Distinct Metamorphic Events Research Articles

Geochronology of granulite‐facies gneisses in the western Musgrave Block, Central Australia

Abstract Rubidium‐strontium total‐rock geochronology of granulite‐facies gneisses in the western Musgrave Block, Central Australia records distinct metamorphic and earlier events. The Tomkinson Ranges are subdivided into two regions with similar relative geological sequences of events, but different primary ages and lithologies; the boundary between the regions is the latitudinally‐trending Hinckley Fault. A 90‐km traverse along regional strike in the northern region produces consistent 1550 m.y. ages. Sampling at several structural levels over a slight range of metamorphic grade in the southern region reveals a single regional isochron with a 1330 m.y. age. Both ages are interpreted as the time of supracrustal genesis of the rocks. Subsequently, granulite‐facies metamorphism occurred throughout the area at 1200 m.y.: attribution of this age to the metamorphism is based upon evidence for outcrop scale isotopic homogenization, and the dating of a granitoid intrusion contemporaneous with the metamorphism.

Isotopic evidence for a terminal lunar cataclysm

Most highland total rock samples define a single U Pb isochron which corresponds to a metamorphism age of ∼ 3.9AE. This age is also obtained for internal U Pb isochrons for some of these samples. The data on 18 rock samples range from concordant samples with 238U/ 206Pb∼ 1.2 to discordant ones with 238U/ 206Pb∼ 0.02. This feature coupled with a correlated pattern of 238U/ 204Pb ratios, indicates that Pb was extensively mobilized at ∼ 3.9AE. The observed Pb U fractionation is essentially due to Pb volatilization during the metamorphic events. Volatile Pb transport is not accompanied by similar effects in Rb and must therefore be attributed to a specific process. Rb Sr internal isochrons for the same rocks determine distinct metamorphic events in the interval 3.85–4.00 AE. We conclude that highland samples from widely separated areas bear the imprint of an event or series of events in a narrow time interval which can be identified with a cataclysmic impacting rate of the moon at ∼ 3.9AE, although differentiation by internal magma generation cannot be excluded. This cataclysm is associated with the Imbrium impact and very possibly the formation of Crisium and Orientale and possibly several other major basins in a narrow time interval (∼2 × 10 8yr or less). The U Pb data indicate formation of the lunar crust at ∼ 4.4AE, which is distinctly younger than 4.6 AE, the time generally associated with planetary formation. If the lunar crust started being formed at ∼ 4.6AE, then this process must have continued until times significantly younger than 4.4 AE. Rb Sr data also indicate formation of the lunar crust around 4.5 AE with only minor additions of high Rb/Sr materials to the crust at times younger than 4.3 AE. Using the U Pb systematics, K/U and the average U concentration of the moon as obtained from heat-flow measurements, we estimate the lunar concentrations: primordial Pb= 35ppb;Rb= 0.5ppm with Rb/Sr= 0.006.