Contribution of alkaline magmas to the petrogenesis of Triassic aluminous A-type granites on Liaodong Peninsula, Northeastern China

Abstract

Aluminous A-type granites are not typically associated with mantle-derived rocks. Their generation is usually linked to crustal melting with mantle-derived magmas primarily acting as a heat source. However, the Triassic Bolinchuan A-type granites on Liaodong Peninsula, northeastern China, coexist with a large mantle-derived alkaline igneous complex, indicating a close genetic relationship that provides new insights into the petrogenesis of aluminous A-type granites. The Bolinchuan granites were formed during 230–226 Ma, coeval with syenites in the alkaline complex (230–224 Ma). The granites are metaluminous to weakly peraluminous, but have relatively high alkali contents (8.61–9.40 wt%). Their high Ga/Al ratios, elevated Nb, Zr, Ce, and Y contents, and high magmatic temperatures (>875 °C) indicate an affinity with aluminous A-type granites. However, the Bolinchuan granites have relatively high MgO contents at intermediate SiO₂ contents, with Mg# values of up to 47; these features are inconsistent with the melting of only crustal rocks. Instead, these granites contain mineral phases similar to those of the syenites and exhibit major and trace element trends that are continuous with those of the syenites. Their geochemical features include enrichments in large-ion lithophile elements (e.g., Ba and Sr), depletions in high-field-strength elements (e.g., Nb and Ta) and heavy rare earth elements, and small negative Eu anomalies, which are also consistent with the features of the associated syenites. Furthermore, the granites and their accessory minerals have high initial ⁸⁷Sr/⁸⁶Sr ratios, and low ε_Nd(t) and ε_Hf(t) values, which are similar to the syenites, apart from being slightly less isotopically enriched. These geochemical and isotopic features suggest the Bolinchuan granites formed from the syenitic magmas by fractional crystallization of alkali feldspar, amphibole, and apatite, in combination with the addition of ∼30 % juvenile crust-derived materials, as indicated by modeling. Our study reveals that differentiation of mantle-derived alkaline magmas can generate magmas parental to aluminous A-type granites. As such, the mantle not only provides the heat for the formation of aluminous A-type granites, but can also be the main source of the magmas.