ISSN 1794-6190 e-ISSN 2339-3459 https://doi.org/10.15446/esrj.v28n3.113685 EARTH SCIENCES RESEARCH JOURNAL Earth Sci. Res. J. Vol. 28, No. 3 (September, 2024): 287 - 295 G EO LO G Y SHRIMP Zircon U-Pb Dating on the Early Cretaceous Volcanics in Zhangwu-Heishan Area, West Liaoning Province, China and its Geological Significance Xinxian Xu, Qiji Fu* Hainan Survey Institute of Mineral Resources, Haikou 571100, China * Corresponding author: 23241252000136@hainanu.edu.cn How to cite item: Xu, X., & Fu, Q. (2024). SHRIMP Zircon U-Pb Dating on the Early Cretaceous Volcanics in Zhangwu- Heishan Area, West Liaoning Province, China and its Geological Significance. Earth Sciences Research Journal, 28(3), 287-295. https://doi.org/10.15446/esrj. v28n3.113685 Record Manuscript received: 26/03/2024 Accepted for publication: 11/10/2024 ABSTRACT Due to special structural location and the output of precious fossils, west Liaoning Province has become a hot subject of research. The horizon and age of volcanic rock play an important role in understanding biostratigraphy, structural and sedimentary evolution of west Liaoning Province and local coal exploration. During this work, over 20 volcanic rock sam- ples were collected from Fuxin-Yixian Basin and Zhangwu-Heishan Area, and 11 samples were performed SHRIMP U-Pb isotope analysis. Combining the new data obtained with the age of Mesozoic volcanic rock in west Liaoning Province, Cre- taceous volcanic activities in west Liaoning Province can be divided into 5 stages, namely 132±1Ma, 126±1Ma, 122±2Ma, 115±2Ma and 100±5Ma. In reference to field observation, the volcanic rock in Zhangwu-Heishan Area includes 3 suites of early Cretaceous volcanic rock and 1 suite of Paleogene olivine-enclave-bearing basalt. Overlying Jiufotang Formation is a volcanic rock series, which occurs at Jianguo of Fuxin, Yaocangtu of Heishan County, Baizitun in the east of Faku County and more, is temporarily named as Gangtaishan Formation or is incorporated into Jiufotang Formation. Igneous rock se- ries of Dalinghe Formation (Daxingzhuang Formation) overlying Sunjiawan Formation of early Cretaceous System wasn’t found in Fuxin-Yixian Basin and Zhangwu-Heishan Area, so it is proposed that this formation is absent in the study area. The stratigraphic succession of Zhangwu-Heishan Area was clarified and the conclusion that no Cretaceous coal seam can be found below volcanic rock in Zhangwu-Heishan Area was drawn. The inherited zircon of Archean Eon was found in volcanic rock samples from Yixian Formation of early Cretaceous System. Keywords: West Liaoning Province; Zhangwu-Heishan Area; Mesozoic Volcanic Rock; SHRIMP U-Pb Isotopic Age; Geological Significance; Palabras clave: occidente de la provincia Liaoning; área de Zhangwu-Heishan; rocas volcánicas del Mesozoico; datación isotópica SHRIMP U-Pb; significación geológica. Datación U-Pb SHRIMP de rocas volcánicas del Cretácico inferior en el área de Zhangwu-Heishan, al occidente de la provincia china de Liaoning y su importancia geológica Debido a su ubicación estructural especial y el desarrollo de fósiles valiosos, el occidente de la provincia china de Liaoning se ha convertido en un centro de interés para la investigación. El horizonte y la edad de las rocas volcánicas juegan un papel importante en el conocimiento de la bioestratigrafía y la evolución estructural y sedimentaria del occidente de la provincia y de la exploración local de carbón. Durante este trabajo más de veinte muestra de rocas volcánicas recolectadas en la cuenca de Fuxin-Yixian y en el área de Zhangwu-Heishan, y a once de estas muestras se les realizaron análisis de datación isotópica SHRIMP U-Pb (Datación U-Pb a través de la Microsonda de Iones de Alta-Resolución). Al combinar la nueva información obtenida con la edad de las rocas volcánicas del Mesozoico en el occidente de la provincia de Liao- ning, las actividades volcánicas del cretácico pueden dividirse en cinco etapas: 132±1Ma, 126±1Ma, 122±2Ma, 115±2Ma y 100±5Ma. En cuanto a la observación de campo, las rocas volcánicas del área de Zhangwu-Heishan incluyen tres patrones de rocas volcánicas del Cretácico inferior y un patrón de basalto paleógeno con enclave de olivino. Una serie de rocas volcánicas cubre la formación Jiufotang, las cuales se encuentran en Jianguo de Fuxin, condado Yaocangtu de Heishan, y Bazitun, en el oriente del condado Faku, y se denomina temporalmente formación Gangtaishan o se incorpora a la formación Jiufotang. Las series de rocas igneas de la formación Dalinghe (formación Daxingzhuang) que se superponen a la formación Sunjiawan del sistema cretácico inferior no fueron encontradas en la cuenca de Fuxin-Yixian ni en el área Zhangwu-Heishan, por lo que se propone que esta formación es ausente en el área de estudio. La sucesión estratigráfica del área de Zhangwu-Heishan fue clarificada y surgió la conclusión de que una veta de carbón del Cretácico podría ser hallada bajo sus rocas volcánicas. El circón heredado del Eón Arcaico se encontró en muestras de roca volcánica de la formación Yixian, del sistema cretácico temprano. RESUMEN https://doi.org/10.15446/esrj.v28n3.113685 mailto:23241252000136@hainanu.edu.cn https://doi.org/10.15446/esrj.v28n3.113685 https://doi.org/10.15446/esrj.v28n3.113685 288 Xinxian Xu, Qiji Fu 1. Geologic Setting Zhangwu-Heishan area is located to the east of Fuxin-Yixian basin. Besides the Quaternary soil, the study area is mostly covered with volcanic rock, which is dominated by the intermediate-basic andesite, trachyandesite and basaltic andesite, with a minority taken by the dacite, rhyolite etc. (Figure 1). In the research history, various workers have proposed different nomenclatures, evolutional processes, and chronostratigraphy for those igneous rocks. It is thus important and necessary to unify those discrepancies, which have brought considerable inconveniences and potential issues for regional correlation, biostratigraphic studies and resources exploration. And this is the main effort this paper aiming to contribute. Field investigation starts with Yixian Basin which has been densely studied by various investigators, and gradually advances to the north and northeast parts of Fuxin-Yixian Basin. Key volcanic outcrops are highlighted and recorded; meanwhile, igneous samples are collected from outcrops or rock cores in different horizons for SHRIMP U-Pb isotopic dating. Combining with data from previous researches, the age and main eruption periods of the above volcanic rock will be generally delineated, and the sequence relationships between volcanic rock and main coal-bearing strata will be identified to provide possible assistances for future geological researches. In the middle and late Jurassic Period, due to southward movement of Siberian Plate and westward subduction of Pacific Plate, west Liaoning Province was extruded in the direction of NW-SE and mainly features NE- and NNE-trending structures (Wang, 2001; Ren, 2002). Adjacent to Inner Mongolia-Xing’an Orogenic Belt and Songliao Basin, West Liaoning Province is bordered by Chifeng-Kaiyuan Fracture to the north, connected to EW- trending Yanshan Orogenic Belt to the west, and separated from Liaodong Uplift Zone by Yilan-Yitong Fracture Zone to the east. Its tectonic position belongs to transition zone of North China Block, Liaodong Uplift Zone and Inner Mongolia-Xing’an Orogenic Belt (Chen et al., 1997). The study area is mainly characterized by NE- and NNE-trending faulted structures. Although nappe structures, extensional structures and strike-slip structures are developed in this area, extensional activities dominated during Yixianian. The major axis of Mesozoic sedimentary basin in this area is mainly NNE-trending. With abundant volcanic rock developed during Yixianian, temporal basin was obviously controlled by basin-margin extensional fractures, which are mainly characterized by NNE-trending fractures. During Jiufotangian Period, graben faulting in west Liaoning Province was further developed; the volcanism calmed though, with tuffaceous materials comprise the majority of this formation. Subsequently, a relatively small-scaled intermediate-basic volcanic eruption occurred and the basin gradually received the sediment from Shahai Formation and Fuxin Formation of early Cretaceous Epoch and Sunjiawan Formation of late Cretaceous Epoch. The transition of Mesozoic structures in west Liaoning Province shows very good spatiotemporal coupling between mantle process and lithosphere thinning in deep part. Figure 1. Geologic Map of Zhangwu-Heishan Area and Sampling Location 289SHRIMP Zircon U-Pb Dating on the Early Cretaceous Volcanics in Zhangwu-Heishan Area, West Liaoning Province, China and its Geological Significance 2. Sampling and Test The reported data on the absolute age of volcanic rock in this area are mainly derived from K-Ar or Ar-Ar method. During this research, 11 volcanic rock samples are collected and analyzed with higher-precision SHRIMP U-Pb isotopic dating method. Specific sample ID, sampling location, lithology determined through slice identification and more are shown in Figure 1 and Table 1. It is should be noted that results of the same sample are subject to different testing methods. Therefore, during this work, 6 samples are collected from Yixian Basin so as to accurately correlate the age of volcanic rock in Zhangwu-Heishan Area with that in Fuxin-Yixian Basin and provide compelling evidence. Test samples used by SHRIMP dating method are the zircon selected from volcanic rock samples. During the testing, outer margin should be distinguished from the core, because the outer margin of zircon is younger than the core. If the test result is derived from chip zircon samples, it may be not the outer margin, result in that the value of absolute age is relatively higher, and can’t reflect the accurate age of volcanic rock. Sample N1 is from the bottom of Borehole BS13 in Badaohao exploration area, Heishan County (Cai et al., 2008 & 2009). Zhangluotun sample (N2) is of quartz trachyandesite, is distributed on the southeast side of Badaohao, obviously intrudes into the tuffite and tuffaceous clastic rock, generally inclines to the inside of volcanic conduit and has large obliquity, and fluidal structures are developed inside the trachyandesite and are almost erective at volcanic conduit (Plate I-7). Liangshan trachyandesite (N3) lies in the northwest of Liangshan Township, takes the form of independent small hill due to weathering and is covered with Quaternary System; lava flow unconformably covers the sedimentary rock in a small hill to the north of Shuangshanzi; and 30-40cm-thick baked layer lies on its top. The trachyandesite is greenish black and has compact and even structure; fluidal structures and columnar jointing are developed, the former of which has almost erective occurrence; and the samples are collected from volcanic conduit (Plate I-1). Sample N4 is collected from Nalishan of Yaocangtu. Sample N5 is from Dongliangshan of Heishan County. The samples for isotopic dating of Fuxin-Yixian Basin are collected from Beizhuanchengzi in the southwest of Qinghemen, Huanghuashan in the southeast of Yi County, Lijiatai in the southeast of Juliangtun of Yi County (Figures 3-4), Xinglonggou in the southeast of Yi County (Figures 3-4), Songbahu in the southeast of Yi County, Tonglvgou, and Luotaishan in the north of Badaohao. Sampling locations are detailed as Figure 1. Experimental procedures are as follows: crush and screen volcanic rock samples, select automorphic zircon crystals which aren’t ground and have no inclusion, and it should be clean and transparent under binocular eyepiece; then combine several selected zircon samples together to make them into resin target; polish the target; and perform the photomicregraph of reflected light, transmitted light and cathode luminescence. Cathode luminescence is performed in Chinese Academy of Geological Sciences and U-Pb isotope analysis is conducted with SHRIMPII-type ionic probe in SHRIMP Center of the Institute of Geology of Chinese Academy of Geological Sciences. During the analysis, standard zircon TEM (417Ma) is used for fractionation correction between elements. For detailed SHRIMP analysis method and process, please refer to Williams (1987). The data are processed with Ludwig SQUID1.0 program and ISOPLOT program (1999 & 2001), and decay constant is the value recommended by Steiger et al. (1997). Common lead correction is implemented through directly determining 204Pb. The dating data used by this paper are the average of 5 values measured at the same survey point and have the error of 1σ. 206Pb/238U age is used for all samples and the weighted average has the confidence level of 95%. 3. Result and Discussion of Isotopic Dating Result of Isotopic Dating During this work, totally 20 volcanic rock samples are collected from Fuxin-Yixian Basin and Zhangwu-Heishan Area, 15 of which are sent to the laboratory for testing. SHRIMP method was performed for 11 samples, within which only 1 sample indicates a Paleogene eruptive origin, the rest 10 samples are all from eruptive rock in the medium term of early Cretaceous Period. Sampling locations are shown in figure 2. Part of zircon is relatively old and the zircon tested may be ancient zircon taken up during magmatic eruption. The breccia in Huanghuashan is the upperest lithological segment (Ren et al., 1997; Zhang et al., 2004) of Yixian Formation and its age is in line with the top boundary of Yixian Formation. During this work, the age determined is 126.3±1.7Ma, which is possible over-estimated the real age since the zircon tested is mainly from volcanic rubble in the samples, and can’t represent the accurate age of this horizon. The ID and sampling location of selected zircon, concordia curve of corresponding age, and statistic histogram are shown as Figure 2. Discussion 1) Age and Sequence of Volcanic Rock in Zhangwu-Heishan Area During field work, volcanic sequence and eruption period are investigated and it is deemed that 3 suites of Cretaceous volcanic rock in Zhangwu-Heishan Area can be distinguished. The first set of volcanic rock lies in the lowest horizon, is of intermediate-basic basaltic andesite, features a large amount of vesicular amygdaloidal structure, is the result of fissure eruption, and was formed during tectonic activities. The second set lies in the middle horizon, is of intermediate trachyandesite, features trachyandesite and bentonite deposit produced by the alteration, and was formed during tectonic stabilization period. The third set lies in the highest horizon, is of compact intermediate-basic basaltic andesite, is the result of fissure eruption, is mainly characterized by hypabyssal andesite and unconformably overlies the bentonite or other older strata, and lava flow produced about-30cm-thick baked layer in underlying strata (Plate I-6). The third set of volcanic rock should be Gangtaishan Formation in Xinlitun named by Tan in 1926, is contemporaneous with Yushugou Formation to the west of Yi County, and is also known as Daxingzhuang Formation in 1:200,000 geologic map by Regional Survey Team of Liaoning Province and Zhanglaogongtun Formation denominated by Wang (2004). From the aspect of lithology and eruptive sequence, correlated with contemporaneous volcanic rock in Yi County, the first set of basaltic andesite is equivalent to the second set of basaltic andesite in Yixian Formation; the second set of intermediate trachyandesite and bentonite are comparable with the breccia in Huanghuashan and should be included in Huanghuashan Formation; and the third set of intermediate-basic basaltic andesite overlies the trachyandesite, underlies Badaohao Formation, should belong to volcanic rock in the medium term of early Cretaceous Period, and is corresponding to Gangtaishan Formation or Yushugou Formation, Daxingzhuang Formation and Zhanglaogongtun Formation. The outcrops of the first set of early Cretaceous volcanic rock include: basaltic andesite in Shijiazi Township to the northeast of Xinlitun (Plate I-4) features a large amount of vesicular and amygdaloidal structures and is abundant in agate. The hypabyssal andesite in Luotaishan overlies the outcrop of coal-bearing series of Beipiao Formation of early Jurassic Epoch and is the lowest intermediate volcanic rock of early Cretaceous Epoch. This set of volcanic rock is named as Daxingzhuang Formation in 1:200,000 geologic map, which may be wrong, has the age of 129.0±2.4Ma determined by SHRIMP method, and should be included in Yixian Formation. The hypabyssal andesite in Shuangshanzi of Liangshan is distributed in Shuangshanzi in the southwest of Liangshan Township in the west of Xinmin County, takes the form of independent small hill due to weathering and is covered with Quaternary System; lava flow unconformably covers the sedimentary rock in a small hill; and 30-40cm thick baked layer lies on its top. The andesite is dark gray and has compact and even structure; fluidal structures and columnar jointing are developed (Plate I-1), the former of which has almost erective occurrence; and the samples are collected from volcanic conduit. This set of volcanic rock is also denominated as Daxingzhuang Formation in 1:200,000 geologic map, has isotopic age of 129.5±1.6Ma and 129.7±1.6Ma determined with SHRIMP method, belongs to older volcanic rock and can be correlated with Yixian Formation in Yi County. The hypabyssal andesite in Zhangluotun to the southeast of Badaohao Coal Mine obviously intrudes into the tuffite and tuffaceous clastic rock, generally inclines to the inside of volcanic conduit and has the obliquity of 40°-50°, and fluidal structures are developed inside the trachyandesite and are almost erective at volcanic conduit. This set of andesite which was originally denominated as Yushugou Formation or Daxingzhuang Formation has the isotopic age of 124.9±2.9Ma and 122.2±4.1Ma determined with SHRIMP method, belongs to older volcanic rock and can be correlated with Yixian Formation (Plate I-7). 290 Xinxian Xu, Qiji Fu Table 1. Summary of SHRIMP U-Pb Isotopic Testing Result of Volcanic Rock in Fuxin and Its Peripheral Areas Sample ID Sampling Location Color Lithology by Slice Identification Age (Ma) Volcanic Eruption Period Horizon in 1:200,000 Geologic Map (1972) Original Horizon in 1:50,000 Geologic Map by the 107th Brigade Horizon Currently Determined N1 1160m in Borehole BS-13 at Badaohao Grayish Green Trachyandesite 125.7±1.7 ②-③ J3t Tuhulu Formation K1y Yixian Formation N2 Zhangluotun to the Southeast of Badaohao Purplish Grey Rhyolite 124.9±2.9 ②-③ J3t Tuhulu Formation K1t Tuhulu Formation N3 To the East of Yaocangtu in Heishan County Grey Black Quartz Trachyandesite 115.5±1.5 ④ J3t Tuhulu Formation Yixian Formation K1g Gangtaishan Formation N4 Nalishan in Yaocangtu Grey Black Basalt 56±2.9 Covered with Quaternary Covered with Quaternary E Paleogene System N5 Dongliangshan in Heishan County Grey Black Trachyandesite 129.7±1.6 ② J3t Tuhulu Formation Daxingzhuang Formation K1y Yixian Formation N6 Beizhuanchengzi in the Southwest of Qinghemen Grey Andesite 132.3±2.3 ① J3t Tuhulu Formation Yixian Formation K1y Yixian Formation N8 Huanghuashan in the Southeast of Yi County Grayish Yellow Dacite 126.3±1.7 ② J3t Tuhulu Formation Yixian Formation K1t Tuhulu Formation N11 Lijiatai in the southeast of Yi County Claret Trachyandesite 125.8±1.9 ② J3y Yixian Formation Dalinghe Formation K1t Tuhulu Formation N12 Xinglonggou in the Southeast of Yi County Claret Andesite 127.0±5.1 ② J3y Yixian Formation Daxingzhuang Formation K1t Tuhulu Formation N13 Tonglvgou of Songbahu in the Southeast of Yi County Caesious Andesite 122.3±4.5 ③ J3t Tuhulu Formation Yixian Formation K1t Tuhulu Formation N14 Luotaishan in the North of Badaohao Offwhite Andesite 129.0±2.4 ①-② J3t Tuhulu Formation Yixian Formation K1y Yixian Formation The second set of early Cretaceous volcanic rock features the trachyandesite and bentonite, and bentonite deposits are found in Hala and Shaojianshanzi in the southwest of Zhangwu County, Xiashitu of Yaocangtu in the northwest of Xinlitun and Douyawopu in the southeast of Zhangwu County. Bentonite deposits are mainly composed of montmorillonite minerals, which can account for over 80% and are of volcanic genesis and sedimentary genesis. Sedimentogenic bentonite is the result of hydrolysis, diagenesis, weathering or low-temperature hydrothermal alteration of intermediate-basic volcanic rock, especially trachyandesitic pyroclast or volcanic ash falling into lake basin, should be contemporaneous with andesitic volcanic eruption in Huanghuashan, and can represent the product of the second set of volcanic eruption in this area. The outcrops include: 1) Bentonite deposits in Hala and Shaojianshanzi areas are located in the southeast of Zhangwu County and between Paozi and Daguben, are covered with the andesite later eroded, and overlie clastic rock (Plate I-5 and Figure 4-2). The bentonite is closely related to the trachyandesite in Huanghuashan in terms of composition and genesis, and shows the interruption in deposition with the overlying basalt andesite, which should be Gangtaishan Formation (Figure 4-1), Yushugou Formation or Daxingzhuang Formation. 2) The benonite in Yaocangtu is distributed in Xiashitu of Yaocangtu in the northwest of Xinlitu (Plate I-8), is covered with the hypabyssal andesite later erupted, and shows the interruption in deposition between them. Bentonite layer overlies a set of clastic rock, which becomes finer and finer bottom-up and is the result of retrogradation during the expansion of lake basin. The importance should be attached to the discovery of precious fossils in sedimentary deposit below this set of volcanic rock. 291SHRIMP Zircon U-Pb Dating on the Early Cretaceous Volcanics in Zhangwu-Heishan Area, West Liaoning Province, China and its Geological Significance Figure 2. ID and Sampling Locations of Selected Zircon, Concordia Curve of Age and Statistic Histogram 292 Xinxian Xu, Qiji Fu The third set of early Cretaceous volcanic rock features intermediate- basic basaltic andesite which overlies the bentonite, is the result of a new round of volcanic eruption, was formed in the medium term of early Cretaceous Epoch, and has the absolute age of less than 110Ma. It crops out at Hala and Shaojianshanzi in the southwest of Zhangwu County, Xiashitu of Yaocangtu in the northwest of Xinlitun and Douyawopu in the southeast of Zhangwu County. The bottom of this set of basaltic andesite often baked the underlying Jiufotang Formation, and has unconformity contact relationship with it. The layer below this set of volcanic rock is abundant in “Jehol Biota” fossils. Bentonite deposit represents an ancient denudation plane and is contemporaneous with andesitic volcanic rock in Huanghuashan. The hypabyssal andesite in Hala and Shaojianshanzi is located in the southwest of Zhangwu County and between Paozi and Daguben and overlies the bentonite, which is underlain by clastic rock. The interruption in deposition occurs between bentonite and overlying andesite (Plate I-5). The andesite in Yaocangtu lies in the northwest of Xinlitun; basaltic andesite in Xiashitu of Cangtu Township overlies the bentonite, indicating that the interruption in deposition occurs between them; and overlying basaltic andesite is the third set of volcanic rock in this area (Plate I-8) and has the absolute age of 115±1.5Ma. Extravasation-facies basaltic andesite in Douyawopu of Zhangwu County crops out at Douyawopu in the southeast of Zhangwu County, is the result of a new round of volcanic eruption after trachyandesitic eruption in this area, and shows the interruption in deposition between it and underlying sedimentary rock, and a baked layer was formed between fissure-eruption extravasation-facies basaltic andesite and underlying argillaceous rock (Plate I-6). Alkaline basalt (Plate I-3) in Jianguo of Wuhuanchi Township is observed on site and sampled, and has low zircon content due to being basic volcanic rock, so U-Pb isotopic dating analysis isn’t performed. Paleogene basalt crops, which are out of Nalishan in Dagubentaogai Township, are composed of olivine basalt. According to U-Pb isotopic dating analysis, it has the age of 56±2.9Ma and belongs to basic effusive rock of Paleogene Period. 2) Age and Sequence of Basalt in Jianguo There are 2 sets of volcanic rock in Jianguo Village, Wuhuanchi Township, Fuxin City: the upper section is black compact basalt, is of volcanic conduit facies, is developed with columnar jointing (Plate I-3) and contains the peridotite and other enallogene xenoliths, and a small amount of vesicular structures at the bottom are the result of cooling lava flow; and the lower section is grayish purple basaltic trachyandesite or augitic andesite with many pores and features high silicon, high alkali, low magnesium and low titanium (Shao et al., 2006). The basalt in the upper section has angular unconformity contact relationship with the andesite in the lower section; red brown weathered crust between them is 15-20 cm thick; and a layer of conglomerate lies below it. This set of basalt is close to Chifeng-Kaiyuan Lithospheric Fault and is adjacent to famous Tanlu Fault System to the east, and its special structural location makes the geologists perform the dating and research the sequence, genesis, magma sources and geotectonic significance of this set of basalt and underlying andesite (Zhu et al., 2002 & 2004; Yang et al., 2004; Zhang & Zheng, 2003; Zheng et al., 2004). The results of dating are greatly different from each other. Among these tests, Zhu RX et al. sampled the rock in great detail. The result of test conducted by Berkeley Geochronology Center shows that its age is 105.5±0.5Ma, which is hardly different from the age of volcanic rock at the same place or that with the same lithology at Niutoushan of Pingzhuang determined by other geologists (Shi et al., 2005; Zhang et al., 2003) and has relatively high confidence level. Therefore, this paper takes 105.5±0.5Ma determined by Zhu et al., and Yang et al. as its age. There are 3 opinions about the sequence of alkaline basalt in the upper section of Jianguo: Xu (1999), Wang et al. (2002) and Bing et al. (2003) thought the basalt in Jianguo belongs to Dalinghe Formation and is the strata above Sunjiawan Formation. Shao et al. (2006) thought that this set of basalt lies at the bottom of Fuxin Formation. Wang and Jiang (1999) investigated Fuxin Basin and adjacent Dalinghe Formation and thought that the basalt in the upper section of Jianguo should overlie Jiufotang Formation, instead of Fuxin Formation. Chang et al. (1999) also thought that volcanic rock of Yushugou Formation in Fuxin Basin can be correlated with the basalt in Jianguo and is a horizon between Jiufotang Formation and Shahai Formation. Shao et al. (2006) placing alkaline basalt in the upper section of Jianguo to the bottom of Fuxin Formation is based on insufficient evidence, because they used the recommended value in International Stratigraphic Chart, instead of test result. Based on field investigation, the author of this paper found that calcirudite rock below the basalt in Jianguo is of tuffaceous cementation, and the gravel has complex composition, but contains no granite gneiss gravel frequently found in Shahai Formation and Fuxin Formation and gets lithological features of Jiufotang Formation. No volcanic rock in Fuxin-Yixian Basin or between Shahai Formation and Fuxin Formation is reported; the basalt not only lies above Jiufotang Formation in this area, but also overlies or intrudes into Jiufotang Formation in Dawulaba of Zhangwu County (Figure 3-1), Jianshanzi of Heishan County (Figure 3-2) and Dongbaizitun of Faku County (Figure 3-3). Isotopic dating shows that this set of basalt is the product in the late term of early Cretaceous Epoch, so the possibility that it belongs to Daxingzhuang Formation (Dalinghe Formation) of late Cretaceous Epoch is eliminated. Based on the above comprehensive analysis, this set of basalt in the upper section of Jianguo lies above Jiufotang Formation and below Shahai Formation, is the result of volcanic eruption in the medium term of early Cretaceous Epoch, and has far smaller distribution range than Yixian Formation. Figure 3. Field Sketch of Study Area. 1 - Conglomerate, 2 - Sandstone, 3 - Mudstone, 4 - Marlite, 5 - Trachyandesite, 6 - Andesite, 7 - Volcanic Agglomerate, 8 - Volcanic Breccia, 9 - Breccia-bearing Tuff, 10 - Rhyolitic Tuff, 11 - Ferriallophanoid, 12 - Quaternary System, 13 - Lower Pleistocene Series; 14 - Fuxin Formation, 15 - Shahai Formation, 16 - Yixian Formation, 17 - Hypabyssal Trachyandesite, 18 - Jiufotang Formation 3) Volcanic Cycle of Early Cretaceous Epoch in Fuxin and Zhangwu-Heishan Area The early Cretaceous volcanic rock in Fuxin and Zhangwu-Heishan Area can be divided into 2 cycles: the first cycle starts with basic basalt, is represented by narrow-sense Yixian Formation and ends with intermediate trachyandesite and crater-lake deposit, namely Huanghuashan breccia and Jiufotang Formation. Lacustrine-facies deposits of Jin’gangshan Formation formed during volcanic dormant period lie between basic and intermediate volcanic rock. Isotope geologic age of volcanic rock of this cycle starts with 145±4Ma of early Cretaceous and ends with 120±2Ma or 112±3Ma, and its peak period is 126Ma to 120Ma. The strata include Yixian Formation and Jiufotang Formation. Yixian Formation can be divided into small volcanic cycles of basic basalt - intermediate andesite - acidic rhyolite and is mixed with lacustrine facies tuff. Volcanic rock of the second cycle is represented by basic basalt, has unconformity contact relationship with underlying volcanic rock or sedimentary rock, baked the underlying strata, gets the isotope geologic age of 106Ma or so, and features alkaline basalt in the upper section of Jianguo, Wuhuanchi Township, and is mainly distributed in Douyawopu and Yaocangtu in Zhangwu County. Stratigraphic units include Gangtaishan Formation (Tan, 1926), Dalinghe Formation, Daxingzhuang Formation, Yushugou Formation, Zhanglaogongtun Formation (Wang et al., 1989) and Fuxin Formation (Shao et al., 2006). 293SHRIMP Zircon U-Pb Dating on the Early Cretaceous Volcanics in Zhangwu-Heishan Area, West Liaoning Province, China and its Geological Significance 4) Sequence Relationship between Volcanic Rock and Coal-bearing Strata in Zhang- wu-Heishan Area It is previously thought that the so-called Dalinghe volcanic rock which is distributed along Dalinghe Fracture overlay red bed of Sunjiawan Formation and belonged to upper Cretaceous Series, so someone suggested searching the coal below the volcanic rock. The investigation on Lijiadian- Baimiaozi to the southeast of Juliangtun shows that the above volcanic rock has the faulted contact relationship with red bed and doesn’t overlie the red bed, that the red bed originally defined doesn’t belong to Sunjiawan Formation and is the conglomerate at the bottom of Shahai Formation, and that andesitic volcanic rock of so-called Dalinghe Formation has the age of 125.8±1.9Ma (N11) and 127.0±5.1Ma (N12) according to SHRIMP method and should belongs to Yixian Formation. During the field reconnaissance in the study area, the expected Dalinghe Formation isn’t found, so it is thought that there is no volcanic rock of Dalinghe Formation. Early Cretaceous volcanic rock in Zhangwu-Heishan study area, despite of Yixian Formation of the first cycle, Gangtaishan Formation of the second cycle, or contemporaneous volcanic rock with different names (such as Daxingzhuang Formation, Yushugou Formation, Zhanglaogongtun Formation and more), has older and lower horizon than Shahai Formation (Badaohao Formation), and Jiufotang Formation which underlies Gangtaishan Formation is of crater-lake deposit after volcanic eruption, is distributed in a limited area and is less wide-spread than fault-trough lake basin genetic Shahai Formation and Fuxin Formation after intermediate- basic volcanic eruption. Jiufotang Formation barely shows the sign of coal accumulation, so it is impossible to find the coal of Shahai Formation or Fuxin Formation below the volcanic rock. 4. Conclusions Based on the research of volcanic rock in Fuxin Basin and its peripheral areas, the following conclusions could be drawn: 1. Based on the age of Mesozoic volcanic rock obtained from SHRIMP U-Pb isotope analysis of 11 volcanic rocks in west Liaoning Province in this study, Cretaceous volcanic activities in the research area can be divided into 5 stages, namely 132±1Ma, 126±1Ma, 122±2Ma, 115±2Ma and 100±5Ma. 2. There are 2 sets of early Cretaceous volcanic rock and 1 set of Paleogene volcanic rock in Fuxin Basin and Zhangwu-Heishan area: early Cretaceous volcanic rock is thick and widespread, has various lithologies and complicated denominations, while Paleogene basic basalt is mainly located at Nalishan to the southeast of Yemataohai. Early Cretaceous volcanic rock in the studying area can be divided into 2 cycles. The first cycle starts with basic basalt, is represented by narrow-sensed Yixian Formation and ends with intermediate trachyandesite and crater-lake deposit, namely Huanghuashan breccia and Jiufotang Formation. Several lacustrine sediments were deposited during volcanic dormant period lying between basic and intermediate volcanic rock. Basic basalt, with the isotopic age of about 106Ma-108Ma, represents the second cycle; it unconformablly underlain volcanic rock or sedimentary rock and has baked the strata beneath it. This sequence is featured by alkaline basalt in the upper section of Jianguo, Wuhuanchi Township, and is mainly distributed in Douyawopu and Yaocangtu in Zhangwu County. 3. The expected Dalinghe Formation is absent in west Liaoning Province, and thus leads to the postulation that the Dalinghe Formation doesn’t exist in the study area and should be canceled. 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Physics of the Earth and Planetary Interiors, 2004, 142: 49-59. 295SHRIMP Zircon U-Pb Dating on the Early Cretaceous Volcanics in Zhangwu-Heishan Area, West Liaoning Province, China and its Geological Significance Appendix 1: Plate I 1) Hypabysal Andesite at Xiaoliangshan in the Northwes of Liangshan Community, Xinmin County 2) Pillow Lava and Fragmented Breccia at Dagebentaogai 3) Alkaline Basalt Developed with Cleavages at Jianguo of Wuhuanchi Township, Fuxin City 4) Basaltic Andesite with Vesicular Amygdaloidal Structure at Shijiazi Township to the Northeast of Xinlitun 5) Bentonite Deposit at Jianshazi Unconfrmably Covered with Hypabissal Andesite Later intruded 6) Baked Layer Formed through Extravasation-facies Basaltic 7) Hypabyssal Trachyandesite at Zangluotun 8) Andesite at Xiashitou of Cangtu County Overlying the Bentonite Plate 1 _heading=h.gjdgxs _heading=h.30j0zll bookmark=id.1fob9te