Ca. 1050 Ma intra-continental rift-related A-type felsic rocks in the southwestern Yangtze Block, South China

Abstract

Late Mesoproterozoic igneous rocks have been traditionally thought to mark Grenvillian orogens and are thus widely used for the reconstruction of Rodinia. In the southwestern Yangtze Block, South China, ∼1050Ma dacites and rhyolites in Huili and granites in Julin are rich in large ion lithophile elements (LILE; e.g., Rb, Ba, Th and U) and poor in Nb, Ta, and Ti relative to the primitive mantle. These felsic rocks contain similarly high Zr, Hf, Ga, and HREE contents and Ga/Al ratios, and have high calculated zircon saturation temperatures (∼900°C in average), thus resembling A-type granitoids. However, the dacites have whole-rock εNd(t) (−7.2 to −4.5) and zircon εHf(t) values (−20.8 to +1.0) much lower than the rhyolites/granites, and were interpreted to be derived from partial melting of an ancient continental crust. In contrast, rhyolites and granites have much higher εNd(t) (−0.4 to +1.8) and εHf(t) (−3.2 to +11.2) values, indicative of their derivation from primary magmas generated by partial melting of newly formed underplating crust, with possibly minor contributions from mantle-derived materials. These A-type felsic rocks have high Y (20–100ppm) and Nb (10–40ppm) contents similar to “within-plate” magmatic rocks in the frequently-used Y vs. Nb and Rb vs. Y+Nb plots for tectonic discrimination. Such a feature, in combination with rift-related, coeval mafic rocks in the region, indicates that they have formed in an intra-plate rifting setting. Our work thus argues against the existence of the so-called Grenvillian Orogen in the southwestern Yangtze Block. In view of the subsequent Neoproterozoic arc magmatism (950–730Ma) in the region, we propose that the ∼1050Ma A-type granitoids and coeval mafic rocks were produced under a continental rifting basin that was transformed to an compression setting during early Neoproterozoic.