Hon Trung Lon, a granite island off the coast of South Vietnam, is composed solely of a unique, extremely differentiated, high-silica alkali granite of Cretaceous age. An adjacent island, Hon Trung Nho, is composed solely of rhyolitic rock of similar composition. The Hon Trung Nho rhyolite may be a fine-grained equivalent of the granite. Whole-rock potassium-argon analysis suggests that the age of the Hon Trung Lon granite is greater than 70 m.y. and less than 100 m.y. This age implies a previously unrecognized period of intrusion for the zone of granitic rocks outcropping along the northeast side of the Gulf of Thailand. Extensive investigation of the Mesozoic and Tertiary igneous rocks of Southeast Asia, particularly Indochina, including reliable geochronological work, is needed to trace the geologic events that produced the complicated structural features and orogenic history of this region. Existing data suggest that Southeast Asia has been the site of active orogeny since the Paleozoic and that the Hon Trung Lon granite was emplaced through a sialic crust which has been evolving in this area for at least the last 400 m.y.
