Metamorphic petrology and geology in China: A review

Yuan-sheng Geng, Qi-han Shen, Hui-xia Song

Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China

A B S T R A C T

The development of metamorphic petrology to metamorphic geology in China has a long history. Ancient basement metamorphic rocks are distributed primarily in the North China Craton, the Yangtze Block and Tarim Craton. They are mainly made up of plutonic gneiss and metamorphosed supercrust rock, transformed to granulite facies through Archean Paleoproterozoic. Many of the Paleoproterozoic metamorphic rocks have undergone high-pressure granulite facies metamorphism with a clockwise metamorphic evolution path. The ultrahigh temperature (UHT) granulites from the Late Paleoproterozoic are found in North China Craton. Many high-precision chronological data have allowed preliminary construction of the formation and evolutionary framework of different metamorphic basements. Primarily there are low-temperature and high-pressure blue schist, high-temperature and high-pressure granulite and ultrahigh-pressure(UHP) eclogite facies metamorphic rocks in the Phanerozoic orogenic belt. The discovery of eclogite in the Sulu orogen and a large quantity of coesite in its country rocks show that there was a deep subduction of voluminous continental materials during the collision process between the Yangtze block and the North China Craton in the Early Mesozoic phase. From the studies of, for instance, organic matter vitrinite reflectance, illite crystallinity, illite (muscovite) polytype and illite (muscovite) b dimension, the Late Paleozoic strata in the eastern region of Inner Mongolia and the north-central region of NE China have only experienced diagenesis to an extremely low-grade metamorphism. The discovery of impact-metamorphosed rocks in Xiuyan area of Liaoning province has enriched the type and category of metamorphic rocks in China. The phase equilibrium method has been widely used in the study of metamorphism of middle and high-grade metamorphic rocks. On the basis of existing geologic surveys and monographic study results,different scholars have respectively compiled 1:1500000 Metamorphic Geological Map and Specifications of Qinghai Tibet Plateau and its Adjacent Areas, 1:2500000 Metamorphic Tectonic Map of China,and the 1:5000000 Metamorphic Geological Map and Specifications of China, among others repectively,which have systematically summarized the research results of metamorphic petrology and metamorphic geology in China.

Keywords:

Metamorphic petrology

Metamorphic geology

Metamorphic P-T-t path

Ultrahigh pressure

Metamorphic pock

Granulite

1. Introduction

Metamorphic petrology is an important sub-discipline of petrology and geology. Metamorphic geology is an integrated discipline that researches metamorphic formation, metamorphic rocks, metamorphism evolution, and metamorphic mineralization, and it develops on the basis of metamorphic petrology. Metamorphic petrology and metamorphic geology are two essential aspects of studying in the composition and evolution of the Earth.

The exposed area of metamorphic rocks in China accounts for one fifth of the land area. Precambrian metamorphic rocks are primarily exposed in the basement of North China Craton, the Tarim Craton and the Yangtze Craton. Phanerozoic metamorphic rocks are mainly distributed in the Dabie-Sulu orogenic belt, the Qin-Qi Kun orogenic belt, the Tianshan-Xingmeng orogenic belt, the Songpan-Ganzi orogenic belt,the Qiangtang orogenic belt, and the Himalayan orogenic belt,among others. Some very low grade metamorphic rocks are developed in some of these and other orogenic belts and large basins (Fig. 1).

Fig. 1. Distribution of the metamorphic rocks formed at distinct geological stages in China. H:Himalayan metamorphism, Y:Yanshanian metamorphism, I:Indosinian metamorphism, V:Variscian metamorphism, C:Caledonian Metamorphism, X:Xingkaian (Pan-African) metamorphism,PєE:Neoproterozoin metamorphism, PєD:Mesoproterozoic metamorphism, PєC:Paleoproterozoic metamorphism, PєB:Neoarchean Metamorphism, PєE+H:Neoproterozoic+Himalayan metamorphism, I-Y:Indosinian–Yanshanian metamorphism, V-I:Variscian – Indosinian metamorphism, C-V:Caledonian–Variscian metamorphism, PєC+V:Paleoproterozoic+Variscian metamorphism, PєD+C:Mesoproterozoic+Caledonian metamorphism, PєE+C:Neoproterozoic+Caledonian metamorphism, PєD-E:Meso-Neoproterozoic metamorphism, PєC-E:Paleo-Neoproterozoic metamorphism, PєC-D:Paleo-Mesoproterozoic metamorphism, PєB-C:Neoarchean-Paleoproterozoic metamorphism.

The independent research of modern geology in our country began in 1922, and before that, there were some few geological works, but those were conducted by foreigners. From 1922 onwards, China constructed the first national geology institute, and started to conduct related researches on geology mineral survey, biostratigraphy and paleontology, but did not include metamorphic petrology. During 1940 to 1949, a few types of metamorphic rocks were researched (Cheng YQ,1940, 1948; Song SH et al., 1948), but these researches were limited.

For the seventy years from the building of People’s Republic of China, metamorphic petrology experienced from the laying down of its discipline foundation to experiencing its progressive development. Concerning the research content,the early researchers, focused on simple mineral rock combination, structure description, metamorphic facies, and general temperature-pressure condition, later transferring to the integrated researches on geochemistry, genetic mineralogy, magma petrology, laboratory mineralogy, isotope geology, deformation structure, etc. The formation mechanism of metamorphic rocks, metamorphism evolution, geodynamics, and geotectonics have gradually been combined in recent years to develop as a new direction of metamorphic geology.

This paper will introduce and discuss the researches and achieved results in the past 40 years (focusing on the last 20 years) from five aspects on metamorphic petrology and metamorphic geology in the basement of cratons, metamorphic rocks in orogenic belts, the metamorphic geology integration and mapping, very-low-grade metamorphic rocks, and other special metamorphic rocks in China.

2. Research and development in the craton basement

2.1. Basic characteristics of the basement of Chinese cratons

The North China craton (NCC) is one of the most important areas in the Precambrian high grade metamorphic region.Its basement is composed of five sets of high grade metamorphic rocks, which experienced multiple-stages of tectonic activity magma activities, metamorphism activities, migmatization, and anataxis, representing a complex evolutionary history (Shen QH et al., 2016). The NCC, formed during the Archean-late Paleoproterozoic, primarily experienced five regional metamorphisms. The rocks of Paleo-Meso Archean in the Anshan area experienced amphibolite facies metamorphism, but the age has still not been determined. The metamorphism age of the oldest TTG (trondhjemite-tonalite-granodiorite) with 3.8 Ga has still not been found, but the early metamorphism age of trondhjemite with 3.77 Ga was identified as 3560 Ma, and the metamorphism age of the gneiss of the Meso-Archean was 3000-3300 Ma. It has now been identified that there were magmatic activities of 3.8 Ga, 3.6 Ga, 3.3 Ga,3.0 Ga and 2.5 Ga (Liu DY et al., 2008; Zhou HY et al., 2007;Wan YS et al., 2009a). The age information of two-phase metamorphisms for 2676-2792 Ma and 2671-2651 Ma has been achieved in the Meso Archean amphibolites of the Taihua Complex in the Lushan area of Henan (Liu DY et al.,2009). A metamorphism age of 2.6 Ga has also been achieved in the Taishan mountain of Luxi (Ren P et al., 2016), confirming early Neoarchean era metamorphism. The Neoarchean granulites-TTG gneisses and the Neoarchean granite - green rock series both respectively experienced metamorphism of late Neoarchean (about 2.5 Ga) and Paleoproterozoic (Grant ML et al., 2009; Fu JH et al., 2016; Yang C and Wei CJ ,2017). In the Paleoproterozoic era, low-medium pressure or high-pressure granulite-facies metamorphism was occurred,producing the Khondalite series of the north boundary of NCC during 1965-1900 Ma, and ultrahigh-temperature metamorphism was locally occurred (1920-1900 Ma), and the high-pressure metamorphism with P-T-t clockwise evolution path of this era was related to the subduction collision between land masses. However, the mechanism of ultrahightemperature metamorphism is currently controversial. In the late Paleoproterozoic (1890-1800 Ma), a regional metamorphism with a clockwise P-T-t path of high-pressure granulite facies- amphibolite facies happened in the Jiao-Liao-Ji area of the central and east of the NCC, representing collision and joining between landmasses. The metamorphisms experienced by the different types of metamorphic rocks show different tectonic environments. Many TTG rocks and planardistribution medium-low pressure granulites of late Archean age were mainly exposed in the central and northern NCC,mostly with anticlockwise P-T-t, reflecting the tectonic environment of mantle plume and underplating. The metamorphism experienced by Neoarchean green rocks during the Late Neoarchean and Early Paleoproterozoic eras was mostly clockwise P-T evolution path, reflecting that it might have been related to the tectonic environment of a combination between post-arc and mantle plume. The Late Proterozoic metamorphism is characterized by the evolution track of clockwise P-T path in high-pressure granulite facies, reflecting the collision and combination between different landmasses, which might have been related to the joining of the Colombia supercontinent.

An advanced metamorphic body with an age of 3.2-2.8 Ga is also exposed in the Huangling area of north Yichang in the Yangtze Block, composed of TTG rock series, metamorphic sedimentary rock, plagioamphibolite, and few basic granulites. The metamorphic mud rock experienced a three-stage evolutions. The coexisting minerals of the M1 stage are Bt+Ms+Chl+Pl+Qtz, with temperatures between 400-550°C,belonging to the low-grade greenschist facies. The typical balance paragenetic composition of the M2 stage is And+St+Alm+Bt+Ms+Pl+Qtz, with temperatures between 520-580°C and pressure between 0.3-0.5 GPa, belonging to low amphibolite facies metamorphism. The material paragenetic composition of the M3 stage is Sil+Alm+Pl+Qtz, with the metamorphic temperature between 640-700°C and metamorphic pressure between 0.3-0.5 GPa. Due to the granulites found, the metamorphic facies is the high amphibolite facies and granulite facies, which reflects a gradual metamorphic process. Two stage metamorphic ages of 2715±9 Ma and 2588±40 Ma have been identified in the amphibolites, and there is not any confirmed evidence whether the metamorphism is only one in the Archean Eon, but the anticlockwise metamorphic evolutionary track is very similar to the metamorphic characteristic of granulite terrane of the Late Neoarchean in NCC. The metamorphic supracrustal rock in the Huanglin area, some basites, and Badu Group experienced metamorphism in late Paleoproterozoic, which has a clockwise P-T-t evolutionary path (Wu YB et al., 2009; Yin CQ et al., 2013).

Neoarchean TTG gneiss and a small amount of supracrustal rocks were found in the Kurotague area of the Tarim craton, the Arktashtarg region of the Altun Mountain and the Dunhuang area (Geng YS et al., 2016a). In the Arktashtarg region, these Archaean rocks mostly were metamorphozed to high amphibolite facies, and experienced migmatization. Archaean rocks in the Altun Mountain and the Dunhuang area have experienced metamorphic transformation of granulite facies, the metamorphic temperature of which is between 682-889°C, and pressure is 0.75 GPa (Lu SN et al., 2006), belonging to medium-pressure granulite facies. Being similar to the metamorphic condition of granulite terrane of the Late Neoarchean in the NCC, there are no evidences of metamorphic condition, evolutionary process, and metamorphic era, for which we are awaiting further confirmation. Some Precambrian metamorphic rocks in the Dunhuang area experienced high-pressure granulite metamorphism of the Late Pa-leoproterozoic, with a clockwise P-T-t path (Zhang JX et al.,2012, 2013).

2.2. High-pressure granulites and ultrahigh-temperature granulites

High-pressure granulites and degrading eclogites were successively found at the area of Hengshan and northwest Hebei province etc. (Wang RM et. al., 1991; Zhai MG et al.,1992; Guo JH et al., 1993). On the basis of metamorphism evolution, Precambrian granulites are divided into three types:a. regional granulite facies type with medium-low pressure and lager distribution; b. local granulite facies in mediumhigh temperature and hot-spot area; c. dynamical heat-flow metamorphic granulite facies in the linear regional (Shen QH et al., 1992). Then, Zhao CG et al., (2002, 2005) integrated the metamorphic research results to state that the metamorphism of west and east parts of the NCC were characterized by anticlockwise path, whereas the metamorphism of central zone from Chengde through Henshan to the south boundary of NCC, was characterized by a clockwise path. Considering the distribution of high-pressed granulites, they proposed that three tectonic units of the eastern block, western block, and central orogenic zone compose the NCC, and this brought about a new research tide. Afterwards, high-pressure granulites were successively found in Helashan-Qianlishan-Jining areas in northwestern margin of NCC, and the Jiaobei terrain(Zhou XW et al., 2004; Yin C Q et al., 2011; Jiao S J et al.,2013; Tam PY et al., 2012; Liu PH et al., 2013). Apart from the many places where high-pressure granulites were found,ultrahigh temperature granulites including orthopyroxene+sillimanite+ quartz, sapphirine+quartz, spinel+quartz were also found in the Tuguiwula of inner Mongolia, Dongpo of Daqingshan mountain, where the metamorphic temperatures were up to 1000 °C (Santosh M et al., 2007; Liu SJ et al.,2012; Guo JH et al., 2012). Although the genesis mechanisms of ultrahigh temperature granulites are also controversial, the finding of such a type of metamorphic rock in the north of North China enriches the research content of metamorphic rocks.

2.3. Zircon chronology research of in-situ microzone

Ever since Liu DY et al. (1992) proposed the existence of 3.8 Ga granites in the Baijiafen area of Anshan city of NCC,our geologists have carefully studied this area, and discovered 3811±4 Ma banded trondhjemites and 3794±4 Ma metamorphic quartz diorites in the Dongshan Park of Anshan city, and discovered 3800±5 Ma mylonitic trondhjemites in the Baijiafen area, 3777±13 Ma banded trondhjemites in the Shengouxi area (Wan YS et al., 2005; Liu DY et al., 2007,2008; Wan YS et al., 2009a), and 3814±2 Ma weak-banded trondhjemites (Wang YF et al., 2015). These new chronological data have further identified the oldest rock of our country with the age of 3.8 Ga in the Anshan area. Inherited zircons from the Hadean Eon with the age of 4174±48 Ma, have recently been found in amphibolites of Cigou Formation, Anshan group, Waitoushan, Anshan-Benxi area. This contributes to the search and research of the oldest materials on the earth.

A large amount of overseas data show that, the growth of the ancient crust occurred mainly around 2.7 Ga (Condie KC ,2000; Rasmussen B et al., 2005; Rino S et al., 2004; Hofmann A et al., 2004; Sandeman HA et al., 2006), but such tectonic thermal events were only found in local areas of near western Shandong and surrounds in North China (Zhuang YX et al., 1997; Jahn BM et al., 1998; Wan YS et al., 2011), and they do not appear to correspond to the global crustal growth time. Recently, evidence of magmatic events were found successively in the Taihua area of the south boundary of NCC(Liu DY et al., 2009), Zhongtiao area (Zhu XY et al., 2013),Taihangshan area (Han BF et al., 2011; Yang CH et al., 2013;Lu ZL et al., 2014), Yinshan area (Dong XJ et al., 2012; Ma MZ et al., 2013), and Jiaobei area (Jahn BM et al., 2008; Liu JH et al., 2013). These findings of early Neoarchean rocks have important meaning for understanding the timing of crustal growth of the NCC.

The khondalite series of the north margin of the NCC, experienced granulite metamorphism, which was traditionally thought to have formed in the Archean due to the high metamorphism (Yang ZS et al., 2000). Wu CH et al., (2006) proposed that the khondalite series formed in Paleoproterozoic era by in-situ dating data of detrital zircons. But there are some different opinions because of the limited number of zircons. Recently, more geochronological and metamorphic researches have been carried out, showing that the khondalite series with associated basic rocks and granites mainly developed in the middle-late Paleoproterozoic (2000-1950 Ma),but the metamorphism of granulite facies happened in late Paleoproterozoic era (1950-1850 Ma) (Wan YS et al., 2006,2009b, 2013; Xia XP et al., 2006a, b; Dong XJ et al., 2012;Yin CQ et al., 2009, 2011; Li XP et al., 2011; Jiao SJ et al.,2013, Liu PH et al., 2014; Li WJ et al., 2017). Maybe there are much older khondalite series in some local area, and they experienced complex metamorphic evolution process (Wan YS et al., 2013; Dong CY et al., 2014).

The oldest rocks of the Yangtze Craton (YC) are exposed in the Kongling area of Hubei province, and its main body is composed of dioritic-tonalitic-trondhjemite-granodioriticgranitic gneisses. Secondly it also includes amphibolite enclaves and a small amount of supracrustal rocks. At the beginning of this century, it was thought that it was formed around 2.9 Ga using the method of SHRIMP zircon U-Pb (Qiu YM et al., 2000; Gao S et al., 2001). More recently, descriptive chronology researches have shown that, the Kongling area experienced three stages of magmatic events during the Archean:stage 1 happened during 3.4-3.2 Ga; stage 2 happened during 3.0-2.9 Ga, and the last stage happened about 2.6 Ga (Zhang SB et al., 2006; Chen K et al., 2013; Guo JL et al., 2014,2015; Li LM et al., 2014) with two regional metamorphic transformations of granulite facies-amphibolite facies (Qiu YM et al., 2000; Wei JQ and Wang JX , 2012; Yin CQ et al.,2013). Otherwise, 2.6 Ga granites of the Yangpo group were found in the Zhongxiang area of Hubei province (Wang ZJ et al., 2013, Zhou GY 2015), and high-precision zircon dating shows that the main rocks of the Yudongzi group of the YC northwestern boundary were developed in the Archaean with some TTG rocks formed around 2.8 Ga (Zhang X et al., 2010;Hui B et al., 2017). These new chronological data enrich our understanding of the composition and evolution of the old basement of the YC.

2.4. Research on basement metamorphism

In the 1980s, along with the introduction of micro-zone analysis such the use of electron probe, much new research on the chemical composition of metamorphic minerals in the rocks was carried out, and the condition of metamorphism was researched by the temperature-pressure meter of coexisting mineral pairs. Both the metamorphic condition and evolution were studied in rocks below the Liaohe group in Liaoning province, the Archean granulites of the Miyun area in Jidong, and metamorphic sedimentary rocks in the Daqingshan-Wulashan area (He GP et al., 1994, He GP an Ye HW ,1998; Lu LZ et al., 1996; Liu XS et al., 1993). Along with the introduction of phase balance simulation technology on metamorphism and with the new understanding of NCC tectonics,many researches have been conducted in the north of North China on high-pressure granulites, ultrahigh-temperature granulites, high-pressure granulites of Jiaodong area (Guo JH et al., 2012; Cai J et al., 2014; Liu PH et al., 2013, 2014; Jiao SJ et al., 2015; Yin CQ et al., 2015; Zhang DD et al., 2016;Tam PY et al., 2012). Recently, granulites with a peak metamorphic pressure of about 1.0 GPa, have been found in Ji'an area of Jilin province, which have a clockwise metamorphic path (Cai J et al., 2017). It is generally believed that highpressure granulites experienced metamorphic reformation of late Paleoproterozoic with the characteristic of clockwise evolutionary track, which is similar to the orogenic belt metamorphic evolution (Fig. 2).

Fig. 2. The P-T paths of the Paleoproterozoic middle-high pressure granulites in North China Craton. a:Khondalite belt, 1:MP granulite,Helanshan Group (Zhao et al., 1999), 2:HP pelitic granulte, Qianlishan Group (Yin et al., 2011), 3 and 4:HP pelitic granulte in Helanshan (Zhou et al., 2010; Yin, 2010), 5:Khondalite series in Daqingshan-Wulashan (Liu et al., 1993), 6:Khondalite series in Wulashan (Xu, 1991), 7:Khondalite series in Daqingshan-Wulashan (Jin et al., 1991), 8:Khondalite series in Daqingshan (Cai et al., 2013); b:UHT granulites in khondalite belt, 1 and 3:Dongpo, Daqingshan (Tsunogae et al., 2011; Guo et al., 2012), 2 and 4:Tuguiwula, Jining (Santosh et al., 2009; Liu et al., 2008),5:Shaerqin, Daqingshan, (Jiao et al., 2015); c:Jijing-Datong gegion, 1:Jining khondalite (Zhao et al., 1999), 2:Jining basic granulite (Zhao et al.,1999), 3:P-T path of first metamorphic epoch (Lu et al., 1992), 4:P-T of secant metamorphic epoch (Lu et al., 1992), 5:khondalite in eastern Datong (Liu et al., 1997), 6:HP pelitic granulite in Jining khondalite (Wang et al., 2011), 7-HP garnet basic granulite in Gushan, Datong (Wang et al., 2011), 8:granulite in Jining khondalite (Jiao et al., 2013), 9:khondalite in Sanchakou, Jining (Cai et al., 2014); d:Chengde-Hengshan Gegion, 1and 2-Sanggan area (Guo et al., 2012), 3 and 4:Hengshan area (O’Brien et al., 2005), 5:basic granulite in Huai’an area (Zhang et al.,1994; Liu, 1995), 6:rich-Al gneiss in Huai’an area (Liu, 1995).

Medium-pressure granulites found in the Daqingshan area show the characteristics of Barrow metamorphic belt with clockwise evolutionary path, and can be divided into staurolite belt, sillimanite belt and kyanite belt (Huang GY et al.,2016). Also in recent years, some researchers have begun studying the metamorphism of high grade-metamorphic rocks around eastern Hebei area, and found that this area might expose the superposition of two granulite metamorphisms: one Archean era and the other Paleoproterozoic era. The Late Archean metamorphism, represented by granulites, has the characteristic of anticlockwise PT path (Duan ZZ et al., 2017;Kwan LCJ et al., 2016), with the highest metamorphic condition up to 860-920°C and 11-14 kbar (Yang QY et al., 2016);The Paleoproterozoic metamorphism, is represented by basic dikes, also with the characteristic of clockwise P-T path(Duan ZZ et al., 2016). Due to the superposition of the two stage granulites, it is more difficult to understand the early metamorphism. Other recent the research on metamorphic mud granulites of Louzishan in the eastern Hebei province shows that some Archean rocks experienced early granulite metamorphism of the Late Archean-Early Paleoproterozoic age, with peak metamorphic temperature and pressure of 1.2-1.3 GPa/820-850°C, and they have clockwise metamorphism evolutionary path (Lu JS et al., 2017). All these results provide some important visions to understand the complexity and tectonic background of the Archean metamorphism.

The metamorphic sedimentary rocks in the Kongling area of Yangtze block, and some basic rocks also experienced the Late Paleoproterozoic metamorphism, among which the metamorphic process for high-pressure basic granulites, garnet amphibolites, and metapelites all show clockwise P-T paths (Yin CQ et al., 2013, Wu YB et al., 2009). The Badu rock group of the Cathaysia block also experienced this same Late Paleoproterozoic metamorphism transformation, among which sillimanite-garnet-biotite gneiss, grarnet-amphibole-plagioclase gneiss, and garnet-clinopyroxene-orthopyroxene granulite all have clockwise P-T paths (Zhao L and Zhou XW , 2012).

The khondalite series of the Dunhuang Group, exposed in the eastern of Tarim Craton, and basic granulites, invading in the Milan rock group, also experienced the Late Paleoproterozoic metamorphism, with metamorphic material association as follows: the early stage is Grt+Hbl+Pl+Qtz, and metamorphic temperature and pressure of 660-700°C, 0.85-0.92 GPa; peak stage of metamorphism is Grt+Cpx+ Pl +Qtz with metamorphism temperature of 760-820°C and pressure of 1.10-1.25 GPa; the pressure-decreasing stage is Grt+Opx+ Hbl+Pl+Qtz with metamorphism temperature of 700-770°C and pressure of 0.6-0.7 GPa, having a clockwise metamorphism evolutionary path (Zhang JX et al., 2012).

The above researches show that, the metamorphic basement of the NCC experienced the Late Neoarchean metamorphic events, where the temperature and pressure were higher, belonging to granulite-facies metamorphism. The khondalite belt, northern North China, Jiao-Liao-Ji belt of East China, the metamorphic sediment rock of the Konglin area of the Yangtze block, the Badu rock group of the Cathaysia block, and the metamorphic rock in the Dunhuang area of the eastern Tarim Craton, all experienced the Late Paleoproterozoic metamorphism. They usually have the characteristic of high-pressure granulites, with clockwise evolution path. The differences of the various metamorphic stages show that the tectonic system changed significantly in time.

2.5. Background of key new understanding in North China Craton formation

Due to the discovery of high-pressure granulites noted above and date achievement to date of the new geochronology, a new understanding of the tectonic system and formation process of NCC. Some scholars proposed that NCC was developed from the collision between the east block and west blocks and joining along the central orogenic belt (Zhao WY et al., 2001), but there are some huge controversies on the di-vision of central orogenic belt, the polarity of collision and combination, and their timing. Other scholars considered that the central orogenic belt, extending in a NEE direction, dived from southeast to northwest, and had only joined at 2.5 Ga (Li JH et al., 2002; Kusky TM and Li JH, 2003; Polat A et al.,2005). However, most scholars believed that the central orogenic belt, extending at the direction of north-northeast, dived from northwest to southeast, and finally joined at about 1.85 Ga, forming North China Craton (Zhao GC et al., 2005;Kröner A et al., 2005; Faure M et al., 2007). The proposition of khondalite belt, the Inner Mongolia suture zone, and the Jiao-Liao-Ji belt in North China, changed the understanding of the tectonics of the NCC. Then the khondalite belt of Late Paleoproterozoic era (Zhao GC et al., 2005), the Jiao-Liao-Ji belt (Li SZ et al., 2005; Zhai MG et al., 2010), and the north Hebei (Kusky T et al., 2007) were successively proposed.These viewpoints have caused a new research tide on the formation and evolution of the NCC.

3. Research development of metamorphic rocks and metamorphic geology in the Phanerozoic orogenic belt

3.1. Blueschist research

In the 1960s and 1970s, as the systematic 1:50000 and 1:200000 regional geological surveys were widely carried out,some blueschist habitats were discovered, but they were not studied in depth. Since the 1980s, tectonologists and petrologists successively found blueschists in the areas of north Qilian (Wu HQ et al., 1990, 1993; Zhang JX et al., 2007; Song SG, 2009), Brahmaputra river in Tibet (Gao YL, 1984; Li C et al., 2007), at Wenduermiao inner Mongolia (Hu X and Niu SY, 1986; Gao CL, 1990), in south Qinling (Tao HX et al.,1986), Luobei-Yilan in Heilongjiang province (Ye HW, 1987;Wu FY et al., 2007; Zhou et al., 2009), Zhangshudun in the eastern of Jiangxi province (Zhou GQ , 1989; Gao S, 2001),Tangbale in Xinjiang province (Xiao XC and Ganham SA,1990), Lancang river in western Yunnan province (Zhang RY et al., 1989; Zhao J et al., 1994), Tongbai-Dabieshan (Zhang et al., 1987; Zhou GZ et al., 1989), Aksu in Xinjiang province(Xiao XC and Ganham SA , 1990), and at south Tianshan mountain and southwest Tianshan mountain in Xinjiang province (Gao et al., 1995, 1999; Zhang LF et al., 2000).Some researches were carried out, related to the problems of metamorphic petrology, tectonics, material composition, and metamorphism. Based on the tectonic background, the places where the blueschists were produced, and the average gradient of temperature and pressure, the chronologic time and distributed characteristics of the rocks in China were summarized (Dong SB, 1989; Liou JG et al. (1989). In the present century, our scholars continued to study blueschist in orogenic belts in depth, and achieved some developments.

(1) Some new high-pressure materials and mineral association have been discovered. e.g., the aragonite inclusions were found in blueschist in Mulanshan, Dabieshan mountain and blueschist in northern Jiangsu province (Zhao WY et al.,2001; Qiu HJ et al., 2002, 2003), and meanwhile, in the conjuncture of clinozoisite and low-iron epidote (Zhao WY et al.,2002). Lawsonites were found of the eclogites in the blueschist belt of Altyn Tagh-north Qilian area (Zhang JX and Meng FC, 2006; Zhang JX et al., 2007; Song SG et al., 2007).High-pressure Mg-carpholite were found in mud rock of the eclogite facies (Yu XN et al., 2009). Coesite illusions were found in garnet crystal in eclogites in the southwestern of Tianshan mountain, and coesite melts and coesite inclusions were also found in omphacites (Zhang LF et al., 2002; Lü Z et al., 2008). Deerites were found in the blueschist of Aksu,Xinjiang and Wenduermiao, Inner Mongolia province. Coesites were found in the garnet phenocryst of a mud rock in the high-pressure and ultrahigh pressure metamorphic belt in the southwestern of Tianshan mountain, which has a mineral association of garnet + phengite + albite +Paragonite + glaucophane + barroisite + quartz, and we classified it to blueschists because of included glaucophane and barroisite. This shows that these blueschists had experienced metamorphic transformation of eclogite facies (Tian ZL et al., 2016). The discovery and composition of these high-pressure metamorphic materials and its association greatly enriches the research on blueschists.

(2) The P-T-t path of blueschists metamorphic evolution,and the formation and metamorphism age of high-pressure and ultrahigh-pressure metamorphic rock have been studied in depth. The metamorphic age of eclogites and blueschists can be measured by in-situ dating of zircons. The P-T-t path of metamorphic temperature, the pressure condition of highpressure blueschists and accompanying eclogites have been measured using mineral temperature-pressures meter and phase balance simulation (Liu L et al., 2013a, b; Shen QH and Geng YS, 2012 and references). On the basis of studying of metamorphic P-T-t path, the exhumation mechanism of blueschist has been discussed (Xu ZQ et al., 2006).

(3) Certain researches that have been seldom deeply studied were enhanced. e.g. as the monographic study and geology survey was further developed, the researches on blueschists in the area of Qiangtang and Shuanghu, Tibet, and on eclogites have been enhanced (Li C et al., 2007; Deng XG et al., 2007; Zhai QG et al., 2009; Shen QH and Geng YS ,2012, and references). Based on the study of minerology, petrology, and geochemical characteristics of blueschists and eclogites, the metamorphic age could be measured, and the metamorphism evolutionary history and tectonic background could be rebuilt. Especially, the researches on geochemistry and tectonic environment of source rocks from the Mudanjiang-Yilan-Luobei blueschists belt, have been enhanced(Huang YC et al., 2008; Zhao LL and Zhang XZ, 2011; Zhou JB et al., 2009; Wang Y et al., 2009; Zhao YL et al., 2010).

Shen QH and Geng YS (2012) preliminarily summarized blueschists in China, providing two key cognitions: 1. the blueschists in China can be divided into “four ages and seventeen rock belts”; 2. the genesis mechanism of blueschists can be divided into two types, namely, the rock belt of east Qinling and Dabie-Sulu belongs to the type of inter-continent subduction-collision, the so-called 'West Alpes' type, , and most other blueschists, which are exposed in the west, belong to ocean crust subduction-collision, 'Cordillera' type.

3.2. Research on granulite of orogenic belt

Since the beginning of the 21st century, research on metamorphic rocks, such as granulite in the Phanerozoic orogenic belt, has continued to deepen on the basis of previous work,and gradually formed an upsurge of new results. The Altai Orogenic Belt, Tianshan Orogen, Altyn-Qilian Orogenic Belt,East Kunlun Orogenic Belt, Qinling-Tongbai-Dabie Orogen Belt, South Qinling-Mianlue Orogenic Belt, Tibet Banggong Lake-Nujiang Orogen, and Himalaya orogenic belt all have specific studies. The keys are still metamorphic petrology of metamorphic rocks, such as granulite, metamorphism P-T-t path and dynamics, as well as geotectonics background. After more than a decade of research, the following major progresses have been made (Shen QH et al., 2014).

The existing state of granulites in the Phanerozoic orogenic belt is different. Some granulite was produced in the orogenic belt alone; for example, there is only one type of highpressure granulite in the South Qinling orogenic belt and the Bangong Lake-Nujiang orogens in Tibet. Low-pressure granulite only occurs in the Muzart area of the Western Tianshan orogenic belt (Gou LL and Zhang LF, 2009). The different granulite types include that exposed in the Altai orogenic belt in Xinjiang is relatively complete; low-pressure argillaceous granulite, high-pressure argillaceous granulite, medium-low pressure-high pressure basic granulite, and ultra-high temperature (UHT) granulite. A muddy high-pressure granulite is seen in the eastern part of the Himalayan orogenic belt in Tibet, and there are also eclogitic garnet pyroxenites and retrograssively formed high-pressure and medium-low pressure granulite (Zhang ZM et al., 2007).

There are more metamorphic periods and certain regularities. The age of granulite in a few orogenic belts remains to be accurately determined. However, as a whole, the period of deterioration has basically been clarified. Apart from the metamorphic period of low-pressure granulite in individual orogenic belts belonging to the Jinning period, Caledonian, Hercynian, Indosinian-Yanshannian have all been seen until the Himalayan period; however, granuliate is more developed in the Caledonian and Himalayan periods with metamorphic periods of granulite from the original Tethys in the north to the Neo-Tethys orogenic belt in the south in the Western China Orogenic Belts, the formation of which is consistent with the evolution of the metamorphism.

Origin and geotectonic background of granulite in the Phanerozoic orogenic belt. Low-pressure granulite occurs in the West Tianshan orogenic belt, and it shows the anticlockwise path of an initial rapid heating-up to isobaric cooling(IBC) after the peak period; it may have formed in the stretching environment of the plate subduction process, and it is affected by a lower magmatic heat source. High-pressure argillaceous granulite, high-pressure felsic granulite, and highpressure mafic granulite are present in most orogenic belts(main bodies). The metamorphic P-T-t path shows the clockwise path of the ITD (isothermal decompressive ), and the tectonic background where it forms may be linked to a continental collision model. Most of the high-pressure granulite in the western Phanerozoic orogenic belt is rich in ophiolite or ophiolitic melange. These ophiolite types represent oceanic or oceanic shell remnants, so strictly speaking, they should be the product of oceanic subduction, continental subduction and ocean-continent collision. Some granulites in the orogenic belt, metamorphic argillaceous rocks and metamorphic basic rocks exposed in the Tula area of south Altyn Tygh, has generally experienced the metamorphic effect of medium-pressure granulite facies. This have a typical character of combination with phase transition function in a medium pressure granulite. That is, the "Barro type" metamorphism zone is shown, which is usually a magmatic and regional evolutionary metamorphic event that occurs after 50 Ma of high pressure-ultrahigh pressure metamorphism (Yu SY, et al., 2016).

Some hyperbaric granulite forms double metamorphic belts with eclogite and high pressure/ultra-high pressure belts.High-pressure granulite and high-pressure eclogite in the Dulan and Qinling orogenic belts in the southeast of Qaidam, coexist in different structural parts of the same orogenic belt.Each have their own independent metamorphic evolutionary history, and they show clockwise P-T-t paths. However, there are some differences in form; some are nearly parallel. Eclogite is formed in oceanic or continental subduction. Highpressure granulite can form in the subduction zone, and may also exist in the thickened orogeny in the lower crust. The maximum pressure of high-pressure granulite is generally 1.5-2.0 GPa, so the corresponding depth is about 50-80 km, and it is equivalent to the thickened orogenic belt bottom environment. Zhang JX et al., (2009) thought that the tectonic environment that it forms in might have been the thickening of the crust beneath the subduction zone. Such a tectonic environment has a hotter structural environment than the subduction zone, that is, it has a high geothermal gradient. Symbiosis of high pressure granulite an ultra-baisic rocks, it may reflect the position of the mantle wedge formed in the crust-mantle transition zone or near the subduction zone. High-pressure eclogite and high-pressure granulite constitute as double metamorphic belts. The formation is mainly related to the subduction of the continent, and partly formed in the dynamic environment of oceanic crust and continental crust subduction. The research on metamorphism in orogenic belts is increasingly linked with orogenicity. For example, Zhang JX et al., (2015)proposed that there are two different types of orogenesis in the Early Paleozoic of the Altyn Tygh-Qilian-northern Qaidam,that is, hyperplasia and collisional orogenicity. The main sign for this is the north Qilian-north Altyn Tygh of HP/LT metamorphic belt, with ophiolite and oceanic crust and subduction-related magmatism, as well as UHP metamorphic belts, regional Barro metamorphism, deep-melting, related magmatic activities, and extended collapse, all related to subduction and con-tinental collisions in the northern Qaidam Basin and southern Allty Tygh Basin. A tectonic model reflecting the subduction,hyperplasia, closure and collisional orogenicity of the original Tethys Ocean has been established.

Part of the granulite metamorphism formed in the thermal relaxation mode of the eclogite facies metamorphic evolution process. The formation of granulite in the North Qinling orogenic belt, the Dabie orogenic belt, and the Himalayan orogenic belt are all applicable to this model. The high-grade granulite from Songshugou in the North Qinling Orogenic Belt was converted into medium and low-pressure granulite by retrograde metamorphism (Liu L et al., 1996). The peaks of garnet pyroxenite in North Dabie of the Dabie orogenic belt are metamorphosed into eclogite facies or high-pressure granulite facies. Then they were converted to medium-pressure granulite due to thermal relaxation. Zhang ZM et al.,(2007) thought that the garnet pyroxenites that were exposed in the eastern structure of the Himalaya orogenic belt have undergone high-pressure granulite facies and metamorphic magma-facies, which can be found in the P-T-t path of highpressure granulite in the southeastern Himalayan orogenic belt, southern Himalayas (Ding L and Zhong DL, 1999). The high Himalayan zone also underwent a metamorphic modification of high-pressure granulite facies, with a clockwise metamorphic evolutionary path (Zhang ZM et al., 2017).

3.3. Research on ultrahigh pressure(UHP) metamorphic petrology

The study of eclogite in China first began in 1963 (Wang HN, 1963). With the discovery of coesite (Xu ZQ, 1987) and particulate diamonds (Xu ST et al., 1992) in eclogites in the Dabie Mountains, many foreign scholars have collaborated with Chinese scientists to study the UHP metamorphic rocks in China. From the 1980s to the end of the last century,Chinese and foreign scholars have discovered many UHP metamorphic rocks in the Dabieshan-Sulu region (Fig. 3). In regards to the types of rocks, not only eclogites that have undergone UHP metamorphism have been discovered, but also ultrahigh-pressure jadeite quartzite, marble, schist, granitic gneiss, and ultramafic rocks have been discovered. The UHP metamorphic rock outcrops west from Xinxiang, Henan province to Rongcheng, Shandong province, with a long distribution range of more than 1000 km, it is also large in scale,with many types of rocks and of the most abundant geological phenomena in the world. During this period, coesites were found between granules in eclogites (Xu ST et al, 1992; Ye et al., 1996; Liou JG and Zhang RY, 1996). The P-T calculation of the pomegranate peridotite in northern Jiangsu (Yang et al.,1993) indicates that the maximum metamorphic pressure can reach 6 GPa, (corresponding to a dive to a depth of 200 km).The discovery of exsolution lamellae of clinopyroxene+rutile+apatite in the garnet in Yangkou, Qingdao city further indicates that low-density continental crust rocks have been subducted to a depth of at least 200 km (Ye K et al., 2000).Detailed petrographic studies have shown that coesite inclusions appear in a series of hydrous ultrahigh-pressure metamorphic minerals or mineral associations (epidolite, eolianite,talc, magnesium staurolite, micorite, kyanite, and talc). This shows that there could be fluid involvement during ultrahigh pressure metamorphism (Zheng YF et al., 1996; Baker J et al.,1997; Wang K and Rumble D, 1999). At the same time, the results of various dating methods show that the UHP metamorphism in the Dabie-Sulu region occurred early Mesozoic.

From 2001 to 2005, the "China Continental Scientific Drilling Project" of the UHP metamorphic belt was implemented in the Maobei area of Donghai, Jiangsu Province.Many scientists and researchers led by Wang Da and Xu Zhiqin have successfully drilled 5155m, the first scientific drilling well in Asia and the third in the world. This is a landmark in the opening up of the scientific drilling business in China. It also marks the beginning of a new phase in China’s research on ultrahigh-pressure metamorphic petrology and geology.

Through the implementation of this project, high-precision serial sections such as lithology, geochemistry, oxygen isotopes, tectonic deformation, petrophysical properties, etc.have been established in important parts of the Sulu ultrahighpressure metamorphic belt. This has revealed the continuous material composition of the plate convergence boundary and the deep structure of the UHP metamorphic zone. It was also confirmed that there was a plate tectonic event before the 0.2 Ga in the Sulu region, and voluminous continental materials subducted into the depths of the mantle in suture zone. Many UHP metamorphism hypothesis and research methods have emerged due to the deep drilling and the in-depth study of special topics. According to the summary of Liu FL et al.,(2016), the four main aspects can be summarized as follows:

(1) The conclusive evidence for the deep subduction of massive continental crust material in the Dabieshan-Sulu ultrahigh-pressure metamorphic belt was discovered. Using laser Raman and electron probe comprehensive analysis methods, ultra high pressure mineral inclusions represented by coesite and their mineral association are commonly found in the zircons of the Dabie-Sulu UHP metamorphic belts in various types of strong retrogressive rocks (Fig. 3). The deep subduction of voluminous continental materials into mantle (ultrahigh-pressure metamorphism) in the Dabie-Sulu Terrane has been proved. It has further been confirmed that the Dabie-Sulu Terrane is the largest UHP metamorphic belt in the world with a length greater than 2000 km. With this, the dynamic evolution model of time-subduction-reentry of the Dabie-Sulu landmass has been established.

Fig. 3. The locations of Qinling-Dabie-Sulu Orogen zone (a) and Simplified geological map of HP-UHP metamorphic belt in the Sulu region (b).

(2) a new research method is used to define the P-T conditions at different stages of strong retrograde metamorphic rocks in the UHP metamorphic belt. More than 90%. The combination of early ultrahigh-pressure metamorphic minerals and P-T evolution information has been completely destroyed, which has brought great difficulties to the study of ultra-high pressure metamorphism evolution. However, detailed study found: that in the different evolutionary stages of strong retrograde metamorphic rocks, different phases of mineral association are preserved in the zircon microdomains. Using the comprehensive research methods of electron probe analysis, traditional geothermometer and geobarometer and phase equilibrium simulation, the temperature and pressure conditions for the deep subduction metamorphic rock stage,the peak ultrahigh-pressure stage, and the late retrograde metamorphic stage were first defined. This work also accurately established the metamorphic evolutionaryP-Tpath. This innovative result has aroused strong international repercussions. It not only fills the gaps in the traditional deep-dipping process of UHP metamorphism studies, but also explores a new theory and method for the study of strong retrograde metamorphic rocks evolution.

(3) The laser Raman ultrahigh-pressure technique is used to systematically study the properties and distribution of mineral inclusions in zircon. After determining the nature and properties of different micro-zones of zircon, in-situ U-Pb dating was performed by using SHRIMP or LA-ICP-MS dating techniques in different zircon microregions. It can also accurately define the Dabie-Sulu ultrahigh-pressure zone as into the metamorphic era (240-244 Ma), the peak UHP metamorphic era (215-225 Ma), and the late amphibolite metamorphic era(215-208 Ma). Based on the results of temperature and pressure studies, a continuous and complete metamorphic evolutionary P-T-t path of the Dabie-Sulu UHP metamorphism was established.

(4) Ultra-high pressure diamond inclusions were found in the zircons of eclogites and gneisses around the Guanpo area of the North Qinling Mountains, and the ultrahigh-pressure metamorphic age was determined to be 527±75 Ma. It is of great scientific significance to reconsider the attributes of the Central Orogenic Belt and its evolution.

The study of ultrahigh-pressure metamorphic petrology in the southern Altyn Tygh-northern Qaidam has also made great progress, mainly reflected the following: pseudomorphic crystals of stihovite (Liu L et al., 2007, 2013b) and coesite(Zhang LF et al., 2002) were found to reflect petrographic markers used in ultrahigh-pressure metamorphism; based on above, combined with the calculation of temperature and pressure of minerals and the simulation of phase equilibrium, it was determined that the peak pressure of UHP metamorphism reached 3-7 GPa (Liu L et al., 2007; Zhang LF et al.,2009); the in-situ dating of zircon shows that the UHP metamorphism in this area occurred in the Early Paleozoic (Zhang JX et al., 2005; Song SG et al., 2006; Liu L et al., 2013b). In addition, in the Songduo area of the Lhasa massif in the southern Tibetan plateau, high-pressure eclogites with T=730-800°C and P=>2.7 GPa were found (Yang JS et al., 2009). It is believed that there may be an east-west high pressure-ultrahigh-pressure zone in the area. As a result, a plate suture was identified in the Lhasa area. Research in this area is already at the forefront of international research.

4. Research on special metamorphic rocks

4.1. Research on very low-grade metamorphism

Very low-grade metamorphism is one of cutting-edge topics of Metamorphic Petrology-Metamorphic Geology in the contemporary era. Because very low-grade metamorphism research has played a major role in the study of prospecting, environmental protection, restoration of oil and gas and coal basin history, hydrocarbon generation mechanism, and hydrocarbon accumulation laws, it has received extensive attention.

In the 1980s, a small number of people had conducted sporadic studies on diagenetic-metamorphic boundary minerals (Zhao ZP, 1984; Ren LF and Chen YQ , 1984). In addition, the petroleum sector had also conducted research on rock diagenesis and metamorphism in some basins, but it has not been published. Zhang LF, (1992) researched the burial metamorphism of the Ordos Basin in northern Shaanxi. Suo ST et al., (1995, 1998) researched very low-grade metamorphism of the Youjiang River Mesozoic. Suo ST et al. (1995), Bi XM et al. (1998) introduced very low-grade metamorphism and research status of very low-grade metamorphism. Since 1994,Chinese Taiwan scholars have used the crystallinity of mica,carbon laser Raman spectroscopy, vitrinite reflectance, and the temperature of the stable area of pyrophyllite to perform deep low-grade metamorphism in areas such as the Central Mountain Ranges, Snow Mountains, and Taipei, Taiwan province (Shan YH and Yang HY, 1994; Yang HY et al.,1994; Chen CH, 1994).

Due to the Late Paleozoic metamorphic or non-metamorphic strata in the Northeast China being closely related to hydrocarbon resources, China’s research on very low-grade metamorphism has been concentrated in this region since the beginning of this century. Dong SB. (1986) summarized the metamorphism of the Paleozoic strata in the northeastern region, and concluded that the Variscan metamorphism was relatively minor and that most of the rocks remained in their original state. Some strong folds show slate and phyllite. The mineral combination is equivalent to a sericite-chlorite grade and belongs to the phyllie level of low-greenschist facies. According to the above data, these formation rocks were often classified as greenschist facies-low greenschist facies.Through recent studies, some scholars have suggested that the chlorite in the Late Paleozoic strata that is widely distributed in this area is only partially distributed. Jiao YG et al., (2005),Ren ZL et al., (2006), Li JK et al. (2007), Zhang XZ et al.,(2008) and Hu DQ et al(2010, 2011, 2012) studied the organic vitrinite reflectance, illite crystallinity, illite/muscovite polymorphism, and illite (muscovite) b0 (b0=9.011A) in late Paleozoic argillite and clastic rock. It is thought that the Late Paleozoic strata, widely distributed in this area, experienced only quasi-very low-grade metamorphism. According to the general classification of metamorphism in the past, the metamorphic range where the metamorphic condition temperature is lower than 350°C is divided into a sub-green schist facies,and further divided into zeolitic phase (T = 180 - 250 °C, p =<0.4 GPa) and prehnite-pumpellyite faices (T = 250 - 350 °C,P = 0.2 - 0.5 GPa). The metamorphic temperature and pressure conditions of these two phases are roughly equivalent to the very low-grade metamorphic zone A and very low-grade metamorphic zone B proposed by Bi XM and Mo XX, (2004).In addition, the chlorite may have had metamorphic origins,so we think that the Late Paleozoic strata in the Northeast are equivalent to very low-grade metamorphic zones.

4.2. Types of impact metamorphism first discovered by China

In recent years, China has discovered and confirmed the existence of extreme metamorphism-impact metamorphism formed by the impact of vermiculite. Xiuyan Crater is located in Luoquanli Village, Suzigou Town, Xiuyan Manchu Autonomous County, Liaoning province. The structure of the bowl-shaped circular pit is very clear, with a diameter of 1.8 km and a depth of 150 m. It has PGE(platinum group elements) which is not common so it caused widespread concern(Chen M. 2007; You ZD and Liu R, 2008). In recent years,Chen Ming and others have studied the structure, rock, and mineral microstructure of the ring pit in Luoquanli. Seismic cones, ultrahigh-pressure minerals, quartz grain internal surface defects (PDFs), shock homogenizers, and impact glass were discovered. A diagnostic marker was found to determine the origin of the crater impact. Thus, it was established that the ring structure in Luoquanli in Xiuyan was the first confirmed case of meteorite impact metamorphism in China(Chen M, 2007; You ZD and Liu R, 2008; Chen M et al.,2010a, b, 2011; You ZD, 2011). The Xiuyan crater was de-veloped in the Paleoproterozoic metamorphic complex (Chen M et al., 2011). The primary constituting rocks are granulite,amphibolite, gneiss, tremolite marble and marble, etc. The main signs of impact metamorphism are: surface distortions in quartz (PDFs), shock cones, impact breccia with melt, and impact molten glass. The metamorphic recrystallized quatz and coesite can be seen in some molten glasses in the rock metamorphic rocks in Xiuyan. Chen M et al., (2011) estimated impact pressures of >20 GPa and >30 GPa, respectively, based on the quasi-planetographic features of quartz and coesite.You ZD, (2011) estimated that the impact pressure might have been 45-60 GPa based on the coesite formation reaction.Based on these data, it can be considered that the pressure in the area should be greater than 30 GPa. The discovery of impact metamorphic rocks enriches the types and varieties of metamorphic rocks in China.

5. Comprehensive research on regional metamorphic rocks and compilation of the metamorphic geological map

In 1986, a task force led by Academician Dong Shenbao published the Metamorphic Geological Map of China(1:4000000) and the Metamorphism and Crustal Evolution in China. The metamorphic facies and facies series of metamorphic rocks in China were studied for the first time. The study systematically summarized the genetic types and evolution of metamorphism in China. The division of the evolutionary stage of China's metamorphism is detailed. The preparation of the national metamorphic map not only changed the understanding of regional metamorphism in the past, but also proved significant for regional metamorphism and genesis types in studying global tectonic settings and it greatly improved the level of Metamorphic Geology research. This map came about for the first time in our country. Many aspects were innovative and therefore have certain milestone significances.

Since the Metamorphic Geological Map of China (1:4000000)was published in 1986, regional geological surveys and special studies have filled gaps in the research of non-metamorphic rocks in some areas of the Tibetan Plateau. This work has accumulated much new data on Metamorphic Petrology and Metamorphic Geology. From the beginning of this century, the preparation of the Metamorphic Geological Map started with a summary of all of these new data. First, Dong Yongsheng compiled the Metamorphic Geological Map(1:1500000) on the Gangdisi and Himalayan regions of the southern Tibetan Plateau. Based on the background of plate tectonics, according to the characteristics of spatiotemporal evolution of metamorphism, the regional metamorphic units were re-divided into a metamorphic area, three metamorphic blocks and nine metamorphic zones. This classification comprehensively reflects the new research progress on metamorphic rocks in the 1:250000 regional geological survey and related special studies, especially regarding high-pressure granulite, blueschist and eclogite. The history of the evolution of the metamorphic rock series and metamorphism and the temporal and spatial distribution on the Qinghai-Tibet Plateau were discussed from the opening history of the Paleo-Tethys Ocean to the Neo-Tethys Ocean. But this map was not formally published, since then, Mao Xiaodong of Chengdu University of Technology compiled the Metamorphic Geological Map (1:1500000) of the entire Tibetan Plateau and published the 2014 Metamorphic Map of Tibet Plateau and Adjacent Region and Specifications (1:5000000) Mao XD et al.,2014). This Metamorphic Geological Map comprehensively summarizes the latest data from geological surveys and studies of metamorphic rocks in the Qinghai-Tibet Plateau since the 1:250000 regional geological surveys. It highlights the temporal and spatial changes of metamorphism and metamorphic belts during the evolution from the Paleo-Tethys Ocean to the Neo-Tethys.

From 2008, the Geological Institute of the Chinese Academy of Geological Sciences took the lead in associating with Peking University, Tianjin Geological Survey Center,and Jilin University personnel to perform metamorphic analysis of Metanolphic Geological Map of China (1:5000000).In 2016, Metamorphic Geological Map and Specifications of China (1:5000000) (Shen QH et al., 2016) and the corresponding paper - Regional Metamorphic Rocks and Metamorphism Evolution in China (Geng YS et al., 2016b) were officially published. In this compilation, the type of metamorphism was reclassified with the plate structure as the background. According to the new data, the degeneration unit was re-divided; For the first time, metamorphic rocks of eclogite facies have been expressed on the map, and the high-pressure and ultra-high-pressure metamorphic rocks in the West Kunlun-Qinling orogenic belt have been expressed in detail.Based on the past 20 years of detailed chronological data, the formation and metamorphic eras of some metamorphic rock series were re-determined; the history of the evolution of the Tibetan Plateau as outline above was discussed. The main developments in the research of Metamorphic Geology in China in the past 20 years are reflected on the map, for example, ultrahigh-pressure granulite and ultra-high-temperature granulite discovered in North China and other places, the Paleoproterozoic Khondalite series, and the late-Paleoproterozoic metamorphism of the Cathaysia block have all been fully reflected in the map; It also summarizes the latest research progress of granulite in the blueschist zone and orogenic belt in China. For the first time in the drawing, lines are used to indicate the type of deteriorating pressure, making the drawing more concise and intuitive.

Almost at the same time, Lu SN and Hao GJ, (2015) edited and officially published a geotectonics map of metamorphic rocks in China (1:2500000) in 2015. This mapping has broken away from the concept of previous metamorphic facies and proposed a new concept of geotectonic facies of metamorphic rocks. It is divided into four grades, namely, facies series (orogenic zone facies series and continental facies series), large facies (including large facies of the bonding zone, large facies of the arc-basin, large facies of the crustal block, and large facies of the block), phases (including ophiolitic melange, magmatic arc, and metamorphic basal complexes are equal to more than 20 phases) and the subfacies(e.g. the ophiolite subfacies, the Archean terrestrial subfacies,volcanic arcs, etc.). The metamorphic units were divided according to geotectonic of tectonic facies of metamorphic rocks. Different ages of which are mainly represented by hue in the on the map, and numbers representing different metamorphic facies. So the map is different from the previous Metamorphic Geological Map, both in terms of guiding ideology and drawing expression. A bold and helpful attempt was made to compile metamorphic geological maps from a geotectonic background.

6. Research on phase equilibrium of metamorphism

Metamorphic phase equilibrium is one of the core issues of Metamorphic Petrology, and is also the basis for the study of the P-T-t path. The progress in the study of metamorphic phase balance has greatly deepened the understanding of the relationship between metamorphism and phase equilibrium,and opened up a new stage for the quantitative study of metamorphism. An important development in granulite research since the 1990s is the use of quantitative methods of metamorphic phase equilibria to simulate the anatectic metamorphism, changes in melt composition, and melt loss in metamorphic minerals. The metamorphic phase equilibrium study uses of the thermodynamics database, the Thermocalc program, and the mineral phase activity model to calculate the rock genetic grid (P-T projection), mineral association diagram, P-T profile and T-X and P-X profile quantitatively. This study indicates the phase equilibrium relationship of a specific whole rock component. With the help of these diagrams,the P-T conditions of natural mineral association can be determined to explain the mineral inclusions, zoning and reaction relationships, etc., and then the P-T path of the rock can be determined. The P-T profile can be used to quantitatively calculate the contours of various mineral components, mineral molar content, and rock saturated water content. Thus not only can the P-T conditions and P-T path of the rock be more precisely defined, but the influence of fluid (or CO2) during the metamorphic evolution of the rock can also be discussed.The advantage of this method is that a mineral component can be used to determine the P-T condition of the rock. This is because when the mass balance equation is included, the degree of freedom of any combination of minerals is equal to two,therefore, as long as two component variables are determined,all other changes (including P-T and other mineral phase components and their contents, etc.) can be solved, so as to avoid the problem of whether or not the mineral compositions are balanced. In addition, the phase equilibrium method can be used to quantify the metamorphosed anatectic reaction, melt behavior, and compositional changes of the melt and residue in high-grade metamorphic rocks. This is not only important for understanding the role of fluids in the process of metamorphism, but also vital for the study of the cause of granite.Up until this century, China's metamorphic petrologists has widely used the metamorphic phase equilibrium method to study the P-T-t locus of metamorphic rocks in various metamorphic regions such as the Archaean high-grade metamorphic zone, the Paleoproterozoic orogenic belt, and the ultrahighpressure metamorphic zone, and has made significant progress. Relevant content is detailed in the relevant discussion of Wei CJ, et al. (Wei CJ and Zhou XW, 2003; Wei CJ, 2011,2012, 2016; Wei CJ and Zhu WP, 2016)

7. Conclusions

Metamorphic petrology, especially metamorphic geology coves a wide range of content and has made great progress in various fields during recent years. We can only briefly summarize here the progress made in these subjects in China in the past 40 years. It is inevitable that there will be some omissions. The literature on metamorphic rocks and metamorphic geology is wide ranging and our literature search for this article far exceeded the citations here; we apologize if any important references have been omitted.

We have made great progress through long-term research on the research into metamorphic petrology and metamorphic geology in China. However, the development is still unbalanced in the different regions, especially in the North China Craton, Dabie-Sulu area, which have had a high degree of research. In recent years some of the areas have gradually become hot spots (such as the eastern Himalaya syntaxis of the Tibetan Plateau). In some areas, metamorphic geology is relatively low. Research into some area, such as those with very low-grade metamorphism are still in their infancy. Based on the summary of this article, the following understandings can be drawn:

(1) using long-term research, we preliminary summarize the composition characteristics of different cratonic metamorphic basements in China. A large number of new highprecision chronological data preliminarily constructed the history of the formation and evolution of the metamorphic basements of the North China Craton and Yangtze Blocks from the Archean to the Paleoproterozoic. Much high-pressure granulite formed in the late Paleoproterozoic was discovered in the NCC. There are also high-pressure granulites with a clockwise P-T evolution path on the Yangtze block, the Cathaysia block, and the eastern edge of Tarim. The granulite with anticlockwise P-T path in the Archean basement has obvious differences to late-Paleoproterozoic granulite. This shows that there has been a significant change from the Archean to late Paleoproterozoic tectonic regimes.

(2) metamorphic rocks in the Phanerozoic orogens in China have undergone a metamorphic transformation of lowgreen schist facies. There are blueschist belts with low temperature and high pressure, high-pressure granulite associated with blue schist or eclogite, and eclogite belts undergoing ultra-high pressure metamorphism. In some eclogites and re-lated surrounding rocks, coesite and diamonds that reflect UHP deterioration have been discovered. This shows that the lower density continental crust can subduct large-scale to the mantle depth. New blueschist and eclogites have been discovered on the Tibetan Plateau, providing more practical information for studying its formation process.

(3) comprehensive study of metamorphic geology and the preparation of the Metamorphic Geological Map summarize the distribution, composition and evolution of metamorphic rocks in China, as well as the tectonic setting of metamorphic rock formation and metamorphic transformation. From the perspective of metamorphic geology, the basic structure of China's continental structure was also analyzed.

(4) very low-grade metamorphism study is relatively weak due to its being an extremely small research object. In the early days, some studies were conducted in Ordos and Youjiang Basin. In recent years, research has been conducted in eastern and northeastern Inner Mongolia, and the depth of the research still needs to be further deepened.

(5) impact metamorphic rocks have been found in Xiuyan,Liaoning. Coesite was found in the molten glass and its impact pressure could reach 30 GPa. The discovery of impact metamorphic rocks has enriched the types of metamorphic rocks in China.

(6) the theory and method of metamorphic phase balance have been promoted. These have been widely used not only in the study of metamorphism of high-pressure and ultra-highpressure metamorphic rocks, but also in Archaean basement metamorphic rocks. At the same time, the use of the phase equilibrium quantitative research method can also properly simulate the types of in-coming anatectic reaction, P-T conditions, melt content and its loss behavior, and changes in the chemical composition of the melt and residue in the melting process, etc. This is very important in exploring the process of high temperature-ultrahigh temperature metamorphism and the cause of granite.

Acknowledgements

This paper could not be done without the support of the National Natural Science Foundation of China (41203025),the Work Project of the China Geological Survey (12120-10811048) and the Fundamental Research Fund of the Institute of Geology of the Chinese Academy of Geological Sciences (J1615). Also thanks to the reviewers and responsible editors for reviewing and proposing revisions and improving the English language.