Cumulate granites and extraction of interstitial high-silica melts: The case of leucogranites in the northern Tibetan Plateau

Abstract

Accumulation of crystals and extraction of melt play a crucial role in generating diverse intermediate to felsic rocks. However, the process of separating crystals and melt in granitic rocks with high silica content remains a topic of debate. In this paper, detailed petrographic, geochemical, and isotopic observations reveal that the ca. 424 Ma Aoyiyayilake S-type granitic batholith, located in Eastern Kunlun of northern Tibetan Plateau, is composed of low- and high- SiO<inf>2</inf> units that resulted from crystal accumulation and melt extraction respectively. Petrographically, the presence of fractures filled with quartz and oligoclase indicates compaction and fracturing occurred in the presence of melt, resulting in the extraction of melts to produce the high-SiO<inf>2</inf> granites. Geochemically, the high-SiO<inf>2</inf> granites have higher depletions in Eu, lower light rare earth elements, large ion lithophile elements (e.g., Ba and Sr) and P content than the low-SiO<inf>2</inf> granites. This is due to the fractional crystallization of rock-forming minerals (e.g., oligoclase and orthoclase) and accessory minerals (e.g., apatite), consistent with the proportions of minerals in the two units, e.g., ∼0.3 % apatite in the low-SiO<inf>2</inf> granites and undetected apatite in the high-SiO<inf>2</inf> granites. Isotopically, the high-SiO<inf>2</inf> granites display lower δ¹¹B of −16.1 ‰ and − 17. compared to −15.7 ‰ to −7.14 ‰ in low-SiO<inf>2</inf> granite, which is attributed to the crystallization of muscovite. These characteristics serve as significant indicators of efficient magma fractionation during the formation of high-SiO<inf>2</inf> granites.