Titanite geochemistry traces extreme differentiation of granitic magma in the collisional orogen

• Published in: Lithos 2024-03, Vol.468-469, p.107515, Article 107515
• Main Authors: Zhou, Kun, Ma, He-Zhi, Chen, Yi-Xiang, Wang, Fang-Yue, Zhao, Zi-Fu
• Format: Article
• Language: English
• Subjects: Collisional orogen; Fractional crystallization; Granitic magma; Nb/Ta fractionation; Titanite
• ISSN: 0024-4937; 1872-6143
• DOI: 10.1016/j.lithos.2024.107515

The genesis of granitic magma is important for the growth and evolution of continental crust. However, whether and how granitic magma can undergo efficient crystal fractionation from partial melting at the source to magma emplacement in the shallow crust level are still controversial. To address this problem, a combined study of whole-rock and titanite geochemistry was carried out for syn-exhumation granites from the Sulu ultrahigh-pressure metamorphic belt in eastern China. These high-Si granites were produced by partial melting of the deeply subducted continental crust. Significant geochemical differentiation has been revealed in these granites. Titanite in the granites can be divided into anatectic and magmatic origins, according to the Fe/Al ratios and trace element compositions. Both types of titanite yield similar UPb ages ranging from 209 ± 23 Ma to 233 ± 26 Ma, corresponding to the late collisional stage of the Sulu orogen. These titanite domains generally have consistent εNd(t) values, suggesting that they formed in a nearly closed system without significant magma mixing of different sources. The highly variable Nb/Ta ratios of titanite show good correlations with FeO, Al2O3, Ta, Sr, LREE and (Lu/Gd)N. This interesting feature is ascribed to the extensive crystal fractionation of titanite, plagioclase and allanite/epidote during magma evolution. The results suggest that the highly silicic granitic melts can experience extensive crystal separation from the liquid during granitic magma differentiation. This study shows that accessory minerals such as titanite play a key role in constraining the geochemical differentiation of granitic melts. Furthermore, titanite geochemistry has the potential to decode the complex processes from partial melting at the source to magma differentiation at shallow crust level in collisional orogens. •Titanite in high-Si syn-exhumation granites can be divided into anatectic and magmatic origins.•Highly variable Nb/Ta ratios of magmatic titanite suggest extensive crystal fractionation.•Titanite chemistry decodes partial melting and magma differentiation of continental crust.