U-Pb dating of Mesozoic igneous rocks from Hong Kong

Abstract

Twenty-two new zircon and monazite U-Pb ages have been obtained for Mesozoic plutonic and volcanic rocks of Hong Kong. These have revealed the existence of four distinct periods of Mid-Jurassic to Lower Cretaceous volcano-plutonism: 164.6 ±0.2 to 159.3 ±0.3 Ma, 146.6 ±0.2 to 146.2 ±0.2 Ma, 143.7 ±0.3 to 142.5 ±0.3 Ma, and 140.9 ±0.2 to 140.4 ±0.2 Ma. A concordant monazite age of 236.3 ± 0.8 Ma has also been obtained from one intrusive body representing the first known occurrence of a Mid-Triassic granitoid in the Territory. Among these rocks, discordant U-Pb data projected to the concordia curve have given a broad range of inheritance ages from early Phanerozoic to late Archaean. A xenocrystic zircon from one sample has yielded a concordant age of 1872 ±3 Ma. The new U-Pb ages have refined the results of recent Rb-Sr whole-rock age dating of the granitoid rocks and have considerably improved correlation between intrusive and extrusive units. The occurrence of inherited components in many of the analysed rocks indicates that they were derived from, or erupted through, older crust containing a wide variety of age components, either as primary igneous or detrital material.