Complex Precambrian crustal evolution beneath the northeastern Yangtze Craton reflected by zircons from Mesozoic volcanic rocks of the Fanchang basin, Anhui Province

Abstract

Our knowledge of the Precambrian accretion and reworking history of deep crust beneath the northeastern Yangtze Craton is still sparse. Zircons from volcanic rocks can not only give accurate crystallization ages for zircons within the host magmas, but also provide unique information on the history of deep crustal rocks sampled during magma ascent. In situ U–Pb ages and Hf-isotopes were analyzed on zircons from two trachyandesites from the Late Mesozoic Zhongfencun and Kedoushan Formations in the Fanchang basin, Anhui Province. The zircons exhibit relatively complex internal structures. Four zircons in the Zhongfencun trachyandesite yield a lower intercept 206Pb/238U age of 133.0±3.2Ma, with Hf model ages of 2.8–2.4Ga (Tcrust) and significantly negative ɛHf(t) (−25.15 to −18.72), demonstrating that this lava formed in Early Cretaceous time. Zircons in the Kedoushan biotite trachyandesite yield a 206Pb/238U age of 126±1Ma, as well as negative but relatively high ɛHf(t) values (up to −4.34), revealing it formed roughly coeval with the Cretaceous A-type granites and bimodal magmatism in this region. More significantly, U–Pb ages and Hf isotopes for Proterozoic zircon xenocrysts in the Fanchang volcanic rocks reveal that the northeastern Yangtze deep crust probably contains unexposed Archean (e.g. 3.4–2.9Ga and 2.8–2.5Ga) components, rarely identified from the outcrops in this area, and has undergone complex evolutional history, including the addition of juvenile mantle-derived materials at 3.4–2.9Ga, ca 2.65Ga and ca 0.8Ga, and reworking of the Precambrian crust at ca 2.0–1.8Ga, ca 0.8Ga and 133–126Ma. The history of the continental crust's accretion and reworking of the northeastern Yangtze Craton may not fully coincide with the major episodes of crustal evolution in the wider Yangtze Craton, especially during the Archean to Paleoproterozoic periods.