[1] Exceptional exposures of four, precisely dated, Middle Jurassic to Early Cretaceous, silicic volcanic centers and their plutonic equivalents in Hong Kong have provided an excellent opportunity to examine close connections in time and space between magma chambers and their overlying calderas. Here, we describe a ∼14 km crustal section through a collapsed caldera in southeastern Hong Kong where the intracaldera fill suggests that the magmatic discharge was of supereruption scale. The main subvolcanic components that link a magma chamber with surface are revealed by well-established field relationships, supplemented by high precision geochronology, whole-rock geochemistry, and geophysical data. Exposures and outcrop patterns reveal kilometer-scale caldera subsidence and evidence of the simultaneous evacuation of hundreds of cubic kilometers of high-silica rhyolite magma through dike-like conduits from a shallow subcrustal reservoir. The resultant volcanotectonic depression, within which is preserved a single cooling unit of massively columnar-jointed densely welded tuff (High Island tuff), is interpreted to form part of a larger tilted Early Cretaceous nested caldera complex. The High Island eruption signaled the end of a 24 Myr-period of voluminous, pulsed Middle Jurassic to Early Cretaceous silicic magmatism in the Hong Kong region characterized by four discrete ignimbrite 'flare-ups'.

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.

Results 115Samples in Tables 1-3 Table 194 3). Geochemically, the South Lamma Granite is similar to granites belonging to the Kwai 195Chung Suite, although prior to obtaining the new age it had been assigned to the Cheung 196Chau Suite (Table 3)

No abstract

Middle Jurassic to Lower Cretaceous intermediate to silicic plutonic and volcanic rocks of Hong Kong record a transition from calc-alkaline, through high-K calc-alkaline, to transitional shoshonitic compositions with time. Close spatial and temporal associations among comagmatic volcanic-plutonic pairs indicate that magmatism occurred in discrete episodes, mostly of less than one million years duration. Synchronous high-K calc-alkaline and transitional shoshonitic magmatic activity during at least one pulse suggests a relatively rapid transition from a subduction-related to an extension-related tectonic setting. Geochemical signatures indicate that the magmatic suites have a mantle origin with a decreasing crustal contribution from two distinct sources. The earliest mantle-derived magmas interacted strongly with a dominantly Archaean crustal protolith. Younger magmas show evidence for interaction with a dominantly Proterozoic crustal protolith. The strongest mantle influence is shown by magmas which were intruded along the boundary between the two dominant crustal sources. This interface marks a deep crustal discontinuity which promoted the passage of magmas to the surface.