Age constraint on Burmese amber based on U–Pb dating of zircons

Amber from northern Myanmar has been commercially exploited for millennia, and it also preserves the most diverse palaeobiota among the worldsʹ seven major deposits of Cretaceous amber. Recent estimated ages vary from Albian to Cenomanian, based on palynology, an ammonoid, and Mesozoic insect taxa preserved within the amber. The burmite-bearing rock is sedimentary and consists mainly of rounded lithic clasts (0.03 ∼ 0.15 mm in diameter), with minor fragments of quartz and feldspar. Among the lithic clasts are mostly volcanic rocks. Zircons separated from the amber matrix form two groups: Group-I zircons are overgrown and have variable CL patterns, experienced slight geological disturbances after they formed, and their Ion microprobe 206Pb/238U ages fall into a very narrow range of ∼102 Ma–∼108 Ma; Group-II zircons are typical magmatic ones with rhythmically flat zones, inferred to be derived from volcanic rock clasts, and yielded a concordia 206Pb/238U age of 98.79 ± 0.62 Ma. The dating on Group-I zircons is only for their interiors, thus hiding what age excursion might come from the overgrowth. Considering the nearshore marine environment and 1-m thickness of the burmite-bearing sediments, and the syn- and post-eruption deposition of volcanic clasts, the age of 98.79 ± 0.62 Ma therefore can be used as a maximum limit for the burmite (either at or after), establishing an earliest Cenomanian age for the fossilized inclusions. The age also indicates that volcanic eruption occurred at 98.79 ± 0.62 Ma in the vicinity of the Hukawng Valley.