Genesis of High-Mg Adakites in the Southeastern Margin of North China Craton: Geochemical and U-Pb Geochronological Perspectives

In the eastern North China Craton (NCC), Mesozoic tectonics was dominated by the Paleo-Pacific subduction rollback and the Tanlu crustal-scale fault movement. The regional transtension had generated extensive adakitic magmatism, some Cu-Au ore-forming but others not. To establish the geodynamic setting and any metallogenic link for the adakites from the southeastern NCC margin, we analyzed the ore-barren adakitic rocks from underground mines in the Huaibei-Linhuan coalfield (where surface igneous outcrops are scarce), and compared their ages and geochemistry with other mineralized and ore-barren adakites across Eastern China. Zircon U-Pb dating reveals two magmatic episodes in the Huaibei-Linhuan coalfield: 1) early-Early Cretaceous (ca. 130–129 Ma) (quartz-)diorite and granodiorite, and 2) late-Early Cretaceous (ca. 115.8 and 105.8 Ma) microgabbro and dolerite. Whole-rock geochemistry indicates that the (quartz-)diorite and granodiorite are high-Mg adakitic, featured by low K2O/Na2O (avg. 0.33), high Sr/La (avg. 44.3), and lack of correlation between SiO2 (fractionation index) and Sr/Y (avg. 56.55) and MREE/HREE (avg. 1.09), resembling typical adakites derived from oceanic-slab partial melting. Geochronological correlation with the regional tectonic events suggests that the slab-melting may have been caused by the Paleo-Pacific subduction rollback. Further extension and crustal thinning in the late-Early Cretaceous along the southern Tanlu fault may have formed the gabbro-dolerite in the coalfield. Geochemical comparison suggests that parental magma of the Huaibei-Linhuan adakites may have had similar water content [similar zircon 10,000*(Eu/Eu*)/Y and Eu/Eu* ratios] to typical porphyry Cu-Au ore-forming magmas, yet the former may have been considerably more reduced (lower zircon Ce/Nd and whole-rock V/Sc ratios). We considered that the assimilation of Carboniferous-Permian coal seams in the area may have further lowered the magma fO2 and thus its potential to form Cu-Au mineralization.

In the southeastern NCC margin (Anhui province) where the North China Alluvial Plain is currently situated, intrusive rocks are rarely exposed and thus less studied. In this study, we collected samples from various underground coal mines across the Huaibei-Linhuan coalfield (Figure 1). Through zircon U-Pb dating and whole-rock geochemical analyses, and by comparing geochemically with the coeval Cu-Au mineralized/barren adakites across Eastern China (esp. in the MLYRB and along/ near the southern Tanlu fault), we discuss why some adakites are ore-forming and some are not.

Geological Background
Archean to Mesozoic geological record is well-preserved in the NCC, except locally in its eastern part due to later decratonization. The craton was largely formed by extensive Archean tonalite-trondhjemite-granodiorite (TTG) emplacement, and remained relatively stable throughout the Paleozoic-Early Triassic. Afterward, the convergence between the Yangtze and North China plates (collided in the Early-Middle Jurassic) had caused widespread Mesozoic magmatism, forming the Cretaceous intermediate-mafic and bimodal (sub-) volcanic rocks in the southern NCC margin (Li et al., 2017). Some intrusive rocks were suggested to have formed by mantle underplating and had a crustal-mantle mixed source. Geochemistry of these rocks implies that the lithospheric mantle beneath the NCC is highly heterogenous (Yang et al., 2008;Zhao et al., 2009;Xia et al., 2013).
Although igneous rock exposure is generally poor in the southeastern NCC margin, many intrusions are encountered in the underground coal mines there, notably those in the Huaibei-Linhuan coalfield. The Huaibei coalfield is located close to the Xuzhou-Suzhou fault system and between the EW-trending Fengpei and Bangpu uplift zones ( Figure 1B). Faults of various sizes and types were documented in the coalfield, but consist mainly of high-angle normal faults and rare reverse faults. The regional Huangji and Subei faults were interpreted to have formed in the same stage as the regional Carboniferous-Permian folding (which formed the Tongting syncline and Nanping anticline) ( Figure 1C) (An et al., 2018).
The Linhuan coalfield is bounded by the N-S-trending Taihe-Wuhe and Subei faults, and by the E-W-trending Nanping and Fengwo faults, giving a rectangular shape to the coalfield. The Taihe-Wuhe fault is a steeply-dipping normal fault, which connects to the Tanlu fault zone in the east. Secondary faults and interlayer structures are well-developed in the coalfield: the former is mainly NE-trending and less-commonly NW-trending, whilst the latter is developed on both hanging-wall and footwall of the coal seams, which locally damaged the coal seams. Major fault activities likely commenced in the Jurassic (early-stage Yanshanian orogeny), during which the regional stress transition (to NW-SE-trending) may have formed the Xuzhou-Suzhou imbricated structure. Tectonic regime in Eastern China changed again to active extension during the middle-and latestage Yanshanian orogeny, forming many normal faults and fault-bounded basins. Normal faulting likely reached its peak during the Cenozoic Himalayan orogeny, during which many ancient structures were also reactivated/modified (Yao and Liu, 2012). Intermediate volcanism and plutonism were active during the Yanshanian movement, and were mainly controlled by the NNE-trending and EW-trending faults (Xu et al., 2004;Yang et al., 2010).
Previous geological survey documented that stratigraphy in the Linhuan coalfield comprises Ordovician and Carboniferous-Permian sequences (#3 Exploration Team of Anhui Bureau of Coal Geology, 2007). The coal-bearing sequences include the Upper Carboniferous Benxi and Taiyuan formations, Lower Permian Shanxi Formation, Middle Permian Shihezi Formation, and the Middle-Upper Permian Shiqianfeng Formation. The Carboniferous sequences comprise mainly greyish mudstone in the lower part, and crystalline limestone and sandy shale-mudstone in the upper part. The Permian sequences comprise mainly sandstone and mudstone (An et al., 2018, and ref. therein). Plutonic emplacement is locally controlled by ancient faults and interlayer fractures, which likely served as magma conduits. Many Jurassic-Cretaceous intermediate-mafic dykes were identified in the Huaibei coalfield, and are controlled mainly by NNE-trending faults and minor by EW-trending and NW-trending ones. Middle Jurassic-Early Cretaceous intermediate-felsic plutons are widespread in the southern NCC margin, and eclogite xenoliths and adakitic rocks were reported in the Xuhuai area as a rectangle tectonic region in the Southern Tan -Lu Fault (STLF) showing in Figure 1B Xu et al., 2009). Plutonic rocks intruding the coal-bearing sequence in the Huaibei coalfield include mainly diorite porphyry stock, dykes, and sills. All coal mines in the Linhuan coalfield were affected (metamorphosed) by various degrees of plutonism, which intensified toward the Subei and Daliujia faults, e.g., at the Yangliu and Haizi mines (#3 Exploration Team of Anhui Bureau of Coal Geology, 2007;Yangliu Coal Industry Co. Ltd, 2017;Zhao et al., 2019).
Intrusive rock samples in this study were collected from coal mines in the Huaibei-Linhuan coalfield. Plutons are mainly dioritic in the coal mines along the NE-trending Qiluzhou fault (Taiping, Haizi, Yangliu), whilst those in the coal mines along the NS-trending Daliujia fault (Yuandian-1, Yuandian-2, Qingdong) are mainly gabbroic and minor peridotitic.
The XRF analysis follows the Chinese national standard procedure GB/T 14506.28-2010. The powdered samples were dried at 105°C for 2-4 h and then cooled down in a desiccator. For each sample, 0.700 g was weighted and mixed with 5.200 g Li 2 B 4 O 7 , 0.400 g LiF, and 0.300 g NH 4 NO 3 . The mixture was placed in platinum crucibles (95% Pt + 5% Au). Afterward, 1 ml LiBr (1.5%) was added to the crucibles, dried on a hot-plate, and then heat (with lid closed) at 1,200°C for 10 min to form a glass disc.
Solution ICP-MS sample preparation follows the Chinese National Standard Procedure GB/T 30714-2014. Weighted dry sample powder (0.1000 g) was placed in polytetrafluoroethylene (PTFE) bottles, mixed successively with 10 ml HNO 3 , 10 ml HF, and 2 ml HCl, and then heated on hot-plate for 2-3 h to completely dissolve the samples. After cooling, the residue was washed with ultrapure water, dried again, and then redissolved in 2.0 ml aqua reqia and 1.0 ml LH 3 BO 3 on hot-plate for around 16 min. The solution was then transferred to 50 ml bottles, diluted with water and then measured (with two blanks) on an Agilent 7700e ICP-MS. The analysis follows the Chinese national standard procedure DZ/T0223-2001.
LA-ICP-MS zircon U-Pb dating was performed at the Wuhan Sample Solution Analytical Technology Co. Ltd. (China) with an Agilent 7900 ICPMS, coupled to a GeoLas HD 193 nm excimer laser ablation system. Zircon 91500 and glass NIST610 were used as external standards for U-Pb dating and trace element calibration, respectively. Each analysis incorporated a background acquisition of approximately 20-30 s followed by 50 s of data acquisition from the sample. An Excel-based software ICPMSDataCal was used to perform off-line selection and integration of background and analyzed signals, time-drift correction and quantitative calibration for trace element analysis and U-Pb dating (Liu et al., 2008;Liu et al., 2010 Y.). Concordia diagrams and weighted mean calculations were made using Isoplot/ Ex_ver3 (Ludwig, 2003). Analytical conditions include: 80 mJ laser energy, 5 Hz frequency, and 32 µm laser beam size. The standards were analyzed twice for every six samples. The data processing and concordia plot production were completed with ICPMSDataCal 10.9 and Isoplot 3.0, respectively.

Whole-Rock Elemental Compositions
For the 12 samples analyzed, their whole-rock major element contents were back-calculated to 100 wt% to remove the volatile influence. A few samples yielded very high LOI (e.g., TT07 12.36 wt%, Y2-23 10.19 wt%), which we considered to be caused by the natural coke inclusions (instead of alteration/ weathering), as demonstrated by the fresh igneous texture and partially-assimilated natural coke fragments observed under the SEM (Figure 3) In the Total Alkali-Silica (TAS) diagram, the gabbro-dolerite samples fall inside the (olivine-)gabbro fields (some marginally alkali), whilst the diorite-granodiorite samples fall inside the (quartz-)monzonite and syenite fields ( Figure 7A). Similar results are shown in the immobile Zr/Ti vs. Nb/Y diagram, suggesting that alteration/weathering has no major influence on the major element geochemistry ( Figure 7B). Both the gabbro-dolerite (ASI 0.94-1.38) and diorite-granodiorite (ASI 1.09-1.42) are peraluminous ( Figure 7C), with the former being calc-alkaline and latter high-K calc-alkaline ( Figure 7D). The Early Cretaceous Huaibei-Linhuan diorite-granodiorite are coeval with the high-Mg adakite-like diorite-granodiorite in the Liujiawa pluton (∼128 Ma), which is located further southwest in the eastern Dabie Orogen close to the Tanlu fault zone (Figure 1) (Jiang et al., 2020). Also along the Tanlu fault, the ore-related Nvshan syenogranite and the Huaiguang granodiorite in the Bengbu-Wuhe goldfield (Anhui Province) were both dated to be ∼130 Ma (Li C. et al., 2020). The Huaibei-Linhuan (quartz-)dioritegranodiorite are also of similar age (slightly younger) than the Tongguanshan Cu-Au ore-forming adakites (∼136.7 Ma; Tongling orefield), and the ore-barren high-Mg adakitic plutons at Fangjiangzhuang (∼129.1 Ma) and Qiaotouji (∼131.7 Ma) in the MLYRB (Liu S.-A. et al., 2010;Jiang et al., 2020). This suggests that the emplacement of the Huaibei-Linhuan (quartz-)diorite and granodiorite was part of the extensive Early Cretaceous (high-Mg-)adakitic magmatism, which extended from the along the eastern MLYRB northward along the Tanlu fault. From Eastern China, the Tanlu fault zone likely continues NE-ward across the Bohai sea basin into NE China, although its northeasternmost end is yet to be well identified (Ye et al., 2021). Early Cretaceous adakitic granitoids were documented at Xiuyan (ca. 129-126 Ma) in the Liaodong Peninsula (NE China) (Dong et al., 2020), and Krinichnoye (131.4 Ma) in the southern Sikhote-Alin (SE Russia Far East). As to be discussed in the later section, we attributed this major regional adakitic magmatism as a product of Paleo-Pacific subduction and rollback and/or tearing, facilitated by the reactivation of the Tanlu fault zone movement.

Petrogenesis of the Huaibei-Linhuan Magmatic Rocks
In this study, we have identified an earlier adakitic (quartz-) diorite-granodiorite phase and a younger gabbro-dolerite phase for the Early Cretaceous magmatism in Huaibei-Linhuan. For the adakitic (quartz-)diorite-granodiorite, they have relatively high Mg (MgO 2.87-4.63; Mg# 0.65-0.76) and low K 2 O/Na 2 O (0.35-0.54), resembling typical adakites derived from oceanic slab melting. Such features are distinct from the adakites derived from melting of the thickened lower crust, notably those from the Bengbu-Wuhe goldfield situated in the interior of the North China Craton (Figure 8) (Li C. et al., 2020). It is not to say that there is no crustal influence in the Huaibei-Linhuan adakitic rocks, as evidence from the presence of Precambrian inherited cores (2,317-2,632 Ma) in the Cretaceous zircons. The weighted average ages of the inherited cores for the (quartz-)dioritegranodiorite samples from Haizi (2,475 ± 33 Ma) and Taiping (2,463 ± 23 Ma) are similar to the relict magmatic zircons in (ca. 2,455 and 2,716 Ma) from the Cenozoic (∼1 Ma) Nushan basalts in the southeastern North China Craton (Ping et al., 2019), suggesting clear NCC crustal input for the Huaibei-Linhuan adakite formation. The lack of the clear correlation between the adakitic signature (Sr/Y) and fractional crystallization (SiO 2 ) suggest that the adakites were not formed by highpressure fractionation (garnet and/or amphibole) ( Figure 7D). This conclusion is also supported by the lack of upward-concave MREE-HREE pattern in the chondrite-normalized REE plots (Figure 9), or the lack of correlation between MREE/HREE (Dy/Yb) and fractionation (SiO 2 ), which would be expect for amphibole-garnet fractionation-derived adakites or normal calcalkaline rocks (such as the Huaibei-Linhuan gabbro-dolerite) (Castillo et al., 1999;Kamvong et al., 2014) (Figure 10A). Previous studies suggested that if the adakites were formed by fractional crystallization, substantial amphibole (85%) and garnet (15%) fractionation are needed (Mori et al., 2007). Our (quartz-) diorite-granodiorite samples do not fall on these fractionation trends ( Figures 10A-B), and no garnet (and very few amphibole) was found in the rocks, suggesting that the adakites were not formed from fractional crystallization. In the Rb/Y vs. Nb/Y and Ba/Th vs. Th discrimination plots, the Huaibei-Linhuan adakite samples fall between the slab-derived and melt-derived enrichment trends, suggesting that the magmas were sourced from both slab-derived fluids and melts ( Figures 10C-D). The Sr/La ratio has been used to distinguish slab-derived adakites Frontiers in Earth Science | www.frontiersin.org October 2021 | Volume 9 | Article 731233 from lower-continental crustal (LCC)-derived ones, because altered oceanic crust (with MORB-type LREE depletion and Sr enrichment by seawater) has much higher Sr/La than the LCC (Liu S.-A. et al., 2010). Our adakitic (quartz-)diorite-granodiorite samples have elevated Sr/La ratios ( Figure 11A), indicating a slab-derived origin. The slab-derived origin is also supported by Frontiers in Earth Science | www.frontiersin.org October 2021 | Volume 9 | Article 731233 the zircon trace element compositions. The zircons show in general increasing Yb/Gd with Hf content, which demonstrate a magma cooling and fractionation trend ( Figure 12A) (Barth and Wooden, 2010). The Cretaceous zircons (or zircon rims) have elevated U/Yb ratio (a proxy of LILE-enrichment), suggesting a more enriched magma source resembling that of typical continental arc (Grimes et al., 2015). This is markedly different from the Precambrian cores, which have lower U/Yb ratio that mimics zircons from typical ocean islands (e.g., Iceland). The Cretaceous and Precambrian zircons have similar Hf and Nb/Yb, showing their host magmas have similar degree of fractionation and alkalinity, respectively (Figures 12B-C) (Grimes et al., 2015). After the magma formation, the lack of distinct Ce or Eu anomalies (some even with weakly positive Eu anomaly), indicates that plagioclase fractionation was insignificant. In the chondrite-normalized REE and primitive mantle-normalized multi-element plots (Sun and McDonough, 1989) (Figures 9,  13), the adakitic (quartz-)diorite-granodiorite and the non-adakitic gabbro-dolerite have generally similar arc-type patterns, e.g., LREE enrichments (between EMORB and OIB), HREE depletions (relative to MORB), negative Nb-Ta-Ti anomalies, and positive Pb-Sr anomalies, suggesting that both rock suites were formed in a subduction-related setting. The only difference between them (apart from the adakitic characters such as the Sr-Y contents), i.e., higher LILE and LREE contents for the (quartz-) diorite-granodiorite, is likely attributed to its more fractionated nature compared with the gabbro-dolerite.

Tectonic Evolution of Cretaceous Eastern China
Cretaceous adakitic magmatism in Eastern China has been variably attributed to the progressive decratonization of North China, and/or the Paleo-Pacific subduction and rollback. As discussed in the previous section, our Huaibei-Linhuan (quartz-)diorite-granodiorite samples show geochemical features typical of oceanic slab melting-derived adakites, but FIGURE 6 | Chondrite-normalized zircon REE patterns for the Huaibei-Linhuan intrusive rock samples: Chondrite normalizing values are from Sun and McDonough (1989 (Zhou et al., 2019). The Huaibei-Linhuan and Banjing are geochemically similar in terms of their low Th content and low Rb/Y and Nb/Y ratios (suggestive of minor crustal meltderived enrichment), which are in marked contrast with the thickened lower crust-derived Bengbu-Wuhe gold ore-forming granitoids (Liu S.-A. et al., 2010) (Figures 10C-D).
Frontiers in Earth Science | www.frontiersin.org October 2021 | Volume 9 | Article 731233 subducted slab. A slab-tearing may also be possible (Zhao et al., 2016) to generate the adakites, although more work will be needed to constrain when and how it actually took place. A detailed review of the Early Cretaceous tectonics across the North Pacific by Liu et al. (2021) has revealed that the Eastern Eurasia, as facilitated by the Paleo-Pacific subduction rollback (via eastward migration of the shallow mantle convection system), has undergone three major extensional events at around 160-145 Ma, 135-120 Ma (peak), and post-120 Ma. These extensional events were accompanied by synextensional magmatism at around 160 Ma, 130-120 Ma (magmatic flare-up), and 100-80 Ma. It is thus clear that the Huaibei-Linhuan high-Mg adakitic magmatism, like many Cu-Au ore-fertile or ore-barren adakitic rocks in the southern Tanlu fault and the LMYRB, were formed during this 130-120 Ma magmatic flare-up during the peak extension episode. Meanwhile, the late-Early Cretaceous gabbro-dolerite magmatism in Huaibei-Linhuan (Qingdong: 115.8 Ma; Yuandian-1: 105.8 Ma) was formed in the post-peak extension. The highly attenuated North China continental crust around the southern Tanlu fault may have enabled higher degree of partial melting with lower degree of crustal assimilation and fractionation, thereby forming more mafic rocks in Huaibei-Linhuan. The presence of highly-attenuated crust is also supported by the complete absence of inherited zircon core or xenocrystic zircon in the Huaibei-Linhuan gabbro-dolerite.

Barren Adakites
From a metallogenic perspective, it is still unclear on why some Early Cretaceous adakites in Eastern China are Cu-Au ore-fertile (e.g., those in the MLYRB), whereas some others are ore-barren (e.g., those along the southern Tanlu fault). Based on whole-rock elemental and Sr-Nd-Pb isotope data, Liu S.-A. et al. (2010) argued that the ore-fertile MLYRB adakites were formed from partial  Frontiers in Earth Science | www.frontiersin.org October 2021 | Volume 9 | Article 731233 melting of the seawater-metasomatized Paleo-Pacific slab, whereas the ore-barren adakites along the southern Tanlu fault were derived from the delaminated North China eclogitic lower continental crust. This explanation, however, is unlikely applicable to the orebarren Huaibei-Linhuan adakitic (quartz-)diorite-granodiorite, as their formation was likely slab-melting-related similar to the MLYRB adakites. This suggestion is supported by the elevated Sr/La values, which are comparable to the ore-fertile MLYRB adakites but considerably higher than the ore-barren adakites along the southern Tanlu fault ( Figure 12A). Porphyry-type Cu-Au fertility is controlled by a number of factors, including the contents of metals, water and sulfur of the magma, the magma oxygen fugacity (fO 2 ), as well as thickness of the crust into which the magma was emplaced (e.g., Audétat et al., 2008;Richards et al., 2012;Park et al., 2019Park et al., , 2021. Comparing with our Huaibei-Linhuan samples, both the mineralized adakites in the MLYRB and the barren adakites in the southern Tanlu fault are more fractionated (Liu S.-A. et al., 2010) (Figure 7D). The MLYRB adakites have distinctly higher Sr/Y (67.96-121.89) than their barren counterparts in Huaibei-Linhuan (15.26-106.95) and the southern Tanlu fault (42.49-81.19). The "more-adakitic" (higher Sr/Y) character in the MLYRB rocks may have caused by the higher magma water content (i.e., "wetter magma"), which can suppress plagioclase fractionation and promote early amphibole ± garnet fractionation (Richards, 2011, and ref. therein).
Previous studies suggested that elevated zircon Eu/Eu* (>0.3) can reflect elevated magma water content, as widely reported from oreforming porphyries at El Salvador and Chuquicamata-El Abra (Chile), Yanacocha (Peru), Yerington (Nevada, US), Batu Hijau (Indonesia), Tampakan (Philippines), and Dexing, Qulong, and Jiama (China) (Ballard et al., 2002;Dilles et al., 2015;Lu et al., 2016). Lu et al. (2016) proposed that wet fertile magmas have commonly high 10,000*(Eu/Eu*)/Y and Eu/Eu* ratios, with both ratios being positively correlated with (Ce/Nd)/Y. Zircon Ce/Nd ratio can be used as a proxy of magma fO 2 in replace of Ce 4+ /Ce 3+ , which is hard to be accurately determined due to the very low (often below the detection limit) zircon La and Pr contents, and the zircon La content is also susceptible to contamination by minute apatite inclusions (Lu et al., 2016;Loucks et al., 2020). Yttrium is added to the denominator of 10,000*(Eu/Eu*)/Y and (Ce/Nd)/Y because its content is commonly low in fertile magmas, caused probably by early hornblende fractionation (Lu et al., 2016, and ref. therein).
From Figures 14A-B, it is seen that the Cretaceous Huaibei-Linhuan adakites have comparable 10,000*(Eu/Eu*)/Y and Eu/Eu* ratios to those of the ore-forming granodiorite porphyries in the MLYRB (Jiurui orefield; Xu et al., 2021), as well as to many other fertile adakitic suites (Lu et al., 2016). Similar to the MLYRB oreforming porphyries, the zircon 10,000*(Eu/Eu*)/Y and Eu/Eu* ratios of the Huaibei-Linhuan adakites are correlated positively with (Ce/Nd)/Y. This suggests that the Huaibei-Linhuan adakitic magma was not H 2 O-poor, as also supported by the considerable amount of hornblende present in our samples ( Figure 14C).
Therefore, we considered that the cause of Cu-Au infertility for the Huaibei-Linhuan adakites may lie in the lower fO 2 of their parental magma. In Figure 14, the Huaibei-Linhuan adakites have generally lower (Ce/Nd)/Y (avg. 0.035) than their MLYRB counterpart (avg. 0.051), suggesting lower fO 2 in the former. For the case of Huaibei-Linhuan, the Ce/Nd ratio can serve as good proxy for the Ce anomaly, as indicated by certain positive correlation (R 2 0.29) between the two ( Figure 14D). The more reducing nature of the Huaibei-Linhuan adakitic magma is further supported by the whole-rock V/Sc content, which serves as a proxy for the original magma fO 2 (Lee et al., 2005). In the V/Sc vs. Mg discrimination plot ( Figure 11B), the Huaibei-Linhuan adakites have distinctly lower V/Sc values than the Cu-Au ore-related porphyries from Quimsacocha (Ecuador) (Beate et al., 2001) and  (after Lee et al., 2005;Kamvong et al., 2014). V/Sc proxy for fO 2 of parental magma. Data from the ore-related porphyries from Quimsacocha (Ecuador) (Beate et al., 2001) and Dexing (Eastern China)  and ore-barren calc-alkaline rocks from the Sunda arc (Reubi and Nicholls, 2004) and Aleutian arc (Jicha and Singer, 2006) are also shown for comparison.
Frontiers in Earth Science | www.frontiersin.org October 2021 | Volume 9 | Article 731233 Dexing (Eastern China) . The V/Sc values remain constant with decreasing MgO, indicating that the V/Sc is not dependent on fractionation. Elevated magma fO 2 has been suggested to be critical in forming porphyry-style Cu-Au mineralization, as it would inhibit early sulfide crystallization and allow chalcophile metals to concentrate in late-stage hydrothermal FIGURE 12 | Plots of (A) Yb/Gd vs. Hf (base map from Barth and Wooden, 2010), (B) U/Yb vs. Hf (base map from Grimes et al., 2015), and (C) U/Yb vs. Nb/Yb (base map from Grimes et al., 2015) for zircons from the Huaibei-Linhuan intrusive rock samples. Primitive mantle normalizing values are from Sun and McDonough (1989).
Frontiers in Earth Science | www.frontiersin.org October 2021 | Volume 9 | Article 731233 20 fluids (e.g., Mungall, 2002;Sillitoe, 2010;Zhang et al., 2017). Mungall et al. (2015) further proposed that sulfide liquids can ascend to the shallow crust through attaching with vapor bubbles, rather than only residing in the deep crust as traditionally believed. The reasons why the parental magma of the Huaibei-Linhuan adakites was particularly low-fO 2 (even lower than that of the ore-barren adakites along the southern Tanlu fault) is unclear, but may partly be attributed to the NCC lithospheric heterogeneity, especially along/near the southern Tanlu fault (e.g., Lü, 2019;Wang et al., 2020). In Huaibei-Linhuan, assimilation of the Carboniferous-Permian coal seams (as shown by the partiallyassimilated natural coke fragments in our samples; Figure 3) would likely further reduce the fO 2 of the intruding dioriticgranodioritic magma, and hence further decrease its potential to form porphyry-style/-related Cu-Au mineralization.
CONCLUSIONS 1) Petrography, zircon U-Pb dating and whole-rock geochemistry suggests that the Huaibei-Linhuan intrusive rocks fall into two types, early-Early Cretaceous (ca. 130-129 Ma) high-Mg adakitic (quartz-)diorite and granodiorite, and late-Early Cretaceous (ca. 115.8 and 105.8 Ma) calc-alkaline microgabbro and dolerite. 2) The Huaibei-Linhuan high-Mg adakitic (quartz-)diorite and granodiorite are featured by low K 2 O/Na 2 O, high Sr/La, and lack of correlation between SiO 2 (fractionation index) and Sr/ Y and MREE/HREE. This suggests that the adakites were not derived from high-pressure garnet-amphibole fractionation or partial melting of the thickened lower crust, but from partial melting of the subducted slab. Age correlation with the regional tectonic events suggests that the slab-melting may have caused by rollback of the subducting Paleo-Pacific slab instead. Further extension and crustal delamination along the southern Tanlu fault may have led to higher-degree partial melting (and lower-degree assimilation-fractionation) that formed the gabbro-dolerite in the Huaibei-Linhuan coalfield. 3) Parental magma of the Huaibei-Linhuan high-Mg adakites was considerably less oxidized than typical porphyry Cu-Au ore-forming magmas, although their water content may have been similarly high. The low magma fO 2 nature is supported Frontiers in Earth Science | www.frontiersin.org October 2021 | Volume 9 | Article 731233 by both the low zircon Ce/Nd and whole-rock V/Sc ratios. The assimilation of the Carboniferous-Permian coal-bearing sequences in the area may have further decreased the magma fO 2 , and thus its potential to form Cu-Au mineralization.

DATA AVAILABILITY STATEMENT
The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author.

AUTHOR CONTRIBUTIONS
SZ: investigation, funding and writing. YA: supervision and writing. CL: data interpretation and writing. HW: analysis and academic support. YL: analysis and academic support. All authors have contributed to the article and approved the submitted version.

FUNDING
The project is jointly funded by the Natural Science Foundation of China (Grant No. 41802244) to the corresponding author.