Geochronological , Sedimentary , Structural , and Metallogenic Characteristics of Southeast China during the Mesozoic : A General Review

The tectonic evolution of Southeast China during Late Mesozoic is a prominent topic. Numerous tectonic models on Late Mesozoic evolution of Southeast China have been published in the past 50 years. We synthesized many up-to-date and precise zircon U-Pb ages, sedimentary strata, and regional structures and discussed the oxygen fugacity of magmas and related ore deposits. We also analyzed the most current tectonic models published by some scholars. A multistage tectonic stress evolution history during Late Mesozoic was constructed, which included the following stages: 1) Early-Middle Jurassic (196 175 Ma) extension, in which many bimodal volcanics formed; 2) Middle-Late Jurassic (165 140 Ma) compression, which generated largescale gneissic granites, garnet-bearing granites, stratigraphic hiatus, and nappe structures; 3) Early Cretaceous (140 ± 5 120 Ma) extension, which formed weakly deformed or undeformed granites, alkali granites, metamorphic core complexes, graben basins, and basic dike swarms; 4) Early Cretaceous (120 110 Ma) compression, which generated nappe structures, volcanic hiatuses, and garnet-bearing granites; and 5) Early-Late Cretaceous (110 80 Ma) extension, which generated largescale bimodal volcanics, basic dike swarms, alkali granites, and graben basins. The Late Mesozoic tectonic evolution of Southeast China may be attributed to the drifting history of the Paleo-Pacific plate. The drifting direction of the Paleo-Pacific plate has changed several times since 140 Ma, which led to major changes in the tectonicphenomena from Jurassic to Cretaceous and to the formation of Late Mesozoic mineral deposits. Corresponding author.


Introduction
The tectonic evolution of Southeast China during Late Mesozoic is a prominent topic.For more than half a century, geologists worldwide have undertaken studies on igneous rocks in Southeast China, and numerous tectonic models on Late Mesozoic evolution of the region were published.These models can be mainly divided into four categories: an active continental margin related to the subduction of the Paleo-Pacific plate, with flat subduction [1] or with a low-angle to a medium-angle subduction [2] [3]; a composite of orogenic belts formed due to continental collisions within the South China block during the Mesozoic period [4]- [6]; rifting along the entire eastern margin of China that commenced in the Middle Mesozoic [7]; and a hot mantle plume in South China block during Mesozoic [8].Despite the number of studies on this problem, this issue remains unresolved.The reasons for this research gap may be attributed to the use of outdated equipment or the lack of comprehensive study on multiple geological elements.Some traditional dating methods, such as K-Ar and Rb-Sr systems, can be easily disturbed by subsequent hydrothermal events or cannot obtain the exact ages of geologic events.Therefore, a precise and accurate determination of the temporal-spatial distribution of igneous rocks is essential to explore the tectonic evolution of Southeast China.Furthermore, the analysis of sediment and regional structure is required.
This study synthesizes the up-to-date and precise zircon U-Pb ages on the basis of recent studies by our team and other groups.This study also discusses the geological features of Mesozoic basins, nappe structures, angular unconformity, and the oxygen fugacity of magmas and related ore deposits to analyze the plate tectonic setting and petrogenesis of Late Mesozoic igneous rocks.Subsequently, this study reveals the tectonic evolution history of Southeast China during Late Mesozoic.

Temporal-Spatial Distribution of Igneous Rocks
Igneous rocks are often emphasized by researchers.Zhou and Li [2] suggested that the magmatic activity of the Southeast China continental margin migrated oceanward to the southeast because the slab dip angle of the Paleo-Pacific plate subduction underneath Southeast China increased from low angle to medium angle.Li and Li [1] proposed that the ages of synorogenic magmatism, thrusting, and metamorphism showed a trend of younging toward the cratonic interior during the Permian-Triassic orogeny.They then put forward a flat-slab subduction model.The age of igneous rocks is one of the most critical factors in determining the tectonic evolution of Southeast China.However, some disagreements exist among the ages obtained via different dating methods.Currently, U-Pb dating is accepted as the most accurate dating method available.Therefore, we consulted approximately 1000 papers and collected hundreds of up-to-date U-Pb dating data [87] to reveal the temporal-spatial distribution of igneous rocks during Late Mesozoic.As shown in Figure 1, we conducted statistics on most zircon U-Pb ages of igneous rocks in Southeast China.Basing on the temporal-spatial distribution of these rocks, we did not observe obvious tendency for igneous rocks to grow younger from inland areas toward the coastal areas.Specifically, Jurassic orogeny igneous rocks are distributed throughout both the coastal areas and inland areas (Figure 1), and younger rocks coexist with the older rocks in many areas [9]- [12].
Magmatic rocks can be used to indicate the setting [46]- [50], such as bimodal volcanics that are generally formed in an extensional tectonic regime.Large-scale volcanic sequences, alkali granites, metamorphic core complexes, and basic dike swarms are the typical products of extensional tectonics [48].Muscovite, cordierite, garnet-bearing granites, and gneissic granites generally form in syn-orogenic settings [29] [49].

Stratigraphic Analyses
Each tectonic event is accompanied by corresponding sediments.The Late Mesozoic stratigraphic sequences in Southeast China show an angular unconformity between Middle Jurassic (J 2 ) and overlying Early Cretaceous (K 1 ) (Figure 3).Thus, the hiatus of Late Jurassic (J 3 ) is general in the study area.The hiatus indicates that Southeast China underwent an intense tectonic event in Late Jurassic when the crust was lifted and eroded.This phenomenon also exists in the adjacent areas, such as Qinling region in North China.This finding suggests that the acme of compression in Eastern China occurred during Late Jurassic.Another angular unconformity occurred between Early Cretaceous (K 1 ) and the overlying Late Cretaceous (K 2 ) in Southeast China (Figure 3).[51]; Fujian province from [43]; Zhejiang province from [36]; Qinling region from [52]).
Granites formed during the Proterozoic Eon were cut and transported to the present location, in which the root can no longer be seen nowadays.This finding denotes that the nappe structure was large-scale.Late Jurassic thrust structures also outcrop in Jiangxi Province (Figure 4).The NE-trending thrust structure in Lengshuikeng deposit thrusts from NW to SE.Therefore, the Jurassic Daguding (J 3 d) formation was overlaid by the Proterozoic Laohutang formation (Pt 3 l), and synchronous granitic rocks formed (Figure 4).The U-Pb ages of 160.8 Ma for the Daguding formation and 154.3 Ma for the granitic rocks [53] indicate that this thrust structure formed at or after 154.Metamorphic core complexes, granitic dome structures and graben basins are always related to the extensional settings [54]- [56].Wugongshan in Jiangxi Province was a Mesozoic granitic dome-type extensional structure (Figure 6(A)) that is composed of metamorphic core complexes [54], the youngest Zircon U-Pb age of Wugongshan granitic dome constrain the final formation time of these metamorphic core complexes at 126. 3 Ma [57].Many other metamorphic core complexes, such as Lushan metamorphic core complex in Jiangxi Province (Figure 6   Another series of thrusts formed between Early to Late Cretaceous in Southeast China.For example, Mesoproterozoic strata thrust over Jurassic strata in Huangbi, Jiangxi Province, during Middle-Late Jurassic (Figure 7(A)).By contrast, in Early-Late Cretaceous, the Mesoproterozic and Jurassic strata overlay the Early Cretaceous Ehuling formation (K 1 e) (Figure 7(A)).The same phenomenon can be seen in Anhui and Hainan Provinces, where the Mesoproterozoic and Paleozoic overlay the Lower Cretaceous (Figures 7(B)-(D)).Compared with the Late Jurassic thrust structures, the thrust structures formed in Early-Late Cretaceous are smaller in scale.
Basing on the temporal-spatial distribution of igneous rocks, stratigraphic sequence, and regional structures (Figure 9), we can infer that two compressional and three extensional events happened in Southeast China during Late Mesozoic (200 -80 Ma).

Distribution of Mineral Deposits and Magmatic Oxygen Fugacity
One of the most important mineralization events occurred in the Mid-Late Jurassic in Southeast China.Numerous Cu-Au-Mo deposits, which are associated with I-type granites, formed during this period [66].In addition, these ore deposits distributed widely from coastal areas to inland (Figure 8) were probably caused by the subduction of the Paleo-Pacific plate on the Pacific margin [67]- [69].The Early Cretaceous is also an important metallogenic epoch in which many Cu-Au-Mo deposits formed.For example, the famous Zijinshan Cu-Au deposit formed during 110 Ma to 100 Ma [11].However, the Early Cretaceous Cu-Au deposits differ from the Late Jurassic deposits because the former deposits are distributed only along the coastal areas (Figure 8).Both the Jurassic and Cretaceous Cu-Au deposits are featured by extremely high magmatic oxygen fugacity [67] [70] [71].
Studies on controlling factors of high fo2 have received much attention for a long time [74] [75].Ballard et al. [76] reported that the Chuquicamata calc-alkaline intrusions of northern Chile had high magmatic oxygen fugacity, which is probably associated with subduction of plates.Lee et al. [74] found that the oxygen fugacity of arc-related magmas is higher than that formed in other tectonic environments.Wang et al. [75] stressed the influence of subducting oceanic sediments and slab dehydration fluids on magmatic oxygen fugacity through the  [54]; (B) Cross-sections through the Lushan metamorphic core complex, Jiangxi province [27]; (C) Profile of Jinhua-Quzhou (Jinqu) tectonic basin, Zhejiang province [64] (K-Ar ages from [65]).study of zircon Hf-O isotopes and trace elements.Therefore, fluids released from subduction slabs were commonly invoked to explain the elevation of oxidation state of the arc lavas.Thus, we concluded that high oxygen fugacity can indicate that magmas formed in a subduction-related setting to a certain extent.

Dynamic Mechanism of Tectonic Evolution
The tectonic evolution of Southeast China during the Late Mesozoic period has always been a topic of debates [77]- [83].The following are the mainstream theories regarding this issue: the flat-slab subduction model presented by Li and Li [1]; the slab dip angle of Paleo-Pacific plate subduction underneath Southeast China increased  from a very low angle to a median angle [2].An important geological event in the flat-slab subduction model shows that the age of synorogenic magmatism represents a trend of younging toward the cratonic interior.However, this event is not evident in South China in Mesozoic magmatism.The increasing slab dip angle model demonstrates a gradual change process.However, the geologic phenomenon is not evident in this gradual change process, which reflects two tectonic events instead.The first event occurred in Late Jurassic and was largescale, thereby leading to the formation of nappe structures, magmatic rocks, and hiatus from coastal to inland areas; the second event occurred in Early Cretaceous and was smaller than the first event.Therefore, the geologic phenomenon in the second event can be seen along the coastal areas.Some small-scaled nappe structures, brief hiatus, and magmatic rocks formed during the second geological event.
The change in the drifting direction of the Paleo-Pacific plate can explain the above geological events (Figure 9).Under the drastic NW-SE compression during Late Jurassic, NE trending nappe structures thrust from SE to NW in Southeast China.The compressional stress came from the southeast of Eastern China.However, no continent is located to the southeast of Eastern China [6].Consequently, the NW-striking subduction of the Paleo-Pacific plate is the origin of compressional stress.Thus, a series of compressional phenomena formed during The subduction direction changed by ~80˚ at ~125 -122 Ma and persisted until ~110 Ma [86].Correspondingly, the subduction of the Paleo-Pacific plate turned out to be NW-striking.The second compressional event occurred at ~125 Ma to 110 Ma, when some nappe structures and a brief hiatus formed.
From ~110 Ma to ~100 Ma, the subduction direction changed by ~30˚ and changed again by ~75˚ at ~100 Ma.When the extensional event occurred again in South China during 110 Ma to 80 Ma, largescale bimodal volcanics, basic dike swarms, and alkali granites formed.
The subduction direction of the plate varied with time.The Jurassic-Cretaceous tectonic evolution of Southeast China is coupled with the subduction of the Paleo-Pacific plate.With the changes in the subduction direction of the Paleo-Pacific plate, the tectonic evolution demonstrated an alternation between compression and extension, which generated numerous geological events (Figure 9).
The Late Mesozoic tectonic evolution of Southeast China may be attributed to the drifting history of the Paleo-Pacific plate.The drifting direction of the Paleo-Pacific plate has changed several times since 140 Ma, and this may lead to prominently different tectonic phenomena between Jurassic and Cretaceous.

Figure 2 .
Figure 2. Cumulative diagram of all ages of Yanshanian plutons in ten eastern provinces of China; the diagrams are plotted by CGDK software [13].

3
Ma.As shown in Figure 5, largescale thrust structures formed in the whole South China during Late Jurassic.Examples of thrust structures include Xiaoxi thrusts in Anhui, Shuikoushan thrusts in Hunan, and Jinzhukeng thrusts in Fujian.These thrust structures indicate that South China underwent an intense compression in late Jurassic.
(B)), developed in South China during Early Cretaceous.Zircon U-Pb ages constrain the formation time of these metamorphic core complexes between 133 and 123 Ma [27] [58].Graben basin is commonly seen in South China during Early Cretaceous, filled by Lower Cretaceous-Upper Cretaceous sediments (Figure 6(C)).This observation indicates that Early Cretaceous was a significant extensional stage in South China, and the zenith of extension was between 135 Ma to 120 Ma.

Figure 8 .
Figure 8. Distribution of Cu, Mo, and Au deposits in southeast China.

Figure 9 .
Figure 9. Tectonic evolution diagram of southeast China during Late Mesozoic.(Tectonic stress from[51]; Subduction direction from[84]-[86]).Middle-Late Jurassic (170 Ma to 145 Ma).The drifting direction of the Paleo-Pacific plate has changed several times since 140 Ma[84]-[86].First, the Paleo-Pacific plate moved roughly southward during 140 Ma to 120 Ma, with the drifting direction nearly parallel to the east boundary of the Eurasian continent.Therefore, many undeformed granites, alkali granites, metamorphic core complexes, graben basin, and some basic dike swarms formed under the extensional background in Southeast China during Early Cretaceous.Meanwhile, the synchronous widespread volcanic series formed.The subduction direction changed by ~80˚ at ~125 -122 Ma and persisted until ~110 Ma[86].Correspondingly, the subduction of the Paleo-Pacific plate turned out to be NW-striking.The second compressional event occurred at ~125 Ma to 110 Ma, when some nappe structures and a brief hiatus formed.From ~110 Ma to ~100 Ma, the subduction direction changed by ~30˚ and changed again by ~75˚ at ~100 Ma.When the extensional event occurred again in South China during 110 Ma to 80 Ma, largescale bimodal volcanics, basic dike swarms, and alkali granites formed.The subduction direction of the plate varied with time.The Jurassic-Cretaceous tectonic evolution of Southeast China is coupled with the subduction of the Paleo-Pacific plate.With the changes in the subduction direction of the Paleo-Pacific plate, the tectonic evolution demonstrated an alternation between compression and extension, which generated numerous geological events (Figure9).