Water-Flux Melting of Amphibolite in Paleozoic Leucogranite from the North Qinling, Central China: Implication for Post Collisional Setting

The differentiation of the continental crust could further promote our understanding about the evolution of the silicate planets (Wang et al., 2016; Niu et al., 2013; Condie et al., 2011). The North Qinling terrane (NQT) is situated between the Luonan-Luanchuan fault and the Shangdan suture zone (Figs. 1a, 1b). The ophiolite (Dong et al., 2015, 2013; Wu and Zheng, 2013), Paleozoic granites (Wang H et al., 2016; Qin et al., 2015, 2014; Wang X X et al., 2013; Wang T et al., 2009) and ultrahigh-high pressure metamorphic rocks (He et al., 2018; Liu et al., 2016 and references therein) in the NQT indicate the Paleozoic northward subduction and closure of the Shangdan Ocean. Paleozoic granites in North Qinling were subdivided into three stages: 507-470 Ma (oceanic subduction), 460-422 Ma (syn-collision) and 415-400 Ma (post-collision) (Chen et al., 2018; Wang et al., 2009). Paleozoic granites that intruded into the Erlangping Group display depleted zircon Lu-Hf isotopic compositions and low δ¹⁸O ratios, which indicates significant crustal growth in the NQT (Wang et al., 2016). Thus, the Petrogenesis of Paleozoic granites is vital so that we can better understand the nature of the basement in the Northern Qinling and crustal differentiation.

Figure 1.  Geologic maps of the (a) Qinling orogenic belt; (b) North Qinling (NQ) orogenic belt (Dong et al., 2015).

This paper reports the zircon ages, major and trace elements, whole rock Sr-Nd isotopes and zircon Hf isotopes of the Dafanggou granite which intruded into the Proterozoic Qinling Complex to trace its source region and discuss its geodynamic implication.

The orogenic belt of Qinling is located within the center of China continent (Ren et al., 2019; Dong et al., 2015; Bader et al., 2013; Ratschbacher et al., 2003; Meng and Zhang, 2000), which is a result of collisions of multiple stages between the South and North China blocks. According to the tectono-litho stratigraphy, the Qinling orogenic belt was subdivided into four tectonic unites, consisting of the southern border of the North China Block the North and South Qinling terranes and finally the northern border of the South China Block, as depicted in Fig. 1a.

The NQT consists of Paleozoic groups of Kuanping, Erlangping, Qinling and Danfeng spanning from the north to south (Fig. 1b). Meta-basalts in the Danfeng Group represent the ophiolite mélange in the Shangdan Ocean (Dong et al., 2015). The Qinling Group contains Proterozoic metasedimentary rocks, metamorphosed igneous rocks, and metabasic blocks (Dong et al., 2015; Wu and Zheng, 2013). The ultrahigh-pressure eclogite has two stage retrograde age of 450 and 420 Ma (Liu et al., 2016; Dong et al., 2015). Voluminous Paleozoic granites intruded into the Proterozoic Qinling Complex. The Kuanping Group is made up of metamorphic greenschist-facies basites and sedimentary rocks (Wang et al., 2016; Dong et al., 2015; Wu and Zheng, 2013). The Erlangping Group locates between the Kuanping and Qinling groups, which is composed of Paleozoic volcanic-sedimentary succession, and the mafic rocks present are considered to be generated in the back-arc basin (Dong et al., 2015; Shi et al., 2013; Zhang et al., 2001).

The Dafanggou pluton occurs in the Canghuang area, northwestern part of the Shima reservoir (Fig. 2). The pluton is 3 km in length and 2 km in width, covering an area of about 2 km². The pluton commonly displays irregular and lenticular shaped and intruded into the transition zone between the Early-Paleozoic Danfeng Group and the Proterozoic Qinling Group, with local mylonitization. The leucogranite occurred in the Dafanggou pluton and display light gray medium-grained leucogranite without mafic enclaves (Fig. 3a). The main minerals include strip or long plate shapes and subhedral plagioclase (20% to 25%), subhedral K-feldspar (35%-40%), quartz (20% to 25%), subhedral biotite (5%-8%) and accessory minerals (i.e., zircon, titanite and apatite) (Figs. 3b, 3c). The xenotopic biotite is partly replaced by chlorite.

Figure 2.  Geologic map of the Dafanggou pluton (modified after SBGMR, 1989).

Figure 3.  Microphotograph of the Dafanggou leucogranite, showing plagioclase (Pl), K-feldspar (Kfs) and quartz (Qz).

The detail analytical methods of identifying major and trace elements, whole-rock Sr-Nd isotope system and zircon U-Pb analyses and in situ zircon Hf are found in the Supplementary Analytical Method.

The cathodoluminescence images of zircon crystals and the U-Pb concordant diagrams are presented in Fig. 4, and the data of the U-Pb isotope are detailed in Table S1. The zircon samples obtained from the Dafanggou leucogranite are between 70 and 150 μm in size, and most grains display stubby columns and subhedral shapes. Most grains were dark and show poor-established oscillatory zoning (Fig. 4a). Most grains display unconcordant ages, six concordant spots display variable Precambrian ages of 630 to 1 107 Ma. Nine grains have concordant ²⁰⁶Pb/²³⁸U ages of 401±7 to 413±5 Ma (Fig. 4b), giving a weighted average age of 404±9 Ma (MSWD=0.13, n=9), with Th=38.7 ppm to 398 ppm, U=411 ppm to 3 399 ppm, and low ratio of Th/U between 0.02 and 0.33.

Figure 4.  Cathodoluminescent images and laser ablation-inductively coupled plasma mass spectrometry (LA-ICP MS) U-Pb concordia diagram of the Dafanggou leucogranite.

The elemental analyses of the Dafanggou leocogranite are listed in Table S2. The leucogranite has SiO₂=68.4 wt.% to 71.7 wt.%, TiO₂=0.06 wt.% to 0.14 wt.%, Al₂O₃=15.6 wt.% to 17.9 wt.%, Na₂O=4.18 wt.% to 5.59 wt.%, K₂O=1.98 wt.% to 3.92 wt.%, with high Na₂O/K₂O ratios of 1.07 to 2.74. The aluminous saturation index (A/CNK) is between 1.04 and 1.09 (Fig. 5a), CaO=2.10 wt.% to 3.09 wt.%, Fe₂O₃^T=0.64 wt.% to 1.28 wt.%, MgO=0.17 wt.% to 0.31 wt.%, P₂O₅=0.02 wt.% to 0.03 wt.%. In the SiO₂ vs. FeO/(FeO+MgO) diagram, the leucogranite plot in the transition zone between Magnesian and Ferroan series (Fig. 5b). It is evident from the trace element spider diagram in Fig. 6b, the leucogranite is rich in Rb, Ba, Th and U, deficient in Nb, Ta, Zr and Hf. Additionally, the leucogranite has a large amount of Sr (448 ppm to 498 ppm) and low amounts of Y (7.64 ppm to 19.3 ppm), giving an elevated range of Sr/Y ratios from 42 to 65 (there are two exceptions have low Sr/Y ratios of 22 to 24). It has Rb contents of 44.5 ppm to 70.9 ppm and Nb/Ta ratios of 7.7 to 13.3. The leucogranite displays right slope LREE patterns (Fig. 6a), showing low total REE (∑REE) contents of 52.6 ppm to 90.4 ppm. It has (La/Yb)_N ratios of 8.7 to 17.8 and LREE/HREE ratios of 6.67 to 11.0, with negligible Eu anomalies (Eu/Eu*=0.79 to 1.25).

Figure 5.  (a) A/NK vs. A/CNK diagram; (b) SiO₂-FeO/(FeO+MgO) (Frost et al., 2001) for the Dafanggou leucogranite.

Figure 6.  (a) Chondrite-normalized rare earth element patterns and (b) mid-oceanic ridge basalt-normalized trace element spider diagrams of the Dafanggou leucogranite (after Sun and McDonough, 1989).

The isotopic analyses of the whole rock Sr-Nd for the leucogranite are detailed in Table S3, in which the initial isotopic indicators were determined using its zircon U-Pb age. The leucogranite has initial (⁸⁷Sr/⁸⁶Sr)_i=0.711 412 to 0.711 515, ¹⁴³Nd/¹⁴⁴Nd=0.511 588 to 0.511 592, and ε_Nd(t) values of -10.4 (Fig. 7a), with the corresponding two-stage Nd model age ranging between 2.46 and 2.56 Ga.

Figure 7.  (a) (⁸⁷Sr/⁸⁶Sr)_i against ε_Nd(t) diagram of the Dafanggou leucogranite (after Liu, 2014; data see Table S3); (b) plot of age-corrected zircon ε_Hf(t) of the Dafanggou leucogranite.

Seventeen grains in the leucogranite were selected for zircon Lu-Hf isotope analyses, and six grains display older zircon U-Pb ages of 615 to 1 002 Ma. They have initial ¹⁷⁶Hf/¹⁷⁷Hf=0.282 272 to 0.282 426, and ε_Hf(t) varied from -3.74 to +2.30 (Table S4), and the corresponding T_DM2 ages is 1 314 to 1 629 Ma. The Paleozoic zircon U-Pb ages of the other 11 grains were 381 to 413 Ma. They have initial ¹⁷⁶Hf/¹⁷⁷Hf=0.282 264 to 0.282 524, with ε_Hf(t) values between -0.25 and -9.20, and the corresponding two-stage model (T_DM2) ages is 1 212 to 1 667 Ma (Fig. 7b).

The Dafanggou leucogranites display moderate SiO₂ (68.4 wt.% to 71.7 wt.%) contents and high Na₂O/K₂O (1.07 to 2.74) and A/CNK (1.04 to 1.09) ratios, with insignificant Eu anomalies (Eu*/Eu=0.79 to 1.25) and low Rb/Sr ratio (0.09 to 0.16). Generally, the hypotheses of the genesis of these leucogranites have been proposed: dehydration or water-flux melting of amphibolite in middle-lower crust (Rapp and Watson, 1995); derived from biotite/muscovite dehydration melting of metasediments (Dario and Lombardo, 2002).

The Dafanggou leucogranite display lower A/CNK ratios (1.04 to 1.08) than that of the peraluminous melts that derived from meta-pelite or metagraywackes. In addition, the leucogranite derived from muscovite dehydration melting of meta-pelitic rocks commonly have high Rb/Sr ratio (> 2). Harris and Inger (1992) proposed that the melting of muscovite/biotite in pelites in the absent of a vapour phase will result in a small melt fraction and increase in the restitic feldspar, would result in high Rb/Sr and low Sr/Ba ratios, this is inconsistent with the low Rb/Sr (0.09 to 0.16) ratios of the Dafanggou leucogranite (Fig. 8).

Figure 8.  Plots of (a) Rb/Ba-Rb/Sr (after Sylvester, 1998) and (b) Q-Ab-Or (Zhang, 2004) diagram of the Dafanggou leucogranite, the yellow circle is the tested samples in this study.

The Dafanggou leucogranite has high Sr (448 ppm to 498 ppm) and low amounts of Y (7.64 ppm to 19.3 ppm), with high Sr/Y ratios of 42 to 65 (there are two exception have low Sr/Y ratios of 22 to 24). Combined with (La/Yb)_N ratios of 8.7 to 17.8, these features are identical with the adakites (Defant and Drummond, 1990), suggesting the garnet and amphibole residue in their source region (Martin et al., 2005). The variable Nb/Ta (7.7 to 13.3) and Zr/Hf (30.7 to 34.0) ratios of leucogranite are similar with that of the middle-lower crustal materials (Nb/Ta=8.3 to 16.6, Zr/Hf=33.9 to 35.8; Rudnick and Gao, 2003). In the Q-Ab-Or phase diagram, the leucogranite plot in the field between 300 to 1 000 Mpa, corresponding to ~10 to 30 km, which also indicate the leucogranite were derived a relatively deep source. Combined with negative ε_Nd(t) (-10.4) and ε_Hf(t) (-0.25 and -9.20) and ancient T_DM2 ages, these evidences indicate the Dafouggou leucogranite were derived from the ancient amphibolitic crust in the Qinling Group.

Furthermore, both dehydration melting and water flux melting of amphibolite can produce Na-rich granitic melt (Gao et al., 2017). Dehydration melting of amphibolite in middle to lower crust need extremely high temperature (> 950 ℃), which need underplating of mafic melts beneath the lower crust (Annen et al., 2006). Because of the absence of coeval mafic igneous rocks within the Qinling Group (Dong et al., 2015) and low T_Zrn (~700 ℃) of Dafangogu leucogranite, we preclude the possibility that high temperature melting of amphibolite. Several lines of evidence approve the low temperature melting for the leucogranite.

(1) There is a large amount of inherited zircon grains in the leucogranite and some young grains display diverse zircon U-Pb ages, indicating that the zircons were crystallized from low temperature granitic melts (Miller et al., 2003). (2) Biotite in the leucogranite also displays xenomorphic shapes, suggesting a xenocrystal origin. (3) The extremely low CaO (2.10 wt.% to 3.09 wt.%), TiO₂ (0.06 wt.% to 0.14 wt.%), Fe₂O₃^T (0.64 wt.% to 1.28 wt.%) and MgO (0.17 wt.% to 0.31 wt.%) contents, indicating limited dissolution of mafic minerals in the granitic melts (Clemens and Stevens, 2012). (4) the leucogranite displays extremely low P₂O₅ (0.02 wt.% to 0.03 wt.%) and total REE (∑REE) contents of 52.6 ppm to 90.4 ppm, indicating limited dissolution of accessory minerals in granitic melts (Hoskin et al., 2000). The above features indicate that the Dafanggou leocogranite was formed by melting of amphibolite in relatively low temperature condition. Then the water flux melting of amphibolite may be the most feasible model for the genesis of the leucogranite.

In the case of water-flux melting of amphibolite, plagioclase first dissolves into melts, which release Sr and Ba into the melts and cause the low Rb/Sr ratios of the granitic melts (Gao et al., 2017). During the water-flux melting of amphibolite, the melting reaction as follows: Amp+Pl+Qtz+H₂O=Grt+Cpx+Titanite+M (Zeng and Gao, 2017; López et al., 2005; Rapp and Watson, 1995), and the formation of garnet in the residue would cause the high Sr/Y ratios of the melt.

In summary, we propose that the Dafanggou leocogranite was derived from water-flux melting of ancient amphibolite under moderate to high pressure condition.

The Dafanggou leocogranite has zircon U-Pb age of 404±6 Ma (MSWD=0.13, n=9), this age is younger than the high-pressure metamorphic rocks within the Qinling Group which have a second retrograde age of ~420 Ma (Liu et al., 2016; Wu and Zheng, 2013), indicating post-collisional magmatism. Wang et al. (2009) also suggested that the formation of 415 to 400 Ma granites in the NQT occurred through this mechanism. In general, the formation of granites through post-collision setting need additional heat from mantle, and the decompression melting of lower crust is the most popular model (Brown, 2010). Some simulation results suggest that increasing pressure may play important role in the melting of water-statured crustal rocks (Fig. 9), indicating that water-statured crustal rocks in compression setting can melt and form leucogranite. This conclusion has great significance to understand the collision process and granite generation (Niu, 2013 and references therein). As mentioned above, we propose that the leucogranite was derived from water-flux melting of ancient amphibolite in middle-lower crust. In combination with the metamorphic works in the NQT, we propose that the following model may be the most plausible to explain the origin of the Dafanggou leocogranite: (1) during early collision, the water-statured amphibolite in the middle crust may be buried deep without significant dehydration process; (2) in the following extension stage, due to the absonant deformation behavior of the soften lower crust, the local compression setting may cause the water-fluxing melting of amphibolite and produce the leucogranite melts. This result suggests that water-fluxing of middle-lower crust in post-collision setting plays an important role in crust differentiation and granite generation.

Figure 9.  Solid curve for the water-saturated crustal rocks in different P-T condition, the asterisk is the primitive P-T condition for the potential source rocks (after Niu, 2013 and reference therein).

(1) The Dafanggou leucogranite intruded into the Proterozoic Qinling Complex has zircon U-Pb age of 404±6 Ma (MSWD=0.13, n=9), which is slightly younger than that of the retrograde age of high-pressure metamorphic rocks within the Qinling Group, indicating a post-collisional setting.

(2) The Dafanggou leucogranite displays evolved Sr-Nd isotopic (ε_Nd(t)= -10.4) compositions and negative zircon ε_Hf(t) values (-9.35 to -0.25). In combination with its low Rb/Sr ratios and high Sr/Y ratios, it is concluded that the leucogranite was formed by water-flux melting of ancient amphibolite.

(3) The occurrence of the Dafanggou leucogranite indicates that water-flux melting of middle-lower crust in post-collision setting may be a potential model for the genesis of granites in collisional orogenic belt.

This work was supported by the Project of the Youth Science and Technology New Star in Shaanxi Province (No. 2017KJXX-94), the National Natural Science Foundation of China (Nos. 41102037, 41421002), the Project of Investigation and Evaluation of Uranium Resources (No. DD2016013623), the National Excellent Doctoral Dissertation of China (No. 201324). The final publication is available at Springer via https://doi.org/10.1007/s12583-020-1335-5.

Electronic Supplementary Materials: Supplementary materials (Tables S1, S2, S3, S4 and Supplementary Analytical Method) are available in the online version of this article at https://doi.org/10.1007/s12583-020-1335-5.