Magma mixing in granite petrogenesis: Insights from biotite inclusions in quartz and feldspar of Mesozoic granites from South China

• Biotite inclusions in quartz and feldspar are used to trace the origin of granitic magmas.
• Matrix and inclusion biotites in two-mica granite show consistent compositions with those from typical S-type granite.
• Inclusions biotites in biotite granite show a compositional transition between typical S- and I-type granites.
• The biotite inclusions in the biotite granite record the mixing between granitic magmas of different compositions.

Magma mixing is a common process in granite petrogenesis. The major element composition of biotites in granites is primarily controlled by the composition of magmas from which they crystallized. Biotite grains enclosed in quartz and feldspars of granites are naturally protected by their host minerals, so that their compositions are likely original and can potentially be used to track the magma mixing. This is illustrated by a combined study of matrix and inclusion biotites from Mesozoic granites in the Nanling Range, South China. Three granite samples have been used in this study: one two-mica granite and two biotite granites. The biotites of different occurrences in the two-mica granite have no compositional distinctions. Biotites in the two-mica granite have higher Al2O3 and lower MgO than those in the biotite granites. The former is consistent with biotites from typical S-type granites of metasedimentary origin. In contrast, biotites from the biotite granites can be categorized into different groups based on their paragenetic minerals and geochemical compositions. They have relatively low aluminous saturation indices but higher Mg numbers, falling in the transitional field between typical S- and I-type granites. In addition, there are two contrasting zircon populations with nearly identical U–Pb ages in the biotite granites. One shows clearly oscillatory zonings in CL images, whereas the other is totally dark and often overgrew on the former one. The zircons with oscillatory zonings have higher δ18O values than the dark ones, indicating their growth from two compositionally different magmas, respectively, with different sources. An integrated interpretation of all these data indicates that mixing of two different magmas was responsible for the petrogenesis of biotite granites. Therefore, the study of biotite inclusions provides insights into the magma mixing in granite petrogenesis.