A review of new detrital zircon U-Pb ages from the Mogok area of Myanmar: Implications for the stratigraphy and early tectonic evolution of the Mogok Metamorphic Belt (MMB)

The Mogok Metamorphic Belt (MMB) in Myanmar is composed of low- and high-metamorphic grade (upper amphibolite-granulite facies) metasediments and igneous intrusions. The low-grade MMB metasediments range in age from Neoproterozoic to Cretaceous but the precursor stratigraphy of the high-grade metasediments and gneisses is not clearly understood. The new detrital zircon U-Pb stratigraphy of high-grade metasediments in the Mogok area, constructed from detrital zircons that have survived the Mesozoic and Cenozoic metamorphism is comparable to the Neoproterozoic-Palaeozoic sequence on the Shan Plateau. A basal unit in the Mogok U-Pb zircon stratigraphy is the Kingsbridge psammitic gneiss containing detrital zircon cores 20-70 & mu;m in size mantled by Mesozoic and Cenozoic zircon growths. The 1100-500 Ma zircons had rhyo-dacite compositions and were sourced from tephra derived from the nearby Tawgpeng batholith and the Bawdwin Volcanic Formation. The rare xenocryst zircons (2000-1200 Ma) had granitic compositions with alteration attributed to abrasion during river passage across Gondwana and offshore when they became mingled with the younger volcanic zircons. The youngest volcanic zircon had a preferred 206Pb/238U age of 514.5 & PLUSMN; 11 Ma near the end of the life of the Cambrian volcanic arc that was terminated by the Indo-Australia accretionary orogenesis (510-450 Ma). The youngest volcanic zircon has a similar age and chemistry to zircons from the Nyima centre in the Lhasa Block. This relationship is taken as evidence that during the Cambrian the volcanic arcs in the Lhasa and Sibumasu blocks were adjacent, and had a linear orientation and so the Lhasa block was not inboard of Sibumasu on the West Australian Gondwana margin.