First report of Paleoproterozoic incipient charnockite from the North China Craton: Implications for ultrahigh-temperature metasomatism

Abstract

The assembly of crustal blocks within the North China Craton (NCC) at the terminal stage of craton building during Paleoproterozoic witnessed major subduction-collision events. Here we illustrate a case where hot and anhydrous magmas emplaced in the lower crust during the early Paleoproterozoic subduction realm in the NCC led to the formation of orthopyroxene-bearing dehydrated zones of incipient charnockites within the adjacent TTG (tonalite-trondhjemite-granodiorite) suite. The coarse greenish anhydrous zones of incipient charnockite show an assemblage of antiperthitic plagioclase+orthopyroxene+K-feldspar+quartz with minor clinopyroxene, ilmenite+magnetite and secondary biotite. The peak P–T conditions of the incipient charnockite are estimated as 890–970°C and 3.5–6.5kbar based on mineral chemical analysis and pseudosection modeling in the system NCKFMASHTO, suggesting ultra-high temperature metasomatism, and the ‘hottest incipient charnockites’ so far reported. Zircons from the massive charnockite show oscillatory-zoned magmatic cores with high Th/U values rimmed by unzoned and luminescent thin metamorphic overgrowths. A concordant group of magmatic zircons yields weighted mean 207Pb/206Pb age of 2401±29Ma, and a lower intercept age of ca. 1834±81Ma. A similar concordant group of zircons from the TTG yields a weighted mean 207Pb/206Pb age of 2446±18Ma and a weighted mean lower intercept age of 1834±47Ma. The ca. 2.45Ga age from the TTG and 2.40 age from the charnockite are taken to represent the magma emplacement ages, and the identical 1.83Ga from zircons in both rocks denoting the timing of late Paleoproterozoic metamorphism. Lu–Hf analysis of magmatic cores of the zircons from the charnockite and the TTG show clear positive ɛHf(t) values (1.2–5.9), suggesting juvenile sources.The formation of incipient charnockite zones close to the contact with the charnockite pluton is consistent with the quantitative numerical models that predict CO2 migration in anhydrous silicate melt and metasomatic fluid from a rapidly cooling pluton. The ultra-high temperature metasomatism and solid-state reaction generating the incipient charnockites provide important insights on CO2 advection from lower crustal magmas during Paleoproterozoic orogeny in the North China Craton.