Unravelling an allochthonous, subaqueously deposited volcanic–epiclastic to subaerial andesitic lava assemblage in Hong Kong: age, stratigraphy and provenance studies of the Middle Jurassic Tuen Mun Formation

The Middle Jurassic Tuen Mun Formation, Hong Kong, is a fault-bounded block of rare andesitic and related rocks that preserve a snapshot of the developing SE China continental arc system during the late Mesozoic. The forearc depositional setting is considered to have been dominated by an emergent andesitic volcanic massif, which shows a transition to a fluvialdominated volcanic plain, and then to an offshore marine environment. The youngest laser ablation inductively coupled plasma mass spectrometry U–Pb detrital zircon ages obtained from the three packages define a coherent group (n = 396; MSWD = 1.8) suggesting a maximum depositional age of 169.5 ± 0.3 Ma. The dominance of large euhedral and concentrically zoned zircons reflects a common volcanic provenance. Hf isotope data (ε_Hf(t) = 0 to –11) on the youngest representative zircon grains imply derivation of magmas from dominantly recycled crustal sources. The detrital zircon age signatures and Hf isotope data show no indication of magmatic interaction with adjacent late Palaeozoic and Early to Middle Jurassic sedimentary rocks, or Middle Jurassic to Early Cretaceous magmatic rocks of Hong Kong. On the basis of the structural, zircon age and isotope data, the Tuen Mun Formation is interpreted to be an allochthonous block, emplaced in the Hong Kong region during the late Middle Jurassic.