Petrology and geochemistry of Late Triassic rapakivi-type granites in the Dupangling area, South China: Role of fluid-rock interaction in the origin of rapakivi texture

Abstract

Rapakivi-type granites not only document magmatic processes but also archive subsolidus interactions between granitic rock and fluid, and thereby provide crucial information about fluid infiltration in the Earth's crust. However, the key process to generate rapakivi texture is still debated. Here, we report comprehensive mineralogical, geochemical and Hf isotopic compositions of the Dupangling rapakivi granites (210.4–214.9 Ma) in the South China Block to investigate the effect of fluid-rock interaction on the origin of rapakivi feldspar. Dupangling rapakivi granites have high SiO2 (72.63–78.21 wt%) and alkali contents (Na2O + K2O = 6.14–9.06 wt%) with high K2O/Na2O ratios (1.37–2.64), characteristic of weakly peraluminous (A/CNK = 1.04–1.10) high-K calc-alkaline granites. Samples have weakly fractionated REE patterns with negative Eu anomalies (δEu = 0.05–0.38). They have negative whole-rock εNd(t) (−8.45 to −7.91), variable zircon εHf(t) (−9.7 to −1.0) and two-stage Hf isotope model ages (1.32–1.84 Ga), due to their derivation from a major early Mesoproterozoic crustal source with a subordinate late Paleoproterozoic component. Rapakivi feldspars in Dupangling granites are composed of an alkali-feldspar core and a plagioclase mantle. Fractures, pores and secondary inclusions (biotite, sericite, epidote and hematite) are closely associated with the mantles and oligoclase patches within alkali-feldspar cores. These pervasive microtextural features and variations of Be, Rb, Ba, Sr and LREE within rapakivi feldspars are interpreted to have formed through subsolidus fluid-induced dissolution-reprecipitation replacement processes. This study highlights that fluid-rock interaction plays an important role in generating rapakivi texture.