Late Archean Sm-Nd isochron age for Mafic-ultramafic supracrustal amphibolites from the Northeastern Sino-Korean Craton, China

Abstract

Isotopic data published by previous workers document the presence of Early Archean (≈ 3.5 Ga) metabasaltic enclaves in grey gneisses west of Bohai Bay, eastern Hebei Province, P.R.C. Metabasaltic and metakomatiitic amphibolites crop out as lower supracrustal members of the Archean section northeast of Bohai Bay in southern Jilin/eastern Liaoning Provinces. Six analyzed specimens of the latter unit contain more magnesium on analysis (averaging approximately 13 vs. 7 wt. percent MgO) but, although they exhibit a wider range of REE concentrations, they average a similar range of REE abundance patterns (La-Lu=24-7X chondrite) as the former (24-9 × chondrite). New Sm-Nd isotopic and REE analyses, combined with available petrologic/geologic information, indicate that metakomatiitic amphibolites from the Sino-Korean craton northeast of Bohai Bay are Late Archean in emplacement age. A wholerock Sm-Nd isochron age of 2660 ± 75 (2 δ) Ma was obtained, with ϵ Nd( T) = + 4.4 ± 0.5 . As in many parts of the Sino-Korean craton, the highly positive ϵ Nd(T) value suggests a prior extended history (⩾ 1.0 Ga) of depletion of subcontinental mantle sources in some incompatible elements and in LREE relative to HREE. A seventh analyzed sample, a mafic inclusion in tonalitic gneiss, of unknown age, has an unusual chemical composition with strong enrichment in LREE, Sr and Zr, and severe depletion in Ni and Cr. Judging from isotopic data, which yield a much younger depleted mantle model age of 1.67 Ga, it could represent a cognate xenolith in a mid Proterozoic tonalitic intrusion (shown on present maps as Archean in age); the protolith might have been an evolved alkali basalt.