Distinction between S-type and peraluminous I-type granites: Zircon versus whole-rock geochemistry

Abstract

Biotite and two-mica granites are common in continental crust. Although they are generally peraluminous in lithochemistry, their petrogenesis has been controversial. Because they often show a negative correlation between P2O5 and SiO2 and a positive correlation between A/CNK and SiO2, they are commonly considered as the I-type granites of metaigneous origin. However, such lithochemical consideration is not certain in view of their other geochemical characteristics. To constrain the source nature of peraluminous granites, we performed a combined study of in situ U–Pb age, O isotope, and trace element for synmagmatic and relict zircons from Triassic biotite and two-mica granites in the Nanling Range, South China. Zircon U–Pb dating yields concordant ages of 230±3 to 237±3Ma for synmagmatic zircons, and 335–2379Ma for relict zircons with two clusters at ca. 440Ma and ca. 800Ma, respectively. Both the synmagmatic zircons and the ~440Ma relict zircons are characterized by high δ18O values of 8.8–11.4‰ and 8.6–10.3‰, respectively. In contrast, the majority of the other relict zircons show relatively low δ18O values of 5.1–7.9‰. The high δ18O values for synmagmatic zircons indicate that the Triassic granites were originated from metasedimentary sources. The two age clusters for relict zircons overlap with two episodes of granitic magmatism, respectively, in the early Paleozoic and the middle Neoproterozoic in South China, suggesting their inheritance from the metasedimentary sources. Thus, these Triassic granites were derived from partial melting of metasedimentary rocks rather than metaigneous rocks; they belong to S-type granite although their lithochemical relationships are akin to common I-type granites. As such, the zircon in situ geochemical analyses have the capacity to unravel the source nature of controversial granites. Our data indicate that fractional crystallization of heterogeneous magmas is the possible mechanism for the decoupling between major element and stable isotope compositions in the peraluminous granites. Such heterogeneities are inherited from the heterogeneous metasedimentary source.