The Mesozoic-Cenozoic volcanic activities in East China present obvious cyclicity. The Mesozoic volcanism began in Early Jurassic, bloomed in Early Cretaceous, and ended in Late Cretaceous. The Cenozoic volcanism was relatively weak and mainly concentrated in Paleocene-Eocene and Miocene-Quaternary. According to their differences in age, rock association, temporal-spatial distribution and regional unconformity, the Mesozoic-Cenozoic volcanism can be divided into 6 volcanic cycles (Ⅰ-Ⅵ): Early Jurassic, Middle-Late Jurassic, early Early Cretaceous, late Early Cretaceous-Late Cretaceous, Paleogene and Neogene (Figs. 1, 2). The volcanic rocks of 6 cycles have different temporal-spatial patterns and unconformable contacts from each other, corresponding to different stages of tectonic evolution in East China (Cao et al., 2020; Xing and Feng, 2015).
Figure 1. (a) Simplified tectonic sketch of eastern China; (b) distribution map of the six volcanic cycles in East China.
The volcanic rocks of the Cycle Ⅰ are mainly distributed in EW-trending along the eastern Nanling area. They include the Fankeng Formation (J1f) of Fujian Province and Changpu Formation (J1c) of Jiangxi Province, and are bimodal volcanic associations of tholeiitic series. Their volcanic facies are dominated by the effusive facies, followed by the pyroclastic flow facies.
The Fankeng Formation exposes in the Yongding area of southwestern Fujian and consists of bimodal tholeiite and plagiorhyolite, the zircon U-Pb and whole-rock Re-Os isochron ages of tholelites are 178-183 Ma (Cen et al., 2016) and 175 Ma (Zhou et al., 2005), respectively. The Changpu Formation is also bimodal tholeiite and plagiorhyolite association with zircon U-Pb ages of 195-173 Ma (mainly 191-175 Ma), and crops out mainly in the Linjiang, Dongkeng, Baimianshi and Changpu basins in southern Jiangxi (Cen et al., 2016; Zhang et al., 2002; Chen et al., 1999). Their acid end-member rocks show A2-type granite-like feature. Combined with contemporary bimodal intrusive rock and layered mafic-ultramafic association, the volcanic rocks of this cycle were inferred to have formed in the post-orogenic rift setting of the Indosinian Movement (Chen et al., 1999; Gilder et al., 1996). In addition, there is a few dacitic tuff of the Maonong Formation (J1m) in central Zhejiang Province, Chen et al. (2007) and Liu L et al. (2012) reported its SHRIMP and LA-ICP-MS zircon U-Pb ages to be 180 and 177 Ma, respectively (Table 1). Except in East China, the volcanic rocks of this cycle are also distributed in other regions of South China and have been dated to be 195-175 Ma in age (Cao et al., 2020).
Formation Rock type Method Age (Ma) Location Data source Maonong Dacitic crystal tuff SHRIMP zircon U-Pb 180±4 Songyang, Zhejiang Chen et al., 2007 Dacitic crystal tuff LA-ICP-MS zircon U-Pb 177.4±1 Songyang, Zhejiang Liu L et al., 2012 Fankeng Basalt Total rock Rb-Sr 177 Yongding, Fujian Xing et al., 2002 Rhyolite 179 Yongding, Fujian Xu, 1992 Basalt LA-ICP-MS zircon U-Pb 170±0.8 Yongding, Fujian Deng et al., 2004 Basalt Total rock Re-Os 175±3 Yongding, Fujian Zhou et al., 2005 Basalt LA-ICP-MS zircon U-Pb 189±6 Yongding, Fujian Xu et al., 2019 Rhyolite LA-ICP-MS zircon U-Pb 184±2 Yongding, Fujian Xu et al., 2019 Changpu Basalt Total rock Rb-Sr 178±7.2 Longnan, Jiangxi Chen et al., 1999 Basalt 173.7±2.5 Longnan, Jiangxi Zhang et al., 2002 Basalt 175.6 Longnan, Jiangxi Lai and Wang, 1996 Basalt 176 Xunwu, Jiangxi Basalt LA-ICP-MS zircon U-Pb 181±1 Huichang, Jiangxi He et al., 2008 Dacite SHRIMP zircon U-Pb 191±2 Changpu, Jiangxi Ji and Wu, 2010 Rhyolite SIMS zircon U-Pb 188.9±1.3 Dongkeng, Jiangxi Zhu et al., 2020
Table 1. Ages of the Cycle Ⅰ (Early Jurassic) volcanic rocks in the southeast coast of China
It is noteworthy that the mafic end-member rocks of this cycle become less significantly from southern Jiangxi to southwestern Fujian, and even absent in central Zhejiang, which indicates that the intracontinental rifting was weakening eastwardly during Early Jurassic.
This Cycle is the weakest eruption period during Mesozoic in East China. The volcanic rocks occur sporadically in northern Fujian, southern Zhejiang and central Jiangxi provinces, and are (andesite)-dacite-rhyolite association of high-K calc-alkaline series. Their volcanic facies are dominated by the pyroclastic flow facies, followed by the effusive facies. According to zircon U-Pb dating, the andesite-dacite association in Fu'an and Zhenghe of northern Fujian and dacitic-rhyolitic association in Qingyuan of southern Zhejiang are 162-150 Ma (Xing et al., 2008) and 173-169 Ma (Xing et al., 2017; Li et al., 2015), respectively; volcanic rocks of the Changlin Formation in eastern Fujian are 148-160 Ma (Liu et al., 2016), and rhyolitic rocks in the Lengshuikeng area of central Jiangxi are 161-147 Ma (Yan et al., 2016; Yu et al., 2015; Qiu et al., 2013). In addition, olivine basalt of the (olivine) basalt-andesitic tuff association of the Antang Formation (J2a) in Ji'an of central Jiangxi was dated a 39Ar-40Ar age of 168 Ma (Wang et al., 2004) (Table 2).
Formation Rock type Method Age (Ma) Location Data source Douling Dacite LA-ICP-MS zircon U-Pb 168.2±2.0 23°54′25″N, 116°23′01″E, Guangdong Guo et al., 2012 Dacite 165.0±1.0 24°05′21″N, 117°05′18″E, Fujian Rhyolite 157.5±2.0 24°23′35″N, 117°13′17″E, Fujian Rhyolite 157.8±1.0 24°47′15″N, 117°50′01″E, Fujian Dacite 148.9±1.1 23°57′50″N, 116°11′39″E, Guangdong Rhyolite Zircon SIMS 145.8±2.0 23°49′40″N, 116°07′26″E, Fujian Changlin Andesite SHRIMP zircon U-Pb 162.3±3.7 Fu'an, Fujian Xing et al., 2008 Rhyolitic crystal tuff 149.8±4.5 Rhyolitic crystal tuff LA-ICP-MS zircon U-Pb 173.6±0.8 Zhenghe, Fujian Li et al., 2015 Changlin Ignimbrite 148±1 Minqing, Fujian Liu et al., 2016 Ignimbrite 153±2 Tuff 157±1 Ignimbrite 160±2 Daguding Rhyolitic ignimbrite LA-ICP-MS zircon U-Pb 160.8±1.9 Lengshuikeng, Jiangxi Qiu et al., 2013 Ehuling Rhyolitic ignimbrite LA-ICP-MS zircon U-Pb 146.6±2.2 Antang Olivine basalt 39Ar-40Ar 168±0.3 Ji'an, Jiangxi Wang et al., 2004
Table 2. Ages of the Cycle Ⅱ (Middle-Late Jurassic) volcanic rocks in the coastal volcanic zone of southeastern China
It is worth mentioning that, although the volcanism of this cycle was very weak, the contemporary Late Jurassic (160-150 Ma) peraluminous granites were very extensive in the Nanling Mountains.
This Cycle is the peak eruption period during Mesozoic volcanism in East China. The volcanic rocks of this cycle are known as "the Lower Volcanic Series" and spread overall NE-trending across the entire area. They contain common basal conglomerates at the bottom which show obvious angular unconformity contact with the underlying strata. The "Lower Volcanic Series" are mostly between 140-120 Ma in age (Table 3), and consist of predominant dacitic-rhyolitic association of high-K calc-alkaline series as well as local bimodal basaltic-rhyolitic association at the initial stage, which manifests that they originated in the extensional lithosphere setting.
Formation Rock type Method Age (Ma) Location Data source Eastern Zhejiang Dashuang Lapilli-bearing lithic tuff LA-ICP-MS zircon U-Pb 140±1 Yiwu, Zhejiang Liu L et al., 2012 Crystal tuff 138±1 Yiwu, Zhejiang Lapilli-bearing lithic tuff 135±0.9 Yiwu, Zhejiang Gaowu Rhyolitic crystal tuff LA-ICP-MS zircon U-Pb 132±2 Tiantai, Zhejiang Liu L et al., 2012 Rhyolitic tuff 133±1 Tiantai, Zhejiang Lithic ignimbrite 135.4±0.9 Yiwu, Zhejiang Rhyolitic ignimbrite LA-ICP-MS zircon U-Pb 136±0.8 Qingyuan, Zhejiang Duan et al., 2013 Rhyolitic ignimbrite 133.2±0.6 Xishantou Rhyolitic ignimbrite LA-ICP-MS zircon U-Pb 131.9±1.9 Qingyuan, Zhejiang Duan et al., 2013 Trachydacite 127.9±0.8 Rhyolitic ignimbrite 128±1 Tiantai, Zhejiang Liu L et al., 2012 Ignimbrite 129±1 Ignimbrite 130±1 Balsalt Total rock Rb-Sr 120 Tiantai, Zhejiang Yang et al., 1999 Balsalt Total rock K-Ar 121.4±2.4 Xianju, Zhejiang Li et al., 1989 Balsalt 121.7±3 Tiantai, Zhejiang Dacitic ignimbrite Biotite 39Ar-40Ar 122.8±2 Tiantai, Zhejiang Dacitic ignimbrite Total rock Rb-Sr 122 Leqing, Zhejiang Tao et al., 2000 Dacitic ignimbrite Biotite K-Ar 124.4±2.9 Xianju, Zhejiang Rhyolitic tuff Total rock Rb-Sr 128 Hutanggang, Zhejiang Dacitic tuff 130 Rhyolitic tuff 132 Linhai, Zhejiang Xie et al., 1996 Rhyolitic tuff 125 Tiantai, Zhejiang Chawan Sedimentary tuff LA-ICP-MS zircon U-Pb 122±2 Tiantai, Zhejiang Liu L et al., 2012 Basalt SHRIMP zircon U-Pb 120±1 Qingtian, Zhejiang Cui et al., 2010 Basaltic andesite 118±2 Tiantai, Zhejiang Jiuliping Rhyolite Total rock Rb-Sr 121±1.2 Leqing, Zhejiang Tao et al., 2000 Rhyolite LA-ICP-MS zircon U-Pb 120.7±0.9 Tiantai, Zhejiang Liu L et al., 2012 Western Zhejiang Laocun Rhyolitic tuff LA-ICP-MS zircon U-Pb 132.3±1.6 Shouchang, Zhejiang Li et al., 2011 Rhyolitic tuff 130.3±3.3 Li et al., 2011 Rhyolitic tuff Biotite K-Ar 128.7±2.9 Shouchang, Zhejiang Wang et al., 2016 Ignimbrite LA-ICP-MS zircon U-Pb 131±1 Pujian, Zhejiang Liu et al., 2014 Ignimbrite 136±1 Ignimbrite 136±2 Shouchang, Zhejiang Huangjian Tuff LA-ICP-MS zircon U-Pb 127±1 Shouchang, Zhejiang Liu et al., 2014 Lithic crystal tuff 129±1 Crystal ignimbrite 130±1 Rhyolite 130±1 Pujian, Zhejiang Liu et al., 2014 Rhyolitic tuff LA-ICP-MS zircon U-Pb 122.6±3.5 Shouchang, Zhejiang Li et al., 2011 Rhyolitic tuff Sanidine K-Ar 127±6 Shouchang, Zhejiang Li et al., 1990 Rhyolitic tuff Biotite K-Ar 126±6 Rhyolitic tuff Sanidine K-Ar 127±4 Rhyolitic tuff Total rock Rb-Sr 127.9±4 Rhyolitic tuff LA-ICP-MS zircon U-Pb 130±6 Western Zhejiang Shouchang Rhyolitic tuff LA-ICP-MS zircon U-Pb 124.3±2.3 Shouchang, Zhejiang Tao et al., 2000 Rhyolitic tuff Total rock Rb-Sr 121.7±2.7 Shouchang, Zhejiang Li et al., 1989 Rhyolitic tuff K-feldspar 39Ar-40Ar 121.6±2.9 Shouchang, Zhejiang Tao et al., 2000 Rhyolitic tuff LA-ICP-MS zircon U-Pb 120.9±1.2 Shouchang, Zhejiang Li et al., 2011 Rhyolitic tuff 123.8±2 Rhyolitic tuff 133±2.4 Tuff 123±2 Shouchang, Zhejiang Liu et al., 2014 Ignimbrite 130±1 Hengshan Tuff K-feldspar 39Ar-40Ar 117.4±1.2 Shouchang, Zhejiang Li et al., 1989 Trachydacite LA-ICP-MS zircon U-Pb 118±1 Pujiang, Zhejiang Liu et al., 2014 Trachy basalt 120±1 Trachy basalt 121±1 Fujian Nanyuan Rhyolitic tuff SHRIMP Zircon U-Pb 130±4 Xianyou, Fujian Li et al., 2009 Basalt 143±7 Basalt Zircon SIMS 143±2 Xianyou, Fujian Guo et al., 2012 Rhyolite LA-ICP-MS zircon U-Pb 136±1 Rhyolitic tuff 133±2.5 Rhyolite 132±2 Dacitic tuff 131±1 Rhyolite Zircon SIMS 132±1 Dacitie 134±1 Rhyolite LA-ICP-MS zircon U-Pb 141±1 Dehua, Fujian Dacitie Zircon SIMS 133±1 Dehua, Fujian Rhyolitic ignimbrite LA-ICP-MS zircon U-Pb 143.1±0.8 Shouning, Fujian Duan et al., 2013 Rhyolitic porphyroclastic lava 140.1±1.0 Zhouning, Fujian Rhyolitic ignimbrite 140.1±1.0 Xianyou, Fujian Liu et al., 2016 Dacite, ignimbrite 142-145 Dehua, Fujian Ignimbrite LA-ICP-MS zircon U-Pb 139 Minqing Xiaoxi Rhyolitic ignimbrite LA-ICP-MS zircon U-Pb 126.2±1.7 Shouning, Fujian Duan et al., 2013 Rhyolite 127-128 Zherong, Fujian Liu et al., 2016 Crystal tuff 129-130 Zherong, Fujian
Table 3. Ages of the Cycle Ⅲ (the early stage of Early Cretaceous) volcanic rocks in the coastal volcanic zone of southeastern China
This Cycle gave rise to the coastal volcanic zone of southeastern China and the volcanic zone of the Middle-Lower Yangtze reaches. The strata in the coastal volcanic zone of southeastern China include the Moshishan Group (K11m) in eastern Zhejiang and the Jiande Group (K11j) in western Zhejiang, the Nanyuan (K11n) and Xiaoxi (K11xx) formations in Fujian, the Wuyi (K11w) and Huobashan (K11h) groups in Jiangxi. While those in the Middle-Lower Yangtze reaches include the Xihengshan (K11xh), Longwangshan (K11lw), Dawangshan (K11dw), Gushan (K11gs), Niangniangshan (K11nn) and Maotanchang (K11mt) formations in Jiangsu and Anhui provinces (Xing and Feng, 2015). The volcanic rocks of this cycle is also distributed in the whole South China, and their ages are mainly concentrated in 145-115 Ma (Cao et al., 2020).
The Moshishan Group in eastern Zhejiang consists of the Dashuang (K11d), Gaowu (K11g), Xishantou (K11x), Chawan (K11c) and Jiuliping (K11j) formations from bottom to top. Their volcanic facies are very complex, with dominant pyroclastic flow facies, followed by air-fall facies, eruption-sedimentary facies, explosion-collapse facies and effusive facies. The Dashuang Formation consists of main acidic pyroclastic rocks and minor interlayered intermediate-acidic lavas as well as pyroclastic-sedimentary rocks, with zircon U-Pb ages of 138-140 Ma (Liu L et al., 2012). The Gaowu Formation is widely distributed and mainly composed of thick, massive intermediate-acid ignimbrites, with zircon U-Pb ages of 132-136 Ma (Duan et al., 2013; Liu L et al., 2012). The Xishantou Formation is thick intermediate-acid volcanic rocks with minor volcanic-sedimentary rocks whose zircon U-Pb ages are 132-125 Ma (Duan et al., 2013; Liu L et al., 2012; Tao et al., 2000). The Chawan Formation is dominated by pyroclastic sedimentary rocks as well as minor interbedded tuff and basalt, from which zircon U-Pb ages of tuff and basalt are 122 Ma (Liu L et al., 2012) and 120-118 Ma (Cui et al., 2010), respectively. The Jiuliping Formation is composed of sparsely exposed rhyolites, with the ages concentrated around 120 Ma (Liu L et al., 2012; Tao et al., 2000). In contrast, the Jiande Group in western Zhejiang consists of volcanic-sedimentary rocks in some small volcanic-tectonic depressions, but has similar lithofacies to the Moshishan Group. It is subdivided into the Laocun (K11l), Huangjian (K11h), Shouchang (K11sc) and Hengshan (K11hs) formations from the bottom up. The Laocun Formation is dominated by tuffaceous sandstone and tuff interlayers whose ages are 129-136 Ma (Liu et al., 2014; Li et al., 2011). The Huangjian Formation, similar to the Gaowu Formation in lithology, is mainly composed of thick rhyolitic ignimbrite with ages of 123-130 Ma (Liu et al., 2014; Li X H et al., 2011; Li K Y et al., 1990). The Shouchang Formation is an equivalent stratum to the Chawan Formation, and its age is of 120-124 Ma (Liu et al., 2014; Li et al., 2011; Tao et al., 2000). The Hengshan Formation is mainly composed of trachy-dacite with a few trachy-basalt, with ages of 120-121 Ma (Liu et al., 2014).
The Nanyuan Formation, equivalent to the Gaowu and Huangjian formations of Zhejiang Province in stratigraphic horizon, is the most widely distributed volcanic stratum in Fujian and has similar lithofacies to the Moshishan Group, but with less eruption-sedimentary facies. It contains mainly intermediate-acid to acid ignimbrite and lava association, with occasional basalt interlay at the bottom (Xing et al., 2008), and their zircon U-Pb ages range from 145 to 130 Ma (Liu et al., 2016; Duan et al., 2013; Guo et al., 2012; Li et al., 2009). The Xiaoxi Formation is equivalent to the both contemporaneous Xishantou and Shouchang formations in stratigraphy, and comprises variegated sandstone and pyroclastic rock with ages of 130-126 Ma (Liu et al., 2016; Duan et al., 2013). In addition, there is limited volcanic rocks of the Xiadu Formation (K11xd) in western Fujian which is an equivalent stratum to the Xiaoxi Formation in eastern Fujian, from which the acidic rocks are of 132-135 Ma in age (Liu et al., 2016).
Cretaceous volcanic rocks in the Middle-Lower Yangtze reaches erupted between 135-125 Ma (mostly between 128-130 Ma; Yan et al., 2009), and are distributed within several volcano-tectonic depressions such as Lu-Zong, Fanchang and Huanning basins in Anhui, Lishui and Liyang basins in Jiangsu, and Ningwu Basin in the boundary area of Jiangsu-Anhui provinces (Table 4). This volcanic zone is an important volcanic Fe-Cu polymetallic belt and is famous for "porphyrite-type iron deposit". It is characteristic of the shoshonite series and minor alkaline series, obviously different from high-K calc-alkaline series in the coastal volcanic zone of southeastern China. The volcanic facies include predominant effusive facies and minor pyroclastic flow, air-fall as well as eruption-sedimentary facies.
Formation Rock type Method Age (Ma) Location (volcanic basin) Data source Longwangshan Hornblende trachyandensite SHRIMP zircon U-Pb 131±4 Ning-Wu Basin Zhang et al., 2003 134.1±1.1 Xue et al., 2015 LA-ICP-MS zircon U-Pb 134.8±1.3 Zhou et al., 2011 Breccia tuff 134.0±2.7 Wang et al., 2014 Dawangshan Andesite SHRIMP zircon U-Pb 127±3 Zhang et al., 2003 Andesite LA-ICP-MS zircon U-Pb 130.3±0.9 Hou et al., 2010 Andesite 132.2±1.6 Zhou et al., 2011 Andesite 131.4±1.8 Wang et al., 2014 Dawangshan Pyroxene dioritic porphyrite SHRIMP zircon U-Pb 127.8±1.8 Yinshan Ning-Wu Basin Xue et al. 2010 LA-ICP-MS zircon U-Pb 128.2±1 Jishan Hou et al., 2010 LA-ICP-MS zircon U-Pb 131.7±0.7 Aoshan Duan et al., 2011 126.1±0.5 Aoshan Pyroxene dioritic porphyrite LA-ICP-MS zircon U-Pb 130.2±2 Aoshan Fan et al., 2010 130.7±1.8 Taocun 131.1±1.5 Heshanqiao 131.1±3.1 Dongshan 130±1.4 Baixiangshan 131.1±1.9 Hemushan 129.2±1.7 Gushan Gushan Trachyandensite LA-ICP-MS zircon U-Pb 128.2±1.3 Ning-Wu Basin Hou and Yuan, 2010 Andesite 129.5±0.8 Zhou et al., 2011 Niangniangshan Hauyne phonolite SHRIMP zircon U-Pb 128.9±1.1 Unpublished data 129.8±1.1 Hauyne phonolite LA-ICP-MS zircon U-Pb 130.6±1.1 Yan et al., 2009 126.6±1.1 Zhou et al., 2011 Longmenyuan Hornblende trachyandesite SHRIMP zircon U-Pb 131.1±1.1 Lu-Zong Basin Xue et al., 2012 LA-ICP-MS zircon U-Pb 134.8±1.8 Zhou et al., 2011 Hornblende trachyandesitic porphyrite SHRIMP zircon U-Pb 132±1 Xue et al., 2015 Diorite porphyrite LA-ICP-MS zircon U-Pb 134.4±2.2 Zhou et al., 2011 Zhuanqiao Pyroxene trachyandesite SHRIMP zircon U-Pb 132.8±2.4 Xue et al., 2012 132.9±0.8 LA-ICP-MS zircon U-Pb 134.1±1.6 Zhou et al., 2011 Zhuanqiao Pyroxene trachyandesitic porphyrite LA-ICP-MS zircon U-Pb 133.3±0. 6 Luohe iron deposit Lu-Zong Basin Zeng et al., 2010 133.2±1.1 132.8±2.6 Nihe iron deposit Andesitic porphyrite LA-ICP-MS zircon U-Pb 133.2±1.7 Jingbian copper deposit Zhang et al., 2010 40Ar-39Ar of the inclusion 133.3±8.3 Shuangmiao Trachybasalt SHRIMP zircon U-Pb 130.1±1.2 Lu-Zong Basin Xue et al., 2012 LA-ICP-MS zircon U-Pb 130.5±0.8 Zhou et al., 2011 Fushan Trachyte 127.1±1.2 Trachytic porphyry SHRIMP zircon U-Pb 127.2±1.3 Xue et al., 2012 Quartz syenite porphyry LA-ICP-MS zircon U-Pb 127.4±2.8 Zhou et al., 2011 Syenite 126.2±1.8 Longwangshan Hornblende trachyandesite LA-ICP-MS zircon U-Pb 128.7±1.8 Lishui Basin Yu and Xu, 2009 Hornblende trachyandesite SHRIMP zircon U-Pb 129.7±1.5 Xue et al., 2015 Dawangshan Pyroxene diorite porphyrite 127.2±1.2 Penjiakou Trachyandesite LA-ICP-MS zircon U-Pb 131.6±0.6 Huaining Basin Xue et al., 2015 Trachyandesite SHRIMP zircon U-Pb 129.4±1.6 Trachyte LA-ICP-MS zircon U-Pb 128.7±0.8 Unpublished data Felsophyre 126.3±0.7 Tuff 130.0±1.7 Yan et al., 2013 Jianzhen Rhyolite 122.3±0.7 Xue et al., 2015 Rhyolite 123.5±0.7 Unpublished data Zhongfencun Rhyolite LA-ICP-MS zircon U-Pb 134.4±2.9 Fanchang Basin Yuan et al., 2010 Rhyolite 131.2±1.1 Liu C et al., 2012 Rhyolite 129.1±1.3 Liu C et al., 2012 Chisha Biotite trachyandesitic porphyrite 131.3±1.8 Yuan et al., 2010 Kedoushan Rhyolite 130.7±1.1 Yan et al., 2009 Rhyolite 130.8±2.2 Yuan et al., 2010 Huangshiba Trachyandesite SHRIMP zircon U-Pb 128±1 Chuzhou Basin Ma and Xue, 2011 Trachyandesite LA-ICP-MS zircon U-Pb 116-132 Xie et al., 2007 Longwangshan Trachyte LA-ICP-MS zircon U-Pb 140.0± 0.7 Liyang Basin Xue, 2016 Dawangshan Dacitic ignimbrite SHRIMP zircon U-Pb 129.1±1.1 Crystal tuff LA-ICP-MS zircon U-Pb 132.3±0.7 Diorite porphyrite LA-ICP-MS zircon U-Pb 132.5±0.7 Maotanchang Trachyandesite LA-ICP-MS zircon U-Pb 120-130 Hefei Basin Wang et al., 2017
Table 4. Ages of the Cycle Ⅲ (the early stage of Early Cretaceous) volcanic rocks in the Middle-Lower Yangtze River reaches
Lu-Zong Basin: The volcanic strata are divided into the Longmenyuan (K11lm), Zhuanqiao (K11zq), Shuangmiao (K11sm) and Fushan (K11fs) formations from bottom to top, with ages ranging from 140 to 123 Ma (mostly in 130-125 Ma). Their rock associations are amphibole trachyandesite and minor basaltic-trachyandesite in the Longmenyuan Formation, pyroxene trachyandesite and minor basaltic-trachyandesite in the Zhuangqiao Fomation, trachybasalt-basaltic trachyandesite and minor trachyandesite-pseudoleucite phonolite-phonolitic tephrite in the Shuangmiao Formation, alkaline series trachyte and minor phonolite-tephritic phonolite-phonolitic tephrite in the Fushan Formation (Xue et al., 2015; Fan et al., 2014; Deng et al., 2011; Zhou et al., 2011; Du et al., 2007; Xie et al., 2007).
Fanchang Basin: The volcanic rocks are shoshonite series with the zircon U-Pb ages of 135-126 Ma, and are subdivided into the Zhongfencun (K11zf), Chisha (K11cs), Kedoushan (K11kd) and Sanliangshan (K11sl) formations from bottom up. The Zhongfencun Formation is trachyandesite-dacite-rhyolite association, the Chisha Formation is trachyandesite-andesite-rhyolite association, the Kedoushan Formation is bimodal basalt-rhyolite association, and the Sanliangshan Formation is biotite trachytic lava and pyroclastic rock association (Xue et al., 2015; Liu C et al., 2012; Zhou et al., 2011; Yuan et al., 2010; Yan et al., 2009).
Huaining Basin: The volcanic rocks belong to shoshonite series and are 130-127 Ma in age. They are divided upward into the Pengjiakou (K11pj), Jiangzhen (K11jz) and Wanggongmiao (K11wg) formations. The Pengjiakou Formation includes early low-Si rhyolitic-quartz trachyte association and late trachyandesitic-trachytic association. The Jiangzhen Formation is bimodal trachybasalt-rhyolite association. The Wanggongmiao Formation is mainly composed of tuffaceous breccia (Xue et al., 2015; Yan et al., 2013).
Ningwu Basin: The volcanic rocks are subdivided into the Longwangshan, Dawangshan, Gushan and Niangniangshan formations from lower to upper, and their zircon U-Pb ages are 135-126 Ma with a peak of ~130 Ma. The Longwangshan Formation is amphibole trachyandesite and minor basaltic-trachyandesite-trachyte association. The Dawangshan and Gushan formations are pyroxene basaltictrachyte-andesite-trachyte association and minor andesite-dacite association. The Niangniangshan is leucite phonolite-noselite phonolite association of alkaline series (Xue et al., 2015; Wang et al., 2014; Zhou et al., 2011).
Lishui Basin: The volcanic rocks belong to shoshonite series with zircon U-Pb ages of 137-124 Ma, and are subdivided into the Longwangshan, Dawangshan, Gushan and Niangniangshan formations from bottom to top. Their rock associations include amphibole trachyandesite and minor shoshonite-trachyte in the Longwangshan Formation, pyroxene trachyandesite-trachyte-trachyandesite in the Dawangshan Formation, basaltic trachyandesite-andesite in the Donglushan Formation, high-K trachyte in the Guanshan Formation (Xue et al., 2015, 2010; Zhang et al., 2015; Wang et al., 2014). Besides, the latest dacite-liparite association of the Jiashan Formation (K11js) was formed in 110 Ma (Dou et al., 2015).
Liyang Basin: The volcanic rocks incorporate both high-K calc-alkaline series and shoshonite series with zircon U-Pb ages of 131-123 Ma. Their rock associations are close to those in the coastal volcanic zone of southeastern China, including amphibole andesite-trachyandesite-rhyolite association of the Longwangshan Formation, pyroxene andesite-dacite-rhyolite (porphyry) association of the Dawangshan Formation (Zhou et al., 2011; Xue et al., 2010).
The volcanic rocks of this cycle are named as "the Upper Volcanic Series" and are in unconformity contact with overlying "the Lower Volcanic Series". They are mainly distributed within the faulted basins (red-bed basins) controlled by regional NE-trending or compound NE-NW-trending faults, and crop out generally as thin interlayers within red-beds. This Cycle is characteristic of bimodal mafic-acid association with ages of 110-90 Ma (Table 5). For the whole South China, the ages of this cycle mainly concentrated in 113-85 Ma (Cao et al., 2020). In the coastal volcanic zone of southeastern China, the Upper Volcanic Series includes the Yongkang Group (k12y), Tiantai Group (k12t), Qujiang Group (k12q) and the Xiaoxiong Formation (K12xx) in Zhejiang, the Shimaoshan Group (k12s) and the Shiniushan Formation (K12sn) in Fujian, and the Ganzhou Group (k12g) in Jiangxi. In the Middle-Lower Yangtze reaches, it includes the Pukou (K12pk) and Chishan formations (K12cs) in the Jiangsu and Anhui areas along the Yangtze River, and the Xiaotian Formation (K12xt) in the North Huaiyang area of Anhui (Xing and Feng, 2015).
Formation Rock type Method Age (Ma) Location Data source Guantou Basalt LA-ICP-MS zircon U-Pb 102 Xinchang, Zhejiang Cui et al., 2010 Basalt Total rock Rb-Sr 104 Jinxian, Zhejiang Yu, 1994 Basaltic andesite K-feldspar 39Ar-40Ar 110 Fenghua, Zhejiang Li et al., 1990 Basalt Biotite K-Ar 102.9±3.1 Yongkang, Zhejiang Basalt Biotite K-Ar 109.6±0.5 Jiangshan, Zhejiang Chaochuan Basalt LA-ICP-MS Zircon U-Pb 104 Jinxian, Zhejiang Cui et al., 2010 Rhyolitic ignimbrite Total rock Rb-Sr 101.5±9.4 Pingyang, Zhejiang Xing et al., 1993 Basalt 105.2±9.9 Pingyang, Zhejiang Rhyolite 96.3±1.8 Xinchang, Zhejiang Zhou et al., 1994 Basalt 104.6 Wuyi, Zhejiang Li et al., 1990 Basalt Total rock K-Ar 110 Wuyi, Zhejiang Xiaopingtian Rhyolitic ignimbrite SHRIMP Zircon U-Pb 97.2±2.3 Yandangshan, Zhejiang Yu et al., 2006 Rhyolite 105.6±4.3 Yandangshan, Zhejiang Rhyolitic ignimbrite 99.3±3.9 Rhyolitic crystal tuff LA-ICP-MS Zircon U-Pb 114.9±1.2 Taishun, Zhejiang Duan et al., 2013 Taishun Rhyolitic crystal tuff 112.7±1.2 Taishun, Zhejian Duan et al., 2013 Rhyolitic ignimbrite 111.2±1.5 Xiaoxiong Rhyolitic tuff 93.8±0.6 Xiaoxiong, Zhejiang Liu L et al., 2012 Rhyolitic tuff 87.9±1.2 Xiaoxiong, Zhejiang Xing et al., 2009 Tangshang Rhyolitic crystal tuff 111±1 Tiantai, Zhejiang Liu L et al., 2012 Rhyolitic tuff Biotite 39Ar-40Ar 109.2±1.9 Tiantai, Zhejiang Li et al., 1990 Rhyolitic tuff Biotite K-Ar 110.9±2.3 Tiantai, Zhejiang Liangtoutang Rhyolitic tuff Biotite K-Ar 103-105 Tiantai, Zhejiang Li et al., 1990 Zhongdai Basalt Total rock K-Ar 105 Quzhou, Zhejiang Zhang, 1987 Maodian Basalt Total rock K-Ar 101.8 Xinjiang, Jiangxi Li et al., 1989 Huangkeng Basalt Total rock K-Ar 113.2 Yongtai, Fujian Feng et al., 1991 Basalt LA-ICP-MS Zircon U-Pb 107 Yongtai, Fujian Xie et al., 2001 Zhaixia Basalt Total rock K-Ar 106.8 Yongtai, Fujian Feng et al., 1991 Rhyolite 107.9 Yongtai, Fujian Shimaoshan Group Rhyolite, dacite Zircon SIMS 104-94 Dehua, Fujian Guo et al., 2012 Rhyolite, cystal tuff LA-ICP-MS Zircon U-Pb 111-105 Fuqing, Fujian Liu et al., 2016 Shiniushan Porphyroclastic lava SHRIMP Zircon U-Pb 93.8±1.3 Dehua, Fujian Xing et al., 2009 Taizhou Basalt Total rock K-Ar 83.4 Subei Basin, Jiangsu Qian and Li, 1996
Table 5. Ages of the Cycle Ⅳ (the late stage of Early Cretaceous to Late Cretaceous) volcanic rocks in the coastal volcanic zone of southeastern China
In Zhejiang Province, the Yongkang Group is subdivided into the Guantou (K12gt), Chaochuan (K12cc) and Xiaopingtian (K12xp) formations from bottom to top, and their volcanic facies are mainly pyroclastic flow facies, effusive facies and eruption-sedimentary facies. The Guantou Formation is bimodal basalt-rhyolitic association with ages of 102-110 Ma (Cui et al., 2010; Yu, 1994; Li et al., 1989). The Chaochuan and Xiaopingtian formations are mainly rhyolitic pyroclastic rocks with local basalts, and their ages range from 97 to 114 Ma (Duan et al., 2013; Yu et al., 2006; Xing et al., 1993; Li et al., 1989). The Xiaoxiong Formation is the top stratum of Mesozoic strata in Zhejiang, and it is distributed in the coastal Xiaoxiong and Ninghai basins and contains mainly purplish rhyolitic tuff-ignimbrite with zircon U-Pb ages of 87.9-93.8 Ma (Liu L et al., 2012; Xing et al., 2009). The Tiantai Group exposes mostly in the Tiantai Basin, the Tangshang Formation (K12ts) at its bottom and the Liangtoutang Formation (K12lt) at its top are both rhyolitic pyroclastic rocks with zircon U-Pb ages of 111 Ma and 103-105 Ma, respectively (Liu L et al., 2012; Li et al., 1989).
In Fujian Province, the Shimaoshan Group is subdivided into the Huangkeng (K12hk) and Zhaixia (K12zx) formations from bottom up. The two formations are bimodal basalt-rhyolite associations with ages of 105-113 and 107-108 Ma, respectively (Liu et al., 2016; Xie et al., 2001; Feng et al., 1991). Their volcanic facies are mainly pyroclastic flow facies, effusive facies and eruption-sedimentary facies. Similar to the Xiaoxiong Formation in Zhejiang, the Shiniushan Formation is the top stratum of Mesozoic strata in Fujian and dispersed mostly in the Yunshan Basin, it is dominated by extrusive facies rhyolitic porphyroclastic lavas with age of 78-94 Ma (Liu L et al., 2012; Xing et al., 2009). In addition, there is a small amount of rhyolitic ignimbrite from the Chongan Formation (K12ca) in the Taining Basin of western Fujian whose zircon U-Pb age is 98.5±1 Ma (Xing et al., 2013). There are also many mafic dike swarms of 96-87 Ma in the coastal area (Yang et al., 2010).
In southern Jiangxi Province, the Ganzhou Group contains bimodal basaltic andesite-rhyolite association of the Maodian Formation (K12md) exposed in the Huichang and Xinjiang basins, and their ages are 93-105 Ma (Wu et al., 2014; Xie et al., 2006; Li et al., 1989).
In addition, in northern Jiangsu Province, there are Late Cretaceous (83.4 Ma) basalts of the Taizhou Formation (K12tz) in the Subei Basin (Qian and Li, 1996).
The volcanic rocks of this cycle occurred in the faulted-depressed basins, such as the Subei Basin in northern Jiangsu Province and the Hefei Basin in northern Anhui Province. They are all concealed basalts intercalating among the sedimentary rocks and erupted in Paleocene-Oligocene (Table 6). Their volcanic facies include dominant effusive facies and minor subvolcanic facies, volcanic neck facies as well as eruption-sedimentary facies.
Epoch Rock type Method Age (Ma) Location Data source Paleocene Basalt K-Ar 64.5-48.1 Subei Basin, Jiangsu Qian and Li, 1996 Oligocene Basalt K-Ar 37.3-29.4 Subei Basin, Jiangsu Paleocene Basalt K-Ar 55 Luhe, Jiangsu Liu, 1992 Eocene Tholeiite K-Ar 47.1±1 Puqiao, shengzhou of Zhejiang Eocene Basic dike K-Ar 41.4-40.6 Guangfeng, NE Jiangxi Yu et al., 2001 Paleocene Tholeiite K-Ar 58.7-59.7 Hefei Basin, Anhui Cong et al., 1996 Oligocene Alkali-olivine basalt K-Ar 22.7-28.8 Hefei Basin, Anhui Cong et al., 1996 Eocene Alkali basalt Ar-Ar 37.8-38.3 Hefei Basin, Anhui Wang et al., 2011 Miocene Alkali-olivine basalt K-Ar 20.7±0.7 Xinchang, Zhejiang Liu, 1992 Miocene Basalt K-Ar 19.20-16.57 Longhai, Fujian Chen and Zhang, 1992 Miocene Alkali basalt K-Ar 18.3-4.8Ma Subei Basin, Jiangsu Qian and Li, 1996 Miocene Alkali-olivine basalt K-Ar 19.2±0.7 Liuhui of Longhai, Fujian Liu, 1992 Miocene Tholeiite K-Ar 18.1±0.8 Liuhui of Longhai, Fujian Miocene Tholeiite K-Ar 17.6±0.5 Liuhui of Longhai, Fujian Miocene Tholeiite K-Ar 19.2±0.6 Mt. Tian Ma of Longhai, Fujian Miocene Alkali-olivine basalt K-Ar 18.0±0.5 Mt. Tian Ma of Longhai, Fujian Miocene Tholeiite K-Ar 16.6±0.6 Mt. Tian Ma of Longhai, Fujian Miocene Tholeiite K-Ar 17.9±4.0 Mt. Niutou Longhai, Fujian Miocene Alkaline olivine basalt K-Ar 16.7±0.6 Mt. Niutou of Longhai, Fujian Miocene Basanite K-Ar 13.8±0.4 Mt. Niutou of Longhai, Fujian Miocene Tholeiite K-Ar 11.7±0.4 Mt. Niutou of Longhai, Fujian Miocene Tholeiite K-Ar 17.7±0.5 Xiangshan of Longhai, Fujian Miocene Alkaline olivine basalt K-Ar 17.5±0.5 Xiangshan OF Longhai, Fujian Miocene Tholeiite Ar-Ar 17.1-14.9 Longhai, Fujian Ho et al., 2003 Miocene Basalt Ar-Ar 15.6±0.7 Xiangshan of Longhai, Fujian Miocene Alkali basalt Ar-Ar 15.4±0.6 Longhai, Fujian Miocene Tholeiite Ar-Ar 14.9±0.6 Zhangpu, Fujian Miocene Alkaline olivine basalt Ar-Ar 14.1±0.3 Minqing, Fujian Liu, 1992 Miocene Alkali basalt Ar-Ar 11.9±0.4 Minqing, Fujian Ho et al., 2003 Miocene Basalt Ar-Ar 10.4±0.5 Linhai, Zhejiang Miocene Tholeiite Ar-Ar 10.5±0.5 Ninghai, Zhejiang Miocene Tholeiite Ar-Ar 9.4±0.1 Xinchang, Zhejiang Miocene Tholeiite Ar-Ar 8.4±0.4 Xinchang, Zhejiang Miocene Alkaline olivine basalt K-Ar 7.13±0.23 Xinchang, Zhejiang Liu, 1992 Miocene Alkaline olivine basalt K-Ar 6.79±0.13 Xinchang, Zhejiang Miocene Tholeiite K-Ar 6.48±0.18 Xinchang, Zhejiang Miocene Tholeiite K-Ar 5.91±0.78 Shengzhou, Zhejiang Miocene Tholeiite K-Ar 5.63±0.14 Wenling, Zhejiang Miocene Basalt Ar-Ar 5.4±0.3 Tiantai, Zhejiang Ho et al., 2003 Pliocene Basalt K-Ar 4.96-0.72 Mingxi, Fujian Chen and Zhang, 1992 Pliocene Basalt K-Ar 4.92±0.14 Mingxi, Fujian Liu, 1992 Pliocene Alkali basalt Ar-Ar 4.9±0.3 Xinchang, Zhejiang Ho et al., 2003 Pliocene Basanite K-Ar 4.64±0.07 Wenling, Zhejiang Liu, 1992 Pliocene Basanite K-Ar 4.45±0.13 Mingxi, Fujian Pliocene Tholeiite Ar-Ar 3.5±0.1 Shengzhou, Zhejiang Ho et al., 2003 Pliocene Tholeiite K-Ar 2.92±0.9 Mingxi, Fujian Liu, 1992 Pliocene Alkali basalt Ar-Ar 2.9±0.1 Shengzhou, Zhejiang Ho et al., 2003 Pliocene Alkali basalt Ar-Ar 2.9±0.1 Shengzhou, Zhejiang Ho et al., 2003 Pliocene Basalt Ar-Ar 2.5±0.1 Zhuji, Zhejiang Pliocene Alkali basalt Ar-Ar 2.2±1 Mingxi, Fujian Pliocene Tholeiite Ar-Ar 2.0±1 Qingliu, Fujian Pleistocene Tholeiite K-Ar 1.39±0.6 Mingxi, Fujian Liu, 1992 Pleistocene Andesite-basalt K-Ar 0.72±0.3 Mingxi, Fujian
Table 6. Ages of the Ⅴ (> 20 Ma) and Ⅵ (< 20 Ma) cycles of Cenozoic volcanic rocks in the East China
In the Subei Basin, the Paleocene-Oligocene volcanic centers were located at the Jinhu-Hai'an area. The concealed basalt interlayers have been found in many drilling cores with a total thickness of over 300 m, and were classified to the Funing Formation (E1f) (K-Ar ages of 64.5-48.1 Ma) and Sanduo Formation (E2s) (Ar-Ar ages of 37.3-29.4 Ma) (Yang et al., 1998; Qian and Li, 1996). The Funing Formation basalt (55 Ma) was also encountered in the Luhe-Yizheng area at western edge of the Subei Basin (Liu et al., 1992).
The Hefei Basin is bounded by the Tanlu Fault to the west side of the Subei Basin. Cretaceous trachyte-trachyte-trachyte dacite-rhyolite association (120-130 Ma, Wang et al., 2017) of the Maotanchang or Baidafan (K1bd) formations and Paleogene Dingyuan Formation (E1d) basalts are concealed in this Basin. According to their K-Ar ages, the Dingyuan Formation includes Palaeocene (59.7-58.7 Ma) tholeiite and minor alkaline olivine basalt association, Oligocene (28.8-22.7 Ma) alkaline olivine basalt and basanite association (Cong et al., 1996), and Eocene alkaline basalt association (37.8-38.3 Ma, Wang et al., 2011).
In addition, Eocene tholeiite (47 Ma) was also found in the Xinchang Basin of Zhejiang Province (Liu et al., 1992). In the Taiwan Strait, there are Palaeocene andesite and minor basalt-dacite-rhyolite association of calc-alkaline series on Huayu Island of the southwestern Penghu Islands, and the zircon U-Pb ages of the altered andesite and rhyolitic dike are of 65-63 Ma (Chen et al., 2010).
The Cenozoic volcanic activities in East China took place mainly in Neogene, with the eruption peak of Miocene-Pliocene (Table 6). The volcanic rocks of the Cycle Ⅵ are alkaline series basalts which were controlled distinctly by regional faults and developed in a continental margin rift setting. Their volcanic facies are dominated by the effusive facies, followed by the subvolcanic facies.
In the Middle-Lower Yangtze reaches, the spatial distribution of Neogene volcanic rocks is from Mingguang of Anhui Province to Luhe of Nanjing, Jiangsu Province, and obviously controlled by NW-trending faults. They erupted in Miocene to Pliocene, without Pleistocene or later volcanism. They are subdivided into two volcano groups: the Luhe-Yizheng Volcano Group in the Nanjing area of Jiangsu Province and the Jiashan Volcano Group in the Mingguang area of Anhui Province. The Luhe-Yizheng Volcano Group includes more than 10 cinder cones and flat-topped cones erupted in 16-8 Ma, and is classified into the Fangshan Formation (N2f). Its rock association is olivine basalt-alkali olivine basalt-basanite of alkaline series; they contain abundant peridotite xenoliths (spinel-lherzolite and few of spinel-harzburgite), pyroxene megacrysts and corundum (sapphire), and produced clay deposits such as attapulgite. The Jiashan Volcano Group appears as table-shaped lava platforms and is alkaline olivine basalt-tholeiite association; its latest eruptive rocks are Pleistocene (K-Ar age of 0.73-0.55 Ma) basalt-basanite-nephelinite association of alkaline series in Nushan, which are rich in mantle-derived xenoliths such as spinel lherzolite (Liu, 1999; Liu et al., 1992; Chen and Peng, 1988). Besides, in the Subei Basin also engenders Miocene-Pliocene (4.8-18.3 Ma) basalt of the Yancheng Formation (N2yc) (Qian and Li, 1996).
In the southeast coast, the Cenozoic volcanic rocks include tholeiite, alkaline basalt and picrite-nepheline basalt. They mainly originated from the depleted asthenosphere mantle, but show different geochemical characteristics due to their different sources, indicating the existence of multiple mantle end-members (Zhang G S et al., 2009; Zhao et al., 2004). In Zhejiang Province, Neogene volcanic rocks are the Shengxian Formation (N2s) basalts; they are tholeiite-alkaline olivine basalt-basanite association with ages of 21-2.5 Ma (mainly in 10-5 Ma, Ho et al., 2003; Liu et al., 1992) and distribute mostly in the coastal Shengzhou-Xinchang area. In Fujian Province, Neogene volcanic rocks, named as the Fotan Formation (N2f), make up three nearly parallel NE-trending volcanic belts in western Fujian (2.2-0.72 Ma), central Fujian (12 Ma) and eastern Fujian (19.2-15 Ma), respectively (Ho et al., 2003; Chen and Zhang, 1992; Liu et al., 1992), the eastward younger trend of these three volcanic belts implies that the rift extension was gradually strengthened from coast to inland. Combined with the spreading time of the South China Sea (30-16Ma, Chung et al., 1997) and the Sea of Japan (15-19 Ma, Jolivet et al., 1994; Uto et al., 1994), it is referred that Cenozoic rifting in the southeastern China was closely related to the spreading of marginal seas. In the Taiwan Strait, the Penghu Islands are mainly composed of Cenozoic basalts with K-Ar ages of 18-8 Ma (the age peak of 14-10 Ma; Wang, 2013; Juang and Chen, 1992) which are basanitoid-alkaline olivine basalt association of alkaline series, tholeiite association of tholeiitic series and contain mantle-derived xenolites such as spinel-lherzolite (Chen, 2001; Lee, 1994; Li et al., 1990).
1.1. Volcanic Cycle Ⅰ (Early Jurassic, 183-175 Ma)
1.2. Volcanic Cycle Ⅱ (Mid-Late Jurassic, 173-146 Ma)
1.3. Volcanic Cycle Ⅲ (Early Stage of Early Cretaceous, 143-117 Ma)
1.3.1. Coastal volcanic zone of southeastern China
1.3.2. Volcanic zone of in the Middle-Lower Yangtze reaches
1.4. Volcanic Cycle Ⅳ (Late Stage of Early Cretaceous to Late Cretaceous, 114-84 Ma)
1.5. Volcanic Cycle Ⅴ (Paleogene, 65-23 Ma)
1.6. Volcanic Cycle Ⅵ (Neogene to Quaternary, 23-0.7 Ma)
The hydrocarbon reservoir is the accumulation of oil and gas in a single trap. The igneous reservoir is one of new and great potential domains of oil and gas exploration. More than 300 igneous oil and gas reservoirs have been discovered in the world, and they were mainly formed in continental marginal basins, especially around the Circum-Pacific region (Jiang et al., 2011; Konyukhov, 2010). Igneous oil and gas reservoirs are relatively abundant in China, especially in eastern China Continent where Mesozoic faulted basins and Cenozoic depressed-rift basins have good prospects for volcanic reservoirs (Fu, 2017; Hou et al., 2012). Compared with the normal sedimentary reservoir, the volcanic reservoir is a special lithologic reservoir which takes volcanics as reservoir rock or is closely related to volcanism. Volcanic rocks can not only provide the accumulation space for oil and gas, but also serve as the caprock for oil and gas reservoirs. The discovery of hydrocarbon source rocks under volcanic reservoirs has also changed the traditional understanding that volcanic rocks act as only the basin basement, and made oil and gas exploration further to the deep (Wang et al., 2015; Jin et al., 2013). In China, more and more volcanic reservoirs have been discovered in Mesozoic-Cenozoic and minor Late Paleozoic strata (for example, the Songliao Basin, Dzungar Basin) since the beginning of this century, and volcanic rocks have been carried out a comprehensive exploration as an important oil and gas exploration field (Zhang et al., 2019; Feng et al., 2014; Zou et al., 2013, 2008; Wu et al., 2006).
There are more than 50 Mesozoic-Cenozoic basins in East China, most of them are volcanic-sedimentary basins and have become the main potential areas of volcanic reservoirs. The representative volcanic reservoirs in East China are briefly described as follows.
In East China, Mesozoic basins of volcanic reservoir are mainly faulted basins developed in late Early Cretaceous to Late Cretaceous with a few early Early Cretaceous volcano-tectonic depressions. During the early rapid extension of faulted basins, organic-rich dark mudstones were generally formed and brought about certain hydrocarbon-generating capacity. In Late Cretaceous, red beds were widely developed in faulted basins, so they are also called "red basin". Since the Cenozoic, these faulted basins have evolved into inherited depressed basins to rift-style basins, some of which deposited very thick sedimentary strata and became important oil and gas reservoirs. The volcanic oil and gas shows have been discovered in the Subei, Jurong and Changzhou basins of Jiangsu Province, the Hefei Basin of Anhui Province and the Yiyang Basin of Jiangxi Province. Among them, the Subei Basin is the only Mesozoic-Cenozoic basin that has obtained industrial oil and gas flows in East China. Besides, the Jurong Basin has been also tested to yield underproductive oil and gas flow (Wu et al., 2015; Ye et al., 2010; Guo and Lei, 1998; Jiang, 1985).
In Zhejiang Province, Mesozoic-Cenozoic basins are generally small in area and mainly filled by fluvial or shallow lacustrine deposits with relatively local deep lacustrine sediments, which preserved low organic matter contents and became argillaceous source rocks. Most Mesozoic sedimentary basins have volcanic basements and are in the stage of petroleum maturity, and some of them have reached the stage of high maturity to over maturity because of the influence of magmatic activities. The sandstones in basins have low porosity of generally 5%-7% due to their poor sorting and contain few oil-bearing pores, although they have been observed extensive fracture-type oil-bearing shows on the surface and underground (Jiang, 1985). The Cretaceous Jinhua-Quzhou Basin is the largest faulted basin in western Zhejiang, with an area of 3 500 km2, where prevalent surface and downhole oil-gas shows have been discovered. The Qujiang Group (k2q) of this basin is subdivided from the bottom to top into the Zhongdai Formation (K2zd) volcanic rock, the Jinhua Formation (K2jh) lacustrine facies mudstone and the Quxian Formation (K2qx) red mudstone. The three formations make up the typical "three-layer structure" consisting of lower volcanic rock, central dark rock and upper red bed cover. The Paleozoic basements (Carboniferous to Permian mudstone and limestone) of the Jinhua-Quzhou Basin are also good marine hydrocarbon source rocks, thus the volcanic rocks of the Zhongdai Formation and the clastic rocks of both the Jinhua and Quxian formations become reservoir rocks, and overlying purplish red mudstone (red bed) of the Quxian Formation acts as the cap rock (Xu et al., 2017; Zu et al., 2011). In the Cretaceous Yongkang Basin of eastern Zhejiang, the volcanic rocks, argillaceous rocks and red bed of the Guantou, Chaochuan and Fangyan formations also form a good source-reservoir-cap assemblage of "three-layer structure" similar to that in the Jinhua-Quzhou Basin. As for the Cenozoic basins, sedimentary strata contain only low saturated hydrocarbon and aromatic hydrocarbon and have been still in the immature stage of petroleum due to the shallow burial depth and small thickness, which causes that these Cenozoic basins have no oil-bearing show though wide natural gas display. Besides, the Hangzhou-Jiaxing-Huzhou Plain and the coastal plain preserve extensive shallow biogenetic methane gas in Quaternary sediments.
In Anhui Province, the Hefei Basin is a huge basin with a total area of 23 000 km2 and is the largest, but the least studied exploration area in eastern China Continent (Liu et al., 2004). This basin has been found oil-gas shows in many places and brought out great potential for oil and gas resources, althouth it has not been made any important oil-gas breakthrough so far. There exist three sets of dark mudstone and coalbed source rocks within this basin, including Carboniferous to Permian basements, Lower-Middle Jurassic and Lower Cretaceous ones (Zhang et al., 2008; Zhao et al., 2001). The volcanic rocks can act as reservoirs or caprocks to form favorable hydrocarbon traps.
In Jiangxi Province, the Yiyang Basin is also a large-scale faulted basin with an area of 3 600 km2 and is tempo-spatially connected with the Jinhua-Quzhou Basin in Zhejiang Province. This basin has obtained drilling oil-gas shows and also shows similar "three-layer structure". Among its "three-layer structure", the lower layer is rhyolitic-dacitic volcanic rocks of the Early Cretaceous Daguding (K1dg) and Ehuling (K1e) formations; the middle layer is thick dark mudstones of the Early Cretaceous Lengshuiwu Formation (K1ls) which is the overall matured hydrocarbon source rock with Ⅱ- and Ⅰ-type kerogens and has reached their oil-generating peak at the late stage of Late Cretaceous; the upper layer forms the late Early Cretaceous to Late Cretaceous caprock, including volcanic rock of the Maodian Formation, red coarse sandstone, mudstone and conglomerate of the Zhoujiadian (K2zj) and Nanxiong (K2nx) formations (Shi et al., 2015; Zhou, 2000).
In Jiangsu Province, the Subei Basin is located north of the Yangtze River and is the largest Late Cretaceous to Cenozoic petroliferous basin in East China, with an area of 35 000 km2 and the maximum sedimentary thickness of 7 000 m. This basin is the sole onshore industrial oil area in East China and the only basin of Upper Cretaceous source rocks in eastern China Continent, where more than 60 medium and small oil-gas fields have been found (Fig. 4, Qiu et al., 2006). The major hydrocarbon source rocks are terrestrial organic-rich shales of both the Upper Cretaceous Taizhou Formation and the Paleogene Funing Formation, and they have reached the mature evolution stage and contain generally > 10% mass fractions of TOC. In addition to petroleum and natural gas, the basin also has attractive exploration potential of unconventional hydrocarbon resources such as shale oil-gas (Zheng et al., 2013; Chen et al., 2008). The dark mudstone of the Taizhou Formation began to generate a great amount of hydrocarbons from Late Paleocene to Eocene, and experienced continuous hydrocarbon charge and accumulation during Late Eocene to Pliocene (Song et al., 2010). The frequent volcanic activities from Late Cretaceous to Pliocene produced effusive facies basaltic lava and tuff, subvolcanic facies basaltic porphyrite and diabase, with a total area of more than 1 000 km2. There are more than ten layers of interbedded basalt, diabase, sandstone and mudstone in many areas of this basin, including the Late Cretaceous Taizhou Formation, Paleocene Funing Formation, Eocene Sanduo Formation and Miocene-Pliocene Yancheng Formation (Qian and Li, 1996). The spatial distribution of volcanic rocks is closely related to the regional faults; the magmas ascended along the faults and formed stratiform volcanics via effusive eruption or bed-parallel intrusion (Zuo et al., 2012). Volcanic rocks act as not only the good hydrocarbon caprock, but also the favorable reservoir spaces. Volcanic and shallow subvolcanic (diabase) reservoirs have encountered oil-gas shows in more than 300 wells and became important exploration targets. For example, basalts and the contact alteration zone between diabase and sand-mudstone of both the Funing and Sanduo formations have high reservoir property and have obtained industrial oil and gas, and become the main volcanic reservoirs in the Subei Basin (Zan et al., 2017; Yang, 2010). The volcanic rocks have dual reservoir spaces of both fractures and pores, for example, in the Minqiao Oilfield of the Jinhu sag, the quenched volcanic breccia comprises abundant fractures and pores and can contain up to 60% oil (Tao et al., 1998). Besides, in the Taixing area on the south side of the Subei Basin, there are the largest CO2 gas fields and important shallow helium reserves in China, both of which are mantle-derived gas accumulations and are closely related to Neogene basaltic volcanic activities of the Yancheng Formation (Ren, 2005; Ye, 2003). The Jurong Basin of southern Jiangsu Province has also obtained low-production oil and gas flow. In this basin, the source rocks are mainly marine mudstones of Permian to Lower Triassic and dark mudstone of Early Cretaceous Gecun Formation (K1gc), and the volcanic reservoirs include overlying Early Cretaceous Longwangshan and Dawangshan formations and Late Cretaceous (~90 Ma, Zhang P et al., 2009) Pukou Formation (Hua, 2014; Wang, 2003).
Volcanic rocks with good permeability are conducive to oil and gas accumulation, while the dense volcanic rocks are favorable for oil and gas sealing. Both volcanic reservoir and caprock may be either mafic, intermediate or acid rocks, and are independent of the lithology and burial depth. The permeability is mainly related to primary and secondary pores. The volcanic rocks are generally low to ultra-low pore and extra-low permeable reservoirs because of their strong heterogeneity and low porosity. The physical properties of volcanic reservoirs may become worse from explosive facies, extrusive facies, effusive facies, volcanic neck facies, eruption-sedimentary facies to subvolcanic-intrusive facies in turn. Among them, the eruptive facies (pyroclastic flow facies, explosion-collapse facies, and air-fall facies), extrusive facies as well as effusive facies are the dominant facies for oil and gas. Two volcanic reservoir-forming models of near-source type and far-source type have been proposed. The volcanic reservoirs in eastern China Continent are mainly of near-source type, in which the volcanic rocks are distributed in or near the hydrocarbon-generating depressions, and the source rocks are above, below or lateral to the volcanic reservoirs so that the resulting oil and gas have the largest contacts to the reservoir (Zheng et al., 2018; Zou et al., 2013, 2008; Liu J Q et al., 2010). Here take the Subei Basin as an example to make a brief description as follows. The volcanic rocks can directly contact with underlying hydrocarbon source rock, and they together make up the volcanic reservoir of "lower generation-upper storage" type. Some hydrocarbon reservoirs are situated at the top of the volcanic channel and covered by the overlying mudstones, which form a good reservoir-cap assemblage. The sandstone can also be as reservoir and the overlying volcanic rock or layered diabase as cap rocks, and they together compose a large trap area. Subvolcanic dykes and diabase also act as barrier layers of hydrocarbon migration (Yang, 2010; Li, 2000). It should be pointed out that volcanic rocks can act usually as the reservoirs together with permeable interbedded sandstones, or as the caprocks with interbedded mudstone.
The volcanism can bring the basins into the peak period of abnormal temperature, which leads to the increase of geothermal gradient and the decrease of the maturity threshold of source rock. Here also take the Subei Basin as an example. Multi-phase volcanic activities in the Gaoyou, Jinhu and Qintong sags reduced the maturity thresholds of both Ⅰ-Ⅱ-type and Ⅲ-type kerogens to be only 2 000 and 2500 m deep, respectively, which brought thses sags into preferred hydrocarbon enrichment areas of the Subei Basin. The Yancheng and Haian sags underwent only 1-2 periods of intense volcanic activities, their maturity thresholds of both Ⅰ-Ⅱ-type and Ⅲ-type kerogens to be 2 500 and 2 800 m deep, respectively; so that the two sags have been found only four small oil and gas fields, and their oil-geology reserves merely account for 3% of the total reserves in the Subei Basin. In the Funing and Lianshui sags, the volcanic activities were very weak, the maturity thresholds of both Ⅰ-Ⅱ-type and Ⅲ-type kerogens are 2 800 and 3 000 m deep, respectively, which indicates that the two sags have a poor prospecting potential (Zan et al., 2017; Jiang et al., 2010).
Volcanic ash and its decomposition products can provide minerals beneficial to the development of microorganisms and plants in the lake basins, which may increase the abundance of organic matter in the sediments and promote the formation of high-quality source rocks. On the other hand, olivine and secondary minerals such as zeolite, montmorillonite and illite formed by hydration of volcanic ash, as well as H2 from volcanic gas, can catalyze the conversion of organic matter into oil and gas. The transition metal elements such as Ni, Fe, V, Co, Cr, Mn and other trace elements from volcanic rocks (especially basalt) can also play significant catalytic effects on the conversion of organic matter into light hydrocarbons and natural gas, and accelerate the conversion of source rocks to oil and gas (Shang et al., 2020; Zou et al., 2013; Jin et al., 2006). For example, in the basalt eruption zone of the western Jinhu sag in the Subei Basin, both the 2nd and 4th members of the Funing Formation deposited in a relatively isolated, semi-saline lake with exuberant organisms (especially fatty lower algaes), and transition metal elements were also migrated from the basalts to the lake, which increased the abundance of oil and gas resources and turned the Funing Formation rocks into excellent source rocks (Zuo et al., 2012; Jiang et al., 2010).
It is worth mentioning that with the ending of Mesozoic volcanic activity in Late Cretaceous (~80 Ma), East China experienced correspondingly large-scale crustal uplifting and consuming of faulted basins, which is also conducive to the basin-wide hydrocarbon expulsion and accumulation (Zhao et al., 2013).
According to the spatio-temporal distribution of volcanic reservoirs and their hydrocarbon source rocks (Table 7), the volcanic oil and gas accumulation of each volcanic cycle can be summarized as follows:
Volcanic cycle Location Volcanic reservoir Lithology Age (Ma) Hydrocarbon source rock Basin type Ⅲ Hefei Basin in Anhui Early Cretaceous Maotanchang Formation Trachy-andesite 120-130 Carboniferous-Permian, Lower-Middle Jurassic and Lower Cretaceous strata Volcano-tectonic depression Jurong Basin in Jiangsu Early Cretaceous Longwangshan and Dawangshan formations Trachy-andesite 130 Permian and Lower Triassic strata, Lower Cretaceous Gecun Formation Volcano-tectonic depression Ⅳ Jinqu Basin in Zhejiang Early Cretaceous Zhongdai Formation Basalt 93-105 Lower Cretaceous Jinhua Formation Faulted basin Yongkang Basin in Zhjiang Early Cretaceous Guantou Formation Basalt 103 Lower Cretaceous Guantou and Chaochuan formations Faulted basin Yiyang Basin in Jiangxi Early Cretaceous Maodian Formation Basalt 102 Lower Cretaceous Lengshuiwu Formation Faulted basin Jurong Basin in Jiangsu Late Cretaceous Pukou Formation Basalt 90 Permian and Lower Triassic marine strata, Lower Cretaceous Gecun Formation Faulted basin Subei Basin in Jiangsu Late Cretaceous Taizhou Formation Basalt 83 Lower Cretaceous Taizhou Formation Faulted basin Ⅴ Hefei Basin in Anhui Paleogene Dìngyuan Formation Basalt 23-60 Carboniferous-Permian, Lower-Middle Jurassic and Lower Cretaceous strata Depressed basin Subei Basin in Jiangsu Palaeocene Funing & Eocene Sanduo formations Basalt 30-64 Upper Cretaceous Taizhou and Palaeocene Funing formations Depressed basin Ⅵ Subei Basin in Jiangsu Miocene-Pliocene Yancheng Formation Basalt 5-18 Rift basin
Table 7. Correlation of volcanic cycle, volcanic reservoir and hydrocarbon source rock
(1) The cycles of Ⅰ and Ⅱ have not producesd volcanic reservoir, which may be ascribed to the lack of both contemporaneous deep sedimentary basin and source rock.
(2) There are only a few volcanic reservoirs in the Cycle Ⅲ, which may be related to the intense volcanic explosions, too high thickness and density of volcanic rocks and the less sedimentary basins. However, volcanic reservoirs can still be formed in some volcano-tectonic depressions where developed Early Cretaceous thick lacustrine mudstone and underlying Paleozoic marine mudstones, which is worthy of attention in the future.
(3) Volcanic reservoirs of the Cycle Ⅳ are relatively well-developed, which may be due to obvious weakening of volcanism and forming of faulted basins. Based on analysis of their source rocks, the faulted basins with Early Cretaceous thick dark mudstones are important exploration areas for volcanic reservoirs in the future.
(4) The volcanic reservoirs of the Cycle V are the largest ones in Mesozoic, such as the industrial oil area discovered in the Subei Basin. They were formed in the depressed basins evolved from the prevenient faulted basins, where multil-stage and permeable basalts can become good accumulation layers, and thick mudstones turn into good source rocks. This cycle should be the focus of exploration for volcanic reservoirs in East China, especially in the Hefei Basin of Anhui Province.
(5) There are finite volcanic reservoirs in the Cycle Ⅵ, which may be related to the too shallow burial of Neogene-Quaternary basalts. However, such volcanic reservoirs are worthy of attention in basins with good caprocks. In addition, more attention should also be paid to CO2 and helium reservoirs associated with the volcanism of this cycle.