Geochronology and petrogenesis of the early Cretaceous granitoids in the Jingde-Guangde area, South China: Implications for its tectonic setting and evolution

The eastern Jiangnan Orogen comprises two tectonic units: the initial defined Jiangnan Orogen (IJO) and the eastern extension area of the Jiangnan Orogen (EEJO). Ten granitoid intrusions with ages from 152 to 130 Ma in the EEJO, were selected for integrated studies. Therein, I-type granodiorites and monzogranites (152-135 Ma) had SiO2 contents of 62.5-69.0 wt%, total alkali contents of 6.2-7.9 wt%, and a narrow range of A/CNK (0.91-1.03). They also showed enrichment of LILE, depletion in HFSE, slightly Eu negative anomalies. I-A transform-type monzogranites and K-feldspar granites (135-130 Ma) exhibited a systematic higher SiO2 (64.6-74.5 wt%) and total alkali contents of 7.8-10.1 wt%, but low Al2O3 (13.42-15.75 wt%), Fe2O3T (1.29-4.00 wt%), MgO content (0.16-1.32 wt%) with more significant Eu and Sr negative anomalies. Whole rock epsilon(Nd)(t) and Sr-87/Sr-86(t) values of the granodiorite-monzogranites fall into the ranges of -8.8 to -6.1 and 0.7080-0.7112, respectively. Their epsilon(Hf)(t) and delta O-18 values are from 11.4 to 3.1 and 7.12 parts per thousand to 10.03 parts per thousand. The monzogranites and K-feldspar granites show relatively higher eNd(t) values (-6.1 to -5.4) and narrow Sr-67/Sr-86 (t) ratios (0.7086-0.7104). Their epsilon(Hf)(t) and delta O-18 values range from 9.3 to 3.3 and 7.23 parts per thousand to 10.09 parts per thousand. These rocks dominantly derived from partial melting of the early Neoproterozoic accretionary crust, indicating that the EEJO should belong to the Jiangnan Orogen. The presence of I-A transform-type rocks implies that the tectonic geodynamic frame transformed from compression to extension might have took place at 135 Ma.