A preliminary study of zircon LAMC-ICP-MS U-Pb ages of the Shuangjingzi complex in Linxi, Inner Mongolia

Abstract: The Shuangjingzi complex occurs on the southern and northern sides of the Xar Moron River in Linxi County, eastern Inner Mongolia and is composed of schist, gneiss and gneissic granite. There exists controversy over timing of the Shuangjingzi complex. Some researchers hold that it belongs to Precambrian metamorphic rocks，while others argue that it belongs to the Early Mesozoic strong structural deformation products. In this paper, the authors studied zircon geochronology of the main rocks in the Shuangjingzi complex by using the LA-MC-ICP-MS method with the purpose of constraining the timing of the complex. The zircon dating results show that the 206Pb/238U ages of the chlorite schist range from 146.3 Ma to 1970 Ma, with no reliable weighted mean ages obtained due to the too scattered ages. The 206Pb/238U ages of the biotite plagioclase gneiss vary from 146.3 Ma to 1736 Ma, with two reliable weighted mean 206Pb/238U ages of （290.5±2.1） Ma (MSWD = 0.36, n=10) and （247.5±3.4） Ma (MSWD = 2.9, n=5), respectively, indicating that two epochs of magmatic emplacement events took place in the original rocks of the complex during Early Permian and Early Triassic, respectively. The zircons from the granite pegmatite that intruded into the complex do not give a reliable weighted mean age, with the scattered 206Pb/238U ages ranging from 206.9 Ma to 2122 Ma, while the zircons from the gneissic granite occurring within the complex yielded a weighted mean 206Pb/238U age of （278.4±1.4） Ma (MSWD=1.9，n=16), suggesting that the emplacement of the original rock occurred during Early Permian. Geochemical analyses show that the Shuangjingzi complex, including chlorite schist, biotite plagioclase gneiss and gneissic biotite granite, is characterized by complicated constituents, with SiO2 content from 61.97% to 77.18%. All samples collected from the complex have the features of LREE differentiation, and are enriched in LILE elements with relative depletion of U, Nb, Sr, P and Ti, and enriched in Th, K, Ce, Nd, Hf, Sm, Y and Yb. A comparison with the Shuangjingzi complex shows that the granite pegmatite exhibits similar enrichment and depletion trends of the trace elements, except for the distinctive relatively depleted Ba and enriched Ta. Therefore, the Shuangjingzi complex is composed of the mixture of rocks (including strata and magmatic rocks) of different compositions and ages,. Based on the age data available and the zircon ages obtained by the authors, it is held that Shuangjingzi complex consisting of a set of low-medium grade metamorphic rocks occurring along the Xar Moron River deep-seated fault was formed between 230 and 206 Ma, resulted from the collision between North China and Siberian Plates, and might have been reworked during Middle-Late Yanshanian period. Therefore, no Precambrian basement exists in the study area.