The ultra-potassic rocks on the southwestern margin of the Ordos basin and their tectonic significance

Abstract：Located in the northern margin of the Jiangnan orogen (south of the Yangtze River)，Zhanggongshan area of Anhui-Jiangxi border region contains very thick low greenschist-facies metamorphic flysch rocks dominated by argillo-calcareous fine-grained elastic rocks with minor volcanic materials. High precise isotopic dating shows that it was probably formed in Neoproterozoic between 821~840 Ma. It was bordered by obliquely crossed Jindezhen-Yintan structural belt with Xikou Group located in the southern part and Shuangqiaoshan Group in the northern part. REE and HFSE in these two groups, such as Nb, Ta, Zr, Hf and Nd, show similar characteristics, indicating that epimetamorphic rocks within the area have the same or similar original crust compositions, while the data (La/Yb)N＝4.66～13.63，δEu=0.09～0.3 and (Gd/Yb)N=1.22～1.66 suggest that these low-grade metamorphic rocks mainly came from Late Archean upper crust composed of felsic rocks. Detrital zircon U-Pb ages are largely 826~2600Ma, showing evidently 5 peaks, i.e., 2500Ma, 2000~2200Ma, 1800Ma, 958Ma and 827~841Ma, dominated by young detrital zircons whose ages are very close to ages of volcanic rocks interbeded with clastic rocks, indicating that epimetamorphic rocks mainly resulted from contemporary igneous rocks. Differences in content and ratios of large-ion lithophile elements such as (LILE), Rb, Cs, Sr, Ba and Th, together with variations of Sr isotopic characteristics, suggest that Xikou Group and Shuangqiaoshan Group detrital rocks experienced different tectonic evolutions. Epimetamorphic volcanic-margin fine-grained clastic rocks were formed in a plate tectonic system related tectonic-sedimentary environment in which Xikou Group shows depositional characteristics of extensional continental margin while original rocks of Shuangqiaoshan Group display a continental island arc closed tectonic depositional environment, suggesting plate subduction depositional characteristics.