Precambrian blueschist terranes of the world

Abstract

Blueschists are most common in post-Paleozoic orogenic belts; higher geothermal gradients and a thinner lithosphere in the Precambrian are widely thought to have disfavored blueschist formation. However, several Precambrian blueschist belts have been reported from Eurasia. On the basis of established geologic and geochronologic constraints, these belts are grouped as suspect, possible and definite Precambrian blueschists. Suspect ones include those in Alaska, in Anglesey (U.K.), in the Scandinavian Caledonides, and in Yunnan (China). Possible Precambrian blueschist belts are those in Delhi (India), in central China, in Heilongjiang (China) and in the Urals and Altai-Sayan-Tien Shan (U.S.S.R.). Definite Precambrian blueschist terranes are those in western Africa and in Aksu (China). These blueschist belts are described with emphasis on the age constraints. The recently investigated Aksu blueschist terrane is presented as an example. This high- P/ T coherent blueschist-greenschist terrane at Aksu is unconformably overlain by a thick, unmetamorphosed sedimentary sequence ranging in age from Late Sinian (ca. 600–800 Ma) through Eocambrian to Paleozoic. The basal Sinian conglomerates contain subrounded to rounded clasts of the underlying schist, clearly demonstrating the pre-Sinian age of blueschist facies metamorphism. Characteristic features of the above-mentioned blueschist belts include: 1. (1) the blueschist terranes are coherent and primary compositional-lithological layers are well preserved; mèlange belts are not common; 2. (2) protoliths include pelite, sandstone, bimodal volcanics, limestone and small quantities of pillow basalt and pelagic sediment; ultramafics are rare; 3. (3) the protoliths were subjected to multiple deformation and transitional blueschist-greenschist facies metamorphism. Typical Precambrian blueschist contains the assemblage crossitic amphibole + epidote + chlorite + albite + quartz ± actinolite); rare lawsonite, jadeitic pyroxene and aragonite have been described in few belts; 4. (4) the metamorphic facies series belongs to the high-pressure intermediate type of Miyashiro (1961). The Precambrian blueschist and other pre-Mesozoic intracontinental blueschist of Eurasia formed before, rather than during, final closure of intervening ocean basins between pairs of continents. They do not usually mark the location of terminal sutures, but are the result of earlier accretion and continental growth by subduction, underplating and imbrication of accretionary wedges including both oceanic and continent-derived elastics. The rare preservation of Precambrian blueschist is due to greenschist overprinting when the heat flow and geothermal gradient was high, rather than reflecting lack of production. Their existence in China and Africa demonstrates that requisite conditions for blueschist facies metamorphism existed at least 700 Ma ago.