AN UPPER CRUSTAL SECTION THROUGH A TILTED EARLY CRETACEOUS NESTED CALDERA COMPLEX IN HONG KONG

Exceptional exposures of at least four large Middle Jurassic to Early Cretaceous silicic volcanic centers and their plutonic equivalents in Hong Kong have provided a rare opportunity to examine the close connections in time and space between magma chambers and their overlying calderas. Here we describe an upper crustal section, from the magma chamber through to the overlying intracaldera fill, of a collapsed caldera probably formed by a supervolcanic eruption. The exposures, which are interpreted to form part of a tilted Early Cretaceous nested caldera complex in southeastern Hong Kong, reveal kilometer-scale earth movements and the simultaneous fissure-fed evacuation of hundreds of cubic kilometers of high-silica rhyolitic magma from a shallow subcrustal reservoir. The eruption signaled the end of Early Cretaceous silicic magmatism in the Hong Kong region and led to formation of a large volcanotectonic depression infilled by pyroclastic flow deposits now preserved as a single cooling unit of spectacularly columnar-jointed (up to 2m diameter), densely welded tuff (High Island Formation). The volcanotectonic depression is nested within an older Early Cretaceous collapsed caldera. U–Pb zircon ages indicate an approximate two million year period of repose between the two caldera-forming events. The exposures in Hong Kong have revealed new insights into the three-dimensional architecture of overlapping ‘supervolcano’ plumbing systems. In a regional context, the Hong Kong volcanic–plutonic assemblages may represent a microcosm of the colossal scale of silicic ‘supervolcano’ activities straddling the Jurassic–Cretaceous boundary along the eastern seaboard of China. Such voluminous magmatic centers would have had global environmental impact and may collectively provide a valid explanation for the Jurassic–Cretaceous boundary, the definition of which remains an outstanding problem on the international geologic timescale.