Archean crustal evolution in China: The Taishan complex, and evidence for juvenile crustal addition from long-term depleted mantle

Abstract

The main framework for crustal evolution of the late Archean Taishan Complex in the Shandong Province of China is constructed using data from field investigation, petrographic examination, geochemical analysis and isotopic age determination. The history of the complex began with emplacement of basic and tonalitic magmas about 2.7–2.75 Ga ago, which were metamorphosed (designated as metamorphism I) to amphibolites and grey gneisses shortly after their emplacement. These rocks constitute a metamorphic basement termed the Wangfushan Gneisses. The basement gneisses were later intruded by a plutonic association including (1) alkali- and light rare earth element (LREE) enriched dioritic rocks and their differentiates ∼ 2.6 Ga ago, (2) granites and trondhjemites, collectively termed the Hushan granites, also ∼ 2.6 Ga ago, and (3) the Aolaishan-type granites (sensu stricto) 2.4–2.5 Ga ago. The plutonic association, in conjunction with the older basement gneisses, was later subjected to a second period of deformation and metamorphism (metamorphism II) ∼ 2 Ga ago as inferred from published K-Ar biotite dates. The chronological order of all the above events has been determined by a multichronometric approach, and the age results (in Ma, with 2σ uncertainties) are summarized as follows: ▪ The relatively low I Sr (initial 87Sr: 86Sr ratio) and high positive ϵ Nd ( T) values for the Wangfushan Gneisses and the diorite group provides strong evidence for continental growth by addition of juvenile material ultimately separated from long-term depleted mantle sources. By contrast, the Sr and Nd isotopic data for the Aolaishan-type granites coherently suggest that they are likely to be derived by anatexis of the basement gneisses at shallow depths. Crustal evolution of the Taishan Complex offers an additional example supporting the concept of a continental accretion-differentiation superevent: that during such a worldwide tectonothermal episode, particularly in the Archean, important differentiation of the upper mantle has contributed to the growth of the continental crust by addition of juvenile material.