IN a comprehensive review of the world's blueschists, Ernst1 noted an apparent absence of these high-pressure, low-temperature rocks in the Precambrian, and ascribed it to a cooling of the Earth in the Phanerozoic eon. As blueschists are thought to be formed in subduction zones, the absence of Precambrian blueschists has been adduced as evidence that plate tectonics did not operate on the early Earth2. Although this conclusion is not inevitable, in that ancient mobile belts are more likely to have been obliterated than younger ones3, the presence of paired metamorphic belts of kyanite-sillimanite and andalusite-sillimanite grade in Proterozoic Gondwanaland supports the existence of a subducted slab, but with ancient geothermal gradients too high to form blueschists along the convergent plate boundary4,5. On the other hand, data from inclusions in African diamonds support the opposing view, that Archaean continental geotherms were similar to those of today6. Here we present Rb-Sr and K-Ar ages of 698-718 Myr for phengitic mica from high-pressure pelitic schists from the Aksu region of China, which are interleaved with blueschists. We interpret these dates as representing the time of the blueschist-facies metatmorphism, suggesting that subduction was occurring in the late Proterozoic.
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