Elsevier

Cretaceous Research

Volume 37, October 2012, Pages 155-163
Cretaceous Research

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

https://doi.org/10.1016/j.cretres.2012.03.014Get rights and content

Abstract

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  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.

Introduction

Amber from northern Myanmar, called Burmese amber or merely “burmite”, is the only Cretaceous amber deposit in the world that is exploited commercially, as well as the first to have been studied scientifically. The history of its use has been reviewed by Zherikhin and Ross (2000), Grimaldi et al. (2002), Cruickshank and Ko (2003), and Ross et al. (2010). Briefly, burmite had been used primarily in carvings for at least two millennia by Chinese people, for which the material is ideally suited (Grimaldi, 1996). The deposits in the Kachin state, northern Myanmar, are productive (an estimated 83 tons were exported between 1898 and 1940), and some amber pieces are very large (the largest is 15 kg, in the Natural History Museum, London). Moreover, colours vary from a transparent yellow to a highly desirable deep red (Fig. 1, Fig. 2, Fig. 3), the amber resists fracturing and is relatively hard (1.2 times harder than Baltic amber), and it receives a glassy polish. Burmite mining lapsed from just before the independence of Burma from Britain in 1947, and did not resume until the late 1990s. The greatest value of burmite, however, is scientific.

Amber in general preserves biological inclusions with microscopic fidelity, so as a mode of fossilization it is unparallelled for phylogenetic and palaeontological studies of Cenozoic and late Mesozoic terrestrial life forms (Grimaldi and Engel, 2005). Amber from the Cretaceous is further significant since it coincides with the radiation of the angiosperms and major tectonic shifts in continental positions, and precedes the famous end-Cretaceous impact event. Of the seven major deposits of amber from the Cretaceous Period (Table 1), Burmese amber contains probably the most diverse palaeobiota. For example, approximately 228 families of organisms (primarily arthropods) have been reported from burmite, compared to a range of 68–125 families recorded thus far in the other six major amber deposits. Only the much larger, commercially exploited deposits from the Miocene of the Dominican Republic and Mexico, and the Eocene Baltic amber have yielded more families and species. Interestingly, burmite contains an exceptional diversity and abundance of the most diverse order of insects, the Coleoptera (16% of all studied inclusions, representing more than 40 families, vs. 2–8% and around a dozen families in the other Cretaceous ambers).

Among the more significant records of organisms in burmite is the only Mesozoic fossil of the phylum Onycophora (“velvet worms”) (Grimaldi et al., 2002), as well as the oldest definitive Mesozoic records of mosquitoes, family Culicidae (Borkent and Grimaldi, 2004), and the insect orders Embiodea (Engel and Grimaldi, 2006), Strepsiptera (Grimaldi et al., 2005a, b), and Zoraptera (Engel and Grimaldi, 2002) (Fig. 4). Oddly, burmite also preserves the youngest records of several archaic insect groups, notably Postopsyllidium of the hemipteran family Protopsyllidiidae (previously known from the Permian–Jurassic) (Grimaldi, 2003), and Parapolycentropus, of the scorpionfly family Pseudopolycentropodidae (Triassic–Barremian) (Grimaldi et al., 2005a) (Fig. 4). Parapolycentropus is remarkable for the loss of the hind wings, specialized antennae, and long, styletiform proboscis, convergently resembling a mosquito. Burmite also preserves early, primitive species in groups that are highly social today, notably Formicidae (ants) and Isoptera (termites) (Engel and Grimaldi, 2005; Engel et al., 2007). One of these presumably social insects is Haidomyrmex (Dlussky, 1996), arguably the most peculiar ant known (Fig. 4).

Despite its scientific significance, precise dating of Burmese amber has been elusive. For the first 80 years of its scientific study, burmite was widely considered to be Eocene–Miocene in age, although Cockerell (1917) insightfully considered a Cretaceous age based on the insect inclusions. When Alexandr Rasnitsyn of the Palaeontological Institute in Moscow examined the burmite collection in the Natural History Museum, London in 1995, he noticed the presence of some Cretaceous insect groups in this amber, notably Serphitidae and the extinct subfamily of ants Sphecomyrminae (Zherikhin and Ross, 2000). This and other evidence established a Cretaceous age for the material, corroborated by expanded studies of myriad arthropod taxa in the NHML and AMNH collections (Grimaldi et al., 2002). Based on 21 insect taxa found within various stages of the late Mesozoic as well as in Burmese amber, a Cenomanian age was hypothesized by Grimaldi et al. (2002). Cruickshank and Ko (2003) reviewed the geology of the burmite deposits, based on published and original observations, and reported an ammonite specimen taken 2 m above an amber bed at the principal mine at Noije Bum, identified as Mortoniceras, which has a stratigraphic range of Middle–Upper Albian (Wright et al., 1996). Cruickshank and Ko (2003) cited unpublished reports by E.H. Davies (Branta Biostratigraphy Ltd.) of the fossil pollen, spores, and dinoflagellates, which further indicated an age of the sediments and thus the amber as “most likely Albian to early Cenomanian”. The late Albian age proposed by Cruickshank and Ko (2003) is widely cited in original and review papers on burmite (e.g., Ross et al., 2010, and references therein).

Section snippets

Amber/geological samples

Cruickshank and Ko (2003) described and mapped the locality of the amber mines that are the primary sources of the collections in the Natural History Museum, London and the American Museum of Natural History, New York. Chhibber (1934) listed 13 amber outcrops in the valley, some of which are outside the mine area described by Cruickshank and Ko (2003). Briefly, the mines are located in the Hukawng Valley (Hukawng Basin) of Kachin State, northern Myanmar, and specifically on Noije Bum, a hill

Petrography of amber-bearing matrix

The rock matrix containing amber is a greyish to bluish-green volcanoclastic to mudstone, consistent with the description by Cruickshank and Ko (2003) that the amber is located in the finer facies of sedimentary rocks they encountered at Noije Bum. On close inspection, the rocks consist of sub-millimetre black, yellow, grey and light green clasts. Thus it is understandable why Chhibber (1934) described the rocks as appearing blue in colour, while Cruickshank and Ko (2003) said that they are

Methods and results

The selected zircons were then cast in a transparent epoxy mount together with the standard zircon Plešovice. Cathodoluminescence (CL) imaging of zircon grains employed a scanning electron microscope (LEO1450VP with MiniCL instrument) at the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS). Backscattered electron (BSE) images and mineral compositions of the rock matrices were obtained using a JXA-8100 Electron Microprobe Analyzer (EMPA) at IGGCAS with a voltage of 15 kV,

Discussion and conclusions

As the zircons are from sedimentary rocks, the depositional age cannot be older than the youngest ages of Group-II zircons (i.e., 98.79 ± 0.62 Ma). There are two possible interpretations: either the depositional age is essentially the same as the Group-II zircons, implying that the volcaniclastics were airfall which immediately became sediments, or the zircons did not become sediment until some unspecified time later. Petrographic observations do not help in resolving between the two possibilities.

Acknowledgements

We are grateful to L.C. Chen and M.S. Du for the samples, and much appreciate the constructive reviews of A. Ross, E. Peñalver and A. Arillo, and the editorial handling by D.J. Batten. This investigation is financially supported by the National Basic Research Program of China (2009CB421008), the NCET in China (NCET-07-0771) and the Fundamental Research Funds for the Central Universities (2001YXL048).

References (50)

  • H.L. Chhibber

    The Mineral Resources of Burma

    (1934)
  • T.D.A. Cockerell

    Arthropods in Burmese amber

    American Journal of Science

    (1917)
  • G.M. Dlussky

    Ants (Hymenoptera: Formicidae) from Burmese amber

    Paleontological Journal

    (1996)
  • D.A. Eberth et al.

    A geochronology of the non-marine Judith River Formation of southern Alberta

  • M.S. Engel et al.

    The first Mesozoic Zoraptera

    American Museum Novitates

    (2002)
  • M.S. Engel et al.

    Primitive new ants in Cretaceous amber from Myanmar, New Jersey, and Canada (Hymenoptera: Formicidae)

    American Museum Novitates

    (2005)
  • M.S. Engel et al.

    The earliest webspinners (Insecta: Embiodea)

    American Museum Novitates

    (2006)
  • M.S. Engel et al.

    Primitive termites from the Early Cretaceous of Asia (Isoptera)

    Stuttgarter Beiträge zur Naturkunde B

    (2007)
  • M.B. Goodwin et al.

    The first radiometric ages from the Judith River Formation (Upper Cretaceous), Hill County, Montana

    Canadian Journal of Earth Sciences

    (1989)
  • F.M. Gradstein et al.

    A Phanerozoic timescale

    Episodes

    (1996)
  • F.M. Gradstein et al.

    A Geologic Timescale 2004

    (2004)
  • D.A. Grimaldi

    Amber: Window to the Past

    (1996)
  • D.A. Grimaldi

    First amber fossils of the extinct family Protopsyllidiidae, and their phylogenetic significance among Hemiptera

    Insect Systematics and Evolution

    (2003)
  • D.A. Grimaldi et al.

    Evolution of the Insects

    (2005)
  • D.A. Grimaldi et al.

    Raritan (New Jersey) amber

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