Geochemistry of ∼2.7 Ga basalts from Taishan area: Constraints on the evolution of early Neoarchean granite-greenstone belt in western Shandong Province, China

Western Shandong Province (WSP) is one of the typical Neoarchean granite-greenstone belts in the North China Craton (NCC). The Taishan association, consisting of a ∼2.7 Ga komatiite–tholeiite sequence and a ∼2.5 Ga felsic volcanic-sedimentary sequence, is the major supracrustal assemblage in the WSP. The ∼2.7 Ga komatiite–tholeiite sequence can be subdivided into the Yanlingguan and Liuhang units. The 150–800 m thick basaltic lava sequence of the Liuhang unit is exposed in the Qixingtai area, with the bottom unit composed of massive and layered basalts. Pillow basalts and tuff dominate the upper parts of the sequence. We report a weighted mean SHRIMP 207Pb/206Pb zircon age of 2706 ± 9 Ma (MSWD = 0.61) for a trondhjemitic dike that intrudes the base of the pillow basalts, constraining the timing of this sequence to be older than this age. The basaltic lava sequence in the Liuhang unit is characterized by voluminous tholeiites and a minor component of enriched basalts. The tholeiites show relatively high MgO (5.5–9.0 wt.%), but low TiO2 (0.8–1.3 wt.%) and Nb (2.1–3.3 ppm) contents. They display co-variations between Zr and other immobile trace elements and flat REE and trace elements patterns on chondrite- and primary mantle-normalized diagrams, respectively. In contrast, the enriched basalts have low MgO (3.6–5.4 wt.%), but high TiO2 (1.6–2.2 wt.%) and Nb (8.4–11.6 ppm) contents. They display moderately enriched LREE and fractionated HREE patterns on chondrite-normalized diagrams. The basaltic sequence of the Liuhang unit displays a moderate range of initial ɛNd (−0.1 to +3.0) values. The element concentration and Nd isotopes of tholeiitic rocks in the Liuhang unit are comparable with the contemporaneous tholeiites in greenstone belts globally. In particular, the geochemical systematics of the tholeiite and enriched basalts show close similarity with those of the Wawa greenstone belt in the Superior Province.The Nb/Th, La/Smcn and Nb/La ratios of these basalts indicate that the basaltic lava sequence of the Liuhang unit has not been significantly affected by crustal contamination compared to the Yanlingguan komatiite–tholeiite sequence. The variation in lithology and geochemistry of basalts between the Liuhang and Yanlingguan units reflects a lateral change in composition of the ∼2.7 Ga basaltic eruption. We propose that the komatiite–tholeiite sequence in the Taishan association was derived from an upwelling mantle plume with eruption close to the continental margin within an ocean basin. Our study does not provide any evidence to support that the WSP granite-greenstone belt was subjected to significant arc–plume interaction during the early Neoarchean. Our results contribute to the understanding of early Neoarchean crustal growth globally identified from different regions.

► The Taishan basaltic lava sequence is characterized by voluminous tholeiites and minor enriched basalts. ► Geochemical systematics of the tholeiite and enriched basalts show similarities with those of the Wawa greenstone belt. ► The Nb/Th, Nb/La and La/Smcn ratios of the basalts do not favor significant crustal contamination. ► The Taishan komatiite–tholeiite sequence derived from plume with eruption near continental margin within an ocean basin.