late Mesozoic magmatism in eastern South China resulted from subduction of the Paleo-Pacific plate beneath the Eurasian plate (e.g., Jiang and Li, 2014;Jiang et al, 2005;Li and Li, 2007;Y.H.M. Li et al, 2014;Z.…”