Petrogenesis of the Granites in the Yandangshan Area, Southeastern China: Constraints from SHRIMP U-Pb Zircon Age and Trace Elements, and Sr-Nd-Hf Isotopic Data

Abstract

New geochemical and geochronological data of two types of granites, which are located in Yandangshan area, southeastern Zhejiang Province, were presented to constrain their magma condition, origin and the genetic relationship between them. The SHRIMP zircon U-Pb dating of Dongshan and He-sheng granite in Yandangshan area shows that they were formed at 114±1 and 103±2 Ma, respectively. Samples from the Dongshan granite have high SiO₂ (76.4 wt.%–76.9 wt.%) and total alkaline (K₂O+Na₂O=8.35 wt.%–8.47 wt.%) contents, but low FeO^T (0.89 wt.%–1.15 wt.%), MgO (0.21 wt.%–0.22 wt.%), and CaO (0.24 wt.%–0.34 wt.%) contents and high A/CNK (∼1.1) values, belonging to the peraluminous and magnesian granite. The Hesheng granite has high SiO₂ (72.2 wt.%–77.5 wt.%), total alkaline (K₂O+Na₂O=8.05 wt.%–9.41 wt.%) and FeO^T contents (1.20–2.06), and high A/CNK values (1.0–1.1), but low in MgO (0.12 wt.%–0.29 wt.%) and CaO (0.24 wt.%–0.34 wt.%) contents. Samples from the Hesheng granite also have high FeO^T/MgO (6.9–10.0) and 10 000×Ga/Al (2.6–3.4) ratios similar to the ferroan/A-type granite. All the samples are enriched in LREE but have produced negative Eu anomalies (Eu/Eu*_Dongshan=0.45–0.47; Eu/Eu*_Hesheng=0.17–0.55), Ba, Nb, and Ta, while the REE contents of the ferroan/A-type granite (Hesheng) are higher than that of the magnesian granite (Dongshan). The (⁸⁷Sr/⁸⁶Sr)i value of the magnesian granite is slightly higher than that of the ferroan/A-type granite and its ε_Nd(t) value (−6.8) is lower than the latter (−6.0–−5.9). In addition, the ε_Hf(t) value (−11.8–−4.2) of magnesian granite is also lower than that of the ferroan/A-type granite (−8.3–−2.0), indicating that there may be more mantle-derived components in the source area of the ferroan/A-type granite. Zircon saturation thermometer (T_Zr) and Ti-in-zircon thermometer (T_Zircon) are used to estimate the temperature of the magma source, and the results show that the magma temperature of the magnesian granite (average T_Zr=798 °C; average T_Zircon=792 °C) is lower than that of the ferroan/A-type granite (average T_Zr=862 °C; average T_Zircon=859 °C). And the oxygen fugacity of magnesian granite (ΔFMQ=1.16–3.47) are also higher than those of the ferroan/A-type granite (ΔFMQ= −0.41–1.14). Our new data indicate that both granites in this study are derived from a mixed source that consists of mantle-derived and crust-derived material. Based on the previous studies, both of the granitic plutons were formed under extension setting, and the granites transformed from magnesian to ferroan in the study area may indicate the extension was enhanced, which may be caused by the roll-back or delamination of the Paleo-Pacific oceanic slab.