Age and geochemistry of granites in Gejiu area, Yunnan province, SW China: Constraints on their petrogenesis and tectonic setting

Gejiu is one of the largest polymetallic tin ore districts in the world. Located at the westernmost end of the South China tungsten–tin province (or Nanling tungsten–tin province), it is a granite-related (Gejiu granite) magmatic-hydrothermal system. Nine samples from two phases of Gejiu granitic intrusions have been analyzed by SHRIMP and/or LA-ICPMS zircon U–Pb techniques, yielding ages ranging from 77.4 ± 2.5 Ma to 85.0 ± 0.85 Ma. Whole rock analysis shows that both phases are high-K and alkali-rich granites and their ACNK values fall mainly into a small range of 1.0–1.1. Moreover, Harker diagrams indicate that granites experienced strong fractional crystallization during magmatic evolution. Most granites display relative enrichment in LREE and strong Eu depletion. The whole rock average εNd(t) values of the Gejiu granites vary from − 9.3 to − 6.9, whereas a range of − 8.12 < εHf(t) < 1.21 is defined by magmatic zircons. Sr–Nd–Hf isotope data indicate that the granites have been mainly derived from crustal melts with minor input of mantle component. Two stage Nd and Hf model ages, together with isotopic characteristics, indicate that the Gejiu granite magmas were possibly derived from partial melting of Mesoproterozoic continental crust, with minor input of mantle-derived melts, followed by extensive fractional crystallization.

Research Highlights
► SHRIMP and LA-ICP-MS U–Pb zircon dating indicate that the granites in the Gejiu area formed at around 80 Ma, which keeping consistent with magmatism and mineralization event in the western south China block.
► Geochemical and Sr–Nd–Hf isotopic data suggest that Gejiu granite magmas experienced a high degree of fractional crystallization after they have formed by partial melting of Mesoproterozoic crust with minor input from mantle-derived magmas. This will provide another example for understanding the intraplate magmatism, which is a hot topic in recent years.
► Granites in the Gejiu area are attributed to the “crust-mantle syntectic type” that display strong signs of mantle-crust interaction in an intraplate setting. This depart from the conventional I-, S-, M- and A-petrochemical types as treated in the literature.