The genesis of ca. 1.78 Ga granitoids in the Xiong’er large igneous province: Implications for continental crust generation

Abstract

The genesis of intermediate-felsic rocks and the implications for crustal growth are highly controversial. These issues are particularly relevant to large igneous provinces, which are one of the most effective methods of crustal generation. Mafic magmas, even those with continental marks, have been considered as the contribution from mantle to crust, whereas the intermediate-felsic series in large igneous provinces has uncertain implications for crustal growth due to uncertainty about their modes of generation. In this paper, we focus on the intermediate-felsic granitoids, i.e., Gushicun diorites and granites, in the Xiong’er large igneous province (ca. 1780‒1750 Ma) through detailed study of their petrology, chronology, and geochemistry. Our new data show that both diorites and granites have the same emplacement age of ca. 1780 Ma and were synchronized with the Xiong’er large igneous province. The granitoids are characterized by enrichment in light rare earth elements (LREEs) and large ion lithophile elements (LILEs; e.g., Li, Ba, and Rb) but depletion in high field strength elements (HFSEs; e.g., Nb, Ta, and Ti). They have almost the same ΣREE (rare earth element) contents and TTi-Zircon (∼850 °C), whole-rock Nd (εNd(t) from −8.24 to −5.76), zircon Hf (εHf(t) isotope ratios of −14 to −8 isotope ratios), and O-isotope compositions (average δ18O = 5.3‰). The fine-grained Gushicun diorites have low Sr/Nd (<10) and Eu/Sm (<0.3) but high Si/Al (>3.2) ratios and are similar to the basaltic andesites in the Xiong’er large igneous province, which suggests that the diorites are crystallization melts that differentiated along the liquid line of descent. The Rayleigh fractionation model based on the REE contents shows that the Gushicun diorites were formed by 40%‒50% fractional crystallization of Xiong’er basaltic melts. The characteristics of high K2O (3.8‒4.7 wt%) and Yb contents (4.0‒6.5 ppm), low Sr/Y (1.4‒3.3), and low metaluminous values (A/CNK = 0.8‒1.1) of the Gushicun granites with mantle-like O-isotope values rule out the melting genesis of low K2O (<0.5 wt%) amphibolite and trondhjemite−tonalite−granodiorites (TTGs) of the surrounding Taihua Complex. The Gushicun granites show a geochemical affinity with andesites in the Xiong’er large igneous province. The alphaMELTs simulation based on the major elements argues against the partial melting of amphibolite and TTGs but supports the melting genesis of Xiong’er intermediate igneous rocks with low water contents (<1 wt%). These findings shed light on the formation of the felsic series in large igneous provinces, which remelted the previous mafic-intermediate rocks via underplating basaltic magmas. Despite having enriched Nd- and Hf-isotope features, the Gushicun diorites represent juvenile crust, while the granites were formed by the reworking of juvenile crust instead of ancient crust. This paper also suggests that radiogenic isotopic ratios cannot be reliably used to assess types of continental crust.