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Elsevier, Earth and Planetary Science Letters, 1-2(270), p. 41-53, 2008

DOI: 10.1016/j.epsl.2008.03.008

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Recycling deep cratonic lithosphere and generation of intraplate magmatism in the North China Craton

This paper is available in a repository.
This paper is available in a repository.

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Data provided by SHERPA/RoMEO

Abstract

Early Cretaceous alkaline picrites and high-magnesium basalts from the North China craton provide evidence for recycling of continental lithosphere by density foundering. Both the picrites and basalts contain xenocrystic olivines with high Fo92–93 and low CaO (< 0.10%), consistent with the lavas' derivation from, or interaction with Archean mantle lithosphere. Most importantly, both the picritic and basaltic lavas contain unusual, reversely zoned clinopyroxene phenocrysts whose cores have low MgO, high Na2O (up to 2.4 wt.%, or 17.3 mol% Jd), and frequently contain ilmenite exsolution lamellae, consistent with their crystallization from an eclogite-derived melt (tonalite or trondhjemite). In contrast, the clinopyroxene exteriors have low Na2O (< 0.92 wt.%, or < 6.5 mol% Jd) and are lamellae-free, suggesting crystallization from a mantle-derived melt (picrite or basalt). Both the cores and exteriors have high Al2O3 contents (up to 6.9 wt.%). These features reflect crystallization of the cpx from an aluminous melt at mantle depths, with the cores forming at a significantly greater depth (≥ 2.5 GPa) than the surrounding cpx (≥ 1.5 GPa). Calculated primary melt compositions further constrain the magmas' formation at 3–4 GPa, in the presence of garnet. The unusually low CaO, high Ni/MgO and low 100Mn/Fe of primary melts indicate derivation of both the picritic and basaltic lavas from pyroxenite sources containing limited or no olivine. High Sr/Y, LaN/YbN and Th/U and low Lu/Hf, together with radiogenic initial 87Sr/86Sr and 187Os/188Os ratios and negative εNd values implicate contributions from melts derived from foundered eclogitic lower continental crust. Modelling suggests that the basalt source region contained a variable proportion (30–40%) of eclogite-derived component whereas the source of the picrites, on average, likely contained a generally higher proportion (60–70%) of a different eclogite-derived component.