| China Geological Survey Chinese Academy of Geological Sciences | Host |
| Citation: |
Xiao-qing Zhu, Ming-jian Wang, Xi Mei, Zhi-xian Tian, Shi-pu Bi, Long Huang, 2022. Discovery of the Mid‒Late Jurassic volcanic rocks (163.4 Ma) in Nanhuangcheng Island, Shandong Province, China, China Geology, 5, 191-192. doi: 10.31035/cg2021028
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Changshan Islands are located on the geographical boundary between the Bohai Sea and the North Yellow Sea, China. Intensively tectonic deformation developed in this area, which is an important connection point to study the tectonics of the Shandong Peninsula and Liaoning Peninsula. Previous studies have shown that the lithologies of the three northern islands (Beihuangcheng Island, Nanhuangcheng Island, and Xiaoqin Island) of the Changshan Islands include Neoproterozoic quartzite, phyllite, and slate (Fuzikuang Formation of the Penglai Group), and a few areas are covered by Quaternary slope deposits, marine deposits and loess (Fig. 1a; Qiao EW et al., 2019). Recently, a set of volcanic rocks was firstly discovered in Nanhuangcheng Island (Fig. 1a). The volcanic rocks are about 0.7 m thick, contacting with a conglomerate layer with a thickness of about 1.5 m (Fig. 1b). The basal Neoproterozoic quartzite (Fuzikuang Formation) is overlain by the assemblage unit of the volcanic rocks and conglomerate layer (Fig. 1b). This study focuses on the first evidence for the discovery of the volcanic rocks and presents a set of high-precision chronological data on the volcanic rocks in Nanhuangcheng Island.
Samples were taken from the lower part of the volcanic rocks (Fig. 1b). Thin section grinding, zircon separation, zircon target preparation, and cathodoluminescence (CL) imaging were carried out at Langfang Chengxin Geological Service Co., Ltd, Hebei Province, China. Zircon U-Pb analyses were performed at the Beijing Quick-Thermo Science & Technology Co., Ltd, using an ESI New Wave NWR 193UC (TwoVol2) laser ablation system connected to an Agilent 8900 ICP-QQQ. Individual zircon grains (mounted and polished in epoxy) were ablated in a constant stream of He that is mixed downstream with N2 and Ar before entering the torch region of the ICP-QQQ. After a warmup of the ICP-QQQ and connection with the laser ablation system, the ICPMS is first tuned for robust plasma conditions by optimizing laser and ICP-QQQ setting, monitoring 232Th16O+/232Th+ ratios (always ≤0.2%) and 238U+/232Th+ ratios (always between 0.95 and 1.05) while ablating NIST SRM 612 in line scan mode. The 91500 zircon was used as primary reference material for all U-Pb age determinations, while the zircon PleŠovice was used as a secondary reference. Background subtraction and correction for laser downhole elemental fractionation for the ratio were performed using the Iolite data reduction package within the Wavemetrics Igor Pro data analysis software (Paton C et al., 2010). Concordia plots were processed using ISOPLOT 4.15 (Ludwig K et al., 2011).
The volcanic rocks have a porphyritic texture (Figs. 1c, d). Phenocrysts mainly consist of plagioclase, biotite, and quartz. The plagioclases are subhedral to euhedral, lath-shaped, badly altered, and have been totally pseudomorph by sericite aggregate. Quartz phenocrysts show xenomorphic granular structure, rounded or embayed shape. The matrix has micropoikilitic texture and mainly consists of plagioclase, a small amount of quartz, and opaque minerals (Figs. 1c, d). As shown in the CL images (Fig. 1e), all the dated zircons have euhedral and prismatic with clear oscillatory zoning. Additionally, most zircons (20/30) have Th/U rations >0.4 (0.27‒1.05;
A set of volcanic rocks was firstly discovered in Nanhuangcheng Island, which can be identified as a set of intermediate-acid igneous rocks based on characteristics of the photomicrographs. High precision zircon U-Pb dating results indicate that this set of volcanic rocks erupted in the Mid‒Late Jurassic boundary (163.4±0.8 Ma). These dating results imply that the Changshan Islands had experienced intense volcanic activities during the first stage of the Yanshan movement (the volcanism). In addition, the assemblage of Mid‒Late Jurassic volcanic rocks and conglomerates in Nanhuangcheng Island may have been related to the conversion of tectonic regimes in East Asia during the Mid‒Late Jurassic.
Xiao-Qing Zhu and Ming-Jian Wang conceived of the presented idea. Xi Mei encouraged Xiao-Qing Zhu to investigate the volcanic rocks in Nanhuangcheng Island. Zhi-Xian Tian, Shi-Pu Bi, and Long Huang participated in field sampling. All authors discussed the results and contributed to the final manuscript.
The authors declare no conflict of interest.
This study was financially supported by the project of the China Geological Survey (DD20190208).
| Ludwig K. 2011. Isoplot v. 4.15: A Geochronological Toolkit for Microsoft Excel. Berkeley Geochronology Center Special Publication, 4, 75. |
| Qiao EW, Peng H, Ma XM. 2019. The Surrounding rock condition exploration and construction method analysis of cross-Bohai strait passage. Journal of Geomechanics, 25(4), 563–573 (in Chinese with English abstract). doi: 10. 12090/j.issn.1006-6616. 2019.25.04.055. |
| Paton C, Woodhead JD, Hellstrom JC, Hergt JM, Greig A, Maas R. 2010. Improved laser ablation U-Pb zircon geochronology through robust downhole fractionation correction. Geochemistry Geophysics Geosystems, 11, Q0AA06. doi: 10.1029/2009GC002618. |
Figures(1)
Xiao-qing Zhu, Ming-jian Wang, Xi Mei, Zhi-xian Tian, Shi-pu Bi, Long Huang, 2022. Discovery of the Mid‒Late Jurassic volcanic rocks (163.4 Ma) in Nanhuangcheng Island, Shandong Province, China, China Geology, 5, 191-192. doi: 10.31035/cg2021028
a‒Simplified geological map of the study area; b‒field photo and the sketch of J2-3 volcanic rocks in Nanhuangcheng Island; c, d‒photomicrographs of volcanic rocks in Nanhuangcheng Island, Qtz‒quartz, Pl‒plagioclase, Bit‒biotite; e‒CL images; f‒zircon U-Pb age Concordia diagram and weighted average age diagram.
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