Neoarchean siliceous high-Mg basalt (SHMB) from the Taishan granite–greenstone terrane, Eastern North China Craton: Petrogenesis and tectonic implications

Abstract

Siliceous high-Mg basalt (SHMB) is a rare rock type that occurs mainly at or near the Archean–Proterozoic boundary. It shares some geochemical similarities with Phanerozoic boninites, but there is a clear distinction. Whether the petrogenesis of SHMB resembles that of Phanerozoic boninites or is related to the komatiitic magmatism is controversial. Neoarchean SHMBs are identified for the first time from the Taishan granite–greenstone terrane within the Eastern Block of the North China Craton (NCC). Zircon U–Pb dating indicates that they were emplaced at ∼2.54Ga, contemporaneous with the generation of sanukitoids and adakitic rocks, but later than the eruption of the Late Archean (2.71Ga) komatiites and komatiitic basalts in the area. The high MgO (>8%), high SiO2 (>52%), and Al2O3/TiO2 ratio (12.4–45.0), together with low TiO2 (<0.5%) and HFSE contents and strong enrichment in LREE and LILE in the Taishan SHMBs are comparable with typical Phanerozoic boninites, except for distinct HREE depletion, lack of U-shaped REE patterns and conspicuous positive Zr anomalies. In conjunction with their more depleted Nd isotopic characteristics (ɛNd(t=2.54Ga)=+4.42 to +1.05) relative to Late Archean komatiites in the region, it suggests that these SHMBs were derived from partial melting of refractory depleted mantle which experienced earlier basalt extraction and was subsequently enriched in LILE and LREE by subduction-related metasomatization, rather than the products of assimilation–fractional crystallization (AFC) of komatiitic magma. A slab-derived adakitic melt was likely the metasomatizing agent, along with minor aqueous fluids released from the subducting oceanic slab. In combination with regional studies, the generation of these magmas was probably related to slab rollback, which is ascribed to the arrival of an oceanic plateau and/or residual thickened lithospheric keel at the subduction zone at that time. This mechanism might have played a crucial role in the formation of Archean granite–greenstone belts and was an important factor in continental crustal growth, particularly during the Late Archean.