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Introduction
Due to the discovery of precious fossils, West Liaoning Province has been drawing extensive interests in recent years (Chen et al., 1998; Sun, 1998; Swisher, 1999; Xu, 2001; Zhou, 2003; Zhou, 2003). Meanwhile, the researches on the stratigraphy, paleo-geography, paleoclimate, and paleomagnetism of the Mesozoic volcano-sedimentary basin in west Liaoning Province are gradually performed (Pan et al., 2001; Zhang et al., 2002; Zhu et al., 2001 & 2002; Gong et al., 2007; Li et al., 2002; Ren, 1997). As an essential terrane in North China where Meso-Cenozoic volcanic activities were developed, outstanding achievements have also been made in the researches on the chronology (Figure 1) and geochemistry of volcanic rock in west Liaoning Province (Jia et al., 1999; Zhou, 2001; Zhang & Zheng, 2003; Yang, 2007; Wu et al., 2003). Whereas SHRIMP Zircon U-Pb isotopic dating of volcanic rock in west Liaoning Province mainly concentrates on Yixian Basin, there are only sporadic isotopic reports dating of volcanic rock in Zhangwu-Heishan Area (Chen et al., 1997; Huang, 2006 & 2007; Xiao, 2008).
West Liaoning Province is located in the east section of the Yanshan Platformal Fold Belt in the north of Sino Korean Paraplatform and belongs to the east extension of Yanshan Orogenic Belt (Figure 1). Yanshan Orogenic Belt lies inside the North China Platform and has experienced intense tectonic deformation, magmatic and volcanic activities, and syn-orogenic sedimentation Jurassic-Cretaceous Yanshan Movement, and finally becomes a typical intraplate orogenic belt (Zhang CH, 1999) far away from any contemporary plate boundary. The subduction of the Pacific Plate has resulted in NE-trending thrust nappe structure of late Jurassic Epoch and extensional structure of the early Cretaceous Epoch. Before the Mesozoic Period, the study area is a large tectonic uplift area. At the first stage of the Yanshan Movement, the fault-folding formed lower-middle Jurassic Series in the NNE-trending depression area, including coal-bearing strata - Beipiao Formation. At the Yanshan Movement's medium setting, a large-scale fracture occurred and was accompanied by a magmatic eruption, and then a Mesozoic tectonic framework was formed.
Denomination and Age of Volcanic Rocks
Although numerous detailed stratigraphic researches have been carried out in West Liaoning Province, the knowledge of stratigraphic sequence, the horizon of volcanic rock, and volcanic eruption periods are unified yet. For instance, whether volcanic rock in Yixian Formation is of late Jurassic Epoch (Wang, 1987; Ren et al., 1997; Wang, 1998; Luo et al., 1999), late Jurassic to early Cretaceous Epoch (Zhang et al., 1992; Chen et al., 1998; Ji et al., 1998; Sun & Zheng, 2000; Jiang et al., 2000; Wang et al., 2002) or early Cretaceous Epoch (Li et al., 2001; Wang et al., 1999; Wang et al., 2001; Xu et al., 1999; Ji et al., 2002; Wang et al., 2005; Zhang et al., 2006) is an open question. For more details of the stratal division in this area and volcanic rock's denomination, see the references Wang et al., 2004; Cai, 2009). It should be noted that the Scientific Research Institute of Liaoning Bureau of Coalfield Geological Exploration (1970) advocates that extensive volcanic rock in this area belongs to Dalinghe Formation and is of Cretaceous Epoch and concluded that there might be coal seams of Fuxin Formation under this suite of volcanic rock. Likewise, the 107th Coalfield Geological Brigade of Liaoning Province held in 1974 suggested that coal-bearing series of Badaohao Formation could not be correlated with Shahai Formation or Fuxin Formation in Fuxin Basin, and Gangtaishan volcanic rock overlies Badaohao Formation and also drew the conclusion that the coal possibly could be found below those volcanic rocks.
Isotopic Dating and Eruption Period of Volcanic Rock
Isotopic dating of volcanic rock in west Liaoning Province starts in the 1980s (Diao, 1983; Wang, 1984 & 1985). Some fundamental findings have been made since the 1990s (For the data on isotopic dating, see Figure 2). By far, over 120 pieces of data derived from various dating methods have been published. These data are systematically collected, and the distribution map (see Figure 2) was plotted in this work. From the figure, the age of volcanic rock which crops out in west Liaoning Province varies between 56 Ma and 147 Ma. Yixian Formation of early Cretaceous Epoch in the Yixian Basin contains four suites of volcanic rock and sedimentary rock (Ji, 2004), which represent four volcanic-sedimentary activities. As shown in Table 1, the periods of volcanic activities are 132±1 Ma, 126±1 Ma, 122±2 Ma, and 115±2 Ma. Also, a suite of alkaline basalt was formed in the period of 90-105 Ma. This is identical to the east part of North China (Zhai et al., 2003). There is also a Paleogene olivine-inclusion-bearing basalt suit with the age of about 56Ma in the Zhangwu-Heishan Area. In 2008 the International Commission on Stratigraphy adjusted the Jurassic-Cretaceous boundary from originally 135 Ma (140 Ma) to 145 Ma±4 Ma and the age of lower Cretaceous-upper Cretaceous limit from 95 Ma to 99.6 Ma. Therefore, previously delineated volcanic rock of late Jurassic Epoch is now of early Cretaceous Epoch, and the peak period of volcanic activities is the medium stage (130 Ma-122 Ma or so) of early Cretaceous Epoch, namely Barremian to Aptian stage.
Note: Data source includes: Diao, N. C.; Wang, D. F.; Guo, H.Z.; Zhang, Z.C.; Smith; Li, P. X.; Wang, S. S.; Luo, Z. K.; Wang, D. Y.; Zhu, R. X.; Zhu, Q. J.; Zhang, X. H.; Peng, Y. D.; Zhang, H. F.; Zhen, J. P.; Wang, W. L.; Ji, Q.; Yang, W.; Zhang, H.; Li, D. M.; Huang, H.; Shao, J. A.; Xiao, G. Q.; Wang, Q. L.; Meng, F. X., and this paper.
Figure 2 Summarizing of Mesozoic and Paleogene volcanic rock dating in western Liaoning
It is an indisputable fact that the lithospheric mantle in North China Craton was thinned on a large scale in the Mesozoic and Cenozoic Periods (Gao et al., 2004; Liu et al., 2004). The earth's crust in west Liaoning Province was also thinned when volcanic rock in the Yixian Formation was formed (Zhang, 2003; Zhang, 2005). The time when Yixian Formation was formed is also the critical period when the earth's crust (lithosphère) in Northeast China was thinned, so the age of top and bottom boundaries of volcanic rock in Yixian Formation is the critical period when the earth's crust (lithosphere) in west Liaoning Province, even Northeast China, was thinned.
Conclusions
The horizon and age of volcanic rock play an essential role in understanding fossil beds, structures, and sedimentary evolution of West Liaoning Province and coal seeking. During this work, 11 volcanic rock samples were measured by SHRIMP U-Pb isotope analysis. Based on the reported data on the age of the Mesozoic volcanic rock in West Liaoning Province, in combination with new measurement data, Cretaceous volcanic activities in West Liaoning Province can be divided into five stages, namely 132±1 Ma, 126±1 Ma, 122±2 Ma, 115±2 Ma, and 100±5 Ma. Based on the statistical result, this paper concluded that the thinning time of the crust in Northeast China is from 132±1 Ma to 115±2 Ma.
