In this study, zircon U–Pb dating was performed on sample 19D136-1 via LA-ICP-MS, and the results are presented in Supplementary Table S1. The zircons separated from the quartz diorite are light-yellow to colorless and are mostly prismatic with length-to-width ratios of 2–1.5. In the cathodoluminescence images, the zircon grains are relatively transparent with clear internal features and well-developed oscillatory zoning (Figure 3), possessing high Th/U ratios of 0.4–0.9. The lines of evidence suggest that zircons from the quartz diorite can be classified as magmatic origin (Wu and Zheng, 2004). Twenty-five dating experiments were conducted on 25 different zircon grains; all analyses were concordant or nearly concordant and yielded upper intercept and mean ²⁰⁶Pb/²³⁸U ages of 444 ± 3 Ma and 445 ± 3 Ma (mean-squared weighted deviation (MSWD) = 0.06), respectively (Figure 4), which are interpreted as the intrusion time of the Honghuapu quartz diorite.

The quartz diorite in the Honghuapu pluton has a moderate amount of SiO₂ (58.26%–63.49%) as well as relatively high amounts of MgO (2.45%–3.20%), Al₂O₃ (14.19%–15.74%), Fe₂O₃T (5.37%–6.09%), and Na₂O (3.29%–5.02%) with a variable amount of K₂O (1.36%–3.00%) (Supplementary Table S2). The total alkali content ranges from 5.97% to 6.85% and is characterized by low K₂O/Na₂O ratios (0.69–0.87), except for the sample 19D136-2 (K₂O/Na₂O = 0.27) whose value may be attributed to alterations (Figure 5a). The Honghuapu quartz diorite has relatively high Mg# values of 47–51 and moderate aluminum saturation index (A/CNK = 0.82–0.97) that are indicative of a metaluminous nature (Figure 5b). In the SiO₂-K₂O diagram, the majority of the analyzed samples appear to belong to the high potassic calc-alkaline series except for the sample 19D136-2 (Figure 5c).

The Honghuapu quartz diorite shows significant fractionation between the different rare earth elements (REEs), where the light rare earth elements (LREEs) are enriched while the heavy rare earth elements (HREEs) are relatively depleted (Figure 6a); it has a slight-to-moderate negative Eu anomaly (δEu = 0.61–0.81). The total REE content varies from 118 to 177 ppm, while the LREE and HREE contents vary in the ranges of 103–153 ppm and 14–24 ppm, respectively, with the (La/Yb)_n ratios ranging from 5 to 7. On the primitive mantle normalized trace element diagram (Figure 6b), the quartz diorite samples are rich in large ion lithophile elements (LILEs) like Rb, Ba, Th, U, and Pb but depleted in high-field strength elements HFSEs like Nb, Ta, and Ti.

The Honghuapu quartz diorite is likely not significantly affected by assimilation and contamination from country rock based on the following reasons. First, all the analytical samples show relatively homogeneous whole-rock major and trace element characteristics, especially consistent REE and trace element patterns. Second, the geochronological results indicate the absence of inherited zircons while capturing relatively older zircons and yield consistent type and age findings for the zircons. Third, the field outcrops do not have documented xenoliths from country rock.

The Honghuapu quartz diorite has a relatively narrow range of SiO₂ content from 58.26% to 63.94%. In combination with published data for quartz diorite in the adjacent areas (Ren, 2019), the rocks show a certain evolutionary trend where SiO₂ has negative correlations with K₂O, CaO, Al₂O₃, Fe₂O₃T, MgO, and P₂O₅ (Figure 7). Therefore, it is suggested that the magma may have undergone a certain degree of fractional crystallization of minerals, such as plagioclase and biotite, as well as a small amount of apatite during its evolution. However, the Honghuapu quartz diorites have no linear relationships with the Huangniupu, Tangzang-Yangjiazhuang, and Fanpu plutons (Figure 7), indicating that the Honghuapu quartz diorite may not have the same source as the latter.

The Honghuapu quartz diorite samples have relatively low MgO contents (2.45%–3.2%) and high FeOt/MgO ratios (1.71–2.02) compared to typical high-magnesium andesite (Tatsumi et al., 2001). In addition, the diorite shows high contents of Y (21.1–39.8 ppm) and Yb (2.45–3.03 ppm) with low ratios of Sr/Y (5–13) and La/Yb (12–14), unlike those of the classic adakite formed by melting of the subduction environment (Hastie and Kerr, 2010). The Honghuapu quartz diorite has high Ba (357–886 ppm) and Sr (198–382 ppm) contents with Ba/Sr ratios in the range of 1.5–2.7, in addition to low Rb content (50.3–102 ppm), indicating high Ba–Sr rock affinity (Figure 8).

Partial melting of the subducted oceanic island arcs or oceanic plateaus (Tarney and Jones, 1994), fractional crystallization of basaltic magmas derived from the enriched mantle and crustal contamination (Fowler et al., 2001; Peng et al., 2013; Wang, 1982), partial melting of the mafic lower crust under high-pressure conditions in the garnet stability field (Choi et al., 2009; Ye et al., 2008), and magma mixing (Zhang et al., 2006) are believed to account for the formation of the high Ba–Sr rock. The melts formed by partial melting of the subducted oceanic islands and oceanic plateaus have consistent REE distribution patterns and weak negative Eu anomalies. However, experimental petrology has shown that the melts produced by the melting of basalt with mid-ocean ridge basalt characteristics are rich in sodium (Beard and Lofgren, 1991), which is inconsistent with the relatively potassium-rich characteristics of the Honghuapu quartz diorite.

Previous studies have shown that the ⁸⁷Sr/⁸⁶Sr and ε_Nd(t) values of the Honghuapu pluton are in the ranges of 0.7046–0.7057 and −0.65 to −1.63, respectively (Ren, 2019); however, the two-stage model ages of the in situ zircon Hf isotopes (TDM₂ = 1.18–1.22 Ga) suggest that they may have originated from an ancient crustal source. The north Qinling terrane outcrops the Precambrian basement comprising the Qinling, Kuanping, Erlangping, and Danfeng groups (Zhang et al., 2002). However, it is worth noting that these groups preserve not only Precambrian rocks but also outcropped Paleozoic rocks associated with evolution of the Shangdan and Mianlue oceans (Dong et al., 2015). The Qinling group has highly evolved Sr–Nd isotopic compositions (⁸⁷Sr/⁸⁶Sr₄₄₀ = 0.7116–0.7481; ε_Nd(t) = −8.4 to −10.7), while the Mesoproterozoic metamorphosed basalts in the Kuanping, Erlangping, and Danfeng groups have low Ba and Sr contents (Ba = 70.3–76.1 ppm; Sr = 133–165 ppm) (Liu, 2014). The lines of evidence suggest that these are unlikely to be the sources of the Honghuapu high Ba–Sr rock. Therefore, partial melting of the lower crust of the north Qinling terrane is probably not the main mechanism of formation of the Honghuapu quartz diorite.

The Rogart high Ba–Sr rocks in northern Scotland were formed by the partial melting of an enriched mantle and experienced fractional crystallization as well as crustal contamination (Fowler et al., 2001; Fowler et al., 2008). The Honghuapu quartz diorite has petrological and geochemical characteristics similar to those of the Rogart pluton, indicating that it may have a similar origin. Previous studies have shown that the north Qinling terrane had an enriched mantle in the early Paleozoic but that its Sr–Nd isotopic composition is more enriched than that of the Honghuapu pluton; this precludes that an enriched mantle is the main source for the pluton. In addition, the mafic rocks derived from the enriched mantle have high Ba and Sr contents of 537–3,322 ppm and 315–1,418 ppm, respectively (Fowler et al., 2008), which are caused by metasomatism of the overlying mantle wedge by subduction-derived sedimentary melts/fluids. Thus, the Honghuapu quartz diorite may have originated from a relatively depleted enriched mantle source.

The partition coefficient of elements in fluids and melts is an effective measure for distinguishing the main factors causing mantle enrichment (Wang et al., 2014). Generally, Th and Nb have higher contents in sediments but relatively lower contents in hydrous fluids, and Nb is more compatible than Th in these sediments. Rb and Ba are fluid-mobile elements that preferentially enter the dehydration fluids during the subduction process. The Honghuapu quartz diorite has relatively high values of the Th/Zr (0.2–0.3), Rb/Y (2.0–2.6), Ba/La (14.7–26.5), and Ba/Th (49.3–89.9) ratios, which are indicative of the fluid metasomatism affinity (Figure 9). However, a few of the samples were enriched in Th and Nb, implying that the enriched mantle source experienced metasomatism with primary subduction slab-derived hydrous fluid and minor sedimentary melt characteristics.

It is widely accepted that the Shangdan ocean subducted northward beneath the north Qinling terrane during the Cambrian and early–middle Ordovician (Wu and Zheng, 2013; Liu et al., 2016); however, the tectonic background of the Qinling orogenic belt during the late Ordovician to early Silurian remains controversial, and there are four scientific viewpoints, namely, subduction of the oceanic crust (Yao et al., 2017), syn-collisional setting (Liu et al., 2016; Wang et al., 2009; Wang et al., 2013; Zhang et al., 2013), transition framework from oceanic–continental subduction to continent–continent collision (Pei et al., 2009; Zhao et al., 2015), and transition background from continent–continent subduction to collision (Liu et al., 2016).

The Honghuapu high Ba–Sr quartz diorite near the Shangdan suture zone of the north Qinling terrane shows enriched LILEs and depleted HFSEs. The lines of evidence indicate that the high Ba–Sr quartz diorite has a significant fingerprint of arc magmatic rocks (Figure 6). Previous studies show that coeval high Ba–Sr rocks adjacent to the Honghuapu pluton display similar arc-affinity geochemical characteristics (Figures 6, 9), such as the 488–484 Ma hydrous mafic rock (Wang et al., 2014), ca. 435 Ma Baihua pluton (Yan et al., 2007a), and ca. 442 Ma Xiaowangjian leucogranite (Yan et al., 2007b). In addition, the early Paleozoic Caotangou group is a volcanic–sedimentary rock that was formed in an oceanic subduction setting (Yan et al., 2007a; Xu et al., 2014; Chen et al., 2019) and preserves a large number of Ordovician paleontological fossils. The zircon U–Pb dating results of the mafic and felsic volcanic rocks in the Caotangou group show that they were formed at ca. 456–435 Ma (Ren, 2019); these results are consistent with formation age of the Honghuapu high Ba–Sr quartz diorite in this study and adjacent arc-affinity pluton (Wang et al., 2014; Yan et al., 2007a; Yan et al., 2007b).

Trace elements are generally used to discriminate the tectonic environment (Pearce et al., 1984). The Honghuapu high Ba–Sr quartz diorites have low contents of Nb and Y, as shown in the volcanic arc and syn-collision settings in the Nb-Y diagram (Figure 10a). In addition, the high Ba–Sr quartz diorites show relatively low amounts of Rb and display volcanic arc granite affinities in the Rb-Nb+Y diagram (Figure 10b). The lines of evidence indicate that the Honghuapu high Ba–Sr quartz diorites in this contribution could be formed in a subducted environment. Therefore, the outcropped arc-affinity high Ba–Sr quartz diorites and adjacent magmatic rocks could indicate that the Shangdan oceanic crust subducted northward beneath the north Qinling terrane during the late Ordovician to early Silurian; intense mantle convection is also believed to have triggered partial melting of the metasomatized mantle wedge, promoting crustal growth in the north Qinling region during the early Paleozoic.