U-PB DATING OF MESOZOIC IGNEOUS ROCKS FROM HONG-KONG

Twenty-two new zircon and monazite U-Pb ages have been obtained for Me sozoic plutonic and volcanic rocks of Kong Kong. These have revealed t he existence of four distinct periods of Mid-Jurassic to Lower Cretace ous 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 fir st known occurrence of a Mid-Triassic granitoid in the Territory. Amon g these rocks, discordant U-Pb data projected to the concordia curve h ave given a broad range of inheritance ages from early Phanerozoic to late Archaean. A xenocrystic zircon from one sample has yielded a conc ordant age of 1872 +/- 3 Ma. The new U-Pb ages have refined the result s of recent Rb-Sr whole-rock age dating of the granitoid rocks and hav e considerably improved correlation between intrusive and extrusive un its. The occurrence of inherited components in many of the analysed ro cks indicates that they were derived from, or erupted through, older c rust containing a wide variety of age components, either as primary ig neous or detrital material.