Geochronology, Geochemistry, Whole Rock Sr-Nd And Zircon Hf-O Isotopes Of The Early Neoproterozoic Volcanic Rocks In Jiangshan, Eastern Part Of The Jiangnan Orogen: Constraints On Petrogenesis And Tectonic Implications

This paper presents the results of combined studies of geochronology, geochemistry, whole rock Sr-Nd and zircon Hf-O isotopes carried out upon the rhyodacite and ignimbrite of Shangshu village, Shangyu town and Shanghupeng village of Jiangshan City in Zhejiang Province, along the northwestern side of the Jiangshan-Shaoxing suture. SHRIMP zircon U-Pb dating of samples in the three areas yielded weighted mean Pb-206/U-238 ages of 842.8 +/- 6.9 Ma and 850.0 +/- 7.3 Ma, 839 +/- 9 Ma and 832.2 +/- 8.1 Ma, 828.3 +/- 8.5 Ma and 836.9 +/- 9.9 Ma, respectively. These ages are older than the volcanic rocks of the Shangshu Formation dated at around 780 Ma distributed in Fuyang City, Hangzhou City, Kaihua County, etc. The volcanic rocks generally have high SiO2 (54.08-76.80 wt%) and Al2O3 (12.40-21.31 wt%), low Fe2O3 (0.68-8.92 wt%), MgO (0.29-2.49 wt%), CaO (0.12-2.86 wt%), TiO2 (0.10-1.59 wt%) and P2O5 (0.01-0.39 wt%), with variable total alkalis (K2O + Na2O =5.42-8.29 wt%). There exists a clear negative correlation between SiO2 and P2O5. The volcanic rocks have A/CNK ratios of 1.03-2.77 and thus are peraluminous. They are characterized by enrichment in LREE, Rb, Ba, Zr, Hf, K, Th, La, U and depletion in Nb, Sr, P, Ti, with distinct LREE and HREE fractionation of (La/Yb)(N) values of 5.68-11.67, and with a moderate negative Eu anomaly (&Eu=0.58-0.89). Whole-rock geochemical data shows that the Jiangshan volcanic rocks are possibly I-type granitic rocks, even though they have some characteristics of A&S-type granites due to the magma fractional crystallization and water-rock interaction. Zircon delta O-18 values are 3.97 parts per thousand -5.49 parts per thousand (average 4.50 parts per thousand), 2.90 parts per thousand -5.21 parts per thousand (average 4.32 parts per thousand) for ignimbrite from Shangshu village section, and Shanghupeng village section, respectively. They are slightly lower than the average delta O-18 values of igneous zircons in equilibrium with mantle magmas (5.3 +/- 0.6 parts per thousand (2 sigma)), the lower delta O-18 value also demonstrating the presence of high temperature water-rock interactions. The ignimbrite rocks have positive (Nd)(t) (4.02, 3.37, 3.91, 4.74, 2.85, 4.39, totals from the three areas) and (Nd)(t) (in-situ zircon) (4.3-14.6, a weighted mean of 8.4; 6.6-12.7, a weighted mean of 9.0; 8.1-12.0, a weighted mean of 9.5, respectively, from the three areas). In conjunction with the trace element studies, they indicate that the source region of the Jiangshan volcanic rocks was mainly composed of juvenile lower crustal material, mixed with some mantle-sourced magma. Detailed elemental and isotopic data suggest that the Jiangshan volcanic rocks were formed in a continental arc setting. There is a series of ca. 860-830 Ma volcanic rocks formed in a back-arc extensional setting in the southern margin of the eastern Jiangnan Orogen, along the northwest side of the Jiangshan-Shaoxing suture. The first stage rift-related anorogenic magmatism may have occurred as early as ca. 860 Ma in the eastern Jiangnan Orogen.