Two episodes of the Indosinian thermal event on the South China Block: Constraints from LA-ICPMS U–Pb zircon and electron microprobe monazite ages of the Darongshan S-type granitic suite

The Darongshan granitic suite (~ 10,000 km2) consists of five major units (Taima, Nadong and Jiuzhou plutons, and Pubei and Darongshan batholiths) typical of peraluminous S-type granitoids containing abundant granulite inclusions in the Cathaysia block, South China. Six samples from these plutons and batholiths have been investigated using both LA-ICPMS U–Pb age dating on zircon cores and EMP U–Th–Pb chemical age dating on monazite cores and rims. LA-ICPMS zircon results give similar major age populations ranging between 260 ± 3 and 250 ± 3 Ma for all units, with apparent older age peaks concentrated at 1020, 800, 430 and 330 Ma. On the other hand, EMP monazite results yield younger ages of 231–229 Ma for Nadong, Taima, Pubei and Darongshan and 224 Ma for Jiuzhou samples, with older age groups of 264 Ma for Taima and 256–250 Ma for Pubei units. Since the older monazite ages are similar to the majority of zircon ages, the latter are considered as inherited ages. Further because such zircon ages are similar with the emplacement time of the Emeishan large igneous province in western South China, they likely reflect the timing of metamorphism for the included fragments of granulitic crusts that had been formed by invasion of the Emeishan plume. The younger monazite ages, as present for all plutons and batholiths in the entire Darongshan area, are taken as the formation age of the host granites. Combining U–Pb zircon and EMP monazite ages known for Permo-Triassic high temperature and high pressure metamorphic rocks and granites in the Indochina block (e.g., the Kannack Complex of the Kontum massif), it is suggested that the Indosinian thermal activity had set records over both the Indochina (plus Simao) and South China blocks in two main episodes, one is 260–250 Ma and the other is 231–229 Ma. One plausible explanation is that these two blocks were one united continent before the Emeishan plume activity and an opening was triggered by this plume at ~ 260 Ma. Due to forces of the approaching Sibumasu block, both the South China and Indochina blocks were amalgamated again at ~ 230 Ma. We, therefore, advocate that double subduction of the plume-triggered oceanic crusts in opposite directions is responsible for the generation of the Darongshan granitic suite in the South China block and its counterpart in the Indochina block.