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[1] The Mogok metamorphic belt (MMB) extends for over 1500 km along the western margin of the ShanThai block, from the Andaman Sea north to the eastern Himalayan syntaxis. Previous geochronology has suggested that a long-lasting Jurassic-early Cretaceous subduction-related event resulted in emplacement of granodiorites and orthogneisses (171-120 Ma) and a poorly constrained Tertiary metamorphic event. On the basis of new U-Pb isotope dilution thermal ionization mass spectrometry and U-Th-Pb laser ablationmulticollector-inductively coupled plasma mass spectrometer geochronology presented here, we propose two Tertiary metamorphic events affected the MMB in Burma. The first was a Paleocene event that ended with intrusion of crosscutting postkinematic biotite granite dikes at $59 Ma. A second metamorphic event spanned late Eocene to Oligocene (at least from 37, possibly 47, to 29 Ma). This resulted in the growth of metamorphic monazite at sillimanite grade, growth of zircon rims at 47-43 Ma, sillimanite + muscovite replacing older andalusite, and synmetamorphic melting producing garnet and tourmaline bearing leucogranites at 45.5 ± 0.6 Ma and 24.5 ± 0.7 Ma. These data imply high-temperature sillimanite + muscovite metamorphism peaking at 680°C and 4.9 kbar between 45 and 33 Ma, to around 606-656°C and 4.4-4.8 kbar at 29.3 ± 0.5 Ma. The later metamorphic event is older than 24.5 ± 0.3 Ma, the age of leucogranites that crosscut all earlier fabrics. Our structural and geochronological data suggest that the MMB links north to the unexposed middle or lower crust rocks of the Lhasa terrane, south Tibet, and east to high-grade metamorphic core complexes in northwest Thailand.

One sentence summary:238 U/ 235 U ratios for U-bearing accessory minerals from a diverse suite of terrestrial rocks indicate a >5‰ range with an average zircon value of 238 U/ 235 U = 137.818 ± 0.045 (2σ) which is consistent with the composition of other terrestrial and meteoritic reservoirs. 2, 3). However, a robust λ 235 U can only be determined with U-Pb analyses using a tracer calibration that is traceable to SI units and free of other potential sources of bias, so we refrain from suggesting this value be adopted at present and urge caution in abandoning the Jaffey et al (1) λ 235 U determination until such a dataset has been generated and evaluated.An emerging 238 U/ 235 U dataset for a wide range of rocks, minerals, and meteorites is now available (17,18,25,(30)(31)(32)(33) and compiled here (Fig. 3)

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[1] The Ama Drime Massif (ADM) is an elongate north-south trending antiformal feature that extends $70 km north across the crest of the South Tibetan Himalaya and offsets the position of the South Tibetan Detachment system. A detailed U(-Th)-Pb geochronologic study of granulitized mafic eclogites and associated rocks from the footwall of the ADM yields important insights into the middle to late Miocene tectonic evolution of the Himalayan orogen. The mafic igneous precursor to the granulitized eclogites is 986.6 ± 1.8 Ma and was intruded into the paleoproterozoic (1799 ± 9 Ma) Ama Drime orthogneiss, the latter being similar in age to rocks previously assigned to the Lesser Himalayan Series in the Himalayan foreland. The original eclogite-facies mineral assemblage in the mafic rocks has been strongly overprinted by granulite facies metamorphism at 750°C and 0.7-0.8 GPa. In the host Ama Drime orthogneiss, the granulite event is correlated with synkinematic sillimanite-grade metamorphism and muscovite dehydration melting. Monazite and xenotime ages indicate that the granulite metamorphism and associated anatexis occurred at <13.2 ± 1.4 Ma. High-grade metamorphism was followed by postkinematic leucogranite dyke emplacement at 11.6 ± 0.4 Ma. This integrated data set indicates that high-temperature metamorphism, decompression, and exhumation of the ADM postdates mid-Miocene south directed midcrustal extrusion and is kinematically linked to orogen-parallel extension. Citation: Cottle,

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Gas hydrates stored on continental shelves are susceptible to dissociation triggered by environmental changes. Knowledge of the timescales of gas hydrate dissociation and subsequent methane release are critical in understanding the impact of marine gas hydrates on the ocean–atmosphere system. Here we report a methane efflux chronology from five sites, at depths of 220–400 m, in the southwest Barents and Norwegian seas where grounded ice sheets led to thickening of the gas hydrate stability zone during the last glaciation. The onset of methane release was coincident with deglaciation-induced pressure release and thinning of the hydrate stability zone. Methane efflux continued for 7–10 kyr, tracking hydrate stability changes controlled by relative sea-level rise, bottom water warming and fluid pathway evolution in response to changing stress fields. The protracted nature of seafloor methane emissions probably attenuated the impact of hydrate dissociation on the climate system.