Redefinition of depositional ages of Neoarchean supracrustal rocks in western Shandong Province, China: SHRIMP U–Pb zircon dating

Abstract

Western Shandong Province is an important terrane in the eastern part of the North China Craton where 2.75–2.5Ga tectono-thermal events are well recognized. Supracrustal rocks, named the Taishan “Group”, experienced greenschist- to amphibolite-facies metamorphism and occur as lenses and strips in granitoids and are subdivided into the Yanlingguan, Shancaoyu and Liuhang “Formations”. The Yanlingguan “Formation” is mainly composed of amphibolite but also contains fine-grained biotite-hornblende gneiss, komatiitic rocks and schist. The Liuhang “Formation” is similar in rock association to the Yanlingguan “Formation” but contains more metasedimentary rocks. The Shancaoyu “Formation” is mainly composed of fine-grained biotite gneiss (leptynite) preserving bedding and is interlayered with schist. Its protoliths are mainly sedimentary rocks with some mafic to felsic volcano-sedimentary varieties. We undertook SHRIMP zircon dating on eighteen metasedimentary and volcanic samples, mainly from the Shancaoyu “Formation” and the upper part of the Liuhang “Formation”. Ages of detrital and volcanic zircons vary from 2.75 to 2.54Ga and 2.55 to 2.525Ga, respectively, and the age distribution patterns vary in different samples. We conclude that the Shancaoyu “Formation” and the upper part of the Liuhang “Formation” were deposited in the late Neoarchean (2.55–2.525Ga) and not in the early Neoarchean (2.8–2.7Ga) as thought before. Western Shandong is the only terrane in the North China Craton where both early and late Neoarchean supracrustal rocks have been identified. A magmatic arc model may be applied to explain the tectono-thermal evolution at the end of the Neoarchean.