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

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• 作者：Guanghai Shi^a ; ^{shiguanghai@263.net.cn} ; ^{shigh@cugb.edu.cn} ; David A. Grimaldi^b ; George E. Harlow^b ; Jing Wang^a ; Jun Wang^a ; Mengchu Yang^a ; Weiyan Lei^a ; Qiuli Li^c ; Xianhua Li^c
• 关键词：Amber ; Myanmar ; Cretaceous ; Palaeobiota ; Zircons ; 206Pb/238U ages ; Volcanic clasts ; Cenomanian
• 刊名：Cretaceous Research
• 出版年：2012
• 出版时间：October, 2012
• 年：2012
• 卷：37
• 期：Complete
• 页码：155-163
• 全文大小：2485 K

文摘

Amber from northern Myanmar has been commercially exploited for millennia, and it also preserves the most diverse palaeobiota among the worlds' 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 ?#xA0;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 ?02 Ma-?08 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 ¡À 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 ¡À 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 ¡À 0.62 Ma in the vicinity of the Hukawng Valley.