Major and trace element concentrations of the analyzed samples are presented in Supplementary Table S2 and Figure 4. The retrograded eclogite samples have SiO₂, Al₂O₃ contents of 47.52–49.35 wt%, 12.25–15.13 wt%, respectively. They plot within the basalt field and sub-alkaline basalt field in TAS and Nb/Y vs. Zr/TiO₂*0.0001 diagrams (Figures 4A,B). In other diagrams, the retrograded eclogite samples show tholeiitic and E-MORB compositions (Figures 4C,D), and they belong to ocean floor basalts (Figure 4E). The spider diagram and chondrite normalized REE distribution patterns of the retrograded eclogite samples exhibit similarities with E-MORB (Figures 4F,H).

The mafic granulite samples exhibit large variations in major elements, and they plot in picro-basalt—basalt—andesite fields (Figure 4A). Group A granulite samples have higher SiO₂, Al₂O₃, and lower MgO, Fe₂O₃T contents compared with samples of Group B, consistent with the above observations that the ferromagnesian mineral contents of Group A granulite samples are lower than those of Group B. In the Nb/Y vs. Zr/TiO₂*0.0001 diagram, these samples plot within the sub-alkaline—andesite—dacite fields (Figure 4B). The mafic granulite samples are also tholeiitic and belong to ocean floor basalts as shown in discrimination diagrams (Figures 4C–E). The spider diagram and chondrite normalized REE distribution patterns of the mafic granulite samples exhibit similarities with OIB (Figures 4F,H).

The three high-grade rocks have all been dated in a previous study, using the LA-ICPMS zircon U-Pb method, which gave metamorphic ages of 245–251 Ma, and also a protolith age of 997 ± 27 Ma for the mafic granulite sample (Zhao L. et al., 2017). In this study, zircons of the two retrograded eclogite samples exhibit similar appearance and internal structures (Figure 5). The zircon grains are rounded, small ellipsoidal and are multi-faceted. They are unzoned or show fir-tree zoning patterns, implying metamorphic origins (Vavra, 1990). These two retrograded eclogite samples give uniform apparent ²⁰⁶Pb/²³⁸U ages with low Th/U ratios and they constrain metamorphic ages of 248–249 Ma (Figure 5; Supplementary Table S3). Some of the zircon grains of the mafic granulite sample show core-rim structures with cores exhibiting oscillatory zoning while rims without zoning or showing fir-tree zoning (Figure 5). The zircon cores that do not show core-rim structures are mostly unzoned or show fir-tree zoning or sector zoning. The analytical results define a Discordia with an upper intercept age at 953 ± 17 Ma and a lower intercept age of 246 ± 6 Ma. The zircon grains with ages of ∼246 Ma show uniformly low Th/U ratios while those with older ages have high Th/U ratios (Figure 5 and Supplementary Table S3), which imply their metamorphic and magmatic origins. These results are consistent with previous studies that the high-grade metamorphism occurred during the Early Triassic (Zhao L. et al., 2017; Xia et al., 2021).

The zircon grains of the two retrograded eclogite samples show significant differences in O isotopic compositions (Supplementary Table S4 and Figure 5). The δ¹⁸O (V-SMOW) values are 3.85–6.08 for zircons from the sample WY1215-2, and 8.46–11.34 for those of the sample WY1216-3. The distinct O isotopic compositions of these metamorphic zircons suggest zircon precipitation from different metamorphic fluids. The δ¹⁸O (V-SMOW) values of zircon cores and rims from the mafic granulite sample show large variations, with cores having low δ¹⁸O values of ∼5.5 while rims having high δ¹⁸O values around ∼8 (Supplementary Table S4 and Figure 5).

The zircon Hf isotopic compositions of the two retrograded eclogite samples are relatively homogenous, with ¹⁷⁶Hf/¹⁷⁷Hf ratios of 0.282646–0.282521 for WY1215-1 and of 0.281223–0.282806 for WY1216-2 (mostly within the range of 0.282605–0.282798, Supplementary Table S5). Ɛ_Hf(t) values are all negative for zircons from the sample WY1215-1 (ranging from −5.9 to −3.4) and mostly positive for zircons from the sample WY1216-2 (ranging from −16.1 to +6.6, mostly ranging from 0.9 to 6.6). The single-stage depleted mantle model ages of the sample WY1215-1 are 1011–1088 Ma and 619–2787 Ma (mostly within the range of 619–892 Ma). For the reason that the zircons from the granulite sample exhibit complex internal structures, their Hf isotopic compositions also show large variations, with ¹⁷⁶Hf/¹⁷⁷Hf ratios ranging from 0.281980 to 0.282654 and related Ɛ_Hf(t) values ranging from −22 to +5.6 (Supplementary Table S5). The single-stage depleted mantle model ages are 829–1772 Ma. In the zircon U-Pb age vs. Ɛ_Hf(t) value diagram, the analyzed results plot above and below the CHUR evolutionary line, based on calculations using apparent zircon U-Pb ages (Figure 6). However, if using the protolith age of the mafic granulite (∼960 Ma) as the starting age, Ɛ_Hf(t) values of most analyses plot above the CHUR evolutionary line (Figure 6).

The six analyzed samples show quite different whole-rock Sm-Nd isotopic compositions (Supplementary Table S6). The retrograded eclogite have higher ¹⁴³Nd/¹⁴⁴Nd ratios (0.512487–0.512694) than the granulite samples (0.511343–0.512144). The single-stage depleted mantle model ages are mostly Mesoproterozoic for the retrograded eclogite samples (1360–1790 Ma), except one sample with an Archean model age of ∼2620 Ma, while those of the mafic granulite samples are older, at 1860–3020 Ma. The Ɛ_Nd(t) values (assuming that the protolith ages are Neoproterozoic based on the protolith age of the mafic granulite, Zhao L. et al., 2017) of the retrograded eclogite samples are positive, ranging from 1.94 to 5.46 while those of the two mafic granulite samples are negative, one at −1.35 and the other at −7.74.

As mentioned earlier, the Indosinian orogeny affected large areas of the SCB. Although previous studies claimed that many of the Neoproterozoic sequences belonging to the Banxi Group represent the Precambrian basement of the SCB, Indosinian reworking of these sequences are significant, represented by folding, shearing and long-distance thrusting within the Jiangnan belt (Chu et al., 2012a; Chu et al., 2012b; Chu et al., 2012c; Liu L. et al., 2012; Wang Y. et al., 2005). These deformation features are typical of thin-skinned tectonics, involving only upper crustal components. The occurrences of the high-grade rocks in the northern Wuyi terrane suggest that the orogeny of this region affected deep lower crustal components. The peak pressure conditions of the retrograded eclogite samples indicate the crustal thickness of the northern Wuyi terrane was almost doubled during the orogeny. The decompressional reaction textures and the isothermal (or slow heating) decompressional P-T stage in the P-T trajectories imply that the orogenic belt experienced quick collapse and the high-grade rocks experienced fast uplifting during the late thermal relaxation stage (Figure 3, Zhao L. et al., 2017). The metamorphic features of these high-grade rocks are strongly reminiscent of those found in plate margin contractional settings (Brown, 2009; O'Brien, 1993). New SIMS zircon U-Pb dating results confirm the time of metamorphism at 246–249 Ma (Figure 5). Indosinian metamorphism has also been reported from the Yunkai terrane in the southwestern Cathaysia Block (Figures 1, 7, Chen C.-H. et al., 2012; Lin et al., 2008; Wan et al., 2010). However, the occurrences of Indosinian metamorphism are not continuous from the Wuyi terrane to the Yunkai terrane, implying that they might not belong to the same Indosinian orogenic belt. Tectonic geometry as well as metamorphic features of the Yunkai terrane exhibit close affinities to the Indosinian tectonothermal events in southwestern SCB and Indochina, which are inconsistent with those of the Wuyi terrane (Lin et al., 2008; Faure et al., 2014; Faure et al., 2016a; Faure et al., 2016b).

The Indosinian high-grade metamorphism in east Eurasia that attracted the most international interests occurs along the Central China Orogenic belt, which is further divided into small segments of the Qinling, Tongbai, Hong’an, Dabie and Sulu (Figure 1). This composite orogenic belt records the complete amalgamation history of the two major continental parts of east Eurasia, namely the SCB and the North China Craton, and besides, the identification of ultrahigh-pressure metamorphic minerals like coesite and diamond from eclogites with continental crustal chemical compositions imply deep subduction of the SCB continental crust (Dong et al., 2016; Okay and Celal Şengör, 1992; Wang et al., 1989; Wu and Zheng, 2013; Xu et al., 1992; Zhai et al., 1995; Zhao Z.-F. et al., 2017). Multiple geochronological studies constrain the time of eclogite facies metamorphism of the Sulu and Dabie segments to be at 255–200 Ma, mostly Triassic (Figure 7, An et al., 2018; Cheng et al., 2011; Li S. et al., 2017; Liu et al., 2004, Liu et al., 2006; Wu and Zheng, 2013), synchronous with these of the Wuyi terrane. As to the Qinling segment of the Central China Orogenic belt, its high-grade metamorphism occurred mainly during Paleozoic and Triassic overprintings are not very strong (Figure 7, Dong et al., 2011). The influence of such an extreme contractional orogen is significant and many of the Indosinian metamorphic events in East Asia have been correlated with this orogenic belt, like those in the Korean Peninsula and Japan (Kwon et al., 2009; Liu, 1993; Oh and Kim, 2021; Ree et al., 1996; Tsujimori, 2002; Ernst et al., 2007). As shown in Figure 1, the striking direction of the Sulu segment of the Central China Orogenic belt is parallel with the Indosinian orogenic belt in the Wuyi terrane of the Cathaysia Block, both following northeast-southwest direction. Almost synchronously, the collision between the SCB and the Indochina Peninsula resulted in high-grade metamorphism in southwest SCB and also Indochina (Faure et al., 2014, 2016a, b), whose major tectonic geometry is, however, mainly northwest-southeast (Faure et al., 2016b).

Another important plate that greatly affected the Phanerozoic SCB and which has been overlooked in many previous studies is the one facing its southeastern continental margin, the Paleo-Pacific plate (Figure 1). Although still controversial, many geologists suggested the onset of the SCB active continental margin facing the Pacific plate during Mesozoic (Zhou and Li, 2000; Zhou et al., 2006), or during Paleozoic (Li et al., 2006; Li and Li, 2007; Sun et al., 2011). However, pre-Triassic geological records of interactions with the Pacific oceanic plate are now absent in the southeastern SCB continental margin, which is mainly composed of late Mesozoic magmatic rocks (volcanics and intrusive rocks, Shu, 2012; Xu et al., 2007). The Japanese islands, which preserve various geological records of oceanic subduction, accretion, subduction erosion and the formation of the continental crust due to interactions with the Pacific plate, ranging from Paleozoic to Cenozoic, have been suggested to show consanguineous with the SCB (Figure 8, Taira, 2001; Wakita, 2013; Pastor-Galán et al., 2021; Isozaki et al., 2010; Wallis et al., 2020). Indeed some of the early Precambrian lithologies in the Hida belt of Southwest Japan show consanguineous with those of the North China Craton as previously suggested (Figure 8, Horie et al., 2010; Kawabata et al., 2021; Harada et al., 2021a, 2021b; Kimura et al., 2019), but these sporadically occurred Paleoproterozoic intrusive rocks (∼1.85 Ga) as well as the Archean-Paleoproterozoic detrital and/or inherited zircon grains are also comparable to those of the Badu Complex in the Wuyi terrane of the Cathaysia Block (Li et al., 2014; Yu et al., 2009, 2012; Zhao et al., 2015; Isozaki, 2019). Besides, the Paleozoic (∼450–410 Ma) and/or Indosinian metamorphic overprintings, as well as the Neoproterozoic lithologies of the Wuyi terrane in northeast Cathaysia Block also provide arguments for a close consanguinity of Japan with the SCB (Zhao et al., 2016, Zhao et al., 2020; Xu et al., 2007; Shu et al., 2008a, Shu et al., 2008b; Horie et al., 2010; Kimura et al., 2019; Harada et al., 2021b). These correlations support the inference that the ancient continental fragments preserved in Japan probably originated from the SCB (Aoki et al., 2015; Isozaki et al., 2010, Isozaki et al., 2014; Tsutsumi et al., 2017; Isozaki, 2019; Pastor-Galán et al., 2021). Besides, the Paleozoic and Mesozoic geological components, like the Paleozoic accretionary complexes, the Mesozoic accretionary complexes, as well as the arc batholiths and ophiolitic mélange in Japan, are quite likely to be the missing continental marginal components of the SCB during the Indosinian orogeny(Isozaki et al., 2010, Isozaki et al., 2014; Isozaki, 2019; Pastor-Galán et al., 2021). Metamorphism of the Japanese islands are mainly low-temperature, high-pressure subduction type and continuous from Paleozoic onwards (Figure 7, Ishiwatari and Tsujimori, 2003; Erst et al., 2007; Matsunaga et al., 2021; Takahashi et al., 2018; Tsujimori, 2002; Tsujimori et al., 2006; Tsujimori and Liou, 2004). Alongside with these subduction zone metamorphism, the low (and medium) dP/dT metamorphism can also often be seen (Takahashi et al., 2018), which collectively termed as the ‘paired metamorphic belts’ (Miyashiro, 1961, 1973). One thing worthy of noticing is the preservation of geological records for an accreted oceanic plateau in Southwest Japan (Tatsumi et al., 2000). These geological records are critical for a better understanding the geological history of the Phanerozoic SCB.

The eclogite and granulite facies mineral assemblages indicate that the studied mafic rocks experienced high-grade metamorphism which might render their whole-rock chemical compositions unreliable, especially the major elements and the fluid-mobile elements like Th, K, etc. The large spread of these samples in Figure 4A partly attests to this inference. However, some overall variation trends can still be extracted from the relatively immobile element plots (Figure 4), like their sub-alkaline basaltic and tholeiitic compositions. They show close affinities to oceanic crust, with the retrograded eclogite samples resembling E-MORB while the mafic granulite samples resembling OIB. The protoliths of these rocks, therefore, are interpreted to represent the remnants of a disappeared ocean.

Zircon O-Hf isotopic compositions of these metamorphosed mafic rocks exhibit large variations as well, but the magmatic zircon cores preserved in the mafic granulite sample have mantle-like O isotopic signatures (Figure 5, Valley et al., 1994). The Neoproterozoic age of 946 Ma constrained by these magmatic zircon cores is interpreted to represent the protolith age of the mafic granulite. Both the zircon Lu-Hf and whole-rock Sm-Nd isotopic compositions of these mafic rocks show large variations. Considering that the hosting rocks of these mafic rocks are the Paleoproterozoic sedimentary rocks of the Badu Complex, any influence from which during high-grade metamorphism will elevate model ages while lowering Ɛ(t) values for both Lu-Hf and Sm-Nd isotopic systems, the high-grade samples of this study with the youngest single-stage model ages and the highest Ɛ (t) values should be regarded to give the best constraints on the features of these meta-mafic rocks. As described above, the youngest single-stage Hf model ages for both the retrograded eclogite and mafic granulite samples are Neoproterozoic, 619–892 Ma for the retrograded eclogite, and 829—∼1000 Ma for the mafic granulite. Whole-rock Sm-Nd isotopic compositions for these high-grade rocks seem to have been disturbed during metamorphism because they exhibit large variations on one hand and on the other hand, the single-stage model ages are Mesoproterozoic to Archean, significantly older than the Hf model ages. The Neoproterozoic protolith ages (990–950 Ma) for these metamorphosed mafic blocks are, therefore, supported by both the magmatic zircon core U-Pb age of the mafic granulite, and the Hf - Nd isotopic compositions.

Previous studies have revealed the occurrences of two episodes of Neoproterozoic magmatism in the Cathaysia Block, one at ∼970 Ma and the other at 860–800 Ma (Shu et al., 2008a; Shu et al., 2011; Wang Y. et al., 2014). They have been interpreted to occur in the active continental margin and within plate rifting environments, respectively (Shu et al., 2008a; Shu et al., 2011). If taking the zircon core age of ∼950 Ma as the protolith age, the metamorphosed blocks of this study can correlate well with the ∼970 Ma magmatism in the Cathaysia Block, representing continental margin components. The protoliths of these metamorphosed mafic blocks, generated during Neoproterozoic, can also be compared with the widespread Neoproterozoic lithologies in the Jiangnan belt (Wang et al., 2013b; Wang Y. et al., 2014; Yao et al., 2014). All these Neoproterozoic magmatic rocks are related to the tectonothermal events that amalgamated the Yangtze and Cathaysia Blocks, and the subsequent rifting (Nanhua Rift) (Li et al., 2009a; Li W. X. et al., 2010; Wang X.-L. et al., 2012). Anyway, the Neoproterozoic magmatic rocks are important components of the continental crust of the SCB during the Indosinian orogeny, rather than newly generated oceanic crustal component (Yu et al., 2008; Shu, 2006, 2012).

If based solely on the Indosinian high-grade metamorphism of the Wuyi terrane in northeastern Cathaysia Block, the conclusion of a continental margin orogenic belt can be assigned to the SCB Indosinian orogen, because these metamorphic rocks show identical features with those developed in continental margin orogenic belts (Brown, 2009; O'Brien, 1993). But the geological implications of high-grade metamorphism are not always exclusive and as pointed out by Raimondo et al. (2014), the crustal shortening and thickening, and exhumation high-grade metamorphic rocks from deep crustal levels of intraplate orogens are comparable with their plate-margin counterparts. Therefore, the eclogite and the granulite facies metamorphism such as these of the Wuyi terrane could suggest both intraplate and plate marginal settings.

The spatial occurrence of orogenic metamorphism has been suggested to be a powerful tool in discriminating styles of orogens (Raimondo et al., 2014; Zhai, 2009; Li et al., 2016c, Li et al., 2016d). As summarized by Zhao et al. (2015) and shown in Figure 1, the occurrences of the Indosinian high-grade metamorphism are found to occur only in the northeastern Cathaysia Block (Wuyi terrane) and are absent in the Nanling area. In other words, their distribution is not continuous and does not form a complete belt that can demarcate the continental boundaries. Such a distribution pattern of high-grade metamorphism is inconsistent with those developed in continental marginal settings, but rather indicates an intracontinental setting. Besides the mafic blocks studied in this paper which show continental crust characteristics, other lithologies in the Cathaysia Block exhibiting Indosinian high-grade metamorphic alterations mainly belong to the Paleoproterozoic Badu and Mayuan Complexes of the Wuyi terrane (Yu et al., 2012; Zhao et al., 2015; Zhao L. et al., 2018). The occurrences of all these rocks indicate that the Indosinian orogeny affected only the ancient continental crust components, without the addition of any juvenile components newly generated from mantle which is, however, usually normal for continental marginal orogenic belts due to the consumption of oceanic crust before the final collision (e.g. Jamieson and Beaumont, 2013; Li et al., 2016e; Hou et al., 2021). In summary, although the high-grade rocks in the Wuyi terrane record metamorphism similar to those seen in continental margin settings, they are more likely to have occurred in the intracontinental environment.

The development of intracontinental orogens normally requires the existence of weak lithospheric zones within the continental block (Collins et al., 2011; Raimondo et al., 2014; Sokoutis and Willingshofer, 2011). As mentioned earlier, a Neoproterozoic rifted system termed the Nanhua rift developed in the SCB, as indicated by the wide occurrences of Neoproterozoic magmatism (860–800 Ma) (Shu et al., 2008a, Shu et al., 2011; Li et al., 2005; Wang and Li, 2003; Wang et al., 2007). This rift system is also known to be a failed rift because it aborted soon afterwards and no new ocean was developed. The South China Caledonian orogeny should have solidified most of the failed rift and formed another basin (foreland?) in the Wuyi terrane which behaved as a branch of the Paleo-Tethys Ocean and provided the weak lithospheric zone in the Cathaysia Block (Figure 9A). Another critical cause of the intraplate orogen is the driving force. Stress source of orogenic belts, either occurring in the intraplate or plate margin settings, are originally derived from the viscous mantle convections, which operate as conveyors and transport the lithosphere to different places of the Earth, resulting in the formation of convergent and divergent plate margins (Collins, 2003; Collins et al., 2011). The driving forces of intraplate orogens have been more specifically suggested to include far-field stress from plate margins, and intraplate stress largely related to vertical tectonic processes (Raimondo et al., 2014). The geometry of the Indosinian orogen is inconsistent with intraplate stress and implies stress sources from continental margins. The mantle convection cell in the Paleo-Tethys tectonic realm transported the SCB northward (today’s orientation), closing the Paleo-Tethys Ocean and forming the Central China Orogenic belt during Permo-Triassic (Collins et al., 2011; Ratschbacher et al., 2003; Wu and Zheng, 2013). The subduction of the paleo-Pacific plate underneath the SCB (including Japan) during the Paleozoic represents another mantle convection cell with a northwestward (today’s orientation) conveying direction (Figure 9A, Li et al., 2006; Li and Li, 2007). Previous studies on the Indosinian deformation and magmatic activities of the SCB revealed that during ∼260–250 Ma, an oceanic plateau arrived at the subduction zone which choked the magmatic arc and besides, the stress, that used to localize on the southeastern continental margin of the SCB, propagated from the continental margin northwestward to inland regions (Figure 9B, Li and Li, 2007; Tatsumi et al., 2000). Based on these discussions, we suggest that the far-field stress from the southeast margin of the SCB due to interactions with the paleo-Pacific plate must have facilitated the formation of the SCB Indosinian intraplate orogen. The spatial distribution of the Indosinian high-grade metamorphism in the Cathaysia Block is parallel with the Sulu orogenic belt (Figure 1) and the metamorphic ages of the high-grade rocks within these two orogenic belts greatly overlap (Figure 7), implying that this segment of the Central China Orogenic belt along the northern margin of the SCB is another stress source for the Indosinian intraplate orogeny of the SCB (Figure 9B). This inference is consistent with previous studies which have already proposed that the Indosinian orogeny of the SCB is closely related to the far-field stress from the Central China Orogenic belt (Zhang et al., 2013; Li J. et al., 2017; Wang et al., 2021).

A complete model for the formation of the Indosinian intraplate orogen in the SCB is, therefore, proposed based on the above discussions (Figure 9). After the amalgamation of the Yangtze and Cathaysia Blocks during Neoproterozoic, the SCB soon witnessed rifting and formed the Nanhua Rift (Li W. X. et al., 2010; Li et al., 2005; Wang X.-C. et al., 2012, Wang X.-L. et al., 2012). This rift system aborted soon afterwards without the formation of open oceans. During Paleozoic, the South China Caledonian orogeny closed solidified this rift system and likely formed a foreland basin in the Cathaysia Block which remained there until Permo-Triassic (Figure 9A, Shu et al., 2008a; Shu et al., 2011), providing the lithospheric weak zone for the intraplate orogenic events. The onset of the active continental margin of the SCB facing the paleo-Pacific (Panthalassa) plate dates back to the Paleozoic and during Permo-Triassic, the arrival of an oceanic plateau at the subduction zone choked the magmatic arc and the stress propagated to the inland regions of the SCB. Almost simultaneously, the collisional events between the SCB and the North China Craton slowed the northward moving of the SCB, providing another retroactive stress source to the SCB (Figure 9B). Therefore, stress from both the northwest (Sulu orogenic belt) and southwest (paleo-Pacific) margins during Permo-Triassic acted within the SCB, forming a super-contraction zone in the Wuyi terrane of the northeast Cathaysia Block. The weak lithospheric zone of the Cathaysia Block accommodated the strain and witnessed strong deformation and significant crustal thickening, termed as the Indosinian Wuyi orogenic belt (Figure 9B). After the collision of the SCB with the North China Craton, the major driving force for the convergence of these two continental blocks is believed to be the dragging of the eclogitized mafic crust of northern South China (Dong et al., 2013), providing an extensional environment in the Cathaysia Block which resulted in orogenic collapse and the formation of post-orogenic magmatism (Figure 9C, Li W. et al., 2012; Sun et al., 2011, Sun et al., 2017). This new model is not only consistent with the geological records in and around the SCB but also has geological implications for interpreting intraplate orogens worldwide.