Generation of aluminous A-type granite by partial melting of felsic restite: Evidence from Mesozoic granitoids in the southern margin of the North China Craton

Abstract

Aluminous A-type granites generally show high SiO 2 contents of >70 wt%, and their petrogenesis is still hotly debated with respect to source nature, magmatic processes and tectonic setting . This paper presents a comparison of petrography , geochronology and geochemistry between a Mesozoic syenogranite and its adjacently contemporaneous granitoids in the Xiong'ershan area, the hanging wall of the Paleozoic continental collision zone between the North China Craton and the North Qinling microcontinent. Syenogranites at Taishanmiao are metaluminous to weakly peraluminous, and belong to high-K calc-alkaline to shoshonitic series. They contain embayed bipyramidal quartz crystals , quartz-feldspar intergrowths and high Fe biotite species as well as accessory fluorite , magnetite and ilmenite . They exhibit significantly negative Ba, Sr and Eu anomalies with most δEu values of <0.5, and relatively high contents of Nb, Ta and Zr. They are weakly magnesian to ferroan in lithochemistry and mostly fall into the field of aluminous A-type granite in terms of their high Zr contents and 10,000 × Ga/Al ratios. Furthermore, they have low Y/Nb ratios of <1.20, falling into the A1-type field of the Nb-Y-Ce ternary diagram. The geochronological results indicate that the Taishanmiao syenogranites were emplaced into the late Paleoproterozoic Xiong'er Group volcanics at 119 ± 1 Ma, consistent with emplacement ages of the late stage non-adakitic granitoids at Funiushan in the same area. Whole-rock Nd and zircon Hf O isotope compositions are similar between the two types of granitoids, indicating that they share a common magma source that is composed of the ancient crustal basement in the southern margin of the North China Craton and the juvenile crust of the North Qinling microcontinent. According to their petrographic and geochemical features, the syenogranites are categorized into aluminous A-type whereas the non-adakitic granitoids belong to I-type. Therefore, they were produced by partial melting of felsic restites after extraction of hydrous granitic melts to form the Funiushan I-type granites. As such, the composition of these granitoids is primarily dictated by both source composition and magmatic processes at the ancient convergent plate margin. Although the granitic magmatism would take place in the post-collisional period, the aluminous A-type granite was formed in the terminal stage of post-collisional magmatism. In addition, the post-collisional granites record reactivation of the hanging wall in the continental collision zone. • The Taishanmiao syenogranites belong to aluminous A1-type granite. • The syenogranites were derived from partial melting of the felsic restite. • The felsic restite was produced by extraction of hydrous granitic melts. • The syenogranites are petrogenetically associated with calc-alkaline granitoids. • Granite composition is also dictated by crustal anataxis and melt extraction.