ALBERT

Description: High-Mg andesites (HMAs) and adakitic rocks are purported to occur exclusively in subduction zones in the modern Earth. In the North China Craton, early Cretaceous HMAs and adakitic dacites were erupted in a continental setting, apparently unrelated to subduction given their location distal (〉1000 km) to the trench at that time. Here we report petrological, mineralogical and geochemical data for these rocks with the aim of constraining their petrogenesis and elucidating the role of water in intraplate magmatism and cratonic destruction. The HMAs can be subdivided into olivine (Ol-)HMAs and clinopyroxene (Cpx-)HMAs. The former have high MgO (〉9·8 wt %) and Mg# (〉71), with rare high-Fo (up to 91) olivine phenocrysts, corresponding to (near-)primary magmas that equilibrated with mantle peridotite. The latter have moderate MgO (7·8–8·8 wt %) and Mg# (mostly 〈70) and low-Fo (mostly 〈 83) olivine phenocrysts. The Cpx-HMAs are interpreted as magmas differentiated from the Ol-HMAs by olivine-dominated fractionation at lower-crust levels. P–T–X H2O estimations show that the primary HMAs are melts of shallow (1·1–1·2 GPa), hot (~1250°C) and wet (H 2 O 〉 3 wt %) lithospheric mantle. The coexisting adakitic dacites are hydrous (H 2 O ≥ 5 wt %) magmas with high SiO 2 (〉63 wt %), Sr/Y ratios (≥39) and Yb SN (source-normalized), low (Sm/Yb) SN , and negligible Eu anomalies. They also have unradiogenic whole-rock Nd [ Nd ( t ) = –19 to –9] and zircon Hf [ Hf ( t ) = –23 to –21] isotopic compositions consistent with derivation by melting of ancient lower crust at depths 〈 40 km. Melting may have been induced by heating and addition of H 2 O from underplated HMAs. Mixing between Cpx-HMAs and low-Mg adakitic dacites in magma chambers produced high-Mg adakitic rocks. The petrogenetic model presented here explains the occurrence of intraplate HMAs and adakitic magmas elsewhere in the North China Craton. The P–T–X H2O conditions inferred for HMA generation imply that the subcontinental lithospheric mantle beneath the craton was hot and hydrous in the early Cretaceous, which may have triggered the destruction of the cratonic root. The occurrence of young HMAs and adakitic rocks in an intraplate extensional environment also casts doubts on the common use of a similar igneous rock association as an indicator of subduction processes in Archean time.