Petrochemistry and Sr, Pb, and Nd isotopic geochemistry of early precambrian rocks, Wutaishan and Taihangshan areas, China

Abstract

In the Wutaishan and Taihangshan region, China, the lowermost high-grade metamorphic complex of grey-gneiss and amphibolite, the Fuping Complex, gives a 2.2 ± 0.2 Ga Pb-Pb isochron age with a first stage growth μ = 7.73, and a 2.37 ± 0.07 Ga SmNd isochron age with (143Nd/144Nd)0 = 0.50963 ± 0.00005 or ε{lunate}Nd(T) = 1.50. Nd-depleted mantle model dates (TDM) of amhibolites 2.48 to 2.60 Ga, those of gneisses are 2.43 to 2.46 Ga. The unconformably overlying medium- to low-grade metavolcanic complex, the Wutai Complex, gives a 2.0 ± 0.1 Ga RbSr isochron age with (87/86Sr) 0 = 0.7025 ± 2, a 2.27 ± 0.02 Ga PbPb isochron date with afirst stage growth μ = 7.73, and a 2.26 ± 0.06 Ga SmNd isochron first stage (143Nd/144Nd)0 = 0.5097±2 or ε{lunate}Nd(T)=1.08. Individual sample Nd depleted mantle model date are ∼ 2.5 Ga. Metavolcanic samples from low-grade metasedimentary rocks of the Hutuo Group all lie close to a 2.3 Ga reference line on a RbSr isochron plot, have Nd TDM = 2.32, 2.34, and 2.62 Ga. From previous geological work, published UPb zircon dating, and ourisotopic and geochemical results, we infer that the Fuping rocks formed ∼ 2.6 Ga ago in an environment like a modern island arc. The Wutai Complex formed by 2.5 Ga ago, mostly formed in a tectonic setting similar to that of the Fuping Complex, with the exception that one volcanic cycle formed in an environment like a modern MOR and one unit formed in an environment transitional between modern within-plate and plate margin settings. The Hutuo Group formed 2.4 Ga ago in a within-plate tectonic environment. There is no evidence for continental crust before 2.6 Ga in the Wutaishan and Taihangshan region. This contradicts previous assignment of the Fuping Complex to 2.8 Ga ol older. The Fuping Complex is not an early nucleus of the Sino-Korean Craton. The new data also confirm that the Wutai Complex is older than its original assignment as Early Proterozoic. The SmNd systems for metabasaltic rocks in the region, either in amphibolite facies or in greenschist facies, are all significantly disturbed, in contrast with the undisturbed SmNd reported in previous studies of the Sino-Korea Craton. This study provides further evidence that the Archean igneous rocks in China formed at different times, from heterogeneous and depleted mantle sources. © 1992.